This work was partly supported by EVSSAR (European Veterinary Society for Small Animal Reproduction) Grant 2016 and by Universit&#224; degli Studi di Milano, Piano di Sostegno alla Ricerca 2019 (Linea 2 Azione A). We also wish to thank Dr. MariaGiorgia Morselli for her contribution to the experiments hereby depicted and to image acquisition.

The procedures hereby depicted did not undergo ethical approval since cat ovaries were collected at veterinary clinics as byproducts from owner-requested routine ovariectomy or ovariohysterectomy.
1. Oocyte collection
<ol>
	<li>Before starting the experiments, prepare solutions for ovary and oocyte collection. Prepare ovary collection solution with phosphate buffered saline (PBS) with a mixture of antibiotics and antimycotics (100 IU/mL of penicillin G sodium, 0.1 mg/mL of streptomycin sulph...

<ol>
	<li>Bankowski, B., Lyerly, A., Faden, R., Wallach, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+social+implications+of+embryo+cryopreservation.">The social implications of embryo cryopreservation.</a> Fertility and Sterility. 84 (4), 823-832 (2005).</li><li>Saragusty, J., Arav, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Current+progress+in+oocyte+and+embryo+cryopreservation+by+slow+freezing+and+vitrification.">Current progress in oocyte and embryo cryopreservation by slow freezing and vitrification.</a> Reproduction. 141 (1), 1-19 (2011).</li><li>Luvoni, G. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Gamete+cryopreservation+in+the+domestic+cat.">Gamete cryopreservation in the domestic cat.</a> Theriogenology. 66 (1), 101-111 (2006).</li><li>Thongphakdee, A., Tipkantha, W., Punkong, C., Chatdarong, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Monitoring+and+controlling+ovarian+activity+in+wild+felids.">Monitoring and controlling ovarian activity in wild felids.</a> Theriogenology. 109, 14-21 (2018).</li><li>Parks, J. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hypothermia+and+mammalian+gametes.">Hypothermia and mammalian gametes.</a> Reproductive Tissue Banking. , 229-261 (1997).</li><li>Carabatsos, M. J., Sellitto, C., Goodenough, D. A., Albertini, D. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Oocyte-granulosa+cell+heterologous+gap+junctions+are+required+for+the+coordination+of+nuclear+and+cytoplasmic+meiotic+competence.">Oocyte-granulosa cell heterologous gap junctions are required for the coordination of nuclear and cytoplasmic meiotic competence.</a> Developmental Biology. 226 (2), 167-179 (2000).</li><li>Murakami, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Blastocysts+derived+from+in+vitro-fertilized+cat+oocytes+after+vitrification+and+dilution+with+sucrose.">Blastocysts derived from in vitro-fertilized cat oocytes after vitrification and dilution with sucrose.</a> Cryobiology. 48 (3), 341-348 (2004).</li><li>Merlo, B., Iacono, E., Regazzini, M., Zambelli, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cat+blastocysts+produced+in+vitro+from+oocytes+vitrified+using+the+cryoloop+technique+and+cryopreserved+electroejaculated+semen.">Cat blastocysts produced in vitro from oocytes vitrified using the cryoloop technique and cryopreserved electroejaculated semen.</a> Theriogenology. 70 (1), 126-130 (2008).</li><li>Luciano, A. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effect+of+different+cryopreservation+protocols+on+cytoskeleton+and+gap+junction+mediated+communication+integrity+in+feline+germinal+vesicle+stage+oocytes.">Effect of different cryopreservation protocols on cytoskeleton and gap junction mediated communication integrity in feline germinal vesicle stage oocytes.</a> Cryobiology. 59 (1), 90-95 (2009).</li><li>Comizzoli, P., Wildt, D. E., Pukazhenthi, B. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=In+vitro+compaction+of+germinal+vesicle+chromatin+is+beneficial+to+survival+of+vitrified+cat+oocytes.">In vitro compaction of germinal vesicle chromatin is beneficial to survival of vitrified cat oocytes.</a> Reproduction in Domestic Animals. 44, 269-274 (2009).</li><li>Alves, A. E., Kozel, A. C., Luvoni, G. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Vitrification+with+DAP+213+and+Cryotop+of+ex+situ+and+in+situ+feline+cumulus-oocyte+complexes.">Vitrification with DAP 213 and Cryotop of ex situ and in situ feline cumulus-oocyte complexes.</a> Reproduction in Domestic Animals. 47 (6), 1003-1008 (2012).</li><li>Kuwayama, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Highly+efficient+vitrification+for+cryopreservation+of+human+oocytes+and+embryos:+The+Cryotop+method.">Highly efficient vitrification for cryopreservation of human oocytes and embryos: The Cryotop method.</a> Theriogenology. 67 (1), 73 (2007).</li><li>Pope, C. E., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=In+vivo+survival+of+domestic+cat+oocytes+after+vitrification,+intracytoplasmic+sperm+injection+and+embryo+transfer.">In vivo survival of domestic cat oocytes after vitrification, intracytoplasmic sperm injection and embryo transfer.</a> Theriogenology. 77 (3), 531-538 (2012).</li><li>Galiguis, J., G&#243;mez, M. C., Leibo, S. P., Pope, C. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Birth+of+a+domestic+cat+kitten+produced+by+vitrification+of+lipid+polarized+in+vitro+matured+oocytes.">Birth of a domestic cat kitten produced by vitrification of lipid polarized in vitro matured oocytes.</a> Cryobiology. 68 (3), 459-466 (2014).</li><li>Fernandez-Gonzalez, L., Jewgenow, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cryopreservation+of+feline+oocytes+by+vitrification+using+commercial+kits+and+slush+nitrogen+technique.">Cryopreservation of feline oocytes by vitrification using commercial kits and slush nitrogen technique.</a> Reproduction in Domestic Animals. 52, 230-234 (2017).</li><li>Colombo, M., Morselli, M. G., Tavares, M. R., Apparicio, M., Luvoni, G. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Developmental+competence+of+domestic+cat+vitrified+oocytes+in+3D+enriched+culture+conditions.">Developmental competence of domestic cat vitrified oocytes in 3D enriched culture conditions.</a> Animals. 9 (6), 329 (2019).</li><li>Colombo, M., Morselli, M. G., Apparicio, M., Luvoni, G. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Granulosa+cells+in+three-dimensional+culture:+A+follicle-like+structure+for+domestic+cat+vitrified+oocytes.">Granulosa cells in three-dimensional culture: A follicle-like structure for domestic cat vitrified oocytes.</a> Reproduction in Domestic Animals. , (2020).</li><li>Wood, T. C., Wildt, D. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effect+of+the+quality+of+the+cumulus-oocyte+complex+in+the+domestic+cat+on+the+ability+of+oocytes+to+mature,+fertilize+and+develop+into+blastocysts+in+vitro.">Effect of the quality of the cumulus-oocyte complex in the domestic cat on the ability of oocytes to mature, fertilize and develop into blastocysts in vitro.</a> Journal of Reproduction and Fertility. 110 (2), 355-360 (1997).</li><li>Cobo, A., Kuwayama, M., P&#233;rez, S., Ruiz, A., Pellicer, A., Remoh&#237;, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Comparison+of+concomitant+outcome+achieved+with+fresh+and+cryopreserved+donor+oocytes+vitrified+by+the+Cryotop+method.">Comparison of concomitant outcome achieved with fresh and cryopreserved donor oocytes vitrified by the Cryotop method.</a> Fertility and Sterility. 89 (6), 1657-1664 (2008).</li><li>Mullen, S. F., Critser, J. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+science+of+cryobiology.">The science of cryobiology.</a> Oncofertility - Fertility preservation for Cancer Survivors. , 83-109 (2007).</li><li>Fahy, G. M., Wowk, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Principles+of+cryopreservation+by+vitrification.">Principles of cryopreservation by vitrification.</a> Methods in Molecular Biology. 1257, 21-82 (2015).</li><li>Apparicio, M., Ruggeri, E., Luvoni, G. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Vitrification+of+immature+feline+oocytes+with+a+commercial+kit+for+bovine+embryo+vitrification.">Vitrification of immature feline oocytes with a commercial kit for bovine embryo vitrification.</a> Reproduction in Domestic Animals. 48 (2), 240-244 (2013).</li><li>Brambillasca, F., Guglielmo, M. C., Coticchio, G., Mignini Renzini, M., Dal Canto, M., Fadini, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+current+challenges+to+efficient+immature+oocyte+cryopreservation.">The current challenges to efficient immature oocyte cryopreservation.</a> Journal of Assisted Reproduction and Genetics. 30 (12), 1531-1539 (2013).</li><li>Moussa, M., Shu, J., Zhang, X., Zeng, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cryopreservation+of+mammalian+oocytes+and+embryos:+current+problems+and+future+perspectives.">Cryopreservation of mammalian oocytes and embryos: current problems and future perspectives.</a> Science China Life Sciences. 57 (9), 903-914 (2014).</li><li>Luvoni, G., Pellizzari, P., Battocchio, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effects+of+slow+and+ultrarapid+freezing+on+morphology+and+resumption+of+meiosis+in+immature+cat+oocytes.">Effects of slow and ultrarapid freezing on morphology and resumption of meiosis in immature cat oocytes.</a> Journal of Reproduction and Fertility. Supplement. 51, 93-98 (1997).</li><li>Calvo, J. J., Robert, D., Viqueira, M., Lombide, P., Calvo, J. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Vitrified+and+in+vitro+matured+oocytes+viability+from+adult+housecat+(Felis+catus)+in+reproductive+season.">Vitrified and in vitro matured oocytes viability from adult housecat (Felis catus) in reproductive season.</a> International Journal of Morphology. 33 (4), (2015).</li><li>Nowak, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+viability+of+Serval+(Leptailurus+serval)+and+Pallas+Cat+(Felis+manul)+oocytes+after+cryopreservation+using+the+Rapid-I+Method.">The viability of Serval (Leptailurus serval) and Pallas Cat (Felis manul) oocytes after cryopreservation using the Rapid-I Method.</a> Cryo Letters. 40 (4), 226-230 (2019).</li></ol>

Oocyte cryopreservation is a crucial germplasm conservation technique, especially in taxa where many species are endangered, such as Felidae family. In this manuscript, a simple and field-friendly protocol for the vitrification of immature cat oocytes was presented. Laboratory-made media, minimum volume vitrification supports and trained personnel are the key factors for the success of this method, which allows obtaining viable oocytes consistently and repeatedly, as shown by the representative results hereby reported.</...

Cryopreservation has become a key step of assisted reproduction techniques (ARTs). In humans, it allows preservation of fertility or postponement of parenthood for medical or personal reasons. In animals, it is necessary to overcome distance and time in planned matings, especially in farm animals and pets, or to preserve genetic material of valuable subjects in conservation programs, particularly in wild endangered species. Gamete cryopreservation is the best choice when the individuals to be bred have not been chosen yet or in order to avoid ethical issues associated with embryo freezing, especially in human medicine1. Spermatozoa are relatively easy to preserve and give satisfactory outcomes after thawing, but oocytes, due to their structural features, might be more complex to store. Indeed, the low surface/volume ratio, as well as the presence of the zona pellucida surrounding the ooplasma, limits the movement of cryoprotectants and water across the cell2. Moreover, in domestic animals including felids, they are characterized by a lipid-rich cytoplasm, which is thought to make them more sensitive to cryopreservation3.
Most felids are threatened, and the domestic cat is used as a model to develop protocols for germplasm preservation thanks to the availability of gonads from routine ovariectomy. In wild animals, gonads can be obtained after elective surgeries or (more frequently) post-mortem, and immature (germinal vesicle) gametes can be retrieved. Hormonal stimulation aimed to obtain mature (metaphase II) oocytes is not as common as in human ARTs because of the ethical issues and the species- and individual-specific response to treatments4.
Therefore, the development of cryopreservation strategies has focused on immature gametes, which can usually be retrieved after the unexpected or sudden death of rare individuals. From a biological point of view, there are some differences in the cryopreservation of immature or mature gametes, each having its advantages. Firstly, DNA is more protected in immature oocytes, whose germinal vesicle contains chromosomes surrounded by a nuclear membrane, while the meiotic spindle of metaphase II oocytes could be more vulnerable to cryoinjuries5. Secondly, cold-induced cytoskeleton damages might affect spindle rotation, polar body extrusion, pronuclear migration and cytokinesis, which could have different impacts according to oocyte developmental phase, influencing meiosis progression or post-fertilization events. Finally, and perhaps most importantly, whereas mature oocytes are ready to be fertilized, immature gametes rely on the support of the surrounding cumulus cells to go through nuclear and cytoplasmic maturation6, and this is the reason why whole cumulus-oocyte complexes (COCs) are cryopreserved. However, the loss of cumulus cells and/or the loss of functional connection between the gamete and the surrounding somatic cells are probably the most detrimental effect of cryopreservation of immature COCs.
Among cryopreservation techniques, vitrification is one that can be applied more easily in field conditions. Compared to slow (or controlled rate) freezing, vitrification is faster and does not require specific equipment, such as a programmable freezer. In order to satisfy the three fundamental principles of vitrification (i.e., high viscosity, connected to high cryoprotectant concentration, small volumes and ultra-rapid temperature decrease), several media and supports especially have been developed and used in cats for both immature and mature oocytes. Beginning with simple straws7, devices were then developed to reach the &#8220;Minimum volume&#8221; goal. Cryoloop8, open pulled straws (OPS)9, plastic gutters (modified straw)10 and cryotubes11 have been used, until a more efficient device (i.e., Cryotop) was employed11, improving survival and meiosis resumption. Cryotop (Supplemental Figure 1) is a commercially available support which has become the elective open system for vitrification. Developed for the vitrification of human oocytes and embryos, it consists of a small film strip attached to a hard plastic holder, protected by a plastic tube cap during storage12. Thanks to its usability and to the extreme reduction in vitrification volume (as little as 0.1 &#181;L), which also leads to extremely rapid cooling and warming rates, this vitrification support has been increasingly applied in several species, including the domestic cat, in which it has been used with a variety of media13,14,15,16,17.
The purpose of this manuscript is to describe a collection-vitrification-warming protocol, with minor modifications from the one originally developed for human oocytes, which employs laboratory-made media and commercial supports for minimum volume vitrification and can be easily applied in field conditions for the cryopreservation of immature feline COCs.

<table>
	<tbody>
		<tr>
			<td>Amphotericin B</td>
			<td>Sigma-Aldrich</td>
			<td>A2942</td>
			<td>/</td>
		</tr>
		<tr>
			<td>Automatic pipettes &#38; tips</td>
			<td>/</td>
			<td>/</td>
			<td>Needed to pipette 20, 100, 240 and 300 &#181;L</td>
		</tr>
		<tr>
			<td>Surgical scalpels</td>
			<td>/</td>
			<td>/</td>
			<td>Size 10 is usually ok</td>
		</tr>
		<tr>
			<td>Bunsen beak</td>
			<td>/</td>
			<td>/</td>
			<td>/</td>
		</tr>
		<tr>
			<td>Clamps</td>
			<td>/</td>
			<td>/</td>
			<td>Some small (Mosquito clamp) for ovary isolation, some bigger (Klemmer clamp) to work in liquid nitrogen</td>
		</tr>
		<tr>
			<td>Cryotop</td>
			<td>Kitazato (distributor: MBT - Medical Biological Technologies)</td>
			<td>01.CR</td>
			<td>Distributors and catalog number may change in different countries</td>
		</tr>
		<tr>
			<td>Dimethyl sulfoxide (DMSO)</td>
			<td>Sigma-Aldrich</td>
			<td>D2650</td>
			<td>/</td>
		</tr>
		<tr>
			<td>Ethylene glycol (EG)</td>
			<td>Sigma-Aldrich</td>
			<td>E9129</td>
			<td>/</td>
		</tr>
		<tr>
			<td>Fetal bovine serum (FBS)</td>
			<td>Sigma-Aldrich</td>
			<td>F9665</td>
			<td>/</td>
		</tr>
		<tr>
			<td>Glass Pasteur pipettes</td>
			<td>/</td>
			<td>/</td>
			<td>Advised lenght 230 mm (100+130)</td>
		</tr>
		<tr>
			<td>Gobelets</td>
			<td>/</td>
			<td>/</td>
			<td>According to the canisters of the storage tank</td>
		</tr>
		<tr>
			<td>Heating stage</td>
			<td>/</td>
			<td>/</td>
			<td>All heating stages are ok, as long as they can keep 38&#186;C</td>
		</tr>
		<tr>
			<td>Liquid nitrogen</td>
			<td>/</td>
			<td>/</td>
			<td>/</td>
		</tr>
		<tr>
			<td>Medium 199</td>
			<td>Sigma-Aldrich</td>
			<td>M4530</td>
			<td>/</td>
		</tr>
		<tr>
			<td>Phosphate-buffered saline (PBS)</td>
			<td>Sigma-Aldrich</td>
			<td>D8662</td>
			<td>/</td>
		</tr>
		<tr>
			<td>Penicillin G sodium</td>
			<td>Sigma-Aldrich</td>
			<td>P3032</td>
			<td>/</td>
		</tr>
		<tr>
			<td>Polyvinyl alcohol (PVA)</td>
			<td>Sigma-Aldrich</td>
			<td>P8136</td>
			<td>/</td>
		</tr>
		<tr>
			<td>Repro plate</td>
			<td>Kitazato (distributor: MBT - Medical Biological Technologies)</td>
			<td>01.K-2</td>
			<td>Distributors and catalog number may change in different countries</td>
		</tr>
		<tr>
			<td>Stereomicroscope</td>
			<td>/</td>
			<td>/</td>
			<td>As long as the operator can select the oocytes, other stereomicroscopes are ok</td>
		</tr>
		<tr>
			<td>Storage tank</td>
			<td>/</td>
			<td>/</td>
			<td>Any regularly filled tank is ok</td>
		</tr>
		<tr>
			<td>Streptomycin sulphate</td>
			<td>Sigma-Aldrich</td>
			<td>S9137</td>
			<td>/</td>
		</tr>
		<tr>
			<td>Styrofoam/nitrogen resistant box</td>
			<td>/</td>
			<td>/</td>
			<td>All boxes which can contain liquid nitrogen are ok, as long as the operator is comfortable. Kitazato box is called &#34;Cooling Rack&#34;</td>
		</tr>
		<tr>
			<td>Sucrose</td>
			<td>Sigma-Aldrich</td>
			<td>S1888</td>
			<td>/</td>
		</tr>
		<tr>
			<td>Timers</td>
			<td>/</td>
			<td>/</td>
			<td>All timers which can be set on times until 9 minutes are ok</td>
		</tr>
	</tbody>
</table>

minimum volume vitrification of immature feline oocytes

In wild animals&#8217; conservation programs, gamete banking is crucial to safeguard genetic resources of valuable individuals and rare species and to promote biodiversity preservation. In felids, most species are threatened with extinction, and domestic breeds are used as a model to increase the efficiency of protocols for germplasm banking. Among oocyte cryopreservation techniques, vitrification is more and more popular in human and veterinary assisted reproduction. Cryotop vitrification, which was at first developed for human oocytes and embryos, has demonstrated to be well-suited for cat oocytes. This method offers several advantages, such as the feasibility in field conditions and the speed of the procedure, which can be helpful when several samples need to be processed. However, the efficiency is strongly dependent on the operator&#8217;s skills, and intra- and inter-laboratory standardization are needed, as well as personnel training. This protocol describes minimum volume vitrification of immature feline oocytes on a commercial support in a step by step field-friendly protocol, from oocyte collection to warming. Following the protocol, preservation of oocyte integrity and viability at warming (as high as 90%) can be expected, although there is still room for improvement in post-warming maturation and embryonic development outcomes.

Following cat oocyte vitrification and warming according to the present protocol (Figure 1 and Supplemental Figure 1), the vast majority of gametes survive. After vitrification, among other techniques, viability can be evaluated at the optical microscope as morphological integrity22 or with the use of vital stains. One of the latter is fluorescein diacetate/propidium iodide (FDA/PI), which allows the identification of viable (bright green fluorescence...

Watch this Scientific Journal Video about Minimum Volume Vitrification of Immature Feline Oocytes at JoVE.com

Minimum Volume Vitrification of Immature Feline Oocytes

This manuscript describes a protocol for the minimum volume vitrification of immature cat oocytes with laboratory-made media on commercial supports. It covers every step from oocyte isolation from ex vivo gonads to vitrification and warming.

In wild animals&#8217; conservation programs, gamete banking is crucial to safeguard genetic resources of valuable individuals and rare species and to ...

This protocol is useful for the long-term conservation of the feline oocytes through a cryopreservation technique known as with vitrification, which is crucial for fertility and biodiversity preservation in animal species. The main advantages of vitrification are its speed since it can be completed in less than 17 minutes and its feasibility in field conditions if liquid nitrogen is available. The domestic cat is a viable model for wild endangered felines.The development and optimization of cryopreservation protocols would allow establishing gamete biobanks for feline conservation programs. Begin by preparing a Repro vitrification plate with three conical wells in one row. Add 20 microliters of equilibration solution to the first well and 300 microliters of vitrification solution to the second and third wells.To equilibrate the cumulus oocyte complexes, or COCs, use a small-bore pipette to transfer one or more complexes to the bottom of the first well. Slowly add 20 microliters of equilibration solution to the border of the drop and wait for three minutes, then add another 240 microliters of equilibration solution and wait for nine minutes. While waiting, prepare a box with liquid nitrogen and label a vitrification support with the experiment or cat identification code.Put the box and support near the stereo microscope. After equilibrating the COCs, perform vitrification in less than 90 seconds. To vitrify the COCs, fill the small-bore pipette with the vitrification solution from the second well.Take the COCs from the bottom of the first well and move them to the surface of the second well. Wash the pipette with the vitrification solution, then move the COCs to another area of the well and mix the surrounding solution. Fill the pipette with solution from another area of the well.Move the COCs and mix the medium surrounding them with the pipette. Repeat this process one more time. Next, wash the pipette with the vitrification solution from the third well and use it to move the COCs to the bottom of the third well.Again, mix the surrounding solution. After filling the pipette with solution from another area of the well, move the COCs and mix the solution. Repeat this process.Fill the pipette tip with the vitrification solution from another area of the well and use it to load the COCs on the strip of the vitrification support close to the tip, then aspirate the excess medium. Immediately plunge the vitrification support in liquid nitrogen and use clamps to close it, making sure it remains immersed. Store the loaded vitrification supports in a goblet and keep them in a storage liquid nitrogen tank until warming.Place the heating stage close to the stereo microscope and turn it on to 38 degrees Celsius. Warm the lid of a Petri dish on top of the stage, then transfer the vitrification supports from the storage tank to a box with liquid nitrogen. Prepare one row of a Repro plate for each vitrification support to be warmed.Add 300 microliters of dilution solution to the first well and 300 microliters of washing solution to the second and third wells. Take the thawing solution out of the incubator and drop 100 microliters on the lid of the Petri dish. Use the clamps to open the vitrification support inside the liquid nitrogen.Place the Petri dish lid under the stereo microscope, then quickly pick up the vitrification support and immerse its strip in the drop, moving it until all COCs detach. Remove the support from the drop as soon as it is empty, but leave the COCs in the thawing solution for one minute. Fill a pipette with dilution solution and use it to move the COCs from the thawing solution drop to the bottom of the first well with dilution solution.Leave the COCs there for three minutes. Wash the pipette with the wash solution from the second well, then take the COCs and move them to the bottom of the second well. Wait for five minutes.Wash the pipette with a solution from the third well and use it to move the COCs from the second well to the surface of the third well. Wait for the COCs to touch the bottom of the third well. Then repeat the process with another well area.When finished, wash the pipette with culture medium and move the oocytes to a culture dish. The vast majority of gametes survive after oocyte vitrification and warming according to this protocol. A representative picture of a viable vitrified warmed cat cumulus oocyte complex stained with fluorescein diacetate and propidium iodide is shown here.This table shows the percentage of survival after vitrification, which depicts post-warming data of oocytes intended for in vitro maturation. Out of 435 oocytes, 395 survived, scoring an overall 90.8%post-warming viability. However, some morphological changes can be noticed after warming.The most frequent morphological abnormalities are changes in ooplasm shape and granulation, partial or sometimes total loss of cumulus cells, and zona pellucida fractures. When attempting this procedure, remember to prepare media in advance so that they are the right temperature before use and to follow protocol temperatures and timings to avoid cell toxicity. The protocol could also be applied to oocytes of other species.If successful, this could also contribute to germ plasm banking for biodiversity preservation in endangered felines.

minimum-volume-vitrification-of-immature-feline-oocytes

Research

JoVE Journal

Developmental Biology

6.3K vues.  Università degli Studi di Milano. Ce protocole est utile pour la conservation à long terme des ovocytes félins grâce à une technique de cryopréservation connue sous le nom de vitrification, qui est cruciale pour la fertilité et la préservation de la biodiversité chez les espèces animales. Les principaux avantages de la vitrification sont sa vitesse puisqu’elle peut être achevée en moins de 17 minutes et sa faisabilité sur le terrain si de l’azote liquide est disponible. Le chat domestique est un modèle viable...

Vidéo: Minimum Volume Vitrification of Immature Feline Oocytes

This manuscript describes a protocol for the minimum volume vitrification of immature cat oocytes with laboratory-made media on commercial supports. It covers every step from oocyte isolation from ex vivo gonads to vitrification and...

vitrification of in vitro matured oocytes collected from adult and prepubertal ovaries in sheep

Vitrification of In Vitro Matured Oocytes Collected from Adult and Prepubertal Ovaries in Sheep

The protocol aims at providing a standard method for the vitrification of adult and juvenile sheep oocytes. It includes all the steps from the preparation of the in vitro maturation media to the post-warming culture. Oocytes are vitrified at the MII stage using Cryotop to ensure the minimum essential volume.

bulk droplet vitrification for primary hepatocyte preservation

Bulk Droplet Vitrification for Primary Hepatocyte Preservation

This manuscript describes an ice-free cryopreservation method for large quantities of rat hepatocytes whereby primary cells are pre-incubated with cryoprotective agents at a low concentration and vitrified in large...

human blastocyst biopsy and vitrification

Human Blastocyst Biopsy and Vitrification

Blastocyst biopsy and vitrification are required to efficiently conduct preimplantation genetic testing. An approach entailing the sequential opening of the zona pellucida and retrieval of 7-8 trophectoderm cells in day 5-7 post-insemination limits both the number of manipulations required and the exposure of the embryo to sub-optimal environmental...

fertility preservation through oocyte vitrification: clinical and laboratory perspectives

Fertility Preservation Through Oocyte Vitrification: Clinical and Laboratory Perspectives

Oocyte cryopreservation is recognized by several international scientific societies as the gold standard for fertility preservation in postpubertal women. Appropriate clinical and laboratory strategies ensure maximum efficacy, efficiency, and safety of fertility preservation...

microinjection of xenopus laevis oocytes

Microinjection of Xenopus Laevis Oocytes

Here we demonstrate cytoplasmic microinjection of Xenopus laevis oocytes with a nuclear import substrate, as well as preparation of the injected oocytes for visualization by thin-sectioning electron microscopy.<br...

generation of immature, mature, and tolerogenic dendritic cells from monocytes

This video demonstrates the generation of immature, mature, and tolerogenic dendritic cells, or DCs, from human monocytes. The incubation of monocytes with growth factors and cytokines leads to the production of immature DCs. In response to immune regulatory molecules, immature DCs differentiate into tolerogenic and mature...

Generation of Immature, Mature, and Tolerogenic Dendritic Cells from Monocytes

retrieval of mouse oocytes

Retrieval of Mouse Oocytes

This protocol illustrates the technique for extracting oocytes or early-stage fertilized embryos from the oviduct of mice. The ability to identify the infindibulum and insert a blunt end needle into it is essential to correctly performing the...

cryopreservation of mouse embryos by ethylene glycol-based vitrification

Cryopreservation of Mouse Embryos by Ethylene Glycol-Based Vitrification

An ethylene glycol-based vitrification method for mouse embryos is described. It is advantageous to other methods in its simplicity and low embryonic toxicity, and therefore can be broadly applicable to many strains of mice, including inbred and gene-modified...

vitrification of ovarian cortex tissue to achieve a glassy state of aggregation

Author Spotlight: Advancing Fertility Preservation in Young Female Cancer Patients Through Ovarian Tissue Vitrification and In Vitro Follicular Growth

A protocol for the vitrification of ovarian tissue, as an alternative cryopreservation method to the widely used slow freezing protocol, is...

Vitrification of Ovarian Cortex Tissue to Achieve a Glassy State of Aggregation

rapid isolation of stage i oocytes in zebrafish devoid of granulosa cells

Author Spotlight: Enhanced Method for Isolating Pure Zebrafish Stage I Oocytes

This protocol describes a modified procedure for rapidly isolating clean stage I oocytes in zebrafish devoid of granulosa cells, thereby providing a convenient method for oocyte-specific...

Improved Procedure to Obtain Granulosa Cell-Free Stage I Zebrafish Oocytes

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nuclear transfer into mouse oocytes

Nuclear Transfer into Mouse Oocytes

This movie and the protocol are intended to help learning nuclear...

collection and cryopreservation of hamster oocytes and mouse embryos

Collection and Cryopreservation of Hamster Oocytes and Mouse Embryos

In this video-article we present a step-by-step demonstration on how to collect and cryopreserve hamster oocytes with high post-thaw survival rates. The same procedure can also be applied to successfully freeze and thaw mouse embryos at different stages of preimplantation...

Aspirate the supernatant, and resuspend the pellet in 1 milliliter of cell culture medium. Add the cell mixture back to the culture. On day 5, add 1 microliter of vitamin D3 stock and 1 microliter of dexamethasone stock per milliliter of medium to two of the sets to generate tolerogenic...

generation of immature, mature and tolerogenic dendritic cells with differing metabolic phenotypes

Generation of Immature, Mature and Tolerogenic Dendritic Cells with Differing Metabolic Phenotypes

Immature dendritic cells can be selectively differentiated into tolerogenic or mature dendritic cells to regulate the balance between immunity and tolerance. This work presents a means to generate from immature monocyte derived dendritic cells (moDCs), in vitro tolerogenic and mature moDCs that differ in metabolic...

modified microsecure vitrification: a safe, simple and highly effective cryopreservation procedure for human blastocysts

Modified MicroSecure Vitrification: A Safe, Simple and Highly Effective Cryopreservation Procedure for Human Blastocysts

MicroSecure Vitrification was developed as a non-commercial, aseptic closed vitrification device system that is compliant with FDA good manufacturing and tissue-handling practices. Due to the withdrawal of hydrophobic plugged embryo straws from the industry, the vitrification procedure was modified to include an inner seal before the standard internal cotton...

minimum burning pressures of water-based emulsion explosives

Minimum Burning Pressures of Water-based Emulsion Explosives

We present an apparatus based on hot-wire ignition in a pressurized enclosure and an associated methodology to measure the minimum pressure required to induce sustained combustion in water-based emulsion explosives. This method improves product characterization to allow one to use them more safely during pumping and mixing...

Rapid Isolation of Stage I Oocytes in Zebrafish Devoid of Granulosa Cells

a novel cell injection method with minimum invasion

Author Spotlight: A Novel Cell Injection Method with Minimum Invasion  

This method eliminates any major invasion during cell injections caused by the cell suspension...

A Novel Cell Injection Method with Minimum Invasion

separation of follicular cells and oocytes in ovarian follicles of zebrafish

Separation of Follicular Cells and Oocytes in Ovarian Follicles of Zebrafish

Here, we present a simple method for separating follicular cells and oocytes in zebrafish ovarian follicles, which will facilitate investigations of ovarian development in...

Expression of Fluorescent Proteins in Branchiostoma lanceolatum by mRNA Injection into Unfertilized Oocytes

We report here a robust and efficient protocol for the expression of fluorescent proteins after mRNA injection into unfertilized oocytes of the cephalochordate amphioxus, Branchiostoma lanceolatum. We use constructs for membrane and nuclear targeted mCherry and eGFP that have been modified to accommodate amphioxus codon usage and Kozak consensus sequences. We describe the type of injection needles to be used, the immobilization protocol for the unfertilized oocytes, and the overall injection set-up. This technique generates fluorescently labeled embryos, in which the dynamics of cell behaviors during early development can be analyzed using the latest in vivo imaging strategies. The development of a microinjection technique in this amphioxus species will allow live imaging analyses of cell behaviors in the embryo as well as gene-specific manipulations, including gene overexpression and knockdown. Altogether, this protocol will further consolidate the basal chordate amphioxus as an animal model for addressing questions related to the mechanisms of embryonic development and, more importantly, to their evolution.

Expression of Fluorescent Proteins in Branchiostoma lanceolatum by mRNA Injection into Unfertilized Oocytes

We report here the robust and efficient expression of fluorescent proteins after mRNA injection into unfertilized oocytes of Branchiostoma lanceolatum. The development of the microinjection technique in this basal chordate will pave the way for far-reaching technical innovations in this emerging model system, including in vivo imaging and gene-specific manipulations.

We report here a robust and efficient protocol for the expression of fluorescent proteins after mRNA injection into unfertilized oocytes of the ...

Isolation and Characterization of Mouse Antral Oocytes Based on Nucleolar Chromatin Organization

This protocol describes a simple and quick method to isolate and characterize mouse antral GV (Germinal Vesicle) oocytes as able (SN, Surrounded Nucleolus) or unable (NSN, Not Surrounded Nucleolus) to develop to the blastocyst stage after in vitro maturation (IVM) and in vitro fertilization (IVF). It makes use of Hoeschst33342 (or any other DNA intercalating dye) able to bind to the heterochromatin of the nucleolus showing a ring in the SN oocytes or not, like in the NSN oocytes. This represents the easiest and quickest way to sort both antral oocytes that can be eventually used for IVM or IVF procedures. 
Briefly, the protocol consists of the following steps: hormone injection to stimulate follicular growth; isolation of the oocytes at the GV stage from the antral compartment by puncturing the ovary with a sterile needle; preparation of thin glass pipettes for mouth pipetting of the oocytes; sorting of the oocytes with Hoechst33342 prepared at a supravital concentration; IVM, IVF or any other molecular/cellular analysis.
Unfortunately there are still few evidences to sort SN and NSN oocytes using less invasive techniques. If and once they will be identified, they could be potentially applied to human assisted reproductive technologies, although with several aspects that should be modified. To date, this technique has potential implications to dramatically increase IVM and IVF successful procedures in both endangered and species with economic interest.

Here we present a protocol for the characterization of mouse antral oocytes based on nucleolar organization.

This protocol describes a simple and quick method to isolate and characterize mouse antral GV (Germinal Vesicle) oocytes as able (SN, Surrounded ...

Clinical embryo vitrification evolved with the development of unique vitrification devices in the 21st century and with the misconception that ultra-rapid cooling in an &#34;open&#34; system (i.e., direct LN2 contact) was a necessity to optimize vitrification success. The dogma surrounding the importance of cooling rates led to unsafe practices subject to technical variation and to the creation of vitrification devices that disregarded important quality-control factors (e.g., ease of use, repeatability, reliability, labeling security, and storage safety). Understanding the quality-control flaws of other devices allowed for the development of a safe, secure, repeatable, and reliable &#181;S-VTF method aimed to minimize intra- and inter-technician variation. Equally important, it combined the availability of two existing FDA-compliant devices: 1) a 0.3-mL ionomeric resin embryo straw with internalized, dual-colored, tamper-proof labeling with repeatable weld seal potential; and 2) shortened, commonly-used, 300-&#181;m ID sterile flexipettes to directly load the embryo(s) in order to create a highly-effective global vitrification device. Like other aseptic, closed vitrification systems (e.g., High Security Vitrification (HSV), Rapid-i, and VitriSafe) effectively used in reproductive medicine, microSecure Vitrification (&#181;S-VTF) has proven that it can achieve high post-warming survival and pregnancy outcomes with its attention to simplicity, and reduced technical variation. Although the 0.3-mL embryo straw containing an internal hydrophobic plug was commercially replaced with a standard semen straw possessing cotton-polyvinyl pyrrolidone (PVP) plugs, it maintained its ionomeric resin composition to ensure weld sealing. However, the cotton plugs can wick out the fluid-embryo contents of the flexipettes upon contact. A modified &#181;S-VTF method was adapted to include an additional internal weld seal before the plug on the device loading side. The added technical step to the &#181;S-VTF procedure has not affected its successful application, as high survival rates (&#62; 95%) and pregnancy rates continue today.

MicroSecure Vitrification was developed as a non-commercial, aseptic closed vitrification device system that is compliant with FDA good manufacturing and tissue-handling practices. Due to the withdrawal of hydrophobic plugged embryo straws from the industry, the vitrification procedure was modified to include an inner seal before the standard internal cotton plug.

Clinical embryo vitrification evolved with the development of unique vitrification devices in the 21st century and with the misconception that ultra-rapid ...

Analysis of Chromosome Segregation, Histone Acetylation, and Spindle Morphology in Horse Oocytes

The field of assisted reproduction has been developed to treat infertility in women, companion animals, and endangered species. In the horse, assisted reproduction also allows for the production of embryos from high performers without interrupting their sports career and contributes to an increase in the number of foals from mares of high genetic value. The present manuscript describes the procedures used for collecting immature and mature oocytes from horse ovaries using ovum pick-up (OPU). These oocytes were then used to investigate the incidence of aneuploidy by adapting a protocol previously developed in mice. Specifically, the chromosomes and the centromeres of metaphase II (MII) oocytes were fluorescently labeled and counted on sequential focal plans after confocal laser microscope scanning. This analysis revealed a higher incidence in the aneuploidy rate when immature oocytes were collected from the follicles and matured in vitro compared to in vivo. Immunostaining for tubulin and the acetylated form of histone four at specific lysine residues also revealed differences in the morphology of the meiotic spindle and in the global pattern of histone acetylation. Finally, the expression of mRNAs coding for histone deacetylases (HDACs) and acetyl-transferases (HATs) was investigated by reverse transcription and quantitative-PCR (q-PCR). No differences in the relative expression of transcripts were observed between in vitro and in vivo matured oocytes. In agreement with a general silencing of the transcriptional activity during oocyte maturation, the analysis of the total transcript amount can only reveal mRNA stability or degradation. Therefore, these findings indicate that other translational and post-translational regulations might be affected.
Overall, the present study describes an experimental approach to morphologically and biochemically characterize the horse oocyte, a cell type that is extremely challenging to study due to low sample availability. However, it can expand our knowledge on the reproductive biology and infertility in monovulatory species.

This manuscript describes an experimental approach to morphologically and biochemically characterize horse oocytes. Specifically, the present work illustrates how to collect immature and mature horse oocytes by ultrasound-guided ovum pick-up (OPU) and how to investigate chromosome segregation, spindle morphology, global histone acetylation, and mRNA expression.

The field of assisted reproduction has been developed to treat infertility in women, companion animals, and endangered species. In the horse, assisted ...

Generation of Genetically Modified Mice through the Microinjection of Oocytes

The use of genetically modified mice has significantly contributed to studies on both physiological and pathological in vivo processes. The pronuclear injection of DNA expression constructs into fertilized oocytes remains the most commonly used technique to generate transgenic mice for overexpression. With the introduction of CRISPR technology for gene targeting, pronuclear injection into fertilized oocytes has been extended to the generation of both knockout and knockin mice. This work describes the preparation of DNA for injection and the generation of CRISPR guides for gene targeting, with a particular emphasis on quality control. The genotyping procedures required for the identification of potential founders are critical. Innovative genotyping strategies that take advantage of the "multiplexing" capabilities of CRISPR are presented herein. Surgical procedures are also outlined. Together, the steps of the protocol will allow for the generation of genetically modified mice and for the subsequent establishment of mouse colonies for a plethora of research fields, including immunology, neuroscience, cancer, physiology, development, and others.

The microinjection of mouse oocytes is commonly used for both classic transgenesis (i.e., the random integration of transgenes) and CRISPR-mediated gene targeting. This protocol reviews the latest developments in microinjection, with a particular emphasis on quality control and genotyping strategies.

The use of genetically modified mice has significantly contributed to studies on both physiological and pathological in vivo processes. The pronuclear ...

A Neural Network-Based Identification of Developmentally Competent or Incompetent Mouse Fully-Grown Oocytes

Infertility clinics would benefit from the ability to select developmentally competent vs. incompetent oocytes using non-invasive procedures, thus improving the overall pregnancy outcome. We recently developed a classification method based on microscopic live observations of mouse oocytes during their in vitro maturation from the germinal vesicle (GV) to the metaphase II stage, followed by the analysis of the cytoplasmic movements occurring during this time-lapse period. Here, we present detailed protocols of this procedure. Oocytes are isolated from fully-grown antral follicles and cultured for 15 h inside a microscope equipped for time-lapse analysis at 37 &#176;C and 5% CO2. Pictures are taken at 8 min intervals. The images are analyzed using the Particle Image Velocimetry (PIV) method that calculates, for each oocyte, the profile of Cytoplasmic Movement Velocities (CMVs) occurring throughout the culture period. Finally, the CMVs of each single oocyte are fed through a mathematical classification tool (Feed-forward Artificial Neural Network, FANN), which predicts the probability of a gamete to be developmentally competent or incompetent with an accuracy of 91.03%. This protocol, set up for the mouse, could now be tested on oocytes of other species, including humans.

Here, we present a protocol for non-invasive assessment of oocyte developmental competence performed during their in vitro maturation from the germinal vesicle to the metaphase II stage. This method combines time-lapse imaging with particle image velocimetry (PIV) and neural network analyses.

Infertility clinics would benefit from the ability to select developmentally competent vs. incompetent oocytes using non-invasive procedures, thus ...

Blastocyst biopsy is performed to obtain a reliable genetic diagnosis during IVF cycles with preimplantation genetic testing. Then, the ideal workflow entails a safe and efficient vitrification protocol, due to the turnaround time of the diagnostic techniques and to transfer the selected embryo(s) on a physiological endometrium in a following natural cycle. A biopsy approach encompassing the sequential opening of the zona pellucida and retrieval of 5-10 trophectoderm cells (ideally 7-8) limits both the number of manipulations required and the exposure of the embryo to sub-optimal environmental conditions. After proper training, the technique was reproducible across different operators in terms of timing of biopsy (~8 min, ranging 3-22 min based on the number of embryos to biopsy per dish), conclusive diagnoses obtained (~97.5%) and live birth rates after vitrified-warmed euploid blastocyst transfer (&#62;40%). The survival rate after biopsy, vitrification and warming was as high as 99.8%. The re-expansion rate at 1.5 h from warming was as high as 97%, largely dependent on the timing between biopsy and vitrification (ideally &#8804;30 min), blastocyst morphological quality and day of biopsy. In general, it is better to vitrify a collapsed blastocyst; therefore, in non-PGT cycles, laser-assisted artificial shrinkage might be performed to induce embryo collapse prior to cryopreservation. The most promising future perspective is the non-invasive analysis of the IVF culture media after blastocyst culture as a putative source of embryonic DNA. However, this potential avant-garde is still under investigation and a reliable protocol yet needs to be defined and validated.

Blastocyst biopsy and vitrification are required to efficiently conduct preimplantation genetic testing. An approach entailing the sequential opening of the zona pellucida and retrieval of 7-8 trophectoderm cells in day 5-7 post-insemination limits both the number of manipulations required and the exposure of the embryo to sub-optimal environmental conditions.

Blastocyst biopsy is performed to obtain a reliable genetic diagnosis during IVF cycles with preimplantation genetic testing. Then, the ideal workflow ...

Zebrafish has become an ideal model to study the ovarian development of vertebrates. The follicle is the basic unit of the ovary, which consists of oocytes and surrounding follicular cells. It is vital to separate both follicular cells and oocytes for various research purposes such as for primary culture of follicular cells, analysis of gene expression, oocyte maturation and in vitro fertilization, etc. The conventional method uses forceps to separate both compartments, which is laborious, time consuming and has high damage to the oocyte. Here, we have established a simple method to separate both compartments using a pulled glass capillary. Under a stereomicroscope, oocytes and follicular cells can be easily separated by pipetting in a pulled fine glass capillary (the diameter depends on the follicle diameter). Compared with the conventional method, this new method has high efficiency in separating both oocytes and follicular cells and has low damage to the oocytes. More importantly, this method can be applied to early-stage follicles including at the pre-vitellogenesis stage. Thus, this simple method can be used to separate follicular cells and oocytes of zebrafish.

Here, we present a simple method for separating follicular cells and oocytes in zebrafish ovarian follicles, which will facilitate investigations of ovarian development in zebrafish.

Zebrafish has become an ideal model to study the ovarian development of vertebrates. The follicle is the basic unit of the ovary, which consists of ...

In livestock, in vitro embryo production systems can be developed and sustained thanks to the large number of ovaries and oocytes that can be easily obtained from a slaughterhouse. Adult ovaries always bear several antral follicles, while in pre-pubertal donors the maximal numbers of oocytes are available at 4 weeks of age, when ovaries bear peak numbers of antral follicles. Thus, 4 weeks old lambs are considered good donors, even if the developmental competence of prepubertal oocytes is lower compared to their adult counterpart. 
Basic research and commercial applications would be boosted by the possibility of successfully cryopreserving vitrified oocytes obtained from both adult and prepubertal donors. The vitrification of oocyte collected from prepubertal donors would also allow shortening the generation interval and thus increasing the genetic gain in breeding programs. However, the loss of developmental potential after cryopreservation makes mammalian oocytes probably one of the most difficult cell types to cryopreserve. Among the available cryopreservation techniques, vitrification is widely applied to animal and human oocytes. Despite recent advancements in the technique, exposures to high concentrations of cryoprotective agents as well as chilling injury and osmotic stress still induce several structural and molecular alterations and reduce the developmental potential of mammalian oocytes. Here, we describe a protocol for the vitrification of sheep oocytes collected from juvenile and adult donors and matured in vitro prior to cryopreservation. The protocol includes all the procedures from oocyte in vitro maturation to vitrification, warming and post-warming incubation period. Oocytes vitrified at the MII stage can indeed be fertilized following warming, but they need extra time prior to fertilization to restore damage due to cryopreservation procedures and to increase their developmental potential. Thus, post-warming culture conditions and timing are crucial steps for the restoration of oocyte developmental potential, especially when oocyte are collected from juvenile donors.

The protocol aims at providing a standard method for the vitrification of adult and juvenile sheep oocytes. It includes all the steps from the preparation of the in vitro maturation media to the post-warming culture. Oocytes are vitrified at the MII stage using Cryotop to ensure the minimum essential volume.

In livestock, in vitro embryo production systems can be developed and sustained thanks to the large number of ovaries and oocytes that can be easily ...

Evaluation of the Spindle Assembly Checkpoint Integrity in Mouse Oocytes

Aneuploidy is the leading genetic abnormality causing early miscarriage and pregnancy failure in humans. Most errors in chromosome segregation that give rise to aneuploidy occur during meiosis in oocytes, but why oocyte meiosis is error-prone is still not fully understood. During cell division, cells prevent errors in chromosome segregation by activating the spindle assembly checkpoint (SAC). This control mechanism relies on detecting kinetochore (KT)-microtubule (MT) attachments and sensing tension generated by spindle fibers. When KTs are unattached, the SAC is activated and prevents cell-cycle progression. The SAC is activated first by MPS1 kinase, which triggers the recruitment and formation of the mitotic checkpoint complex (MCC), composed of MAD1, MAD2, BUB3, and BUBR1. Then, the MCC diffuses into the cytoplasm and sequesters CDC20, an anaphase-promoting complex/cyclosome (APC/C) activator. Once KTs become attached to microtubules and chromosomes are aligned at the metaphase plate, the SAC is silenced, CDC20 is released, and the APC/C is activated, triggering the degradation of Cyclin B and Securin, thereby allowing anaphase onset. Compared to somatic cells, the SAC in oocytes is not as effective because cells can undergo anaphase despite having unattached KTs. Understanding why the SAC is more permissive and if this permissiveness is one of the causes of chromosome segregation errors in oocytes still needs further investigation. The present protocol describes the three techniques to comprehensively evaluate SAC integrity in mouse oocytes. These techniques include using nocodazole to depolymerize MTs to evaluate the SAC response, tracking SAC silencing by following the kinetics of Securin destruction, and evaluating the recruitment of MAD2 to KTs by immunofluorescence. Together these techniques probe mechanisms needed to produce healthy eggs by providing a complete evaluation of SAC integrity.

Error in chromosome segregation is a common feature in oocytes. Therefore, studying the spindle assembly checkpoint gives important clues about the mechanisms needed to produce healthy eggs. The present protocol describes three complementary assays to evaluate spindle assembly checkpoint integrity in mouse oocytes.

Aneuploidy is the leading genetic abnormality causing early miscarriage and pregnancy failure in humans. Most errors in chromosome segregation that give ...