This work was supported by the National Natural Science Foundation of China (32170813 and 31871449) and Science and Technology Department of Sichuan (2024NSFSC0651), and 1&#183;3&#183;5 project for disciplines of excellence&#8211;Clinical Research Fund, West China Hospital, Sichuan University (2024HXFH035). The authors would like to thank Zhao Wang and Yanqiu Gao of the Laboratory of Pediatric Surgery for breeding of zebrafish related to this work. The authors would also like to thank all the reviewers who participated in the review, as well as MJEditor (www.mjeditor.com) for providing English editing services during the preparation of this manuscript.

All zebrafish were handled following stringent animal care guidelines outlined by the relevant national and/or local animal welfare bodies. The maintenance and handling of fish received approval from both local authorities and the animal ethics committee of the West China Hospital of Sichuan University (approval No. 20220422003). Zebrafish ovaries contain a mixture of multi-stage oocytes, with each developmental stage being present in adult zebrafish ovaries. However, juvenile zebrafish were selected for this study due t...

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

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In this study, we developed a method for isolating pure and clean stage I oocytes, excluding granulosa cells, for downstream analysis (particularly genomic analyses). Comparing this modified method with the referenced method13, the stage I oocytes obtained using this method are morphologically intact, sufficient in number, and free from contamination with other somatic cells, making them suitable for various subsequent studies and analyses. Furthermore, compared with other mechanical separation me...

The zebrafish is among the most important model systems in developmental biology. In recent years, numerous studies have utilized the zebrafish as a model to study important biological events and regulatory processes from oocyte to embryo. These encompass the intricate processes of oocyte development and maturation1, the functionality of maternal genes2, the regulation of maternal-zygotic transitions3, and extensive omics analyses4.
Granulosa cells, the somatic cells enveloping and nurturing the developing oocyte within the ovarian follicle5,6, play a pivotal role in this developmental process. As primordial germ cells (PGCs) evolve into oogonia, they become surrounded by a monolayer of granulosa cells7. Together with the external thecal cells, the oocyte and its surrounding granulosa cells constitute a mature follicle8. Given the fundamental distinction between germ cells and somatic cells, obtaining a pure oocyte sample is imperative, especially for genome-related analyses.
Within the follicular structure of zebrafish, granulosa cells typically exhibit a diameter of only a few microns8, emphasizing the intimate interconnection between granulosa cells and oocytes9. This close association presents a challenge in achieving complete separation due to the considerable difference in both the number and volume of granulosa cells versus oocytes (hundreds of granulosa cells compared to a single oocyte)10,11. Even minimal contamination with a single granulosa cell can impede downstream analyses specifically targeting oocytes. Therefore, for studies focusing on genomic and epigenetic characteristics, the elimination of granulosa cells is essential.
Benefiting from well-characterized morphological criteria, oocytes at each stage can be distinguished based on diameter11. The oogenesis process in zebrafish is categorized into five stages according to morphology and karyotype11. Stage I oocytes (7-140 &#181;m diameter) encompass oocytes from the onset of meiosis to the early stage of meiosis I. Crucially, these oocytes are transparent, allowing for the observation of the central nucleus through transmitted light (Figure 1Ai). Stage II oocytes (140-340 &#181;m diameter) gradually become foamy and translucent. With the enlargement of follicles and the proliferation of cortical alveoli, the germinal vesicles in the center become difficult to distinguish12 (Figure 1Aii). Stage III oocytes (340-690 &#181;m diameter) progressively accumulate vitellogenin, and fresh follicles become increasingly opaque (Figure 1Aiii). Meiosis continues in stage IV oocytes (690-730 &#181;m diameter) as chromosomes enter the middle of meiosis II, where they stagnate (Figure 1Aiv). Stage V oocytes (730-750 &#181;m diameter) have matured and are ready for ovulation7,11(Figure 1Av).
Based on the unique characteristics of each of the aforementioned stages, a method has been proposed to isolate oocytes from stages I to III by digesting zebrafish ovaries using a digestive solution containing collagenase I, collagenase II, and hyaluronidase, followed by filtration through a specific-sized cell strainer13. However, while this method allows for obtaining oocytes at different developmental stages, it fails to completely separate oocytes and granulosa cells. Other researchers have also suggested methods to separate granulosa cells from oocytes. However, these methods primarily rely on mechanical approaches, which can cause oocyte damage, are time-consuming, and are inadequate for obtaining a substantial number of oocytes for analysis14,15.
Given the limitations of existing methods and specific research requirements, this study aims to establish a procedure to thoroughly separate oocytes and granulosa cells and obtain a sufficient number of clean stage I oocytes for analysis. Expanding upon the referenced method13, we employ an improved digestion buffer (see Table of Materials) that is gentler and facilitates the dispersion of oocytes and dissociation of granulosa cells. Subsequently, the oocytes are passed through a cell strainer, followed by washing and further microscopic selection, enabling the acquisition of a large number of clean stage I oocytes.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Kinger&#39;s cell dissociation solution</td>
			<td>PlantChemMed</td>
			<td>PC-33689</td>
			<td>Kinger&#39;s cell dissociation solution can be stored stably at -20 &#176;C after packaging and can be used after thawing at low temperature (4 &#176;C). It can be used directly for dissociating zebrafish ovaries. The optimal temperature is 28.5 &#176;C, for approximately 2-3 hours. The duration can be adjusted according to the specific dissociation conditions, either shortened or extended (https://www.plantchemmed.com/chanpin?productNo=PC-33689).</td>
		</tr>
		<tr>
			<td>Cell strainers (100 &#956;m )</td>
			<td>Falcon</td>
			<td>352360</td>
			<td></td>
		</tr>
		<tr>
			<td>Fluorescence microscope</td>
			<td>Zeiss</td>
			<td>Axio Zoom.V16</td>
			<td></td>
		</tr>
		<tr>
			<td>Forceps</td>
			<td>Dumont</td>
			<td>#5</td>
			<td></td>
		</tr>
		<tr>
			<td>Glass capillary needle</td>
			<td>/</td>
			<td>/</td>
			<td>Blunted by burning with lighter</td>
		</tr>
		<tr>
			<td>Hoechst</td>
			<td>Yesen</td>
			<td>40732ES03</td>
			<td></td>
		</tr>
		<tr>
			<td>Low adsorption pipette tips (10 &#956;l )</td>
			<td>Labsellect</td>
			<td>T-0010-LR-R-S</td>
			<td></td>
		</tr>
		<tr>
			<td>Leibovitz&#8217;s L-15 medium medium (with L-glutamine)</td>
			<td>Hyclone</td>
			<td>SH30525.01</td>
			<td></td>
		</tr>
		<tr>
			<td>Ice bucket</td>
			<td>/</td>
			<td>/</td>
			<td>Ice-cold water is used to euthanize zebrafish</td>
		</tr>
		<tr>
			<td>Incubator</td>
			<td>WIGGENS</td>
			<td>WH-01</td>
			<td></td>
		</tr>
		<tr>
			<td>Juvenile fish</td>
			<td>/</td>
			<td>/</td>
			<td>5&#8211;6 weeks post-fertilization, standard length [SL] of 10&#8211;15 mm</td>
		</tr>
		<tr>
			<td>Plastic dish (35 mm )</td>
			<td>SORFA</td>
			<td>230101</td>
			<td></td>
		</tr>
		<tr>
			<td>Stereomicroscope</td>
			<td>Motic</td>
			<td>SMZ-161</td>
			<td></td>
		</tr>
		<tr>
			<td>Tissue Culture Plate (6-wells)</td>
			<td>SORFA</td>
			<td>0110006</td>
			<td></td>
		</tr>
		<tr>
			<td>Vannas spring scissors</td>
			<td>Fine Science Toosl</td>
			<td>#15000-00</td>
			<td></td>
		</tr>
	</tbody>
</table>

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

The study of oocyte development holds significant implications in developmental biology. The zebrafish (Danio rerio) has been extensively used as a model organism to investigate early developmental processes from oocyte to embryo. In zebrafish, oocytes are surrounded by a single layer of somatic granulosa cells. However, separating granulosa cells from oocytes poses a challenge, as achieving pure oocytes is crucial for precise analysis. Although various methods have been proposed to isolate zebrafish oocytes at different developmental stages, current techniques fall short in removing granulosa cells completely, limiting the accuracy of genome analysis focused solely on oocytes. In this study, we successfully developed a rapid and efficient process for isolating pure stage I oocytes in zebrafish while eliminating granulosa cell contamination. This technique facilitates biochemical and molecular analysis, particularly in exploring epigenetic and genome structure aspects specific to oocytes. Notably, the method is user-friendly, minimizes oocyte damage, and provides a practical solution for subsequent research and analysis.

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

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

This research is focused on establishing process for rapidly and efficiently isolating pure stage one ooyctes in zebrafish while eliminating granulosa cell contamination. The primary aim is to develop a method that facilities more precise analysis in oocyte specific research areas, particularly in genome and epigenetic research. The oocytes of zebrafish are enveloped by a mono-layer of granule cells.It's difficult to swiftly separate granule cells from oocytes due to the significant disparity in both number and volume, making it challenging to obtain pure oocyte samples for specific analysis, especially in genome related studies. This method has survival advantage over mechanical methods and the previous detection approaches. First, the improved detection buffer is gentler, helping to isolate the granulosa cells while minimizing damage to the oocytes.Plus, it allows us to get plenty of clean stage one oocytes. We hope as research helps scientists get a better look at the detail and growth of the phage oocytes, including cell stack, epigenetics and genome structure. This method work just as well for pompanos, and we think it could be applied to study a lot of different kind of fish.

Figure 1 illustrates the ovarian morphology observed in both adult and juvenile zebrafish, showcasing oocytes at different developmental stages to serve as a reference. Figure 1A provides a graphical representation of oocyte morphology and size at each developmental stage, beginning with the primary growth stage (stage I) and ending with the ovulated egg (stage V). Figures 1Ai-v illustrate the diameters corresponding to stages I to V of the oocy...

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 research.

The study of oocyte development holds significant implications in developmental biology. The zebrafish (Danio rerio) has been extensively used as a model ...