Funding was provided by "Progetto a concorso Fondazione IRCCS Ospedale Maggiore Policlinico 2009-2010" to MD, and Associazione Bambino Nefropatico ABN Onlus, Milano

All animal experiments were performed according to the National and European current regulations regarding the protection of animals used for scientific purposes and were reviewed and approved by the ethical committee of Milan University.
1. Isolation of Primary Osteoblasts
The method, with minor modification, follows the procedure described by Dodig et al14.
<ol>
	<li>Prepare digesting medium, by adding 0.1% Collagenase P and ...

<ol>
	<li>Schneider, P., Meier, M., Wepf, R., M&#252;ller, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Towards+quantitative+3D+imaging+of+the+osteocyte+lacuno-canalicular+network.">Towards quantitative 3D imaging of the osteocyte lacuno-canalicular network.</a> Bone. 47 (5), 848-858 (2010).</li><li>Cheng, F., Hulley, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+osteocyte--a+novel+endocrine+regulator+of+body+phosphate+homeostasis.">The osteocyte--a novel endocrine regulator of body phosphate homeostasis.</a> Maturitas. 67 (4), 327-338 (2010).</li><li>Paic, F., 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=Identification+of+differentially+expressed+genes+between+osteoblasts+and+osteocytes.">Identification of differentially expressed genes between osteoblasts and osteocytes.</a> 45 (4), 682-692 (2009).</li><li>Dallas, S. L., Bonewald, L. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dynamics+of+the+transition+from+osteoblast+to+osteocyte.">Dynamics of the transition from osteoblast to osteocyte.</a> Ann N Y Acad Sci. 1192, 437-443 (2010).</li><li>van der Plas, A., Nijweide, P. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Isolation+and+purification+of+osteocytes.">Isolation and purification of osteocytes.</a> J Bone Miner Res. 7 (4), 389-396 (1992).</li><li>Nijweide, P. J., van der Plas, A., Alblas, M. J., Klein-Nulend, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Osteocyte+isolation+and+culture.">Osteocyte isolation and culture.</a> Methods Mol Med. 80, 41-50 (2003).</li><li>Stern, A. R., Stern, M. M., Van Dyke, M. E., J&#228;hn, K., Prideaux, M., Bonewald, L. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Isolation+and+culture+of+primary+osteocytes+from+the+long+bones+of+skeletally+mature+and+aged+mice.">Isolation and culture of primary osteocytes from the long bones of skeletally mature and aged mice.</a> Biotechniques. 52 (6), 361-373 .</li><li>Kalajzic, I., Matthews, B. G., Torreggiani, E., Harris, M. A., Divieti Pajevic, P., Harris, S. E. <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+and+in+vivo+approaches+to+study+osteocyte.">In vitro and in vivo approaches to study osteocyte.</a> Bone. 54 (2), 296-206 .</li><li>Bonewald, L. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Establishment+and+characterization+of+an+osteocyte-like+cell+line,+MLO-Y4.">Establishment and characterization of an osteocyte-like cell line, MLO-Y4.</a> J Bone Miner Metab. 17 (1), 61-65 (1999).</li><li>Mattinzoli, D., 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=A+novel+model+of+in+vitro+osteocytogenesis+induced+by+retinoic+acid+treatment.">A novel model of in vitro osteocytogenesis induced by retinoic acid treatment.</a> Eur Cell Mater. 24, 403-425 (2012).</li><li>Ross, S. A., McCaffery, P. J., Drager, U. C., De Luca, L. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Retinoids+in+embryonal+development.">Retinoids in embryonal development.</a> Physiol Rev. 80 (3), 1021-1054 (2000).</li><li>Clagett-Dame, M., McNeill, E. M., Muley, P. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Role+of+all-trans+retinoic+acid+in+neurite+outgrowth+and+axonal+elongation.">Role of all-trans retinoic acid in neurite outgrowth and axonal elongation.</a> J Neurobiol. 66 (7), 739-756 (2006).</li><li>Vaughan, M. R., 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=ATRA+induces+podocyte+differentiation+and+alters+nephrin+and+podocin+expression+in+vitro+and+in+vivo.">ATRA induces podocyte differentiation and alters nephrin and podocin expression in vitro and in vivo.</a> Kidney Int. 68 (1), 133-144 (2005).</li><li>Dodig, 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=Identification+of+a+TAAT-containing+motif+required+for+high+level+expression+of+the+COL1A1+promoter+in+differentiated+osteoblasts+of+transgenic+mice.">Identification of a TAAT-containing motif required for high level expression of the COL1A1 promoter in differentiated osteoblasts of transgenic mice.</a> J Biol Chem. 271 (27), 16422-16429 (1996).</li><li>Burry, R. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Immunocytochemistry.">Immunocytochemistry.</a> A practical guide for biomedical research. , (2010).</li><li>Woo, S. M., Rosse, r. J., Dusevich, V., Kalajzic, I., Bonewald, L. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cell+line+IDG-SW3+replicates+osteoblast-to-late-osteocyte+differentiation+in+vitro+and+accelerates+bone+formation+in+vivo.">Cell line IDG-SW3 replicates osteoblast-to-late-osteocyte differentiation in vitro and accelerates bone formation in vivo.</a> J Bone Miner Res. 26 (11), 2634-2646 (2011).</li><li>Gu, G., Nars, M., Hentune, n. T. A., Metsikk&#246;, K., V&#228;&#228;n&#228;nen, H. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Isolated+primary+osteocytes+express+functional+gap+junctions+in+vitro.">Isolated primary osteocytes express functional gap junctions in vitro.</a> Cell Tissue Res. 323 (2), 263-271 (2006).</li><li>Boukhechba, F., 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=Human+primary+osteocyte+differentiation+in+a+3D+culture+system.">Human primary osteocyte differentiation in a 3D culture system.</a> J Bone Miner Res. 24 (11), 1927-1935 (2009).</li><li>Krishnan, V., Dhurjati, R., Vogler, E. A., Mastro, A. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Osteogenesis+in+vitro:+from+pre-osteoblasts+to+osteocytes:+a+contribution+from+the+Osteobiology+Research+Group,+The+Pennsylvania+State+University.">Osteogenesis in vitro: from pre-osteoblasts to osteocytes: a contribution from the Osteobiology Research Group, The Pennsylvania State University.</a> In Vitro Cell Dev Biol Anim. 46 (1), 28-35 .</li><li>Irie, K., Ejiri, S., Sakakura, Y., Shibui, T., Yajima, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Matrix+mineralization+as+a+trigger+for+osteocyte+maturation.">Matrix mineralization as a trigger for osteocyte maturation.</a> J Histochem Cytochem. 56 (6), 561-567 .</li><li>Hirao, 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=Oxygen+tension+is+an+important+mediator+of+the+transformation+of+osteoblasts+to+osteocytes.">Oxygen tension is an important mediator of the transformation of osteoblasts to osteocytes.</a> J Bone Miner Metab. 25 (5), 266-276 (2007).</li><li>Brounais, B., 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=Long+term+oncostatin+M+treatment+induces+an+osteocyte-like+differentiation+on+osteosarcoma+and+calvaria.">Long term oncostatin M treatment induces an osteocyte-like differentiation on osteosarcoma and calvaria.</a> Bone. 44 (5), 830-839 (2009).</li><li>Gupta, R. R., Yoo, D. J., Hebert, C., Niger, C., Stains, J. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Induction+of+an+osteocyte-like+phenotype+by+fibroblast+growth+factor-2.">Induction of an osteocyte-like phenotype by fibroblast growth factor-2.</a> Biochem Biophys Res Commun. 402 (2), 258-264 (2010).</li><li>Poole, K. 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=Sclerostin+is+a+delayed+secreted+product+of+osteocytes+that+inhibits+bone+formation.">Sclerostin is a delayed secreted product of osteocytes that inhibits bone formation.</a> FASEB J. 19 (13), 1842-1844 (2005).</li><li>Dallas, S. L., Prideaux, M., Bonewald, L. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+Osteocyte:+An+Endocrine+Cell+and+More.">The Osteocyte: An Endocrine Cell and More.</a> Endocr Rev. 34 (5), 658-690 (2013).</li></ol>

In the last years the osteocyte has emerged as the most central cell in the bone. Research advances are progressively revealing a number of previously unsuspected or unproven osteocyte properties, which are of enormous value for designing novel and better treatment for a variety of bone diseases. However, investigation of osteocyte biology has been suffering from the limited availability of in vitro models to such an extent that osteoblasts have been used as surrogate cells over a long period before the osteocyt...

Osteocytes, the most abundant bone cell type, are terminally differentiated, highly ramified cells deeply located within the skeleton. The cell body of mature osteocytes are contained in bone lacunae and have diverse shapes; osteocytes with elongated cell bodies are found in the cortical bone, whereas rounded osteocytes are more frequent in the trabecular bone1. Branched dendrites extend from the cell body and reside in tiny channels called canaliculi, forming an intricate web that makes multiple contacts not only with other osteocytes, but also with other bone cell types, bone marrow, blood vessels and associated pericytes. Through the interstitial fluid contained in lacunae and canaliculi, osteocytes are also ultimately connected to the circulating system, therefore they can influence not only local but also systemic events, and vice versa their behavior can be regulated by both local and systemic changes2.
The study of osteocytes has recently gained momentum, thanks to several technical advances, such as the generation of tissue- and cell-specific transgenic animals, the use of powerful microscopy techniques and of high-throughput molecular screening3,4. However, knowledge of these cells is still incomplete, mainly due to the relative scarcity of adequate in vitro models. Actually, osteocytes have been always difficult to obtain and maintain in culture because of the deep location, and the low level of proliferation that characterizes such a differentiated cell type.
Along the years, a number of methods have been developed to isolate primary osteocytes5-7, though they generally produce low cell yields and carry the risk that even a few contaminating fibroblasts will rapidly overgrow the osteocytes8. For this reason, most experimental in vitro work has been conducted so far on the well-established osteocyte cell line MLO-Y4&#160;9.
Additional in vitro methodologies could enhance the possibility of studying these cells and could improve the analysis of osteocyte biology and pathophysiology. To be largely adopted, such methods should be easy to reproduce, and would not require special instrumentation or very long times to reach cell maturation. Importantly, if applicable to primary cells, they would make it possible to take advantage of transgenic animals.&#160; We recently described that treatment of the osteoblast cell line MC3T3-E1 and of primary osteoblasts with retinoic acid induces a rapid phenotype change leading to the development of a homogeneous population of ramified cells bearing morphological and molecular features of osteocytes10.
All-trans retinoic acid (ATRA) is an active metabolic product of vitamin A which regulates gene transcription by binding the nuclear retinoic acid receptors (RARs). RARs bind to DNA as heterodimers with the retinoid X receptors (RXRs), ultimately leading to modulation of retinoic acid (RA)-responsive target genes11. ATRA has been shown to modulate differentiation and maturation of several cell types, among them other ramified cells such as neuronal cells12 and podocytes13.
The method here described is based on addition of ATRA to primary osteoblasts. To be effective, ATRA has to be added at a precise maturation stage on cells plated at a defined density; in our experience these conditions are critical to obtain the phenotype switch from osteoblasts to osteocytes.

<table border="0" cellpadding="0" cellspacing="0" style="width:923px;" width="923">
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	<tbody>
		<tr height="20">
			<td height="20" style="height:20px;width:283px;">Name</td>
			<td style="width:239px;">Company</td>
			<td style="width:219px;">Cat #</td>
			<td style="width:183px;">comments</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Collagenase P</td>
			<td>Roche Applied Science</td>
			<td>11213857001</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Trypsin</td>
			<td>Gibco, Life Technologies</td>
			<td>15400054</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">HBSS</td>
			<td>Gibco, Life Technologies</td>
			<td>14175129</td>
			<td>Pre-warm at 37&#176;C before use</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Alpha-MEM</td>
			<td>Invitrogen, Life Technologies</td>
			<td>22571038</td>
			<td>Pre-warm at 37&#176;C before use</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">FBS</td>
			<td>Sigma-Aldrich</td>
			<td>F4135</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Streptomycin/Penicillin</td>
			<td>Sigma-Aldrich</td>
			<td>P4333</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Ascorbic acid</td>
			<td>Sigma-Aldrich</td>
			<td>A4403</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">glycerol 2-phosphate disodium salt hydrate</td>
			<td>Sigma-Aldrich</td>
			<td>G9422</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">ATRA</td>
			<td>Sigma-Aldrich</td>
			<td>R2625</td>
			<td>Protect ATRA from light</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">paraformaldehyde</td>
			<td>Sigma-Aldrich</td>
			<td>P6148</td>
			<td>Dissolve in PBS and filter before use. Work always under a chemical hood.</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">DAPI</td>
			<td>Sigma-Aldrich</td>
			<td>32670</td>
			<td>Can be added to the secondary antibody</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Alizarin red</td>
			<td>Sigma-Aldrich</td>
			<td>A5533</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Janus Green</td>
			<td>Sigma-Aldrich</td>
			<td>201677</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Perchloric acid</td>
			<td>Sigma-Aldrich</td>
			<td>176745</td>
			<td>Use with caution (skin and eye protection are recommended)</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">HCl</td>
			<td>Sigma-Aldrich</td>
			<td>320331</td>
			<td>Use with caution (skin and eye protection are recommended)</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">glycerol</td>
			<td>Sigma-Aldrich</td>
			<td>G5516</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">fluorsave</td>
			<td>Calbiochem - Merck</td>
			<td>345789</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;"></td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Economy Tweezers #7, 0.40 x 0.5mm tips</td>
			<td>World Precision Instruments</td>
			<td>501981</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Economy Tweezers #4, 0.40 x 0.45mm tips</td>
			<td>World Precision Instruments</td>
			<td>501978</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Dissecting Scissors, straight,10cm curved</td>
			<td>World Precision Instruments</td>
			<td>14394</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Surgical Scissors, 14 cm, straight, S/S</td>
			<td>World Precision Instruments</td>
			<td>501218</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;"></td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Culture flasks</td>
			<td>Corning</td>
			<td>430168</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Well-plates</td>
			<td>Corning</td>
			<td>3335</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Thermanox coverslips</td>
			<td>Thermo Scientific Nunc</td>
			<td>12-565-27</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;"></td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">microscope</td>
			<td>Zeiss Apotome</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">spectrometer</td>
			<td>Safas Xenius</td>
			<td></td>
			<td></td>
		</tr>
	</tbody>
</table>

application of retinoic acid to obtain osteocytes cultures from primary mouse osteoblasts

The need for osteocyte cultures is well known to the community of bone researchers; isolation of primary osteocytes is difficult and produces low cell numbers. Therefore, the most widely used cellular system is the osteocyte-like MLO-Y4 cell line.
The method here described refers to the use of retinoic acid to generate a homogeneous population of ramified cells with morphological and molecular osteocyte features.
After isolation of osteoblasts from mouse calvaria, all-trans retinoic acid (ATRA) is added to cell medium, and cell monitoring is conducted daily under an inverted microscope. First morphological changes are detectable after 2 days of treatment and differentiation is generally complete in 5 days, with progressive development of dendrites, loss of the ability to produce extracellular matrix, down-regulation of osteoblast markers and up-regulation of osteocyte-specific molecules.
Daily cell monitoring is needed because of the inherent variability of primary cells, and the protocol can be adapted with minimal variation to cells obtained from different mouse strains and applied to transgenic models. 
The method is easy to perform and does not require special instrumentation, it is highly reproducible, and rapidly generates a mature osteocyte population in complete absence of extracellular matrix, allowing the use of these cells for unlimited biological applications.

Results were obtained from 5 to 10 independent experiments.
Cell Morphology
AA/GP treated primary cells have mostly cobblestone-like features, characteristic of mature osteoblasts. Interspersed ramified cells (as indicated by red arrows in Figure 1A) can be found, which likely represent some osteocytes.
With ATRA treatment, cells rapidly start displaying ramifications, which are generally observed after 2 days. As shown in <str...

Watch this Scientific Journal Video about Application of Retinoic Acid to Obtain Osteocytes Cultures from Primary Mouse Osteoblasts at JoVE.com

Application of Retinoic Acid to Obtain Osteocytes Cultures from Primary Mouse Osteoblasts

Treatment of primary mouse osteoblasts with retinoic acid produces a homogeneous population of ramified cells bearing morphological and molecular features of osteocytes. The method overcomes the difficulty of obtaining and maintaining primary osteocytes in culture, and can be advantageous to study cells derived from transgenic models.&#160;

The need for osteocyte cultures is well known to the community of bone researchers; isolation of primary osteocytes is difficult and produces low cell ...

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13.3K 조회수.  Fondazione IRCCS Ca' Granda Ospedale Maggiore Policlinico.  레티노 산으로 기본 마우스 조골 세포의 치료는 골 세포의 형태 학적 및 분자 기능 베어링 분지 세포의 균일 한 인구를 생산하고 있습니다. 방법은 획득 및 배양 주 골 세포를 유지의 어려움을 극복하고, 형질 전환 모델 유래 세포를 연구하는 것이 유리할 수있다. 