Onset of meiosis in the chicken embryo; evidence of a role for retinoic acid.
BACKGROUND
Meiosis in higher vertebrates shows a dramatic sexual dimorphism: germ cells enter meiosis and arrest at prophase I during embryogenesis in females, whereas in males they enter mitotic arrest during embryogenesis and enter meiosis only after birth. Here we report the molecular analysis of meiosis onset in the chicken model and provide evidence for conserved regulation by retinoic acid.


RESULTS
Meiosis in the chicken embryo is initiated late in embryogenesis (day 15.5), relative to gonadal sex differentiation (from day 6). Meiotic germ cells are first detectable only in female gonads from day 15.5, correlating with the expression of the meiosis marker, SCP3. Gonads isolated from day 10.5 female embryos and grown in serum-free medium could still initiate meiosis at day 16.5, suggesting that this process is controlled by an endogenous clock in the germ cells themselves, and/or that germ cells are already committed to meiosis at the time of explantation. Early commitment is supported by the analysis of chicken STRA8, a pre-meiotic marker shown to be essential for meiosis in mouse. Chicken STRA8 is expressed female-specifically from embryonic day 12.5, preceding morphological evidence of meiosis at day 15.5. Previous studies have shown that, in the mouse embryo, female-specific induction of STRA8 and meiosis are triggered by retinoic acid. A comprehensive analysis of genes regulating retinoic acid metabolism in chicken embryos reveals dynamic expression in the gonads. In particular, the retinoic acid-synthesising enzyme, RALDH2, is expressed in the left ovarian cortex at the time of STRA8 up-regulation, prior to meiosis.


CONCLUSIONS
This study presents the first molecular analysis of meiosis onset in an avian embryo. Although aspects of avian meiosis differ from that of mammals, a role for retinoic acid may be conserved.
Smith
Craig A
CA
Murdoch Childrens Research Institute and University of Melbourne Department of Paediatrics, Royal Children's Hospital, Melbourne, 3052, Australia. craig.smith@mcri.edu.au
Roeszler
Kelly N
KN
Bowles
Josephine
J
Koopman
Peter
P
Sinclair
Andrew H
AH
D000445:Aldehyde Oxidoreductases / Q000096:biosynthesis / Q000235:genetics; D000818:Animals; D002642:Chick Embryo; D002645:Chickens / Q000235:genetics* / Q000378:metabolism; D003577:Cytochrome P-450 Enzyme System / Q000096:biosynthesis / Q000235:genetics; D005260:Female; D018507:Gene Expression Regulation, Developmental / Q000502:physiology; D008297:Male; D008540:Meiosis / Q000235:genetics / Q000502:physiology*; D051379:Mice; D009924:Organ Culture Techniques; D000072516:Retinoic Acid 4-Hydroxylase; D014212:Tretinoin / Q000378:metabolism / Q000502:physiology*; D015854:Up-Regulation / Q000502:physiology
D014212:Tretinoin; D003577:Cytochrome P-450 Enzyme System; D000072516:Retinoic Acid 4-Hydroxylase; D000445:Aldehyde Oxidoreductases
Dev Biol. 2003 Oct 15;262(2):303-12
14550793
Proc Natl Acad Sci U S A. 2006 Feb 21;103(8):2474-9
16461896
Science. 2006 Apr 28;312(5773):596-600
16574820
Dev Biol. 2003 Oct 1;262(1):1-15
14512014
Proc Natl Acad Sci U S A. 2007 Jan 2;104(1):187-92
17189423
Dev Biol. 2005 Aug 15;284(2):337-50
16005453
J Exp Zool A Comp Exp Biol. 2005 Mar 1;303(3):179-85
15726631
Mol Reprod Dev. 2004 Aug;68(4):422-8
15236325
Dev Biol. 1999 Dec 1;216(1):282-96
10588879
Gen Comp Endocrinol. 1997 Nov;108(2):182-90
9356214
Anat Rec. 1991 Jul;230(3):378-86
1867412
J Morphol. 1951 Jan;88(1):49-92
24539719
Prog Clin Biol Res. 1986;217A:183-6
3749130
Development. 2007 Oct;134(19):3401-11
17715177
Cell Differ. 1972 Dec;1(5):325-34
4670889
Zoolog Sci. 1996 Aug;13(4):559-63
8940910
Bioessays. 2004 Feb;26(2):120-32
14745830
Nat Genet. 2006 Dec;38(12):1430-4
17115059
Dev Biol. 1997 Jul 1;187(1):107-13
9224678
Gene Expr Patterns. 2003 Oct;3(5):621-7
12971996
Exp Cell Biol. 1986;54(2):94-105
3754824
Anat Rec. 1976 Jun;185(2):139-45
945007
Dev Biol. 2001 Jan 15;229(2):468-79
11203703
J Exp Zool. 1983 Nov;228(2):173-93
6663256
J Embryol Exp Morphol. 1967 Dec;18(3):299-304
4171091
Dev Genet. 1999;25(4):353-64
10570467
J Cell Biol. 1996 Oct;135(2):469-77
8896602
J Embryol Exp Morphol. 1963 Sep;11:513-36
14061957
Dev Biol. 1974 Jan;36(1):113-29
4822828
Biol Reprod. 2003 Feb;68(2):560-70
12533420
Dev Biol. 1999 Jun 15;210(2):288-304
10357892
Dev Dyn. 2003 May;227(1):114-27
12701104
Development. 1998 Nov;125(21):4133-44
9753668
Br Poult Sci. 2001 Mar;42(1):134-8
11337963
Dev Dyn. 2004 Jul;230(3):509-17
15188435
Physiol Rev. 1986 Jan;66(1):71-117
3511481