The authors have declared that no competing interests exist.
Conceived and designed the experiments: SI CDS. Performed the experiments: SI. Analyzed the data: SI. Wrote the paper: SI CDS.
Female birds develop asymmetric gonads: a functional ovary develops on the left, whereas the right gonad regresses. In males, however, testes develop on both sides. We examined the distribution of germ cells using
Unlike mammals, which have apparently symmetric gonads, most female bird species develop asymmetrically, generating a functional ovary only on the left side, whereas males develop bilateral testes
The gonads of both sexes contain two layers, cortex and medulla
Several genes underlie sexual differentiation and lie near the top of a genetic hierarchy governing sex specific differences. Among the genes that differ between male and female embryos at an early stage,
In addition to differential expression of molecular components related to sexual differentiation and function, a few genes have been described to display left-right differences in expression. Not surprisingly, given its role as a highly conserved determinant of left-sidedness in many organ systems
Cell lines derived from pre-primitive streak stage embryos (“chick ES cells”) can contribute to all somatic lineages but not to the germ line
As early as 1935, Witschi suggested that the left embryonic ovary contains more germ cells than the right, consistent with the obvious difference in development of these female organs (the right ovary does not develop to adulthood)
Testicular sections exhibit germ cells,
Ovarian sections exhibit germ cells,
Gene | Number ofembryos ( |
Number ofsections (total) | Expressing cells inleft gonad sections | Expressing cells inright gonad sections | Expressing cells in left cortex | Expressing cells in right cortex | Expressing cells in left medulla | Expressing cells in right medulla |
3F/3M | 64F/78M | F(61±30)**a1, M(21±16)**a2 | F(2±4)**a1, M(11±11)**a2 | F(54±34)**b1, M(5±2) | F(3±3)**b1, M(5±4) | F(9±14), M(16±9) | F(12±8), M(16±7) | |
3F/3M | 34F/66M | F (104±47)**a1, M(65±27)**a2 | F(32±30)**a1, M(22±12)**a2 | F(70±33)**b1, M(8±5)*b2 | F(32±29)**b1, M(4±3)*b2 | F(38±29)**c1, M(49±24)**c2 | F(8±7)**c1, M(12±12)**c2 | |
3F/3M | 62F/73M | F(89±33)**a1, M(69±26)**a2 | F(32±16)**a1, M(21±11)**a2 | F(45±21)**b1, M(16±8)**b2 | F(13±9)**b1, M(5±2)**b2 | F(43±15)**c1, M(50±23)**c2 | F(21±13)**c1, M(17±9)**c2 | |
3F/4M | 53F/101M | F(37±16)**a1, M(34±13)**a2 | F(19±8)**a1, M(23±12)**a2 | F(11±22), M(7±6) | F(1±2), M(6±4) | F(22±6)**c1, M(35±13) | F(13±4)**c1, M(28±12) | |
3F/3M | 63F/50M | F(25±14)**a1, M(36±21)**a2 | F(10±8)**a1, M(14±8)**a2 | F(28±9)**b1, M(8±4)**b2 | F(9±5)**b1, M(3±3)**b2 | F(9±4), M(34±18)**c2 | F(8±6), M(15±9)**c2 |
F: Female; M: Male (**
a1: a comparison between female left and right gonad sections; b1: a comparison between female left and right cortices; c1: a comparison between female left and right medullae; a2: a comparison between male left and right gonad sections; b2: a comparison between male left and right cortices; c2: a comparison between male left and right medullae.
Almost all the female germ cells are located in the ovarian cortex (
These results reveal left-right differences in germ cell distribution in the gonads of embryos of both sexes: the left gonad contains a greater number of
Next, we examined embryonic ovaries and testes for expression of three pluripotency-associated genes:
The average number of cells expressing
The average number of cells expressing
In ovarian cortex, there were 45±21
Therefore again the majority of the medullary ovarian cells expressing
In female, the average number of cells expressing
These results reveal that there is little or no correlation between
Embryonic ovaries and testes also express
A distinctive feature of gonadal development in the chick is that female embryos develop gonads asymmetrically: only the left side forms a functional ovary while the right side regresses
Left-right patterning plays important roles for internal organ formation, positioning and embryonic turning
Other previous studies also reported asymmetric gonad development in chick embryos, suggesting that 70% of PGCs are found on the left side of female gonads
That
The present study also provides novel information about the expression of genes associated with pluripotency in embryonic gonads of both sexes and between left and right gonads. For all four genes studied (
At stage HH 35, embryonic gonads are well into their differentiation into testes and ovaries. Studies in humans have reported that both fetal testicular and ovarian germ cells express pluripotency-associated markers including OCT4 and NANOG, suggesting that both male and female fetal germ cells maintain expression of these genes during and after sexual differentiation of the gonads
The present study confirms asymmetric distribution of
All animal experiments were conducted according to UK Home Office guidelines. All embryos were harvested before the 10th day of incubation and the work is therefore exempt from requirement for a licence. Fertilized hens’ (
To generate digoxigenin RNA antisense probes,
To assess the proportion of cells expressing the various genes in embryonic testes and ovaries, expressing cells were counted starting from the first section of the first slide containing gonadal tissue. To avoid counting the same cells more than once, one in three sections were counted until the last section of the gonad was reached. The unpaired Student’s
We thank Sharon Boast for technical assistance.