Different patterns of anti‐Müllerian hormone expression, as related to DMRT1, SF‐1, WT1, GATA‐4, Wnt‐4, and Lhx9 expression, in the chick differentiating gonads

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Abstract
In mammals, anti‐Müllerian hormone (AMH) is produced by Sertoli cells from the onset of testicular differentiation and by granulosa cells after birth. In birds, AMH starts to be expressed in indifferent gonads of both sexes at a similar level and is later up‐regulated in males. We previously demonstrated that, unlike in mammals, the onset of AMH expression occurs in chick embryo in the absence of SOX9. We looked for potential factors that might be involved in regulating AMH expression at different stages of chick gonad differentiation by comparing its expression pattern in embryos and young chicken with that of DMRT1, SF‐1, WT1, GATA‐4, Wnt‐4, and Lhx9, by in situ hybridization. The results allowed us to distinguish different phases. (1) In indifferent gonads of both sexes, AMH is expressed in dispersed medullar cells. SF‐1, WT1, GATA‐4, Wnt‐4, and DMRT1 are transcribed in the same region of the gonads, but none of these factors has an expression strictly coincident with that of AMH. Lhx9 is present only in the cortical area. (2) After this period, AMH is up‐regulated in male gonads. The up‐regulation is concomitant with the beginning of SOX9 expression and a sex dimorphic level of DMRT1 transcripts. It is followed by the aggregation of the AMH‐positive cells (Sertoli cells) into testicular cords in which AMH is coexpressed with DMRT1, SF‐1, WT1, GATA‐4, and SOX9. (3) In the females, the low level of dispersed medullar AMH expression is conserved. With development of the cortex in the left ovary, cells expressing AMH accumulate in the juxtacortical part of the medulla, whereas they remain dispersed in the right ovary. At this stage, AMH expression is not strictly correlated with any of the studied factors. (4) After hatching, the organization of left ovarian cortex is characterized by the formation of follicles. Follicular cells express AMH in conjunction with SF‐1, WT1, and GATA‐4 and in the absence of SOX9, as in mammals. In addition, they express Lhx9 and Wnt‐4, the latter being also found in the oocytes. (5) Moreover, unlike in mammals, the chicken ovary retains a dispersed AMH expression in cortical interstitial cells between the follicles, with no obvious correlation with any of the factors studied. Thus, the dispersed type of AMH expression in indifferent and female gonads appears to be bird‐specific and not controlled by the same factors as testicular or follicular AMH transcription.