The reproductive tract of a 10 month equine fetus
Keywords: uterus, fetus, ovaries, equine, anatomy.
At about 90 days of gestation the fetal gonads of both sexes start to increase in size and reach maximum diameters at about 7 to 8 months of gestation. Interestingly, the growth is due to hyperplasia of interstitial tissue, not oocytes or seminiferous tubules. In fact, by 7 to 8 months the fetal ovaries are larger than the maternal ovaries. After this peak in gonadal growth, the fetal gonads decrease in size to about one tenth of fetal maximum.
Equine fetal ovaries in both sexes have long been known to be a substantial source of androgens. These androgens are aromatized to various estrogens. Increasing and decreasing estrogen production by the equine placenta during gestation (a feature often used in pregnancy diagnosis) parallels the growth and regression of the fetal gonads. However, it has only recently been shown that the gonads are also a substantial source of inhibin (not AMH) but the function of this inhibin production is not yet known.
In this ten month old equine fetus, the ovaries are still relatively massive.In the image below, the right kidney of the fetus lies between left and right ovaries. The uterine tube (fallopian tube) is shown here, crossing the ventral surface of the left ovary. The left and right horns and body of the uterus are also visible, a small fold in the caudal part of the uterine body, indicating the approximate location of the cervix.
At about 90 days of gestation the fetal gonads of both sexes start to increase in size and reach maximum diameters at about 7 to 8 months of gestation. Interestingly, the growth is due to hyperplasia of interstitial tissue, not oocytes or seminiferous tubules. In fact, by 7 to 8 months the fetal ovaries are larger than the maternal ovaries. After this peak in gonadal growth, the fetal gonads decrease in size to about one tenth of fetal maximum.
Equine fetal ovaries in both sexes have long been known to be a substantial source of androgens. These androgens are aromatized to various estrogens. Increasing and decreasing estrogen production by the equine placenta during gestation (a feature often used in pregnancy diagnosis) parallels the growth and regression of the fetal gonads. However, it has only recently been shown that the gonads are also a substantial source of inhibin (not AMH) but the function of this inhibin production is not yet known.
In this ten month old equine fetus, the ovaries are still relatively massive.In the image below, the right kidney of the fetus lies between left and right ovaries. The uterine tube (fallopian tube) is shown here, crossing the ventral surface of the left ovary. The left and right horns and body of the uterus are also visible, a small fold in the caudal part of the uterine body, indicating the approximate location of the cervix.
Image size: 1500 x 1500px
The image below is a dorsal view of the excised tract seen above. The massive size of the ovaries compared to the rest of the reproductive tract is remarkable. The caudal part of the tract (from the cervix caudally) has been twisted anticlockwise to reveal the urethra (yellow arrow) and the clitoral sinus (green arrow). R= Rectum. The small black arrow indicates the position of a partially perforate hymen!
The intercornual ligament (more of a feature of the bovine than the equine tract) is clearly visible here, as are the (middle) uterine arteries.
Image size: 1500 x 1130px
Selected reference:
Tanaka, Y et al. 20013. Localization and secretion of inhibins in the equine fetal ovaries. Biol.Reprod.328-335
