Fetal genital tract and adnexa.
Keywords: ewe, ovine, lamb, uterus, term, ovariesUterine anatomy:
Figure 1. The genital tract of a Texel fetus (one of triplets) at approximately 130 days gestation. For scale, it is superimposed on the fetus itself in the upper image. Despite the presence of a male fetus in this pregnancy, mullerian development was normal in this female. For more on freemartinism in sheep, see another posting in LORI. Image size: 924 x 743 px
Figure 3. Structures surrounding the intra-abdominal portion of the umbilical cord, showing how they are replace by their analogs in the adult animal. Image size: 986 X 517 px
Figure 1. The genital tract of a Texel fetus (one of triplets) at approximately 130 days gestation. For scale, it is superimposed on the fetus itself in the upper image. Despite the presence of a male fetus in this pregnancy, mullerian development was normal in this female. For more on freemartinism in sheep, see another posting in LORI. Image size: 924 x 743 px
In a late gestation ruminant fetus, the author has noticed that the fetal uterus is usually tightly coiled as it is in adults during estrus. That appears to have been the case here too. This may be a response to elevated of placental estrogen secreted towards the end of gestation. Also note the dark fluid accumulation in the expanded infundibulum; reminiscent of hydropsalpinx. This has also been noted occasionally by the author in other ruminant fetal tracts. Its significant is unknown.
The reader is reminded that all the oocytes this fetus would ever have, were present in these ovaries at this time. Meiosis had indeed started but only progressed about half-way through meiosis 1 (reduction division) in those oocytes. The oocytes were still diploid (but had double the amount of DNA compared to somatic cells) when this photograph was taken. If this fetus had survived until puberty, an oocyte would be selected for ovulation (if it had not been wasted by regression during a diestrous follicle wave) and meiosis would resume under the effect of an LH surge during estrus. This division would form a polar body. Almost immediately, the mitotic phase of meiosis (meiosis 2) would follow and in theory, another two polar bodies would have been formed, stimulated by fertilization itself. In most vertebrates, fertilization is required to complete meiosis 2, in others not. Occasionally the first polar body does not divide, leaving only two polar bodies under the zona pellucida at the time of fertilization.
Recall that the mitotic phase of meiosis (meiosis 2) occurs almost immediately after the completion of reduction division. This occurs so quickly that there is not enough time for DNA to double. However, this is not a problem because DNA is doubled in oocytes just as meiosis starts, long before birth, in the early life of an embryo. Again, all the oocytes in the fetal ovaries shown here had already doubled the amount of DNA they would have had at the time of ovulation.
Note: In males, there is no fixed number of gametes; diploid spermatogonia are continually replaced along the basement membrane of seminiferous tubules. Also, males do not experience LH surges. Instead, baseline LH secretion constantly stimulates spermatogenesis.
Ovarian function:
It is interesting to re-visit the quandary of why it is that corpora lutea are present in the ovaries at term, yet ewes can be ovariectomized after 60 days of gestation with no effect on lamb birth weight and survival. The ovaries of the ewe in this case were examined and as expected, at least three corpora lutea were present in the ovaries. See figure 2.
Figure 2. Corpora lutea in the ovaries of a ewe at term. Image size: 800 x 544 px
Another valuable view of this pregnancy was to examine structures that contribute to umbilical anatomy; a theme common to all mammals. In the following image the reader can appreciate how fetal structures are replaced in situ by their equivalents in the adult.
Selected references:
Al-Gubory, K.H. et al. 1999. Effects of luteectomy on the maintenance of pregnancy,
circulating progesterone concentrations and lambing performance in sheep.
Reprod.Fertil.Dev. 11(6):317-22
Clift, D and Schuh, M. 2013 Restarting life: fertilization and the transition from meiosis to mitosis. Nat Rev Mol Cell Biol. 14:549-62
Cooper, G M. The Cell, 2nd edition. A Molecular Approach. Sinauer Associates; 2000.ISBN-10: 0-87893-106-6
