The Role of Ovarian Surface Epithelium in Folliculogenesis during Fetal Development of the Bovine Ovary: A Histological and Immunohistochemical Study

Although many aspects of ovarian differentiation have been
established,comparatively little is known about prenatal follicle
formation anddifferentiation of bovine ovaries. The objective of
this investigationwas to study the role of the surface epithelium
during the developmentof germ cell nests, germ cell cords and
follicle formation in the fetalbovine ovary. Associated important
proliferation and apoptotic featureswere further investigated.
Additionally, the expression pattern of theS100 protein was
detected. A strong increase of mitotic figures wasdetected in the
surface epithelium, germ cell nests and germ cell cordsof ovaries
with a crown-rump length (CRL) of 13.0-58.0 cm. Oocytes
werepositively stained with S100 in bovine ovaries from fetuses
with a CRLof 21.0 cm. The staining intensity enhanced parallel to
increasingoocyte and follicle sizes during the ovary development.
In later stages,a strong staining for S100 was observed in healthy
oocytes incontradistinction to atretic oocytes where no expression
of the S100protein could be found. In conclusion, increasing
mitosis index ofsurface epithelium cells, as well as oogonia
directly beneath thesurface epithelium, in combination with open
surface connection duringstages from a CRL of 11.0-94.0 cm of
bovine fetal ovaries could play animportant role in the period of
time of ongoing folliculogenesis andderivation of granulosa cells.
Additionally, S100-positive oocytes inprimordial and later follicle
stages joined by a high rate ofKi67-positive index in surrounding
granulosa cells indicate that in theoocytes the S100 protein can
perhaps be a useful marker for intactoocytes in bovine ovaries.