Formation of Ovarian Follicles During Fetal Development in Sheep1

Abstract

The origin of follicle (i.e., pregranulosa) cells that become the somatic component of primordial follicles is obscure. In addition, information regarding the structural changes that accompany the concomitant regression of ovigerous cords and the appearance of primordial follicles is lacking. In the present study, ovine ovaries collected at frequent time intervals between Day 38 and Day 100 of fetal life were examined by light and electron microscopy. To gain new information regarding the origin of follicular cells, incorporation of 5-bromo-2'-deoxyuridine was used to identify proliferating cells at selected stages of development. Based on the location and identity of proliferating cells, apoptotic cells, and sequential changes in histoarchitecture, we hypothesize 1) that most (i.e., >95%) of the granulosal cells in newly formed primordial follicles originate from the ovarian surface epithelium; 2) that the sequential events leading to follicle formation take place entirely within ovigerous cords, with the first follicles forming at the interface of the cortex and medulla; and 3) that the loss (i.e., >75%) of germ cells, but not of somatic cells, within the ovigerous cords is a means by which each surviving oocyte gains additional pregranulosal cells before follicle formation. Conceptual models detailing the chronology of developmental events involved in the formation of primordial follicles in sheep are discussed.

Quantification of healthy follicles in the neonatal and adult mouse ovary: evidence for maintenance of primordial follicle supply

“…We noted that the superficial cortex particularly in day 1 ovaries shows smaller oocytes, many still in cell nests, together with primordial follicles. A similar sequence of primordial follicle formation suggesting a medullary-cortical gradient of maturation has been reported in the mouse ovary by Peters (1969) and McClellan et al (2003), the rat-(Hirshfield 1992, Rajah et al 1992, Hirshfield & DeSanti 1995 and the sheep ovary (Sawyer et al 2002) and appears to be a developmental pattern intrinsic to the neonatal mammalian ovary . As oocytes in the ovarian medulla mature earlier than those in the superficial cortex, i.e.…”

Transcriptional Regulation by the Wilms Tumor Protein, Wt1, Suggests a Role of the Metalloproteinase Adamts16 in Murine Genitourinary Development

“…Indeed, three major sources for pregranulosa cells have been under discussion for many decades: 1) mesothelial cells on the surface of the developing ovary (62), 2) cells originating from a centrally located blastema (63), and 3) mesonephric cells derived from the rete ovarii system (64). The results of a more recent histological study in sheep favored the idea that Ͼ95% of the granulosa cells in newly formed primordial follicles arise from the ovarian surface epithelium (65). During this process, mesothelial cells on the outside of the ovary are expected to undergo epithelial-to-mesenchymal transition and migrate into the ovarian cortex, where they differentiate to follicular granulosa cells.…”

Estudo de ovários fetais equinos: uma abordagem histológica1

“…A diminuição gradativa dos cordões ovígeros parece ser uma caracterís-tica conservada ao longo do processo evolutivo e tem sido relatada em outras espécies de mamíferos, como por exemplo, em macacas (Fouquet & Dang 1980), ovelhas (Sawyer et al 2002), bovinos (Kenngott et al 2013) e seres humanos (Konishi et al 1986), bem como na espécie de codorna Coturnix coturnix japonica (Madekurozwa 2012).…”

“…Segundo estudos realizados por Sawyer (2002), ovários fetais de ovelhas também continham cordões ovígeros contendo células primordiais. Visto que o aumento em número e tamanho dos cordões ovígeros no córtex e a visualização da Hay & Allen (1975) relataram hipertrofia e hiperplasia ovariana e correlacionaram estes fatos às células intersticiais localizadas na medula do órgão.…”