Intraovarian regulation of folliculogenesis in the dog: A review

Abstract

Within the ovary, there is a vast reserve of intraovarian follicles. During reproductive life, poorly understood intraovarian mechanisms activate small numbers of dormant primordial follicles that subsequently develop into primary, secondary and antral stages (Gougeon, 2010). Folliculogenesis can be divided into four stages: activation of primordial follicles, growth of early stage follicles, selection of dominant follicle(s) and maturation of pre-ovulatory follicles (Gougeon, 2010) (Figure 1). The first two stages (activation and growth) are regulated by intraovarian factors (Hsueh, Kawamura, Cheng, & Fauser, 2015; McGee & Hsueh, 2000) whereas the final two stages are gonadotrophin-dependent. Most conventional wisdom about mechanisms regulating mammalian folliculogenesis has been derived from extensive studies in rodents (Eppig, 2001; Richard & Pangas, 2010) and some from larger species (Fortune, Cushman, Wahl, & Kito, 2000; van den Hurk & Zhao, 2005). This review focuses on endocrine, paracrine and autocrine controls of folliculogenesis in the domestic dog. Because there is limited information about mechanisms regulating dog folliculogenesis, knowledge gained from mouse and larger mammal studies also will be discussed when it is relevant. 2 | OVARIAN FOLLI CULOG ENE S IS IN THE DOG Mammalian oocytes arise from primordial germ cells (PGCs) developed during embryogenesis (Edson, Nagaraja, & Matzuk, 2009). Once formed, the PGCs proliferate and migrate to the undifferentiated or bipotential gonad at the genital ridge (Edson et al., 2009). Upon reaching the undifferentiated gonad, PGCs continue to rapidly proliferate with incomplete cytokinesis to form clusters of germ cell nest consisting of oogonia connected to each other by intercellular bridges, and the syncytia units are surrounded by pre-granulosa and stromal mesenchymal cells of the ovary (Tingen, Kim, & Woodruff, 2009). Shortly after germ cell proliferation commences, differentiation of the bipotential gonad takes place (Wear, McPike, & Watanabe, 2016). In the dog, ovarian differentiation occurs at Day 36 of gestation (Andersen & Good, 1970; Meyers-Wallen et al.,

Morphometric changes on the endometrium during diestrus stages in bitches with and without histological endometritis

“…The luteal phase lasts approximately the same time in non-pregnant and pregnant females, and up to now, a luteolytic mechanism is not known in the non-pregnant bitch (Concannon 2011;Songsasen 2020). Therefore, knowing that diestrus is prolonged in the bitch, it could be interesting to study the endometrial changes in the different diestrus stages.…”