An autoradiographic study of long-term follicular development in the cyclic hamster

Follicular development in European ground squirrels (Spermophilus citellus) in different phases of the annual cycle

In seasonally breeding mammals, in particular hibernators, reproduction underlies severe energetic and temporal constraints to enable the allocation of sufficient body fat reserves before winter. Thus, the timing of conception in spring can be crucial in terms of reproductive success. This study investigates follicular development in European ground squirrels (Spermophilus citellus) in three phases of the annual cycle: at vernal emergence, after weaning the offspring and shortly before hibernation. The animals were kept in outdoor enclosures within the natural habitat of the species. They were captured in weekly intervals, weighed and reproductive status was determined. Unilateral ovariectomy was scheduled such that the three periods were sampled. Numbers and diameters of tertiary follicles (TF) and corpora lutea (CL) in each ovary were determined, and plasma oestradiol and progesterone levels were analysed. The highest numbers of TF, including Graafian follicles, were found in ovaries at vernal emergence. During post-lactation, the number of TF was lower and active CL appeared in the investigated ovaries. Shortly before hibernation, active CL were present, but luteolysis had started in some individuals. Both oestradiol and progesterone secretion peaked after the termination of lactation and decreased before hibernation. The results demonstrate a second oestrus cycle in European ground squirrels after weaning, including an active luteal phase. This non-reproductive oestrus cycle with its endocrine output is an intriguing phenomenon. It may positively affect both prehibernatory fattening and reproduction in the subsequent season.

Regulation of ovarian follicular development: a review of microscopic studies

Follicular development in the mammalian ovary is a complex process regulated by an orchestrated action of the pituitary gonadotropins, e.g., follicle stimulating hormone (FSH) and luteinizing hormone (LH), and local ovarian factors, such as peptide growth factors and steroids. The mechanism of endocrine/paracrine/autocrine regulation of folliculogenesis (i.e., cell proliferation and functions) has been addressed largely by biochemical means. However, the availability of immunological and molecular tools now enables us to undertake critical microscopic studies revealing the ovarian cell-type specific synthesis and/or accumulation of many of these local peptide modulators, their roles in the proliferation and differentiation of follicular cells, and their regulation by gonadotropins and local factors. The objective of this review is to provide a comprehensive yet complete picture of the endocrine/autocrine regulation of mammalian folliculogenesis as revealed by microscopic studies. Efforts have been made to include adequate research information relevant to update our understanding of the process of follicular development; however, to maintain the brevity, many equally important studies could not be included. This review confirms that FSH and LH are still the primary stimuli for follicular development. However, it is clear that the actions of these hormones at the cell level involve a host of peptide factors which are produced locally by different follicular cell types and which are powerful modulators of gonadotropin actions. The temporal and spatial expression of the genes of these modulators, the synthesis of active factors, their interactions, and the dynamics of their receptors on the follicular cell surface may be the ultimate determinants of cellular events which are crucial to coordinated growth and differentiation of follicular cells leading to folliculogenesis and ovulation.

Deoxyribonucleic acid polymerase-alpha activity in hamster follicles during the estrous cycle: roles of follicle stimulating hormone and luteinizing hormone

In the hamster, DNA polymerase-alpha (Pol-alpha) in follicles at stages 1-4 (1-4 layers granulosa cells and no theca) increased significantly during the proestrous (P) gonadotropin surges, remained high on estrus (E) and then declined to low levels by 09.00 h, proestrus (P). However, Pol-alpha in stages 5-8 (large preantral to small antral stages) remained steady throughout the cycle. Hypophysectomy on metestrus decreased Pol-alpha by 17 h which was reversed by 2.5 micrograms follicle stimulating hormone (FSH) but not by luteinizing hormone (LH) (2.5 micrograms) or human chorionic gonadotropin (hCG) (1 IU). Hypophysectomy on E resulted 72 h later in a fall in Pol-alpha in stages 1-6; FSH (2.5 micrograms) or LH (2 micrograms) restored enzyme activity within 5 h to stages 1-6 and 5-6, respectively. Thus, Pol-alpha in the smallest preantral follicles is induced by FSH; however, for large preantral and antral follicles, steady levels are maintained by tonic FSH and LH resulting in no apparent change in enzyme activity during active DNA synthesis.

A daily rhythm in hCG binding to ovarian follicles of the cyclic hamster

On each day of the estrous cycle hCG binding to follicle increased from 09.00 to 21.00 h; then hCG binding was static until 09.00 h of the next day. FSH binding did not exhibit rhythmicity. This pattern of hCG binding may be related to the pulsing of LH on each cycle day.

Follicular growth and atresia in the mouse

Follicles were classified on the basis of the number of layers of follicle cells, the presence and degree of development of the zone pellucide, and the presence of an antrum. Formation of an antrum in follicles with less than 7 to 8 cell layers and/or presence of necrotic cells was considered indicative of degeneration. When classified in this manner, the data suggest that follicles and their contained oocytes are committed to either normal development or atresia by the time a third layer of granulosa cells is formed.