MAMMALIAN OVARY 



463 



Fig. 7.6. Intact immature rat given progesterone for 3 days. The various-sized follicles in 

 tliis area are in early stages of atresia. The pycnotic granulosa cells are dispersing into the 

 follicular fluid and the oocyte in the small follicle at the upper left is denuded. (Courtesv of 

 Dr. J. T.Bradburv.) 



tive clianges are evident in the oocyte. When 

 vesicular follicles become atretic the granu- 

 losa disintegrates and the cells disperse into 

 the liquor folliculi (Knigge and Leathern, 

 19.56 ( . If the follicle has developed a distinct 

 theca interna, the theca regresses after the 

 granulosa has disintegrated. In rats the 

 atretic follicles leave no recognizable histo- 

 logic remnants. In some species the theca re- 

 gresses back to ovarian interstitial tissue 

 (Dawson and AlcCabe, 1951; Williams, 

 1956). In the ovary of the human female 

 and rhesus monkey the atretic follicle leaves 

 a scar (corpus atreticum) in which the mem- 

 l)rana propria persists for months as a 

 folded hyaline membrane within the loose 

 fibrous remnant of the theca. 



The cause of follicular atresia is not 

 known. Immediately after hypophysectomy 

 there is a wave of atresia in the ovary of the 

 immature rat (Fig. 7.5) and rabbit (Foster, 

 Foster and Hisaw, 1937). In adult rats the 

 postovulatory wave of follicular atresia has 



l)een attrilnited to an action of the corjiora 

 lutea (Atkinson and Leathem, 1946). Injec- 

 tions of androgen or of progesterone increase 

 the incidence of atr(>sia in rat ovaries (Fig. 

 7.6) (Paesi, 1949b; Barraclough, 1955; 

 Payne, Hellbaum and Owens, 1956). Fur- 

 ther study is necessary to determine whether 

 the atresia is due to a direct effect of andro- 

 gen or progesterone on the follicles, or 

 whether the postovulatory decline in gon- 

 adotrophins, like hypophysectomy, with- 

 draws a supi:)orting influence and permits 

 the follicles to degenerate. The supporting 

 influence may be estrogenic because in- 

 jections of estrogen at the time of hypo- 

 physectomy will prevent, or at least delay, 

 the expected follicular atresia (Fig. 7.7). 

 Some months after hypophysectomy the 

 number of remaining oocytes is greater 

 than in the ovaries of normal litter-mate 

 sisters (Ingram, 1953). This suggests that 

 vegetating oocytes are less liable to undergo 



