MAMMALIAN OVARY 



459 



Between the 40th and 150th day of preg- 

 nancy in the mare the ovaries contain nu- 

 merous actively growing follicles and several 

 functional corpora lutea (Cole, Howell and 

 Hart, 1931; Rowlands, 1949). However, 

 from the 150th day until the late stages, 

 there is a regression of all the corpora lutea 

 and an absence of large follicles. In the late 

 stages only minute vestiges of corpora lutea 

 and small follicles remain. If the latter is 

 true, follicular growth must be rapid after 

 parturition, because the first heat following 

 foaling was between the 7th and 10th days 

 in 77 per cent of the many mares Trum 

 (1950) studied. In the African elephant, 

 Loxodonta africana, there is also a replace- 

 ment of the corpora lutea (one plus several 

 accessory corpora lutea j about midway 

 through pregnancy (Perry, 1953). Some are 

 formed following ovulation and some not. 

 They persist until term when they involute 

 rapidly. During the late stages of pregnancy 

 no follicles with antra are founcl. Dawson 

 ( 1946) wrote that the domestic cat does not 

 possess mature follicles at the time of par- 

 turition. In nonlactating animals the pro- 

 estrous level is reached the 4th week after 

 parturition. 



Presumptive evidence exists that the folli- 

 cles in the parturitive chimpanzee are small 

 (Young and Yerkes, 1943 ) . In the human fe- 

 male the appearance of the first ovulatory 

 cycle after pregnancy is irregular (Sharman, 

 1950, 1951 ; AlcKeown, Gibson and Dougray, 

 1954). According to Sharman, it may occur 

 about 6 weeks after delivery in nonlactating 

 women. This suggests that follicles are small 

 at the end of ju-egnancy in the human fe- 

 male. 



Inhibitory effects of lactation on follicular 

 development are indicated by the substance 

 of many of the reports cited above (Dawson; 

 Dukes; Perry; Schwartz; Sharman; Wil- 

 liams, Garrigus, Norton and Nalbandov) 

 and by much other information. As would 

 be expected, the intra- and interspecies vari- 

 ations are great. Studies in progress at Iowa 

 (Bradbury, personal communication) are re- 

 vealing that some women experience an 

 atrophy of the vaginal epithelium during the 

 second and third month of lactation. The 

 atrophy is indicative of a lack of ovarian es- 

 trogen and suggests that follicular develop- 

 ment is not normal. Observations that are 



similarly suggestive have been made in other 

 species. The absence of estrogen in signifi- 

 cant ciuantities during lactation in the mouse 

 (Atkinson and Leathem, 1946) and guinea 

 pig (Rowlands, 1956) is believed to be the 

 reflection of a delay in the resumption of 

 follicular growth and ovulation. Mother rats 

 and mice may copulate and conceive within 

 24 hours after delivering a litter of young. 

 AVhile the mother is nursing the newborn lit- 

 ter, the fertilized eggs of the new pregnancy 

 develop into blastocysts, but these blasto- 

 cysts fail to implant in the uterus at the 

 usual time (Talmadge, Buchanan, Kraintz, 

 Lazo-Wasem and Zarrow, 1954; Whitten, 

 1955; Cochrane and Meyer, 1957). This de- 

 lay in implantation is apparently due to a 

 lack of estrogen, because an injection of es- 

 trogen will result in implantation of the 

 blastocysts. The suppression of estrous cy- 

 cles during lactation in the mouse and rat is 

 influenced in part by the size of the litter. A 

 litter of 8 to 10 young will inhibit cycles, 

 whereas cycles are displayed if the litter is 

 reduced to 2 or 3 young (Parkes, 1926a; 

 Hain, 1935). The cottontail rabbit [Sijlvila- 

 gus floridamis) seems to be a species in 

 which ovarian follicular development is lit- 

 tle if any affected by lactation, for Schwartz 

 ( 1942 ) stated that suckling does not prevent 

 ovulation after coitus, at least in the early 

 stages of lactation. 



III. Corpus Liiteuni 



The formation of the corpus luteum has 

 been described for many species ( see reviews 

 in Corner, 1945; Harrison, 1948a; Brambell, 

 1956). In general, after rupture of the folli- 

 cle and discharge of the ovum, the granu- 

 losa is invaded by blood vessels from the 

 theca interna (Bassett, 1943). They form a 

 rich network among the enlarging granulosa 

 lutein cells. The extent and nature of the 

 contribution from the theca interna varies 

 from species to species, but, as Corner states, 

 the origin of the major part of the epithe- 

 lioid cells of the corpus luteum from the 

 granulosa may now be considered a fact. 



Whereas there may be a fairly uniform 

 pattern of development and control of ovar- 

 ian follicles in mammalian forms, there are 

 diverse mechanisms for the formation and 

 maintenance of corpoi'a lutea; consequently 



