458 



PHYSIOLOGY OF GONADS 



terrelationsliip betwen pituitary, gonads, 

 and placenta. In the mare, and presumably 

 other species in which multiple ovulations 

 occur early in pregnancy, the involvement 

 of chorionic gonadotrophins, pituitary gona- 

 dotrophins, and estrogen of placental origin 

 has been suggested (Rowlands, 1949). When 

 folliculogenesis is inhibited just before the 

 stage of the preovulatory swelling, as it is 

 in many pregnant animals (see below), the 

 nervous system may be involved. An un- 

 usually significant investigation in which 

 the threshold of stimulation to ovulation in 

 the rabbit was correlated with threshold 

 changes in cerebral activity has recently 

 been completed (Kawakami and Sawyer, 

 1959). It was demonstrated that pregnancy 

 or prolonged treatment with progesterone 

 maintains the electroencephalogram (EEGj 

 after-reaction threshold to low frequency 

 stimulation of hypothalamic or rhinen- 

 cephalic nuclei at an elevated level. At this 

 level, gonadotrophin release does not occur 

 in response to coitus or other ovulatory 

 stimuli. The discovery of this fact has pro- 

 vided a basis for understanding the various 

 ovarian conditions associated with preg- 

 nancy and lactation, or at least those in 

 which follicular development proceeds to 

 the point of preovulatory swelling and then 

 stops. It may be that some other mechanism 

 of inhibition accounts for the more severe 

 retardation of folliculogenesis in si)ecies in 

 which tliis occurs. 



The European hares, Lepus tiniidus L., 

 and L. ciiniculus L., are reported as mating 

 during pregnancy with the occurrence of 

 superfetation (Lienhart, 1940). Pregnancy, 

 therefore, has little or no effect on any 

 stage of folliculogenesis in these species. 

 The domestic rabbit appears to he somewhat 

 more affected and perhaps more variable. 

 Claesson, Hillarj), Hogberg and ll()kfeh 

 (1949) state that the ovaiics of pii'gnant 

 rab})its are composed almost entirely of 

 interstitial gland, except for the corpoi'a lu- 

 tea, but, according to Hannnond and Mar- 

 shall (1925) and Dawson (1946), mature 

 follicles ai'e present and pregnant animals 

 will occasionally mate. However, if we may 

 assume that the reaction of i)regnant ani- 

 mals is similar to that of ])seudopregnant 

 animals (Makepeace, Weinstein and Fried- 

 man, 1938), pituitary gonadoti'opliin is not 



released and ovulation does not occur. From 

 examination of the ovaries and from the 

 fact that fertile matings can occur within a 

 very few hours after parturition (Dempsey, 

 1937; Boling, Blandau, Wilson and Young, 

 1939; Blandau and Soderwall, 1941), it is 

 clear that follicular development in the 

 pregnant guinea pig and rat proceeds to a 

 point just short of the preovulatory swell- 

 ing. According to Nelson (1929) and Swezy 

 and Evans (1930), cycles of oogenesis occur 

 in laboratory rats, and, although the folli- 

 cles may form small corpora lutea (Swezy 

 and Evans), ordinarily they do not rupture. 

 The musk-rat. Ondatra zibethica, and the 

 African bat, Xycteris luteola, must display 

 an advanced follicular development during 

 pregnancy because there is evidence of post- 

 l)artum estrus (Warwick, 1940; Matthews, 

 1941, respectively). Brown and Luther 

 (1951) state that postpartum estrus occurs 

 within 3 days after farrowing in the sow, if 

 the young pigs are removed. We assume, 

 from this latter statement and from the re- 

 l^ort that estrus and service may occur dur- 

 ing pregnancy in this species (Perry and 

 Pomeroy, 1956), that large follicles are 

 present in tlie ovaries of the pregnant sow. 



Heat ])eriods in the jjregnant ewe are asso- 

 ciated with follicular growth, but ovulation 

 does not occur, and late in pregnancy fol- 

 licle size decreases significantly (Williams, 

 Garrigus, Norton and Nalbandov, 1956 ) . 

 The first heat after {parturition was an aver- 

 age of 23.9 days later, range 1 to 61 days. 

 According to Harrison (1948b), widespread 

 atretic changes can be seen in all the folli- 

 cles in the goat, beginning the 40th day of 

 |)regnancy. By the (iOth day. no healthy fol- 

 licles can be found. 



Hammond (1927) was of the opinion that 

 during jircgnancy in the cow, follicles de- 

 velop to the size at which the jireovulatory 

 swelling begins, but Dukes (1943), citing a 

 study by Weber, wrote that cows come into 

 heat 3 to 7 weeks after parturition. Support 

 for tliis \-iew comes from the report l)y Hafez 

 ( 1954) that the average interval to the post- 

 partum esti'us in another bovine, the Egyp- 

 tian buffalo, is 43.8 days, range 16 to 76 

 days. The iclatixcly long postpartum inter- 

 \al in these two species is presumptive evi- 

 dence that follicles are relatively small at 

 the end of pregnancy in bovines. 



