HYPOPHYSIS AND GONADOTROPHIC HORMONES 



follicular development. Since ovulation de- 

 pends on an acute discharge of LH (dis- 

 cussed elsewhere), the question is raised as 

 to the role of progesterone in this regard. 

 Firstly, it is important to note that pro- 

 gesterone probably is not in itself a re- 

 leasing agent; rather, it appears to synergize 

 with estrogen to facilitate the triggering 

 mechanism. Moreover, although the pos- 

 sibility exists that progesterone acts di- 

 rectly on the hypophyseal cells to release 

 LH, an impressive body of evidence sug- 

 gests that progesterone promotes LH release 

 (indirectly) by acting on the central nerv- 

 ous system — probably the hypothalamus 

 (for review see Markee, Everett and Saw- 

 yer, 1952 and the chapter by Everett) . Un- 

 der other conditions, quite the reverse of 

 being a stimulus for ovulation, progesterone 

 is a potent inhibitor of ovulation. There can 

 be little doubt that the suppression of ovu- 

 lation during the luteal and gravid phases 

 of the reproductive cycle is due to pro- 

 gesterone. Single doses of progesterone pre- 

 vent ovulation in the mated rabbit, and 

 continued high dosage to cyclic rats (Phil- 

 lips, 1937), guinea pigs (M0ller-Christensen 

 and Fonss-Bech, 1940), sheep (Dutt and 

 Casida, 1948) , and cattle (Ulberg, Christian 

 and Casida, 1951) postpones ovulation in- 

 definitely. 



I. NEUROHYPOPHYSEAL INFLUENCES ON 

 GONADOTROPHIN SECRETION 



Recent evidence reviewed by Benson and 

 Cowie (1957) has tended to link the pos- 

 terior lobe of the hypophysis with the mech- 

 anism for release of prolactin. Many years 

 ago Selye (1934) demonstrated that the 

 nervous excitation of suckling served to 

 maintain lactation. A reflex arc was believed 

 to be involved and this has been partially 

 identified by later studies. The stimulus of 

 suckling has been shown to act through the 

 central nervous system (Eayrs and Badde- 

 ley, 1956) and the hypothalamo-neurohypo- 

 phy.seal axis (Andcrsson, 1951a, b) and to 

 effect the release of oxytocin (Cross and 

 Harris, 1950, 1951). The latter causes con- 

 traction of the myoejiithelial components of 

 the alveoli, thereby effecting milk ejection 

 (let-down). A number of workers (see 

 Donker, Koshi and Petersen, 1954) have 

 obtained evidence that tlic l)cneficial effect 



of oxytocin on lactation is more pro- 

 nounced than can be accounted for on the 

 basis of change in intra-alveolar pressure. 

 The question then arose as to wdiether oxy- 

 tocin has an indirect as well as a direct ac- 

 tion on the mammary structure. Benson and 

 Folley (1956, 1957) noted that continued 

 administration of synthetic or commercial 

 oxytocin retards mammary involution in 

 lactating rats following the interruption of 

 suckling. Inasmuch as a similar effect was 

 obtained by injecting prolactin, Benson and 

 Folley have speculated that oxytocin may 

 provide the stimulus for release of prolactin, 

 ergo LTH, from the anterior pituitary. 



Other lines of evidence have also sug- 

 gested a relationship between the posterior 

 l)ituitary and gonadotrophic functions of 

 the anterior lobe. Desclin (1956a, b) and 

 Stutinsky (1957) have showai in rats that 

 injections of oxytocin at the time of estrus 

 produce a pseudopregnancy reaction. The 

 results suggested enhancement of LTH re- 

 lease, but the significance of these findings 

 has been questioned because the response is 

 not specific for oxytocin. Pseudopregnancy 

 has been induced by the administration of a 

 variety of nonspecific substances, such as 

 plant juice extract (Dury and Bradbury, 

 1942) and ovalbumin, etc. (Swingle, Seay, 

 Perlmutt, Collins, Barlow and Fedor, 195i) 

 to female rats at the time of estrus. 



V. Anatomic Features Important to 



Modern Concepts of Pituitary 



Gonadotrophic Function 



In terms of general morphology there is 

 little to add to the classical knowledge 

 concerning the hypophysis of mammals. 

 Present considerations turn largely on the 

 details of the vascular supply and the in- 

 nervation of tlie gland. In this regard the 

 literature has been notably enriched by 

 Green's (1951a, 1952) comprehensive study 

 of 76 species ranging from amphioxus to 

 man and l)y Wingstrand's monograph 

 (1951 ) on the structure and development of 

 the a\ian jiituitary. The notable advances 

 which have been made in the cytologic and 

 eh'ctron niici'oscopic identification of cell 

 tyi)es in the pars distalis are reviewed in 

 the chapter by Purves. 



A distinct regional lobulation of the pars 

 distalis. fii'st described in the eliicken and 



