CENTRAL CONTROL OF PITUITARY SECRETION 



lOI I 



been published (see tig. 2). The general nature ol' 

 the pituitary vascularization is shown in figure i . 

 The mammalian system consists of small arterial 

 twigs from the internal carotid and posterior com- 

 municating arteries which supply a plexus lying 

 between the pars tuberalis and median eminence; 

 from this plexus capillaries or sinusoids of \'arying 

 shapes and sizes, often in the form of loops, penetrate 

 into the nervous tissue of the median eminence (the 

 primary plexus of the portal vessels) and then drain 

 by the large trunks of the portal vessels which run 

 down the pituitary stalk into the sinusoids of the pars 

 distalis. The phylogenetic constancy of these vessels 

 has been commented upon Ijy Green (125) who 

 examined the pituitary gland in 75 species of verte- 

 brates: "It is a remarkable fact that the hypophvsio- 

 portal circulation shows such minor variations 

 between related species and that the variations 

 described can be followed with such ease in so orderly 

 a manner in a phylogenetic series. Such constancy 

 suggests a functional significance. Were it not so, 

 wide variations might be expected to occur, since 

 the general morphology of the pituitary itself is any- 

 thing but constant." Since the blood in the portal 

 vessels has been seen to flow from the median 

 eminence of the tuber cinereum to the pars distalis 

 of the pituitary in amphibia, mice, rats, cats and 

 dogs (123, 127, 183, 331, 363), the view has been put 

 forward that the hypothalamus may exert its influ- 

 ence over anterior pituitary function through some 

 humoral effect mediated by these vessels. The idea 

 of a humoral control of the adenohypophysis by the 

 hypothalamus or neurohypophysis is not new (see 

 156, p. 164) but has gained force in the last few years 

 in the light of the above anatomical data and recent 

 experimental findings. There are clearly many 

 possibilities as to how such a mechanism might 

 function, but the most likely seems to be that nerve 

 fibers from the hypothalamus liberate some humoral 

 substance or substances into the capillaries of the 

 primary plexus in the median eminence and that 

 this substance is carried by the portal vessels to excite 

 or inhibit the cells of the pars distalis. 



Hypophysial Portal J'essels and Anterior Pituitary Activity 



The data that the hypophysial portal vessels are 

 the anatomical structures by which the hypothalamus 

 controls anterior pituitary secretion may be .summa- 

 rized as follows. 



a) The hypothalamus exerts an important regu- 

 latory effect on adenohypophysial function but the 



gland lacks a nerve supply. Hinsey & Markee (180), 

 Friedgood (iii), Harris (148) and Brooks (43) all 

 suggested, over 20 years ago, that the hypothalamus 

 or neurohypophysis might affect the release of lutein- 

 izing hormone from the anterior pituitary by humoral 

 means. 



b) The finding that the hypophysial portal vessels 

 formed a constant anatomical link connecting the 

 median eminence of the tuber cinereum with the pars 

 distalis, and that the blood flow in these vessels was 

 from the tuber cinereum to the pituitary, led to the 

 suggestion that any humoral control transmitted to 

 the gland down the pituitary stalk was exerted 

 through these vessels. 



c) Electrical stimulation of various regions of the 

 hypothalamus excites gonadotrophic (148, 151, 164, 

 218), aclrenocorticotrophic (72, 188, 271) and thyro- 

 trophic (56, 164) secretion, although similar stimu- 

 lation of the pituitary gland itself is without effect 

 (72, 151, 164, 218). These data are compatible with 

 the view that the discharge of these hormones is 

 controlled by a neurohumoral mechanism involving 

 hypothalamic neurones (excitable by electrical 

 stimulation) and a humoral mechanism in the pitui- 

 tary stalk and gland (not excitable by electric stimu- 

 lation). 



d) The discordant findings of different workers 

 regarding anterior lobe function after pituitary 

 stalk section may i)e explained by varving degrees 

 of regeneration of the portal ves.sels. Harris (153) 

 first observed such regeneration in the rat and noted 

 that it was correlated with the return of reproductive 

 activity. Regeneration of the portal vessels has now 

 been observed in mice (71), rats (153, 160), rabbits 

 (107, i8g), ferrets (81, 329) and monkeys (162), and 

 has been correlated with return of anterior pituitary 

 activity in mice (71), rabbits (107) and ferrets (81). 



e) The pituitary gland transplanted to a site in the 

 body remote from the sella turcica shows at most only 

 fragments of normal activity. However, if the glandu- 

 lar tissue is transplanted in a hypophysectomized 

 donor to the sella turcica or to a site in the sub- 

 arachnoid space below the median eminence (128, 

 161), then the transplanted tissue may become 

 revascularized by the portal system and apparently 

 normal function returns (161). 



The above data show that the hypophysial portal 

 vessels are in some way specifically related to the 

 normal maintenance and regulation of adenohy- 

 pophysial function. The mechanism by which these 

 vessels exert a controlling influence over the anterior 

 pituitary is not certain. The most likely theory is that 



