628 



PHYSIOLOGY OF GONADS 



which has confirmed that pituitary tissue 

 grafted under the kidney capsule in rats ap- 

 parently secretes prolactin and will give 

 slight maintenance of milk secretion in hy- 

 pophysectomized animals, this maintenance 

 being considerably enhanced if ACTH or 

 STH is also administered (Cowie, Tindal 

 and Benson, 1960). It would thus seem 

 that the cells of the anterior lobe have 

 the ability when isolated from the hypo- 

 physeal portal system to secrete prolactin, 

 but the experiments cited above allow no 

 conclusions to be drawn regarding the route 

 by which the galactopoietic function of the 

 pituitary is normally controlled. 



Recent reports that bilateral cervical 

 sympathectomy in the lactating goat causes 

 a fall in the milk yield suggest that the ga- 

 lactopoietic functions of the anterior lobe 

 may be influenced by the sympathetic nerv- 

 ous system (Tsakhaev, 1959; Tverskoy, 

 1960) . Declines in milk yield also occur after 

 section of the pituitary stalk in the goat, but 

 it is not clear in such cases whether the ef- 

 fects are due to the interruption of nervous 

 or vascular pathways within the stalk 

 (Tsakhaev, 1959; Tverskoy, 1960). In these 

 studies on stalk section the cut ends of the 

 pituitary stalk were not separated by a plas- 

 tic plate, so some restoration of the hy- 

 l^ophyseal portal system may have occurred. 

 Further experiments on the effects of sec- 

 tion of the pituitary stalk on lactation in 

 which restoration of the hypophyseal portal 

 is prevented by the insertion of a plate are 

 being conducted in our laboratory and also 

 in the Soviet Union. Another possible mode 

 of communication between hypothalamus 

 and anterior pituitary has been investigated 

 by Benson and Folley (1956, 1957a, b) who 

 have suggested that the oxytocin released 

 from the neurohypophysis in response to the 

 suckling stimulus may directly act on the 

 cells of the anterior lobe and stimulate the 

 release of the galactopoietic complex. The 

 careful anatomic researches of Landsmeer 

 (1951), Daniel and Prichard (1956, 1957, 

 1958) and Jewell (1956) have demonstrated 

 in several species the existence of direct 

 vascular connections from the neurohy- 

 lK)physis to the anterior lobe so that the 

 neurohypophyseal hormones liberated into 

 the blood stream would in fact be carried 



direct to the anterior pituitary cells in very 

 high concentrations. Clearly such a concept 

 would provide a simple explanation of how 

 the hormonal integration, coordination, and 

 maintenance of mammary function is 

 achieved. It has already been noted (see 

 page 607) that a connection between milk 

 ejection and the onset of copious lactation 

 has been suggested. There is considerable 

 evidence that oxytocin is liberated during 

 parturition in sufficient quantities to cause 

 contraction of the alveoli and milk ejection 

 (see Harris, 1955; Cross, 1958; Cross, Good- 

 win and Silver, 1958) ; if, therefore, oxytocin 

 can release the lactogenic and galatopoietic 

 complexes from the anterior pituitary, a 

 simple explanation of the mechanism trig- 

 gering off the onset of copious milk secre- 

 tion, before the application of the milking 

 stimulus, is available. 



We must now consider what experimental 

 evidence there is to support this rather at- 

 tractive theory. First, Benson and Folley 

 (1956, 1957a, b) demonstrated that regular 

 injections of oxytocin can retard mammary 

 regression after weaning in a similar fash- 

 ion to injections of prolactin (see page 

 610), and they have shown that the pres- 

 ence of the pituitary is essential for oxytocin 

 to elicit this effect. Synthetic oxytocin 

 proved equally effective, thus discounting 

 the possibility of a contaminant in natural 

 oxytocin being concerned (Fig. 10.20) . These 

 experiments have so far only been carried 

 out in rats, but they strongly suggest that 

 oxytocin can elicit the secretion of prolactin. 

 In agreement with this concept are several 

 observations that regular injections of oxy- 

 tocin have galactopoietic effects in lactating 

 cows and that oxytocin has luteotrophic ef- 

 fects in rats (see review by Benson, Cowie 

 and Tindal, 1958) . There is, moreover, some 

 evidence that the suckling stimulus may 

 cause the release of vasopressin or the anti- 

 diuretic hormone (ADH) from the neuro- 

 hypoi)hysis (see page 621), and it has been 

 shown that ADH or some material closely 

 associated with it may cause the secretion of 

 ACTH from the anterior lobe (see review 

 by Benson, Cowie and Tindal, 1958) ; so 

 there are some grounds for supposing that 

 the hormones of the posterior lobe evoke 

 the secretion of several components of the 



