CENTRAL CONTROL OF PITUITARY SECRETION 



1009 



pituitary gland. Tiiese authors divide the pituitary 

 into the neurohypophysis ('posterior lobe') and 

 adenohypophysis ("anterior lobe"). The neurohy- 

 pophysis is subdivided into the three parts of the 

 gland — the median eminence of the tuber cinereum, 

 the infundibular stem and the infundibular process 

 or neural lobe. The adenohypophysis, derived 

 embryologically from Rathke's pouch, is subdivided 

 into the pars distalis, pars tuberalis and pars inter- 

 media. The neural stalk, together with its sheath of 

 portions of the pars glandularis, is designated the 

 hypophysial stalk. 



The main secreting portion of the adenohypophysis 

 is the pars distalis and much interest has centered 

 around the anatomical pathway by which the 

 central nervous system influences this part of the 

 gland, and thereby affects also gonadal, thyroidal 

 and adrenocortical function. It has been argued at 

 different times that the pars distalis is controlled by 

 the hypothalamus a) by an inner\ation passing 

 through the peripheral autonomic nervous system 

 (svmpathetic fibers from the cervical .sympathetic 

 trunk and parasympathetic fibers from the facial 

 nerve), h) by a direct innervation passing through 

 the stalk of the gland, c) by a systemic blood supply 

 and d) by the hypophysial portal blood supply. In 

 view of the embryological origin of the pars distalis 

 from buccal epithelium, and its migration to its 

 final and constant attachment in all vertebrates to 

 the floor of the third ventricle, it would seem teleo- 

 logically reasonable to suppose that the influence of 

 the hypothalamus was exerted by a direct pathway 

 to the gland. 



The first account of any nerves passing to the 

 pituitary gland was a brief mention of a sympathetic 

 supply by Bougery (39) in 1845. Many workers in 

 later years published findings in agreeinent (for 

 literature see 152), but the comprehensive and more 

 recent studies of Rasmussen (274) and Green (124, 

 125) fail to lend support to the view that the pars 

 distalis receives a substantial nerve supply of any 

 type. Rasmussen studied the nerve supply to the 

 anterior pituitary in many laboratory animals and 

 found a few sympathetic fibers entering the pars 

 distalis. Since large areas of the gland were found 

 to i)e free of nerve fibers, the conclusion was drawn 

 that the fibers present were vasomotor in nature. 

 Green (125), in a detailed study of the pituitary 

 gland, in many vertebrates ranging froin cyclostomes 

 to iTian, concluded, "In none of the animals studied 

 has an innervation of the pars distalis been found." 

 One of the difficulties in evaluating earlv work is 



due to the capricious nature of silver staining tech- 

 niques. Such staining methods are likely to iin- 

 pregnate reticular connective tissue fibers as well as 

 nerve fibers (154). Therefore, when such stains have 

 been used (241, 337), accounts of a rich innervation 

 to the pars distalis must be viewed with caution 

 unless rigid control procedures have been applied. 

 Wingstrand (357), investigating the avian pituitary, 

 concludes, "In perfectly impregnated slides in which 

 the reticular fibers are unstained the adenohypophysis 

 contains no or very few nerve fibers." The experi- 

 mental data regarding the importance of a cervical 

 sympathetic innervation of the pars distalis are clear 

 in its implications. The fact that complete s\mpa- 

 thectomy does not prevent normal reproduction in 

 female cats (52) and does not cause any very signifi- 

 cant change in the metabolic rate of cats (52 ) or rats 

 (208) demonstrates that a sympathetic innervation 

 of the pituitary plays no appreciable part in the 

 control of the secretion of gonadotrophic or thyro- 

 trophic hormones. Also pseudopregnancy still follows 

 sterile coitus in the partially sympathectomizcd rat 

 (112, 340), and ovulation still follows sterile coitus in 

 the partially or completely sympathectomized rabbit 

 (42, 168). 



A parasympathetic inner\ation of the pars distalis 

 was suggested as a possibility by Hinsey & Markee 

 (180). However, again the experimental data argue 

 against such a supply since it has been found that 

 removal of the sphenopalatine ganglia does not 

 result in any reproductive abnorinality in the rat 

 (313), and that the normal reflex release of gonado- 

 trophin still follows coitus in the rabbit after bilateral 

 avulsion of the facial nerve (143) and after destruction 

 of the petrosal nerves at the geniculate ganglia (341). 



The hypothalamus sends a rich bundle of nerve 

 fibers through the infundibular stem to end in soine 

 part of the neurohypophysis. It is probable that a few 

 nerve fibers pass into the pars intermedia and pars 

 tul^eralis, but it is doubtful if any penetrate into the 

 pars distalis. Detailed references to the literature on 

 this subject have been given (125, 152, 156, 274). 

 Reports in the older accounts of a rich hypothalamic 

 innervation of the pars distalis are most likely to be 

 explained on silver impregnation of reticular con- 

 nective tissue fibers. Recent findings with the electron 

 microscope support this view. Electronmicroscopy 

 clearly differentiates between reticular fibers and 

 nerve fibers, and two independent groups (Palay, 

 personal communication; Farquhar, M. G. & J. F. 

 Rinehart, personal communication) have failed to 

 find any nerve fibers in the pais distalis with the use 



