REPRODUCTIVE ENDOCRINOLOGY IN BIRDS 



1135 



as light, could influence the activity of the 

 pituitary is by way of the nervous system. 

 Considerable evidence from different kinds 

 of experiments implicates the hypothalamus 

 as the structure that is specifically involved 

 in the transmission of the stimuli to the 

 anterior pituitary. The hypothesis has been 

 proposed (Scharrer and Scharrer, 1954; 

 Benoit and Assenmacher, 1955, 1959; Assen- 

 macher and Benoit, 1958) that environ- 

 mental factors cause changes in the activity 

 01 specialized hypothalamic cells, the so- 

 called neurosecretory cells. These cells can 

 be identified by a variety of stains (Assen- 

 macher, 1958; Legait, 1959). They may be 

 considered, on one hand, as nerve cells, on 

 the other, as endocrine cells. Material pro- 

 duced by these cells is transported along 

 their axons to the posterior pituitary. How- 

 ever, loops of these axons in the median 

 eminence come into close contact with the 

 capillary bed of the portal vessels of the 

 hypothalamus where some of the neuro- 

 secretory material (NSM) is picked up by 

 portal vessels which transport it to the 

 anterior pituitary whose cells it stimulates. 

 The evidence in support of this hypothesis 

 will be presented together with the counter 

 argument by Zuckerman (1955), who ques- 

 tioned the validity of this hypothesis. For 

 convenience, the available evidence will be 

 divided into somewhat arbitrary categories. 

 Inasmuch as many details of findings have 

 been published recently in the review papers 

 and chapters of this book cited above, refer- 

 ences will be limited largely to these re- 

 views. Discussion will be limited to the re- 

 sults obtained with birds. 



Anatomic evidence shows that few, if 

 any, nerves reach the glandular tissue of the 

 anterior pituitary. Even the few fibers found 

 l)y Metuzals (1955) do not seem to have 

 significance, because their origin could not 

 be established. On the other hand, NSAI has 

 been observed in the hypothalamico-hy- 

 l)oi)hyseal tract of ducks (Assenmacher, 

 1958; Legait, 1959; Benoit and Assen- 

 macher, 19591, chickens, although rarely 

 ( Legait, 1959) , and the white-crowned spar- 

 row, Zonotrichia leucophrys gamhellii (Oks- 

 che, Laws, Kamemoto and Farner, 1959). 

 The axons of these neurosecretory cells form 

 ''loops" which are in close contact with the 

 portal vessels in the strafion (lUinduUire of 



the "special zone" of the median eminence 

 (Assenmacher, 1958; Oksche, Laws, Kame- 

 moto and Farner, 1959) . The median emi- 

 nence can be divided into three layers: (a) 

 stratum ependymale, (b) stratum fibrosuni, 

 (c) stratum glandulare. The tracts from the 

 hypothalamus to the neurohypophysis are 

 part of the stratum ftbrosimi, and NSM can 

 l3e found here, often in such amounts that 

 the individual fibers of the tracts can be dis- 

 tinguished because of the content of NSM 

 (Oksche, Laws, Kamemoto and Farner, 

 1959). The stratum gkindidare also con- 

 tains large amounts of NSM arranged in 

 arcades (Wingstrand, 1951; Legait, 1959). 

 In this area the hypophyseal portal vessels 

 make contact with the loops of NSM. The 

 demonstration that the blood flow is frojn the 

 median eminence to the anterior pituitary 

 in ducks (Assenmacher, 1958) , together with 

 all the other anatomic evidence cited above, 

 is certainly in accord with the hypothesis 

 that the neurosecretory material from the 

 hypothalamus is the link between the ner- 

 vous system and the anterior pituitary. 

 Zuckerman (1955) has stressed that nerve 

 fibers such as those found by Metuzals 

 (1955) may form the functional connection 

 between the hypothalamus and the pitui- 

 tary. However, it seems to this author that 

 Zuckerman's argument cannot be accepted 

 for birds until it has been established that 

 the fibers come from the hypothalamus. 



Further evidence is provided by interrup- 

 tions of the connections between hypothala- 

 mus and anterior pituitary. 



1. Lesions in a medial region of the ven- 

 tral portion of the paraventricular nucleus 

 in chickens caused a long lasting interrup- 

 tion of ovulation (Ralph, 1959). Lesions in 

 the same area also prevented progesterone- 

 induced ovulations (Ralph and Fraps, 

 1959). In other parts of the hypothalamus 

 lesions did not consistently interrupt either 

 "spontaneous" or progesterone-induced ovu- 

 lations, and when they did, the interruption 

 of "spontaneous" ovulations was temporary 

 rather than long lasting. In drakes {Anas 

 platijrhynchos) , fairly large lesions of the 

 anterior hypothalamus prevented the noi-- 

 mal light-induced increase in testicular ac- 

 tivity (Assenmacher, 1958). 



2. Complete interruption of the hypo- 

 l)hyseal portal system of laying hens re- 



