HYPOPHYSEAL MORPHOLOGY 



223 



cell types differ in size and in the density of 

 their content. The specific differences in the 

 chemical nature of the contents of secretory 

 granules in different cells cannot yet be 

 made apparent by electron microscopy. The 

 electron microscope emphasizes the essen- 

 tial similarity of the specific granules of 

 acidophil and basophil cells and indeed of 

 secretory granules generally whether in 

 endocrine or exocrine cells. 



Lipid droplets are seen in electron micro- 

 graphs as dense bodies, their density being 

 no doubt the result of precipitation of os- 

 mium by the reducing action of the lipid. 

 They are irregular in outline, probably as a 

 result of solution of the lipid and distortion 

 during dehydration and embedding. The 

 lipid droplets do not have an enclosing 

 membrane. 



Farquhar and Rinehart have identified 

 in electron micrographs the five granulated 

 cell types which can be distinguished by 

 light microscopy in the pars anterior of the 

 rat. All five contain the same cytoplasmic 

 structures; there is no distinctive structural 

 element peculiar to any of the types. There 

 are differences in the shape of the cells, in 

 the extent and form of the endoplasmic 

 reticulum, and in the size, density, and dis- 

 tribution of the secretory granules which 

 enable the different types to be distin- 

 guished from one another. 



The secretory granules of the basophil 

 cells are less than 200 mfx in size. Although 

 such granules can be seen individually under 

 good optical conditions with the light 

 microscope, the coarsely granular or floc- 

 culent appearance of the cytoplasm of baso- 

 phil cells in stained sections is produced by 

 the uneven distribution of the fine granules 

 which are not individually resolved. The 

 "basophil granules" of the basophil cells 

 of the rat pars anterior as seen under the 

 light microscope therefore bear the same 

 relationship to the secretory granules as 

 the Nissl granules of nerve cells bear to the 

 Palade granules ; they are in fact clusters of 

 granules made visible by the staining of 

 their specific content. 



E. MICROVILLI 



In addition to the granulated cells the 

 pars anterior of the rat contains cells with 



a distinctive structural feature, namely, 

 microvilli projecting from the free surface 

 of the cell into a space which is enclosed 

 by contiguous cells of the same type. These 

 cells are not accessible to study by light 

 microscopy and their appearance is so dif- 

 ferent from that of known endocrine cells 

 that their function must be admitted to be 

 problematical. 



XV. The Neurohypophysis and Neuro- 

 hypophyseal Secretion 



A. NEUROSECRETORY PHENOMENA IN THE 

 HYPOTHALAMUS AND NEUROHYPOPHYSIS 



Neurohypophyseal tissue contains con- 

 nective tissue, nonmedullated axons, and 

 interspersed neuroglial cells. In the neural 

 eminence and neural stalk the axons run 

 more or less parallel to one another and, 

 although some of the axons terminate in 

 these regions, most of them pass into the 

 neural lobe where they end in the pars ner- 

 vosa. The terminal enlargement which forms 

 the neural lobe results in part from the 

 branching of the axons in the terminal part 

 of their course, in part from the increased 

 number of glial cells in this region of the 

 neurohypophysis. 



In fish and amphibia the axons in the 

 neurohypophysis arise from cells in the pre- 

 optic nucleus in the hypothalamus and form 

 the preopticohypophyseal tract in the intra- 

 hypothalamic part of their course. In 

 reptiles, birds, and mammals the homologue 

 of the preoptic nucleus is separated into 

 two parts, the supraoptic nucleus and the 

 paraventricular nucleus. The axons of the 

 neurons in the paraventricular nucleus pass 

 towards the supraoptic nucleus and after 

 running through or closely alongside this 

 structure join the supraopticohypophyseal 

 tract (Laqueur in discussion, Scharrer and 

 Scharrer, 1954) . Lesions which are produced 

 experimentally to destroy the supraoptic 

 nuclei or interrupt the supraopticohypo- 

 physeal tract also interrupt the paraven- 

 triculohypophyseal fibers and produce a 

 total neurohypophyseal deficiency. It there- 

 fore appeared at one time that neurohy- 

 pophyseal activity in mammals was en- 

 tirely dependent on the supraoptic nuclei 

 alone, but it is now clear that the paraven- 



