



The Endocrine Glands 



The endocrine glands of vertebrates are several in number, 

 and of these the pituitary is perhaps the most complex and 

 important in function. 



\ 

 THE PITUITARY 



The pituitary of the mammal arises as a product of an 

 invaginated pouch from the stomodeum, Rathke's pouch, 

 which meets and fuses with a pouch, the infundibulum, 

 growing down from the diencephalon. These two compo- 

 nents give rise to the adenohypophysis and neurohypo- 

 physis, respectively. As development proceeds, Rathke's 

 pouch loses connection with the stomodeum and becomes 

 fused to the neurohypophysis (the pars nervosa). The adult 

 pituitary has several types of tissues in it, each identified by 

 histological features. 



The anterior lobe is differentiated into pars anterior, in- 

 termedia, and tuberalis. The anterior and tuberalis parts 

 represent the front half of the adenohypophyseal sac; the 

 intermediate lobe is the posterior half in contact with the 

 pars nervosa. In man the pars tuberalis develops as an out- 

 growth around the stalk connecting the pars nervosa to the 

 brain. 



The pituitary is generally considered the master gland of 

 the body, extremely important in the coordination of inter- 

 nal affairs. In the pituitary there is an interesting combina- 

 tion of nerve tissue and glandular tissue. In the supraoptic 

 and paraventricular nuclei of the brain there are neurosec- 

 retory cells, whose axons pass down into the neurohypo- 

 physis. Secretory granules form in the cell body and pass 

 down along the axons. It is assumed that capillary clusters 

 pick up the secretions from the axon endings. The capil- 

 laries unite to form venules which pass into the pars ante- 

 rior where they break up into a plexus of sinusoids. The 

 transport of these secretions thus involves a portal system. 

 The secretions are important in determining events in the 

 adenohypophysis, whose secretions regulate many events. 

 This relationship between the central nervous system and 

 the endocrine system makes possible responses to various 

 environmental conditions. 



Within the other classes of vertebrates, essentially the 

 same story of pituitary development is encountered. The 

 adult gland is very similar to that of the mammal. In the 

 reptiles or amphibians, a pars tuberalis as such is not 

 identifiable. It can be assumed to be included in the ante- 

 rior part. In the fishes there is an extreme anterior part of 

 unknown endocrine function which may represent the pars 

 tuberalis. The middle portion, in terms of its endocrine 

 product, appears to be the homolog of the pars anterior; be- 

 hind this is a pars intermedia which is closely associated 

 with the tissue of the pars nervosa. 



Comparative study of the endocrines produced has been 

 responsible for the indentifications of these areas which his- 

 tologically do not present a uniform picture. Among the 

 fishes the shark is the most aberrant, in that the adenohy- 

 pophysis remains a vesicle with or without differentiation. 

 The sharks, actinopterygians, and Lattmerm also have a saccus 

 vasculosa associated with, or as an outgrowth of, the pars 

 nervosa. It has been suggested that this is a sensory organ 

 functioning in depth perception; it is more likely of endo- 

 crine function. A saccus vasculosa is present in the lamprey 

 as well as a pituitary formed of pars intermedia and pars 

 anterior (plus tuberalis?). In Myxine there is only a pars 

 intermedia and the saccus is rudimentary. 



The development of the pituitary in the fishes appears to 

 be the same as that in the higher forms; a Rathke's pouch 

 and infundibular pocket rise to the definitive gland. The 

 exception to this picture is observed in the lamprey (Figure 

 12-1). Here the nasohypophyseal pouch extends back un- 

 derneath the brain to contact the only slightly evaginated 

 infundibulum. This posterior pouch forms a dense mass of 

 tissue, without an internal lumen, connected by a strand of 

 tissue with the nasal pouch. At the time of metamorphosis a 

 new sac-like outgrowth from the nasal pouch extends poste- 

 riorly below the adenohypophysis to give rise to the adult 

 "hypophyseal" sac. The subhypophyseal sac is thus a sec- 

 ondary structure, not a part of the original Rathke's pouch. 



THE THYROID 



The thyroid develops in the lamprey from the subpharyn- 

 geal gland, which seems to be the homolog of the endostyle 



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