HYPOPHYSEAL MORPHOLOGY 



167 



the presence of a mixed cell population in- 

 cluding several types of chromophil cells. 



In man, whales, porpoises, armadillo, 

 manatee, elephant, pangolin, beaver, and in 

 the whole class of birds, there is no pars in- 

 termedia (Fig. 3.3). The adenolobe contains 

 throughout a mixed population of cells re- 

 sembling that of the pars anterior. This 

 structure differs from the pars anterior in 

 containing intermedin in addition to the an- 

 terior lobe hormones, and in all probability 

 contains an additional cell type correspond- 

 ing to the pars intermedia cells. This struc- 

 ture I will call pars distalis to indicate that 

 it is not strictly homologous wth the pars 

 anterior as previously defined (Geiling, 

 1935). In these species, therefore, the parts 

 of the hypophysis are two: (1) Pars tuber- 

 alis — as in other species. (2) Pars distalis — 

 the whole adenolobe containing a mixed cell 

 population throughout its extent. 



It may be noted here that, although in 

 mammals the absence of a pars intermedia is 

 correlated with the absence of the hypo- 

 physeal cleft, in lower vertebrates a pars in- 

 termedia may be present despite the absence 

 of the cleft (Green, 1951). 



C. ZONES IN THE PARS ANTERIOR AND 

 PARS DISTALIS 



The different cell types in the pars ante- 

 rior or pars distalis are not uniformly dis- 

 tributed. The inequalities in the distribution 

 of the acidophil cells, although not more 

 marked than those of other cell types, are 

 more conscipuous because of the larger num- 

 ber of these cells and their conspicuous stain- 

 ing. Zones rich in acidophil cells are called 

 acidophil zones, zones poor in acidophil cells 

 are called basophil zones. The zona tuberalis 

 in the pars anterior adjacent to the junction 

 of the latter with the pars tuberalis is a baso- 

 phil zone. 



This zonation, seen in many pituitaries, 

 aids in the distinction between specific cell 

 types and functional variants. When two 

 cell groups with different stainability are 

 shown to have different distributions char- 

 acteristic for each, the groups so differen- 

 tiated may be accepted as types. When there 

 is no difference in the distribution of cells 

 with minor differences in coloration, etc., 

 the possibility remains that these different 



appearances are those of a single cell type 

 in different functional states. 



IV. A General Statement of the Problems 



of the Special Cytology of the 



Hypophysis 



Nine distinct hormones are secreted by 

 the hypophysis. Some simplification of the 

 problem of relating the secretion of each 

 hormone to specific cytologic details results 

 from the fact that 2 of these hormones are 

 secreted from the neurohypophysis and 1 

 from the pars intermedia, leaving only 6 to 

 be secreted from the pars anterior. A com- 

 plete revolution in the physiology of neuro- 

 hypophysis has recently resulted from the 

 discovery of methods for staining neuro- 

 secretion in this structure. It is indeed a 

 source of wonder and delight that so much 

 new and fascinating knowledge should re- 

 sult from the application of a simple techni- 

 cal discovery. The last 20 years have also 

 seen great advances in the cytology of the 

 pars anterior. Indeed, the progress here can 

 be claimed to be more extensive, if less spec- 

 tacular, than in the neurohypophysis. For 

 all this the author, constrained to review the 

 comparative cytology of the pars anterior, 

 approaches his task with foreboding. A pe- 

 culiar difficulty arises here which is not en- 

 countered in the neurohypophysis. The 

 staining reactions of the granules of func- 

 tionally equivalent cells are not consistent 

 from species to species. This variation is eas- 

 ily understandable. The hormones them- 

 selves vary in chemical composition from 

 species to species, even to the extent of the 

 hormone from one mammalian species being 

 inactive in another species. The secretory 

 granules of pars anterior cells contain the 

 hormones in association with hormonally in- 

 active proteins no less variable than the 

 hormones themselves. Functionally equiv- 

 alent cells in different species may therefore 

 have no specific component common to both 

 despite the fact that they serve an identical 

 function in the two species. 



In this situation generalizations concern- 

 ing the cytology of the pars anterior are diffi- 

 cult and hazardous. The treatment of the 

 subject given here seems to be consistent 

 with the recorded observations in a numl)cr 

 of species. Without doubt further in\'o>tiL:;t- 



