THE PANCREAS. 



185 



FIG. 230. Portion of pancreas, 

 treated with Golgi method, show- 

 ing secretion-canaliculi extending 

 between the gland-cells. X 50. 



mediate ducts pass directly to the tubular alveoli, into most, but not all, of 

 which their attenuated epithelium protrudes as the centroacinal cells. The 

 relation of the latter to the secretory elements within the alveolus is such 

 that the thinned-out duct-cells are invaginated into the alveolus, so that the 

 glandular epithelium is partly excluded from 

 direct relation with the lumen. 



The tubular alveoli, often tortuous and 

 sometimes divided, possess a well-defined base- 

 ment membrane, against which lie the gland-cells. 

 The latter are usually blunt pyramidal in shape, 

 with a length of about 12 p.. Their cytoplasm 

 exhibits two well differentiated zones, an inner 

 granular one, next the lumen, filled with highly 

 refracting zymogen granules, and an outer clear 

 one, next the basement membrane, which is free 

 from granules and contains the spherical nucleus. 

 The relative width of these zones varies with the functional condition of the 

 cells. During rest, when the cells are stored with zymogen particles, the 

 granular zone is very broad and the outer homogeneous one correspondingly 

 narrow. With discharge of the pancreatic secretion during digestion, the 

 granular zone diminishes and reaches its minimum, almost disappearing, 

 when the gland is exhausted. The return to a condition of rest is accom- 

 panied by the formation and accumulation of a new store of zymogen particles 

 until the granular zone is again at its maximum. Intercellular secretion- 

 canaliculi are present in all alveoli in which the centroacinal cells exclude 

 the gland-cells from the lumen. They extend between the cells almost, bur> 

 not quite, to the basement membrane (Fig. 230) and serve to convey the 



secretion-products through the 

 obstructing central cells into 

 the lumen of the alveolus. 



The interalveolar cell- 

 groups, or islets of Lan- 

 gerhans, appear as small 

 collections of modified gland- 

 tissue, some 3 mm. in diameter, 

 lying between the tubular 

 alveoli, from which they are 

 separated by delicate envelopes 

 of fibrous tissue. These bodies 

 consist of anastomosing solid 

 cords or trabeculae of small 

 polyhedral cells, faintly gran- 

 ular and without zone differ- 

 entiation, separated by wide 

 capillary blood-vessels, with 

 which the interalveolar cells 

 come into intimate relation, the 

 vascular endothelium alone intervening. While some of the cell-groups are 

 isolated and do not open into the ducts of the alveolar tissue, others are con- 

 nected with the alveoli and their ducts, but the relation between the inter- 

 alveolar cells and the ordinary glandular parenchyma is still uncertain. 

 Developmentally, the islet-cells and the ordinary glandular elements are 

 derived from a common source, but differentiation having once been estab- 



Blood-vessel 



Modified 

 'cells 



Alveolar 

 'cells 



FIG. 231. Section of pancreas, showing details of island 

 of Langerhans. X 190. 



