A histological study of the caecal epithelium of Fasciola hepatica L.

Abstract

A secretory cycle has been revealed in the intestinal epithelial cells of Fasciola hepatica. The basal regions of tall cells contain mitochondria-like bodies which stain deeply with basic dyes. These granules become numerous and are then arranged in vertical arrays of variable height which extend into the middle region of their cells. The distal halves of the epithelial cells contain numerous granules or droplets which have no affinity for basic stains and tend to swell, with consequent disarray of the secretory products. In the terminal parts of the cells vacuole-like formations appear, usually one to each cell, and eventually the lumen of the caecum becomes occluded by a froth-like mass of vacuoles. When food enters the caecum in small amount, the vacuoles are broken down and some secretion is liberated and mixed with the incoming homogenate. Meanwhile the vacuole-like structures encroach more deeply into the cell and eventually the residual secretion is discharged. New cell boundaries are being established near the nuclear zone and after complete discharge of secretion the distal parts of the cells consist of aggregates of long cytoplasmic filaments, which form a kind of fringe on the free borders of the now much shortened cells. These delicate cytoplasmic fringes, are abraded, probably by the physical action of streams of food, and typical short cells in caeca which contain food have much shorter fringes no more than one-half the height of the shallow cells. Ultimately all traces of such residual cytoplasm remaining after completion of the secretory cycle disappear. Short cells containing few granules are presumed to increase in height subsequently by regeneration of cytoplasm and, possibly, nuclear changes also, until they attain the form seen in cells commencing the secretory cycle. Such changes are especially difficult to observe because of variability in the heights of cells normally forming the epithelium. The especially tall cells which are seen, isolated or in small groups, at the height of the secretory cycle—the so-called pyramidal cells—are believed to be merely cells which have had their secretory activities delayed; ultimately, they are denuded in the same way as other cells during the secretory cycle. The type of secretory activity disclosed is therefore of the apocrine type, which is considered appropriate to the suctorial method of feeding employed by the fluke.