Histology and ultrastructure of the gut of the black mollie (poecilia spp.), a hybrid teleost.

Abstract

A light and electron microscopic (TEM and SEM) study of the gut of the black mollie (Poecilia spp.) shows that the gut is a relatively undifferentiated muscular tube lined with a simple columnar epithelium. The mucosa and underlying lamina propria are formed into folds. The apices of the folds form obtuse angles at the cranial end and more acute ones caudally. The mucosal epithelium is a simple columnar sheet with PAS-positive goblet cells interspersed throughout. The enterocytes are covered apically with uniform microvilli in most of the tract, and the plasma membrane deeply infolded basally. Mitochondria are located in the basal folds. A continuous basal lamina separates the epithelial mucosa from the underlying lamina propria. Enterocytes have a PAS-positive Golgi apparatus in the supranuclear region. The enterocytes of the cranial and middle thirds of the gut are morphologically indistinguishable, but those in the caudal third display shorter and sparser microvilli. There is no evidence of pinocytotic activity in the enterocytes of the cranial two thirds of the gut tube, but some apical pinocytotic vesicles are seen in the cells in the caudal third. The tunica muscularis consists of two layers of smooth muscle, inner circular and outer longitudinal. We observe lateral separation of enterocytes from each other deep to the junctional complexes in the caudal and midgut regions. The gut of the mollie lacks some of the specialized cell types commonly found in other fishes. It cannot create an acidic gastric environment; overall control of digestive activity, osmolarity, and ionic balance presumably rest in the structural relationships between epithelial cells and between the epithelium and its supporting tissues. Regional specialization of cells and of the gut as a whole may confer digestive function equivalent to that found in more complicated systems.