The functional organization of the salivary gland of Biomphalaria straminea (Gastropoda: Planorbidae): Secretory mechanisms and enzymatic determinations

Abstract

A detailed morphological analysis of the salivary gland of adults of the freshwater snail Biomphalaria straminea was carried out by light and electron microscopy, complemented by the biochemical assay of key digestive enzymes in gland homogenates. The salivary gland is a paired tubular organ with the ante-rior portion (the tubular ducts) inserted into the buccal mass and the posterior region (the secretory portion) joined by their tips, forming a loop. The entire gland is made up of a simple columnar epithelium, resting on a thin connective tissue, and three regions can be recognized: the duct, the intermediary and the secretory regions. The duct is formed by a single cell type, the duct epithelial cell, covered apically by cilia and microvilli. The intermediary region presents two cell types: one similar to the duct cells and the other to the secretory cells found in the secretory region. Two cell types are also observed in the secretory region: the intercalary (non-secretory cells) and the secretory cells. The intercalary cells have many cilia and microvilli on their apical surface which, along with the cilia present in the duct epithelial cells, should act in mixing and propelling the salivary secretion in the gland lumen from the secretory region into the buccal cavity. According to the general cell morphology, size and secretory activity, five phases of differentiation (I-V) can be identified in secretory cells. Based on the ultrastructural aspect of the secretion vesicles and the rough endoplasmic reticulum, two cell sub-types can be further distinguished in secretory cells: L-cells and H-cells. The mechanism of elimination of secretion in secretory cells is initially apocrine (phases III-IV), ending as a holocrine mechanism in the final (phase V) steps of the secretory process. To detect the presence of digestive hydrolases in the salivary gland, five enzymes (amylase, cellulase, maltase, aminopeptidase and trypsin) were tested in gland homogenates. Except for trypsin, the other enzymes were found to be significantly active, indicating an important role of the salivary secretion in the initial digestion of food ingested by these animals.