Fine structure of the midgut epithelium in the developing brown shrimp, Penaeus aztecus.

The midgut epithelium of larval and early postlarval brown shrimp has been studied with light and electron microscopy. Ultrastructurally the features of the midgut do not change during these stages of development. On the basis of electron density, two epithelial cell types can be distinguished, and these are referred to as light and dark cells. The dark cells contain more rough endoplasmic reticulum and more free ribosomes than the light cells. Mitochondria in the dark cells have a matrix which is less electron dense than the mitochondrial matrix of the light cells. Both cell types have a microvillous border with a surface coat. The microvilli lack microfilaments within their core, and a terminal web is not differentiated in the stages examined. Tubular smooth endoplasmic reticulum is abundant in the basal portions of the cells. Electron dense, membrane bound vesicles are consistently seen in association with the Golgi apparatus, apical cell surface, and gut lumen and therefore are believed to be secretory granules. Cells in the anterior portion of the midgut often contain very large lipid droplets in the cytoplasm.