Light and electron microscopic observations on some environment-organism interfaces in the developing brown shrimp, Penaeus aztecus

The histology and ultrastructure of the following regions in larval and early postlarval brown shrimp have been studied: 1) midgut, 2) epidermis modified for muscle insertions, 3) epidermis modified for support, 4) epidermis modified for osmoregulation, 5) epidermis modified for osmoregulation and support. The results of these observations are summarized in the following paragraphs. The fine structure of the midgut does not change during larval and early postlarval 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. [...]