ER-annulate lamellar associations of mucosal epithelial cells of the rat jejunum.

Mucosal cells of the rat jejunum vary greatly in the relative abundance and kind of endoplasmic reticulum (ER). Many cells have few rough surfaced cisternal ER elements; these cells frequently are located in basal villous regions and generally are less differentiated than those near apices. Profiles of rough ER cisternae were often encountered in stacks of 3-4 parallel units in apical cells. A number of differentiating basal cells were found to contain ER-annulate lamellar associations arranged similarly to the ER stacks of apical cells. The annulate lamellae of this complex resemble those described for other rapidly differentiating or embryonic cells and may be derived from the nuclear envelope. They could be involved in the formation of the ER.