Three-dimensional characteristics of the endoplasmic reticulum in the columnar cells of the rat small intestine: an electron microscopy study in thick section.

The endoplasmic reticulum (ER) of the columnar cells of the rat jejunum was studied with a specific block-staining technique and standard transmission electron microscopy. A new three-dimensional model, based on the analysis of stereo pair photographs, is proposed; this model suggests that the endoplasmic reticulum may constitute a transcellular route. Thick sections (0.5-1 micron) of columnar cells were made after a 5-day impregnation with osmium tetroxide and were examined by standard transmission electron microscopy at 80-100 kV. The evolution of the ER during the cellular ascent of cells from the crypt to the top of the villus is toward a greater complexity. At the base of the crypt of Lieberkühn, no definite organization is noted, and most often only the nuclear envelope and canalicular elements are stained. The endoplasmic reticulum of the mature columnar cells forms a continuous network of canaliculi and fenestrated saccules; it extends from the apex, below the microvilli, to the lateral and basal plasma membranes, and sends many projections to the nuclear envelope. In the basal part of the cell, below the nucleus, the ER consists mainly of tubular canaliculi, whereas mostly saccules are observed in the supranuclear part. The canaliculi have a diameter of 40-60 nm. Fenestrated saccules appear to form a continuous tubular structure surrounding mitochondria; the saccules have a thickness of 25-40 nm and possess irregular perforations of 35-60 nm. Finally, in some cells, the endoplasmic reticulum seems to show functional differences as reflected by the absence of reaction in cells adjacent to well-stained cells in the same part of the villus; thus the osmium impregnation technique appears to be a valid tool for studying the ER organization.