A new specialization of the granular endoplasmic reticulum in the uterine glands of the pig in the later stages of pregnancy.

Abstract

ABSTRACT A new specialization of the granular endoplasmic reticulum (GER) which develops within the uterine gland cells of the pig during the final third of pregnancy is described. The ‘tubular bodies ‘are interpreted as sheets of GER rolled up to form open-ended cylinders enclosing cytoplasm which is distinctly less electron-dense than the surrounding external cytoplasm, and in which large clear vesicles and free ribosomes are frequently found. The tubular bodies are not present during the first two thirds of pregnancy, but by day 100 of gestation they are readily discerned. They increase in number within the last week of pregnancy and are still found in glands from the day 1 post-partum uterus. They are distributed, either singly or in groups of up to 30 or more units, throughout the cells. The cisternal space which surrounds them is continuous with that of the granular endoplasmic reticulum. A tentative developmental sequence is suggested, based on morphological similarities between the wall structure of the early tubular body and the paired cistemae, which are seen scattered as single, short strands within the gland cells. The final form is achieved by modification of the inner apposed, ribosome-free cisternal membranes into the straightened-out, thickened membranes characteristic of the tubular body wall, together with a rolling-up of the ‘sheet’ of paired GER cistemae. Subsequent loss of the innermost unmodified cisternal membrane which is now in contact with the centrally enclosed cytoplasm results in the characteristic tubular body wall structure. The characteristic appearance of the central cytoplasm and the presence of modified surrounding membranes suggests the possibility of a localized, modified internal environment required for some unspecified functional activity; possibly a synthetic process or some type of ionic regulation.