Ultrastructure of the uterine glands of the pig

The ultrastructure of the uterine glands is described from material taken from nulliparous, pregnant and post-partum pigs. The glands are of a simple tubular type, lined by a single layer of glandular epithelium standing on a well developed basal lamina. Apart from those which are ciliate, the cells are of one type, with a large basal nucleus and a well developed supranuclear Golgi region. By the fifth week of pregnancy, saccules and secretion droplets, filled with material which appears to contain acid phosphatase, are associated with the still developing Golgi apparatus, and glycogen accumulates in the cytoplasm. By the ninth week the glandular cells are densely packed with parallel arrays of granular endoplasmic reticulum. As pregnancy continues the glandular epithelial cells enlarge further, the granular endoplasmic reticulum increases in amount, dense droplets collect in the cytoplasm and characteristic tubular organelles develop. In late pregnancy, small globular cells are found sparsely distributed within the basal lamina of the glandular epithelium, each almost completely enfolded within the basal portion of one of the large epithelial cells. They are referred to in this account as 'basal' cells. They are crowded with densely staining membrane-bound droplets but it is not clear whether, or in what way, they may contribute to the glandular secretions. They were not found elsewhere in the material studied and their possible origin is discussed. After parturition the uterine glands undergo very rapid involution; many of the lining cells atrophy and are shed into the uterine lumen.