The fine structure of the rat metrial gland in relation to the origin of the granulated cells.

The fine structure of the metrial gland of the rat was examined at days 12, 14, 16, 18 and 20 of pregnancy and at all these stages two well differentiated cell types were seen. A relatively pale-staining cell which contained numerous dense granules was readily distinguished from a darkly stained fibroblast-like cell. Numerous cells could be identified, however, which did not fit either of these categories. On the basis of their morphological appearances these cells were placed in a series, with the members of the series showing a transition from a small cell with a high nuclear/cytoplasmic size ratio to a readily identifiable metrial gland cell containing a few typical granules. The small cell, apparently the precursor of the granulated metrial gland cell, was characterized by a markedly heterochromatic nucleus and few cytoplasmic organelles, although there were occasional mitochondria and Golgi bodies. On morphological grounds this cell appeared to be from the lymphocyte series and was probably a small lymphocyte.