Histochemical and ultrastructural investigation on the cytotypes in the mouse Harderian gland.

The presence of melatonin and other biogenic indoleamines in the Harderian gland has been proposed by various authors. In the present work we would have investigated which of the cytotypes of the mouse Harderian gland might be involved in melatonin turnover. For this aim we employed the osmium tetroxide-zinc iodide (ZIO) histochemical technique that has been proposed useful to identify indole monoamines. Harderian glands of male, female and pregnant female mice were studied by light and transmission electron microscopy (TEM). Secretory type B cells were selectively stained by the ZIO-mixture, much more in female than male; type A cells were much more numerous than type B ones and never ZIO-positive. Ultrastructural examinations showed that type B cells contain numerous granules in the whole cytoplasm, in the perinuclear cisterna and around the cytoplasmic vacuoles and vesicles. Myoepithelial cells were sometimes found weakly ZIO-positive. Endothelial cells of capillary blood vessels presented ZIO-precipitates in the cytoplasm, as well as in the perinuclear cisterna. These data may suggest an uptake of melatonin from the blood stream and an involvement of type B secretory cells in melatonin catabolic turnover, or a "in situ" melatonin biosynthesis and endocrine secretion by the same B cells. The higher ZIO-positivity of type B cells in female than male may be related also to the influence of sexual steroid hormones, that characterizes the porphyrin pigment amount involved in melatonin oxidation.