Morphological findings relating to the problem of cortex and medulla in the pineal glands of rat and hamster.

Because, in previous investigations on the rat pineal gland, karyometric studies of pinealocytes from cortical and medullary regions had yielded contradictory results, experiments were carried out to resolve this problem. In immersion-fixed, paraffin-embedded pineal glands, nuclear size of cortical regions was invariably larger than that in the medulla, the nuclear size clearly depending on the plane of sectioning. The differences between cortex and medulla were abolished in (a) pineal glands sagittally bisected prior to immersion fixation and (b) perfusion-fixed, epoxy resin-embedded pineal tissue, suggesting that unequal pinealocyte nuclear size in cortex and medulla is artefactual. In the hamster pineal gland, cortical and medullary regions are separated by a narrow band of interstitial cells, indicating that different structural features account for this subdivision in different species.