Previous studies aimed at elucidating day/night changes of pineal gland size and pinealocyte nuclear volume have yielded contradictory results. The aim of the present investigation was to examine the above parameters over a period of 10 successive days, at 6-hour intervals under a lighting regimen of LD 12:12 (7 a.m. to 7 p.m.). It was found that changes in pineal gland volume could not be correlated with the light/dark phases; instead, a rhythm with a two-day period was encountered. Pinealocyte nuclear volume, by contrast, showed clear circadian changes, exhibiting troughs during photophase and peaks during scotophase. In this experimental series, the karyometric results conformed to the melatonin forming rhythm in the rat pineal gland as reported in the literature.

A method for the estimation of the size and total number of calcitonin-containing cells (C cells) in the rat thyroid gland has been devised. The total area, the number of C cells per unit area, and the areal fraction of C cells were determined for the C cell region using step serial sections. From these data it was estimated that from 0.3 X 10(6) to 1.0 X 10(6) C cells were evenly divided between the two thyroid lobes. Approximately 150 micron3 of cytoplasm were associated with each of these cells. In comparison with sham-operated rats, pinealectomy had little effect on the number of C cells. In an experiment terminated in the summer, there was a statistically insignificant decrease 6 weeks postsurgery; no effect was seen at 12 weeks. On the other hand, a slight increase in the number of C cells was seen in January, 12 weeks postsurgery. The volume of cytoplasm per cell was not altered by pinealectomy.

In a study of the effect of pinealectomy on thyroid C-cell number, 8 animals out of 66 were found to have thymic tissue in close association with the thyroid. Cells containing immunoreactive calcitonin were found in all of the thyroids but in only one of the 8 pieces of thymus. These cells found in a piece of thymic tissue associated with the right thyroid lobe were located immediately under the capsule and did not form or associate with follicles. Unlike the other animals the rat with thymic calcitonin cells had an unequal distribution of C-cells between the left and right thyroid lobes, but the total number of thyroidal C-cells was the same as that of the other rats. Since the thymus proper was not examined in these 66 animals, ten additional rats were taken for such a study. Thyroid-associated thymic tissue was found in three of these, but none of these thymi showed any immunoreactive cells.

Previous studies have shown that in rodents the pineal complex may consist of two parts, i.e. the small deep pineal and the large superficial pineal. As commonly used techniques of pinealectomy remove the superficial pineal only, it was decided to examine what effect this operation has on the deep pineal in male Sprague-Dawley rats. 3 and 6 weeks after superficial pinealectomy, the deep pineal exhibited a clear decrease in volume, due to an atrophy of the intrinsic pinealocytes. An atrophy was still noticeable after 6 or 12 months. It is concluded that after superficial pinealectomy the deep pineal is not an important factor with respect to melatonin formation, in particular, as there are extrapineal sites of melatonin formation.