The aim of this paper is to summarize some of our quantitative descriptive and experimental studies, to discuss them in view of the literature data, and to present a synthesis of the topic. The results of stereological analysis of some tissue components of the rat thyroid gland have been compared with the results of topological studies on the parafollicular cells of various mammalian species. Localization of the parafollicular cells in the central regions of the thyroid gland lobes, where the follicular cell activity seems to be greater than in the periphery of the lobes, has led to the hypothesis that the parafollicular cells regulate (stimulate and/or suppress) the activity of the follicular cells. Long-term application and antithyroid drugs to mice and rats has shown that excessive concentrations of thyrotropin provoke hyperplasia of both the follicular cells and the intrathyroid mast cells and, transiently, of the parafollicular cells. This and some of the literature data are congruent with the hypothesis that the parafollicular and mast cells also stimulate the follicular cells by their paracrine secretions. Long-term application of antithyroid drugs to mice and rats has shown that excessive concentrations of cular cells but also probably stimulation of the follicular cells, as judged by the stereological measurements. The biological meaning of the spatial integration of follicular and parafollicular cells seems to be a functional coordination of both epithelial cell lines, supported by intrathyroid mast cells.

It has been shown that compound 48/80 evokes acid secretion in the isolated mouse stomach by releasing histamine from mast cells. The aim of the present work was to study the distribution and type of mast cells in the isolated mouse stomach and to examine the effect of compound 48/80 and of electrical field stimulation (EFS) on these cells. Histological examination of the stomachs showed only the presence of connective tissue mast cells (CTMC) in all parts of the stomach except in the glandular mucosa where mucosal mast cells (MMC) predominated (MMC -71%, CTMC -29%). Compound 48/80 and EFS did not affect MMC, whereas CTMC showed marked degranulation in all parts of the stomach including the glandular mucosa. It can be concluded that CTMC, present in the gastric mucosa and being sensitive also to EFS, may be involved in the regulation of gastric secretion in the mouse.

Histamine was found to be stored in the submandibular gland of the cat mainly in the mast cells. The amine is released from mast cells by compound 48/80 and pilocarpine. This was demonstrated in the in vivo and in vitro experiments and by histological examination. During the physiological stimulation of the gland, via the electrical stimulation of the chorda tympani nerve, significant changes of histamine content were not found. This could be explained by the increased synthesis of histamine during physiological stimulation. The physiological role of histamine in salivary secretion was demonstrated. The similarity between the roles of histamine, mast cells and chromaffine-like cells in salivary and gastric secretion is discussed.

It has been shown that histamine is present in guinea pig hearts. In the present work, the effect of some substances, known to liberate mast cell histamine, on the isolated guinea pig atria was studied. Compound 48/80 (100 micrograms/ml), pentagastrin (10(-6) M) and substance P (10(-5) M) were added 2-3 times to the isolated organs and the frequency of contractions was measured. At the end of experiments, the atria were examined histologically for mast cell degranulation. Compound 48/80 and pentagastrin increased the frequency of contractions of isolated atria. Substance P provoked a dose-dependent decrease of contraction frequency; this effect was diminished by atropine (10(-5) M). All three substances provoked pronounced degranulation of mast cells present in the atrium, the effect of substance P being significantly greater than the effects of the other two substances. It can be concluded that mast cells, present in guinea pig atrium, are sensitive to the histamine liberators used; histamine is released in quantities high enough to produce an effect.

In this experiment of 11 month-duration, 18 female Wistar rats received controlled amounts of calcium with food and water, to produce a state of either hypocalcemia or hypercalcemia. After a long-term low calcium diet hypocalcemia disappeared. This group of animals showed insignificant increases in the nuclear-cytoplasmic ratio of the parathyroid cells as well as in the total volume of the parathyroid glands, and a significant increase in the volume density of the osteoid. In the group receiving a high calcium diet, hypercalcemia was still present after 11 month. The insignificantly smaller and more numerous C-cells produced more calcitonin than normally. The parathyroid cells were significantly smaller and the numerical areal density of the osteoclasts was significantly lower than in the control group. It can be concluded that the parathyroid glands and C-cells are involved in the maintenance of blood calcium homeostasis during a long-term experiment on rats receiving low or high calcium diet.