Synchronization of the Pineal and Pituitary Morphophysiology by Photoperiodic Environmental Signals

Terrestrial environments have cyclical characteristics, and therefore, there must be a temporal harmonization between environmental cycles and biological rhythmicity. Thus, seasonal changes in lighting and temperature synchronize circannual rhythms to regulate the biological rhythms of organisms. This synchronization is carried out through biological clocks, which control the activity of numerous endocrine glands, such as the pineal gland and the secretion of its main hormone, melatonin. This chapter describes the morphophysiological variations of the pineal and pituitary glands of male vizcachas (Lagostomus maximus maximus), captured in San Luis, Argentina. It is a nocturnal rodent mammal that lives in underground caves in semiarid areas of central Argentina and, in its natural habitat, is a photoperiod-dependent seasonal breeder. The pineal gland is ovoid in shape, elongated, and connected by a stalk to the base of the brain. In it, two regions (proximal and distal) and three cell types are distinguished: pinealocytes (P), interstitial cells (IC), and pigmented cells. Seasonal morphological changes have been reported in the main parenchyma cells in adult male animals. At ultrastructural level, winter P showed an oval nucleus with dispersed chromatin and a large nucleolus. The cytoplasm exhibited numerous dense matrix tubular mitochondria, developed Golgi complex and RER, free polyribosomes, and dense bodies of various sizes suggesting active metabolism and increased cellular activity. Immunohistochemical studies made it possible to identify IC, through the expression of S-100, GFAP, and vimentin proteins. These cells were mainly located in the proximal region and exhibited an irregular cell body with abundant cytoplasmic processes surrounding the P and/or reaching blood vessels. Likewise, the existence of IC in different functional and maturation stages was demonstrated, probably in relation to the needs of the glandular microenvironment or the presence of cell subpopulations with different functions. The expression of S-100 and GFAP was maximum during winter, in parallel to the P activity. This indicates a synchronized activity between both cell types through an intraglandular paracrine communication network that ensures optimal neuroendocrine regulation. In the pituitary gland of adult male vizcachas, specifically in the adenohypophysis, different cell populations and follicular structures that present seasonal morphological variations were described. The cell types identified in pars distalis (PD) were corticotrophs, thyrotrophs, gonadotrophs, lactotrophs, somatotrophs, and follicle-stellate cells. All these cell populations presented a defined regionalization in the PD parenchyma and histomorphometric variations in relation to cell activity during the reproductive cycle. In addition, they exhibited a specific cytological configuration that demonstrates the existence of an intrapituitary communication network to respond to the different physiological requirements of the animals. Besides, cell proliferation and androgen receptor expression exhibit seasonal changes according to the glandular activity. In conclusion, the morphological changes in these endocrine glands, central components in the neuroendocrine control system, demonstrate the synchronization of glandular activity with the main environmental signals, promoting reproductive and adaptive behaviors that guarantee the perpetuation of this species.