Immunohistochemical and tracer studies of macrophages/microglia in the pineal gland of postnatal rats

Abstract

The pineal gland of rats of various ages (1-21 days old) was examined by immunohistochemistry and electron microscopy. Numerous widely distributed cells identified as macrophages/microglia were immunoreactive with the monoclonal antibodies OX-42, OX-18, OX-6, and ED1, indicating that they expressed complement type 3 (CR3) receptors, major histocompatibility complex class I and II antigens, and antigens of monocyte/macrophage lineage as detected by the antibodies, respectively. Following an intraperitoneal injection of rhodamine isothiocyanate (RhIC) in all age groups, the cells emitted a bright fluorescence. They were also labeled by horseradish peroxidase (HRP), as demonstrated in both light and electron microscopy. An HRP reaction was observed in vesicles and lysosomes at the ultrastructural level. A remarkable feature was the uptake of these tracers by pinealocytes. In light microscopy, the pinealocytes showed a punctate reaction product 3-24 hours after HRP injection. By electron microscopy, the reaction product was observed in vesicles, lysosomes, and some rod-like structures in the cytoplasm. On the basis of their immunophenotypic features, it is suggested that the macrophages/microglia in the pineal gland are active phagocytes which are also probably involved in the immunoregulatory function in the gland. The avid uptake of RhIC and HRP from the circulation by these cells suggests that serum-derived substances that may gain access to the parenchyma of the gland are being constantly monitored. The labeling of pinealocytes with HRP suggests that the functional activities of these cells are being modulated by serum-derived substances.

The Pineal Gland and Beneficial Effects of Melatonin

“…The phagocytic nature of these cells was further evidenced by their removal of degenerating elements in the pineal gland in blast injuries [39]. It was concluded from their study as well as ours [2] that macrophages/microglia in the pineal are in constant surveillance of serum-derived substances and blood borne pathogens that may activate the cells to exert their potential immunoregulatory functions. It was concluded from their study as well as ours [2] that macrophages/microglia in the pineal are in constant surveillance of serum-derived substances and blood borne pathogens that may activate the cells to exert their potential immunoregulatory functions.…”

“…These cells express complement type 3 (CR3) receptors and major histocompatibility complex (MHC) classI and II antigens [1,2]. Intravenously administered horseradish peroxidase leaked into perivascular spaces in the pineal gland and was taken up avidly by the macrophages/microglia supporting their macrophagic nature [2] and indicating that they are constantly monitoring serum-derived substances. Intravenously administered horseradish peroxidase leaked into perivascular spaces in the pineal gland and was taken up avidly by the macrophages/microglia supporting their macrophagic nature [2] and indicating that they are constantly monitoring serum-derived substances.…”

“…the pinealocytes and interstitial cells classified as the astrocytes and microglia/ macrophages [1,2] The pinealocytes synthesize the neurohormone melatonin [3] which acts as an anti-oxidant [4][5][6] as well as a biologic modulator of mood, sleep, sexual behaviour, reproductive alterations and circadian rhythms, [7,8], and has regulatory actions on the immune system [9,10]. It is a central structure in the circadian system involved in synchronizing the chronobiology of organisms through synthesis of melatonin.…”

Cellular Basis of Pineal Gland Development: Emerging Role of Microglia as Phenotype Regulator

“…Functionally, pineal microglia have been reported to serve as: 1) cytokine-dependent mediators of pinealocyte neurite extension and in vitro neurotrophic effects [27–30]; 2) antigen-presenting cells via MHC class II (OX6), and blood-derived substances selectors [20,22]; 3) sensors of injury following a non-penetrative blast, intravenous injections of proinflamatory bacterial wall components, or hypoxic exposure [21,31,32]; and 4) regulators and targets of pineal melatonin [31–35]. Together, these studies support the concept that pineal microglia mediate innate immune responses within the CNS by altering melatonin levels via dynamic interactions with astrocytes and pinealocytes.…”

“…Microglia have been identified as one of the pineal interstitial cell types via OX6 (MHCII), OX42 (CD11b), IL-1β, ED1 (CD68), and TNF-R1 expression, among other markers [20–26]. Microglia have been reported to play several roles in the pineal gland, including regulation of pinealocyte neurites in a cytokine-dependent manner [27–30]; serving as antigen-presenting cells [20,22]; sensing physical injury, bacteria, and hypoxia [21,31,32], and modulating pineal melatonin [31–35]. Our data expand the repertoire of microglial functions in the developing and adult pineal gland.…”