The ultrastructure of mammalian pinealocytes: a systematic investigation

Genomic evidence for the parallel regression of melatonin synthesis and signaling pathways in placental mammals

Background: The study of regressive evolution has yielded a wealth of examples where the underlying genes bear molecular signatures of trait degradation, such as pseudogenization or deletion. Typically, it appears that such disrupted genes are limited to the function of the regressed trait, whereas pleiotropic genes tend to be maintained by natural selection to support their myriad purposes. One such set of pleiotropic genes is involved in the synthesis ( AANAT, ASMT) and signaling ( MTNR1A, MTNR1B) of melatonin, a hormone secreted by the vertebrate pineal gland. Melatonin provides a signal of environmental darkness, thereby influencing the circadian and circannual rhythmicity of numerous physiological traits. Therefore, the complete loss of a pineal gland and the underlying melatonin pathway genes seems likely to be maladaptive, unless compensated by extrapineal sources of melatonin. Methods: We examined AANAT, ASMT, MTNR1A and MTNR1B in 123 vertebrate species, including pineal-less placental mammals and crocodylians. We searched for inactivating mutations and modelled selective pressures (dN/dS) to test whether the genes remain functionally intact. Results: We report that crocodylians retain intact melatonin genes and express AANAT and ASMT in their eyes, whereas all four genes have been repeatedly inactivated in the pineal-less xenarthrans, pangolins, sirenians, and whales. Furthermore, colugos have lost these genes, and several lineages of subterranean mammals have partial melatonin pathway dysfunction. These results are supported by the presence of shared inactivating mutations across clades and analyses of selection pressure based on the ratio of non-synonymous to synonymous substitutions (dN/dS), suggesting extended periods of relaxed selection on these genes. Conclusions: The losses of melatonin synthesis and signaling date to tens of millions of years ago in several lineages of placental mammals, raising questions about the evolutionary resilience of pleiotropic genes, and the causes and consequences of losing melatonin pathways in these species.

The pineal gland of the shrew (Blarina brevicauda and Blarina carolinensis): a light and electron microscopic study of pinealocytes

The pineal gland structure and ultrastructure in the Northern (Blarina brevicauda) and Southern short-tailed shrew (Blarina carolinensis) are described by light and electron microscopy. Results observed were similar to other mammals of Insectivora described previously, specifically, the hedgehog (Erinaceus europaeus) and the Old World mole (Talpa europea). Two different types of pinealocytes were noticed by electron microscopy, in addition to relatively few glial cells. Granular vesicles were not noticed in abundance. The granular endoplasmic reticulum was observed and studded with vesicles. The golgi apparatus was well developed and appeared often. Synaptic ribbons were observed in several different formations consisting of ribbons and/or rods. The ciliary derivative, the rudimentary photoreceptor structures found in the pinealocytes of population I, was noticed in a 9 + 0 tubular pattern. Within these semifossorial shrews, the relationship between specific intracellular organelles and their function was discussed.

Pigmented Cells in the Pineal Gland of Female Viscacha (Lagostomus maximus maximus): A Histochemical and Ultrastructural Study

The presence of pigment has been demonstrated in different nervous structures such as those of retina, substantia nigra, and locus coeruleus. These pigments have also been described in the pineal gland of different mammal species. Histochemical and ultrastructural studies of the pineal gland of female viscacha (Lagostomus maximus maximus) were performed to analyze the presence of pigmented cells under natural conditions and to evaluate a probable relation between pigment content and glandular activity during pregnancy. The following techniques were applied: hematoxylin-eosin, phosphotungstic acid-hematoxylin, Masson-Fontana silver, DOPA histochemistry, Schmorl's reaction and toluidine blue. Estradiol and progesterone serum levels were determined by RIA. The ultrastructural features of the pineal pigment granules were also analyzed. Pigment granules were observed in a random distribution, but the pigmented cells were frequently found near blood vessels. The pineal pigment was histochemically identified as melanin. Differences in the amount of pigmented cells were found between pregnant and nonpregnant viscachas. The ultrastructural analysis revealed the presence of premelanosomes and melanosomes. Estradiol and progesterone levels vary during pregnancy. In conclusion, the changes in the amount of pigment content and hormone levels may indicate that the pineal gland of female viscacha is susceptible to endocrine variations during pregnancy.

Histopathology of tumors of the pineal region

Pineal region tumors are heterogeneous lesions and include mainly pineal parenchymal tumors (PPTs), papillary tumors of the pineal region (PTPRs) and germ cell tumors (GCTs). This article describes the cystic pineal gland compared with normal tissue and histopathological features of the most frequent pineal region tumors. PPTs are subdivided into pineocytoma (grade I), pineoblastoma (grade IV) and tumors with intermediate differentiation (PPTIDs; grades II-III). A grading system based on the number of mitoses and neurofilament protein expression distinguishes low- from high-grade PPTID. PTPR is a new tumoral entity thought to originate from the subcommissural organ. GCTs include germinoma, embryonal carcinoma, teratoma, yolk sac tumor and choriocarcinoma and are often of mixed histologic composition. New histogenetic data for GCTs are presented.

Annual reproductive synchronization in ovary and pineal gland function of female short-nosed fruit bat, Cynopterus sphinx

We studied the annual correlation of ovarian activity and pineal gland in relation with seasonal variation and gestation of a tropical zone short-nosed fruit bat Cynopterus sphinx. Female bats showed bimodal polyestry (February/March and September/October) in their reproductive cycle. Plasma estradiol concentration ran parallel with ovarian activity and had an inverse relation with pineal mass and peripheral melatonin concentration. Due to the delayed embryonic development in the uterus (October-March) of female bats, interestingly, the uterine activity did not show a parallel relation with ovarian activity and estradiol level. Further, compared with normal non-pregnant females, melatonin level was high during gestation and delayed embryonic development phase. This suggests that the reproductive synchrony and annual variation in ovarian activity of this nocturnal flying mammal differ from other common tropical mammals. The delayed embryonic development in bats might be an adaptive strategy for the unfavorable conditions of the seasons and might be regulated by high peripheral estradiol and melatonin concentration.

Evidence for implication of tryptophan hydroxylase in the regulation of melatonin synthesis in ovine pinealocytes in culture

1. Tryptophan hydroxylase (TPOH) is the first enzyme in the melatonin synthesis pathway and the rate-limiting enzyme in serotonin synthesis. We established in this study an in vitro model of ovine pinealocytes to investigate the role of TPOH in melatonin production. 2. We demonstrated that TPOH is highly expressed both in vivo and in vitro at the protein and mRNA levels. In vitro pinealocytes show ultrastructural features similar to those previously described in vivo. 3. Moreover, our in vitro model allowed us to study the regulation mechanisms for melatonin synthesis in sheep pinealocytes and to demonstrate that both transcriptional and posttranscriptional mechanisms are involved. 4. In particular, our results suggest that TPOH plays an essential role in the regulation of melatonin synthesis.

Melatonin rhythms and pineal structure in a tropical bat, Anoura geoffroyi, that does not use photoperiod to regulate seasonal reproduction

It has been hypothesized that pineal structure and function might differ between temperate zone and tropical species of mammals because of lower amplitudes of seasonal change in photoperiod and, in some areas, less seasonal climatic variation. Anoura geoffroyi produce a single offspring in November or December of each year on the Caribbean island of Trinidad, at 10 degrees N latitude in the deep tropics. Previous work has shown that this population lacks reproductive responses to photoperiod, and must be enforcing seasonal breeding using a non-photoperiodic cue. Anoura geoffroyi have a minute, thin, and rod-like pineal gland. Throughout much of its length, the pineal courses irregularly within the ventrolateral wall of the great cerebral vein. This intimate relationship may have functional implications. Despite having a very small pineal gland, this species produced a nocturnal rise in serum melatonin. Serum melatonin levels in most individuals were below or near undetectable levels during the light period and rose to a peak averaging 100 pg/ml in the last third of the dark period. Our results indicate that, although the pineal gland of A. geoffroyi is extremely small, serum melatonin levels are comparable to those of other mammals.

Three dimensional culture of pineal cell aggregates: a model of cell-cell co-operation

Three dimensional (3-D) cultures of pineal cell aggregates were obtained by constant gyratory shaking the heterogenous cell populations, obtained from the rat pineals, in the DMEM (Dulbecco's modified Eagle's medium). Within 4 days, the pineal cells became organized into a tissue like configuration appearing as a compact ball, evidenced by the scanning electron microscopy. The 3-D aggregates seemed to be mainly composed of pinealocytes (round-oval cells), glial (elongated cells) and other unknown cells. The heterogenous cells were separated by intercellular spaces. The ultrastructural characteristics revealed by transmission electron microscopy exhibited the presence of granular lysosomes, typical of pinealocytes actively involved in the secretion. These pineal cell aggregates secreted melatonin and other indole amines i.e. 5-methoxytryptamine (5-MT), indole acetic acid (IAA), 5-methoxy-3-indole acetic acid (5-MIAA), tryptophol (TOL) and 5-methoxytryptophol (5-MTL) in the culture medium, indicating the functional aspect of pinealocytes. The 3-D aggregates cultures had advantages over the pineal monolayer cultures as, after 4 days of culture, the amounts of indole amines secreted by 3-D aggregates were higher than those secreted by monolayer cultures. Besides, the 3-D aggregates remained functional till 24 days in the gyratory culture conditions. In the continuous perifusion system, the 3-D aggregates secreted melatonin while challanged with isoproterenol. This 3-D model of pineal cell aggregates might be useful, in future, to perform other kinetic studies of the release of indole amines in perifusion experiments as this system allows the maintenance of pineal cells for a long period of time.

Calbindin-D28k, calretinin, and S-100 immunoreactivities in rat pineal gland during postnatal development

Profound morphological modifications occur during postnatal development of the rat pineal gland. We have immunohistochemically followed those events from postnatal day 1 to 20 by using three cytoarchitectonic markers (S-100, calbindin-D28k, and calretinin) that belong to the calmodulin/troponin C calcium-binding protein family. In the developing rat pineal, anticalbindin-D28k antibody labels three cell types: immature and mature astrocytes and perivascular type II pinealocytes. During development, calbindin-D28k positive cells migrate from the base of the pineal stalk into the superficial part of the pineal. Calbindin-D28k, usually used as a neuronal marker in the central nervous system, recognizes in rat pineal precursor astrocytes 5 days before S-100 and labels a subpopulation somewhat different from S-100 positive astrocytes. Calretinin immunoreactivity appeared in the postero-superior part of the pineal and was abundant until postnatal day 5, then its density dramatically felt to leave, after postnatal day 20, an occasional population of cells whose morphology is compatible with neuron-like cells.

Structure of the pineal gland in the adult cat

The ultrastructure of the pineal gland in the adult cat is described and compared with that of other mammals. Connective tissue spaces showed capillaries with nonfenestrated endothelia and numerous unmyelinated nerve fibers. In the proximal region of the gland, myelinated nerve fibers coming from the anterior commissure were also found. Cat pinealocytes showed a nucleus with prominent nucleoli, a well developed Golgi apparatus, centrioles, granular endoplasmic reticulum, ribosomes, abundant microtubuli and enlarged mitochondria. Pinealocytes showed several long processes with bulbous endings filled with clear vesicles and scarce "synaptic" ribbons. Pineal astrocytes and their processes were characterized by the presence of abundant filaments.

Rat pineal cell aggregates: ultrastructural and functional characteristics

The aggregates were obtained by constant gyratory shaking of suspension cells freshly isolated from adult rat pineal glands. Their sizes ranged from 60 to 120 microns. Within 4-5 days, the aggregates formed by pinealocytes, astrocytes, and other unidentified cells became organized in a tissue-like configuration. There was no proliferation of the fibroblast cells. Ultrastructural characteristics of the aggregates were revealed by the presence of granular lysosomes, which are typical of pinealocytes, and are actively involved in the secretion. Functional characteristics were studied in static incubation. The aggregates secreted melatonin and other indole amines in culture medium. Basal melatonin release was detected until Day 24 of culture. This secretion was stimulated 230% with Isoproterenol (beta-adrenergic agonist), 725% with Epinephrine (alpha- and beta-adrenergic agonists), and 140% with Vasoactive Intestinal Peptide after 5 days in culture, then > 1200% with Forskolin 9 days later (14-day-old aggregates). The results indicate that three-dimensional aggregates obtained from isolated pineal gland cells were the functional multicellular structures with in vivo characteristics.

Observations on the pineal gland of the big brown bat, Eptesicus fucus: possible correlation of melanin intensification with constant darkness

BACKGROUND

Our initial observation of the macroscopically pigmented pineal gland of the big brown bat, Eptesicus fuscus, led to this study. Information has been lacking on pigmentation in the pineal and its significance in mammals in general and bats in particular. This report begins to address this situation.

METHODS

Bats were examined both in the wild and after exposure to various experimental conditions. The pineals were examined macroscopically as well as with light and electron microscopy. The pigment was identified as melanin by its color, the ultrastructure of its granules, and their reaction with hydrogen peroxide.

RESULTS

Gross observations showed the pineals to be variably pigmented, which were subjectively scored from unpigmented to heavily pigmented. Pineals from bats exposed to a continuous 24 h light regimen or those from a summer population contained very little, if any, externally visible melanin. Such pineals are considered unpigmented in this study. In contrast, pineals from 74% of 156 animals taken together, either subjected to constant darkness or hibernation (simulated or natural), exhibited very heavily pigmented pineals. The pigment in these cases even extended to the juxtapineal meningeal covering. The pineal was pigmented even in a newborn Eptesicus.

CONCLUSIONS

The pineal pigmentation in the big brown bat appears to intensify with constant darkness and may vary seasonally. The observation of macroscopically pigmented pineals in some other bats (Myotis lucifugus, Pipistrellus subflavus, and Lasiurus borealis) suggests that this phenomenon may be of taxonomic value for the family Vespertilionidae (Order Chiroptera).

Structural and ultrastructural characteristics of human pineal gland, and pineal parenchymal tumors

We have studied 20 pineal parenchymal tumors (PPT) and 4 normal or cystic pineal glands both by light and electron microscopy and immunohistochemistry with antibodies against glial markers [glial fibrillary acidic protein (GFAP) and protein S-100] or neural/neuroendocrine markers [neurofilaments (NF), synaptophysin and chromogranin A]. Light microscopy revealed the cellular organization of pinealocytes in the normal gland and in different morphological types of pineal tumors (typical pineocytomas, PPT with intermediate differentiation, mixed PPT exhibiting elements of both pineocytoma and pineoblastoma and pineoblastomas). Immunohistochemistry showed the presence of GFAP and protein S-100 in interstitial cells in non-neoplastic pineal gland. Cell processes were labeled with anti-synaptophysin and anti-NF antibodies. No immunoreactivity was found for chromogranin A in non-neoplastic pineal gland. In pineocytomas, GFAP and protein S-100 were observed in interstitial cells. Synaptophysin and NF were present in the large rosettes of pineocytomas. Synaptophysin, NF and chromogranin A were present in pineocytomas with a lobular arrangement of cells. Anti-chromogranin A immunoreactivity was also seen in lobular areas of some PPT with intermediate differentiation. Analysis of normal human pineal gland by electron microscopy showed the presence of vesicle-crowned rodlets (VCR or synaptic ribbons), fibrous filaments (F), paired twisted filaments but few dense-core vesicles (DCV) in normal pinealocytes. Tumoral pineal cells appeared to differentiate either towards a neurosensory pathway characterized by the presence of sensory cells elements (VCR and F), or towards a neuroendocrine pathway, with the occurrence of many DCV. Immunogold labeling demonstrated the presence of chromogranin A in neurosecretory granules.

Fine structure of the free-living parakeet pineal in relation to the breeding cycle

Seasonal changes in the ultrastructure of the free-living Rose-Ringed Parakeet Psittacula krameri pineal were examined in relation to the sub-tropical environment and seasonal reproduction. Dark and light pinealocytes of the presumptive neuroendocrine cell line predominated, while supporting cells, ependymal cells, myelinated and non-myelinated nerve fibers with nerve endings, and regressed photoreceptor elements were also observed. Unlike in pineals of many animals, particularly mammals, the presence of dense-core vesicles (DCVs) with varying core density, and absence of clear vesicles and vacuoles with flocullent material, indicate the involvement of DCVs in the synthesis and secretion of pineals principle/s. In November (pre-breeding) when the day length registered a drop to LD 10:14, pinealocytes showed significantly decreased and smaller DCVs and mitochondria, nuclei with heterochromatin, and greater distribution of glycogen and lipid droplets, all indicating low pineal metabolic activity. During the shortest day regime from December to March, when the birds peaked breeding, the number and size of DCVs and mitochondria increased, and Golgi body-endoplasmic reticulum-lysosome complex (GERL) was very well defined. Images of DCVs suggested possible secretion of pineal principle/s by dissolution, and exocytosis. Coincidence of these features with peak gonadotrophic (circulating LH) and spermatogenic and testicular endocrine activity described previously suggested an active turnover of pineal products during this short day length regime when parakeets breed. In contrast, during the post-breeding season (April onwards), when the day-length increased to LD 13:11 and hypophyseal-gonadal function was down, nuclei and RER continued to show active profile, the Golgi body and associated complex were moderately seen, and the DCVs and mitochondria were significantly smaller and lesser. It is therefore probable that the pineal is an important relay to translate cues related to less drastic sub-tropical environmental change into DCV-linked neurohormones that in turn may be involved in modulating seasonal breeding in parakeets.