Pedro Amat

Hypothalamic tanycytes: a key component of brain-endocrine interaction

Tanycytes are bipolar cells bridging the cerebrospinal fluid (CSF) to the portal capillaries and may link the CSF to neuroendocrine events. During the perinatal period a subpopulation of radial glial cells differentiates into tanycytes, a cell lineage sharing some properties with astrocytes and the radial glia, but displaying unique and distinct morphological, molecular, and functional characteristics. Four populations of tanycytes, alpha(1,2) and beta(1,2), can be distinguished. These subtypes express differentially important functional molecules, such as glucose and glutamate transporters; a series of receptors for neuropeptide and peripheral hormones; secretory molecules such as transforming growth factors, prostaglandin E(2), and the specific protein P85; and proteins of the endocytic pathways. This results in functional differences between the four subtypes of tanycytes. Thus, alpha(1,2) tanycytes do not have barrier properties, whereas beta(1,2) tanycytes do. Different types of tanycytes use different mechanisms to internalize and transport cargo molecules; compounds internalized via a clathrin-dependent endocytosis would only enter tanycytes from the CSF. There are also differences in the neuron-tanycyte relationships; beta(1,2) tanycytes are innervated by peptidergic and aminergic neurons, but alpha(1,2) tanycytes are not. Important aspects of the neuron-beta(1) tanycyte relationships have been elucidated. Tanycytes can participate in the release of gonadotropin-releasing hormone (GnRH) to the portal blood by expressing estrogen receptors, absorbing molecules from the CSF, and providing signal(s) to the GnRH neurons. Removal of tanycytes prevents the pulse of GnRH release into the portal blood, the peak of luteinizing hormone, and ovulation. The discovery in tanycytes of new functional molecules is opening a new field of research. Thus, thyroxine deiodinase type II, an enzyme generating triiodothyronine (T(3)) from thyroxine, appears to be exclusively expressed by tanycytes, suggesting that these cells are the main source of brain T(3). Glucose transporter-2 (GLUT-2), a low-affinity transporter of glucose and fructose, and ATP-sensitive K(+) channels are expressed by tanycytes, suggesting that they may sense CSF glucose concentrations.

A second look at the barriers of the medial basal hypothalamus

Abstract. The cell bodies of hypothalamic secretory neurons are localized in areas protected by the blood-brain barrier (BBB), whereas their axon terminals are localized in the median eminence, which lacks a BBB. This implies a complex barrier system, allowing neurons of the central nervous system to secrete into the blood stream without making the BBB leaky. In the present study, three experimental protocols were applied to clarify certain relevant aspects of the barriers operating in the medial basal hypothalamus of the rat. We established that the milieu of the arcuate nucleus is exposed to both the ventricular and the subarachnoidal cerebrospinal fluid (CSF).The median eminence milieu, the perivascular space of the portal vessels, and the subarachnoid space appear to be in open communication; also, β2-tanycytes establish an efficient barrier between the median eminence milieu and the ventricular CSF. Similarly, β1-tanycytes establish a lateral barrier, separating the intercellular space of the median eminence from that of the arcuate nucleus. We also found that the glucose transporter I (GLUT I), a BBB marker, is localized throughout the whole plasma membrane of β1-tanycytes, but is missing from β2-tanycytes. Expression of GLUT I by tanycytes progressively develops during the first postnatal weeks; while the degree of damage of the arcuate nucleus by administration of monosodium glutamate, at different postnatal intervals, parallels that of the GLUT I immunoreactivity of β1-tanycytes. An explanation is offered for the selective destruction of the arcuate neurons by the parenteral administration of monosodium glutamate to infant rats.

A quantitative morphological study of human Leydig cells from birth to adulthood

SummaryHuman testicular specimens were obtained from biopsies and autopsies covering the period from birth to adulthood. The number of testosterone-containing Leydig cells was determined using the peroxidase-anti-peroxidase method. This number decreased markedly from 3–6 months of age to the end of the first year of life and, up to 6 years of age, only a small number of testosterone-containing cells was found. From 6 years onwards the number of Leydig cells progressively increased. Ultrastructural examination revealed four types of Leydig cells: (1) fetal-type Leydig cells (from birth to 1 year of age) with round nuclei, abundant smooth endoplasmic reticulum and mitochondria with tubular cristae; (2) infantile-type Leydig cells (from birth to 8–10 years of age), showing a multilobated nucleus, moderately abundant smooth endoplasmic reticulum, some lipid droplets and mitochondria with parallel cristae; (3) prepubertal, partially differentiated Leydig cells (from 6 years of age onwards) with regularly-outlined round nuclei, abundant smooth endoplasmic reticulum, mitochondria with tubular cristae, and some lipid droplets and lipofuscin granules; and (4) mature adult Leydig cells (from 8–10 years of age onwards). The ultrastructure of the infantile-type Leydig cells and the lack of delay between the disappearance of the fetal-type Leydig cells and the appearance of infantile-type Leydig cells suggest that fetal-type Leydig cells give rise to the infantile-type Leydig cells. Before puberty, myofibroblast-like precursor cells differentiate into the prepubertal, partially differentiated Leydig cells, which complete their differentiation into the adult Leydig cells.

Polarized endocytosis and transcytosis in the hypothalamic tanycytes of the rat

Four types of tanycytes can be distinguished in the rat hypothalamus: α1 and α2 tanycytes establish an anatomical link between the ventricular cerebrospinal fluid (CSF) and the arcuate nucleus, whereas β1 and β2 tanycytes establish a link between CSF and portal blood. Endocytosis and transcytosis in these cells have been investigated by (1) immunocytochemistry with antibodies against molecular markers of the endocytotic and transcytotic pathways; (2) the administration of wheat germ agglutinin (WGA) into the ventricular or subarachnoidal CSF and following its internalisation by and its routing through tanycytes. The four populations of tanycytes show marked differences concerning the expression and subcellular location of proteins involved in endocytosis and transcytosis, such as clathrin, caveolin-1, Rab4 and ARF6. Thus, β1,2 tanycytes express caveolin-1 at the ventricular cell pole and at their terminals contacting the portal capillaries, whereas α1,2 tanycytes do not, suggesting that caveolae-dependant endocytosis does not occur in the latter and that, in β1,2 tanycytes, it may occur at both cell poles. In β1,2 tanycytes, clathrin is only expressed at the ventricular cell pole indicating that clathrin-dependant endocytosis operates for compounds present in the ventricular CSF and not for those exposed to the terminals. This agrees with the property of β1,2 tanycytes of internalising WGA through the ventricular cell pole but not through the terminals. The subcellular distribution in β1,2 tanycytes of WGA and of the proteins clathrin and Rab4 indicates that part of the internalised WGA follows the degradative pathway and part is sorted to a transcytotic pathway and that the transcytotic and the secretory pathways might intersect.

Testicular involution in elderly men: comparison of histologic quantitative studies with hormone patterns *

An endocrinologic and quantitative histologic study was carried out in 64 elderly men who underwent orchidectomy owing to prostatic carcinoma. The men were classified into age groups (decade of life), and each group was subdivided into group A (testes with complete spermatogenesis in most tubules) and group B (testes showing maturation arrest of spermatogenesis in most tubules). Up to 80 years of age, men of group A showed hormone levels and testicular parameters similar to those of young control men. From 50 to 60 years of age, men of group B showed a significant decrease in testicular volume, tubular volume, tubular length, number of germ cells, Sertoli cells and Leydig cells per testis, and plasma testosterone levels, whereas the tunica propria thickness and plasma levels of both follicle-stimulating hormone and luteinizing hormone were increased.

Ultrastructural observations on nucleoli and related structures during human spermatogenesis

SummaryThe ultrastructural study of nucleoli and ribonucleoprotein-containing structures in human seminiferous tubules revealed that the nucleoli of spermatogonia, spermatocytes and Sertoli cells exhibited a tripartite structure consisting of: (1) a fibrillar center, (2) a compact granular portion, and (3) a reticular portion containing both pars fibrosa and pars granulosa. The nucleoli of primary spermatocytes showed a developed reticular portion. At pachytene, the compact granular portion enlarged and lost its connection with the fibrillar center and the reticular portion which decreased in size. This suggests a nucleolar segregation similar to that of ovocytes in many species. Two similar developmental stages of nucleoli were observed in spermatogonia. In addition to nucleoli, there were other ribonucleoprotein-containing structures such as intranuclear closely-packed granules in Ap spermatogonia, coarse granules in the chromatin rarefaction zone of Ad spermatogonia, the nuage and Lubarsch crystals of spermatogonia, the chromatoid body of spermatids, the annulate lamellae of both spermatids and Sertoli cells, and many structures of the spermatid neck region.

Mitosis in adult human Leydig cells

SummaryThe ultrastructural study of testicular biopsies from 87 adult men revealed mitosis in two mature Leydig cells, each from a different man. The men showed normal hormone levels and had received no previous chemotherapy or hormone treatment, nor had they been exposed to known toxic agents. The presence of mitotic Leydig cells suggests that differentiated Leydig cells may divide and contribute either to the increase in the number of Leydig cells or to the formation of multinucleate Leydig cells.

Changes in surface area and number of Leydig cells in relation to the 6 stages of the cycle of the human seminiferous epithelium

SummaryIn order to evaluate the occurrence of a Leydig cell cycle related to the cycle of the seminiferous epithelium in man, the numbers of peritubular Leydig cells and surface area of these cells along 1 mm of tubular basement membrane at each stage of the cycle were calculated on histological sections of young adult testes. The Leydig cells that were located separated from the tubules (perivascular Leydig cells) were also classified according to the stage of the cycle shown by the nearest seminiferous tubule; the surface area and number of these cells were also calculated. The total surface area and numbers of Leydig cells (peritubular plus perivascular) along 1 mm of tubular basement membrane did not change during the cycle of the seminiferous epithelium. Both the surface area and the numbers of peritubular Leydig cells were greater in stages I and II of the cycle, when spermatozoa are released; they decreased in stages III and IV and increased again in stages V and VI, whereas the contrary occurred in perivascular Leydig cells. The average surface area of each Leydig cell type remained constant throughout the stages of the cycle.

The ultrastructure of the herring bodies in rats subjected to different experimental conditions

SummaryA study of the morphology of Herring bodies of the posterior pituitary lobe of rats treated with colchicine and/or exposed to low temperatures has been performed. After treatment with colchicine (20 μg in distilled water injected intracisternally) a predominance of Herring bodies with a large number of small “synaptic-like” vesicles surrounded by neurosecretory granules is found. Exposures to low temperature (4–6° C) result in an increase in the neurosecretory material and the Herring bodies show many neurosecretory granules of different densities. After treatment with colchicine and subsequent exposure to low temperatures, the Herring bodies are characterized by having a great number of autophagic bodies which become more numerous as the length of the exposure is increased; later autophagic vacuoles and lamellar bodies become evident.