The innervation of the pars intermedia of the pituitary gland in the toad

Summary The pars intermedia of the pituitary in Bufo arenarum Hensel is innervated by two types of fibers, both coming from the adjacent neurohypophysis: 1. The neurosecretory nerve endings, originating in the neurohypophysis and found along the limit of the pars intermedia with the neurohypophysis. 2. The common nerve endings, which penetrate the pars intermedia as a single bundle from the neurohypophysis and distribute throughout the gland.The present observations report morphological evidence that in the pars intermedia of the pituitary gland in the toad a double nervous control might be involved.     

The ultrastructure of the pars intermedia and the subdivision of its glandular cells in the young sheep pituitary

The ultrastructures of the pituitary pars intermedia of 11 male sheep about 1-13 months of age were observed. The following cells were distinguished in the sheep pars intermedia: pars intermedia glandular cells, marginal cells, interstitial cells, follicular cells, pars distalis-like granular cells, degenerating cells and mast cells. The pars intermedia glandular cells contain a well-developed Golgi apparatus and many mitochondria. The secretory granules had various densities and sizes (100-425 nm in diameter). No image of exocytosis was found. The pars intermedia glandular cells were classified into the following three subtypes: "PI-I, PI-II and PI-III type cells." PI-I type cells contain rather few secretory granules, but their rough endoplasmic reticulum is well developed in parallel arrays. PI-II type cells contain a few secretory granules, and the rough endoplasmic reticulum consists of scattered short strands. PI-III type cells contain abundant secretory granules of high density mixed with a few granules with low density. The rough endoplasmic reticulum is scattered strands. This type of cell was distinguished from granular cells like those of the pars distalis, although the granular cells also contained many dense granules. The marginal cells contacted the glandular cells directly, not being separated by a basal lamina. The degenerating cells were filled with myelinated lysosomes. Mast cells were often found in the interlobular connective tissue.     

Mitosis in pituitary MSH/endorphin cells of adult male rat pars intermedia: Light and electron microscopic observations

Small numbers of dividing cells were observed in the pituitary intermediate lobe of normal and p-chloroamphetamine-treated adult male rats. Ultrastructural examination reveals that the mitotic elements are granulated MSH/endorphin cells. Nerve fibers providing direct innervation could be found on dividing cells. The observations illustrate a limited potential for proliferation in the adult MSH/endorphin cell population.     

Characterization of calcium and sodium currents in porcine pars intermedia cells

The whole-cell configuration of the patch-clamp technique was applied to porcine pars intermedia cells. A tetrodotoxin-sensitive sodium current was recorded. Three types of calcium current were observed. Depolarizations from a holding potential of 100 mV elicited a transient current (ICaT), whereas depolarizations from a holding potential of -40 mV evoked a sustained current (ICaS). A third current (ICaN; N for neither) was activated by strong depolarization to +10 or +20 mV from holding potentials of - 100 mV. Increasing internal Ca2+ significantly reduced the amplitude of ICaS.     

Morphological, developmental and immunohistochemical observations on the opossum pituitary with emphasis on the pars intermedia.

Development of the pituitary in Didelphis follows the general pattern of that described for most mammalian species. The dorsal region of a multichambered pituitary vesicle, which forms from Rathke's pouch, comes to lie adjacent to the presumptive infundibulum by the 10 1/2 d of gestation. The epithelial wall of this vesicle consists of spindle-shaped cells. The dorsal wall of the upper chamber of the pituitary vesicle forms the pars intermedia; the ventral wall of this chamber gives rise to cells of the pars distalis. Corticotropes, somatotropes, and lactotropes are seen in the presumptive adenohypophysis of the 11 1/2 d embryo. Gonadotropes and thyrotropes appear about 1 d later, shortly after birth. By the 2 postnatal week, the adult distribution of all 5 cell types within the pars distalis appears to have been established. The wall bounding the pituitary cleft in the adult represents an epithelial continuum limited by a basal lamina and corresponds to the upper chamber of the original pituitary vesicle. Ultrastructurally, the limiting walls of the pituitary cleft consists of stellate (marginal) cells, large, bulbous cells, and granulated cells. The latter correspond to the various endocrine cell types normally associated with the pars distalis. Non-granular folliculo-stellate cells also are observed within the epithelial cords of the pars distalis.     

The pars intermedia of the hypophysis during salt loading

Summary Albino rats were subjected to the action of hypertonic NaCl solution for 20 days. Complete loss of neurosecretion in the pars posterior of hypophysis was noted. Basophilia of the cytoplasm of the majority of the pars intermedia cells increased exceedingly; the latter stained intensely with paraldehyde-fuchsin by Schiff-iodic acid method. Differentiation of intermedial cells into two types (differing structurally and tinctorially) was intensified. Formation of oxyphilic colloid (which is later resorbed) at the border of pars intermedia and anterior is characteristic of the early experimental periods; at the same time cytoplasmic basophilia of the cells of the first type in the pars intermedia is reduced, whereas by the end of the experiment it is seen to increase markedly. The changes observed demonstrate the relationship between the pars intermedia function and the humoral effects of the neurohypophysis.(Presented by Active Member AMN SSSR A. V. Lebedinskii) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 55,No. 5, pp. 108–111, May, 1963     

The effect of hypothalamic lesions on the melanocyte-stimulating hormone content and histology of the pars intermedia of the rat pituitary gland

1. The effect of bilateral lesions in the anterior hypothalamus on the histology and content of melanocyte-stimulating hormone (MSH) of the pars intermedia has been studied in the adult male rat.2. By 7-15 days striking histological changes, suggestive of hyperactivity, were evident. These changes were accompanied by an elevated MSH content, which was particularly noticeable at 7 days.3. By 7 days there was an increase in the acid phosphatase activity of pars intermedia cells, as demonstrated histochemically.4. It was concluded that a hypothalamic control system exists for the mammalian pars intermedia and may involve both inhibitory and excitatory components.     

Innervation of thyrotropin-releasing hormone-containing neurons in the bullfrog pars intermedia

We studied immunocytochemical localization of thyrotropin-releasing (TRH) in the bullfrog pats intermedia. Many TRH-like immunoreactive terminals containing immunoreactive granular vesicles 80-150 nm in diameter (116 nm mean diameter) were found in the pars intermedia. Some TRH-like immunoreactive terminals were in synaptic contact with the intermedia cells. These findings support the hypothesis that TRH directly activates the intermedia cells via synapses to secrete alpha-melanocyte-stimulating hormone.     

Advantages and limitations of the equine disease, pituitary pars intermedia dysfunction as a model of spontaneous dopaminergic neurodegenerative disease

Parkinson's disease (PD) is a leading cause of neurologic disability in the aged population. Remarkable progress has been made in the past decade to understand the cellular and molecular events that occur in PD. However attempts to unravel the early, initiating factors in the pathogenesis of dopaminergic neurodegeneration and PD have been limited by the lack of a suitable animal model. Models in which there has been genetic or environmental manipulation are not of use in determining the natural cause of a disease. While a large scale prospective human study would be ideal, the relatively low prevalence of PD makes this approach economically and logistically infeasible. Equine pituitary pars intermedia dysfunction (PPID) is a spontaneous, progressive neuroendocrine disease that commonly affects aged horses and ponies. PPID results from neurodegeneration of the dopaminergic periventricular neurons that innervate the intermediate lobe of the pituitary. PPID is 10-20 times more prevalent than PD and may be readily diagnosed without a need for advanced imaging technology. Although the diseases are anatomically distinct, recent evidence suggests the pathogenesis of dopaminergic neuronal damage in PPID may have significant similarities to that of PD. In this review, the similarities and differences in the pathology of neurodegeneration in PPID and PD are compared. The potential utility of the horse as a model of spontaneous dopaminergic neurodegeneration is discussed.     

Ultrastructure of the pars intermedia of pigmented rats: Observations of melanin-like granules within secretory cells

The pars intermedia of non-obese pigmented rats of the Zucker strain is described using transmission electron microscopy. Secretory cells appear similar to those described elsewhere in albino rodent hypophyses. Cells which may be equivalent to ACTH cells of non-pigmented mice and rats are observed near theborder of the pars distalis. Melanocytes are observed to surround lobules of the pars intermedia and to penetrate among secretory cells. In addition, large granules which appear to be melanin are present within some pars intermedia cells. Portions of some cells near melanin or melanin-like granules exhibit expanded rough endoplasmic reticulum and Golgi zones containing dense granules. Observations are discussed in relation to origin of the melanin, and its possible significance near cells known to produce melanocyte-stimulating hormone (MSH).     

The pars tuberalis of the human pituitary

Summary Forty autopsy pituitaries were studied to elucidate the histology, immunocytology and ultrastructure of pars tuberalis in subjects with normal and abnormal endocrine homeostasis. Pars tuberalis consisted mainly of gonadotrophs interspersed with few corticotrophs and thyrotrophs, histologically resembling those of pars distalis. Somatotrophs and lactotrophs were not identified. There were no histologic differences attributable to age or sex. In cases of glucocorticoid excess, pars tuberalis corticotrophs showed Crooke's hyalinization. Following castration or hypophysectomy, pars tuberalis gonadotrophs exhibited more intense immunostaining for FSH¹ and LH than did normals. Ultrastructural analysis revealed gonadotrophs and corticotrophs showing no evidence of active secretion; immunoelectron microscopy demonstrated FSH, LH and ACTH in secretory granules. By light microscopy, squamous nests, often identified in pars tuberalis, were positive for immunoreactive keratin; cells at their periphery contained FSH, LH or ACTH, indicating derivation of nests by squamous metaplasia from gonadotrophs and corticotrophs. By electron microscopy, clusters of epithelial cells containing desmosomes and tonofilaments were surrounded by granulated gonadotrophs.Human pars tuberalis cells represent mainly a subpopulation of gonadotrophs possessing all organelles required for synthesis and storage of hormones but showing ultrastructural features of functional inactivity; the reasons for this inactivity and for the formation of squamous nests remain unexplained.     

Small-conductance chloride channels activated by calcium on cultured endocrine cells from mammalian pars intermedia

Porcine intermediate lobe (IL) endocrine cells maintained in primary culture have been studied using patch-clamp derived configurations to record unitary activity on outside-out vesicles. Solutions were devised so as to record Cl current in isolation and to fix cytoplasmic Ca concentration [Ca]_i between 0.1 M and 3 M. Between [Ca]_i 0.5 and 1 M, the chloride permeability was restricted to single events with a small amplitude, that varied linearly with the membrane potential. Mean slope conductance of this chloride channel was 2.5 pS. Single channel analysis yielded two mean open time values of 10 and 55 ms at –80 mV. Relaxations of chloride currents on outside-out patches was examined at different [Ca]_i. Relaxation was negligible at 0.15 M [Ca]_i, whereas at higher [Ca]_i, the current exhibited relaxation in response to voltage jumps the kinetic of which could be fitted by two exponentials. At 0.5 M [Ca]_i, the fast relaxation time constant was shown to be voltage insensitive with a value of about 10 ms. The slow relaxation time constant had a mean value of 75 ms at –60 mV and increased with membrane depolarization with a twofold change over 120 mV. Another voltage effect was to favour the slow opening mode at the more depolarized potentials: the ratio of fast to slow relaxations being 5:1 at –60 mV as compared to 11 at +80 mV). Finally the estimated probability of opening (p _o) linearly increased with voltage.p _o displayed a bell-shaped dependence on [Ca]_i, so that full activation of the channels was not achieved.     

Extracellular spaces of the rat pars intermedia as outlined by lanthanum tracer

The extracellular spaces of the rat pars intermedia were examined after perfusion with fixative containing lanthanum. The tracer reveals complex and extensive interdigitations among secretory cells. Spaces appear continuous with extracellular “channels” between the epithelial cells lining the hypophyseal cleft. Some suggestion for close contiguity between endocrine cells was present as “narrowing” or discontinuous areas within lanthanum-filled spaces. Extracellular regions surrounding nerve fibers and terminals appear continuous with the spaces between secretory cells. Areas between cells are thought to provide access to vascular and local neurotransmitter input, as well as to provide an extensive area for extrusion of peptide hormones and endogeneous opiates.     

Proliferation and differentiation of rat anterior pituitary cells

Studies on the proliferation and differentiation of the cells in the rat anterior pituitary were reviewed. The mitotic rate of anterior pituitary is low in the control adult animal, but it increased by stimulation, such as by ablation of the target organ. A high mitotic rate was also reported during ontogenesis of the pituitary. Concomitant with this augmented mitosis, the number of those cells that are double-labeled with the marker of proliferation and the antibody to pituitary hormones increased as well. The percentage of these double-labeled cells in all the proliferating cells is less than 10%, suggesting that about 1/10 of the proliferating cells are involved in producing pituitary cells. This percentage for GH cells is 30-40% at most, suggesting very active production of them. The percentage of the double-labeled cell in all the hormone-producing cells is within 10% in all cell-types of the pituitary, including GH cells. When the proliferation is detected by a more sensitive method, this percentage increased to 20-40%, suggesting that the self-mitosis of the pituitary cells contributes considerably to their proliferation at a certain period during their ontogenesis.     

Effects of veratridine, tetrodotoxin and other drugs that alter electrical behaviour on secretion of melanocyte-stimulating hormone from melanotrophs of the pituitary pars intermedia

Since melanotrophs are electrically active and exhibit spontaneous Na spikes, a study was made of the effects, on melanotroph secretion, of drugs known to influence electrical properties. The output of melanocyte-stimulating hormone was measured from perifused neurointermediate lobes of mice or melanotrophs dispersed from such lobes of mice or rats. Veratridine (200 microM), which is known to increase Na permeability in a variety of cells, caused a large, although transient, increase in secretion from the melanotrophs that required extracellular Ca2+ and was blocked by the Na-channel blocker tetrodotoxin (1 microM). Tetraethylammonium (10 mM), which blocks K channels and thus prolongs the duration of the action potential in many cells, also stimulated secretion in the melanotrophs in a Ca-dependent manner. This response was not, however, blocked by tetrodotoxin, and is thus not attributable to prolongation of Na spikes in these cells. Moreover, tetrodotoxin did not inhibit basal secretion. The stimulant effect of veratridine on secretion in melanotrophs and its suppression by tetrodotoxin suggests that voltage-dependent Na channels can participate in the regulation of hormone output in these cells of the pituitary pars intermedia. However, the apparent lack of effect of tetrodotoxin on basal secretion suggests that the spontaneous Na spikes previously observed in these cells are not required for promoting the Ca influx which other evidence shows is important for basal secretion.