Kazumasa Kurosumi

Electron Microscopic Analysis of the Secretion Mechanism

Publisher Summary This chapter discusses the ultrastructure of secreting cells. “Secretion” usually means the action of cells by which a new substance is produced in the cell and then eliminated from it. Secretion is a very widespread phenomenon among various kinds of animal cells, not only in tissues specifically differentiated for this activity—namely, the glands-both exocrine and endocrine, but also in some of the mesenchymal and nervous tissues. A considerable number of investigations with an electron microscope have revealed many important facts on a number of aspects of the mechanism of secretion. In spite of its superior resolving power, the electron microscope has some unavoidable disadvantages for its biological application. These are the relative difficulty in preparing the specimen, the extreme narrowness of the observable field, the lack of the technique of selective staining, and the impossibility of observation on living material. Except the last, most of these defects may be overcome to some extent by the comparison of both images by electron and light microscopy with accompanying cytochemical tests. Moreover, biochemical assays using ultracentrifugation may aid the chemical identification of ultrastructural entities revealed by electron microscopy.

Electron microscope studies on the fine structures of the pars nervosa and pars intermedia, and their morphological interrelation in the normal rat hypophysis

Abstract Pars nervosa and pars intermedia in the hypophyses of the adult male albino rats fed in normal condition were observed with the electron microscope. In nerve terminals in the neural lobe, various steps of dissolution of neurosecretory substance could be observed. Neurosecretory axons may appear completely surrounded by the cytoplasm of a pituicyte. It is not certain whether the axon may completely pierce through the pituicyte or not. Similar invasion of axons with or without secretory granules occurs into the glandular (epithelial) cells of the pars intermedia. This finding may serve as ultrastructural evidence of one of the ways through which the neurosecretory substance may be transported from the neuro- to the adenohypophysis. The neurosecretory granules exclusively appear inside the axons, and no isolated granule can be observed in the pituicyte and intermedia cells, nor in the intercellular spaces in the both lobes. The secretion has apparently been dissolved prior to the discharge and after that it is no longer electron dense. Small numbers of dense granules, probably of secretion, were observed at the Golgi apparatus of the light cells of pars intermedia, suggesting secretory activities which have not been completely clarified at the present state of investigation. Ciliated cells could be seen at the marginal zone of the pars intermedia facing the residual lumen of Rathke. The arrangement of cilia is frequently irregular, but their ultrastructures are the same as those of kinocilia of ciliated epithelium hitherto reported.

The fine structure of mammary glands of lactating rats, with special reference to the apocrine secretion.

Abstract The mammary gland of lactating rats was observed with the electron microscope. The cytoplasm of glandular cells contains parallel-arranged cisternae of well-developed rough endoplasmic reticulum. The Golgi apparatus is also well developed and contains small dark secretion granules of a protein nature. In the basal cytoplasmic area small fat droplets occur in the vicinity of an accumulation of tubular or vesicular elements of smooth endoplasmic reticulum, which is thought to be correlated with the elaboration of fat droplets. Fat droplets push up the luminal surface membrane, and protrude into the lumen. The plasma membrane constricts the cytoplasm slightly below the fat droplet. Some proteinaceous secretion granules encased in vacuoles are contained in the cytoplasmic husk around the fat droplet. Therefore, the protein granules as well as fat droplets are released by the mechanism of apocrine secretion, in which a small amount of the cytoplasm and plasma membrane may be also extruded.

Electron microscopy of two types of gonadotrophs in the anterior pituitary glands of persistent estrous and diestrous rats

SummaryPersistent estrus and diestrus was produced in rats by the administration of estrone for either 5 days or 30 days, respectively, immediately after birth. Female rats without any treatment were used for control. After these rats grew up, the anterior pituitaries were examined by electron microscopy. The identification criteria for two types of gonadotrophs, FSH-and LH-cells, proposed by Barnes were adopted. In the persistent estrous rats, FSH-gonadotrophs were almost normal, but LH-gonadotrophs were filled with an abundance of secretory granules which were probably suppressed in discharge. On the other hand, in the persistent diestrous rats, FSH-cells were few in number and strongly atrophic, containing a few secretory granules, while LH-cells were almost normal or rather slightly activated. These electron microscopic findings well coincide with the results of light microscopy of ovaries, which suggested that in the persistent estrous rats FSH secretion might be almost normal but the secretion of LH might be inhibited, while in the persistent diestrous rats FSH secretion might be almost totally abolished but LH might be moderately secreted. From these findings, identification of FSH-and LH-gonadotrophs in the anterior pituitary of the rat well coincides with that proposed by Barnes in mice.

Cytology of the Secretion in Mammalian Sweat Glands

Publisher Summary This chapter explains the cytology of the secretion in mammalian sweat glands. The mammalian skin glands are repeatedly used as material for the study of cytology of secretion. Among the different kinds of skin glands, the sweat glands are widely distributed through almost all the species of mammals and vary remarkably in fine structure according to species. The human sweat glands are best differentiated as compared with those of nonhuman mammals. The secretory portion of the apocrine sweat glands is a glomerulum composed of secretory tubules whose cross diameters are different from place to place and often form strong dilatations. The secretory tubules sometimes send out a branch, which may end blindly or anastomose to the main tubule to form a ring. In the human skin, the apocrine sweat glands in the axillary region are best developed and have the most complicated glomerulum. The wall of the secretory tubule of the apocrine sweat glands consists of two different kinds of cells, the glandular secretory cells and the contractile myoepithelial cells. Another characteristic of the sweat glands is wide variations in structure and function, which are explained in the chapter.

Immunocytochemical Delineation of Thyrotrophic Area: Origin of Thyrotropin-Releasing Hormone in the Median Eminence

To identify the specific thyrotropin-releasing hormone (TRH)-containing neurons projecting to the median eminence (ME), a retrograde tracing method with horseradish peroxidase (HRP) was combined with immunocytochemical staining for TRH. Three days after HRP injection restricted to the ME, several TRH-positive neuronal perikarya were found to contain HRP. Such double-stained cells were exclusively distributed in the anterior parts of the periventricular nucleus and the most medial parts of the paraventricular nucleus. Few double-stained cells were observed in other parts of the brain examined. The present observations appear to demonstrate that the specific TRH neurons projecting to the ME are located along the border of the third ventricle, anterior to the ME.

Ultrastructural immunocytochemical localization of LH and FSH in the pituitary of the untreated male rat

SummaryRapid freeze-substitution fixation was employed in immunocytochemical studies on the localization of LH and FSH in the typical gonadotrophs of the anterior pituitary in the untreated male rat; a modification of a recently described ferritin antibody method (Inoue et al. 1982) was used in these studies. It was shown that rapid freeze-substitution fixation provides good preservation not only of the ultrastructure but also of the antigenicity. Both LH and FSH were clearly demonstrated in the same gonadotrophic cells, but the subcellular localization of these gonadotrophins differed: (i) LH was mainly located in small secretory granules, 250–300 nm in diameter; (ii) FSH was mainly present in large secretory granules, up to 500 nm in diameter. In the pituitary gland of the adult male rat, all gonadotrophs that react to antibodies against gonadotrophins are characterized by small and large secretory granules. Other types of cells of the anterior pituitary containing either small secretory granules or resembling corticotrophs with secretory granules assembled at cell periphery did not react to either anti-LH beta or anti-FSH beta serum.For light microscopy, the peroxidase antibody method was used. All of the gonadotrophin-positive cells contain both LH and FSH. None of the pituitary cells reacted to antibody against only one gonadotrophin. However, some cells are “LH-rich” while other cells are “FSH-rich”.

Differential subcellular localization of ACTH and α-MSH in corticotropes of the rat anterior pituitary

SummarySpecific antisera to α-melanotropin (α-MSH) and corticotropin (ACTH 1-39) were used to obtain immunocytochemical evidence for the differential localization of α-MSH and ACTH in the secretory granules of corticotropes of rat anterior pituitary. The specificity of the antisera was established by binding 131I-labeled α-MSH and ACTH 1-39 to their respective antisera. Double-labeling immunocytochemistry (for α-MSH, ferritin; for ACTH, colloidal gold) was performed. Some secretory granules were labeled with ferritin particles (α-MSH), whereas others contained gold particles (ACTH). Only a few granules showed both ACTH and α-MSH. In typical corticotropes (stellate in form with a small number of secretory granules aligned along the cell periphery) only some of the secretory granules that were labeled with anti-ACTH serum were also immunoreactive to anti-α-MSH. In atypical corticotropes (polygonal in shape and containing a large number of secretory granules) almost all of the immunoreactive ACTH secretory granules were also positive to anti-α-MSH serum. An intermediate type of corticotrope was observed containing a small number of secretory granules, almost all of which were labeled with anti-α-MSH. Thus, rat anterior pituitary corticotropes may be classified into three types according to the distribution and content of α-MSH. The light-microscopic immuncytochemistry provided similar results.

Origin of Septal Thyrotropin-Releasing Hormone in the Rat

Through the combined demonstration of retrograde transport of horseradish peroxidase (HRP) and immunocytochemical staining of thyrotropin-releasing hormone (TRH), the TRH neurons that project to the lateral septum were identified. Following the injection of HRP into the lateral septum, retrogradely labelled neurons were detected in the bed nucleus of stria terminalis (BNST), the ventral pallidum, the anterior commissural nucleus, the lateral preoptic nucleus and the perifornical region. Some neurons in the BNST and the perifornical region were found to contain both TRH and HRP. In contrast, no TRH-containing neurons were labelled with HRP in the paraventricular nucleus, the putative thyrotropic area. These data suggest that TRH neurons projecting to the lateral septum were different from those which send fibers to the median eminence.

Mitochondrial deformation and apocrine secretory mechanism in the rabbit submandibular organ as revealed by electron microscopy

SummaryThe submandibular organ (a sort of apocrine sweat glands) of the rabbit was observed with the electron microscope. The cell structure of glandular tubules varies depending upon the secretory activity; there are three functional stages. The secretory cells at the resting stage are characterized by low height, absence of secretory substance, and presence of small and slender mitochondria.In the synthesizing stage, enlargement and peculiar deformation of mitochondria are observed. Secretory substance always occurs near the deformed mitochondria. The part of a mitochondrion closely abutting on the secretion mass is extremely thin, and contains longitudinally oriented cristae. Sometimes a direct continuity is observed between the thinned portion of the deformed mitochondria and the mass of secretory substance. It is presumed that the secretion is initially produced in the mitochondria and then discharged from them. The Golgi apparatus and the rough surfaced endoplasmic reticulum may be involved indirectly. Smooth surfaced vesicles, probably related to the transport of raw material, are extremely abundant in the cells of this stage.The development of a generally homogeneous projection into the gland lumen is characteristic of the stage of secretion discharge. The mitochondria are again small and slender, and the secretion is liquefied. At the base of the full-grown projection, cytoplasm is condensed to form a demarcation zone from which the projection may become detached. This mechanism of release of secretory product is quite the same as the so-called apocrine secretory process long postulated by light microscopists.