The milk gland of the sheep ked, Melophagus ovinus: A comparison with Glossina

Structurally, the milk gland of the sheep ked, Melophagus ovinus, is quite similar to that of the tsetse fly, Glossina morsitans. In both insects the highly branched gland consists of two cell layers. An extracellular reservoir associated with each secretory cell initially receives the secreted milk. Milk then passes into the gland lumen through a dense cuticular rete. Gram-negative bacteria, presumably symbionts, are abundant in the lumen. Unlike tsetse, the secretory reservoir of the sheep ked is bi-lobed, and the secretory cell nucleus remains centrally located throughout the pregnancy cycle. Lipid droplets are much more abundant in the cytoplasm of the ked secretory cell, and analysis of larval milk shows 5–6% higher lipid content in the sheep ked. Results of histochemical analysis of ked milk are consistent with the analysis of tsetse milk. Four major milk proteins are detectable with polyacrylamide gel electrophoresis. Changes in abundance of ER and sizes of the secretory cell nucleus and secretory reservoir reflect a dramatic cycle of glandular activity during pregnancy. Unlike tsetse, the sheep ked remains constantly on its host and appears to take frequent, but small, blood meals. This strategy implies that the demand for nutrient storage is less than in tsetse.     

Reciprocity in the Developmental Regulation of Aquaporins 1, 3 and 5 during Pregnancy and Lactation in the Rat

Milk secretion involves significant flux of water, driven largely by synthesis of lactose within the Golgi apparatus. It has not been determined whether this flux is simply a passive consequence of the osmotic potential between cytosol and Golgi, or whether it involves regulated flow. Aquaporins (AQPs) are membrane water channels that regulate water flux. AQP1, AQP3 and AQP5 have previously been detected in mammary tissue, but evidence of developmental regulation (altered expression according to the developmental and physiological state of the mammary gland) is lacking and their cellular/subcellular location is not well understood. In this paper we present evidence of developmental regulation of all three of these AQPs. Further, there was evidence of reciprocity since expression of the rather abundant AQP3 and less abundant AQP1 increased significantly from pregnancy into lactation, whereas expression of the least abundant AQP5 decreased. It would be tempting to suggest that AQP3 and AQP1 are involved in the secretion of water into milk. Paradoxically, however, it was AQP5 that demonstrated most evidence of expression located at the apical (secretory) membrane. The possibility is discussed that AQP5 is synthesized during pregnancy as a stable protein that functions to regulate water secretion during lactation. AQP3 was identified primarily at the basal and lateral membranes of the secretory cells, suggesting a possible involvement in regulated uptake of water and glycerol. AQP1 was identified primarily at the capillary and secretory cell cytoplasmic level and may again be more concerned with uptake and hence milk synthesis, rather than secretion. The fact that expression was developmentally regulated supports, but does not prove, a regulatory involvement of AQPs in water flux through the milk secretory cell.     

Proteomic analysis of two functional states of the Golgi complex in mammary epithelial cells

Organellar compartments involved in secretion are expanded during the transition from late pregnancy (basal secretory state) to lactation (maximal secretory state) to accommodate for the increased secretory function required for copious milk production in mammary epithelial cells. The Golgi complex is a major organelle of the secretory pathway and functions to sort, package, distribute, and post-translationally modify newly synthesized proteins and membrane lipids. These complex functions of the Golgi are reflected in the protein complement of the organelle. Therefore, using proteomics, the protein complements of Golgi fractions isolated at two functional states (basal and maximal) were compared to identify some of the molecular changes that occur during this transition. This global analysis has revealed that only a subset of the total proteins is up-regulated from steady state during the transition. Identification of these proteins by tandem mass spectrometry has revealed several classes of proteins involved in the regulation of membrane fusion and secretion. This first installment of the functional proteomic analysis of the Golgi complex begins to define the molecular basis for the transition from basal to maximal secretion.     

Secretory membranes of the lactating mammary gland

Summary The lactating mammary gland is one of the most highly differentiated and metabolically active organs in the body. Membranes of the lactating mammary cell have important roles in transmitting from one membrane to another of hormonal information and in milk secretion, which is the final event. During milk secretion, the projection of the surface membrane into the alveolar lumen by enveloping intracellular lipid droplets with the apical plasma membrane is one of the most remarkable aspects of biological membrane action throughout nature.This review focuses on current knowledge about membranes in the lactating mammary gland. (1) Advances in the isolation and properties of membranes, especially the plasma membrane and Golgi-derived secretory vesicles, concerned with milk secretion from the lactating mammary gland are described. (2) Milk serum components are secreted by fusing the membranes of secretory vesicles that condense milk secretions with the plasma membrane in the apical regions. This occurs through the formation of a tubular-shaped projection and vesicular depression in a ball-and-socket configuration, as well as by simple fusion. (3) Intracellular lipid droplets are directly extruded from the mammary epithelial cells by progressive envelopment of the plasma membranes in the apical regions. (4) The balance between the surface volume lost in enveloping lipid droplets and that provided by fusion of the secretory vesicle and other vesicles with the apical plasma membrane is discussed. (5) The membrane surrounding a milk fat globule, which is referred to as the milk fat globule membrane (MFGM), is composed of at least the coating membrane of an intracellular lipid droplet, of the apical plasma membrane and secretory vesicle membrane, and of a coat material. Consequently, MFGM is molecularly different from the plasma membrane in composition. (6) MFGM of bovine milk is structurally composed of an inner coating membrane and outer plasma membrane just after segregation. These two membranes are fused and reorganized through a process of vesiculation and fragmentation to stabilize the fat globules. Hypothetical structural models for MFGM from bovine milk fat globules just after secretion and after rearrangement are proposed.Abbrevations MFGM milk fat globule membrane - HEPES N-2-hydroxylpiperazine-N-2-ethanesulfonic acid - INT 2-(p-indophenyl)-3-(p-nitrophenyl)-5-phenyltetrazolium - SDS-PAGE polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate - Sph sphingomyelin - PC phosphatidyl choline - PE phosphatidyl ethanolamine - PS phosphatidyl serine - PI phosphatidyl inositol - PAS periodic acid-Schiff reagent - CB Coomassie brilliant blue R-250 Dedicated to Professor Stuart Patton on the occasion of his 70th birthday.     

Development of female accessory glands of Chrysomya putoria (Wiedemann) (Diptera: Calliphoridae) during oogenesis

Females of Chrysomya putoria (Diptera: Calliphoridae) have two sexual accessory glands, which are tubular and more dilated at the distal extremity. The glands open independently into the common oviduct. Two morpho-physiological regions were distinguished in the longitudinal semi-thin sections of the glands. The secretory region is constituted by three layers: a cuticular intima, lining the lumen, followed by a layer of small cells, and then a layer of very large secretory cells. The ductal region of the gland presents only two layers: the cuticular intima and a cellular layer. In both regions a basement membrane is present. Each secretory cell has in its apical region a reservoir, which enlarges throughout oogenesis; in its basal region there is a large nucleus. The ductal cells are cylindrical and smaller than the secretory cells. The glandular secretion is synthesized in the cytoplasm of the secretory cells, stored and/or modified in the reservoir, then drained to the lumen through an end apparatus seen in the apical region of the secretory cell. Histochemical tests indicate that this secretion is a glycoprotein. Measurements of the glands from females at different physiological conditions and fed on different diets correlate with the results obtained for changes in the ovary during oogenesis. Cell number averaged 561.2 ± 77.54 per gland. There was no increase in cell number during oogenesis.     

Nutrient transfer during the reproductive cycle in Glossina austeni Newst.: histology and histochemistry of the milk gland, fat body, and oenocytes

Using thick sections cut from Epon-embedded tissues fixed in Karnovsky's fixative, the cytological changes visible in the light microscope were described for milk gland, fat body and oenocytes during the pregnancy cycle of the female of Glossina austeni. Histochemical procedures on paraffin sections were used to explore changes in nucleic acids, proteins, and tyrosine-containing proteins. The milk gland undergoes a cycle of secretion correlated with the pregnancy cycle and the results are consistent with the view that the protein component of the milk is synthesized in the secretioy cells of the gland. Tyrosine is particularly abundant in the oenocytes which appear to play a crucial role in the metabolism of this amino acid.     

Milk lipid and protein traffic in mammary epithelial cells: joint and independent pathways

In mammary epithelial cells, milk lipids and proteins are synthesised in the same compartment, the endoplasmic reticulum. Lipids, carried through the cytoplasm, associate with the apical membrane which then pinches off and releases the lipid globule. Proteins, carried through membrane compartments are released in the lumen after fusion of secretory vesicles with the apical membrane. These processes assure a relatively constant composition of milk but it is not known whether lipid and protein secretion are linked. The protein composition of the milk fat globule membrane and the stimulatory effects of prolactin and oxytocin on lipid and protein secretion suggest that these processes are coupled and co-regulated. However, it is possible to observe a dissociation between the formation and the secretion of the two constituents, during differentiation and in various experimental conditions, and this suggests that coupling is not strictly required.     

Changes in the secretory activity of the Golgi apparatus during the cell cycle in root tips of maize (Zea Mays L.)

Epidermal cells of maize roots were studied to determine the distribution of Golgi apparatus-derived secretory vesicles in various stages of cell division. The following conclusions were reached: 1) The pattern of Golgi apparatus secretion varies with the cell cycle. 2) Large numbers of secretory vesicles are incorporated into the cell plate. 3) Secretory vesicles from the Golgi apparatus are incorporated primarily in walls undergoing expansion. 4) Secretory vesicles are smaller during mitosis and the first part of cytokinesis than they are during interphase. 5) Secretory vesicles account for at least 12–23% of cell-plate plasma membrane and an estimated 25% of cell-plate volume.     

Functional differentiation and alveolar morphogenesis of primary mammary cultures on reconstituted basement membrane

An essential feature of mammary gland differentiation during pregnancy is the formation of alveoli composed of polarized epithelial cells, which, under the influence of lactogenic hormones, secrete vectorially and sequester milk proteins. Previous culture studies have described either organization of cells polarized towards lumina containing little or no demonstrable tissue-specific protein, or establishment of functional secretory cells exhibiting little or no glandular architecture. In this paper, we report that tissue-specific vectorial secretion coincides with the formation of functional alveoli-like structures by primary mammary epithelial cells cultured on a reconstituted basement membrane matrix (derived from Engelbreth-Holm-Swarm murine tumour). Morphogenesis of these unique three-dimensional structures was initiated by cell-directed remodelling of the exogenous matrix leading to reorganization of cells into matrix-ensheathed aggregates by 24 h after plating. The aggregates subsequently cavitated, so that by day 6 the cells were organized into hollow spheres in which apical cell surfaces faced lumina sealed by tight junctions and basal surfaces were surrounded by a distinct basal lamina. The profiles of proteins secreted into the apical (luminal) and basal (medium) compartments indicated that these alveoli-like structures were capable of an appreciable amount of vectorial secretion. Immunoprecipitation with a broad spectrum milk antiserum showed that more than 80% of caseins were secreted into the lumina, whereas iron-binding proteins (both lactoferrin and transferrin) were present in comparable amounts in each compartment. Thus, these mammary cells established protein targeting pathways directing milk-specific proteins to the luminal compartment. A time course monitoring secretory activity demonstrated that establishment of tissue-specific vectorial secretion and increased total and milk protein secretion coincided with functional alveolar-like multicellular architecture. This culture system is unique among models of epithelial cell polarity in that it demonstrates several aspects of epithelial cell polarization: vectorial secretion, apical junctions, a sequestered compartment and formation of a basal lamina. These lumina-containing structures therefore reproduce the dual role of mammary epithelia to secrete vectorially and to sequester milk proteins. Thus, in addition to maintaining tissue-specific cytodifferentiation and function, a basement membrane promotes the expression of tissue-like morphogenesis.     

Compound exocytosis of casein micelles in mammary epithelial cells

Ultrastructure of lactating bovine and rat mammary epithelial cells was studied with emphasis on secretory vesicle interactions. In the apical zone of the cell, adjacent secretory vesicles formed ball and socket configurations at their points of apposition. Similar configurations were formed between plasma membrane and secretory vesicle membrane. These structures may be formed by the diffusion of water between vesicles with different osmotic potentials. Frequently, vesicular chains consisting of 10 or more linked secretory vesicles were observed. Prior to the exocytotic release of casein micelles, adjacent vesicles fused through fragmentation of the ball and socket membrane. These membrane fragments and the casein micelles appeared to be secreted into the alveolar lumen after passing from one vesicle into another and finally through a pore in the apical plasma membrane. Emptied vesicular chains appeared to collapse and fragmentation of their membrane was observed. Based on these observations, we suggest that most vesicular membrane does not directly contact or become incorporated into the plasma membrane during secretion of the nonfat phase of milk.     

A comparative study of the chemical defensive system of tenebrionid beetles III. Morphology of the glands

The morphology of the abdominal defensive glands and associated structures of 115 species of tenebrionid beetles was studied on KOH cleared material. The glands and reservoirs of all Tenebrionidae are homologous and evolved as a pair of sacs from the intersegmental membrane between sternites VII and VIII. On the basis of reservoir morphology and secretory cell tubule termination, seven provisional gland types were established. Several of the types include species from several tribes, and several tribes contain several gland types, indicating possible incongruencies between the taxonomy and phylogeny of the family. Morphological trends in the evolution of the glands include: increase of reservoir capacity, constriction of the proximal portion of the sacs into distinct exit ducts, release of secretion by exuding or spraying rather than everting, and concentration of the secretory cell tubule terminations into restricted fields, collecting ducts or ampullae. The morphology of the glands of 58 species is illustrated and the results are discussed in light of the current taxonomy of the Tenebrionidae.     

Ultrastructural changes of the teleostean hatching gland cell during natural and electrically induced precocious secretion.

Ultrastructural changes of the hatching gland during electrically induced precocious secretion were compared with those during natural secretion in the medaka, Oryzias latipes. The gland cells are covered by a layer of epithelial cells, which adjoin one another just on the apical center of each gland cell. When the natural as well as the precocious secretion occurred, each gland cell was swollen upward and rounded, and separation of the epithelial joints occurred, giving rise to an exposure of the apical portion of the gland cells. There were marked differences between these two kinds of secretion process in the behavior of the secretory granules prior to secretion and in the mode of discharge of the secretory substances. The changes which occurred during both types of secretion and which, therefore, seemed to be essential to the secretory processes of this gland cell were the swelling up of the gland cells in the initiation of secretion and the reduction of the electron density of the zymogen granules. These secretion-associated ultrastructural changes are discussed in view of the difference in the maturation of the gland cells.     

Apocrine secretory mechanism: recent findings and unresolved problems

Cell secretion is an important physiological process that ensures smooth metabolic activities, tissue repair and growth and immunological functions in the body. It occurs when the intracellular secretory materials are released to the exterior; these may be in the form of lipids, protein or mucous and may travel through a duct system or via blood to reach the target organ. To date three types of secretory mechanisms have been characterized, they include apocrine, holocrine and exocytosis. Apocrine secretion occurs when the release of secretory materials is accompanied with loss of part of cytoplasm. The secretory materials may be contained in the secretory vesicles or dissolved in the cytoplasm that is lost during secretion. In holocrine secretion, the entire cell is secreted into the glandular lumen, and it is presumed that the intended secretory materials are contained in the cell cytoplasm. Exocytosis is the most commonly occurring type of secretion; here the secretory materials are contained in the secretory vesicles and released without loss of cytoplasm. Apocrine secretory mechanisms have not been properly discussed; for example the biochemical and physiological pathways that regulate apocrine secretory process are not clearly known. Similarly, the plasma membrane dynamics during apocrine secretion has not been researched. In other glands morphological features during apocrine secretion have not been documented. The current paper reviews what is known about apocrine secretion, recent findings and highlights on the unresolved areas for future research.     

Secretory activity of gonadotropin and the responsiveness of gonadotrophs to gonadotropin-releasing hormone during the annual reproductive cycle of male bats, Rhinolophus ferrumequinum: analysis by cell immunoblot assay

The purpose of this study was to examine secretory activity of gonadotropin (Gn) and the responsiveness of Gn secretion to Gn-releasing hormone (GnRH) in male horseshoe bats, Rhinolophus ferrumequinum, during the annual reproductive cycle. Anterior pituitary cells were monodispersed and subjected to cell immunoblot assay for Gn. Cell blots specific for follicle stimulating hormone (FSH) or luteinizing hormone (LH) were quantified using a microscopic image analyzer. The percentages of LH- or FSH-secreting cells detected as immunoreactive cell blots were markedly increased in the spermatogenic period (summer) and decreased in the hibernation period (winter). The mean Gn secretion from individual cells and total Gn secretion per unit area of the transfer membrane also showed similar changes. The responsiveness of Gn secretion to GnRH was greater in the spermatogenic period than in other seasons. On the other hand, although the secretory activity of Gn was markedly decreased during hibernation, a stimulatory effect of GnRH on Gn secretion was observed. These findings suggest that seasonal changes in the release of Gn required for gametogenesis and gonadal steroidogenesis varied depending on the reproductive activity and seasonal changes in Gn sensitivity to stimulatory effects of GnRH due to alterations in GnRH receptor numbers and/or in postreceptor events of gonadotrophs.     

Daily changes of neurosecretory type-II cell structure of Pieris larvae entrained by short and long days

The neurosecretory type-II cell (NS-II cell) group of each brain hemisphere consists of three kinds of cells: two small cells, six large ones, and two others having characteristic vacuolated endoplasmic reticulum (ER).Ultrastructural changes of large NS-II cells were observed through the fifth instar and diurnally when short-day and long-day larvae were compared. There were little differences between short-day and long-day larvae in cell structures on corresponding developmental days except for daily changes, but remarkable changes were observed every day through the instar. A secretory cycle through the instar was supposed being based on the ultrastructural changes in NS-II cells: reduced secretory activity on the first day, formation of organelles necessary for the synthesis of secretory materials throughout the instar on the second day, active synthesis and secretion of secretory material during the middle stage (third-fourth day), and reversion to a reduced level of cell activity after the cessation of feeding.In short-day larvae on the third to fourth day, NS-II cells contained large aggregates of secretory granules during the day except for the time of 13 hr after the onset of photophase when a decrease of secretory granules occurred. In long-day larvae, only a small amount of secretory granules was observed at 8 and 13 hr after the onset. Rough ER changed daily paralleling with the quantitative change of the secretory granules.Based on these differences of daily changes in NS-II cell activity between short-day and long-day larvae, it was concluded that photoperiodic time measurement of diapause induction depends on the daily secretory cycle entrained by the photoperiods during the larval stage.     

The terpene-producing glands of a Phasmid insect. Cell morphology and histochemistry

The defensive glands of Anisomorpha buprestoides produce the terpene toxicant anisomorphal. Each gland consists of a cuticular secretion reservoir surrounded by the secretory epithelium and the musculature which serves to compress the gland and expel the secretion. Two types of cells make up the secretory epithelium: a squamous layer next to the cuticular reservoir and a layer of larger secretory cells responsible for production of the toxicant. The microvilli-laden plasma membrane of each secretory cell is invaginated to form a central cavity. It appears that secretory products pass into the central cavity and then flow out to the gland reservoir via an efferent cuticular ductule contained within the squamous epithelial cell. Histochemical techniques demonstrate lipid reserves, carboxylic esterases, a variety of phosphatases, and an alcohol dehydrogenase, within the secretory cells. It is suggested that the lipid reserves are precursors of the terpenoid toxicant, that a mevalonic kinase has been histochemically demonstrated by the phosphatase test, and that an unusual alcohol dehydrogenase is active in the final steps of toxicant synthesis. The histochemical evidence is consistent with the hypothesis that anisomorphal is produced via the mevalonic acid pathway.     

Identification of Genes Controlling Growth Polarity in the Budding Yeast Saccharomyces Cerevisiae: A Possible Role of N-Glycosylation and Involvement of the Exocyst Complex

The regulation of secretion polarity and cell surface growth during the cell cycle is critical for proper morphogenesis and viability of Saccharomyces cerevisiae. A shift from isotropic cell surface growth to polarized growth is necessary for bud emergence and a repolarization of secretion to the bud neck is necessary for cell separation. Although alterations in the actin cytoskeleton have been implicated in these changes in secretion polarity, clearly other cellular systems involved in secretion are likely to be targets of cell cycle regulation. To investigate mechanisms coupling cell cycle progression to changes in secretion polarity in parallel with and downstream of regulation of actin polarization, we implemented a screen for mutants defective specifically in polarized growth but with normal actin cytoskeleton structure. These mutants fell into three classes: those partially defective in N-glycosylation, those linked to specific defects in the exocyst, and a third class neither defective in glycosylation nor linked to the exocyst. These results raise the possibility that changes in N-linked glycosylation may be involved in a signal linking cell cycle progression and secretion polarity and that the exocyst may have regulatory functions in coupling the secretory machinery to the polarized actin cytoskeleton.     

Cell secretion: an update

This past decade has witnessed the publication of a flurry of scientific papers and reports on the subject of cell secretion, following discovery of a permanent plasma membrane structure termed 'porosome' and its determination as the universal secretory machinery in cells. This discovery has led to a paradigm shift in our understanding of the secretory process, demonstrating that membrane-bound secretory vesicles transiently dock and fuse at the porosome base to release their contents to the cell exterior. The regulated release of intravesicular contents during cell secretion is governed by dilation of the porosome opening to the outside, and the extent of vesicle swelling. In agreement, a great number of articles have been written and studies performed, which are briefly discussed in this article.     

The effects of colchicine on milk secretion, mammary metabolism and blood flow in the goat

Mammary function in the conscious goat was studied during colchicine-induced depression of milk secretion in one mammary gland. Milk yield of the treated gland was reduced to approximately a quarter of previous, while there were significant increases in afternoon milk yield from the untreated glands on the 2nd and 3rd days after treatment in goats in late lactation. Milk composition in the untreated glands was not significantly affected. In the treated gland, milk [Na+], [Cl-], [citrate] and [protein] increased while [K+] and [lactose] decreased, although the time course of these changes differed; milk [fat] was unaffected. Mammary extractions ((A-V)/A) of glucose, acetate and most amino acids were significantly decreased during the period of maximal inhibition of secretion. There were no significant changes in arterial plasma concentrations of glucose, acetate or any essential amino acids. In another series of experiments, mammary blood flow increased and then returned to normal after colchicine treatment even though milk yield and mammary glucose uptake decreased markedly; oxygen uptake was not significantly affected. The results are discussed in relation to the actions of colchicine on the mammary secretory cell, to the normal control of mammary blood flow and to the mechanism of compensation by the untreated gland.