Electron microscopic study of the anterior midgut in Cenocorixa bifida Hung. (Hemiptera: Corixidae) with reference to its secretory function

The epithelium of anterior midgut of adult Cenocorixa bifida was examined with light and electron microscopy. The folded epithelium is composed of tall columnar cells extending to the lumen, differentiating dark and light cells with interdigitating apices and regenerative basal cells in the nidi surrounded by villiform ridges that penetrate deeply into the epithelium. The columnar cells display microvilli at their luminal surface. Microvilli lined intercellular spaces and basal plasma membrane infoldings are associated with mitochondria. These ultrastructural features suggest their role in absorption of electrolytes and nutrients from the midgut lumen. The columnar cells contain large oval nuclei with prominent nucleoli. Their cytoplasm is rich in rough endoplasmic reticulum, Golgi complexes and electron-dense secretory granules indicating that they are also engaged in synthesis of digestive enzymes. The presence of secretory granules in close proximity of the apical plasma membrane suggests the release of secretion is by exocytosis. The presence of degenerating cells containing secretory granules at the luminal surface and the occurance of empty vesicles and cell fragments in the lumen are consistent with the holocrine secretion of digestive enzymes. Apical extrusions of columnar cells filled with fine granular material are most likely formed in response to the lack of food in the midgut. The presence of laminated concretions in the cytoplasm is indicative of storageexcretion of surplus minerals. The peritrophic membrane is absent from the midgut of C. bifida.     

Ultrastructure of the sexual segments of the kidney in male and female lizards,Cnemidophorus l. lemniscatus (L.)

Summary Kidneys of adult male and female lizards were studied by electron microscopy, in order to understand the ultrastructure of the collecting duct and a differentiated part thereof, the sexual segment, which is an important accessory sexual organ. First portion of sexual segment in males: The cells are filled with large secretory granules of a wide range of opacities. The granular endoplasmic reticulum is abundant; basal formations of superimposed flat cisternae are frequent. Distended vesicles and microvesicles prevail in the supranuclear, well developed Golgi apparatus. Evidences indicate that secretion of these cells is holocrine. Second portion of sexual segment in males: All of the secretory granules are apical in location and relatively electron-opaque; they show a denser core. This core is formed by a substance which, after lying in contact with ribosomes, enters the secretory vesicles of the highly developed Golgi apparatus. A lighter substance is then condensed around it. The secretion of the granules is merocrine. The granular endoplasmic reticulum is very abundant in these cells, but basal ergastoplasmic formations are lacking. Sexual segment in females: The cells show features similar to those of the male first portion, but they are smaller. Undifferentiated collecting duct: Most of the cells are mucigenic. They have small ovoid, apical secretory granules. The density of the granules varies from cell to cell; when they are electron-lucent, they exhibit laminar or dotted opaque figures. Moderately developed Golgi apparatus and granular endoplasmic reticulum, as well as elongated mitochondria, occur in mucigenic cells. Intercalated among the latter are non-secretory cells. They have very abundant mitochondria, numerous microvilli, many pinocytic and smooth-membrane vesicles, whereas the organelles participating in synthetic processes are poorly developed; their function is most likely related to active solute transport.     

Ultrastructure of the ultimobranchial follicles of the laying chicken

Summary The ultimobranchial gland of the laying chicken consists of groups of C cells interspersed among a collection of intercommunicating follicles and ducts of variable size and shape. The epithelium lining this system ranges from squamous to columnar and includes stratified squamous and pseudostratified columnar elements. Four cell types are distinguished in this epithelium: F, mucous, C, and basal cells. F cells show microvilli and microfilaments. Pinocytotic activity and images of fusion of coated vesicles with the plasma membrane are evident. The rough-surfaced endoplasmic reticulum (RER) and the Golgi complex are moderately developed. Dense bodies are encountered apically in some cells. Mucous cells possess microvilli and secretory material in the typical form of partially fused droplets. C cells contain secretory granules and are invariably separated from the follicular lumen by other cell types. The smaller, pyramidal basal cells contain filaments, RER, small Golgi complexes, free ribosomes and hemidesmosomes. The lumina contain flocculent or granular material, cellular debris and desquamated cells. Morphological evidence demonstrates that features of the pharyngeal epithelium are retained and that the majority of the cell types, with the exception of C cells, are presumably nonendocrine.Supported by grant HES 75-09030 from the National Science FoundationThe technical assistance of Quan Nguyen is gratefully acknowledged     

Cytodifferentiation in the accessory glands of Tenebrio molitor I. Ultrastructure of the tubular gland in the post-ecdysial adult male

The tubular accessory reproductive glands of the male mealworm beetle consist of a secretory epithelium surrounded by a thin muscular sheath. Each columnar secretory cell is divisible into three zones: basal which is adjacent to the muscle layer and contains rough endoplasmic reticulum and Golgi, intermediate, which contains endoplasmic reticulum and Golgi zones in the immature gland and is filled with secretory vesicles in the mature gland, and apical. Maturation also involves proliferation and organization of the rough endoplasmic reticulum in the basal and intermediate zone. The process appears to be complete at four days after ecdysis. Parallels with other insect glands and with the mammalian prostate are striking.     

The ultrastructure of the zymogen cells in Hydra viridis

Summary The zymogenic secretory cells of Hydra viridis are scattered between the digestive muscle cells of the gastric region. The mature zymogenic cells are located along the apical surface of the gastrodermal epithelium and contain numerous spherical secretory droplets. They appear to differentiate from stem cells located near the mesoglea. These stem cells resemble epidermal interstitial cells and are filled with free ribosomes. They differ from the interstitial cell in that they usually possess a small amount of granular endoplasmic reticulum. During the process of differentiation they elaborate a highly organized system of granular endoplasmic reticulum. This system becomes dispersed into vesicles as the secretory product is synthesized. There is no indication that the Golgi apparatus participates directly in the formation of the secretory droplets, and there is no indication of a membrane bounding the mature secretory droplet.The fate of the zymogenic cell following the discharge of its secretory product was not determined. It is possible that these cells revert back to a stage resembling the stem cell before resynthesizing a new supply of secretion. In this case the normal secretory process would be very similar to the events described in the dedifferentiation of the zymogenic cells during regeneration.This work was supported by Grant number GB-3262 from the National science Foundation.     

Light and electron microscopy studies of the oesophagus and crop epithelium in Aplysia depilans (Mollusca, Opisthobranchia)

The oesophagus and crop epithelium of Aplysia depilans consist in a single layer of columnar cells with apical microvilli, and some of them also possess cilia. Cell membrane invaginations, small vesicles, multivesicular bodies and many dense lysosomes were observed in the apical region of the cytoplasm. In most cells, a very large lipid droplet was observed above the nucleus and a smaller one was frequently found below the nucleus; glycogen granules are also present. Considering these ultrastructural features, it seems that these cells collect nutritive substances from the lumen by endocytosis, digest them in the apical lysosomes and store the resulting products. The cell bodies of mucus secreting flask-shaped cells are subepithelial in the oesophagus and intraepithelial in the crop. Histochemistry methods showed that the secretion stored in these cells contains acidic polysaccharides. Secretory vesicles with thin electron-dense filaments scattered in an electron-lucent background fill most of these cells, and the basal nucleus is surrounded by dilated rough endoplasmic reticulum cisternae containing small tubular structures. Considering the relatively low number of secretory cells, mucus production cannot be high. Moreover, since protein secreting cells were not observed in either oesophagus or crop, extracellular digestion in the lumen of these anterior segments of the digestive tract most probably depend on the enzymes secreted by the salivary and digestive glands.     

Endocytic routes to the Golgi apparatus

 The endocytic routes of labelled lectins as well as cationic ferritin were studied in cells with a regulated secretion, i.e. pancreatic beta cells, and in constitutively secreting cells, i.e. fibroblasts and HepG2 hepatoma cells, paying particular attention to routes into the Golgi apparatus. Considerable amounts of internalised molecules were taken up into the trans Golgi network (TGN) and into Golgi subcompartments in all three cell types as well as in secretory granules of the pancreatic beta cells. The internalised materials did not pass rapidly the TGN and Golgi stacks, but were still present hours after internalisation, being then particularly concentrated in TGN-elements and in the transmost Golgi cisterna. Endocytosed materials reached forming secretory granules present in the TGN. Further, direct fusion between endocytotic vesicles and mature secretory granules was observed. Golgi subcompartments as well as endocytic TGN containing endocytosed materials were in close apposition to specialised regions of the endoplasmic reticulum. The Golgi apparatus including its parts containing endocytosed materials were transformed into a tubular reticulum upon treatment with the fungal metabolite Brefeldin A. Rarely, internalised material was observed in the lumen of the endoplasmic reticulum, thus providing evidence for an endocytic plasma membrane to endoplasmic reticulum route. Accepted: 9 March 1998     

The histochemistry and ultrastructure of the clitellum of the enchytraeid Lumbricittus rivalis (Oligochaeta: Annelida)

The monostratified clitellar epidermis of Lumbricillus rivalis consists of supporting cells, granular secretory cells, and globular secretory cells, together with the acid mucous cells typical of normal skin in the anterior and posterior transition zones. The secretion of the granular cells is a neutral glycoprotein with low levels of bound lipid, and that of the globular cells is a sulphated acid mucopolysaccharide lacking detectable protein or lipid.
Ultrastructurally the granular cells possess 1 μm, membrane-bound granules of variable electron density suggestive of maturation changes. The formative granules arise from Golgi vesicles and are moderately electron translucent. They contain parallel-aligned, tubule-like inclusions of 14 nm diameter, observed also in the mature regions of the Golgi. Granules in the mid or apical part of the cell show increased electron density, the formative pattern being wholly or partially obscured and the subunit alignment, when discernable, no longer uniform. Moiré fringe patterns are evident in some granules. The patterns and electron density are lost after pronase digestion. The globular cells contain electron translucent, membrane-bound globules of 2–3 μm diameter with finely fibrous contents. They arise from mature Golgi vesicles and are unaffected by pronase treatment.
The results suggest that the granular cells secrete the cocoon wall and the globular cells the material that surrounds the developing embryos, and are compared with published accounts of other microdrile clitella and with those of the clitellum of lumbricid earthworms.     

Intracellular transport of albumin through the secretory apparatus of rat liver parenchymal cells. An immunocytochemical study

The fine structural localization of albumin in rat liver parenchymal cells was determined by an improved immunocytochemical method and serial sectioning. Albumin in the secretory apparatus of the parenchymal cells was present in segments of the rough endoplasmic reticulum, interrupted with negative segments, in transport vesicles, Golgi saccules, finely anastomosed tubules and vesicles on the trans side of the Golgi complex, and in secretion granules. Horizontally sectioned Golgi saccules contained lipoprotein particles on one side and albumin on the other side. After transport, the vesicles that contained albumin fused with the so-called rigid lamellae on the trans-side of the Golgi complex. Ultrathin serial sections revealed no true structural continuity between the endoplasmic reticulum and the cis-aspect of the Golgi complex. We concluded that secretory proteins are transported from the endoplasmic reticulum to the Golgi complex by transport vesicles that bud from the endoplasmic reticulum and fuse with the Golgi saccules. These vesicles fuse regularly with the Golgi saccules on the cis-side and occasionally with tubular elements on the trans-aspect that may belong to the so-called GERL.     

The amylase-secreting cells of the stomach of the scallop, Pecten maximus: ultrastructural, immunohistochemical and immunocytochemical characterizations

(1) alpha-amylase was extracted and purified from the stomach/digestive gland complex of the scallop Pecten maximus and an anti-serum was induced against the purified amylase by rabbit immunization. (2) The anti scallop amylase was used to localize the amylase-secreting cells in the stomach of Pecten maximus by immunofluorescence and immunogold labelling. The amylase-secreting cells are glandular cells particularly numerous in the main sorting area of the stomach. Their secretory granules were found strongly positive for anti-amylase. Three types of glandular cells were observed, actually corresponding to the three stages of the glandular-cell activity, synthesis, secretion and excretion. (3) The synthesizing cell shows the characteristic features of a protein-synthesizing cell: a conspicuous nucleolus and abundant granular endoplasmic reticulum. In the secretory cell, the secretory granules are formed by the Golgi apparatus and accumulate in the apical part of the cell. The secretory cell is filled with two types of secretory granules which are released in the stomach lumen by apocrine excretion. (4) The present study brings the first demonstration of the synthesis and extracellular release of amylase by glandular cells of the stomach epithelium of a bivalve.     

An electron microscopic study of hemoglobin synthesis in the marine annelid,Amphitrite ornata (Polychaeta: Terebellidae)

The heart-body of the marine worm Amphitrite, located within the supraesophageal dorsal vessel, is in the form of a cylinder the thin wall of which is deeply corrugated by luminal projections and folds along its entire length. It is anchored in places to the luminal surface of the dorsal vessel by an extracellular matrix containing collagen fibers. The luminal surfaces of both the heart-body and the dorsal vessel are covered by a basement membrane-like vascular lamina which in turn supports a discontinuous pseudoendothelium of littoral hemocytes. The cells of the heart-body constitute a pseudostratified, high columnar epithelium. They possess extensive rough endoplasmic reticulum (RER), a well developed Golgi zone, ferritin particles and granules, and several types of membrane-bound inclusions. Hemoglobin molecules identical to those in the circulation lie within cytoplasmic, membrane-bound vesicles. Analysis of our electron micrographs suggests the following sequence of hemoglobin production and secretion: Large quantities of a moderately dense flocculent material, probably globin, are synthesized in RER and move to the Golgi zone within partly rough- and partly smooth-surfaced transitional cisternae; small transport vesicles, formed from Golgi cisternae that have fused with transitional cisternae, convey the flocculent material from the convex to the concave face of the Golgi complex; a similar flocculent material and an amorphous, highly dense material are processed in the Golgi complex and are transferred to condensing vacuoles in which clearly identifiable hemoglobin molecules are first observed. Mature secretory vesicles containing only hemoglobin migrate to the cell periphery and discharge their contents by exocytosis. Hemoglobin molecules then cross the vascular lamina to reach the circulation.     

The fine structure of the collar cells in the optic tentacles of Helix aspersa

Summary At the base of the optic tentacular ganglion there is a group of large monopolar cells containing numerous secretory inclusions. These are the collar cells. Secretory material can be seen accumulating in swollen portions of the granular endoplasmic reticulum. It is postulated that this material is transported to the Golgi bodies and thus the limiting membrane of the inclusions is derived from the Golgi membranes. The Golgi bodies appear to be polarized and small vesicles resembling secretory inclusions are associated with one face of these organelles. The secretory inclusions fuse together to form large membrane-bound secretory pools in the perikaryon. The collar-cell processes are packed with secretory inclusions. These processes traverse the digital extensions of the tentacular ganglion and pass into the epithelium covering the tip of the tentacle. The secretory inclusions do not resemble neurosecretory inclusions in other situations. The collar cell processes receive a nerve supply from single axons containing granular and agranular vesicles. The evidence that these cells may be modified neurons is only minimal.This work was supported by the Australian Research Grants Committee.     

The fine structure of the midgut in the mite Anystis baccarum (L.) (Acari, Actinedida: Anystidae)

The ventriculus and the midgut caeca of the fed females of Anystis baccarum (L.) were investigated by using light and electron microscopy. In addition to the main type of polyfunctional digestive cells, special secretory cells were detected in the anterior region of the ventriculus. The shape and the ultrastructure of the digestive cells vary depending on their physiological state. Intracellular digestion, absorption or excretion processes prevail at different stages of the cell cycle. The secretory cells are characterized by the presence of extensive rough endoplasmic reticulum, filling whole space of the cell. These cells do not contain the apical network of pinocytotic canals, which are typical for the digestive cells. Three types of secretory granules were found in the cytoplasm of the secretory cells that probably correspond to three sequential stages of granulogenesis. The primary secretory granules are formed by the fusion of Golgi vesicles. The primary granules fuse to form complex vesicles with heterogeneous contents. These secondary granules aggregate to form very large inclusions of high electron density (tertiary secretory granules), which probably represent the storage of the secretory product. All types of secretory granules were observed close to the apical plasmalemma.     

Life-cycle and functional cytology of the hepatopancreatic cells of Astacus astacus (Crustacea,Decapoda)

Summary The hepatopancreas of the freshwater crayfish Astacus astacus was reinvestigated by means of light and electron microscopy using refined techniques of tissue preservation. The results contribute significantly to the solution of controversial problems of the decapod hepatopancreas such as cell genealogy, cellular interdependences, elimination of senescent cells and functional interpretation of the cell types. The three mature cell types of the organ, R-, F- and B-cells, are shown to originate independently from embryonic E-cells which are located at the blind-ending tips of the hepatopancreatic tubules. The less abundant M-cells are supposedly of non-hepatopancreatic origin since they are also found in other epithelia of the digestive tract. Differentiating cells can be assigned at an early stage to one of the three hepatopancreatic cell lines if the ultrastructural appearance and distribution pattern of their organelles are used as distinguishing features. The most sensitive markers are the Golgi bodies which have a cell-specific architecture and secretion product not only in mature cells but also in early differentiating stages. Later conversion of one cell type into another, as has often been proposed in literature, does not occur. Senescent cells are preferably expelled from the epithelium at the junction of neighbouring hepatopancreatic tubules and at the antechamber which links the hepatopancreas to the main digestive tract. Cellular discharge in the antechamber occurs by sliding of the oldest parts of the hepatopancreatic epithelium across a particular antechamber epithelium that was thus far unknown. New ultrastructural findings are described with respect to the absorptive apparatus of nutrient absorbing R-cells, the formation of Golgi vesicles and retrieval of membranes in digestive enzyme synthesizing F-cells, and the involvement of Golgi body and endoplasmic reticulum in the formation of heterophagic vacuoles in B-cells. The discovery of these ultrastructural features enables a more sophisticated functional interpretation of the hepatopancreatic cells of Decapoda.     

Fine structure of the corpuscles of stannius in the toadfish.

The micro-anatomy of the corpuscles of Stannius of the toadfish, Opsanus tau, an aglomerular marine teleost, has been studied by light and electron microscopy. The corpuscles are composed of extensively anastomosed cords of epithelial cells which maintain intimate contact with blood capillaries. Most of the epithelial cells contain acidophilic granules which also show a positive reaction with the periodic acid-Schiff technique and aldehyde fuchsin. On the basis of fine structural criteria, three cell types can be recognized. The granular cells contain abundant quantities of granular endoplasmic reticulum, ribosomes, Golgi apparatus with prosecretory granules, coated vesicles, polymorphic mitochondria with lamellar cristae, filaments, microtubules, a cilium, a variety of lysosome-like dense bodies, glycogen particles, lipid droplets, secretory granules and intranuclear lipid-like inclusions. One variety of agranular cell (type I) is characterized by the total absence of secretory granules, but it contains large amounts of granular endoplasmic reticulum and ribosomes, conspicuous profiles of Golgi apparatus, coated vesicles and sometimes an abundance of glycogen. Another variety of agranular cell (type II) has poorly developed cytoplasmic organelles. The perivascular space between the capillary and parenchyma contains connective tissue cells and abundant nerve fibers. The different types of epithelial cells observed in the corpuscles of Stannius of this fish may represent functional stages of the secretory cycle in a single cell type.     

Fine structure of chicken thymic epithelial vesicles.

This paper describes the fine structure of epithelial vesicles as observed in the thymus of the chicken. Three types of vesicles are found. The largest is made up of a stratified columnar epithelium containing lymphoid cells. The lumen contains a fairly light material and cell debris. The lumen-bordering cells are columnar and their microvilli are sometimes developed to such an extent as to appear as a brush border. The wall of the vesicles contains lymphocytes and granular cells with dense granules 100-500 nm in diameter. Smaller vesicles are found which contain no lymphoid cells. They are made of a simple or stratified epithelium containing typical mucous cells, and columnar cells similar to the bordering cells of the large vesicles. The third type is an intracytoplasmic vesicle in epithelial reticular cells, which differs from usual vacuoles by the presence of microvilli and its often unusually large size. The morphology of the first 2 types of vesicles indicates a secretory capability, the function of which is unknown. Data from the literature suggest that the third type produces the factor(s) conferring cellular immunological competence.     

The ultrastructure of secretion in the spermatheca of the salamander, Manculus quadrigitatus (Holbrook)

Spermathecae of mature salamanders (Manculus quadridigitatus) were studied by light and electron microscopy. Gross morphology exhibits a complex of muscle, connective tissue and pigment cells surrounding a cluster of tubules, which empty into a ciliated central duct leading to the cloacal cavity. The tubules are composed of myoepithetial cells and secretory cells, anchored to an encompassing basal lamina. Secretion products appear to be initiated as crystalline deposits, seen in mitochondrial repositories, which are subsequently sequestered in the perinuclear region. Observations show these vesicles to be precursors of the Golgi synthesis of carboxylated polysaccharides, as determined by histochemical tests using toluidine blue, ruthenium red, and alcian blue. The secretory products are emptied into the tubule lumen. thereby bathing the stored sperm and maintaining them in a viable state for approximately 8 months. The storage tubules appear to be a complete complex of varying epithelial cells specifically designed to support viable sperm and to resorb non-functional forms.     

Radioautographic analysis of the secretory pathway for glycoproteins in principal cells of the mouse epididymis exposed to [3H] fucose

The secretory process for glycoproteins in principal cells of the mouse caput epididymis was studied by electron microscope radioautography at intervals after exposure to [3H] fucose in vitro. The large Golgi apparatus showed very heavy labeling at the initial interval, followed by a steady decline in percent of grains and relative grain concentrations. Conversely, the epididymal lumen and the apical cell surface began low and increased in radioactivity at the 30-min interval. The extensive sparsely granulated endoplasmic reticulum showed modest increases in percent of grains and relative grain concentrations 30 min after administration of the percursor. Subdivision of the sparsely granulated reticulum into "intermediate" profiles (some ribosomes attached to the membranes) and "smooth" profiles (lacking ribosomes) showed that this increase was due to silver grains assigned to the smooth portions. After the initial interval, high relative grain concentrations were calculated for vesicles. The results indicate that glycosylation of epididymal secretory glycoproteins occurs in the Golgi apparatus, which is, therefore, not bypassed as its morphological features had suggested. The kinetics of the secretory process in the principal cells includes 15 to 30 min for synthesis of the polypeptide parts of secretory products and addition of sugars in the Golgi apparatus, and a similar time for subsequent release from the Golgi apparatus, transport to the apical end of the cell and discharge to the lumen. Ribosome-studded (intermediate) portions of the sparsely granulated endoplasmic reticulum are probably involved in synthesis of polypeptide parts of secretory products, while vesicles or smooth portions of the sparsely granulated reticulum may play a role in intracellular transport of glycoproteins.     

Electron microscopic and cytochemical studies of the mouse subcommissural organ

Summary In mice most of the ependymal cells of the subcommissural organ (SCO cells) are densely packed with dilated cisternae of the endoplasmic reticulum (ER) containing either finely granular or flocculent materials. The well developed supra-nuclear Golgi apparatus consists of stacks of flattened saccules and small vesicles; the two or three outer Golgi saccules are moderately dilated and exhibit numerous fenestrations; occasional profiles suggesting the budding of coated vesicles and formation of membrane-bound dense bodies from the ends of the innermost Golgi saccules are seen. A few coated vesicles and membrane-bound dense bodies of various sizes and shapes are also found in the Golgi region.The contents of the dilated ER cisternae are stained with periodic acid-silver methenamine techniques. In the Golgi complex the two or three inner saccules are stained as deeply as the dense bodies, and the outer saccules are only slightly stained. The stained contents of ER cisternae are more electron opaque than those of the outer but less opaque than those of the inner Golgi saccules and the dense bodies.Acid phosphatase activities are localized in the dense bodies, some of the coated vesicles in the Golgi region, and in the one or two inner Golgi saccules.On the basis of these results the following conclusions have been reached: (1) In mouse SCO cells the finely granular and the flocculent materials in the lumen of ER cisternae contain a complex carbohydrate(s) which is secreted into the ventricle to form Reissner's fiber; (2) the secretory substance is assumed to be synthesized by the ER and stored in its cisternae, and the Golgi apparatus might play only a minor role, if any, in the elaboration of the secretory material; (3) most of the dense bodies in the mouse SCO cells are lysosomal in nature instead of being so-called dark secretory granules.Sponsored by the National Science Council, Republic of China.     

An ultrastructural study of the clitellar epithelium of the earthworm Metaphire posthuma (vail.)

The ultrastructure of clitellar epithelium of Metuphire posthuma revealed mainly three types of secretory cells. Most prominent among these are the large slender granular cells which contain a large number of secretory granules filling in the entire columncr region of the cell. The secretory granules are 2-4mu in diameter with a limiting membrane and containing numerous tiny vesicles in a matrix of varying electron density. Basolateral rough endoplasmic reticulum and extensive Golgi cisternae were seen interspersed with the secretory granules. The Golgi cisternae in these cells were quite prominent extending all around the secretory granules. The secretory granules of type 2 cells are spheroid bodies with motley appearance due to varying electron density of the matrix. The immature granules contain fibrillar material. Type 3 cells contained electron lucent membrane-bound mucous like secretory granules which are reticulated with filamentous materials. All the three cell types open to the exterior at the cuticular region which is characterised by the presence of numerous microvilli.