Ultrastructure of langur monkey epididymidis prior to and following vasectomy and vasovasostomy.

The ultrastructural changes in langur monkey epididymis prior to and following vasectomy or vasovasostomy were studied. The epididymal epithelium of the intact langur monkey was found to consist mainly of principal cells and basal cells and frequently apical or mitochondria rich cells were found. Besides these cells intraepithelial lymphocytes were also a consistent feature of the epididymal epithelium. Principal cells identified by means of the tuft of the stereocilia on their apical surface, bear well developed Golgi bodies, endoplasmic reticulum, vesicles, vacuoles and multivesicular bodies. This suggests their active involvement in absorption and secretion. Basal cells present at the base of the lamina bear a few cellular organelles and strong interdigitations with the adjacent cells. Apical or mitochondria rich cells were characterized by clusters of mitochondria in the apical region of the cell and few microvilli on their apical surface. Lymphocytes with a large nucleus and a pale rim of cytoplasm were also found at the base of the epithelium. Secretory and absorptive functions of principal cells of the epididymal epithelium were found to be increased after vasectomy, as indicated by bulging of the apical portion of the principal cells and membrane bound structure in the lumen. An extensive increase in the number of lysosomes, vesicles and vacuoles was also observed. An increase in the number of macrophages with spermatozoa remnants in the lumen of epididymis suggests that the principal mechanism for spermatozoa disposal following vasectomy is intraluminal endocytosis by macrophages. Changes following vasectomy persisted in vasovasostomized animals even after 12 months of recanalization, which may contribute to the failure of functional reanastomosis.     

Light and electron microscopic observations of fabrication, release, and fate of biphasic secretion granules produced by epididymal epithelial principal cells of the fan-throated lizard Sitana ponticeriana cuvier

The epididymis of the fan-throated lizard Sitana ponticeriana was examined with light and transmission electron microscopy to understand the cellular mechanisms of fabrication of secretion granules in epithelial principal cells, granule release into the lumen, and the fate of the dense structured granules after reaching the lumen. Principal cells of the ductus epididymis, except at the cauda, secrete electron-dense biphasic granules copiously, which decrease in abundance from the initial segment to corpus. The principal cell possesses a prominent Golgi apparatus and all versions of endoplasmic reticulum (ER), rough, smooth, and sparsely granulated. The material of the dense portion of the secretion granules, after processing at the Golgi apparatus, appears to accumulate in large ER cisternae in the supranuclear cytoplasm. It undergoes condensation when the cisternae become condensing vacuoles. Mitochondria appear to play a role in dense granule formation. The condensing vacuoles are displaced toward the apical cytoplasm when the material of the less dense portion is added to the condensing vacuoles at the Golgi area. Thus, the less dense and dense portions of the secretion granules are secreted and added to the condensing vacuoles separately. The composite granules are released into the lumen by exocytosis when the less dense portion merges with the luminal content, whereas the dense portion maintains its structured identity. The latter, initially measuring 1-2 microm in diameter, increases in size several times. It is inferred that these granules release their content gradually, resulting in the appearance of vacuoles, and suggesting that the granules have an insoluble matrix in which there is a sparingly soluble material. The substance leaching out of the granules appears to contribute to keeping the sperm quiescent and alive during storage in the male reproductive tract.     

Morphological features of the epididymal epithelium of gerbil, Meriones unguiculatus

Principal cells of the ducts epididymis of the Mongolian gerbil showed ultrastructural characteristics of lining epithelium cells close related to processes of protein secretion, and transcytosis occurring between adjacent principal cells which were mainly verified in the initial segment. Principal cells also presented roles of fluid phase and adsorptive endocytoses, as well as autophagic and heterophagic lysosomal activities mainly observed in the caput epididymis. Columnar (principal) cells of the corpus epididymidis presented great number of variable vesicles and vacuoles distributed in all the cytoplasmic levels occurring a progressive coalescence pattern among them, which help to guarantee formation of cytoplasmic channels for fluid phase transport between the tubular lumen and epididymal interstitium. Clear cells were presented in the initial segment and predominately in the cauda epididymis epithelium of the gerbil and showed marked ultrastructural characteristics of endocytosis activities occurrence, perhaps directly related to the turnover of fluid phase of spermatozoa stored into the lumen of the distal tail. Other epididymal epithelium cells were verified and described such as basal, halo, apical and dark cells, but they did not presented special ultrastructural features.     

Light and electron microscopic cytochemistry of glycoconjugates in the rectosigmoid colonic epithelium of the mouse and rat

The several cell types in mouse and rat rectosigmoid colon have been examined with light and electron microscopic methods for localizing and characterizing complex carbohydrates. Mucous cells, also termed vacuolated cells, and goblet cells comprised most of the deep crypt epithelium in both species, and absorptive columnar cells and goblet cells mainly populated the more superficial epithelium of the upper crypts and main lumen. Occasional tuft cells and enteroendocrine cells were also encountered. Transitional cells structurally intermediate between mucous cells and absorptive cells contained granules characteristic of mucous cells and vesicles like those of columnar absorptive cells. These intermediate cells supported the concept of replacement of mucous by absorptive cells through transformation of mucous into absorptive cells. The intermediate cells also contained numerous lysosomes often in apparent fusion with mucous granules, indicating crinophagic disposal of mucous granules as a mechanism in the cell transformation. Glycoconjugate in absorptive cell vesicles resembled that coating the apical plasmalemma and appeared to represent the source of the glycocalyx of the brush border. Complex carbohydrate in these vesicles differed cytochemically from that of the mucous cell granules, which release their content into the crypt lumen. The absorptive cell vesicles, therefore, constitute an organelle distinct from the mucous cell granules rather than an atrophic form of the latter in a more mature cell. Goblet cells differed in failing to transform morphologically with age but changed in the cytochemical characteristic of their secretion during migration up the crypts. Terminal N-acetylglucosamine residues diminished, while terminal sialic acid-galactose dimers increased during the upward migration, indicating activation of glycosyl transferase synthesis in relation to goblet cell maturation. Glycoconjugate in secretion of mucous cell granules differed markedly from that in goblet cell granules, and content of both organelles differed from that of absorptive cell vesicles. However, secretion in mucous cell granules appeared generally similar for mice and rats with minor exceptions, and secretion in goblets of mice generally resembled that in goblets of rats. Cells interpreted tentatively as Kulchitsky cells stained for high content of fucose with the Ulex europeus I lectin. Globoid leukocytes infiltrating the epithelium of the rat but not the mouse rectosigmoid colon resembled globoid leukocytes in rat tracheal epithelium and, like the latter, appeared to derive from mast cells.     

Nature's ingenuity: bypassing the classical secretory route via apocrine secretion

Although it has been suggested that epithelial cells of the male reproductive system are involved in apocrine secretion, this method of secretion is not fully understood. In the present study, apocrine secretion was investigated in epithelial principal cells lining the epididymis and vas deferens (VD) of adult mice. The tissues were fixed by cardiac vascular perfusion with glutaraldehyde for routine electron microscope (EM) analysis and Bouin's fixative for light microscope (LM) immunocytochemistry to access functional roles. In the epididymis and VD, the apex of principal cells revealed protrusions of cytoplasm referred to as apical blebs (ABs). The latter contained solely numerous free ribosomes, 20 nm vesicles and few ER cisternae, suggesting segregation of their contents. While some ABs displayed wide areas of contact with the apical principal cell cytoplasm, others showed thin stalk-like attachment points as well as fissures at the junction of the two areas. Together with images of ABs and their contents deep in the lumen, it is suggested that ABs detach from principal cells whereupon they breakdown to release their contents therein. As ABs of the epididymis were immunoreactive for glutathione-S-transferases (GSTs) and ubiquitin, it is proposed that these proteins are synthesized on free ribosomes in ABs and that apocrine secretion represents the manner whereby they enter the lumen to effectively protect sperm from free radical injury and ubiquitinate proteins for degradation, respectively. ABs of the VD were immunoreactive for 3beta-HSD, suggesting that they are also capable of synthesis of steroids with their release via apocrine secretion. Taken together the data provide evidence for apocrine secretion in the adult mouse epididymis and VD that could play important roles in relation to sperm maturation, protection and viability.     

Morphology of the bovine epididymis

The epididymis of the bull was divided into six regions, and morphological differences between regions were studied. The epithelium of all regions contained four cell types: principal and basal epithelial cells, and intraepithelial lymphocytes and macrophages. The epithelium of regions II-V also contained a few apical cells. Principal cells of all regions possessed an endocytotic apparatus including stereocilia underlain by canaliculi, coated vesicles, and subapical vacuoles (up to 1 micron in diameter); however, large vacuoles with a flocculent content and multivesicular bodies (up to 5 microns in diameter) were most numerous in regions II, III, and IV. The unique features of principal cells of region I were the presence of well-developed Golgi bodies, few lipid droplets, and whorls of smooth endoplasmic reticulum in the supranuclear cytoplasm. Numerous mitochondria, distended cisternae of rough endoplasmic reticulum, and dense granules characterized the infranuclear cytoplasm of the principal cells of regions II-VI; however, these features were more developed in region V. Apical cells were characterized by the apical location of the nucleus, many mitochondria in the apical cytoplasm, and few microvilli at the luminal border. Basal cells with few cytoplasmic lipid droplets were present throughout the length of the epididymis but appeared more numerous in region V. Intraepithelial lymphocytes were present at all levels of the epithelium but were never seen in the lumen. Intraepithelial macrophages containing heterogeneous granules, eccentric nuclei, and pseudopods were invariably seen near the basal area of the epithelium in all regions. These observations are discussed in an effort to define the role of each cell type in the epididymal epithelium.     

Fine structure of the mandibular gland in pika (Ochotona rufescens rufescens)

The mandibular gland of the pika was examined by light microscopy, and transmission and scanning electron microscopies. The acinar cells were noted to be composed of serous cells and seromucous cells. The serous cells containing granules of moderate and high densities were slightly basophile and strongly positive to PAS, but were not stained with AB. The seromucous cells possessing less dense granules were light and moderately positive to PAS and AB. A sexual dimorphism was observed between these cells: Serous cells were considerably more frequent in males and seromucous cells were more numerous in females. Intercalated duct cells consisted of cuboidal light cells containing a few vesicles in the apical region. Striated ducts were comprised of two portions--a secretory portion and a typical striated portion without secretory granules. The secretory portion was composed of light and dark cells having secretory granules varying in size and density. The epithelium of typical striated portion consisted of light and dark cells containing fine vacuoles and vesicles.     

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.     

Immunocytochemical localization and lectin-binding properties of the 22 kDa secretory protein from rat ventral prostate

The distribution of the 22 kDa secretory protein from rat ventral prostate was studied by light and electron microscopic immunocytochemistry. An anti-22 kDa protein antiserum was raised in rabbits and its specificity was tested by Western blotting. With the immunofluorescence technique, the 22 kDa protein was detected in the luminal secretions and intracellular apical granules of the ventral prostate. No reaction was observed in the seminal vesicle or dorsolateral prostate. After castration, no intracellular immunoreactivity was detected in ventral prostate, although positively labeled secretory material was retained within the acinar lumen. Restoration of normal intracellular staining pattern was incomplete after 5 daily testosterone injections. At the ultrastructural level, labeling was confined to apical secretory granules and condensing vacuoles. The 22 kDa protein separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and transferred to nitrocellulose was shown to bind intensely to wheat germ agglutinin (WGA) but only faintly to Concanavalin A. This protein was thus demonstrated to contain N-acetylglucosamine residues. Accordingly, on tissue sections, WGA reacted intensely with condensing vacuoles and secretory granules.     

The fine structure and histochemistry of the caecal epithelium of Calicotyle kröyeri (Monogenea: Monopisthocotylea)

The caecal epithelium of Calicotyle kröyeri consists of a single cell type which functions in the uptake and intracellular digestion of host epidermis and associated mucus. Each cell is columnar with a small basal nucleus and prominent nucleolus. Perinuclear cytoplasm contains narrow profiles of GER and mitochondria with numerous cristae. Golgi complexes are small and indistinct. Most of the cell is filled with vacuoles of heterogeneous content, the largest occupying the cell apex. There is in each cell an apical endocytotic complex comprising cell surface lamellae, apical vesicles and numerous tubular invaginations of the plasmalemma. The limiting membrane of all these components is structurally modified and bears a highly organized array of peg-like structures on its luminal surface. The complex is capable of ingesting particulate food material from the gut lumen for transfer, via vesicles, to the vacuoles for digestion. Most of the vacuoles represent the digestive elements of the cell and, histochemically, are reactive for protein, mucus and carboxylic esterases. Indigestible residues and lipid droplets accumulate in the large apical vacuole and are periodically released to the lumen by exocytosis. Small, undifferentiated caecal cells were occasionally observed in the epithelium, but their development has not been recorded.     

DYNAMIC CHANGES IN THE ULTRASTRUCTURE OF THE ACINAR CELL OF THE RAT PAROTID GLAND DURING THE SECRETORY CYCLE

Synchronization of the secretory cycle in vivo was obtained by injecting isoprenaline as an inducer of secretion. A quantitative correlation between enzyme release, its subsequent reaccumulation, and the sequence of ultrastructural changes was found. At the ultrastructural level secretion was paralleled by depletion of zymogen granules through fusion of the granule membrane with the lumen membrane and discharge of the content. Each zymogen granule membrane, once connected with the lumen, acted as a lumen membrane. Fusion was thus sequential and resulted in a dramatic enlargement of the lumen space. During the entire process the passage between the lumen and the intercellular space remained blocked by the tight junctions, as shown by their impenetrability to ferritin. Reduction of the lumen size following enzyme discharge seemed to be achieved by withdrawal of lumen membrane in the form of small smooth vesicles which appeared mostly in the apical part of the cell. At the same time, the cell retracted towards the lumen, the whole process being completed within 2 hr from onset of secretion. Disappearance of the smooth vesicle followed, concomitant with formation of many condensing vacuoles and appearance of mature zymogen granules. The fate of the zymogen granule membrane, including its fusion with the lumen membrane, resorption in the form of small smooth vesicles, and its eventual reutilization mediated by the Golgi system, is discussed.     

The aglomerular nephron of antarctic teleosts: a light and electron microscopic study.

Complete serial sections demonstrated that ten species of Antarctic teleost fishes representing two families had aglomerular kidneys. The aglomerular nephron of such kidneys consists of two distinct regions: (1) a highly contorted principal segment and (2) a system of collecting tubules and ducts. Throughout the principal segment the cells are characterized by densely packed microbilli and a single cilium projecting into the lumen. Within the cytoplasm, lysosomes are rarely encountered, as would be expected if there is little or no reabsorption of protein from the urine. At the base of these cells, the plasma membrane is prominently infolded in close association with abundant mitochondria. The overall fine structure of the principal segment cells is consistent with their probable function in the secretion of ions into the formative urine. Between the principal segment and the collecting tubule is a very short transitional zone characterized by transitional mucus cells and multiciliated cells. The collecting tubule and duct system is lined entirely by mucus cells. In comparison with principal segment cells, the mucus cells have a well-developed Golgi complex and abundant secretory granules in the apical cytoplasm; these granules presumably contain the non-sulfated acid mucopolysaccharide demonstrable by light microscopic histochemistry.     

Reinternalization of secretory proteins during membrane recycling in rat pancreatic acinar cells

Membrane recycling in pancreatic acinar cells involves endocytic vesicle formation at the apical cell surface and rapid membrane traffic to the Golgi complex. During this process a small amount of extracellular content is taken up from the acinar lumen. In order to determine whether secretory proteins already released into the pancreatic acinar lumen are reinternalized during membrane retrieval, 3H-labeled amylase or 125I-labeled secretory proteins were reinfused through the pancreatic duct until the lumina were reached. Tissue samples from various time points were prepared for light and electron microscope autoradiography. The observations showed that [3H]amylase and, to a lesser extent, the 125I-labeled secretory proteins were internalized at the apical cell surface and rapidly (within 2-5 min) transferred to the Golgi cisternae and the condensing vacuoles; only a minor proportion of silver grains was observed over lysosomes. In addition, at later time points, mature secretion granules close to the Golgi complex became labeled. The results indicate that exocytosis in the rat exocrine pancreas does not operate at 100% efficiency; part of the exported amylase and part of the total secretion product are reinternalized concomitantly with the endocytic removal of plasma membrane and are copackaged together with newly synthesized secretory proteins.     

Transcytosis in the epididymis studied by local arterial perfusion

Summary The transport of protein across the cells of the epididymal epithelium was studied using horseradish peroxidase. Transient vascular perfusion of the epididymis of the rat and golden hamster was achieved by pulsatile retrograde infusion into the testicular artery. Peroxidase was found in the interstitium and in the epithelium, located in vesicles, vacuoles and multivesicular bodies of principal, clear and apical cells. Similar findings were obtained in mice after systemic injection of the tracer. In the rat, discharge to the lumen was confirmed by the appearance of enzyme activity in luminal fluid from the caput epididymidis after local injection. The extent of transport amounted to no more than what has been considered leakage in physiological experiments, and the time-course of appearance complemented that found by electron microscopy. The level of transcytosis after pulsatile administration of peroxidase in vivo, as judged from the occurrence of tracer in the epithelium, was much less than that obtained during constant immersion in vitro. The protein was present in multivesicular bodies of principal cells and in vesicles of clear cells at short times after presentation in vitro, when it could not have arrived by endocytosis from the lumen. This suggests that routing of basal endocytic vesicles to the lysosomal apparatus occurs.     

Ultrastructure of the perfused rat epididymis: Effect of luminal sodium ion concentration

Summary The appearance of the rat epididymal epithelium changed when it was perfused in vivo through the lumen with unphysiologically high sodium ion concentrations; dilatation of intercellular spaces (ICS) at threshold concentrations of 30mM-Na⁺ in the cauda and about 55mM-Na⁺ in the corpus was associated with absorption of water from the lumen. Despite the distended ICS, junctional complexes appeared intact, and their integrity was confirmed by the exclusion of luminal horseradish peroxidase (HRP) from the ICS, and by demonstrating that circulating [³H]inulin did not enter the lumen. Smooth ER and lipid droplets in the principal cells of the corpus epididymidis were well maintained, and the preservation of granular ER in principal cells of the cauda epididymidis lent morphological support to the continued secretion of protein in this segment. However, occasional distension or involution of inner Golgi cisternae was evident in principal cells after 3–6 h perfusion. In contrast to multivesicular bodies of principal cells, the apical and basal vacuoles characteristic of clear cells changed in size with different perfusing solutions. When low Na⁺ concentrations were perfused large translucent vacuoles were frequently found in the apical cytoplasm of clear cells in the corpus and cauda epididymidis, and filled vacuoles became larger and showed a decrease in content density in the cauda epididymidis. These large vacuoles were absent from tissue perfused with high Na⁺ concentrations. Normal pinocytotic activity of both cell types was demonstrated by perfusing HRP which was taken up by the normal route in principal cells, with some transfer to the Golgi cisternae. By far the most HRP was accumulated in clear cell vacuoles irrespective of the composition of the perfusing solution.     

CELL MEMBRANE RESORPTION IN THE RAT EXOCRINE PANCREAS CELL AFTER IN VIVO STIMULATION OF THE SECRETION, AS STUDIED BY IN VITRO INCUBATION WITH EXTRACELLULAR SPACE MARKERS

Pancreatic secretion in the rat was stimulated in vivo by pilocarpine injection causing 90% of the storage granules to be discharged within 2 h. Incubation in vitro with [¹⁴C]sorbitol indicated that maximal ingestion of this extracellular space marker occurred 3 h after secretogogue injection. Morphological cell membrane measurements on cells with stimulated secretion revealed a simultaneous decrease in amount of membrane bordering the microvilli at the cell apex, lamellar processes, and infoldings present at the latero-basal face of these cells. In 3-h stimulated cells, having the average zymogen granule content characteristic for that phase of secretion, ferritin treatment in vitro showed that the infoldings and related fragmentation vesicles had ingested ferritin and could consequently be considered as being transport vehicles for redundant cell membrane. During stimulated secretion numerous vesicles and vacuoles appeared in the apical cytoplasm. Part of these structures were postulated to be related to the Golgi complex and were discussed in relation to secretory protein transport. Another part of these structures was assumed to have an endocytotic nature, although they never contained ferritin.     

Fine structural changes in the epididymal epithelium of moles (Talpa europaea) throughout the year.

The epididymis of the European mole (Talpa europaea) was studied by light and electron microscopy. In the sexually active animal, spermatozoa mature during their passage through the epididymis and the structure of the cells lining the duct suggests a clear regional division into initial, middle and terminal segments. Numerous intra-epithelial vesicles were present in the distal part of the middle segment of sexually active moles and the lining epithelium in the terminal segment appeared to be secretory. Variation in the sensitivity of different regions of the epididymis to androgens was apparent: the principal cells of the initial segment were morphologically active only during the peak of the breeding season in spring, while the cells of the terminal segment became active earlier and remained so for longer. During sexual regression, many autophagic vacuoles were found in the principal cells, and these became transformed into lipofuscin pigment granules. Cells heavily laden with these granules appeared concurrently in the lining epithelium. It is suggested that such cells may be involved in the regression of principal cells by means of heterophagic activity. A similar situation was also observed, but to a lesser extent, at the beginning of the breeding season. Outside the breeding season, the organelles of the principal cells were poorly developed throughout the epididymis, and lipofuscin pigment granules remained in the principal and basal cells of adults. Such granules were seldom seen in immature animals.     

An electron microscopical study of absorbing cells in the posterior caput epididymidis of rabbits

Summary The columnar cells in regions 3 and 4 of the ductus epididymidis in rabbits display ultrastructural features characteristic of absorbing cells. The stereocilia show basal anastomoses and often a fibrillar core continuous with a fibrillar web in the apical cytoplasm. Numerous invaginations of the slightly downy apical cell membrane and many thick-walled apical vesicles and vacuoles contain an opaque substance similar to that seen in the lumen. The vacuoles often contain small vesicles or bodies, probably formed from the vacuolar wall by budding. Numerous bodies or vacuoles with moderately dense contents are seen in the Golgi area and in the supranuclear and intranuclear cytoplasm in region 3. In region 4 they are denser and mainly seen above the nucleus. A high acid phosphatase activity was demonstrated in most dense and some light bodies. India ink introduced by way of the rete testis was taken up from the lumen into apical invaginations, vesicles and vacuoles and slowly transferred to denser bodies below the Golgi apparatus.These observations are interpreted as evidence for a resorption of substances from the lumen by a pinocytotic process, and for their storage and perhaps digestion in the dense bodies, which appear to have a lysosomal character. The Golgi apparatus is large with many vesicles of two types and empty cisternae but few typical Golgi vacuoles. The partly granular endoplasmic reticulum is very well developed and has opaque contents. Microtubules run from the terminal bar region into the Golgi area. Thick-walled vesicles occur throughout the cytoplasm, sometimes in continuity with the cell membrane. The basal parts of the cell borders often interdigitate.Supported by a grant from the Swedish State Medical Research Council.     

Cyto-epitheliochorial placenta of the viviparous lizard Pseudemoia entrecasteauxii: a new placental morphotype

The structural features of the uterine epithelium of the chorioallantoic placenta and omphalloplacenta in the viviparous Australian skink, Pseudemoia entrecasteauxii, were investigated using SEM and TEM techniques. In particular, the structural characteristics that would allow interpretation of function were analyzed, particularly those of gas exchange in the chorioallantoic placenta and histotrophy in the omphaloplacenta. Pseudemoia entrecasteauxii has a complex placenta consisting of a placentome, paraplacentome, and omphaloplacenta. The paraplacentome has a well-vascularized lamina propria in which projecting uterine capillaries displace the overlying uterine epithelial cells, reducing them to attenuated cytoplasmic extensions. Associated cell nuclei and organelles are lost from this region, to provide a capillary lumen to uterine lumen barrier of 0.5-1.0 microm. Hence, the paraplacentome is likely a prominent site for gaseous exchange via simple diffusion. The omphaloplacenta has a similar cytology to that of the placentome, but the uterine epithelial cells are hypertrophied and the apical plasma membrane actively secretes vesicles into the uterine lumen. The omphaloplacenta shows features that are associated with histotrophic transport of nutrients via vesicle secretion, very similar to that of lipid apocrine secretion. The placentome consists of cuboidal cells in the uterine epithelium, with large centrally located nuclei overlying the well-vascularized lamina propria. Although the placentome has a similar cytological structure to that of the omphaloplacenta, granules or active vesicle secretion were not observed. Thus, the placentome may be associated with histotrophy, but not via apocrine secretion. Squamate placentation is epitheliochorial; however, we propose a new term be used to describe the type of placentation in P. entrecasteauxii: "cyto-epitheliochorial," because of the extreme attenuation of uterine epithelial cells of the paraplacentome.     

Morphometric and ultrastructural features of the mare oviduct epithelium during oestrus

Morphometric, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) investigations have displayed regional differences in the mare oviductal epithelium. The entire mucosa of the oviduct was lined with a pseudostratified epithelium, which consisted of two distinct cell types, ciliated and non-ciliated. Ciliated cells were predominant in the three different segments of the oviduct and their percentage increased from fimbriae to ampulla and significantly decreased in the isthmus. SEM revealed in the infundibulum finger-like mucosal folds, some of them interconnected, in the ampulla numerous and elaborated branched folds of the mucosa, whereas the isthmus displayed a narrow lumen, short and non-branched mucosal folds. In the ampulla and isthmus the majority of non-ciliated cells showed apical blebs provided or not of short microvilli. TEM displayed different ultrastructural features of ciliated and non-ciliated cells along the oviduct. Isthmus ciliated cells presented a more electron-dense cytoplasm than in infundibulum and ampulla cells and its cilia were enclosed in an amorphous matrix. The non-ciliated cells of infundibulum did not contain secretory granules but some apical endocytic vesicles and microvilli coated by a well developed glycocalyx. Non-ciliated cells of ampulla and isthmus contained secretory granules. Apical protrusions of ampulla displayed two types of secretory granules as well as occasional electron-lucent vesicles. Isthmus non-ciliated cells showed either electron-lucent or electron-dense cytoplasm and not all contained apical protrusions. The electron-dense non-ciliated cells displayed microvilli coated with a well developed glycocalyx. Three types of granules were observed in the isthmus non-ciliated cells. The regional differences observed along the epithelium lining the mare oviduct suggest that the epithelium of the each segment is involved in the production of a distinctive microenvironment with a unique biochemical milieu related to its functional role.