Profiles Of Rough Endoplasmic Reticulum Research Articles

Apoplasmic barrier in the extrafloral nectary of Citharexylum myrianthum (Verbenaceae)

The cytological changes underlying the formation of an apoplasmic barrier in the multi-layered extrafloral nectaries of Citharexylum myrianthum are compatible with the synthesis, transport and deposition of suberin. In terms of ontogenesis and function, the intermediate layers of these nectaries are homologous with the stalks of nectar-secreting trichomes. Anticlinal cell wall impregnations are common in trichomatic nectaries and their functions as endodermis-like barriers have been discussed because of possible direct effects on the nectary physiology, mainly in the nectar secretion and resorption. However, the cytological events linked to nectary wall impregnations remain little explored. This study documents the ontogenesis and the fine structure of the EFN cells, and cytological events linked to the wall impregnations of multi-layered extrafloral nectaries (EFNs) in Citharexylum myrianthum Cham. (Verbenaceae). EFNs are patelliform, and differentiated into (a) a multicellular foot, which is compound in structure and vascularised with phloem strands, (b) a bi-layered intermediate region with thickened cell walls and (c) a single-layered secretory region with palisade-like cells. EFNs are protodermal in origin, starting with a single protodermal cell and ending with the complex, multi-layered structure. The cell wall impregnations first appear in the very young EFN and increase towards maturity. Lipid patches (assumed to be suberin) are deposited on the inner faces of the primary walls, first along the anticlinal walls and then extend to the periclinal walls. On both walls, plasmodesmata remain apparently intact during the maturation of the EFNs. In the peripheral cytoplasm there are abundant polymorphic plastids, well-developed Golgi bodies often close to rough endoplasmic reticulum profiles, mitochondria and polyribosomes. Cytological events linked to the wall impregnations are consistent with suberin synthesis, transport and deposition. Our findings offer new insights into the structure-properties of specialised nectary cell walls and so should contribute to our knowledge of the physiological and protective roles of this structure in nectar glands.

Vesicular trafficking in abscission zone cells during ethylene-promoted fruit abscission in citrus

Abscission is a cell separation process by which plants are able to shed aerial organs. This physiological process proceeds through a breakdown in adhesion between a group of specialized cells that usually differentiate in particular positions of the plant body named abscission zones (AZs). The occurrence of fruit abscission has special relevance in fruit tree crops such as citrus since it may have negative consequences on yield. To identify AZ-specifically expressed genes involved in citrus fruit abscission, we used a combined strategy of laser microdissection and microarray analysis to compare the global expression profiles of calyx abscission zone (AZ-C) cells of citrus fruits and adjacent fruit rind cells during ethylene-promoted fruit abscission. The comparison enabled to identify a set of potentially specific genes including those associated with vesicular transport and intracellular transport vesicles. The first ultrastructural observations on AZ-C cells during fruit abscission (Iwahori and van Steveninck, 1976) showed an increase in rough endoplasmic reticulum profiles and dictyosome stacks with associated vesicles that fused with the plasma membrane to release their contents to the cell wall. In this comparative transcriptomic survey, we highlighted specific over-expression in AZ-C cells of genes belonging to the major transport steps along the secretory and the endocytic pathways of vesicle trafficking. In particular, it is likely that some of the genes identified in this study would represent major candidate genes for further biotechnological approaches aimed at trying to solve or at least mitigate agronomic issues regarding citrus fruit abscission.

In-vitro differentiation of adult human bone marrow-derived mesenchymal stem cells into bone-forming cells

Background Bone tissue constitutes one of the most important units of the locomotive system. However, its regenerative capacity remains limited and bone diseases are still major socioeconomic issues. Stem cells have been the subject of much research over the recent years and comprise a promising tool for cellular therapy. Osteoblastic differentiation from human bone marrow-derived mesenchymal stem cells (hBM-MSCs) shows the best regenerative potential; thus, they are an ideal candidate as a source for osteogenic cells. Aim The aim of this study was to evaluate hBM-MSCs histologically to highlight the morphological changes occurring during their osteogenic differentiation. Materials and methods Cells from passage 9 were seeded in complete culture medium. When cells became confluent the osteogenic medium was applied. Osteogenic differentiation was detected histologically on days 7, 14, and 21 using light and electron microscopes. Results Cells in osteogenic medium became cuboidal in shape with extracellular deposits, which stain positively red with Alizarin red stain. Ultrastructurally, they revealed the picture of active secreting cells. Their cytoplasm showed euchromatic nucleus, multiple mitochondria, profiles of rough endoplasmic reticulum, well-developed Golgi complex, and lysosomes. Conclusion The hBM-MSCs represented a source of osteogenic differentiating cells. Moreover, passage 9 secured cells for further studies while maintaining their osteoblastic differentiation. In addition, the study shed light on the ultrastructure of these cells during differentiation, which revealed the characteristics of active secreting cells.

Integument cell differentiation in dandelions (Taraxacum, Asteraceae, Lactuceae) with special attention paid to plasmodesmata.

The aim of the paper is to determine what happens with plasmodesmata when mucilage is secreted into the periplasmic space in plant cells. Ultrastructural analysis of the periendothelial zone mucilage cells was performed on examples of the ovule tissues of several sexual and apomictic Taraxacum species. The cytoplasm of the periendothelial zone cells was dense, filled by numerous organelles and profiles of rough endoplasmic reticulum and active Golgi dictyosomes with vesicles that contained fibrillar material. At the beginning of the differentiation process of the periendothelial zone, the cells were connected by primary plasmodesmata. However, during the differentiation and the thickening of the cell walls (mucilage deposition), the plasmodesmata become elongated and associated with cytoplasmic bridges. The cytoplasmic bridges may connect the protoplast to the plasmodesmata through the mucilage layers in order to maintain cell-to-cell communication during the differentiation of the periendothelial zone cells.

Structure of the ovary of the articulate brachiopod Hemithyris psittacea (Rhynchonelliformea, Brachiopoda) with special reference to connective tissue pillars

Summary The microscopical anatomy of the ovary of the articulate brachiopod Hemithyris psittacea was investigated by light and transmission electron microscopy. The ovary has a reticulate structure with connective tissue pillars, which pierce it. The pillars have a wine-glass form and consist of two parts—a column that is attached to inner mantle wall and a broader region that is attached to outer mantle wall. The coelomic epithelium of the column contains myofilaments orientated across the gonad cavity. The broader part contains parenchymal tissue with cells that are filled by the expanded profiles of rough endoplasmic reticulum. Globular cells with membrane-free inclusions were found in the mantle wall. The connective tissue pillars may prevent the collapse of gonad during gametogenesis and/or contract to facilitate spawning. It is assumed, that the parenchymal tissue and the globular cells take part in the nutrition of the oocytes and in the storage of the nutrients.

Primary Culture of Skeletal Muscle Cells as a Model for Studies of Toxoplasma gondii Cystogenesis

Toxoplasma gondii is a protozoan pathogen of birds and mammals, including humans. The infective stage, the bradyzoite, lives within cysts, which occur predominantly in cells of the central nervous system and skeletal and cardiac muscles, characterizing the chronic phase of toxoplasmosis. In the present study, we employed for the first time primary mouse culture of skeletal muscle cells (SkMC) infected with bradyzoites, as a cellular model for cystogenesis. The interconversion of bradyzoite and tachyzoite was analyzed by immunofluorescence using 2 stage-specific antibodies, i.e., anti-bradyzoite (anti-BAG1) and anti-tachyzoite (anti-SAG1). After 24 hr of interaction only bradyzoites were multiplying, as revealed by anti-BAG1 incubation; interconversion to tachyzoites was not observed. After 48 hr of infection, 2 types of vacuoles were seen, i.e., BAG1+ and SAG1+, indicating the presence of bradyzoites as well as their interconversion to tachyzoites. After 96 hr of infection, BAG1+ vacuoles presented a higher number of parasites when compared to 48 hr, indicating multiplication of bradyzoites without interconversion. Using ultrastructural analysis, bradyzoites were found to adhere to the cell membranes via both the apical and posterior regions or were associated with SkMC membrane expansions. During bradyzoite invasion of SkMC, migration of the rough endoplasmic reticulum (RER) profiles to the parasite invasion site was observed. Later, RER profiles were localized between the mitochondria and parasitophorous vacuole membrane (PVM) that contained the parasite. After 31 days of parasite-host cell infection, RER profiles and mitochondria were not observed in association with the cyst wall. Alterations of the PVM, including increased thickness and electrondensity gain on its inner membrane face, were observed 48 hr after infection. Cystogenesis was complete 96 hr after infection, resulting in the formation of the cyst wall, which displayed numerous membrane invaginations. In addition, an electron-dense granular region enriched with vesicles and tubules was present, as well as numerous intracystic bradyzoites. These results show that the in vitro T. gondii model and SkMC are potential tools for both the study of cystogenesis using molecular approaches and the drug screening action on tissue cysts and bradyzoites.

Giant Cell Osteosarcoma in the Calvarium of a Cat

Feline primary osteosarcomas involving the skull are extremely rare. When they occur, orbit, mandible, and maxilla are the most common sites. Microscopically, scattered multinucleated giant cells (MGCs) are not an uncommon occurrence in osteosarcoma (OSC), but they are generally in low number. Only in a rare variant, the giant cell-rich OSC, are MGCs the prevalent cell type. Although osteoclast and osteoblast origin have been postulated in human and veterinary literature, the origin of MGCs in osteosarcomas is poorly understood. This report describes a giant cell-rich OSC in the calvarium of a 13-year-old spayed female shorthair cat. The animal exhibited a range of neurologic signs, including left circling, compulsive gait, lack of proprioception, and bilateral absence of menace reaction, with indication of left forebrain involvement. Gross lesions were characterized by a multilobate, spherical mass located in the left calvarium, compressing the left forebrain. Histologically, the tumor was characterized by scattered nests of MGCs separated by small bundles of pleomorphic, fusate to polygonal cells. Between spindle cells, osteoid was very sparse and arranged in thin strands. Immunohistochemical stains for vimentin were positive, with no detectable cellular staining for cytokeratin, S-100 protein, or Class II major histocompatibility complex. Ultrastructurally, MGCs contained profiles of rough endoplasmic reticulum; no lysosomes were observed. The origin of MGCs in osteosarcoma remains obscure, and our results confirm their ambiguous identity.

Extramedullary Plasmacytoma of the Salivary Gland in Two Syrian Hamsters (Mesocricetus auratus)

Two Syrian hamsters developed marked swelling of the ventral neck. Histologic examination of both masses revealed that the submaxillary salivary glands were effaced by large numbers of neoplastic plasma cells. In one hamster, neoplastic cells had infiltrated the adjacent lymph node. The neoplastic cells expressed CD79a antigen and were negative for CD3, lambda, and kappa light chains. Ultrastructural features of neoplastic cells in the salivary gland of one hamster included abundant cytoplasmic rough endoplasmic reticulum profiles, and peripherally displaced nuclei that contained marginated heterochromatin, consistent with plasma cells. Salivary gland plasmacytomas are extremely rare in humans and have not previously been reported in nonhuman species. The occurrence of such neoplasms in two hamsters suggests that this species may be predisposed to developing tumors of this type.

Epithelioid cellular chordoma of the sacrum: a potential diagnostic problem

We describe a sacral chordoma composed of solid nests of epithelioid cells. The tumor originated in the presacral area. Extensive clinical workup did not reveal any other lesion in the patient. In contrast to typical chordomas, this tumor contained only rare physaliferous cells, had no myxoid stroma, and was immunohistochemically unreactive with the antibody to S-100. The diagnosis of chordoma was supported by electron microscopy, which showed that the tumor cells contained numerous mitochondria surrounded by profiles of rough endoplasmic reticulum. The abundance of mitochondria, the narrow intercellular spaces, combined with a lack of glycogen, and a lack of extracellular myxoid material accounted for the epithelioid appearance of the tumor. We report this case to point out that the cellular chordomas can appear epithelioid in the sacrum and they may resemble metastatic squamous or transitional cell carcinomas.

Morphological and enzymatic analysis of the midgut of Anopheles darlingi during blood digestion

The midgut of adult female Anopheles darlingi is comprised of narrow anterior and dilated posterior regions, with a single layered epithelium composed by cuboidal digestive cells. Densely packed apical microvilli and an intricate basal labyrinth characterize each cell pole. Before blood feeding, apical cytoplasm contains numerous round granules and whorled profiles of rough endoplasmic reticulum. Engorgement causes a great distension of midgut. This provokes the flattening of digestive cells and their nuclei. Simultaneously, apical granules disappear, the whorls of endoplasmic reticulum disassemble and 3 h post bloodmeal (PBM), nucleoli enlarge manyfold. An intense absorptive process takes place during the first 24 h PBM, with the formation of large glycogen inclusions, which persist after the end of the digestive process. Endoproteases activities are induced after bloodmeal and attain their maximum values between 10 and 36 h PBM. At least two different aminopeptidases seem to participate in the digestive process, with their maximum activity values at 36 and 48 h PBM, respectively. Coarse electrondense aggregates, possibly debris from digested erythrocytes, begin to appear on the luminal face of the peritrophic membrane from 18 h PBM and persist during all the digestive process, and are excreted at its end. We suggest that these aggregates could contain some kind of insoluble form of haem, in order of neutralize its toxicity.

Electron Microscopic Studies on the Adrenal Gland of Hipposiderid Male Bat, Hipposideros lankadiva

Adrenocortical and medullary zones are distinct in Hipposideros lankadiva . The cortex is further differentiated into three distinct zones-zona glomerulosa, fasciculata and zona reticularis. The cells of zona glomerulosa are polyhedral and are arranged in the form of acini or group of cells. Mitochondria are rod shaped with lamelliform or tubular cristae and are randomly distributed in the cytoplasm. Golgi apparatus is indistinct. Profiles of rough endoplasmic reticulum are scarce. Few profiles of vesicular agranular endoplasmic reticulum are observed. Few lipid droplets and lysosomes are also encountered in the cytoplasm.

Characterization of foam cells in nipple aspirate fluid.

Foam cells with abundant vacuolated cytoplasm are prominent in most samples of spontaneous nipple discharge, nipple aspirate fluid, and ductal lavage. Although several investigators have attempted to characterize these cells, there is no consensus about whether these cells are derived entirely from macrophages or from both ductal epithelial cells and macrophages. Using immunocytochemical methods, we studied 20 paired specimens of nipple aspirate fluid containing abundant foam cells obtained from the involved breast of women with in situ or invasive carcinoma and from the contralateral normal breast. We used a cocktail of anticytokeratin antibodies including AE1, AE3, and CAM5.2 and the macrophage marker KP1 (CD68). In addition, we examined samples by electron microscopy. The foam cells were consistently negative for cytokeratin and positive for CD68. In every case electron microscopy of these cells revealed irregular outlines with short cytoplasmic processes. The cytoplasm was abundant and contained numerous lysosomes, a small Golgi complex, lipid droplets, mitochondria, and short profiles of rough endoplasmic reticulum. There was no evidence, however, of cell junctions or tonofilaments. The immunocytochemical and electron microscopic findings of our study together clearly support a macrophage derivation for foam cells in nipple aspirate fluid.

Morphological influence of ascorbic acid deficiency on endochondral ossification in osteogenic disorder Shionogi rat.

The influences of chronic deficiency of L-ascorbic acid (AsA) on the differentiation of osteo-chondrogenic cells and the process of endochondral ossification were examined in the mandibular condyle and the tibial epiphysis and metaphysis by using Osteogenic Disorder Shionogi (ODS) rats that bear an inborn deficiency of L-gulonolactone oxidase. Weanling male rats were kept on an AsA-free diet for up to 4 weeks, until the symptoms of scurvy became evident. The tibiae and condylar processes of scorbutic rats displayed undersized and distorted profiles with thin cortical and scanty cancellous bones. In these scorbutic bones, the osteoblasts showed characteristic expanded round profiles of rough endoplasmic reticulum, and lay on the bone surface where the osteoid layer was missing. Trabeculae formation was deadlocked, although calcification of the cartilage matrix proceeded in both types of bone. Scorbutic condylar cartilage showed severe disorganization of cell zones, such as unusual thickening of the calcification zone, whereas the tibial cartilage showed no particular alterations (except for a moderately decreased population of chondrocytes). In condylar cartilage, hypertrophic chondrocytes were encased in a thickened calcification zone, and groups of nonhypertrophic chondrocytes occasionally formed cell nests surrounded by a metachromatic matrix in the hypertrophic cell zone. These results indicate that during endochondral ossification, chronic AsA deficiency depresses osteoblast function and disturbs the differentiation pathway of chondrocytes. The influence of scurvy on mandibular condyle cartilage is different from that on articular and epiphyseal cartilage of the tibia, suggesting that AsA plays different roles in endochondral ossification in the mandibular condyle and long bones.

Ultrastructure of the normal adult human female prostate gland (Skene's gland).

The predominant cells of female prostatic glands lining their lumen were found to be tall cylindrical secretory cells with short stubby microvilli, protuberances of the apical cytoplasm, and with bleb formation. Abundant secretory vacuoles and granules, rough endoplasmic reticulum, developed Golgi complexes and numerous mitochondria are characteristic of their active secretory configuration with apocrine (apical blebs) and merocrine (secretory vacuoles and granules) type of secretion. Basal (reserve) cells were seen to be located between the secretory (luminal) cells and the basement membrane. Their ground cytoplasm is dense with rough endoplasmic reticulum and mitochondria. Their nuclei, unlike those of secretory cells, possess more peripheral condensed chromatin, denser dispersed chromatin and sporadic nucleoli. Besides the two basic types of mature prostatic cells intermediary cells were also seen, located between the basal and secretory cells or in their close vicinity. Their cytoplasm exhibits numerous profiles of rough endoplasmic reticulum and free ribosomes. Secretory vacuoles and granules were mostly practically absent (type 1 intermediary cells) so that they resembled basal (reserve) cells. In some of them, however, as in secretory cells, such secretory elements do gradually appear (type 2 intermediary cells). The finding of intermediary cells in the lining of prostatic glands supports the role of basal (reserve) cells in the renewal of cells in glands of the female prostate. The first ultrastructural analysis of the normal female prostate performed by transmission electron microscopy showed that, as in the postpubertal male, the prostatic glands in the adult female display mature secretory and basal cells. The results of the presented study further corroborate the contemporary concept of the female prostate as a functional genitourinary organ.

Ontogeny of NADPH-d expression in the thymic microenvironment of the chick embryo.

Nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) histochemistry was used to demonstrate the presence of nitric oxide in the developing chicken thymus. NADPH-d was first expressed in the epithelial cells located at the corticomedullary junction of the thymic rudiment on day 13 of incubation. The number of labelled cells gradually increased from day 13 to day 21. Ultrastructural evidence showed that the labelling was localized in a heterogeneous population of cells in the medulla near the corticomedullary junction, comprising the cystic, undifferentiated, myoid, lymphoid and epithelial reticular cells. At this age, the vascular endothelium was NADPH-d positive. Labelling was also detected in some macrophages. The reaction product primarily labelled profiles of rough endoplasmic reticulum and to a lesser extent the outer membranes of mitochondria, portions of the nuclear envelope and the Golgi apparatus. By day 18/19, NADPH-d-labelled nerve fibres were occasionally observed in the interlobular connective tissue. By day 21, these fibres formed perivascular plexuses. Labelled nerve fibres were occasionally observed in the medullary parenchyma. Possible functions of nitric oxide in the embryonic thymus are discussed.

Development of the Rough Endoplasmic Reticulum in Incompatible Pollen Tubes of Brugmansia suaveolens (Solanaceae)

After self-pollination of Brugmansia suaveolens, a characteristic stacking of cisternae of the rough endoplasmic reticulum (RER) was seen in pollen tubes. The stacking commenced within less than 6 h after pollination and was the earliest noticeable ultrastructural change. This phenomenon was roughly coincident with a retarded growth rate of the pollen tubes. The RER formed intricate complexes consisting of combinations of two seemingly different arrangements, longitudinally stacked cisternae and concentric stacks. We present a model that interprets the stacked RER profiles as longitudinal sections through cylindrical formations with rounded ends. The appearance of profiles of RER aggregates in electron micrographs is determined by the angle of sectioning. Concentric profiles might be cross sections through such cylinders or sections through rounded or doughnut-shaped appendages.

Quantitative Ultrastructural Evaluation of Satellite Cells in Soleus Muscle from Rats Kept in Hypokinesia

The aim of the present study is a morphometric, ultrastructural evaluation of satellite cells (SC) derived from the soleus muscle (SOL) of rats exposed to conditions of hypokinesia for a period of 7 or 21 days. Qualitative and quantitative analysis of 320 electron micrographs of SC from each group was carried out. After 7 and 21 days of immobilization, profiles of the SC, in contrast to the control group, had a lower mean surface area, had a cylindrical shape, and exhibited more folded membrane. Analysis of electron micrographs of SC showed that after immobilization, a lower number of SC contained profiles of mitochondria, rough endoplasmic reticulum (RER), and Golgi. Volume fractions of RER were twofold lower after 7 days of hypokinesia and fivefold lower after 21 days compared with the control group. The SC of SOL of rats subjected to hypokinesia differed from the control group by a markedly decreased number of ribosomes and RER profiles. After 21 days of immobilization the ultrastructural characteristics of SC were typical for cells in an inactive state showing various degrees of degeneration. The results of our study presented here permit the conclusion that 21 days of hypokinesia induce a depression of SC activity, whereas subtle changes in SC appear only after 7 days of immobilization.

Unique Intracerebral Tumor with Divergent Differentiation in a Patient Presenting as NF2: Report of a Case with Features of Astrocytoma, Ependymoma, and PNET

Patients with neurofibromatosis 2 (NF2) are predisposed to a variety of neoplastic and dysplastic lesions, including schwannomas, neurofibromas, meningiomas, astrocytomas, and ependymomas, as well as entities such as meningioangiomatosis, schwannosis, and hamartomas. This study reports a unique intracerebral frontotemporal tumor in a 6-year-old boy with presumed NF2, on the basis of bilateral cerebellopontine tumors consistent with acoustic neuromas. The intracerebral tumor revealed a variety of histological patterns, including foci of primitive neuroectodermal tumor (PNET), low-grade astrocytoma and ependymoma, as well as neuroepithelial rests with immature ganglion cells and hamartomatous areas. The MIB-1 labeling index ranged from 63% in the foci of PNET to 4–7% in other foci. The PNET component revealed immunopositivity for synaptophysin and neurofilament and showed cells with delicate intercellular junctions, profiles of rough endoplasmic reticulum, mitochondria, and dense core granules, and cell processes with microtubules and neurofilaments. The glial and ependymal components showed bundles of glial filaments and prominent cell junctions, cilia, and microvilli. The hamartomatous component also included aggregates of cells with hyaline eosinophilic cytoplasm. By EM these cells contained abundant amorphous flocculent material. This constellation of pathologic findings, especially the finding of PNET, is unique and not previously reported in the setting of NF2.

Ultrastructural Changes in Cotyledons of Pineapple Guava (Myrtaceae) During Somatic Embryogenesis

Ultrastructural studies (SEM and TEM) were performed on cotyledons of pineapple guava ( Feijoa sellowianaBerg, Myrtaceae) induced to form embryos on medium containing 1.0 mg l −1(4.5μ M2,4-D) and 0.3 Msucrose. At the time of culture, the cells were filled with protein and lipid bodies. Microbodies and poorly differentiated organelles could also be seen. In contrast to germinating cotyledons, where lipid and protein reserves were quickly metabolized, cells of the embryogenically induced cotyledons showed evidence of reserve consumption only after 5 d of culture. Subepidermal cells of the upper cotyledonary surface underwent several divisions giving rise to a meristematic layer of several cells thickness from which somatic embryos developed. Embryos could also be formed directly by successive divisions of epidermal cells. Cells involved in somatic embryo formation contained a large nucleus with a conspicuous nucleolus and dense cytoplasm where numerous ribosomes, mitochondria, plastids with starch and short profiles of rough endoplasmic reticulum were present. Plasmodesmata were present both in cell walls of the meristematic cells and in few celled embryos whereas in degenerating embryos or in more advanced stages of somatic embryo development no plasmodesmata could be found. Although oil bodies were not observed in the meristematic cells they were identified in very young embryos, being the first reserve compounds to appear. Cells not involved in somatic embryo differentiation were characterized by the presence of several microbodies containing a crystalloid inclusion and elongated mitochondria.

Ultrastructural immunogold localization of vimentin and S-100 protein in guinea pig pars tuberalis.

Ultrastructural features of vimentin- and S-100 protein-positive cells in guinea pig pars tuberalis were examined by immunoelectron microscopy with the postembedding immunogold method. Interstitial cells that exhibited elongated cytoplasm and partly surrounded the specific secretory cells were filled with intermediate filament bundles on which immunogold particles for vimentin were densely located. Small colloid-containing follicles were occasionally distributed in the cranial region surrounding the median eminence. The follicular cells lining the luminal cavities contained considerable amounts of intermediate filaments immunoreactive for vimentin. Some specific secretory cells displayed various amounts of intermediate filaments immunoreactive for vimentin, which were scattered throughout the cytoplasm or concentrated around the nucleus. The specific secretory cells in which many parallel profiles of rough endoplasmic reticulum (RER) or a large number of mitochondria were dispersed throughout cytoplasm, i.e., being highly metabolically active, were devoid of intermediate filaments. The ventrocaudal region in continuity with the pars distalis was occupied by cells that were packed with vesicular inclusions, considered to be dilated cisternae of RER, and frequently formed colloid-containing follicles. Immunoreactivity for S-100 protein was located exclusively in this novel cell type. Immunogold particles for S-100 protein were distributed in the cytoplasmic matrix and were also associated with the membrane of vesicular inclusions. Gold particles were also detected on the tight junctions, desmosomes, and fibril materials at the apical cell regions of the colloid-containing follicles. The finding of two separate populations of cells containing vimentin and S-100 protein, respectively, supports the idea of functional separation in the pars tuberalis.