Light and electron microscopical observations on the terminal airways and alveoli of the lung of the SA (Cape) fur seal Arctocephalus pusillus

During activities of the Sea Fisheries Research Institute at Kleinzee, lung samples from six South African fur seals were collected. The terminal airways showed pseudostratified ciliated columnar epithelium with numerous goblet cells and occasional brush cells. Smooth muscle, cartilage and submucosal glands were also present. The epithelium changed over a short distance, in the smaller airways, through pseudostratified columnar non-ciliated to simple cuboidal epithelium with no goblet cells. The columnar non-ciliated cells contained secretory granules, which appeared to be serous. No Clara cells were found. Cartilage and muscle were present throughout, up to the origin of the alveolar ducts, but the glands disappeared together with the goblet cells. Alveoli were lined by types one and two alveolar epithelial cells, with subepithelial capillaries. They were divided by an alveolar septum with a well developed alveolar knob. This knob contained elastic fibres and fibroblasts, but not the smooth muscle cells which are present in terrestrial mammals and in Phocidae.     

中华鳖肠道黏膜上皮细胞的超微结构

应用透射电镜技术，对15只正常中华鳖小肠和大肠黏膜上皮细胞的超微结构进行了研究。结果表明，中华鳖小肠黏膜上皮为单层柱状上皮，以高柱状吸收细胞为主，具有发达的微绒毛、线粒体、粗面内质网和膜包小囊泡，其结构的发达程度已接近高等哺乳动物和鸟类。吸收细胞之间可见少量低电子密度的亮细胞，其种类需进一步确定。杯状细胞明显可见，胞质内含大量黏液性颗粒，细胞游离面形成明显的微绒毛。内分泌细胞很少。大肠黏膜上皮细胞排列松散，全部由上下粗细不均的柱状细胞组成。大肠上皮内未见专门的杯状细胞，但上皮细胞顶部胞质内含大量黏液颗粒，细胞器分布于黏液颗粒团之下的胞质中。细胞间隙宽大明显，有利于黏膜免疫细胞出入上皮。小肠和大肠均未见腺体分布。     

Light and electron microscope studies on the nasopharynx and nasopharyngeal tonsil of the horse

Light and electron microscope studies were conducted on the nasopharynx and the nasopharyngeal tonsil of 15 young horses. The nasopharynx and nasopharyngeal tonsil was lined with pseudostratified columnar ciliated epithelium and goblet cells. The lymphoepithelium of the nasopharyngeal tonsil was folded forming crypts, the mucosa of which was modified into follicle associated epithelium characterized by stratified cuboidal epithelium, loss of cilia, absence of goblet cells and infiltration of lymphocytes. The lamina propria mucosae of the nasopharyngeal tonsil contained well-developed lymphoid tissue and clusters of seromucus acini. Scanning electron-microscopy revealed a dense mat of cilia covering the nasopharynx and nasopharyngeal tonsil. The follicle-associated epithelium consisted of different populations of microvillus cells in addition to M cells with very short microvilli and a few squamous and intermediate cells. Microvillus cells in the deeper part of the FAE had larger microvilli and their cytoplasm contained a dense population of mitochondria, smooth and rough endoplasmic reticulum, Golgi complexes and lysosomes. The flat surfaced M cell had a more electron-dense cytoplasm and contained small supranuclear vacuoles in addition to the organelles seen in microvillus cells.     

Histology,histochemistry and ultrastructure of the nasopharyngeal tonsil of the buffalo (Bubalus bubalis)

The light microscopic appearance and ultrastructure of the nasopharyngeal tonsil (tonsilla pharyngea), collected from 12 adult buffaloes of local mixed breed, were explored for the distribution of different types of epithelia, lymphoid tissue and high endothelial venules. The tonsillar mucosa was lined by pseudostratified columnar ciliated epithelium having goblet cells. The respiratory epithelium associated with the underlying lymphoid tissue formed the lymphoepithelium. The epithelium was further modified into follicle‐associated epithelium (FAE) characterized by reduced epithelial height, presence of a few dome‐shaped cuboidal cells equivalent of the M‐cells and absence of goblet and ciliated cells. The lymphoid tissue was distributed in the form of isolated lymphoid cells, diffuse lymphoid tissue and lymphoid follicles, mainly distributed within the propria‐submucosa along with the sero‐mucous glandular tissue. The goblet cells of the respiratory epithelium and the acinar cells contained different mucopolysaccharides. Scanning electron microscopy of the surface mucosa demonstrated a dense mat of cilia, island‐like arrangement of microvillus cells, M‐cells and a few brush‐like cells. The transmission electron microscopy revealed the different cell organelles of the respiratory epithelium and the FAE. Lymphocyte migration via the high endothelial venules in the propria‐submucosa was also observed.     

Microscopic anatomy of the lower respiratory tract of the grey short-tailed opossum (Monodelphis domestica)

The respiratory tracts of seven grey short-tailed opossums were histologically examined. Six opossums were prepared by perfusion with buffered formalin. Opossum seven was perfused with gluteraldehyde. Samples taken from the respiratory passages and lungs of specimens 1-6 were stained with haematoxylin and eosin. A mixture of methylene and azure blue was used for specimen 7. The trachea and right and left principal bronchi are lined with a pseudostratified ciliated columnar epithelium with occasional goblet cells. The secondary and tertiary bronchi and the primary and secondary bronchioles are lined by a simple ciliated columnar epithelium. The terminal bronchioles and a portion of the respiratory bronchioles are lined by a simple ciliated cuboidal epithelium. The terminal portion of the respiratory bronchioles and the alveolar ducts are lined with simple squamous epithelium. Alveoli are lined by type I and II pneumocytes. Tracheal glands are present in the tela submucosa. The fibromusculocartilaginous tunic of the trachea consists of c-shaped cartilage rings and the trachealis muscle. A lamina muscularis mucosa begins in the intrapulmonary portion of the principal bronchus and continues into the respiratory bronchioles. Bronchial glands are present in the propria submucosa and tela submucosa of the principal bronchi. The musculocartilaginous tunic is localized to the extrapulmonary portion of the principal bronchus. The bronchial cartilages are irregular shaped plates and limited to the extrapulmonary portion of the principal bronchus. The visceral pleura is a simple squamous mesothelium covering the outer surface of the lung.     

Viral antigen distribution in the respiratory tract of cattle persistently infected with bovine viral diarrhea virus subtype 2a

Tissues were obtained at necropsy from the nasal vestibule, turbinates, nasopharynx, trachea, tracheobronchial bifurcation, and lung from each of 10 clinically healthy calves persistently infected (PI) with bovine viral diarrhea virus (BVDV) serotype 2a. Tissues from the nasal vestibule were obtained by biopsy from five additional PI calves. Formalin-fixed tissues were processed for immunohistochemistry to localize the distribution of BVDV throughout the respiratory tract. Antigen distribution and intensity were subjectively evaluated. Throughout the respiratory tract, mononuclear leukocytes, vascular smooth muscle, and endoneural and perineural cells had BVDV immunoreactivity (BVDV-IR). Multifocally, squamous and ciliated columnar epithelium throughout the respiratory tract contained weak to moderate BVDV antigen. Viral antigen was not seen in goblet cells. BVDV-IR in mixed tubuloalveolar glands of the nasal cavity was weak to strong in serous secretory cells and ductular epithelium. Chondrocytes of the concha often contained BVDV antigen diffusely. Nasal mucus-secreting and tracheobronchial glands multifocally contained weak viral signal. In all cases, alveolar macrophages had moderate to strong BVDV-IR, whereas BVDV-IR in alveolar epithelial cells was weak to moderate. BVDV was present in interalveolar leukocytes and mesenchymal cells. Results indicate that serous secretions of the nasal cavity, productive viral infection of epithelium, and infected leukocytes in respiratory secretions are likely major sources of infectious BVDV from PI calves. The presence of BVDV antigen in respiratory epithelium is, at least, indirect support for the notion that this virus predisposes PI cattle to secondary microbial infections.     

雌性中华鳖输卵管精子储存结构的研究

应用光镜和电镜技术,系统观察雌性中华鳖输卵管精子储存情况,显示与精子储存有关组织结构与细胞形态。结果表明,精子储存在雌性中华鳖输卵管的蛋白分泌部后部至子宫部,但各段的组织结构及精子储存量存在一定差别。蛋白分泌部后部上皮较发达,由典型的高柱状纤毛细胞和分泌细胞构成,固有膜中腺体多为泡状腺。上皮和腺体中含有大量高密度的膜性分泌颗粒,此段只有少量的精子储存。峡部较窄且固有膜内无腺体,上皮排列紧密并呈迷路样迂回分布,上皮细胞内高电子密度分泌颗粒成团集中分布在核上方。峡部管腔中分布着大量的精子,靠近管腔的精子或精子头部嵌入上皮纤毛之间或顶端凹陷的胞质中,且精子嵌入部分的细胞结构保持完整。子宫前部形成垂直于管腔的储精小管(SST),子宫上皮及此段SST上皮的分泌颗粒电子密度不均,含高密度电子致密斑,腺体中的分泌颗粒也呈现不同的内部结构。子宫部及SST的管腔中储存有大量精子。这些区段复杂的细胞结构及分泌活动,可能在精子储存中发挥重要作用。     

Ultrastructural features of the uterus in the sexually immature ostrich (Struthio camelus) during periods of ovarian inactivity and activity

The ultrastructure of the surface epithelium and tubular glands of the uterus in the immature ostrich is described. In ostriches with inactive ovaries the uterus is lined by a non-ciliated simple columnar epithelium, with basally located heterochromatic nuclei. Scanning electron microscopy revealed that these non-ciliated cells have a dense microvillous cover. A simple columnar to pseudostratified columnar epithelium, comprised of non-ciliated and ciliated cells, lines the uterus in birds with active ovaries. The ciliated cells possess a wide luminal region, which contains a nucleus and various organelles. An accumulation of secretory granules was observed in the apical regions of the non-ciliated cells, as well as in a few ciliated cells. In addition to non-ciliated and ciliated cells, a cell type with rarefied cytoplasm was also identified. These cells appear to correspond to calcium secreting cells identified in other avian species. The results of this study indicate that, although uterine differentiation is present in immature ostriches with active ovaries, the production of secretory product appears to occur mainly in non-ciliated epithelial cells.     

秦岭细鳞鲑消化系统的形态学观察

运用大体解剖和常规组织学方法对秦岭细鳞鲑消化系统的组织形态进行了系统观察。结果表明,秦岭细鳞鲑的消化道由口咽腔、食道、胃和肠等4部分组成,除口咽腔外,管壁由内向外分为黏膜、黏膜下层、肌层和浆膜。口咽腔宽大,上颌、犁骨及腭骨等处布有细齿,黏膜衬以复层扁平上皮,间有较多的黏液细胞和少量的杯状细胞及味蕾;食管粗而短,黏膜向管腔突出形成多个纵行皱襞,上皮由复层扁平上皮逐渐向单层柱状过渡,上皮细胞间可见到数量较多的粘液细胞和杯状细胞,食管腺不发达;胃体积较大,呈“U”形,包括贲门部、胃体和幽门部,胃体部小凹及胃底腺结构明显,肌层发达;胃与肠相接处有63-65个幽门盲囊,肠道粗短而略微盘曲,黏膜上皮为单层柱状,未见明显的肠腺。以上结果显示,秦岭细鳞鲑消化道组织结构与其肉食性功能密切相关。肝小叶界限不清;双列肝细胞围绕中央静脉呈放射状走行,肝细胞个体大、呈多边形,核呈圆形或卵圆形,1-2个。胰脏大部分环绕前肠边缘、呈细长条索状,另可见分散于幽门盲囊及胃周围的弥散部分;其外分泌部为管泡状腺,腺细胞呈锥体形,胰岛结构明显。     

Gross,Histological and Ultrastructural Features of the Bulbourethral Gland in the Greater Cane Rat (Thryonomys swinderianus)

The present study examines the structure and ultrastructure of the bulbourethral glands in 10 sexually matured male greater cane rats raised in captivity. Following anaesthesia, the rats were perfusion‐fixed transcardially and the bulbourethral glands dissected out. Upon morphologic and morphometric analysis, the Cowper's glands were observed to have an average volume of 0.24 ± 0.08 ml, a diameter of 6.3 ± 0.6 mm and weighs 0.199 ± 0.06 g. The paired, gourd‐shaped tubuloalveolar glands were surrounded by dense connective tissues and separated into lobules by capsular septae. Each lobule consists of endpiece/secretory units and excretory ducts lined by simple glandular epithelium and pseudo‐stratified epithelium, respectively. The round end pieces consisted of 8–10 pyramidal to columnar epithelial cells with flattened, basally located nuclei and granule‐filled cytoplasm that bounded a narrow glandular lumen. The striking ultrastructural features of these secretory cells were the presence of some granules with uniform electron density and those with regions of lesser density as well as the absence of secretory vacuoles. Another unique characteristic of these secretory granules is the presence of electron dense strands radiating from their surfaces. The apical surfaces of the cells were also studded with abundant microvilli. From the findings, the structure of bulbourethral glands in the greater cane rat shows more resemblances to that of humans than to its rodent phylogeny. These findings serve as additional knowledge in the structural interpretation of the bulbourethral gland and its secretory products.     

雌性空怀双峰驼生殖道的形态结构

应用组织学方法观察了雌性空怀双峰驼生殖道的形态结构。结果显示，双峰驼生殖道的基本结构与其他哺乳动物相似，但微细结构有差异。双峰驼输卵管粘膜皱襞极其发达，分支多而呈复杂的网状迷路。皱襞基部的迷路酷似固有膜而存在腺体，迷路网格内常见细胞团块。虽然双峰驼怀孕时胎儿位于左侧子宫角，但左、右子宫角以及子宫体的组织结构基本相同。子宫内膜无肉阜，上皮下陷于固有膜内，形成大量长而弯曲的单管状腺。子宫颈固有膜浅层分布有许多小腺体，深层分布有成群的较大腺体。这些腺体为分支管状腺，腺上皮PAS强阳性。阴道粘膜上皮为复层上皮。从输卵管到阴道，粘膜上皮主要为单层柱状上皮，由纤毛细胞和分泌细胞组成，局部可见假复层柱状纤毛上皮。纤毛细胞由前向后逐渐减少，但在子宫颈仍可见到。粘膜上皮和腺上皮内夹有许多淋巴细胞或中性粒细胞，后段局部甚至见到这些免疫细胞浸润于上皮细胞间。固有膜内分布有大量淋巴细胞、中性粒细胞、肥大细胞、浆细胞和巨噬细胞，有时出现淋巴滤泡。     

Morphological and Functional Aspects of Reptilian Ultimobranchial Gland

The intention of this review is to compare studies on the morphology and histology (light and electron microscopic) of ultimobranchial glands of various groups of reptiles. Moreover, experiments (including our investigations) on suppression or stimulation of the ultimobranchial gland are included. Adult reptiles possess one (on the left side) or two ultimo-branchial glands (UBG). The UBG lie just anterior to the heart. Light as well as electron microscopically, the gland has been shown to contain follicles and cell cords (cell aggregates). The follicular epithelium is lined by simple cuboidal or pseudo-stratified columnar cells. Ciliated and goblet cells may be present in the follicular epithelia in some groups. The lumen may contain a colloid-like substance with desquamated cells or debris. The UBG of reptiles seem to be an active secretory organ with influence on calcium regulation. Other functions of calcitonin have also been suggested in reptiles for example in neurotransmission, in volume regulation, phosphate balance and promotion of bone calcification (at least in juveniles).     

Morphological features of the gastrointestinal tract of Garganey (Anas querquedula,Linnaeus 1758)—Oesophagus to coprodeum

The present work was aimed to study the gastrointestinal tract in Garganey from the oesophagus to coprodeum by the aid of the gross morphology, scanning electron microscopy and histology. At the level of the thoracic inlet, the oesophagus had a thick part which had numerous tortuous longitudinal folds carried transverse folds on them. Histologically, the thick part had thick epithelium and appeared more folded with more and larger glands. The mucosa of the proventriculus had mucosal folds appeared as a leaf-like with rounded tips. The openings of the proventriculus glands were projected with papillae on the epithelial surface. The gizzard lined with horizontal layer of cuticle which sent vertical parts of cuticle. The duodenal villi were pentagonal in shape with sharply pointed tips, while the jejunal villi were triangular with rounded ends, and the ileal villi were wide and pentagonal in shape with pointed tips. The mucosa of the duodenum, jejunum and ileum was thrown into villi lined by simple columnar epithelium with goblet cells with difference in the form, length, density and number. The apical points of the duodenal and ileal villi appeared pointed, while the jejunal villi had rounded ends. The jejunal villi were longer and numerous than the duodenal villi, but the ileal villi appeared with different length and less numerous. The mucosa of the caecum had numerous mucous glands which surrounded by circular crypts. The rectal mucosa had longitudinal folds and semilunar and circular crypts. The mucosa of the coprodeum had longitudinal folds surrounded by crypts.     

Normal ultrastructure and histochemical characteristics of canine lacrimal glands

Lacrimal glands of 12 dogs free of ocular disease were examined to determine the normal structure of these glands. The glands consisted of tubuloacinar cells that ultrastructurally and histochemically were of a single type of secretory cell in the tubules and possibly 3 types of secretory cells in the acini. The tubular epithelium contained homogenous electron-dense granules that stained as neutral glycoconjugates (periodic acid-Schiff positive and Alcian blue and high iron diamine negative). The predominant acinar cells contained granules of lesser electron density than those of the tubules, and stained as sialomucin (Alcian blue [pH 2.5] and periodic acid-Schiff-positive, and high iron diamine-negative). A second type of acinar cell was in peripheral lobules that ultrastructurally and histochemically appeared like lipid granules (positive with oil red O and osmium tetroxide). Ultrastructurally, a third type of acinar granule was finely granular, electron-lucent, and frequently coalesced. It was not readily apparent whether the latter was an artifact, a stage in the maturation of the sialomucin granules, or a third type of acinar granule. Individual acinar cells usually had a predominance of 1 granule type, but greater than 1 granule type could be found in some cells. The basal surfaces of the acinar, tubular, and ductal cells were incompletely ensheathed by myoepithelial cells. Plasma cells, lymphocytes, mast cells, endothelial cells, fat cells, and Schwann cells composed the cellular elements of the interstitium. Lymphocytes, mast cells, and nerve endings also were found in the parenchyma.     

Light and electron microscopic studies of the trachea in the one-humped camel (Camelus dromedarius)

Histology of trachea of camel (Camelus dromedarius) was studied using light, scanning (SEM) and transmission electron microscopy (TEM). Tissue samples taken from the trachea (proximal, middle and distal part) were routinely prepared for histology (LM, EM) and stained with haematoxylin and eosin, Van Giesson (VG), Alcian blue, Periodic acid schiff (PAS), Masson's trichrome (MT), Verhof, PAS-VG and PAS-MT. The trachea of camel consists of 66-75 incomplete cartilaginous rings of hyaline. The lamina epithelium is composed of pseudostratified-ciliated columnar epithelium with many goblet cells. Submucosal layers were loose connective tissue with many elastic fibres. The mucosal and submucosal layers were 517.2 +/- 61.6 microm (n = 20) thick. Submucosal glands were tubuloalveolar with mucous (acidic and neutral) secretions. Trachealis muscle was attached to the inside sheet of tracheal cartilage. Ultrastructural studies showed that surface epithelium is pseudostratified with mucus-producing goblet cells, ciliated and basal cells, similar to other mammals. The ciliated cells contained many mitochondria, oval nucleus and many big granules. In scanning electron microscopy (SEM) studies, viscoelastic layers were observed on the epithelial surface of trachea, and there were highly condensed cilia under this layer.     

Morphological Studies on the Development of the Bronchial Epithelium of the Fetal Camel Lung

Light microscopic observations revealed that in camel foetuses of 25 mm crown‐to‐rump length (CRL) the primordial tubular system of the prospective lung was formed of several tubules lined by undifferentiated columnar epithelium. Intra‐epithelial neuroendocrine cells were the first elements to be differentiated in the lining epithelium of the primordial tubular system of the prospective lung as early as 25 mm CRL. On reaching 50–67 mm CRL, the primordial tubular system started to differentiate into two systems of primordial tubules, the prospective bronchial or light tubules and the future respiratory or dark tubules. The lining epithelium of the prospective bronchial tubules revealed a clear evidence of ciliogenesis as early as 80 mm CRL. From 800 mm CRL onwards, the bronchial epithelium demonstrated ciliated and non‐ciliated secretory cells. The non‐ciliated secretory cells of the bronchial epithelium of fetal camel lung showed moderate reaction to AB/PAS technique, for the first time, in fetuses reaching 600 mm CRL.     

An ultrastructural study of adenocarcinoma of the small intestine in sheep

The ultrastructural appearance at primary and metastatic sites of ten ovine small intestinal adenocarcinomas was that of scirrhous tubular adenocarcinoma. Polygonal undifferentiated tumor cells had desmosomes, folded nuclei, and moderate numbers of mitochondria but few other organelles. More differentiated cells were columnar with apical microvilli and basal nuclei. They contained granular endoplasmic reticulum, Golgi and secretory granules. Microvillus-lined intracytoplasmic lumina (5-10 micron diameter), fibrous filaments (10 nm diameter, up to 1.4 micron length) and tubular paracrystalline arrays (hexagonal symmetry, 37-nm periodicity) in lumina and secretory granules were seen in some tumor cells in all ten sheep.     

On the Development of the Feline Skeleton

Light microscopic observations revealed that in camel foetuses of 25 mm crown-to-rump length (CRL) the primordial tubular system of the prospective lung was formed of several tubules lined by undifferentiated columnar epithelium. Intra-epithelial neuroendocrine cells were the first elements to be differentiated in the lining epithelium of the primordial tubular system of the prospective lung as early as 25 mm CRL. On reaching 50–67 mm CRL, the primordial tubular system started to differentiate into two systems of primordial tubules, the prospective bronchial or light tubules and the future respiratory or dark tubules. The lining epithelium of the prospective bronchial tubules revealed a clear evidence of ciliogenesis as early as 80 mm CRL. From 800 mm CRL onwards, the bronchial epithelium demonstrated ciliated and non-ciliated secretory cells. The non-ciliated secretory cells of the bronchial epithelium of fetal camel lung showed moderate reaction to AB/PAS technique, for the first time, in fetuses reaching 600 mm CRL.     

Morphology of the non-sensory tissue components in rat aging vomeronasal organ

With 30 figures, 3 histograms and 3 tables SUMMARY: The vomeronasal organ (VNO) is a chemosensory organ that detects environmental pheromones. The morphology of the 'non-sensory' epithelium (NSE) of the VNO and its lamina propria, as well as how it relates to ageing has received little attention. Histological, histochemical, morphometric and ultrastructural techniques were used to study the morphological structure of the rat NSE in five adult (3 months old) and five aged (2-2.5 years old) male albino rats. In adult rats, the NSE contained dark and light columnar cells with predominance of the latter. The surface of the epithelial cells was covered with microvilli and/or cilia. The lamina propria contained serous vomeronasal glands (VNGs), smooth muscles with numerous variable-sized mitochondria, vessels including lymphatic capillaries and nerve bundles. The following changes were detected in aged rats. The NSE exhibited an increase in number of dark columnar cells. Some cells revealed a prominent cell coat, dense aggregation of filaments in the luminal cytoplasm and appearance of multinucleated cells. Their surface revealed malformed configuration. Large mitochondria (2 μm), formed by fusion, were frequently observed in the smooth muscle cells of the lamina propria. Lipid droplets were frequently detected both in the VNGs acini and in the lymphatic endothelium. Ageing affected both the cells of the tissues and the extracellular matrix.     

The mucosa of the digestive tract in Micropogonias furnieri: a light and electron microscope approach

The histomorphological aspects as well as the histochemical content and distribution of glycoproteins (GPs) in the mucosa of the digestive tract of the white croaker Micropogonias furnieri were studied. The buccopharyngeal cavity and the esophagous showed a squamous stratified epithelium with mucous cells. The stomach presented three portions: cardias, fundus and pylorus. Tubular glands formed by a single type of gland cell were located along the cardias and fundus. Histochemical tests showed that the buccopharyngeal cavity and the esophagous presented the largest amount of the different types of mucosubstances. Both organs showed abundant secretory mucous cells that synthesize large quantities of neutral, sulphated and sialylated GPs. The surface epithelium in the cardias and fundus synthesized and secreted scarce sialylated and neutral GPs whereas the secretions of the apical surface were abundant. The pylorus secreted large amounts of neutral as well as sulphated and sialylated GPs. Gland cells secreted neutral GPs. The ultrastructural features of the gut cells were quite similar to those of other teleosts. The buccopharyngeal cavity and the esophagous surface epithelial cells, identified by their superficial localization, were characterized by cytoplasmic vesicles of different size. Abundant goblet cells with secretory mucous granules were also present. Gastric glands in the stomach contained just one form of cell with a fine structure similar to cells that secrete pepsinogen.