Development of the pigeon bursa of Fabricius. A scanning and transmission electron microscope study

The development of the pigeon bursa of Fabricius was investigated by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Particularly SEM emphasized that the bursa during development progressively undergoes hypertrophia and hyperplasia of lobules constituting the plicae. Moreover modifications of surface epithelium were observed. During early days of development after hatching, epithelium showed evenly distributed microvilli which become shorter and shorter and also unevenly distributed with aging. In addition TEM allowed us to state that the bursa of Fabricius in pigeons, during development, undergoes morphological modifications among which one of the most remarkable is the gradual and continuous increasing of lymphocytes. Another one, as it was already observed in chickens (Frazier 1974), consists in the contemporaneous but independent development of cortex and medulla, even if we observed undifferentiated epithelial cells moving from medulla to cortex and a migration of lymphocytes and mesenchymal cells from cortex to medulla (Tar et al. 1958; Ackerman et al. 1964; Naukkarinen et al. 1978).     

A scanning electron microscope study of vascular development in the dental papilla of prenatal rat molars

Summary Prenatal development of the vascular supply to the dental papilla was studied in the maxillary first molar teeth of rats from 18.5 to 22.0 days gestation, using the vascular casting/scanning electron microscope method. Five pulp horns developed in order, central, distal, mesial, disto-lingual and mesio-lingual with the development of the dental papilla. The first vessels that invaded each pulp horn were slightly depressed and formed an irregular network. The newly developed blood vessels were found to grow by sprouting and loop formation. After the invasion, blood vessels at the top of the horn became wider and then diminished in size to form a dense vascular network. The growth of the blood vessels in the latter stages is thought to take place mainly at the tops of the horns, and it is suggested that narrower capillaries arise from wide vessels. A dense and flattened vascular network consisting of thin blood vessels was formed when mesenchymal cells were beginning to differentiate into odontoblasts. The increase in density is thought to correlate with the differentiation of odontoblasts.     

Apoptosis in mouse primordial germ cells: a study by transmission and scanning electron microscope

A detailed study of the death in vitro of mouse primordial germ cell (PGCs) by means of transmission and scanning electron microscopy is reported. The results show that after 4–5 h of culture 15–20% PGCs assume the typical morphological features of apoptotic cells, including chromatin condensation in dense masses under the nuclear membrane, compaction of the cytoplasm, crowding of organelles and surface protuberances. Cells then break up into discrete fragments (apoptotic bodies) which eventually degenerate by secondary necrosis. It is possible that apoptosis plays a biologically useful role in avoiding uncontrolled PGC proliferation and in eliminating misplaced germ cells whose survivial might be harmful to the animal.     

The formation and final structure of the oocyst wall ofEimeria acervulina: A transmission and scanning electron microscope study

Summary The formation and final structure of the oocyst wall ofEimeria acervulina is described, based on a detailed electron microscope study of the maturing oocysts. After fertilization of the macrogametocyte the wallforming bodies of type I progressively undergo disaggregation into smaller bodies and eventually move into spaces left by the pellicular membranes of the zygote, which simultaneously separate and elevate away from the zygote cytoplasm to form the outer layer of the oocyst wall. A newly formed membrane separates this layer from the cytoplasm. Following the formation of the outer layer, another membrane separates and elevates away from the cytoplasm, and the wall-forming bodies of type II, which by now have migrated to the periphery, move into the spaces and fuse together to form the inner layer of the oocyst wall. A newly formed membrane separates this layer from the cytoplasm. The wall of the young oocyst thus consists of two membrane-bound layers of approximately similar thickness; the outer layer being osmiophilic whilst the inner one is paler. An overlying membrane covers the oocyst. The surface of the oocyst wall was smooth in appearance when viewed with the scanning electron microscope.     

A scanning electron microscope study of myoepithelial cells in the intercalated ducts of rat parotid and exorbital lacrimal glands

Myoepithelial cells in the intercalated ducts of rat parotid and exorbital lacrimal glands were examined by scanning electron microscopy. The basal surface of the intercalated ducts revealed myoepithelial cells running parallel with its long axis. These myoepithelial cells were linked with one another, forming a well-developed network, and numerous wrinkles running transversely were observed on the surface of the myoepithelial cells. Also, some myoepithelial cells in the terminal portion linked with those in the intercalated duct. Based on these findings, it is suggested that myoepithelial cells in the intercalated duct may function as a protective wall against constriction of the narrow lumen of the intercalated duct when it is subjected to pressure by the surrounding tissues.     

Basal lamina fenestrations in the human colon: transmission and scanning electron microscope study

Basal lamina at the interface between colonic epithelial cells and the lamina propria was exposed by incubating colonic specimens in 1% boric acid solutions. Examination of this epithelial-stromal interface by scanning electron microscopy (SEM) showed a smooth, slightly undulating basal lamina covering crypts and luminal surfaces. The basal lamina on the luminal surfaces had numerous round or ovoid fenestrations, most measuring 2.5-4.0 microns. These were continuous with channels in the collagen fiber network of the lamina propria. Except very near the surface, no fenestrations were found in the basal lamina lining the crypts. Transmission electron microscopy (TEM) of serial thin sections of colonic mucosa without the epithelial cells removed showed only a few actual basal lamina fenestrations. Rarely, epithelial cell processes extended into the lamina propria through the basal lamina. Most of the fenestrations seen by SEM appeared to correspond spatially by TEM to foci of close contact between the basal lamina and underlying fibroblastic cell processes. At these sites the basal lamina and fibroblastic cell process might be removed along with the overlying epithelial cells during processing with boric acid. These data support functional differences in epithelial-stromal interaction between cell populations lining the luminal surface and those making up the crypt lining and pericryptal fibroblast sheath. The TEM findings demonstrate that the human colonic basal lamina is not absolutely continuous and that the development of basal lamina fenestrations and epithelial cell processes extending into the lamina propria is not pathognomonic of neoplastic transformation and stromal invasion.     

On the migration of myogenic stem cells into the prospective wing region of chick embryos. A scanning and transmission electron microscope study

In chick embryos undifferentiated myogenic stem cells migrate from the ventrolateral somite respectively dermatome edge into the prospective wing region after the second day of incubation. At first, single cells that are elongated in mediolateral direction, later also small groups of cells, are found in the space between somites and somatopleura at the wing bud level. The leading ends of the migrating cells are formed like finger-shaped lobopodia as well as flattened lamellipodia from which thin filopodia arise. The main structural features of the cell processes are microtubules and microfilaments predominantly oriented parallel to the long axis of the cells. The filopodia are found to be in close connection with the surrounding network of collagen fibrils. Since the main strands of the fibrils show a mediolateral orientation, it may be assumed that the direction of cell migration depends on the arrangement of the collagen fibrils.     

A study of interaction between lymphocytes and tumor cells with the scanning electron microscope

It was shown with the aid of the scanning electron microscope that in the course of interaction between lymphocytes of BALB/c mice with methylcholanthrene-induced sarcoma and autologous tumor cells the state of the surface membranes of the cells underwent substantial changes. Lymphocytes separated from autologous tumor cells by means of a Millipore filter impermeable to the cells formed long cytoplasmic processes which passed through the pores of the filter and established close contact with the tumor cells. Under these conditions the tumor cells became spherical in shape, they swelled, and holes appeared in the surface membrane, leading to lysis of the cell.Department of Immunology of Carcinogenesis, Institute for Problems in Oncology, Academy of Sciences of the Ukrainian SSR, Kiev. Laboratory of Histology, A. N. Severtsov Institute of Evolutionary Morphology and Ecology of Animals, Academy of Sciences of the USSR, Moscow. (Presented by Academician of the Academy of Medical Sciences of the USSR A. D. Timofeevskii.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 79, No. 3, pp. 85–87, March, 1975.     

A scanning and transmission electron microscopic study of contractile trabecules in the rat spleen

The fine structures of contractile trabecules in the splenic red pulp of the rat were examined by electron microscopy to elucidate their participation in the active contraction of the spleen. Numerous fine thready trabecules were developed in the red pulp. They were enveloped with a cytoplasmic layer of reticular cells and consisted of elongated smooth muscle cells, fascicles of collagenous fibrils and elastic fibers. Their fibrous components in the capsular ends extended in a triangular form of fanribs into the fibrous tunica of the capsule. Smooth muscle cell-like interstitial cells (SIC) were situated in the interfibrous spaces. Flattened SICs were affixed with cytoplasmic processes to the elastic lamina. The trabeculocapsular junctions were represented on the elastic lamina by grouped or isolated circular patches with concentrically arranged triangular processes and were also observed on the capsular serosa by plaques with scarce microvilli of serosal cells. Smooth muscle cells of the fine trabecules were equipped on the cell surface with anchoring structures to extracellular fibrous elements as previously described by Gabella (1981). Close associations were also seen between the smooth muscle cells and elastic fibers which were terminated to the fascicles of collagenous fibers. Cell-to-cell connections were expressed by fibrous connections between spiny processes and a small number of puntate intermediate junctions and nexuses. Unmyelinated nerve fibers with adrenergic terminals were seen in the intercellular spaces. We propose that for the rat spleen, the fine trabecules in the red pulp are muscular contractiles which are responsible for the active contraction due to sympathetic stimuli and the administration of alpha 1-adrenoceptor agonists, while the elastic lamina in the capsule plays a role in the comprehensive contraction of the subcapsular vascular bed.     

Microcirculation of the rat adrenal gland: A scanning electron microscope study of vascular casts

Blood vascular beds of the rat adrenal gland were filled with methacrylate resin and observed with a scanning electron microscope. The classical findings on mammalian adrenal glands of Flint (1900), Bennett and Kilham (1940), and Gersh and Grollman (1941) were confirmed. The cortical capillaries arise from the cortical arteries and converge at the corticomedullary junction into the peripheral venous radicles which flow into the tributaries of the central vein. The medullary capillaries originate from the medullary arteries and drain through the deep venous radicles into the tributaries of the central vein. No direct connection between the cortical and medullary capillaries was noted except for rare communications via the peripheral venous radicles. These findings show that most of the cortical blood, rich in glucocorticoids, flows in the medulla, not through the medullary capillary plexus but exclusively through the radicles of the central vein. Evidence for adrenal portal vessels could not be found.     

A scanning electron microscope study of aberrations in the prism pattern of rat incisor inner enamel

The prism pattern in the inner enamel of adult rat incisors was studied with the SEM in unfixed tissues that had been sectioned, ground, polished, and etched. Six different types of aberrations in the prism pattern were encountered: 1. Prism lamellae may be shorter than the mesio-lateral width of enamel. 2. Prism lamellae may deviate from a transverse orientation. 3. Prism lamellae may “fuse” or “bifurcate.” 4. Prisms of two adjacent lamellae may pursue a common course. 5. Prisms may change direction. 6. Variations exist in the outline of transversely cut prism profiles. Aberrations were observed at any distance from the dentino-enamel junction. These observations were used as a basis for an analysis of the movement of ameloblasts during rat incisor amelogenesis. It was concluded that it is physically possible for the ameloblasts to create the observed aberrations as they move along the path of the prisms. However, the aberrations seem to make it more difficult to understand the factors controlling ameloblast movement. Occasionally crystallite bridges connecting adjacent prisms were observed. A configuration resembling a bifurcating prism is presented.     

CSF-contacting pinealocytes in the pineal recess of the Mongolian gerbil: A correlative scanning and transmission electron microscope study

The pineal recess of the Mongolian gerbil was studied using correlative scanning and transmission electron microscopy. The surface of the pineal recess can be subdivided into three distinct zones: (1) central, (2) transitional, and (3) peripheral. In the gerbil, the deep pineal gland is located deep to the central and transitional zones. The ependyma of the peripheral zone is densely ciliated and resembles that of the main ventricular lining. Ependymal cells of the transitional zone are sparsely ciliated but possess numerous microvilli on their apical surfaces. Supraependymal neurons were identified in the transitional zones. These cells appear to make a synaptic-like contact with the underlying ependymal cells. Of the three zones, the central zone demonstrated the greatest amount of morphological variability. Although a number of supraependymal structures could be identified in the central zone, the most remarkable feature was the presence of protruding cells that possessed no significant surface features Employing correlative transmission electron microscopy, the protruding cells were shown to be CSF-contacting pinealocytes. The number of CSF-contacting pinealocytes present in the central zone varied from one cell to large clusters that covered the entire zone. The results of this investigation demonstrate the presence of a direct contact and the potential for interaction between the deep pineal gland and the CSF of the pineal recess in the gerbil.     

The interstitial cells of Cajal of the rat stomach. A light and electron microscope study

Interstitial cells of Cajal (ICC), presumed to have a smooth muscle-like nature and to play a pacemaker role, are usually identified for their peculiar ultrastructural features and specific location throughout the gut muscle wall. A Zinc-Iodide-Osmium (ZIO) impregnation for ICC identification under the light microscope has been proposed. However, controversies as to certain ICC identification under both light and electron microscopes are still present, due to their ultrastructural features somewhat similar to the fibroblast ones and the low specificity of the ZIO-staining. The rat stomach has been studied. Some specimens have been routinely processed for electron microscopy, some others have been ZIO-impregnated and further routinely processed for both light and electron microscopy, in order to assure that all the cells presumed to be ICC for their ZIO-staining affinity are the same cells identified as ICC with routine electron microscope procedures, and not fibroblasts. The routine electron microscope examination made it possible to identify within the rat gastric muscle coat two cell populations, one with the same location and morphology as those reported in literature for the gastric ICC, and a second one with a similar location, but showing undoubted fibroblastic features. ZIO-staining, under both light and electron microscopes, revealed ZIO-stained cells distributed within the muscle coat in a manner identical to that of the ultrastructurally identified ICC. Under electron microscope examination, this cell type only was fully impregnated by the zinciodide deposits, whereas all other cell types, including fibroblast-like cells, were devoid of them. These data confirm that ICC can be electively ZIO-stained and that these cells and fibroblasts are two distinct cell types, as ultrastructural and physiological reports had previously suggested.     

The pelvic epithelium of the rat kidney: A scanning and transmission electron microscopic study

The renal pelvis of the rat is characterized by extensions called specialized fornices that penetrate into the outer zone of the outer medulla (a type II as classified by Pfeiffer, 1968, 1970). The renal pelvic epithelium, therefore, covers areas of the kidney from the inner medulla, the inner and outer stripe of the outer medulla, and the cortex. The renal pelves of seven rats were studied by transmission and scanning electron microscopy. The transitional epithelium on the nonparenchymal surface of the pelvis was three to four cell layers thick (zone 0–1). This epithelium became thinner where it covered the renal cortex (zone 1–2) or the outer medulla. Although the apical cells of the epithelium retained the asymmetric luminal unit-membrane plaques, the number of cytoplasmic fusiform vesicles decreased as one studied the epithelium progressing over the zones from cortex toward papilla. Scanning electron microscopy demonstrated a small number of surface cells of a different morphology that were characterized by apical microvilli. The number of these microvillous lining cells increased as the epithelium covering the outer (zone 2–3) and inner (zone 3–4) stripe regions of the outer medulla was viewed, until the inner medulla was entirely covered by this cell type. In a reciprocal manner, the cells with the asymmetric apical plaques decreased in numbers and in their morphologic specialization in each successive region. The epithelium surrounding the inner medulla (zone 6–7) was completely devoid of this transitional cell type. Judging from the morphologic characteristics of the epithelia, one could surmise that little exchange of urea, water, and salts would occur with the extrarenal connective tissue or the cortical parenchyma. Recycling of urea might become more important physiologically with the outer stripe parenchyma, and even more so with the increased surfaces of the inner stripe parenchyma that lined the secondary pyramid, as well as with the epithelium lining the inner medulla.     

The structure of Schwann cells in unmyelinated fibres. A qualitative and quantitative electron microscope study

The structure, size and distribution of many cytoplasmic components of Schwann cells associated with unmyelinated axons in lizard thoracic spinal roots were analysed under the electron microscope. The percentages of Schwann cell cytoplasmic area occupied by the following cytoplasmic components were determined: mitochondria, Golgi apparatus, granular endoplasmic reticulum, multivesicular bodies, smooth endoplasmic reticulum, lipofuscin granules, peroxisome-like bodies, autophagic vacuoles, dense bodies and lipid droplets. A linear correlation was found between the sectional areas of the mitochondria and granular endoplasmic reticulum of the Schwann cell and both length of Schwann cell plasma membrane profile and size of the related axoplasm. The structure of Schwann cells associated with unmyelinated axons and that of Schwann cells associated with myelinated axons were compared in the same species and in the same region of the peripheral nervous system using the same fixative and the same preparation technique. Some differences were detected in the organization of the granular endoplasmic reticulum, in the presence of cilia and in the percentages of cytoplasm occupied by various components. The hypothesis that Schwann cell mitochondria and granular endoplasmic reticulum are involved in the production and storage of proteins for the plasma membrane of this cell as well as the hypothesis that these organelles are involved in the production and storage of protein metabolites which are subsequently transferred to the related axons seem applicable not only to Schwann cells associated with myelinated axons (Pannese et al., in press), but also to those associated with unmyelinated ones.     

The implantation chamber,blastocyst and blastocyst imprint of the rat: A scanning electron microscope study

Splitting the uterus longitudinally through implantation sites makes it possible to obtain access to blastocysts and implantation chambers during stages of implantation of the blastocyst in the rat. On the afternoon of day 5 of pregnancy, blastocysts lie in a shallow antimesometrial depression and tend to fall free of the uterus when the chamber is opened. On day 6, blastocysts are oriented in a mesometrial-antimesometrial plane, occupy a distinct implantation chamber, and tend to adhere to one side or the other of the uterus, leaving an imprint on the contralateral side. After about noon of day 6, some of the blastocysts split in half laterally, and by day 7 all blastocysts which are exposed are split. In addition to demonstrating increased adhesion of blastocyst to uterine epithelium, the procedure clearly shows the progressive elongation of the implantation chamber. The embryonic cell mass is specifically oriented on day 6, and is clasped but not distorted, whereas the abembryonic trophoblast is slightly compressed and indented by the uterine epithelium. The microvilli of the uterine epithelium within the imprint become progressively flattened when compared to the microvilli of the implantation chamber outside of the imprint. The method provides a means of gaining direct access to the surface of uterine epithelium precisely where it has been in association with the blastocyst not only for scanning electron microscopy but also for studies of the properties of the surface constituents.     

A scanning electron microscope study of human pulp fibroblasts in culture

Human pulp tissue was obtained from the first lower right molar of a 24 year old male patient during oral surgery. The tooth was healthy with no evidence of carious lesions. 6 cultures were initiated by the explant method. The cells were isolated and carried through 8 passages. Samples from different passages were fixed, dehydrated, and prepared for scanning electron microscopic observations. Some cells had a smooth surface, while others showed microvillous projections from their surface. Both cell types were seen in the same culture. However, in the third passage all the cells were of the smooth surface type. The cells were interconnected by a system of fibrils that were related to their surface. These fibrils also appeared to anchor the cells to the surface of the coverslips on which they were growing. This study showed that more than one cell type apparently existed in human pulp fibroblasts culture. Whether these cell types have different functional properties, or whether they are different developmental stages of the same cell type in culture, remain to be determined.