Fine structure of the epidermis of the optic tentacle in a slug, Limax flavus L.

The epidermis at the tip of the optic tentacle in Limax flavus is constructed of columnar epithelial cells, distal processes of nerve cells, and scattered processes of the collar cells. The epithelial cells extend stout microvilli called plasmatic processes by Wright perpendicularly from the free surface. Each plasmic process branches into a few terminal twigs embedded in a fuzzy filamentous substance. Most nerve cells have their nuclei under the basal lamina. The distal processes of these nerve cells reach the free surface and send long microvilli to form the spongy layer under a filamentous covering. At the side surface of the tentacle the epithelial cells are cuboidal or squamous and the neural elements are fewer. Here, no spongy layer is formed; and the collar cell processes are replaced by the lateral cell processes. Peculiar secretion granules are contained in the lateral and collar cell processes as well as in their cell bodies situated beneath the basal lamina.     

Morphologic changes of the basal lamina in the small intestine of Xenopus laevis during metamorphosis

Further investigations of the epithelial and mesothelial basal lamina of the duodenum of Xenopus laevis during metamorphosis were performed by means of scanning electron microscopy (SEM) and histochemical techniques using polyethyleneimine (PEI) to demonstrate anionic sites as well as light- and transmission-electron-microscopic methods involving morphometric analysis. The basal lamina of the duodenal epithelial cells was smooth, and it was occasionally curved along the processes of the epithelial cells (stages 56-59). The basal lamina became thicker by folding, and the thickness of the folded basal lamina exceeded 1 micron (stages 60-62). Subsequently, the folded basal lamina disappeared gradually and became almost smooth again and consisted of only one layer (stages 63-66). After removing the epithelium by boric acid, SEM revealed that the small ridges of the basal lamina protruded like a mesh-work into the luminal side, and the luminal surface of the basal lamina became smooth at later stages of the metamorphic climax. The electron-dense granules of PEI-positive material were localized at both sides of the lamina densa at regular intervals (80-100 nm). The basal lamina of the mesothelial cells was almost smooth at stages 56-59 and started to show occasional slight folding. This folding became continuous and deeper (stages 60-62). The folded mesothelial basal lamina disappeared except for the cell-associated basal lamina and became smooth again at later stages of the metamorphic climax (stages 63-66). These morphologic changes of the basal lamina observed in the epithelium and mesothelium may be induced by common factors. We suggest that physical changes in the small intestine involving the shortening and narrowing should be a main factor to cause these changes in the basal lamina. Furthermore, morphometric analysis proposed that the basal lamina becomes more complex by adding newly synthesized basal lamina material, especially in the epithelium.     

Morphogenetic determination of the dermoepidermal interface during tail regeneration in Rana catesbeiana.

During regeneration of the amputated tadpole tail, reconstruction of the epithelial basal lamina and basement lamella occurs only after the other major morphogenetic processes are well established. At 4 days after tail transection of the bullfrog tadpole, electron microscopy of the internal surface of the basal cell layer of the blastemal epithelium reveals it to be relatively free of extracellular matrix. By 11 days a basal lamina of distinct regularity has formed, and the first rodlets and fibers signaling the replacement of the collagenous basement lamella are identified. At 15 days the basal cells of the epithelium start to exhibit specialization of their internal cell surfaces: Hemidesmosomes and associated tonofilaments appear, and the adepidermal globular layer is formed. Orthogonal packing of collagen plies begins by 19 days after transection, the number of layers exceeding 22 in the latter stages of regeneration.     

Intercellular relationships in the external glial limiting membrane of the neocortex of the cat and rat.

The external glial limiting membrane of the cerebral cortex appears to be a complete astrocytic mantle covering the pial surface of the molecular layer. It consists of flattened cell bodies arranged singly or in small groups spaced about 100 mu apart and multitudes of interdigitating processes arrayed in layers. The glial mantle is thicker in the sulci than on the gyri. It is covered externally by a basal lamina which is associated with collagenous fibrils and cells of the pia mater. The extracellular space in aldehyde-perfused material appears as a regular, electron-lucent interval 150 A wide between adjacent cell membranes. Gap junctions are frequently encountered in the external glial limiting membrane; desmosomes are present between astrocytic processes but are seen much less often.     

The fate of the expected fusion zone in rat fetuses with experimentally-induced cleft palate—An ultrastructural study

The ultrastructure of palatal processes from rat fetuses with cleft palate induced by Meclozine or amniotic sac puncture was studied from gestation days 16 to 20. Degenerative changes involving the cytoplasm of epithelial cells of the expected fusion zone began on Day 16, when palatal fusion occurs normally in the midline seam in untreated fetuses. By Day 17 the cells in the same region showed more advanced degeneration, degraded cytoplasm and occasional dead epithelial cells appearing to be removed via intercellular extrusion or by macrophages. In the expected fusion zone, the basal lamina became discontinuous while in the oral and nasal zones of the palatal processes a firm epithelium-connective tissue attachment developed. As gestation progressed, the zone became narrower until, by Day 20, no degenerating cells remained. Replacement of lost epithelium appeared to have occurred from the nasal side, the termination of the keratinizing oral epithelium having the same location throughout the period studied. The results support the concept of programmed cell degeneration in palatal processes but indicate that this event has the potential to extend over a relatively long period of fetal life.     

Structure and regeneration of the planarian basal lamina: An ultrastructural study

The structure and regeneration of the planarian subepidermal basement membrane or basal lamina have been electron microscopically examined, particularly in relation to the changes of extracellular products at the wounded area. The intact basal lamina consists of three structural elements; namely, an electron-lucent zone, a limiting layer and a microfibrillar layer. Ultrastructural changes during wound healing have suggested that the amorphous material secreted in the interspace between the epidermal cells and blastema contains precursors of the basal lamina. Within the amorphous zone two distinct phases of the basal lamina regeneration are observed: one is a reconstitution of the limiting layer and the other is a polymerization of the microfibrils. The limiting layer arises from areas subjacent to newly developed hemidesmosomes of epidermal cells. The unit microfibrils are formed from an accumulation of the precursors through transitional smaller microfibrils. At the late stage, individual mature microfibrils are regularly lined with the limiting layer and cell membranes of the newly differentiated muscle fibres. On the basis of these observations we suggest that the planarian basal lamina is regenerated by the interaction between epidermal cells and myoblasts.     

An Ultrastructural Investigation on Vitellophage Invasion of the Yolk Mass during and after Germ Band Formation in Embryos of the Stick Insect Carausius morosus Br.

Developing embryos of the stick insect Carausius morosus were examined ultrastructurally with a view to studying vitellophage invasion of the yolk mass during and after germ band formation. Newly laid eggs in C.morosus have a unique yolk fluid compartment surrounded by a narrow fringe of cytoplasm comprising several small yolk granules. Vitellophages originate mainly from a thin layer of stem cells, the so-called yolk cell membrane, interposed between the germ band and the yolk mass. Throughout development, a thin basal lamina separates the yolk cell membrane from the overlying embryo.
Vitellophages extend from the yolk cell membrane with long cytoplasmic processes or filopodia to invade the central yolk mass. Along their route of entrance, filopodia engulf portions of the yolk mass and sequester it into membrane-bounded granules. As this process continues, the yolk mass is gradually partitioned into a number of yolk granules inside the vitellophages.
Later in development, the yolk cell membrane is gradually replaced by the endodermal cells that emerge from the anterior and posterior embryonic rudiments. From this stage of development onwards, vitellophages remain attached to the basal lamina through long filopodia extending between the endodermal cells. Yolk confined in different vitellophagic cells appears heterogeneous both in density and texture, suggesting that yolk degradation may be spatially differentiated.     

STRUCTURAL FEATURES OF THE EPITHELIO-MESENCHYMAL INTERFACE OF RAT DUODENAL MUCOSA DURING DEVELOPMENT

In fetal rats 5–7 days before birth, the duodenal epithelium is separated from mesenchymal cells by a well-defined basal lamina. By 3–4 days before birth, when small rudimentary villi are first seen, direct contact between epithelial and mesenchymal cells occurs by means of epithelial cell cytoplasmic processes which project through gaps in the basal lamina into the lamina propria. At contact sites, the epithelial and mesenchymal cell plasma membranes were less than 100 A apart but membrane fusion was not seen. In number and size these epithelial cell processes increase strikingly during the last 2 days of gestation, and they persist in large numbers until 7–10 days after birth. Thereafter, they decrease gradually in both number and size until 3–4 wk after birth, when the morphology of the epithelio-mesenchymal interface resembles that seen in adult rats, i.e., there are only rare epithelial cell processes which penetrate deeply into the lamina propria. The presence of a large number of epithelio-mesenchymal contact sites during the period of rapid growth and differentiation of duodenal mucosa may reflect epithelio-mesenchymal cell interactions which may facilitate the maturation of the duodenal mucosa.     

Chorioallantoic placenta formation in the rat: I. Luminal epithelial cell death and extracellular matrix modifications in the mesometrial region of implantation chambers.

On days 7 and 8 of pregnancy, mesometrial regions of rat gestation sites were examined by light microscopy and transmission electron microscopy to determine what changes occur before the chorioallantoic placenta forms in that region. By day 7, gestation sites contained a uterine lumen mesometrially and an antimesometrial extension of the uterine lumen, the implantation chamber. The implantation chamber consisted of a mesometrial chamber between the uterine lumen and the conceptus, an antimesometrial chamber that contained the conceptus, and a decidual crypt antimesometrial to the conceptus. Stromal cells that formed the walls of the implantation chamber were closely packed decidual cells, while those that surrounded the uterine lumen were loosely arranged. Late on day 7, a portion of the epithelium lining the mesometrial chamber was degenerating, but this area of initial degeneration was never adjacent to the antimesometrial chamber. By early day 8, most of the epithelial cells lining the mesometrial chamber were degenerating and were being sloughed into the chamber lumen. Although degeneration of these epithelial cells morphologically resembled necrosis, it was precisely controlled, since adjacent epithelial cells lining the uterine lumen remained healthy. The space that separated the denuded luminal surface of the mesometrial chamber from underlying decidual cells became wider and was occupied by an extracellular matrix rich in cross-banded collagen fibrils. Decidual cell processes, that earlier had penetrated the basal lamina beneath healthy epithelial cells, protruded into this matrix and penetrated the basal lamina at the luminal surface. By late day 8, large areas of denuded chamber wall were covered with decidual cell processes, little remained of the basal lamina, and cross-banded collagen fibrils were scarce in the area occupied by decidual cell processes. During the times studied, uterine tissues that formed the walls of the mesometrial chamber were not in direct contact with the conceptus. This study indicates that trophoblast does not play a direct role in epithelial degeneration, basal lamina penetration, or extracellular matrix modifications in the mesometrial region of implantation chambers where part of the chorioallantoic placenta forms, although trophoblast may be required to trigger or modulate some of the changes.     

Ultrastructure of the colon of Locusta migratoria

The ultrastructure of the colon of Locusta migratoria is described. The colon is lined by a thick cuticle that, for the most part, adheres to the underlying epithelium. The cuboid epithelial cells are characterized by moderate invaginations of the apical and, to a lesser extent, basal plasma membranes; the lateral plasma membranes are relatively flat. The bulk of the mitochondria are located in the apical region of the cell and are not particularly associated with any of the plasma membranes. The basal region of the cells contains much rough endoplasmic reticulum, glycogenlike granules, and a predominance of spherical, electron-dense bodies of various sizes. Where muscle fibers make contact with the epithelium, the cells are much reduced; the cytoplasm is usually less electron-dense, and, typically, the nucleus has a thick layer of granular material associated with the inner nuclear membrane. The apical and basal plasma membranes of the reduced epithelial cells contain numerous hemidesmosomes. The apical hemidesmosomes occur in pairs around an extracellular space that contains electron-opaque material. The latter forms tonofibrillae that extend into the endocuticle. Bundles of microtubules are associated with the hemidesmosomes. The tubules traverse the cell from the apical to the basal region. The possible significance of these findings is discussed.     

Rat endometrial stromal-epithelial response to estrogen infusion

Morphologic changes at the interface of rat endometrial luminal epithelial cells and the stromal cells immediately adjacent were examined and correlated with hypertrophy of the epithelial cells during estradiol (E2) infusion (1 microgram E2/24 h). While the lamina densa in castrate endometrium was thread-like, it became thicker and apparently more granular in some areas below the luminal epithelium during E2 infusion. However, no changes were seen in the intensity of laminin-like immunoreactivity at various time points up to 96 hours after beginning infusion, suggesting that these alterations were due to changes in nonlaminin components. The stromal cells adjacent to the basal lamina in the castrate state had cell processes extending toward the epithelium that terminated on the basal lamina. Under estrogen infusion, stromal cell bodies migrated close to and became oriented along the basal lamina. No interruptions were seen in the lamina densa or in the laminin-like immunoreactivity in the basal lamina. Thus, there were no direct morphologic interactions between epithelial and stromal cells induced by estrogen. Some of the stromal cells developed a dilated rough endoplasmic reticulum and some developed multiple elaborate processes within 41 hours after minipump implantation. Within 28 hours, nuclear hypertrophy had occurred in 15% of the epithelial cell layer. If interactions occur between stromal and epithelial cells, and morphologic evidence presented here suggests they do, then all such interactions are through an intact lamina densa-laminin layer, and any chemical mediators affecting cells on opposite sides of the lamina densa must migrate through it.     

Incomplete fusion of the epithelial and endothelial basement membranes in interspecies hybrid glomeruli

Avascular, undifferentiated mouse kidneys transplanted onto quail chorioallantoic membrane differentiate and become vascularized by quail vessels. The glomeruli which form under these conditions consist of mouse podocytes and quail endothelial cells. Immunohistochemistry has shown that the glomerular basement membrane (GBM) has a dual origin, as integral basement membrane components are produced by both podocytes and endothelial cells. In electron microscopy this GBM is composed of two partially separated layers, an epithelial and an endothelial basal lamina which both have a lamina densa and a lamina rara. These two basal laminas are partially fused, but there are large areas where this fusion does not occur. In some places of incomplete fusion, fibrillar extracellular material is seen between and beneath the GBM. It is concluded that basement membrane components derived from the different species can interact partially, but the fusion is incomplete. The abnormal assembly of the epithelial and the endothelial basal laminas might be due to molecular differences between the components produced by the two cell lineages. In spite of the incomplete fusion, the system used serves as a good model-system to study basement membrane formation, since the cells organize in a histiotypic fashion and form true vascularized glomeruli.     

Hypertrophic smooth muscle

Summary In adult guinea-pigs, oral to a partial obstruction to the flow of ingesta in the ileum there is a marked increase in the diameter of the intestine and a hypertrophy of the muscle coat. The features of the intramuscular blood vessels and of the extracellular materials were studied by electron microscopy. There is a small increase in the amount of intercellular space measured morphometrically. The basal lamina surrounding the hypertrophic muscle cells is more prominent than in controls. In the intercellular space between muscle cells, in addition to collagen fibrils, there is abundant amorphous material of medium electron density and streak-like, electron-dense material often similar to thickened basal laminae. The total amount of stroma (intercellular materials) present in a unit length of intestine is greatly increased in hypertrophy; a role of the muscle cells in the production of new collagen and other extracellular elements is suggested by the present observations. Many new intramuscular blood vessels (mainly capillaries, some of which are fenestrated) are formed during hypertrophy of the intestinal wall, so that the circular muscle layer remains as well vascularized in the hypertrophic intestine as in the controls. Blood vessels are not formed within the longitudinal muscle layer.     

Cranial meninges of goldfish: age-related changes in morphology of meningeal cells and accumulation of surfactant-like multilamellar bodies

In the optic tectum of goldfish, the outer, middle and inner layers of the endomeninx were evident in animals ranging in age from 1 month to several years. The outer layer in young animals consisted of closely overlapping cells with intertwined processes, whereas in the older animals it contained large extracellular spaces. The intermediate layer cells were always arranged in a single continuous layer, but in young animals they overlapped extensively with one another toward their edges whereas in the oldest animals they became extremely flat and non-overlapping. The inner layer included an outer tier of cells with their bases adhering to the intermediate layer, and an inner tier of cells detached from both the intermediate layer and the basal lamina overlying the brain parenchyma. Inner layer cells contained many large vacuoles that were in continuity with the extracellular space. With age, the extracellular space and the vacuolar system expanded, and the inner layer evolved into a meshwork of attenuated cytoplasmic processes embedded in the granular extracellular matrix. Another age-related feature was the accumulation adjacent to the basal lamina of uniform disc-shaped membranous structures, resembling multilamellar bodies of lung surfactant. These disc bodies were apparently generated by the coalescence of vesicles formed at the surface of the inner layer cells, possibly as a by-product of protein secretion by these cells.     

Fine structure of differentiating ameloblasts in the kitten.

The fine structure of differentiating ameloblasts was studied in the lower second molar of 1-week-old kittens after perfusion fixation with and without subsequent decalcification. The differentiation zone was divided into three phases. In Differentiation 1, ameloblasts are about 27 mum long and face an uninterrupted basal lamina. The predentin adjacent to the basal lamina contains a few collagen fibrils oriented mainly at right angles to the ameloblast surface. The specialized predentin forms a well-defined layer, up to 1.5 mum thick, referred to as the junctional layer. In Differentiation 2, ameloblast processes extend through the basal lamina and the thickness of the junctional layer. The processes consist of cytoplasmic sheets forming a honeycomb-like network. Dentin starts to calcify after process-formation is underway. Two distinct types of odontoblast processes, having different shapes and contents, come in contact with the ameloblasts and push into the ameloblastic layer. In Differentiation 3, stippled material appears in the extracellular spaces between ameloblasts. Later, stippled material-like substances appear in the predentin close to the ameloblast apex and close to odontoblast processes within the dentin. Ameloblasts now are up to 40 mum high. Enamel secretion starts in small circumscribed areas which gradually enlarge, leading to the disappearance of the ameloblast processes. These findings are compared with results obtained in other species, including man, and their possible functional significance is discussed.     

The electron microscopy of normal human oesophageal epithelium.

Oesophageal biopsies were studied with the electron microscope. Three layers were identified, as in the light microscopy of the oesophageal epithelium: basal, prickle and funtional cell layers. A continuous basement membrane separated the lamina propria from the basal cells. The basal cell membrane carried hemidesmosomes, desmosomes and microvillous processes. Their cytoplasm contained the usual organelles plus free ribosomes and tonofirbrils. Prickle cells contained glycogen rosettes and many tonofilaments, and their cell membrane many microvillous and demosomal processes, in places elaborated into desmosome fields. In both these layers there was a wide intercellular space containing some particulate and membranous debris. The flattened cells of the functional layer had fewer desmosomes and microvilli but abundant glycogen and tonofilaments, and a narrow intercellular space. Membrane coating granules first reaching a maximum in the functional cell layer appeared in the upper prickle cell layer and few persisted into the surface cells. The apical cell membrane of the most superficial cells was thickened and had few small microvillous processes, which were covered with a filamentous "fuzzy" coat. No keratohyaline granules were present. Papillae of lamina propria contained capillaries, some with a fenestrated endothelium.     

Laminin ultrastructural immunolocalization in rat testis during ontogenesis

Summary The synthesis of one of the main glycoproteins of the basement membrane, the laminin, was demonstrated by ultrastructural immunolocalization during rat foetal (16th day to 20th day of gestation) and postnatal development of the testis. The lamina densa, part of seminiferous tubular basement membrane, is labeled uniformly at all studied stages. The lamina lucida is not well defined before the postnatal stages, at which times discrete immunostaining extends from the lamina densa to the adjacent seminiferous epithelial cells (spermatogonia and Sertoli cells). The extracellular matrix around the peritubular cells is not labeled before birth. Intracellular immunostaining was detected as early as the 16th day of gestation in both Sertoli cells and cells around the seminiferous tubules which will transform later into peritubular cells. It was located in rough endoplasmic reticulum (RER) cisternae and secretory vesicles. After 18–20 days of postnatal life, the immunostaining faints progressively. Some positive material is seen in the RER of the gonocytes at all studied stages.Sertoli cells and peritubular cells are the main producing cells of laminin after the 16th of gestation. The laminin secreted by gonocytes may play an important role in adhesion of gonocytes to the lamina densa and adjacent Sertoli cells before their transition from basal compartment to adluminal compartment.     

Invagination of Ectodermal Placodes Is Driven by Cell Intercalation-Mediated Contraction of the Suprabasal Tissue Canopy

Ectodermal organs such as teeth, hair follicles, and mammary glands begin their development as placodes. These are local epithelial thickenings that invaginate into mesenchymal space. There is currently little mechanistic understanding of the cellular processes driving the early morphogenesis of these organs and of why they lead to invagination rather than simple tissue thickening. Here, we show that placode invagination depends on horizontal contraction of superficial layers of cells that form a shrinking and thickening canopy over underlying epithelial cells. This contraction occurs by cell intercalation and is mechanically coupled to the basal layer by peripheral basal cells that extend apically and centripetally while remaining attached to the basal lamina. This process is topologically analogous to well-studied apical constriction mechanisms, but very different from them both in scale and molecular mechanism. Mechanical cell–cell coupling is propagated through the tissue via E-cadherin junctions, which in turn depend on tissue-wide tension. We further present evidence that this mechanism is conserved among different ectodermal organs and is, therefore, a novel and fundamental morphogenetic motif widespread in embryonic development.     

Hemocytes contribute to both the formation and breakdown of the basal lamina in developing wings of Manduca sexta

Monoclonal antibodies (MAbs) raised against wing tissues of Manduca sexta recognize epitopes shared by both hemocytes and basal laminae. During the last larval stadium, the basal lamina of moth wing epithelium forms after hemocytes have migrated into the space adjacent to basal surfaces of epithelial cells. As adult development commences, hemocytes participate in phagocytosis of the same basal lamina; and as dissolution of the basal lamina proceeds (day 2-day 5 post-pupation), wing epithelial cells send forth long basal processes and rearrange within the plane of the epithelium. During this period of cell rearrangement, the immunoreactivity of the basal lamina decreases in concert with an increase in immunoreactive vesicles within hemocytes; and at the ultrastructural level, hemocytes have been observed to engulf fragments of basal lamina. The distribution of immunolabel in the developing moth wing suggests that hemocytes contribute not only to the formation of the wing's basal lamina but also to its breakdown. Since basal laminae are probably important determinants of epithelial form and pattern, hemocytes also contribute to the shaping of epithelial populations.     

Fibronectin and its relation to the basal lamina and to the cell surface in the chicken blastoderm

Summary The ultrastructural distribution of fibronectin immunoreactivity was investigated in the chicken embryo during late gastrulation. Sites of binding of anti-fibronectin antibodies were ascribed to the basal lamina and associated structures, and to the cell surface. The fibronectin-rich basal lamina was resolved into (1) a lamina densa, which appears as a continuous, dense sheet, (2) a lamina lucida, consisting of anchoring cords between lamina densa and epithelial cells, and (3) a lamina intima, closely juxtaposed to the cell surface. Cell-surface labelling was also observed in mesoblast cells, and along the dorsal side of the deep-layer cells. The ventral side of the latter cells was poorly stained in the endophyllic crescent, except in coated pits, and more regularly stained at the level of definitive endoblast. Some structures associated with the basal lamina reacted intensely with anti-fibronectin antibodies. These are (1) the interstitial bodies, which are aggregates of extracellular material, and (2) a kind of fibril or tubule, embedded in a fibronectin matrix and mainly found in the endophyllic crescent. Some intracellular labelling was found in most deep-layer cells, in few epiblast cells, never in mesoblast cells. These results extend previous studies on the localization of fibronectin, and correlate its presence and surface topology with its postulated role in migration of mesoblast cells on the basal lamina which, chemically, constitutes an appropriate substrate.