The development of the tongue in the chick embryo: observation under a scanning electron microscope

The morphological features of the chick embryo tongue from the 8th day of incubation till hatching and during the early post incubation period have been investigated by means of Scanning Electron Microscope. At the SEM, it is possible to observe that already at the 8th day of incubation, the body and the root are separated by a low smooth-surfaced ridge. In the following days this ridge develops, giving rise to the so-called lingual spines, whose significance is still uncertain. As concerns the evolutive pattern of the superficial layer of the epithelium, in the first days of the considered incubation period the cells appear dome-shaped and have microvilli on their apical surface; afterwards they tend to become more flattened, and the microvilli are replaced by a thick net of microplicae. In the last days of incubation and after hatching desquamative phenomena become evident. The above described evolutive process can be regarded as a common feature of the whole dorsal lingual surface; only few regional differences are to be noted, such as the earlier development of the microplicae on the apex and borders of the tongue. In particular, the microplicae observed at the apex of the tongue show a typical aspect and arrangement; they run regularly parallel to each other. On the lingual dorsal surface taste bud-like structures have never been observed.     

Polyvillous cells of the epidermis: transportive elements of lamprey skin

Polyvillous cells are differentiated bipolar cells in the epidermis of Lampetra spp., which are not innervated and are interpreted as a type of ionocyte. They bear numerous apical microvilli, capped by mucus. In some examples the apex bulges at the surface of the epidermis, in others it is sunken into a crypt or may even appear to be internal to the cell. It is supposed that these variations correlate with the maturity of the individual cell. Cytological features aiding recognition of the polyvillous cells by electron microscopy are the presence of numerous membrane fragments in the secretion above the short irregular apical microvilli, coated apical invaginations or vesicles, mitochondria with well developed cristae and internal granules, and in many cases a plasma membrane which appears more electrondense than that of the neighbouring epithelial cells     

The corpus luteum of the pig. Scanning electron microscopic study of surface features at different times of incubation

The surface ultrastructure of porcine early corpus luteum cells (days 1-3 of the luteal phase) was studied in SEM and correlated with progesterone secretion. Luteal cells were divided into 2 groups: small cells (10-20 microns) and large cells (20-30 microns) and their surface features were observed after 1, 3, and 5 h of incubation in the control medium and in a medium supplemented with prolactin (PRL). The surface morphology of control cells was characterized by numerous smooth blebs and the presence or absence of thin microvilli. Small and large cells showed a tendency to adhere to the glass during the experiment, but on the large cells the number of thin adhesive filopodia was greater. After the 1st and 3rd h of incubation with PRL the number of microvilli and numerous filopodia on the small cells increased substantially. Nodular blebs were scattered and appeared to protrude from the cell surface. Many small cells adhered to the glass by thick, layered and thin thread-like cytoplasmic processes. After the 5th h distinct smoothing of the surface of the small cells was seen. The number of microvilli seen on the PRL stimulated surface of the large cells was smaller and in some cases even entirely absent. After the 1st and 3rd h of the experiment the large cell surface was ruffled with minute folds. Numerous nodular blebs protruded from the cell surface. The number of adhesive filopodia attaching the cells to the glass decreased or vanished during the experiment. After the 5 h of incubation most of the cells had smooth surface with smooth blebs. Progesterone secretion was measured by radioimmunoassay. The cells in the medium without exogenous hormone (control) secreted relatively low levels of progesterone throughout 1-5 h of the incubation period. After addition of PRL to the medium the amount of secreted progesterone increased.     

Cyclic changes in the surface structure of the cervix from the ewe as revealed by scanning electron microscopy

Thirty parous ewes were divided into six groups and sacrificed on day 0 (first day of estrus), 1, 2, 10, 15 or 16 of the estrous cycle. The cervices were removed immediately and processed for examination with the scanning electron microscope. Observation of the tissues reveals that the surface of the cervix is highly convoluted, which results in the formation of numerous folds or crypts. Two forms of columnar epithelial cells, a ciliated and a non-ciliated cell with microvilli, line the luminal surface of the cerix in the day 10, luteal-phase ewes. However, on day 15, 2 days before estrus, the non-ciliated cells differentiate into two morphologically distinct types of secretory cells. One type forms when the apex of the non-ciliated cell dilates outward into the lumen of the cervix. Concurrent with apical enlargement, the microvilli are lost and the limiting cell membrane becomes smooth. The other type of cell is characterized by only a slight apical swelling. Consequently, remnants of microvilli along with secretory granules can be observed on the limiting membrane of this cell. Both cells release a particulate component, which is believed to be a precursor of mucus, into the lumen of the cerix. These particles undergo a series of morphological transformations to form a fibrillar layer, generally referred to as 'cervical mucus', that covers the epithelial surface at estrus. One to 2 days following the onset of estrus, the fibers become more closely assoicated with amorphous material that begins to coagulate, thereby revealing the underlying ciliated and non-ciliated cells that characterize the cervix of the luteal-phage ewe. The cyclical variation in secretory cells and factors that may influence that structural transformations which occur in mucus are discussed.     

Oligovillous cells of the epidermis: sensory elements of lamprey skin

The epidermis cf Lampetra spp. contains several kinds of differentiated cell; one innervated variety is characterized by bearing a group of large apical microvilli which project from the surface of the skin. In Lampetra planeri such oligovillous cells are numerous under the oral hood of the ammocoete larva, on the papillae fringing the dorsal fin and bordering the gill vents of the adult, and at the tip of the male genital papilla. Elsewhere on the head, body and fins they are present but more scattered, which appears to be the condition also in adult anadromous Lampetra fluviatilis . There are differences in the number and dimensions of the microvilli found on oligovillous cells, but each is supported by a stout core of actin filaments extending a variable distance down into the cytoplasm. Under the apex of the cell there are microtubules and numerous vesicles which are thought to be concerned in the renewal of the membrane on the microvilli. Beside and proximal to the nucleus is a system of channels of rough endoplasmic reticulum, and a stack of membranous cisternae which appears to have been derived from the endoplasmic reticulum. A nerve fibre is associated with the base of the cell which is indented by a spur-like process from the neurite. Typical "synaptic vesicles" are not found in the cell but irregular vesicular or canalicular profiles are associated with the cell membrane adjoining the neurite spur. The space between the cell and neurite membranes contains extracellular material with a characteristic appearance of prickle-like densities on the cell side meeting densities on the neurite membrane. Variations in the cytology of oligovillous cells can be explained in terms of a cycle of development and de-differentiation. Certain cells with vesicles throughout the cytoplasm and with a narrow apex without microvilli are interpreted as degenerate examples. The oligovillous cells are thought to be chemosensory receptors.     

A scanning electron microscopic study of the luteo-follicular complex. II. Events leading to ovulation.

Morphological changes on the ovarian surface of different mammals both before and during ovulation have been examined by scanning electron microscopy. Preovulatory follicles were blisterlike structures that protruded markedly from the ovarian surface. Basal areas of preovulatory follicles were covered with polyhedral cells containing numerous microvilli, whereas on the lateral surfaces, superficial cells were elongated and possessed few microvilli. At the apex of the follicle, cells were very flattened and possessed few microvilli, which were present only in regions of intercellular contact. In some apical areas, cells appeared to be degenerating, whereas in other regions, groups of cells had "sloughed off." In addition, a fluidlike material was observed to exude from intercellular spaces of the superficial epithelium and to cover some apical cells. By transmission electron microscopy, the same fluidlike material was observed to (1) infiltrate the connective tissue of the tunica albuginea, (2) accumulate under the basal lamina, and (3) distend intercellular spaces of the superficial epithelium. Just prior to ovulation, large, irregular areas of the apex were ruptured and the oocyte, covered with a large amount of fluid, appeared to emerge from the follicle. At ovulation, the oocyte was not completely covered with follicle cells and the zona pellucida was clearly evident. The surface of the zona was quite irregular and contained numerous infoldings, channels and crypts. Follicle cells had polyhedral or star shapes and possessed large cytoplasmic evaginations that obliquely penetrated the zona. Both the zona pellucida and corona cells were covered with a fine layer of granular material. The SEM results and parallel TEM observations suggest that a local increase of fluids (edema) may be an important factor in the final decomposition of the distended and weakened apex of the preovulatory follicle. In addition, the participation of follicle cells, smooth muscle cells and the oviduct in the escape of the oocyte from the ruptured follicle is discussed.     

Scanning microscopy of the developing vagina in postnatal gerbils

Scanning electron microscopy of postnatally developing gerbil vagina (birth to maturity) shows that longitudinal folds form prior to transverse folds; the process of fold formation is initiated on the dorsal wall and proceeds ventrally. From days 1 to 7 postnatally, the vaginal epithelium is composed of either flat or bulging cells, depending on the vaginal region. The luminal cell surface is covered with uniform stubby microvilli and solitary cilia. Between days 9 and 20, the flat cells with distinct cell boundaries spread toward more proximal areas, leading to the formation of mixed patches of cells with flat or rounded apices. Individual elongated microvilli or tufts of forked microvilli may sprout from their surfaces. Solitary cilia gradually disappear. The transition from immature to mature vaginal epithelium starts around day 20, when individual cells recess below the level of neighboring cells. This process spreads throughout the vagina during the following days, reflecting local changes in the subsurface layers of the epithelium preparatory to exfoliation. Around day 40 the actual exfoliation of the luminal cell layer starts. By this time the surface characteristics of many of the desquamating cells have changed. In addition to microvilli, microridges are being formed. The process of exfoliation is finished by about day 60. The newly appearing cell layers now transform into typical cornified cells of the cycling vaginal epithelium.     

Study of the surface of cytolytic T-lymphocytes by scanning electron microscopy

Scanning electron microscopy revealed three types of cytolytic T lymphocytes (CTL) interacting with target cells (TC). The type I cells occurred as smooth spherical lymphocytes with single microvilli; the type II rounded or oval cells were uniformly covered with microvilli; the type III cells were marked by an irregular shape, were densely covered with microvilli, while their surface was folded or tuberous. Within the first several minutes after absorption the surface of TC was largely covered by the type I lymphocytes. The proportion of the type III cells rose with the time of interaction. At the beginning it was 8-9%, reaching 30-71% after 30-60 minutes of incubation. It is assumed that the 3 types of the cells described mirror 3 stages of CTL activation. The increase in number of microvilli and appearance of the membranous folds may be a consequence of exocytosis and incorporation of the membrane of secretory granules into the plasma membrane of lymphocytes. The data obtained support the authors' assumption about the secretory mechanism of the action of CTL, whose contact with TC stimulated secretion activation.     

A study of the anatomy, histology and ultrastructure of the digestive tract of Orthrias angorae Steindachner, 1897

The anatomy, histology and ultrastructure of the digestive tract of Orthrias angorae (Steindachner, 1897) were investigated using light microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The histological structure consists of four layers: mucosa, submucosa, muscularis and serosa. The esophageal mucosa consists of undifferentiated basal epithelial cells, mucous cells and surface epithelial cells. It was observed that the J-shaped stomach had a meshwork of folds in the cardiac region, and longitudinal folds in the fundic and pyloric regions. A single layer of columnar cells, PAS positive only in their apical portions, forms the epithelium. The convoluted tube-shape intestine is lined by simple columnar epithelial cells, which have microvilli at the apical surface. The wall of the esophagus and stomach are thicker than that of the intestine because of the thick muscle layer. There were numerous goblet cells in the intestine. There were numerous gastric glands in the submucosa layer ofthe cardiac stomach, but none were present in the pyloric region of the stomach. There were no pyloric caeca between the stomach and intestine. The enterocytes with microvilli contained rough endoplasmic reticulum, ribosomes and rounded bodies, and the gastric cells contained a well-developed Golgi apparatus.     

An ultrastructural study of the dorsal lingual epithelium of the crab-eating frog,Rana cancrivora

The amphibian tongue contains two types of papilla which are believed to function in gustation and in the secretion of salivary fluid. Scanning electron microscopy reveals that columnar, filiform papillae are compactly distributed over nearly the entire dorsal surface of the tongue of the frog, Rana cancrivora, and fungiform papillae are scattered among the filiform papillae. Microridges and microvilli are distributed on the epithelial cell surface of the extensive area of the filiform papillae. Light microscopy shows that the apex of each filiform papilla is composed of stratified columnar and/or cuboidal epithelium and its base is composed of simple columnar epithelium. Transmission electron microscopy reveals that most of the epithelium of the filiform papillae is composed of cells that contain numerous round electron-dense granules 1–3 μm in diameter. Cellular interdigitation is well developed between adjacent cells. On the free-surface of epithelial cells, microridges or microvilli are frequently seen. Between these granular cells, a small number of ciliated cells, mitochondria-rich cells and electron-lucent cells are inserted. In some cases, electron-dense granules are present in the ciliated cells. At higher magnification, the electron-dense granules appear to be covered with patterns of spots and tubules. Overall, the morphology and ultrastructure of the lingual epithelium of the three species of Rana that have been studied are quite similar, but they can be easily distinguished from those of Bufo japonicus. Therefore, it appears that lingual morphology is phylogenetically constrained among members of the predominantly freshwater genus Rana to produce uniformity of papillary structure and this morphology persists in Rana cancrivora despite the distinct saline environment in which it lives. © 1993 Wiley-Liss, Inc.     

Scanning electron microscopy of pulmonary vascular endothelium in rats with hypoxia-induced hypertension

Scanning electron microscopy was used to study the endothelial surface of the pulmonary trunk, artery, and vein in normobaric control rats as well as in rats exposed to hypobaric hypoxia for 7 and 21 days. The individual endothelial cells of the normobaric pulmonary trunk and hilar artery were flat and slightly elongated with elevated nuclear regions, and those of the intermediate-sized artery were more elongated and had more microvilli than the large arteries studied. Their endothelial cell boundaries were outlined by beaded cytoplasmic projections. The surfaces of the normobaric hilar and intermediate-sized veins were smooth and demonstrated numerous longitudinal streaks. These venous endothelial cells were elongated and their cell boundaries were outlined by low discontinuous marginal folds. Exposure to hypobaric hypoxia caused the following changes on the arterial surface: elevation of the endothelial cells; formation of microvilli-rich cell clusters; formation of hollow defects; and the attachment of leukocytes. Hypobaric hypoxia also caused the disappearance of the longitudinal streaks and the occurrence of microvilli-rich cells in the hilar veins. The endothelial surface modifications in the hypobaric rats could be related to thickening of the endothelium, intimal edema, increased intimal connective tissue, luminal invasion of leukocytes, and increased endothelial cell proliferation, known to occur in systemic arteries of hypertensive animals.     

FINE STRUCTURE OF CELL SURFACE SPECIALIZATIONS IN THE MATURING DUODENAL MUCOSA OF THE CHICK

Cell surfaces in the duodenal mucosa have been studied in maturing tissue of the chick from incubation until hatching. Changes in the distribution of mitoses in this tissue give an indication of its rate of maturation. This rate is paralleled in developmental changes in microvilli and junctional complexes. Concentration of mitotic figures towards the base of villous folds is gradual from day 9 to day 16, then rapid to day 19, after which the mature pattern is acquired. By day 11, microvilli appear in a regular pattern which does not alter through hatching. Their height remains the same to day 16 when it increases gradually to day 19, then very sharply. The angle formed between the microvilli and the cell surface increases gradually to day 16, giving evidence of advancing internal structure. Changes in cell adhesion also occur at day 16. Thereafter, following trypsin treatment cells are held together in patches by the tight junctions of the terminal bar, although the desmosomes are separated. The timing of these morphological changes is compared with that of alkaline phosphatase accumulation in this tissue as reported by Moog (13). Increase in the surface area of the microvilli parallels the increase in the activity of the enzyme.     

A helical,polymeric extracellular protein associated with the luminal surface of Haemonchus contortus intestinal cells

The structure of the intestinal cells of the parasitic nematode Haemonchus contortus is described. The cells have numerous microvilli about 0.09 μ in diameter; most being 5.5–7.5 μ in length. The microvillar (plasma) membrane is coated with a layer of amorphous material (glycocalyx) about 60 Å thick which is electron dense in sectioned preparations. Associated with the surface of this material, and filling the spaces between the microvilli, are filaments in the form of helices about 400 Å in diameter and of variable pitch. The helices appear to be flexible but they are aligned approximately with the long axes of the microvilli. There are up to ten helices per microvillus; they extend beyond the tips of the microvilli and are up to 10 μ long. The material has been obtained nearly pure in small amounts. It is primarily protein and it is proposed that it should be called contortin. The monomeric form (of molecular weight about 60,000) has been identified with a Y-shaped structure with arms about 45 Å long and 25 Å wide seen in negatively stained preparations. The helical filament appears to be formed by lateral polymerization of pairs of these units.     

Histological and histochemical studies of the gall bladder of the graylag goose (Anser anser)

ABSTRACT

We investigated the histological structure of the graylag goose (Anser anser) gall bladder. Sections of the gall bladder were stained with hematoxylin and eosin (H & E), Alcian blue (pH 2.5) for acid mucopolysaccharides, Gomori’s method for reticular fibers, Masson’s trichrome, periodic acid-Schiff (PAS) and Verhoeff’s elastin stain. The goose gall bladder was composed of a tunica mucosa, tunica muscularis and tunica adventitia or tunica serosa. The tunica mucosa formed regularly distributed simple isometric folds plus larger, less numerous, branched folds. The luminal surface was lined by tall columnar epithelial cells that stained for both acid and neutral mucopolysaccharides. The epithelial cells formed a discontinuous striated border of interdigitating microvilli on the luminal surface. Neither a lamina muscularis nor goblet cells were observed in the tunica mucosa. Unusual findings included branched mucosal folds, discontinuous microvilli and absence of an outer longitudinal layer in the tunica muscularis. No marked sex-associated differences were found. The general histochemical and histological structures of the graylag goose gall bladder are similar to those of birds such as chukar partridge and quail, but with some unique elements that may reflect differences in organ function.     

Morphometric and ultrastructural features of the mare oviduct epithelium during oestrus

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

Morphogenesis of intestinal villi. I. Scanning electron microscopy of the duodenal epithelium of the developing chick embryo

Development of villi in the duodenum of the chick was studied in stages ranging from 11 days of incubation to one week after hatching. Formation of definitive villi is preceded by development of a set of previllous ridges that run lengthwise along the duodenum. The first set of 16 previllous ridges (Set I) is complete by about 13 days of incubation; all ridges in the set are fairly uniform and proceed through their subsequent development in synchrony. Previllous ridges in Set I fold into a highly regular zigzag pattern between 14 and 16 days of incubation. Definitive villi develop from Set I ridges beginning at about 17 days when populations of distinct cells appear on the crests of the ridges between angles in the zigzag folds. Cells in these populations lack the rounded appearance of cells seen in earlier stages; their apical surfaces are densely covered with microvilli. A second set of villi (Set II) develops at about 16 days of incubation when about 16 rows of tongue-like flaps erupt between the previllous ridges of Set I. At hatching, Set II villi are still smaller than villi of Set I; this distinction disappears by about the fourth day after hatching. The significance of the morphological changes in epithelial cells is discussed in terms of several hypotheses bearing on the mechanisms of villus formation.     

Die Vaskularisation der Kiemen von Myxine glutinosa L. (Cyclostomata)

The angioarchitecture of the gills in Myxine glutinosa L. was studied by scanning electron microscopy of vascular corrosion casts (methylmethacrylate). It was found that the afferent branchial artery may be connected to the sinus peribranchialis by a papilla. The sinus is connected to delicate vessels and sinuses, which are interposed between meridionally arranged radial arteries. These delicate vascular formations continue into the interior of the gill and form a plexus on both poles of the gill folds. Light and scanning electron microscopical studies on the vascular endothelium of the afferent vessels of the gill lamellae reveal rounded endothelial cells, which are characterized by a high content of granula, by long cellular processes and by bridge-formations towards neighbouring cells. Supporting columns within the vascular system of the lamellae were found to be set up by several spirally arranged cells. SEM observations reveal goblet cells and numerous superficial epithelial cells with various numbers of microvilli and microridges forming the epithelial surface of the gill folds.     

The perforation organ of Thais lapillus L. (Gasteropodes, Prosobranches). Optical and electron microscopic study]

The histologic and cytologic structure of the boring organ of Thais lapillus has been studied with both light and electron microscopy. The organ evaginates from the foot and has a dome-like form. It has a spongy center which contain nerves, muscles and blood vessels, these originate in the foot and continue into the boring organ via a short stalk. The organ is invested externally by a continuous simple epithelium constituted of extremely elongated cells (150 to 300 mu). There are three radial zones from the basal to the apical aspect of the cells : basally the cells are overlapping, here are located the nuclei, the Golgi apparatus, the rough endoplasmic reticulum and secretory granules; organelles which in their character suggest a secretory function for these cells. In the middle zone cells are separated into "groups" by large spaces occupied by blood sinuses; at this level are located numerous elongated mitochondria with closely apposed, angulated cristae. In addition there are microtubules arranged parallel to the main cellular axis, whose role may be to facilitate rapid transit of secretory granules from the basal to the apical zone of the epithelium. In the apical zone the cells expand in tightly interdigitating folds, having a height of 2 mu, which are joined together by junctions of various types. This zone is the only junctional area between the epithelial cells of this expanding and contracting boring organ. A network of microfilaments which penetrates the cellular folds at this level, probably plays an important role in the invagination of the organ. Finally at the apex, the epithelial cells have a high "brush border" of long microvilli whose plasmalemma is coated internally by particulate subunits of 30 A. This peculiarity may be an other fact to support the hypothesis that hydrogen ions are emited by these cells, by the action of carbonic anhydrase located perhaps in these subunits.     

Relationship between insulin stimulation and endogenous regulation of 2-deoxyglucose uptake in 3T3-L1 adipocytes

The occurrence of the endogenous regulatory response to high rates of 2-deoxyglucose (2-DG) uptake, as previously described for C6 glioma cells during incubation with 2 mM 2-DG (Lange et al.: J. Cell. Physiol., 1989), was studied in 3T3-L1 preadipocytes and adipocytes, and the influence of insulin on this endogenous uptake regulation was examined. In contrast to 3T3-L1 preadipocytes, insulin-sensitive differentiated 3T3-L1 adipocytes displayed the time-dependent cyclic pattern of 2-DG uptake rates characteristic of the membrane-limited and endogenously regulated cellular state of hexose utilization. Although insulin induced a threefold stimulation of 2-DG tracer uptake in adipocytes, the hormone did not additionally stimulate the uptake rates or affect the periodic response: maximum and minimum levels of uptake remained unchanged. Scanning electron microscopy (SEM) revealed that the acquirement of the differentiated state is accompanied by a conspicuous transformation of the smooth surface of undifferentiated 3T3-L1 cells into a surface covered by numerous microvilli of uniform size and appearance. Treatment with insulin (10 mU/ml; 10 minutes) converted these microvilli into voluminous saccular membrane protrusions of the same type as had been formed during incubation of 3T3-L1 adipocytes with 2 mM 2-DG, and which have previously been shown to be involved in the endogenous uptake regulation of C6 glioma cells (Lange et al.: J. Cell. Physiol., 1989). These insulin-induced saccated membrane areas appeared to become integrated into the cell surface. Accordingly, insulin treatment caused a twofold increase of the intracellular distribution space of 3-O-methylglucose (3-OMG) in 3T3-L1 adipocytes. This insulin-induced increase of the 3-OMG distribution space exhibited the same time (t1/2 = 2-2.5 minutes) and dose dependence (EC50 = 20 nM) as the insulin-induced stimulation of 3-OMG transport. Glucose deprivation during the differentiation period inhibited the outgrowth of microvilli from the cell surface. Glucose starvation (18 hours at less than 0.5 mM) induced a conspicuous reduction of the length of microvilli on differentiated 3T3-L1 cells. In this state, the stalks of the microvilli are almost invisible and the enlarged spherical tips of the microvilli (with an average diameter of 370 nm compared to 230 nm of fed cells) appeared to protrude directly out of the cell surface. Starvation-induced shortening of microvilli was accompanied by a threefold increase of the basal 3-OMG transport rate and a greater than twofold increase of the intracellular 3-OMG distribution space as compared to fed cells (10 mM; 18 hours).(ABSTRACT TRUNCATED AT 400 WORDS)