Common Basal Lamina Research Articles

Prenatal Development of the Bovine Oviduct

In this study the development of the bovine Fallopian tube was investigated using light microscopic methods. Formation and differentiation of the Müllerian duct were studied in mesonephroi of 16 embryos and fetuses with a crown-rump lengths (CRL) of 0.9-8.4 cm. The funnel field, the rostral beginning of the Müllerian duct was first observed at a CRL of 0.9 cm. It appears as a thickening of the mesothelium on the craniolateral side of the mesonephros. During later development the Müllerian duct emerges by caudal outgrowth from the funnel field. Formation of a common basal lamina surrounding the caudal tips of Müllerian and Wolffian ducts could be observed at all stages up to CRL of 2.7 cm. The mesothelium and the epithelium of the Wolffian duct adjacent to the Müllerian duct showed a modification of epithelium height in all examined stages. Probably the Wolffian duct influences the growth of Müllerian duct by epithelio-mesenchymal interactions. Fetuses from a CRL of 12.0 to 94.0 cm were used for investigation of the prenatal differentiation of the oviductal mucosa. Folding of the oviductal mucosa started at a CRL of 29.0 cm and continued until birth. Individual primary, secondary and tertiary folds are formed in special proliferation zones and epithelium-folding buds. The cellular differentiation of the oviductal epithelium involves the formation of ciliated and secretory cells during different times of prenatal development. Ciliogenesis was first detected at a CRL of 33.0 cm. Active secretory cells could be observed in the oviductal epithelium from a CRL of 64.0 cm onwards.

Sox15 and Fhl3 transcriptionally coactivate Foxk1 and regulate myogenic progenitor cells

The regulation of myogenic progenitor cells during muscle regeneration is not clearly understood. We have previously shown that the Foxk1 gene, a member of the forkhead/winged helix family of transcription factors, is expressed in myogenic progenitor cells in adult skeletal muscle. In the present study, we utilize transgenic technology and demonstrate that the 4.6 kb upstream fragment of the Foxk1 gene directs beta-galactosidase expression to the myogenic progenitor cell population. We further establish that Sox15 directs Foxk1 expression to the myogenic progenitor cell population, as it binds to an evolutionarily conserved site and recruits Fhl3 to transcriptionally coactivate Foxk1 gene expression. Knockdown of endogenous Sox15 results in perturbed cell cycle kinetics and decreased Foxk1 expression. Furthermore, Sox15 mutant mice display perturbed skeletal muscle regeneration, due in part to decreased numbers of satellite cells and decreased Foxk1 expression. These studies demonstrate that Sox15, Fhl3 and Foxk1 function to coordinately regulate the myogenic progenitor cell population and skeletal muscle regeneration.

Dorsal longitudinal stretch receptor of Drosophila melanogaster larva – Fine structure and maturation

Insects possess two types of sensory neurons: ciliated type I sensory neurons that innervate external sensory organs and chordotonal organs, and type II sensory neurons that form a subepidermal plexus or innervate stretch receptors. Among stretch receptors, a dorsel longitudinal stretch receptor is highly conserved in insects, being found in all insect orders investigated. Here we describe the topology and anatomical structure of this receptor in the fruit fly embryo and larva using transmission electron microscopy and single cell staining for fluorescence microscopy. The receptor is composed of the dorsal bipolar dendrite neuron, which arises from an archetypal cell lineage, its sister glial cell and the peripheral glial cell accompanying the nerve. The neuron is situated among the muscles in the dorsal body wall on the intersegmental nerve. Its two dendrites stretch the length of the segment to the segmental folds. The neuron is wrapped by both glial cells and surrounded by a common basal lamina, which fans out at the dendritic tips to attach them to the epidermal cells at the segmental borders.

Early development of the human mesonephros

The mesonephrogenic cord disintegrates into approximately 35-40 provesicular cell masses which are in close contact with the mesonephric (Wolffian) duct (WD) on their lateral side. Here, the epithelium of the WD is columnar and shares a common basal lamina with the provesicular cell masses. This in turn gives rise to a sickle-shaped pseudostratified epithelium. The concavity of the sickle is filled by spherical cells, the transition of which into the surrounding connective tissue is continuous. The sickle is transformed into a distillation flask and becomes separated from the mesonephric duct while the spherical cells maintain a connection to it by a-for the time being-solid outlet pipe. The columnar epithelium of the mesonephric duct becomes a multilayered cone, whose surface is in contact with the outlet tube. Shortly after, a continuous lumen is formed in the cone and the outlet pipe which is delimited by cells becoming columnar and forming a basal lamina. The epithelial anlage of the nephron is clearly separated from the surrounding mesenchyma by these processes. The flask eventually becomes a corpusculum, the outlet pipe a secretory (proximal) as well as collecting tubule, and the cone of the mesonephric duct a mesoureter. The various sections display differentially differentiated epithelia that are clearly distinct from each other. The mesoureter behaves differently during differentiation of epi- and paragenitale: in the epigenitale, it is short and runs into the collecting tubules of the nephrons at the lateral side of the convolved tubules, whereas a long mesoureter crosses the dorsal side of the convolved tubules and joins the corresponding collecting tubules at the far end of the mesonephros in the paragenitale.

Abortive myogenesis in denervated skeletal muscle: differentiative properties of satellite cells, their migration, and block of terminal differentiation

Little is known about the biological properties of myogenic satellite cells during post-denervation muscle atrophy. The present study investigated the differentiative capacity of satellite cells and their involvement in the compensatory regenerative process in long-term denervated rat muscle. Electron microscopy and immunocytochemical labeling of muscle tissue 1-18 months following denervation demonstrated that despite activation of satellite cells, myogenesis in denervated muscle is abortive and does not lead to the formation of normal muscle fibers. Small sizes, poor development of the contractile system in newly formed denervated myotubes, and the absence of satellite cells on the surface indicate that their differentiation typically does not progress to terminal stages. Many immature myotubes degenerate, and others survive but are embedded in a collagen lattice near their parent fibers. Interestingly, newly formed myotubes located on the surface of parent muscle fibers beneath the basal lamina typically did not contain developed myofibrils. This suggests that the contacts of daughter and parent muscle fibers block myofibrillogenesis. Assembly of sarcomeres in most cases occurs following complete spatial separation of daughter and parent muscle fibers. Another manifestation of the involvement of myogenic precursors in abortive myogenesis is the formation of clusters of underdeveloped branching myotubes surrounded by a common basal lamina. We found that myoblasts can also fuse directly with differentiated muscle fibers. The presence of satellite cells near the openings in the basal lamina and in the interstitial space indicates that myogenic precursors can migrate through the basal lamina and form myotubes at a distance from parent fibers. Our data may explain why long-term denervated skeletal muscle has a poor capacity for regeneration and functional restoration.

Disappearance of the post-lesional laminin immunopositivity of brain vessels is parallel with the formation of gliovascular junctions and common basal lamina. A double-labelling immunohistochemical study.

Previous studies revealed that during development the laminin immunopositivity gradually disappeared from the brain vessels, but temporarily re-appeared in them around lesions. The question of the present study was the correlation between the post-lesional vascular immunopositivity to laminin and the glial reaction. Following stab wounds, double fluorescent immunohistochemical labelling was performed against laminin (using a polyclonal antiserum against laminin 1) and glial fibrillary acidic protein. A number of vessels exhibited intense immunopositivity to laminin within the lesioned tissue. Where these laminin immunopositive vessels entered the perilesional brain substance, the astroglia formed contacts on them, and the separate vascular and glial basal laminae fused. The disappearance of the post-lesional laminin immunopositivity seemed to coincide with these phenomena. When monoclonal antibodies were applied against the beta1 and gamma1 laminin chains, vessels proved to be immunopositive at the lesion, but none in the intact brain tissue. No immunoreactivity was detected in the cases of alpha2 and beta2 chains. The results suggest that the disappearance of laminin immunopositivity may be attributed to that the epitopes become inaccessible for antibodies owing to the formation of gliovascular junctions and common basal lamina between astroglia and vessel. The possible role of an alteration in the laminin composition and the effect of fixation are discussed.

MORPHOLOGICAL STUDY ON THE PERICYTES OF GOAT'S PLACENTOMAL CAPILLARIES

Pcricytcs arc connective tissue cells associated abluminally with capillary and postcapillary venulcs, They share a common basal lamina with endothelial cells (Majno, 1965: Schor et al., 1995). Such rnicrovcssels arc involved with the exchangeof material and extracellular space (Nehls and Drenckhahn, 1993). Perleytes were first described in the nictitating membrane of the frog by Rouget (1873). and have been seen surrounding capillaries in a wide variety of tissues(Zimmerman, 1923: Majno, 1965: Lunam and Roger, 1981: Sims, 1986: Hirschi and D'Amore, 1996; Sundberg et al., 1996; Wisse et al., 1996). The most probable function for pcricytcs is the regulation of capillary flow by their contraction (Epling, 1966; Rhodin, 1968: Wieble, 1974: Tilton et al., 1979). A possible functional role of the pericytes is the control of material exchange through the vessel wall (lmayama and Urab, 1984) and its participation in angiogenesis has been proposed (Crocker et at, 1970; Schor et at, 1995). A further role that has been attributed to pericytcs is to stabilize the capillary wall (Cohen et al., 1980; Frank et al., 1990: Jeon et at., 1996). In rat uterus. the increase in pericytes coincides with the time of implantation (Lunam and Roger, 1983). Steven and Samuel (1979) have suggested that there arc few pericytes associated with the maternal capillaries in sheep placenta to perform an important regulatory function on the transport of material from the mother to the fetus. However, Yasear et HI. (1987) found a considerably largenumber of pcricytcs in association with the maternal microvessels in sheep placenta throughout pregnancy. In the gOHt'S placenta. the materno-Ictal interface is characterized by the presence of syncytial tissue covering the maternal side of placentomes. The plcentomes arc ruminant-specific fetomatcrnal contact zones characterized by the presence ofmicrovillar junction between syncytium and fetal trophoblastic layer (Steven, 1975; Wooding and Flint, 1994). Due to the lack of information on the distribution of pericytes in the goat's placenta the present study was initiated to evaluatethe distribution of the pericytes surrounding the materna! capillaries which are located close to the syncytium, i.e. those most closely involved in the materno-fetal exchanges throughout pregnancy.

Postinflammatory changes of the diaphragmatic stomata

Numerous investigations concerning the fine morphology of diaphragmatic stomata have been performed, but its ultrastructural changes in experimental conditions remain unclear. The present study demonstrates the peritoneal side of the diaphragm in adult Wistar rats by transmission electron microscopy. Ten experimental animals were observed 5 and 8 days after Pseudomonas aeuriginosa instillation (PI) into the peritoneal cavity. A control group of 6 rats showed flat mesothelial covering on basal lamina (BL) and connective tissue layer, as well as cubic mesothelial cells, single stomata over underlying lymphatic lacunae (LL). Five days after PI the mesothelial cells had more numerous microvilli, microvesicles, vacuoles, lysosomes and a lesser number of specialized contacts. The multiplication of the extravasal cells and larger intercellular spaces lead to thickenings of the connective tissue around LL. LL were larger and located in close proximity of the mesothelium. Intercellular spaces in the mesothelial layer and different types of contacts between mesothelial cells and endothelial protrusions of LL (with common BL or without BL) were encountered. Eight days after PI the mesothelium, endothelium of LL, their BL and surrounding connective tissue were interrupted and structurally modified to form typical new channels--stomata. The larger portion of the channels were formed of mesothelial cells, while the endothelial cells participated in the submesothelial part. LL were more numerous than in the previous period, and were arranged in groups. LL increased their vertical (50.59 microm) and horizontal (155.57 microm) diameter, as compared with control animals (respectively 12.37 microm and 74.08 microm). Neighbouring LL were separated by thin or thick septae. Peristomatal mesothelial cells or more rarely endothelium formed valve- or bridge-like structures. Valves on the opposite side of LL were observed. Groups of electron-dense bodies characterized some tall endothelial cells of LL. Cubic mesothelium, endothelium of the LL, both BL, the cell connections that formed new stomata, LL and surrounding connective tissue underwent rapid and parallel changes after PI. Among these elements of the lymphatic regions mentioned above, the mesothelium and endothelium of LL had a main role in experimental conditions.

Scanning and transmission electron microscopic study of visceral and parietal peritoneal regions in the rat

The visceral peritoneum of intraabdominal organs (spleen, stomach, liver, small intestine), omentum majus and the parietal peritoneum of the anterior abdominal wall and the diaphragm were studied in adult Wistar rats by combined scanning and transmission electron microscopy (SEM, TEM). In general, the peritoneal surface consisted of a mesothelium composed of cubic, flat or intermediate cell types delimited by a basal lamina. Cubic mesothelial cells predominated in parenchymal organs (spleen, liver) and were characterized by prominent and indentated nuclei, a cytoplasm richly supplied with organelles, a dense microvillous coat, basal invaginations and elaborate intercellular contacts. Flat mesothelial cells were observed in the intestinal, omental and parietal peritoneum (tendinous diaphragm, abdominal wall) and showed elongated nuclei, scant cytoplasm, a poorly developed organelle apparatus and sparsely distributed microvilli. An intermediate mesothelial cell type was described within the gastric peritoneum characterized by a central cytoplasmic protrusion at the nuclear region containing most of the cytoplasmic organelles and by thin finger-like cytoplasmic processes. The submesothelial connective tissue layer was composed of collagen fiber bundles, fibroblasts and free cells (macrophages, granulocytes, mast cells) and contained blood and lymphatic vessels. In the spleen, elastic fibers formed a membranous structure with intercalated smooth muscle cells. Mesothelial openings were observed as tunnel-like invaginations within the hepatic peritoneum and as clusters of peritoneal stomata within the parietal peritoneum of the anterior abdominal wall and the muscular diaphragm. The round or oval openings of the peritoneal stomata were frequently occluded by overlapping adjacent mesothelial cells and their microvillous coat or obstructed by cellular material. At the side of the peritoneal stomata the mesothelial cell layer was interrupted to allow a direct access to the underlying submesothelial lymphatic system. The mesothelium and lymphatic endothelium shared a common basal lamina. The endothelial cells were discontinuous and displayed valve-like plasmalemmatic interdigitations facilitating an intercellular transport of fluids and corpuscular elements from the peritoneal cavity to the submesothelial lymphatic lacunae. The findings underline the morphological heterogeneity of the peritoneum in visceral and parietal regions, suggesting different functional implications, and further support the presence of extra-diaphragmatic peritoneal stomata.

Neomyogenesis in neonatally de-efferented and postnatally denervated rat muscle spindles

The ultrastructure of muscle spindles de-efferented by the extirpation of the lumbosacral spinal cord at the age of 2 days and subsequently deprived of their sensory innervation by the section of the sciatic nerve at 3-4 weeks of age was studied in serial sections of 2-month-old rat hindlimb muscles. De-efferentation leaves the primary sensory neurons and their peripheral axons intact and capable of inducing the muscle spindle morphogenesis during the critical period of their development. In de-efferented and subsequently denervated muscle spindles, new supernumerary intrafusal muscle profiles (SIPs) appeared in the muscle spindle A region. They were formed in intimate spatial relation with the original intrafusal muscle fibres (IMFs) predominantly from activated satellite cells derived from both nuclear bag (larger diameter) and nuclear chain fibres. SIPs, however, lacked the typical nuclear accumulations, as well as other ultrastructural distinctions present in control IMFs. The majority of differentiated SIPs separated from original IMFs, whereas the less differentiated SIPs were usually closely apposed to the surface of the parent IMFs and both were covered by the common basal lamina. In some spindles, the original IMFs and/or new SIPs at different stages of their differentiation were found together and they formed clusters of variable shape and composition. In the majority of clusters, all profiles seemed to be isolated along their entire length, although in few clusters, occasional cytoplasmic connections of variable length between intrafusal profiles were found. This result is important for the interpretation of the forthcoming study of expression of muscle spindle-specific myosin heavy chain isoforms in denervated SIPs in rat muscle spindles gradually deprived of their motor and sensory innervation.

Immunocytochemistry of M‐cadherin in mature and regenerating rat muscle

Cadherins are transmembrane proteins mediating calcium-dependent cell-cell adhesion in a cell type-specific manner by means of homophilic binding. M(muscle)-cadherin is a recently detected member of the cadherin family. We have investigated the localization of M-cadherin in normal and aneurally regenerating skeletal muscle of rat by means of pre-embedding immunocytochemistry. The antibody was directed against the extra-cellular domain of M-cadherin. Myoblasts and myotubes in regenerating muscles tended to be arranged in clusters enclosed by a common basal lamina. Satellite cells of mature muscle fibers were attached to the underlying fiber without separating basal lamina. Reactivity for M-cadherin was restricted to the plasma membranes of myoblasts and satellite cells, and was most intense at the membrane areas facing adjacent myotubes or myofibers. Myoblasts interposed between two myotubes were stained on the entire surface. The adjacent plasma membrane of the otherwise negative myotube or muscle fiber, was stained as well, and extracellular reaction product was in the gap between the cells. The cellular localization of M-cadherin may indicate that M-cadherin is involved in calcium-dependent adhesion between satellite cells and muscle fibers or, by means of interposed myoblasts, between the clustered myotubes in a regenerating muscle.

The frequent occurrence of closely packed intrafusal myotubes during the early postnatal development of muscle spindles in the Chinese hamster.

The formation of intrafusal muscle fibers of developing muscle spindles in the tenuissimus muscle of the Chinese hamster was examined by electron microscopy from birth to 7 days postnatal. At birth, at least two nuclear bag myotubes with a small aggregation of nuclei were already recognizable within a spindle capsule, establishing side-to-side contact with each other without the intercalation of a basal lamina. By the third postnatal day, most muscle spindles contained a full adult set of four intrafusal myotubes effected by the formation of nuclear chain myotubes due to the fusion of myoblasts. The intrafusal myotubes were usually enclosed in a common basal lamina and closely apposed without an interposing basal lamina, thus forming a muscle bundle. Sensory endings were thereby confined to the outer surface of the muscle bundle. After the third day, closely packed intrafusal myotubes generally separated into independent muscle fibers which were innervated by sensory endings and ensheathed in attenuated inner capsule cells. Pairings of fibers and sensory cross terminals between fibers were frequently found in the equatorial region. These phenomena suggest that the incomplete separation of the bundled myotubes at an early stage of postnatal development may persist into adulthood.

Proliferative activity of somatic cells during gonadal development in the Japanese pond frog, Rana nigromaculata

AbstractProliferative activities of gonadal somatic cells were examined autoradiographically during gonadal development in Rana nigromaculata. Both the germinal epithelium and the sexual cord appeared to be of coelomic epithelial origin, as indicated ultrastructurally by the observation that the 2 regions were covered with a common basal lamina. Cell‐proliferative activity was higher in the sexual cord than in the germinal epithelium at stage 29, suggesting that topographically specific active proliferation of coelomic epithelial cells contributed to the formation of the sexual cord. In the course of testicular development, high proliferative activity of the sexual cord was recognized until stage 32 (just after gonadal sex differentiation). At stage 43 (during metamorphosis), higher proliferative activity was noted in the germinal epithelium than in the sexual cord, and the germinal epithelium differentiated into seminiferous cords. There were no differences in proliferative activity between the germinal epithelium and sexual cord at stages 32 and 34 in the ovaries. At stage 43, there was higher proliferative activity in the germinal epithelium than in the sexual cord, and the germinal epithelium differentiated into follicular cells. These results suggest that both seminiferous cords and follicular cells are derived from germinal epithelium and that both pass through similar processes of development.

Implantation of cultured sensory neurons and schwann cells into lesioned neonatal rat spinal cord. II. Implant characteristics and examination of corticospinal tract growth

The purpose of this study was to test the effectiveness of implants derived from peripheral neural tissue to serve as bridges following interruption of the developing corticospinal tract (CST). Implants prepared from purified populations of cultured dorsal root ganglion neurons (DRGNs) and Schwann cells (SCs) (Kuhlengel et al., J. Comp. Neurol. 293:63-73, 1990) were placed into thoracolumbar regions of neonatal rat spinal cord from which a 2-mm length of dorsal columns had been removed by suction. These cords were examined by a number of techniques 10 days to 6 months later. The implants, recognizable by their DRGN content, filled the vacated dorsal columns and survived the longest periods examined. The most effective method to maintain implant position was dorsal placement of collagen-coated Nitex filter. Implants were inserted either at the time of lesioning or 5 days later. The implant survival rate was better (72% vs. 50%) and meningeal scarring was less with immediate implantation, but delayed implantation resulted in better implant-cord fusion and the implant better filled the lesion cavity. DRGN/SC implants became well vascularized without leptomeningeal cells; this may explain why implant survival was not improved with leptomeningeal cell addition. Particularly well-differentiated implants (full extracellular matrix production and myelination) did not fuse as well with cord as did those less well differentiated. The addition of nerve growth factor to the Nitex filter collagen coating led to improved survival of DRGNs in implants. Electron microscopy showed that astrocytes populated the implant-cord junction region and migrated into implants. Typical SCs related to nonmyelinated and myelinated axons were present in implants. Close proximity of astrocytes and central myelin to SCs and peripheral myelin demonstrated good implant integration with cord. Clusters of SCs, astrocytes, and axons, all enclosed within a common basal lamina, were observed in implants. Immunostaining for GFAP and laminin confirmed our microscopy findings that SCs did not migrate from implant into host but that astrocytes left host tissue to enter implants. Neuroanatomical tracing of CST neurons with HRP-WGA showed that labeled fibers were not present in the implant but were fasciculated just beneath in gray matter. These fibers remained clustered in gray matter underneath the ventral dorsal columns caudal to the lesion. In lesioned but not implanted rats, labeled fibers were only diffusely distributed in gray matter. Delayed implantation led to more variation in fasciculation compared with immediate implantation.(ABSTRACT TRUNCATED AT 400 WORDS)

Muscle fibre loss and reinnervation after long-term denervation.

Cutaneous pectoris muscles of frog (Rana temporaria) were investigated 19.5-40 months after denervation. On whole mounts a heavy reduction in size and number of muscle fibres is noticed; in two muscles studied with semithin and ultrathin sections the number of remaining muscle fibres is 149 and around 120, while one of the contralateral muscles contains 250 and control muscles of equal sized frogs between 220 and 320 (n = 18) fibres. By electron microscopy muscle fibres undergoing degeneration or phagocytosis can be seen (3 of 20 muscle fibres present in a single ultrathin cross-section). On the other hand several profiles contained within one common basal lamina sheath are present in 14 of 20 fibres, indicating satellite cell proliferation. In one preparation 40 months after denervation not a single muscle fibre or axon is present, suggesting that eventually, without nerve supply, muscle fibres entirely disappear. Upon spontaneous reinnervation or implantation of the hypoglossal nerve 16 months after denervation, synapses are formed with the remaining muscle fibres. When studied 3.5-24 months after nerve implantation muscles innervated by few axons only (less than 10, 10-20 axons) contain a low number of muscle fibres (mean 44 +/- 41 SD, n = 6), while all muscles with a larger number of axons have more than 150 muscle fibres (n = 6). This indicates that unless large numbers of axons regenerate and/or when reinnervation is delayed muscle fibre loss continues to occur. The presence in one muscle of motor axons but only six muscle fibres 24 months after nerve implantation indicates that muscle fibre loss cannot be reversed, or recovery is extremely slow. This observation is interpreted as evidence for the exhaustibility of the satellite cell pool.

Morphology and morphometry of the normal lung of the adult vervet monkey (Cercopithecus aethiops).

The lungs of four adult specimens of the vervet monkey (Cercopithecus aethiops) have been examined by transmission and scanning electron microscopy. A morphometric evaluation of the structural components directly involved in gas exchange has been carried out and the data have been modelled to estimate the anatomical diffusing capacity of the lung. The upper air-conducting airways of the lung were lined by an epithelium characterized by ciliated cells among which were dispersed goblet cells. The alveolar surface was lined by squamous type I pneumocytes and cuboidal type II granular pneumocytes. The blood-gas (tissue) barrier consisted of an epithelial cell, a common basal lamina, and an endothelial cell in the thin parts of the interalveolar septum. In the thicker parts of the septum, an interstitial space interposed between the basal laminae of the epithelial and endothelial cells contained supportive elements such as collagen, elastic tissue, and fibrocytes. The alveoli, the blood capillaries, and septal tissue composed 73%, 16%, and 11%, respectively, of the parenchyma. The harmonic and arithmetic mean thicknesses of the blood-gas (tissue) barrier were 0.311 micron and 1.048 microns; the surface area of the blood-gas (tissue) barrier per unit body weight was 50 cm2g-1, and the surface density was 117 mm2.mm3-1. The weight-specific total morphometric diffusing capacity was 0.11 mlO2 (sec.mbar.kg)-1. In comparison, the pulmonary morphometric characteristics of vervet monkey lung were superior to those of the other primates (Macaca irus, M. mulatta, and Homo sapiens) for which equivalent data are available. The gas-exchange potential of the lungs of the nonhuman primates as revealed by morphometric studies surpasses that of man, a feature that can be attributed to the relatively less energetic human lifestyle.

Differentiation of striated muscle fibers in pituitary gland grafts transplanted beneath the kidney capsule.

Striated muscle fibers appear with regularity in pituitary isografts transplanted beneath the kidney capsule. These differentiated striated muscle fibers consist of multinucleated cells resembling skeletal muscle fibers. They appear mainly in the pars distalis, though occasionally in the pars intermedia of the grafts. Although most glandular cells disappeared due to necrosis in the center of the grafted anterior pituitary, where the striated muscle fibers occurred, the folliculo-stellate cells were well preserved in this region and formed elongated myotube-like structures. The muscle fibers appearing in the central pars distalis were in close contact to the folliculo-stellate cells and also to the marginal epithelium of the pituitary cleft. In these cases the striated muscle fibers and folliculo-stellate cells were surrounded by a common basal lamina. This close association between heterotopically differentiated muscle fibers and the normally occurring folliculo-stellate cells strongly suggests a close relationship in differentiation and function between these different cells.

ULTRASTRUCTURAL STUDY ON AN APPENDICEAL CARCINOID TUMOR SHOWING INTRA‐NERVE FIBER GROWTH

Ultrastructural and immunohistochemical studies on an appendiceal carcinoid tumor in a 53-year-old man disclosed an intimate association of nerves and tumor cells. Electron microscopy revealed that the tumor nests were composed of neurosecretory cells containing neurosecretory granules and peripherally-located Schwann cells encasing axons. Some tumor nests were continuous with nerve fibers invested in the common basal lamina. These findings suggest that at least some carcinoid tumors of the appendix show intra-nerve fiber growth, and that the neurosecretory cells might be present in the nerve fibers from the beginning or infiltrated into the nerve fibers at a very early stage of neoplastic transformation.

Ultrastructural observations on the lung of the emperor penguin (Aptenodytes forsten)

Of all avian species the emperor penguin is the best adapted bird to attain the greatest diving depths and diving durations. Therefore the lung of this bird was investigated with electron-microscopic, i.e., freeze-fracture and thin-section methods. The parabronchi are surrounded by bundles of smooth muscle cells innervated by varicosities of autonomic nerves. The parabronchial epithelium is flat, bears a few microvilli and does not show any conspicuous ultrastructural specializations; only individual cells contain secretory granules. The atrial epithelial cells bear apical microvilli and are interconnected by adhering and tight junctions (5–10 sealing strands), the latter presumably forming an effective barrier against paracellular fluid movements. The cells contain lamellar inclusions of two types: (i) round membrane-bounded granules, the lamellar content of which is fixation-labile, and (ii) large polymorphic compact deposits of well-preserved lamellae. In both types of inclusions the individual lamellae can be of trilaminar appearance, whereas their fracture faces are smooth. Lamellar material also covers the epithelium of atria, infundibula and air capillaries. In thin areas the diameter of the morphological blood-air barrier measures 220–330 nm. Usually the endothelium of the blood capillaries is thicker (40–180 nm) than the air capillary epithelium (25–150 nm). Both epithelium and endothelium are interconnected by tight junctions, which seem to be more extensive and presumably tighter in the epithelium than in the endothelium. Frequently the common basal lamina is the thickest individual component of the blood-air barrier, measuring between 170–230 nm. Often collagen fibrils occur in this area of the barrier. In comparison with that of other birds the entire blood-air barrier of the emperor penguin is relatively thick, probably owing to an adaptation of the lung tissue which must resist high hydrostatic pressure during diving excursions.

Testicular cell differentiation in fetal mouse ovaries following transplantation into adult male mice

Testicular development is a complicated process involving differentiation and arrangement of several cell types. To analyze the process of testicular organization we examined the sequence of the appearance of testicular structures induced in fetal ovaries following transplantation. Fetal mouse ovaries on the twelfth day of gestation were transplanted beneath the kidney capsules of adult male mice. They continued to develop morphologically as ovaries until the eleventh day after transplantation, when seminiferous cord formation and testosterone production began in addition to follicle development (ovotestes). Between the eleventh and fourteenth day after transplantation, ovarian grafts frequently contained transitional structures consisting of Sertoli cells, pregranulosa cells, a third type of cells which show intermediate characteristics between Sertoli and pregranulosa cells, and oocytes enclosed by common basal lamina. Leydig cells or peritubular myoid cells were not found in the transitional area, whereas these cells were present around seminiferous cords composed only of Sertoli cells. Oocytes were absent or degenerated in the well-developed seminiferous cords. The present findings suggest that, in ovarian grafts, pregranulosa cells can differentiate into Sertoli cells, which are responsible for the organization of the seminiferous cords, degeneration of oocytes, and differentiation of other testicular somatic cell types.