Cell contacts and smooth muscle bundle formation in tissue transplants into the anterior eye chamber

Summary Segments of the taenia coli from guinea-pig were transplanted into the anterior chamber of the eye. Depending on such factors as the total volume of the transplant and the presence or absence of ganglion cells degeneration was either very extensive (90% or more of the total number of muscle cells) or localized (alternating regions of degenerating and normal structure). During days 1–2 muscle cells lost their plasma membranes so that their cytoplasmic contents were dispersed into the intercellular spaces. Many cells produced numerous small processes which were pinched off and dispersed in a similar manner. Following a period of intense mitotic activity (3–8 days) numerous cells with the characteristics of embryonic smooth muscle cells were evident. Within 10–14 days these differentiating cells produced bulbous protrusions and assumed more irregular outlines than at 3–8 days. The protrusions formed close contacts (50–100Å intercellular space) and tight junctions between adjacent muscle cells. Aggregation of muscle cells into bundles was under way between 14–28 days. At approximately 4–6 weeks these developing muscle groups were invaded by nerve fiber bundles. The pattern of the innervation and the form and size of the muscle bundles simulated the normal. These findings are discussed in relation to the possible functions of the intercellular contacts and cellular protrusions which characterise various periods of regeneration.This work was supported by the Australian Research Grants Committee. The transplants were carried out by Dr. T. Malmfors assistant, Miss Ulla Enberg.     

Ultrastructure of interstitial cells of Cajal at the gastro-oesophageal junction of the monkey (Macaca fascicularis)

The ultrastructure of the interstitial cells of Cajal (ICC) in the oesophagus of the monkey resembled that described in the oesophagus of other mammalian species but differed in their paucity and almost lack of smooth endoplasmic reticulum, caveolae and filaments. The plasmalemma of the ICC was in close contact (20- to 30-nm gaps) with that of smooth muscle cells. This may occasionally take the form of a desmosome, but gap junctions have not been observed. Vesiculated axon profiles, containing large granular or agranular vesicles were in close contact (20- to 30-nm gaps) with the plasmalemma of ICC. In a few vesiculated profiles a presynaptic density could be recognized. The intercalation of the ICC between the vesiculated axon profiles and the smooth muscle cells suggest a role in oesophageal motility. Between 3 and 21 days following bilateral vagotomy some ICC showed regressive changes such as increased electron density and shrinkage of the cytoplasm, crowding of the organelles and dissolution of the nuclear chromatin material. Axon profiles in the vicinity of the affected ICC contained glycogen granules suggesting injury. In late stages, the number of ICC and smooth muscle contacts was reduced. The results suggest that the vagus nerves exert a trophic influence on the ICC and that the intercellular relationships between ICC and smooth muscle cells possess a degree of plasticity. It is tentatively suggested that these vagal effects may be mediated via the oesophageal myenteric ganglia.     

Ultrastructural studies of the myenteric plexus and smooth muscle in organotypic cultures of the guinea-pig small intestine

External muscle and myenteric plexus from the small intestine of adult guinea-pigs were maintained in vitro for 3 or 6 days. Myenteric neurons and smooth muscle cells from such organotypic cultures were examined at the electron-microscopic level. An intact basal lamina was found around the myenteric ganglia and internodal strands. Neuronal membranes, nuclei and subcellular organelles appeared to be well preserved in cultured tissues and ribosomes were abundant. Dogiel type-II neurons were distinguishable by their elongated electron-dense mitochondria, numerous lysosomes and high densities of ribosomes. Vesiculated nerve profiles contained combinations of differently shaped vesicles. Synaptic membrane specializations were found between vesiculated nerve profiles and nerve processes and cell bodies. The majority of nerve fibres were well preserved in the myenteric ganglia, in internodal strands and in bundles running between circular muscle cells. No detectable changes were found in the ultrastructure of the somata and processes of glial cells. Longitudinal and circular muscle cells from cultured tissue had clearly defined membranes with some close associations with neighbouring muscle cells. Caveolae occurred in rows that ran parallel to the long axis of the muscle cells. These results indicate that the ultrastructural features of enteric neurons and smooth muscle of the guinea-pig small intestine are well preserved in organotypic culture.     

Changes in Surface Morphology of Myogenic Cells during the Cell Cycle, Fusion and Myotube Formation

The surface morphology of chick myogenic cells during development in cell culture was examined by scanning electron microscopy. Myoblasts at the G₁ and S phases of the cell cycle had a relatively smooth surface. In late G₂ and mitosis, they had many microvilli and some blebs on their surfaces. Ca²⁺-deficient fusion-arrested myoblasts had a relatively smooth surface. When the cells underwent cell fusion, many microvilli, small spherical protrusions, and some blebs appeared on their surface. In newly formed myotubes, the surface over the nucleus was smooth whereas that over perinuclear regions had many flat excrescences and other surface protrusions. This mosaic appearance of the surface was less prominent in striated myotubes. Scanning electron microscopy combined with fluorescence microscopy using rhodamine-labeled erabutoxin b revealed that sites of accumulation of acetylcholine receptor had a smooth surface. These results suggest that changes in surface structure occur in association with the cell cycle, fusion and subsequent development of myotubes.     

Morphology of smooth muscle cells in the rat thoracic duct

Summary The three-dimensional cytoarchitecture and ultrastructure of the smooth muscle cells in the wall of the rat thoracic duct were investigated by scanning and transmission electron microscopy. The muscle layer basically consists of a single layer of circularly arranged cells. The smooth muscle cell is fusiform or ribbon-like in shape, as in veins or venules with a similar or smaller diameter. Connections by spinous processes are observed between adjacent muscle cells along their length. Spot-like membrane contacts frequently occur in areas where facing membranes are closely apposed. These are thought to be gap junctions and may be responsible for electrical coupling and mechanical attachment. Large invaginations arranged regularly in rows on the surface of the smooth muscle cells can be observed. These invaginations are closely associated with a flattened sarcoplasmic reticulum, and caveolae tend to open into the invaginations.     

Myoendothelial junctions in blood vessels of a teleost (Prochilodus scrofa) pancreas and caeca

Summary Close contacts between endothelial and smooth muscle cells in teleost (Prochilodus scrofa) blood vessels are described for the first time in the present study. More frequently are seen finger-like, club-shaped or foot-like endothelial processes that come into close contact with the plasma membrane of a smooth muscle cell. Rarely, some myoendothelial contacts occur between the finger-like protrusions that arise from both the endothelial and from the smooth muscle cells. The functional significance of the myoendothelial connection is discussed.     

Three-dimensional cytoarchitecture of the media of arteriovenous anastomoses in the rabbit ear

Arteriovenous anastomoses in the rabbit ear were examined with scanning electron microscopy to elucidate the structural differentiation of the media of the shunt. Arterial, intermediate, and venous segments in the shunt and two layers of the media in the intermediate segment were differentiated based on cell shape and cell organization. In the arterial segment, smooth muscle cells were spindle-shaped, either elongated or short, with a few branches, and were arranged circularly or diagonally with respect to the vessel's long axis. There were also stellate muscle cells with radiating processes. In the intermediate segment, the smooth muscle cells of the outer layer of the media were also arranged circularly and resembled the elongated cells in the arterial segments, but they were more irregular in shape and had more processes than those of the arterial segment. The epithelioid cells of the inner layer of the media were oval or polygonal and oriented irregularly with respect to the vessel's long axis, clustering to form longitudinal plicae. The smooth muscle cells of the venous segment were flat with many lateral processes and formed a thin, discontinuous layer. The smooth muscle cells in the arterial segment and those of the outer layer of the intermediate segment exhibited a highly rugged surface texture, indicating their strong contractility; the epithelioid cells and the smooth muscle cells in the venous segment exhibited a generally smooth surface, indicating less contractility. The intermediate segments were supplied with a dense nerve plexus. The intermediate segments, therefore, may be actively involved in the regulation of blood flow under neuronal influence.     

Potassium chloride-insoluble myofilaments in vertebrate smooth muscle cells

Actomyosin was extracted from avian gizzard smooth muscle. The residue was then homogenized and fractionated by differential centrifugation. Fractions of the residue that sedimented at 1 000 g and 13 000 g were examined in negatively stained and sectioned preparations with the electron microscope. The major components of both fractions were 100 Å diameter filaments and fusiform dense bodies. The filaments and dense bodies closely resembled their counterparts in sectioned preparations of unextracted smooth muscle cells from Taenia coli. The insolubility of the 100 Å diameter filaments at high ionic strength and their detailed structure suggest that they are not composed of actin and myosin. Their general features indicate that they correspond to the so-called thick filaments observed in the early studies of vertebrate smooth muscle cells.     

A developmental and ultrastructural study of the optic chiasma in Xenopus

The structure of the optic chiasma in Xenopus tadpoles has been investigated by light and electron microscopy. Where the optic nerve approaches the chiasma, a tongue of cells protrudes from the periventricular cell mass into the dorsal part of the nerve. Glial processes from this tongue of cells ensheath fascicles of optic axons as they enter the brain. Coincident with this partitioning, the annular arrangement of axons in the optic nerve changes to the laminar organization of the optic tract. Beyond the site of this rearrangement, all newly growing axons accumulate in the ventral-most part of the nerve and pass into the region between the periventricular cells and pia which we have called the 'bridge'. This region is characterized by a loose meshwork of glial cell processes, intercellular spaces and the presence of both optic and nonoptic axons. In the bridge, putative growth cones of retinal ganglion cell axons are found in the intercellular spaces in contact with both the glia and with other axons. The newly growing axons from each eye cross in the bridge at the midline and pass into the superficial layers of the contralateral optic tracts. As the system continues to grow, previous generations of axon, which initially crossed in the existing bridge, are displaced dorsally and caudally, forming the deeper layers of the chiasma. At their point of crossing in the deeper layers, these fascicles of axons from each eye interweave in an intimate fashion. There is no glial segregation of the older axons as they interweave within the chiasma.     

The adsorptive and transport capacity of tanycytes during the perinatal period of the rat

Summary Transport of ferritin and horseradish peroxidase from the 3rd ventricle to the median eminence was examined in rats during the perinatal life, the time when functional interrelations between hypothalamus and hypophysis are established. Protein tracers injected into the lateral ventricle are adsorbed on the apical surface of the tanycyte, mainly on its protrusions or in indentations. On the 18th day of prenatal life a few small bleblike protrusions are observed. After birth microvilli appear. In time their concentration increases to result in an increase of adsorbed substances. They are taken up by smooth and coated pinocytotic vesicles and transported to the basal portion of the cell or to the intercellular space bypassing junctional complexes. In addition to pinocytotic vesicles protein tracers fill channels of smooth ER or Golgi complex and multivesicular bodies illustrating a process probably involved in metabolic or secretory processes.     

Electron microscopic observations on the innervation of the sphincter of oddi in the dog

The ultrastructure and acetylcholinesterase activity of the intrinsic innervation of the sphincter of Oddi of eight adult dogs was studied by electron microscopy. A rich distribution of unmyelinated axons embedded individually or as groups within Schwann cell cytoplasm ("innervation fasciculee"), is to be observed. A few myelinated fibres were also observed. Many of the axons are acetylcholinesterase-positive. Three main types of nerve terminals are distinguished according to their vesicle populations. Individual nerve cells or small groups of nerve cells were scattered between the smooth muscle bundles and in the lamina glandularis mucosae. The cytoplasm of some neurons contains many electron dense spherical bodies resembling "myeloid bodies", and many lysosomes. Nerve terminals synapse onto both neuronal perikarya and their dendrites. Within the nerve fascicles, close appositions between the terminals occur frequently probably representing the most peripheral inter-neuronal integrative link in the neural regulation of the function of the sphincter of Oddi. -- The gap between nerve terminals and smooth muscle cells usually measures several thousands of A. Closer appositions are seldom seen, and no synaptic complexes can be observed.     

Ultrastructural observations of the tunica muscularis in the small intestine of Xenopus laevis, with special reference to the interstitial cells of Cajal

The distribution and ultrastructure of the interstitial cells of Cajal (ICC) has been examined in the small intestine of the frog Xenopus laevis, as the physiological significance of these cells remains obscure in amphibians and other lower vertebrates. The present study has revealed the existence of a special type of interstitial cell in the tunica muscularis of the small intestine of Xenopus; this cell is characterized by the presence of numerous caveolae, many small mitochondria, and the formation of intercellular connections with the same type of cell. Since these ultrastructural features are shared with mammalian ICC, the cells in the small intestine of Xenopus probably correspond to ICC. These cells also form close contacts with neighboring smooth muscle cells and with nerve varicosities containing accumulations of synaptic vesicles. These cellular networks are likely to be involved in the transmission of nerve impulses to muscle cells, as has been suggested for mammalian tissues. However, true gap junctions have not been detected; they occur neither between the same type of cells nor between the putative ICC and smooth muscle cells. The widespread distribution of ICC or equivalent cells in different groups of vertebrates, together with the conservation of their ultrastructural features, suggests that they differentiated early in vertebrate evolution to play key regulatory roles in gastrointestinal movement.     

Smooth muscle cells, interstitial cells of Cajal and myenteric plexus interrelationships in the human colon

The plane between longitudinal and circular muscle of human colon, as revealed on examination with light and electron microscopes, has no clear-cut border. Some groups of smooth muscle cells, obliquely oriented and with features similar to both circular and longitudinal ones--the connecting muscle bundles--run from one muscle layer to another. Other groups of smooth muscle cells, possessing their own specific ultrastructural features--the myenteric muscle sheaths--, make up envelopes of variable thickness around some myenteric ganglia and nerve strands, partially or completely embedding them in one or other muscle layer. Non-neuronal, non-muscular cells (interstitial cells of Cajal, covering cells, fibroblast-like and macrophage-like cells) complicate the texture of the myenteric muscle sheaths, creating an intricate, interconnected cellular network inside them, widespread among nerve bundles and smooth muscle cells; however, only interstitial cells have cell-to-cell junctions also with the smooth muscle cells and nerve endings. These data document the existence in this colonic area of two different types of muscle cell arrangements, one of which, the myenteric muscle sheath, only contains putative pacemaker cells.     

The interstitial cells in the colon of the rabbit

Summary The interstitial cells associated with the myenteric plexus of the rabbit colon were studied by scanning and transmission electron microscopy. It was demonstrated that the interstitial cells were stellate or fusiform in shape and located over the ganglia, over nerve bundles and between muscle cells. They were characterized by many slender processes, and resemble fibroblasts. No basal lamina was observed between the interstitial cells and muscle cells. It was concluded that structural features of the interstitial cells are distinctly different from those of neurons, Schwann cells, or of smooth muscle cells, while they show clear similarities to those of fibroblasts. By scanning electron microscopy the shapes and the relations of these cells could be demonstrated in great detail.     

Interstitial cells associated with the deep muscular plexus of the guinea-pig small intestine,with special reference to the interstitial cells of Cajal

Summary Interstitial cells associated with the deep muscular plexus of the guinea-pig small intestine were studied by electron microscopy, and three-dimensional cell models were reconstructed from serial ultrathin sections with a computer graphic system. Three types of cells were recognized. The first type was similar in shape to smooth muscle cells, but did not contain an organized contractile apparatus. Many large gap junctions comprising about 4% of the cell surface were present; they connected cells of the first type to each other, to the second type of cell and to smooth muscle cells of the outer circular layer. The second type of cell had a welldemarcated cell body with long slender processes and was characterized by a large amount of glycogen comprising about 9% of the cell volume. The third type of cell was similar to fibroblasts, and contained well-developed Golgi apparatus and rough endoplasmic retiulum. Some of these fibroblast-like cells (a possible subtype) formed small gap junctions. All three types of cells showed close relationships with nerve varicosities. This cellular network consisting of gap-junction-rich cells, glycogen-rich cells and smooth muscle cells may be involved in the pacemaking activity of intestinal movement.     

ULTRASTRUCTURAL STUDIES ON THE CONTRACTILE MECHANISM OF SMOOTH MUSCLE

Fresh taenia coli and chicken gizzard smooth muscle were studied in the contracted and relaxed states. Thick and thin filaments were observed in certain (but not all) cells fixed in contraction. Relaxed smooth muscle contained only thin filaments. Several other morphological differences were observed between contracted and relaxed smooth muscle. The nuclear chromatin is clumped in contraction and evenly dispersed in the relaxed state. The sarcolemma is more highly vesiculated in contraction than in relaxation. In contraction, the sarcoplasm also appears more electron opaque. Over-all morphological differences between cells fixed in isometric and in unloaded contraction were also noticeable. The results suggest a sliding filament mechanism of smooth muscle contraction; however, in smooth muscle, unlike striated muscle, the thick filaments appear to be in a highly labile condition in the contractile process. The relation between contraction and a possible change in pH is also discussed.     

Fibroblasts as a part of the contractile system in duodenal villi of rat

Summary The stroma of duodenal villi of rats was studied by light- and electron microscopy. Fibroblasts are rather evenly distributed within the villus. Their branched processes embrace all blood vessels, the lacteal and the bundles of smooth muscle cells. They are connected to each other and to smooth muscle cells by close contacts. Unmyelinated axons are found close to the fibroblasts where they may show synapse-like formations.The fibroblasts within intestinal villi contain many dilated cisterns of rER similar to normal fibroblasts. In contrast to the latter, there are many aggregated, contractile filaments, being situated mainly below the plasma membrane and within the processes. It is suggested that fibroblasts representing a 3-dimensional contractile network may be activated by smooth muscle cells and/or by innervation. So, they seem to be involved in the diminution of the vascular and stromal spaces within the villus.     

Three-dimensional observation of the fibroblast-like cells associated with the rat myenteric plexus,with special reference to the interstitial cells of Cajal

Summary An extensive cellular network becomes visible over the myenteric plexus of the rat after removal of the overlying tissues under the scanning electron microscope. The cells are mainly stellate and have many slender processes via which they interconnect. They form a three-dimensional network and are closely associated with the ganglia and nerve bundles, and also extend over the smooth muscle cells. They are considered to correspond to the interstitial cells of Cajal because of their peculiar arrangement and their topography. Transmission electron-microscopic evidence demonstrates that the majority of those cells have features of fibroblasts. Gap junctions and intermediate junctions are observed between these fibroblast-like cells, and also between them and smooth muscle cells. Examination of serial thin sections reveals that single fibroblast-like interstitial cells connect to both circular and longitudinal muscle cells via gap junctions. It is suggested that the network of interstitial cells conducts electrical signals.     

Innervation of mouse lymph nodes: nerve endings on muscular vessels and reticular cells

Lymph node nerve endings have been studied in 1- to 48-day-old mice. Serial sections of Epon-embedded lymph nodes were observed under the electron microscope to find the nerve endings. Most lymph node nerve fibers finally reach the smooth muscle cells of arterioles and muscular venules. Both kinds of vascular endings are similar, although endings are less numerous on venules. Nerve endings consist of one or more nerve processes surrounded by a usually incomplete Schwann cell sheath; frequently, axons show wide areas directly facing the muscle cells. The distance between such a naked axon and a myocyte ranges from 100 to 800 nm. Small granulated and clear vesicles are especially abundant in varicosities of nerve processes that are located very close to muscle cells. Nerve endings of lymph node vasculature probably correspond to vasomotor sympathetic adrenergic endings, regulating the degree of contraction of vessels which have a muscular layer. Other kinds of nerve endings also exist in lymph nodes: some axons appear free in the stroma and contact the surfaces of reticular cells; the latter also extend delicate cytoplasmic processes that surround the axons. The functional significance of nerve cell-reticular cell contacts is unknown.