Taisuke Nakao

Some observations on the fine structure of the myotendinous junction in myotomal muscles of the tadpole tail

SummaryMyotendinous junctions in the myotomal tail muscles of the tadpole of Rana rugosa were examined by electron microscopy. At the site of the myotendinous junction, the sarcolemma is covered on its sarcoplasmic aspect by the connecting filament layer and the attachment layer, and on the extracellular aspect by the intermediary layer and the external lamina, with associated collagen fibrils. The intermediary layer consists of filamentous structures which closely resemble “microfibrils” (Hanak and Böck, 1971), “spine-like or thread-like profiles” (Korneliussen, 1973) and “intermediary layer” (Nakao, 1975a, b) in the myotendinous junctions of other vertebrate skeletal muscles.Particularly interesting is the fact that all the coverings and linings of the sarcolemma, including the external lamina, are completely absent in the terminal segment of the finger-like sarcolemmal invagination characteristic of the myotendinous junction. Furthermore, special types of coupling between a sac of sarcoplasmic reticulum and a part of the sarcolemmal invagination are frequently observed. These couplings always occur along the region of the sarcolemma where the external lamina is absent. The couplings show features similar to those of the triad, such as “ SR feet ”, “scalloped SR membranes” and “granular content of the SR sac”, suggesting that they are analogous and functionally similar to the triad and other equivalent structures.

An electron microscopic study of the circulatory system in Nereis japonica

Blood vessels in Nereis japonica were studied by electron microscopy. It was found that blood vessels regardless of location were similar in the basic organization of the basal lamina and the usual presence of collagen fibrils on the vessel wall. Differences arise, depending on whether the outside of the basal lamina is covered by peritoneal cells, by gut epithelium, or by epidermis. These relate to the location of the vessels in mesenteries, gut or epidermis, but do not reflect basic structural differences in the vessels themselves. Furthermore, it was concluded that true endothelial cells do not exist in the circulatory system of Nereis japonica and that, in this respect, the system is essentially different from that of vertebrates, in which endothelial cells line the vessels of a closed circulatory system. These considerations lead to the further conclusion that the vascular lumen in Nereis is essentially interstitial space and that the system, which has been known as a typical “closed” circulatory system in annelids, is actually an open circulatory system. Peritoneal cells covering the walls of internal vessels show various degrees of muscular differentiation and those possessing myofilaments may be called “myomesothelial cells.”

Electron microscopic studies of coated membranes in two types of gill epithelial cells of lamprey.

SummaryCoated membranes in two types of gill epithelial cell of adult lamprey, Lampetra japonica, were studied by electron microscopy. The type 3 gill epithelial cells possess well-developed microvilli or microfolds, apical vesicles and abundant mitochondria. The cytoplasmic surface of the microvillous plasma membrane is covered by a coat of regularly spaced particles with a center-to-center distance of about 15 nm. Each particle consists of a bulbous free end, about 10 nm in diameter, and a connecting piece, about 5 nm long. Apical vesicles are covered by a surface coat which consists of fine filamentous material but lack any special coating on their cytoplasmic surface.The type 4 cells (chloride cells) are characterized by apical vesicles, abundant mitochondria and cytoplasmic tubules. These tubules possess a coat on their luminal surface which consists of spirally wound parallel rows of electron-dense materials. The rows are about 16 nm apart and wound at a pitch of about 45°. The cytoplasmic surface of these tubules does not display a special coat. These coated membranes are assumed to be the sites of active ion transport across the plasma membrane. In particular, particles in type 3 cells and linear coat materials in chloride cells may be either loci of transport enzymes or energy generating systems. Apical vesicles lack any coating on their cytoplasmic surface but a fine filamentous coat is present on their luminal surface. They contain “intraluminal vesicles” and are continuous with apical ends of cytoplasmic tubules.

Development of the spinal nerves in the mouse with special reference to innervation of the axial musculature

Development of the mouse spinal nerves was studied. On E11 (11th day of gestation), the primitive spinal nerve fascicle extended ventrally in the anterior half of the sclerotome. Spinal nerves in the forelimb region united with each other to form the primitive brachial plexus. Their terminal segment was covered by a peculiar cell mass. On E12, five primary branches developed along the primitive spinal nerve trunk. The ramus dorsalis was originally a cutaneous nerve, supplying two series of branches to the skin of the back. The medial series was derived from the dorsal ramus of C2–C8, and the lateral series from C8 and the more caudal dorsal rami. Nerves of the former series took the presegmental course through the intermyotomic space, while those of the latter the postsegmental course. The ramus cutaneus lateralis was a nerve that took the presegmental course to become cutaneous. The ramus intercostalis externus was a muscle branch whose distribution was restricted within the segment. The ramus anterior was a muscle branch from the end of the primitive spinal nerve trunk. The ramus visceralis connected a thoracic nerve with the para-aortic sympathetic cell cord. On E13–16 the ramus anterior secondarily gave off a cutaneous branch (ramus cutaneus anterior). The ramus intercostalis externus extended ventrally deep to the intercostalis externus muscle, crossing just caudal to the ramus cutaneus lateralis that secondarily gave off branches to the obliquus externus abdominis muscle.

Electron microscopic study of the open circulatory system of the shrimp, Caridina japonica. I. Gill capillaries

The ultrastructure of the phyllobranchiate type gill of the shrimp, Caridina japonica, was studied. The most characteristic feature of the open circulatory system of Cardina is the vascular lumen of the gill capillaries which is considered to be the interstitial space. The following observations substantiate this view: (1) a thin fibrous layer forms the innermost structure of the walls of gill capillaries and is in direct contact with the blood stream; (2) filaments in the fibrous layer are assumed to correspond to the reticular fibers in the interstitial space of the alveolar wall of mammals; (3) the absence of the endothelium as well as the endothelial basal lamina which are the essential structural components of the closed circulatory system in vertebrates. The gill epithelium contains intermediate, septate and tight junctions. The first two form a junctional complex near the apical cell border and may function as a permeability barrier by occluding the intercellular space as well as functioning in electrical coupling and cellular adhesion. The tight junction is spot‐like and may serve no role in the function of the permeability barrier.

The fine structure and innervation of gill lamellae in Anodonta.

SummaryGill lamellae of a bivalve Anodonta woodiana lauta (v. Martens) were observed by electron microscopy. The Anodonta gill wall consists of a single layer of epithelial cells, its basal lamina and the underlying connective tissue layer. It was confirmed that there is no true endothelium in the vessels and that the connective tissue layer of the vessel wall is therefore in direct contact with the blood. Cells of a specific type referred to as “trabecular cells” lie in the blood lacunae. These cells closely resemble the pillar cells of fish gills, but show certain fundamental differences. Characteristic features of the trabecular cells are (1) an elongated cell body which lies across the vascular lumen and attaches to the vessel wall by means of the tips of their long processes, (2) two types of myofilaments (thick and thin) in the cytoplasm, (3) external dense plaques at the cell surface which are associated with the insertion of myofilaments into the cell membrane, (4) direct contact between the cell surface and the blood except at the regions where the cell is covered by external plaques and connective tissue fibrils. These facts suggest that the Anodonta trabecular cell is not analogous with the pillar cell of fish gills but rather with muscle cells which show a specific morphological modification and a peculiar relationship to the vessel wall due to the absence of the endothelium. These cells are assumed to regulate blood flow within the gill vessels.As to the permeability of the wall of Anodonta gill vessels, junctional complex consisting of an intermediate and a septate junction between adjacent gill epithelial cells probably plays the main role as a barrier between the blood and the surrounding water. The basal lamina underlying the gill epithelium is assumed to act as a coarse permeability barrier.Numerous nerve endings of unknown function are observed in the gill epithelium. It is strongly suggested, however, that they are associated with the additional function of the Anodonta gill lamellae as a food-sorting device.

An electron microscopic study of autonomic nerve cells in the cloacal region of the lamprey, Lampetra japonica.

SummaryTwo types of autonomic nerve cell in the cloacal region of lamprey,Lampetra japonica have been studied by electron microscopy. Large ganglion cells (LGC) were unipolar and individually invested with a satellite cell sheath. The LGC-satellite cell complex measured 24 μm × 38 μm on average. Granular endoplasmic reticulum and cored vesicles (80–140 nm in diameter) were scattered in the perikaryon. Two kinds of peculiar cytoplasmic filament were seen in LGC: one type was about 20 nm in diameter with periodic dense material on the surface and the other had a diameter of about 8 nm and showed an undulating profile. Nerve endings containing abundant small clear vesicles (30–50 nm in diameter) and a few larger cored vesicles (50–100 nm in diameter) were found in synaptic contact with LGC. Small ganglion cells (SGC) were also unipolar and covered incompletely by a satellite cell sheath. The SGC-satellite cell complex measured 6 and 12 μm on average. The SGC was packed with organelles and the perikaryon appeared more electron dense than that of LGC. SGC perikaryonal cytoplasm contained dispersed granular endoplasmic reticulum and numerous large cored vesicles (55–220 nm in diameter). Nerve endings containing numerous large cored vesicles (70–170 nm in diameter) and variable numbers of small clear vesicles (30–50 nm in diameter) were seen on the surface of SGC.

An electron microscopic study on the innervation of the gill filaments of a lamprey, Lampetra japonica

This paper reports observations on the innervation of gill filaments of the lamprey, Lampetra japonica. Nerve fibers run on each side of the afferent filament artery (AFA nerve) and in the connective tissue compartment along the efferent filament artery (EFA nerve). The AFA nerve supplies vasomotor fibers to the afferent filament artery and arteriovenous anastomoses and special visceral motor fibers to branchial muscle fibers (musculus compressor branchialis circularis). Nerve endings of the vasomotor fibers contain large, cored vesicles (60–180 nm in diameter) with a variable number of small, clear vesicles (30–70 μm in diameter), whereas those of the visceral motor fibers have many small, clear vesicles with few large, cored vesicles. The EFA nerve supplies vasomotor fibers to the efferent filament artery. Their endings, containing mixtures of predominantly large, cored vesicles and small, clear vesicles make close synaptic contacts with reticular cells. The latter in turn are connected with each other or with smooth muscle cells in the wall of the efferent filament artery by nexuses. No nerves are found in the axial plate between the afferent and efferent filament arteries nor in the secondary lamellae of individual gill filaments. No afferent nerve supply to the gill filament has been found.

Elastic fibres in the notochord of Rana rugosa tadpoles

SummaryThe elastica interna was studied in the notochord sheath of the caudal one third of the tail of Rana rugosa tadpoles. Dense amorphous bodies are present in depressions at the surface of superficial notochord cells. These bodies are intimately associated with microfibrils. The notochord basement membrane tends to cover the surface of the dense amorphous bodies but often appears to fuse with them and disappear. There is a striking close resemblance between the amorphous dense bodies and the elastica interna. These observations, together with the observation of dilated cisterns of granular endoplasmic reticulum with dense contents in the cytoplasm of superficial notochord cells, suggest that the amorphous dense bodies and the elastica interna are produced by superficial notochord cells.