Kimiyuki Ikeda

Specialized intercellular junctions and ciliary necklace in rat brain.

SummarySpecialized intercellular junctions in astrocytes and ependymal cells as well as choroid plexus epithelium of the rat were examined with the freeze-fracture or -etch technique. Gap junctions usually showed a discontinuous distribution around the perimeters of astrocytes and ependymal cells, and were composed of closely aggregated particles in fracture face A and polygonally oriented pits in face B. Tight junctions were not usually observed in astrocytes and ependymal cells, but were evident in the choroid plexus epithelium as a ramifying complex of ridges and furrows. This network formed a continuous band on the lateral walls of the plexus epithelial cells. An isolated complex of ridges and furrows, probably a fascia occludens, was rarely found in the ependymal cells of the rat lateral ventricle.A membrane specialization, described as the ciliary necklace, was seen in the basal region of the ependymal cilia, and was usually composed of 5 or 6, but sometimes of up to 13 strands. Each strand was oriented perpendicularly to the axis of the ciliary shaft and consisted of rows of particles in faces A and B.

Specialized intercellular junctions in human intracranial germinomas

SummaryThree human intracranial germinomas were studied using thin section and freeze-fracture techniques. In thin sections, the spheroidal cells were similar to the seminoma cells of the testis. There were three types of specialized intercellular junctions; desmosomes, gap junctions, and tight junctions, were evident between the spheroidal cells. In freeze-fracture replicas, tight and gap junctions were easily differentiated by their characteristic deployment patterns of their subunits. The tight junction revealed a discontinuous, circular, serpentine, or ramifying complex of ridges and furrows, referred to as fascia occludens. The gap junction was also discontinuous and largely confined to the compartment formed by the ramifying ridges and furrows of tight junctions, and revealed aggregated particles and pits on fracture faces A and B, respectively.

Specialized junctional complexes in human meningioma

SummaryFive human meningiomas were studied with techniques of thin section and freeze-fracture. Three specialized junctional complexes, desmonsomes, gap and tight junctions, were found between adjacent plasma membranes. Desmosomes were not shown in freezefracture replicas; they exhibited a charactersstic structure in thin sections. In thin sections the outer leaflets of adjacent plasma membranes of gap junctions appeared to fuse, obliterating the intercellular space. The gap junction in replicas was irregularly ovoid or polygonal in form, mostly 0.1–1.0 μ in diameter, and characterized by a hexagonal aggregation of particles in face A and associated pits in face B. The outer leaflets of adjacent plasma membranes in the tight junction fused at discrete points in thin sections. In replicas, the tight junction was characterized by short complexes of serpentine or ramifying ridges in face A and furrows in face B, and mostly discontinous in distribution. The histogenesis of meningioma cells was discussed from a standpoint of the characteristic features of the specialized intercellular junctions.

Freeze-etching images of central myelinated nerve fibres.

SummaryThe three-dimensional features of myelinated nerve fibres in rat brain were studied with the freeze-etching technique. The cross fracture of the myelin sheath exhibited an alternate orientation of ridges and furrows with a periodicity of about 60–70 Å. The ridge in the myelin sheath was either continuous with the outer ridge of the outer loop plasma membrane or formed by fusion of the inner ridge of the loop membrane. The furrow in the myelin sheath was continuous with the furrow of the loop plasma membrane. Since the ridge of the loop membrane practically corresponded to the leaflet of the loop unit membrane, the ridge in the myelin sheath was roughly equivalent to the major or the intraperiod dense line, and the furrow in the myelin sheath was then localized in the middle between the major and the intraperiod dense lines. The myelin sheath in tangential fractures was composed of repeating membrane layers, which were relatively free of membrane particles. Although the furrow in cross fractures generally corresponded to the potential fracture plane, the main fracture process in the myelin sheath often passed alternately in the potential fracture plane. The fracture face of the myelin sheath, therefore, might represent an extended lipid layer localized in the middle of the lipid bilayer.

Freeze-fracture images of capillaries in normal and neoplastic pituitaries

SummaryThe capillaries in five human chromophobe adenomas were studied by thinsection and freeze-fracture techniques. Fenestrae were found in the attenuated portions of the endothelium, and plasmalemmal vesicles in the nonfenestrated portions. In freeze-fracture replicas, the fenestrae, 600–700 Å in diameter, were revealed as circular holes in fracture face A and circular broken-off necks in face B, some 40–60 occurred per μm2. When the fracture process passed in a stepwise fashion from the luminal into the contraluminal plasma membrane, the fenestrae at the fracture edge involved both plasma membranes. Similar configuration of fenestrae was evident in the capillary endothelium of the rat pituitary. The fenestrae in the chromophobe adenoma were much less frequent than in the normal rat pituitary.The plasmalemmal vesicles occurring in replicas on one side of the cell did not appear to open to the other side of the cell. When the fracture followed the contours of the attached plasmalemmal vesicles, they appeared as smooth depressions in face A and domes in face B. When the fracture did not follow the contours of the vesicles, they were revealed as holes or depressed circular plaques in face A and broken-off necks or circular constellations of particles in face B. The plasmalemmal vesicles were more abundant and obvious in the tumour than in the normal rat pituitary.