Freeze-etching images of central myelinated nerve fibres

Cryo-electron microscopy of nervous tissue. A review

The present work attempts to demonstrate that cryofixation is a valuable method for the study of the nervous tissue. The use of the newly developed methods of cryofixation and freeze-etching without fixatives or cryoprotectants allows new exciting perspectives for the electron microscopical observation of cellular components, emphasizing their three-dimensional morphological structures. Significant contributions have been made on the fine structure of the cytoskeleton, cell membranes and cell organelles. The components of the cytoskeleton are distributed in different composition through the perikarya, dendrites and axon. The ubiquitous presence of the cytoskeleton suggests a crucial role in the functional activities of the neurons, especially in relation to the intracellular communication and to developmental and regeneration processes. Vitrified cellular membranes of myelin sheaths and rod outer segments have been observed in hydrated state by using cryofixation and cryotransfer techniques. These procedures allow new insights into the supramolecular structure and an approximation of morphological data to the present biophysical membrane model including a critical comparison with the current descriptions gained by conventional electron microscopy.

Myelinated axons of ganglion cells in the retinae of teleosts

In the retina of the goldfish and the rainbow trout, the axons of ganglion cells belong to the unmyelinated or the myelinated types. The unmyelinated fibers are either arranged in bundles in direct contact with neighboring fibers or they are separated by intervening lamellae of oligodendroglial cytoplasm. The myelin sheaths of the myelinated fibers differ greatly in thickness. Most fibers show 3 to 5 myelin layers; single fiber elements, however, show 10 or even more layers.

Membrane specializations and cytoplasmic channels of Schwann cells in mammalian peripheral nerve as seen in freeze-fracture replicas

Mammalian Schwann cells in rat, rabbit and human fetal nerves were studied using several cryoprotective agents for electron microscopic study of freeze-fracture replicas. The findings in fixed and unfixed tissue reveal surface plasmalemma caveolar specializations and the outer layer membrane junctional complexes found in non-mammalian species. The plasmalemma also reveals a complex arrangement of contours outlining cytoplasmic channel networks distinct from the long-recognized Schmidt-Lanterman incisures and paranodal cytoplasmic loops. A specialized interconnected channel system in the outer "loose" myelin layer displays relatively uniform dimensions comparable in diameter to nodal microvilli, paranodal loops and some incisures. An adaxonal tubular channel system constituting the "axon-Schwann network" is found in the internodal region in addition to other variants of the adaxonal Schwann plasmalemma. The several forms of sequestration of Schwann cell cytoplasm presumably underlie the specialized needs of cytoplasmic continuity in a dynamic functional entity in which large domains of cytoplasm have been displaced by the formation of compact myelin.

Jimpy mouse myelin revisited with freeze-fracture

Corpus callosum, cerebellum, and spinal cord from Jimpy mice, and control littermates, 15 and 21 days old, were prepared for freeze-fracture in a "cryofract" apparatus. The few myelinated axons in the Jimpy exhibited a striking paucity of particles in myelin P faces, though tight junctions were present. In addition, small maculae of particles were found on these P faces. Peripheral myelin appeared normal, both for the quantity and disposition of particles on their P faces.

The development of a zonula occludens in peripheral myelin of the chick embryo. A freeze-fracture study

Sciatic nerves of chick embryos, 12 to 18 days incubation, were examined in freeze-fracture replicas with special emphasis placed on the development of tight junctional contacts in the myelin sheaths. In stages of beginning myelination short isolated particulate chains (focal tight junctions) appear in fracture faces of the adjacent membranes in the outer myelin lamellae, i.e., the outer mesaxon. In stages of progressing myelination these tight junctional elements elongate and become more numerous. They can also be found in the membranes of the inner mesaxons, the paranodal loops and the intramyelinic cytoplasmic inclusions. In fibers of advanced myelinogenesis a fusion of these isolated tight junctions--either end-to-end or at an angle--gives rise to continuous zonulae occludentes. This contact zone extends in the mesaxonal membranes along the direction of the fiber, whereas in paranodal myelin it acquires a helical course joining the membranes of the paranodal loops. It is proposed that this zonula occludens, which seals the cytoplasmic border of the Schwann cell, separates an intramyelinic from an extramyelinic, extracellular space already during the developmental stages of myelinogenesis.

Electron microscopy of cutaneous nerves and receptors

Recent ultrastructural observations on the connective tissue sheaths of nerves, Schwann cell-axonal relations, and nerve terminals and receptors are reviewed. It seems likely that endoneurial collagen is formed by perineurial cells during development and postnatally. New observations on "collagen pockets" are presented. Attention is drawn to freeze-fracture studies of peripheral nerve, particularly in relation to junctional complexes associated with compact myelin, and further application of the technique is considered. Current views on Merkel cells, encapsulated endings, and free nerve terminals are discussed.

Freeze-fracture properties of central myelin in the bullfrog

The freeze-fractured membrane of the central myelin sheath has three classes of particulate components: (i) Particles inherent to the compact myelin lamellae. These are distributed at random and cleave predominantly with the P (protoplasmic) face. (ii) Particles which comprise the intramyelinic tight junctions. These are arranged in strands and are located at the inner and outer mesaxon, the paranodal loops, the cytoplasmic incisures, and occasionally within the compact regions of the myelin sheath. (iii) Particles localized exclusively at the portion of the paranodal loop membrane involved in the septate-like junction with the axolemma. These are regularly spaced and are organized in parallel rows. In the central myelin sheaths of bullfrogs fixed by perfusion with aldehydes and cryoprotected in 30% glycerol, the randomly distributed particles differ in size and shape from those of the axolemma. They possess a reasonably well defined bimodal distribution with respect to particle shape--most can be described either as globules or as ellipsoids. The globular particles range in diameter from 60 to 150 A. The ellipsoidal particles are 100-200 A long and 15-50 A wide. The total number of particles per square micron on the P face is approximately 1500. About half of these are of the globular type and half of the ellipsoidal type. In poorly fixed specimens, loss of interlamellar adhesion and loss of randomly distributed particles seem to coincide. Evidence is presented against the hypothesis that the tight junctions between compact myelin lamellae represent the radial component of the myelin. The possible relation between the types of particulate components seen in freeze-fracture and the classes of protein isolated from central myelin fractions is briefly discussed.

Plasma membrane structures of medulloblastoma and cerebellar sarcoma

Three medulloblastomas and 1 cerebellar sarcoma were studied on their plasma membrane structures. The average number of membrane particles per mum2 plasma membrane was 710 on face A and 70 on face B of medulloblastoma and 1280 on face A and 160 on face B of cerebellar sarcoma. The membrane particles were often aggregated in medulloblastoma and diffusely scattered in cerebellar sarcoma. Small gap junctions were occasionally found in cerebellar sarcoma and not evident in medulloblastoma. Round membrane protrusions, about 0.5-0.6 mu in diameter and provided with several small depressions on their foot, were often observed in region of narrow perinuclear cytoplasm of cerebellar sarcoma and different in structure from cytoplasmic processes. The present series is too limited in number to allow a definite conclusion, but indicates that the plasma membrane structures are different in medulloblastoma and cerebellar sarcoma.

Zonulae occludentes of the myelin lamellae in the nerve fibre layer of the retina and in the optic nerve of the rabbit: a demonstration by the freeze-fracture method

Ridges and grooves composing extensive zonulae occludentes are revealed by the freeze-fracture method on split myelin lamellae in the nerve fibre layer of the retina and in the optic nerve of the rabbit. The junctions are located immediately internal to the outer loop of the myelin sheath and in corresponding areas of deeper myelin layers. They follow a straight or gently undulating course along the axis of the fibres. Only at the paranodal region of nodes of Ranvier do they deviate and assume a transverse course, The strands of these zonulae occludentes probably represent the radial thickenings of the intraperiod line described in thin sections.