Ultrastructural study of myelinating cells and sub-pial astrocytes in developing rat spinal cord

Attempt to classify glial cells by means of their process specialization using the rabbit retinal Müller cell as an example of cytotopographic specialization of glial cells

The rabbit retinal Müller cell is one of the most widely studied glial cell types, and it has all forms of contacts that a glial cell can express, viz. 1) to a (ventricular) fluid space, 2) to a mesenchymal borderline (basal lamina), and 3) to neuronal compartments. This cell demonstrates the local adaptation of cell processes to the microenvironment with which they are in contact. Summarizing available data on Müller cells and other glial cell types, it is concluded that the structure with which the process is in contact determines the type of glial cell process that develops. The type I process has microvilli, desmosome-like junctions, and high Na+,K+-ATPase activity; this type of process is in direct contact with a fluid such as cerebrospinal fluid. The type II endfoot-bearing process contains gliofilaments and has a high K+ conductivity; this type of process is covered by a basal lamina and is in contact with mesenchyme. The type III sheath-bearing process insulates neuronal compartments and expresses suitable membrane properties for glia-neuronal communication. Since structurally similar processes have been shown to have similar physiological properties, a new systematic classification of glial cells is proposed, based on the presence or absence of defined types of cell processes. This approach is believed to provide new insights into the function of neuroglia in both the central and peripheral nervous systems, in vertebrates and invertebrates, and even during ontogenetic development.

[Myelination, demyelination and re-myelination in the central nervous system]

The myelin sheaths that surround axons in the CNS are made and maintained by oligodendrocytes. These glial cells can form variable numbers of myelin segments (internodules): from 1 to 200 so that when one oligodendrocyte is destroyed with preservation of the axon, many internodules can be lost, constituting a demyelinating process. As a consequence of the destruction of myelin and sheath cells a rapid and abundant cell response takes place. The response is made up by resident (microglia) and haematogenous phagocytes which phagocytose myelin and cellular debris leaving the axons demyelinated. Demyelinated axons may either stay demyelinated and clumped together or they may be separated by astrocytic processes, yet they can be remyelinated. The occurrence of remyelination depends upon the intensity and time of exposition to the demyelinating agent. Remyelination in the CNS with complete restoration of conduction may be made by oligodendrocytes or Schwann cells which invade the CNS when astrocytes are destroyed.

Demyelination in canine distemper encephalomyelitis: an ultrastructural analysis

A morphological study of selected white matter lesions was carried out in three dogs with canine distemper encephalomyelitis. Two dogs had experimental infections while the third was a spontaneous case. Two stages were identified in the process of demyelination. The earliest evidence of myelin injury was a ballooning change in myelin sheaths involving single or multiple axons. This was followed by a progressive stripping of compact sheaths by the cytoplasmic fingers of phagocytic cells which infiltrated and removed myelin lamellae. Some axonal necrosis also accompanied these changes. Where demyelination occurred, canine distemper viral nucleocapsids were found in astrocytes, macrophages, ependymal cells and infiltrating lymphocytes. In contrast, oligodendrocytes were conspicuous by their apparent lack of infection. Thus it seems that myelin loss cannot be ascribed to oligodendrocyte infection. Perturbed astrocyte function following canine distemper viral infection may cause oedema of myelin sheaths, leading to ballooning and primary demyelination. Cells which phagocytosed myelin were mainly identified as microglial cells with lesser involvement by astrocytes. Rarely, oligodendrocytes also acted as macrophages. Myelin debris was engulfed in bulk or as small droplets into coated pits. Remyelination was present in established plaques although not in great abundance, perhaps due to the diminished oligodendrocyte numbers and a relative increase in immature forms of these cells. These observations are compared to similar changes observed in other demyelinating diseases of animals and man.

Gliogenesis in organotypic tissue culture of the spinal cord of the embryonic mouse. II. Autoradiographic studies

Organotypic cultures of the spinal cord of the embryonic mouse were subjected to pulses of tritiated thymidine at various times between explanation and 42 days in vitro (DIV). Autoradiography was performed both on cultures fixed immediately at the end of the pulse and on cultures maintained in radioactive-free medium for various periods after the pulse. Quantitative light autoradiographic studies showed a single peak of glial cell proliferation at 9 DIV equivalent to that demonstrated in vivo. The growth rate of glial cells (related to time in culture) decreased along an exponential decay type curve. All these observations were statistically significant when tested against the corresponding null hypothesis. Ultrastructural autoradiography shows that at early stages of the culture, radial glial cells and immature glial cells divided and eventually gave rise to astrocytes and oligodendrocytes. During the period of maximal cell proliferation, tritiated thymidine was incorporated by differentiated astrocytes and ultrastructurally recognizable immature oligodendrocytes. Oligodendrocytes did not divide beyond the stage of active oligodendrocytes (the cells initiating myelination). They were capable of producing dark oligodendrocytes within a week following the last division. These observations emphasize the similarity of the proliferation during development in organotypic culture to that in vivo, modified by the trauma of explantation and the culture conditions.

Gliogenesis in organotypic tissue culture of the spinal cord of the embryonic mouse. I. Immunocytochemical and ultrastructural studies

The technique of organotypic tissue culture offers an opportunity to observe in vitro complex interactions among glial cells and neurons, leading to the formation of myelin. In the present and accompanying work a combined ultrastructural, immunocytochemical and autoradiographic approach was used in a detailed study of the process of gliogenesis. Using immunocytochemical and ultrastructural criteria, differentiation along the oligodendroglia cell line is seen to be initiated a few days later than along the astroglial line. The sequence and timing of oligodendroglial differentiation both ultrastructurally and chemically follow those described in vivo. Formation of myelin has been demonstrated only by oligodendrocytes in which there is continuity between the perikaryal plasmalemma and myelin membranes. Oligodendroglial maturation culminated with the formation of light, medium and dark oligodendrocytes. The periodic acid Schiff-positive, glial fibrillary acidic protein (GFAP)-negative process of radial glial cells at explantation become GFAP-positive within 3 days, as described in vivo. Many of the astrocytes appear to have been derived from radial glial cells. Large numbers of dark glial cells, similar to the so-called 'intermediate glial cells', were seen. These were found to be astrocytes whose appearance probably reflected reaction to explantation-induced injury.

Perinodal astrocytic processes at nodes of Ranvier in developing normal and glial cell deficient rat spinal cord

This study examined, during normal development and during the development of a glial cell deficient axon population, the nature of astrocyte involvement at the central nodes of Ranvier on spinal cord axons. One condition examined was the ventral funiculus of normal 7-day-old rats. At this age, the lumbar spinal cord underwent an active phase of gliogenesis, and axons were seen in various stages of myelination. Perinodal astrocytic processes were routinely observed at nodes of axons on which myelin sheaths exceeded 8 compact lamellae. Perinodal astrocytic processes were also seen in close proximity to axolemma at most developing nodes. This study also examined the lumbar spinal cords of rats which were X-irradiated on the third postnatal day. This procedure caused a profound reduction in the astrocyte and oligodendrocyte population in 13- and 18-day-old rats, while sparing the neuronal elements. Thus, axo-glial relationships observed in this tissue are unlikely to be random occurrences. Despite the reduction in glial cells, some oligodendrocyte-myelinated axons were observed in the irradiated spinal cords. Perinodal astrocytes were seen at all oligodendrocyte-derived nodes observed in the irradiated cord and appeared to have a specific relationship to the node of Ranvier. The presence of astrocytic processes at the normal, developing node and at the nodes in glial cell deficient spinal cords suggests that astrocytes may be necessary to the function of nodal axolemma. In irradiated spinal cords, where the glial cells are markedly reduced, apposition between astrocytic and oligodendrocytic membrane at the paranode and internode was also seen and was so common that it is highly unlikely to be due to random occurrences. These observations further suggest that in addition to the presumptive role at the nodes, astrocytes may play an inductive or supportive role in the development and maintenance of central myelin.

Glial fibrillary acidic protein (GFAP) in spinal cord of postnatal rat. An immunoperoxidase study in semithin sections

The presence and distribution of gliofibrillary acidic protein (GFAP) was studied in the postnatal spinal cord of the rat. On birth GFAP can be seen in astrocytic bodies and their processes of ventral columns; perivascular glial membrane was initially seen within white and gray matter; most of the fibers were transversally oriented. There are many mitotic cells, some of them with GFAP in their cytoplasm, while others lack it, thus providing strong evidence to identify two cellular populations as astrocytes and, possibly, oligodendrocytes and to state that neuroglial cells differentiate to astrocytes before myelination gliosis. The glia limitans membrane is continuous from the 5th day onwards and in 17-day-old animals the astrocytic framework of the spinal cord has reached its adult appearance.

Expression of glial fibrillary acidic protein in immature oligodendroglia

In the human fetal spinal cord at 15 to 16 weeks, glial fibrillary acidic protein (GFAP) was demonstrated within the cytoplasm and processes of cells having the cytological, ultrastructural, and immunocytochemical features of oligodendrocytes--including processes that extend into and contribute to the formation of myelin sheaths. By 17 to 18 weeks, however, GFAP immunoreactivity was no longer evident within such cells. Thus GFAP is expressed by myelin-forming oligodendroglia early in their development.