E.J.H. Nathaniel

The Reactive Astrocyte

Publisher Summary This chapter explains astrocytes. Astrocytes are star-shaped glial cells in the brain and spinal cord. They are the most abundant cells of the human brain and account for 30–65% of all glial cells. They perform many functions, including biochemical support of endothelial cells that form the blood–brain barrier, provision of nutrients to the nervous tissue, maintenance of extracellular ion balance, and a role in the repair and scarring process of the brain and spinal cord following traumatic injuries.The astrocytes are divided into protoplasmic and fibrous astrocytes. The protoplasmic variety, principally encountered in the gray matter, is characterized by a large but variable number of processes that are radially directed. By contrast, the fibrous astrocytes are located mostly in the white matter and at the surfaces of the brain and spinal cord and are considered to have fewer, longer, thinner, and smoother processes than do protoplasmic astrocytes. Both the protoplasmic and fibrous astrocytes have processes-forming expansions that abut against the capillaries; these are termed vascular end-feet.

Astroglial response to degeneration of dorsal root fibers in adult rat spinal cord

Abstract The morphology of the astrocyte was studied with the electron microscope in the posterior gray and at the edge of the spinal cord, in both normal adult rats and after crush lesion of dorsal roots in adult rats. In general, the astrocyte was characterized by a pale, round nucleus surrounded by a cytoplasm containing small bundles of filaments and few organelles. However, the subpial astrocyte appeared to possess a larger amount of filaments. Within 4 days after injury, the astrocytes responded by accumulating glycogen particles within their cytoplasm even in the absence of morphological evidence of damage to nerve fibers. In addition, the astrocytic nucleus appeared reactive as evidenced by a tendency to increased nuclear irregularity, nuclear pores, and prominent nucleoli. The cytoplasm of several astrocytes also displayed hypertrophy resulting in accumulation of microfilaments, mitochondria, multiple Golgi complexes, and several electron-dense bodies. In animals killed 4 weeks after lesion, clear evidence of astrocytes functioning as macrophages in the removal of degenerating myelin was observed. These reactive astrocytes exhibited multiple Golgi complexes, large amounts of microfilaments, and numerous mitochondria. The subpial astrocytes especially possessed an abundance of microfilaments.

Regeneration of dorsal root fibers into the adult rat spinal cord

Abstract Regeneration of dorsal root nerve fibers into the spinal cord of adult rat was studied with the electron microscope after crushing the roots. Regenerated dorsal root myelinated fibers were observed in the substantia gelatinosa and posterior funiculi around the tenth week after lesion. Cytoplasmic processes of oligodendrocytes were often found close to the young myelinated nerve fibers. The astrocytic response subsided as regeneration progressed. Clusters of small, circular profile of cellular processess were found in the neuropil about the sixth week and are considered to be regenerated unmyelinated axons. At this period, groups of segments of cellular processes containing clear vesicles were also encountered in the substantia gelatinosa and resembled growth cones described in peripheral regenerating nerves.

Growth patterns of rat body, brain, and cerebellum in fetal alcohol syndrome

Pregnant Sprague-Dawley rats were offered protein- and vitamin-enriched, nutritionally balanced, liquid ethanol diet, providing 35% of calories as ethanol, beginning on day 6 of gestation. Control animals were pair-fed identical liquid diet except that maltose-dextrins were substituted for ethanol. At birth the pups of both ethanol exposed and pair-fed mothers were surrogate-fostered by normal dams. Plasma concentrations of ethanol and thyroxine were determined in dams of experimental and control groups. Thyroxine concentrations were significantly reduced in ethanol-fed dams compared with pair-fed. Pups of both groups were killed at 0-1, 5, 7, 10, 14, 21, 28, 35, and 42 postnatal days. The crown-rump, crown-tail lengths, and weights of body, brain, and cerebellum were significantly reduced in ethanol-exposed pups compared with controls at all time sequences studied.

Altered Purkinje cell maturation in rats exposed prenatally to ethanol

Sprague-Dawley pregnant rats received a liquid ethanol diet that was nutritionally balanced and provided 35% of calories in ethanol, beginning on day 6 of gestation. Control animals were pair-fed the same diet except that maltose-dextrins were substituted for ethanol. At birth pups of both ethanol-exposed and pair-fed groups were fostered by surrogate mothers which received normal rat chow ad libitum. An average of eight alcohol-exposed and eight pair-fed pups were killed at 0-1, 5, 7, 10, 14, 21, 28, and 42 days. Their fixed cerebella were bisected in the midvermal plane and one-half embedded in Araldite. Thin sections were cut from lobules II/III and stained with uranyl acetate and lead citrate and visualized with the electron microscope to evaluate the effect of ethanol on Purkinje cell growth and maturation. Although the basic layering of the cerebellar cortex was not altered, alignment of Purkinje neurons to form a monocellular layer was delayed associated with disorientation of some neurons in the ethanol-exposed pups. Delayed differentiation of Purkinje neurons in the experimental animals was also noted as evidenced by the ribosomal accumulation at the basal aspect of the cell at 14 days postnatally, resembling a 7-day control pup. Purkinje neurons of control pups were well differentiated at 14/21 days, with a centralized nucleus surrounded by a cytoplasm containing Nissl bodies, numerous Golgi complexes, and mitochondria. The ethanol-exposed pups at 14/21 days showed delayed maturation indicated by a basal ribosomal mass, with few segments of rough endoplasmic reticulum. At 28/42 days the Purkinje neurons of the controls showed a greater degree of differentiation and maturation in contrast to the reduced protein-synthesizing machinery, i.e., poorly developed endoplasmic reticulum and Nissl bodies, in the experimental pups. These studies showed delayed differentiation and maturation of Purkinje neurons as a consequence of ethanol exposure.

Prenatal ethanol exposure and cerebellar development in rats

Rats of Sprague-Dawley strain were fed a liquid ethanol diet that was nutritionally balanced and provided 35% of calories as ethanol. Control animals were pair-fed and received the same liquid diet with maltose-dextrins substituting for ethanol. At birth the pups of both the experimental and control groups were fostered by surrogate mothers which received normal rat chow ad libitum. An average of eight alcohol-fed and eight pair-fed pups were killed at 0-1, 5, 7, 10, 14, 21, 28, and 42 postnatal days. The fixed cerebella of these pups was bisected in the midvermal plane and one-half was embedded in paraffin and the other in Araldite. Projection drawings of hematoxylin and eosin-stained paraffin sections were made at 50X magnification. Using the graphics plate of an Apple II computer, the circumference and area of the vermal cerebellum was determined. We found that at all the time sequences studied the area and circumference of the cerebellum were significantly reduced in ethanol-exposed pups compared with the pair-fed controls.

Electron microscopic studies of spinal ganglion cells following crushing of dorsal roots in adult rat.

Chromatolytic alterations were induced in the spinal ganglion cells of adult rats by crushing the dorsal roots of the cauda equina. The lesioned animals were sacrificed at 3, 7, 14, 21, and 28 days after lesion. Ganglia of lesioned roots were fixed in aldehydes followed by osmication. The normal ganglion cell possessed a circular or oval, centrally located nucleus. Small aggregates of free and membrane-bound ribosomes were diffusely located in the cytoplasm. Organelles, such as mitochondria, Golgi complexes, and dense bodies, were randomly located in the cell. The earliest reaction of the ganglion cell to injury was seen in the nucleus. Slight irregularity of nuclear outline was observed in 3 days and became progressively more evident by 2–3 weeks. Associated with this feature was the appearance of numerous nuclear pores. Eccentric position of the nucleus was noted by 4 weeks post-lesion. The cytoplasmic response consisted of aggregation and peripheral displacement of massive Nissl bodies. Multiple hypertrophic Golgi complexes and numerous dense bodies, presumably lysosomes, were also found in these cells. The neurofilamentous reaction was marked in the perinuclear zone as well as in the rest of the cytoplasm. Both small and large ganglion cells exhibited evidences of chromatolysis. The present studies clearly indicate that the reactive ganglion cells are capable of actively synthesizing the proteins necessary for dorsal root regeneration observed in our earlier studies.

Altered Purkinje cell maturation in rats exposed prenatally to ethanol: II. Synaptology

Pregnant rats of Sprague-Dawley strain were offered a protein- and vitamin-enriched nutritionally balanced liquid ethanol diet, providing 35% of calories as ethanol. Control animals were pair-fed identical liquid diet except that maltose-dextrins were substituted for ethanol. Pups of both ethanol-exposed and pair-fed groups were surrogate-fostered by mothers which received normal rat chow ad libitum. An average of eight pups of each experimental and control group was killed at 0-1, 5, 7, 10, 14, 21, 28, and 42 days. After fixation, their cerebella were bisected in the midvermal plane and one-half embedded in Araldite. Thin sections cut from lobules II/III were stained with uranyl acetate and lead citrate and the synaptology of the Purkinje neurons was studied with the electron microscope in both the experimental and control groups. Impaired climbing fiber maturation was seen in ethanol-exposed pups as evident from the presence of somatic spines--climbing fiber synapses at 14 days postnatally. In the 14-day-old control pups the climbing fiber maturation was complete or nearly complete as evident by their synapses on spines situated on the smooth dendritic branches and by the absence or paucity of climbing fiber--somatic spine synapse. In addition, a reduction in the number of parallel fibers was evident from the increase in Purkinje cell spines per varicosity in the ethanol-exposed pups. We conclude, therefore, a direct or indirect effect of ethanol occurs on climbing fiber maturation and Purkinje cell parallel fiber relationship.

Development of brain capillaries in euthyroid and hypothyroid rats.

The number of blood vessel profiles per square millimeter in the cuneate nucleus of normal rats, as estimated by light microscopy, doubled between 1 and 3 weeks postnatum (pn), and plateaued thereafter. In hypothyroid rats this postnatal increase occurred only until 2 weeks pn. There was also a significant reduction in blood vessel numbers in hypothyroid rats (P < 0.01) observed as early as 2 weeks pn. Ultrastructurally, capillary maturation in normal rats consisted of the following: the earliest identifiable capillaries displayed a slit-like lumen enclosed by thick edothelial cells and surrounded by an unevenly thick basal lamina. The astrocytic end-feet of these capillaries appeared as pale, dilated profiles. With maturation, the capillary wall became attenuated with a concomitant increase in lumen size. Mature capillaries were surrounded by an evenly thick basal lamina and narrow astrocytic end-feet containing large bundles of filaments. Outward cytoplasmic projections of pericytes and endothelial cells were seen as long as 3 weeks pn. Fine structural alterations in hypothyroid rats consisted of pericytic hypertrophy at 2 weeks pn, followed by the appearance of large inclusion-laden cells, situated within the vessel wall. At 4 weeks pn the astrocytic end-feet of mature capillaries lacked filament bundles but contained numerous glycogen granules. Edema of the astrocytic end-feet was noted frequently at 6 weeks pn.

Degeneration of dorsal roots in the adult rat spinal cord.

Abstract Degeneration in the dorsal root and in the portion of the adult rat spinal cord where the root enters was studied with the electron microscope following a crushing lesion. The earliest sign of damage to the cord was the appearance of glycogen in astrocytic processes 96 hr postoperatively. Glycogen particles were also found at this time within the boutons. Clear evidence of degeneration in the cord was seen at the end of the first week after injury and the peak of degeneration was reached around the fourth week. The pattern of axonal degeneration was variable and was followed by myelin breakdown. Disintegration of myelin commenced at random without any preferred site of orientation. Cleavage of compact myelin usually occurred along the intraperiod line. Elimination of damaged myelin was primarily a function of astrocytes. The reactive astrocytes were characterized by numerous filaments, multiple Golgi complexes, and ribosomes. Degenerating myelin was occasionally observed within cells which resembled oligodendrocytes. Vascular pericytes or mononuclear cells did not function as macrophages.