Hepatic lipocytes were isolated from normal rat liver and established in culture. A virtually pure isolate was obtained by fractionating enzymatically digested liver on a discontinuous gradient of arabinogalactan. Isolated cells displayed prominent rough endoplasmic reticulum and typical cytoplasmic droplets containing vitamin A. Lipocytes in pri-

The plasma protein apolipoprotein (apo) E is an important determinant of lipid transport and metabolism in mammals. In the present study, immunocytochemistry has been used to identify apo E in specific cells of the central and peripheral nervous systems of the rat. Light microscopic examination revealed that all astrocytes, including specialized astrocytic cells (Bergmann glia of the cerebellum, tanycytes of the third ventricle, pituicytes of the neurohypophysis, and Muller cells of the retina), possessed significant concentrations of apo E. In all of the major subdivisions of the central nervous system, the perinuclear region of astrocytic cells, as well as their cell processes that end on basement membranes at either the pial surface or along blood vessels, were found to be rich in apo E. Extracellular apo E was present along many of these same surfaces. The impression that apo E is secreted by astrocytic cells was confirmed by electron microscopic immunocytochemical studies, which demonstrated the presence of apo E in the Golgi apparatus. Apo E was not present in neurons, oligodendroglia, microglia, ependymal cells, and choroidal cells. In the peripheral nervous system, apo E was present within the glia surrounding sensory and motor neurons; satellite cells of the dorsal root ganglia and superior cervical sympathetic ganglion as well as the enteric glia of the intestinal ganglia were reactive. Apo E was also present within the nonmyelinating Schwann cells but not within the myelinating Schwann cells of peripheral nerves. These results suggest that apo E has an important, previously unsuspected role in the physiology of nervous tissue.

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Recent work has demonstrated that apo E secretion and accumulation increase in the regenerating peripheral nerve. The fact that apoE, in conjunction with apoA-I and LDL receptors, participates in a well-established lipid transfer system raised the possibility that apoE is also involved in lipid transport in the injured nerve. In the present study of the crushed rat sciatic nerve, a combination of techniques was used to trace the cellular associations of apoE, apoA-I, and the LDL receptor during nerve repair and to determine the distribution of lipid at each stage. After a crush injury, as axons died and Schwann cells reabsorbed myelin, resident and monocyte-derived macrophages produced large quantities of apoE distal to the injury site. As axons regenerated in the first week, their tips contained a high concentration of LDL receptors. After axon regeneration, apoE and apoA-I began to accumulate distal to the injury site and macrophages became increasingly cholesterol-loaded. As remyelination began in the second and third weeks after injury, Schwann cells exhausted their cholesterol stores, then displayed increased LDL receptors. Depletion of macrophage cholesterol stores followed over the next several weeks. During this stage of regeneration, apoE and apoA-I were present in the extracellular matrix as components of cholesterol-rich lipoproteins. Our results demonstrate that the regenerating peripheral nerve possesses the components of a cholesterol transfer mechanism, and the sequence of events suggests that this mechanism supplies the cholesterol required for rapid membrane biogenesis during axon regeneration and remyelination.