ABSTRACT

administered intraventricularly are taken up by granular pericytes in cerebral fine vessels. These cells are provided with elongated and complicated infoldings. Pinocytotic vesicles occur often along the infoldings, and ends of infoldings are swollen and assume vesicular shapes. The specific morphology of the granular perithelial cells is considered t o favor a removal of metabolic wates in cerebral tissue.Exogenous substances such as trypan blue and horseradish peroxidase It is known that systemically injected trypan blue and horseradish peroxidase d o not penetrate the vascular wall o f cerebral vessels because of the blood-brain barrier. The tracers do, however, spread into cerebral tissues if they are administered intraventricularly. The removal of exogenous molecules from cerebral tissues has scarcely been explored.in the removal of exogenous tracers. MATERIAL AND METHODSWe report here experiments indicating that granule-containing vascular pericytes are involvedThe animals employed in this study were Wistar rats, aged 4-5 months. They were bred in the authors' laboratory and fed on a standard rat chow (CLEA Co. L.T.D., Tokyo). They were divided into two groups (A and B).into the left cerebral ventricle. In the group-B rats, the same volume of physiological saline containing 2 mg horseradish peroxidase was administered similarly.At 60 minutes after the injection, the rats were killed by decapitation. The brains were removed and placed into cold physiological saline. Under a dissecting microscope, the cerebral cortex on the side opposite the injection was sliced with a 1-azor blade. Some sliced specimens were used as stretch-specimens for light microscopic observations (Mato and Ookawara, '79). They were stained with hematoxylin-eosin, PAS (periodic acidschiff reaction) or Sudan black B. The others were immersed in a solution containing 2% paraformaldehyde and 2.5% glutaraldehyde, buffered with 0.1 M phosphate buffer (pH 7.4) for 4 hours. The specimens of group A were postfixed with 1% osmium tetroxide in 0.1 M phosphate buffer for 2 hours. The specimens of group B were again washed with 0.1 M phosphate buffer overnight, and treated with 0.03% dianiinobenzidine and 0.01% hydrogen peroxide solution for 10 minutes. The Specimens were then postfixed in the same manner as those froin group A. Standard procedures were used for embedding and cutting. To observe the uptake of tracer in the light microscope, stretch-specimens were stained with eosin for the trypan-blue group, and for horseradish peroxidase, stretch-specimens were treated with 0.03% diaminobenzidine and 0.01% hydrogen peroxide for 6 minutes.In the group-A rats, 0.05 ml of 1% trypan blue in physiological saline solution was injected

It seems established that under pathological conditions, microglia and blood monocytes (invading the cerebral parenchyma) behave as histiocytic cells in the central nervous system. However, it has not been clear whether or not phagocytic cells are present in normal cerebral tissue. Recently, we found a new type of cell having an uptake capacity for exogenous substance at the bifurcations of small cerebral vessels except for capillaries. According to Imamoto et al. (1982), ameboid microglia, a kind of precursor of microglia, appear at a perinatal stage and can incorporate exogenous material. In the present paper, the developmental sequences of ameboid microglia and the unique cells laden with fluorescent granules are compared at a light and electron-microscopic level. From this study, it is clear that ameboid microglia are already present in the corpus callosum at 5 days after birth and are potent in their uptake capacity for horseradish peroxidase (HRP). However, at 2 weeks, they transform into star cells and the capacity for incorporation diminishes markedly. The finding is also supported by the quantitative analysis of transformation of ameboid microglia. At 3 months, glial cells do not take the administered HRP under the present conditions. On the other hand, fluorescent granular perithelial (FGP) cells arise from a leptomeningeal tissue (pia mater) and become situated in the perivascular spaces. They are not clearly defined at 5 days, and their uptake capacity for HRP has not yet developed. At 2 weeks, the FGP cells take definite forms with several inclusion bodies, and their uptake capacity for HRP attains a certain degree. Often, they are located at bifurcations of small blood vessels. At 3 months, the FGP cells differentiate completely in appearance, and their pinocytotic capacity reaches a high level. Consequently, the FGP cells belong to a different type of cell from that of ameboid microglia in their developmental sequences and assume a principal role of scavenging waste products in normal cerebral tissue.

The fluorescent granular perithelium (FGP) of rats and humans under experimental and pathological conditions was examined with the electron microscope. The FGP incorporated high molecular-weight protein (ferritin) and carbon particles administered intraventricularly. In a case of spontaneous cerebral hemorrhage, the FGP was found to contain lipoidal products and minute fragmented cell debris. The FGP in a patient with lipidosis contained pale inclusion bodies. In aged individuals, the inclusion bodies formed irregular larger aggregates.

The tridimensional appearance and distribution of FGP (fluorescent granular perithelial) cells was studied by means of light and scanning electron microscopy. In young rats they first appeared as hexagonal cells in that were closely associated; later they transformed into slender forms and were loosely arranged. Scanning electron microscope observation gave a general view of FGP cells, their globular vacuolated inclusions, and their hypertrophied protrusion into the luminal surface of blood vessels. The nodular protrusions may be related to the limitation of blood flow in small cerebral blood vessels.