The ultrastructure of brain macrophages in rat neonates

$28 THE ULTRASTRUCTURE OF BRAIN MACROPL~GES IN RAT NEONATES TOSHIHIKO MIYAKE* and TADAHISA KITAMURA*, Dept. of Pathology, Kyoto Prefectural University of Medicine, Kawaramachi-Hirokoji, Kamikyo-ku, Kyoto 602, Japan. The macrophages which are present physiologically in the brains of neonatal animals have long been considered to be the progenitors of microglia. Recent evidence, however, has thrown doubt on this hypothesis. In order to clarify the destiny of the neonatal brain macrophages, we identified these cells in the corpus callosum of rat neonates according to the electronmicroscopic immunoperoxidase technique using anti-(rat)granulomonocyte antiserum, and studied their sequential changes in ultrastructure. On the neonatal days, there were many positive cells which were mononuclear and had large cell bodies. Dense heterochromatin clumps were present in the periphery of the nuclei. The soma of the positive cells contained many vacuoles, variable in size. Debris of necrotic cells was occasionally found in these vacuoles. The products of the staining reaction were deposited in the soma diffusely, but particularly around the vacuoles. Some positive cells had extremely large vacuoles, which occupied the soma almost totally, and seemed to be degenerative. Necrotic positive cells were also found, which showed disintegrated intracellular organelles and pyknotic nuclei. In the second postnatal week, the degenerative or necrotic positive cells were seen more frequently than before. In the third postnatal week, the positive cells decreased in total number, and many of them were degenerative or necrotic. At the end of this week, we could not detect positive cells in the corpus callosum. In this study, we could not encounter the positive cells which were just passing through the vascular wall. These results indicate: I. neonatal brain macrophages phagocytose destruction products actively and contribute toward remodeling the developing brain tissue; and 2. these macrophages die out by the end of the first postnatal month, but do not survive to become microglia.

THE POSTNATAL DEVELOPMENT OF CORTICOSPINAL NEURONS IN THE RAT RITSUKO OHTANI and TOSHIO SHIRA[ Department of Anatomy, Yamagata University, Zao-iida, Yamagata 990-23, Japan

School of Medicine, Aza Nishinomae

The ontogeny of corticospinal (CS) neurons in the rat was studied using the retrograde transport of HRP and the Golgi method. HRP-labeled CS neurons of neonatal rats (1-3 days after birth) were distributed as one continuous band from the medial region to the lateral one in layer V of the frontal and parietal areas of the cerebrum. In the 7-day-old rats, the partial disruption of this band appeared. Then, in 14-day-old rats, no-HRP-labeled cells area was extended and CS neurons were localized into two narrow bands, as those in adult rats. These two bands consisted of : (i) a dorsomedial band of CS neurons in the frontal and the anterior parietal cortices; and (2) a relatively smaller band of CS neurons in the lateral parietal cortex. Pyramidal neurons in layer V of the precentral area (Zilles et al., 1980) were observed by the Golgi method and compared with HRP-labeled CS neurons. In 1-day-old rats, Golgi-impregnated pyramidal neurons had a thinner axonal process, an apical dendrite from which some fine protrusions or processes emerged, several short basal dendrites, and no spines. However, by 7 days, the number and length of basal dendrites increased, and a few spines as well as short branches appeared along the apical dendrite. By 14 days, these pyramidal neurons showed their mature structure, as indicated mainly by the complexity of branching and extension of basal dendrites, and the increase in the number of spines.