Hajnalka Ábrahám

Cell formation in the human hippocampal formation from mid-gestation to the late postnatal period

In the present study cell formation was studied in the human hippocampal formation from the 24th gestational week until the end of the first postnatal year. Proliferating cells were detected with the monoclonal antibody MIB-1. The cytoarchitectonic layers of Ammons horn are formed before the 24th gestational week. In harmony with this observation, cell proliferation in the hippocampal ventricular zone is minimal after the 24th week. In addition, local cell multiplication in Ammons horn is occasional and the proliferating cells are glial or endothelial cells. In contrast, cell formation continues in the hilar region of the dentate gyrus even after birth. Immature cells accumulate in the hilus, and at the border between the hilus and the granule cell layer throughout the first eight postnatal months. The subgranular zone of the dentate gyrus becomes a cell sparse area at about the 11th postnatal month, indicating that immature cells from the hilus have already migrated to the granule cell layer and differentiated into granule cells. There is an increase in glial cell proliferation both in Ammons horn and the dentate gyrus at the 11.5th postnatal month suggesting the onset of myelination by the end of the first year. Our findings indicate that most pyramidal neurons of Ammons horn are generated in the first half of pregnancy and no pyramidal neurons are formed after the 24th gestational week. In contrast, granule cells of the dentate gyrus proliferate in a decreasing rate during the second half of pregnancy and after birth. Proliferating neuronal precursors occur in a low percentage in the dentate gyrus of 3-, 5- and 11.5-month-old children.

Cell formation in the cortical layers of the developing human cerebellum.

Cell proliferation has been studied in the human cerebellar cortex between the 24th gestational week and the 12th postnatal month. Intensive cell formation has been found in the external granular layer (EGL) of the human cerebellum, where the highest cell proliferation rate occurs between the 28th and 34th gestational weeks. This is followed by a gradual decrease that lasts up to the eighth postnatal month. As late in development as the fifth postnatal month, still 30% of cells of the EGL are labeled with the monoclonal antibody Ki‐67, which is specific for dividing cells. The width of the EGL remained unchanged from the 28th gestational week to the end of the first postnatal month, when it starts to decrease and completely disappears by the 11th postnatal month. Large number of Ki‐67 labeled cells occurs in the internal granular layer (IGL) between the 24th and 28th gestational weeks. From the 36th week onwards, the labeling index is less than 1%, although a few labeled cells have always been found in this layer even in the late postnatal period. Labeled cells are distributed in the entire width of the IGL. However, from the 34th gestational week, almost all labeled cells are found among and directly below the Purkinje cells. Their position, the nuclear features, and their occasionally stained cell processes suggest that those are Bergmann glial cells. There are few Ki‐67 labeled cells in the molecular layer (ML) and in the white matter (WM) of the cerebellum throughout the examined period. It is likely that most of these are glial cells. Pyknotic index has been found to be small in all layers of the cerebellum during the examined period.

Early microglial reaction following mild forebrain ischemia induced by common carotid artery occlusion in rats

Early microglial reaction following mild ischemic injury caused by bilateral common carotid artery occlusion has been investigated in rats. The ischemic insults lasted for 10, 15 and 20 min without recirculation, and with several reperfusion intervals from 1 h to 3 days. The resting and activated microglial cells were visualized with immunohistochemistry using monoclonal antibodies raised against the CR3 complement receptor, the MHC class I and class II antigens, the macrophage common antigen and with Bandeiiraea simplicifolia lectin-histochemistry. The neuroprotective effect of hypothermia on the early microglial activation was also studied. Ten minutes bilateral common carotid artery occlusion in hypothermic rats without reperfusion caused a mild microglial reaction in the hippocampus. Strong reaction was seen following 20 min insult without reperfusion. Ischemia followed by recirculation caused milder reaction than without reperfusion. Our results suggest that the microglial cells are very sensitive indicators of a mild, transient ischemic insult that does not result in neuronal cell death.

Myelination in the human hippocampal formation from midgestation to adulthood.

Myelination, one of the last steps of neuronal development, was examined in the human fetal and postnatal hippocampal formation using immunohistochemistry to detect a protein component of the myelin sheath, the myelin basic protein synthesized by oligodendroglial cells. Myelin basic protein‐immunoreactive oligodendroglial cells were first seen at the 20th gestational week in the fimbria fornicis and in the alveus. Between the 21st and 35th weeks, myelinated axons also appeared in the fimbria fornicis. At the age of 39 gestational weeks, short and thin myelinated fibers were present in the fimbria, in the alveus, and less so in the stratum oriens of the hippocampus, while the first oligodendroglial cells appeared in the stratum lacunosum‐moleculare and in the hilus. By the 2nd postnatal week myelinated fibers appeared in the stratum lacunosum‐moleculare of Ammons horn. At the 3rd month, myelination was strong in the alveus, moderate in the strata oriens, lacunosum‐moleculare and radiatum of Ammons horn, while only a small number of myelinated fibers were detected in the hilus. By the 5th month, the first oligodendroglial cells were detected in the molecular layer of the dentate gyrus. Myelination continued in the following years, particularly in the dentate gyrus, where even at the age of 11 years the density of myelinated fibers did not reach the adult level. It appears that the first myelinated axons belong to the long‐projecting large hippocampal pyramidal cells and/or to their subcortical and cortical afferents. The sequence of myelination follows the known developmental pattern of hippocampal afferent and efferent pathways, and the prolonged myelination might be a factor in the prolonged functional maturation of hippocampal circuitry.

A correlative light and electron microscopic study of postnatal myelination in the murine corpus callosum

Oligodendroglial cells differ in their ultrastructural appearance depending on their myelin producing and maintaining activity. To better understand the relationship between light and electron microscopic features of myelination, myelin formation in the corpus callosum was studied in young postnatal mice. Immunostaining for myelin basic protein (MBP), which has an important role in myelin compaction, was compared with conventional Luxol Fast Blue myelin staining and with electron microscopic images of unlabeled tissue.

Impaired myelination of the human hippocampal formation in Down syndrome

Myelination is considered as one of the last steps of neuronal development and is essential to the physiologically matured function of afferent and efferent pathways. In the present study, myelin formation was examined in the human fetal, postnatal and adult hippocampal formation in Down syndrome and in age‐matched controls with immunohistochemistry detecting a protein component of the myelin sheath, the myelin basic protein synthesized by oligodendroglial cells.

Rapid activation of microglial cells by hypoxia, kainic acid, and potassium ions in slice preparations of the rat hippocampus

Microglial activation induced by hypoxia, kainic acid and elevated potassium concentration, all of which alter neuronal function, was studied in hippocampal slices. The activation of microglia was detected by immunostaining with a monoclonal antibody (OX-42) raised against a type 3 complement receptor (CD11b). During activation the phenotype of microglia changes and the intensity of staining of individual cells increases. Oxygen deprivation depressed the focal responses of CA1 neurons to stratum radiatum volleys. Microglial activation was time dependent. Ten minute hypoxia caused mild activation, and after 20 min, a strong microglial reaction could be observed. Although neuronal function returned during reoxygenation, the morphological signs of microglial activation remained. Epileptiform activity of hippocampal neurons, followed by depression, was induced by application of 0.5 mM kainic acid, in a time and dose dependent manner. Washing out kainic acid did not alter microglial reaction. Elevated concentrations of potassium ions induced microglial changes similar to those induced by hypoxia and kainic acid. It is therefore suggested that an elevated extracellular potassium ion concentration may be the common factor in microglial activation observed in these experiments since this is raised both in hypoxia and under the effect of excitotoxins.

Fetal development of membrane water channel proteins aquaporin-1 and aquaporin-4 in the human brain

Aquaporin‐1 and aquaporin‐4, water channel membrane proteins reported in both experimental animals and in adult humans, have been detected in different, non‐overlapping areas of the central nervous system. This immunohistochemical study describes the developmental expression pattern of the water channel membrane proteins, aquaporin‐1 and aquaporin‐4, in various structures of human fetal brain over the gestational period of 14–40 weeks.

Ontogeny of calbindin immunoreactivity in the human hippocampal formation with a special emphasis on granule cells of the dentate gyrus

Calbindin (CB) is a calcium‐binding protein that is present in principal cells as well as in interneurons of the hippocampal formation of various species including humans. Studies with transgenic mice revealed that CB is essential for long‐term potentiation and synaptic plasticity which are the cellular basis of learning and memory. In a previous study we have shown that CB expression in granule cells of the dentate gyrus correlates with the functional maturation of the hippocampal formation in the rat.

Ontogeny of cocaine- and amphetamine-regulated transcript (CART) peptide and calbindin immunoreactivity in granule cells of the dentate gyrus in the rat.

Cocaine‐ and amphetamine‐regulated transcript (CART) peptide was first discovered in the rat striatum following cocaine and amphetamine administration. However, even without psychostimulant treatment, many neuronal groups of the central nervous system, including granule cells of the dentate gyrus, express CART peptide. Earlier studies, based on the prenatal expression of CART peptide in the mesencephalon, suggest that it exerts neurotrophic effects. In the present study, ontogenetic expression of CART peptide in dentate gyrus granule cells was studied using immunohistochemistry in rats from 5 days to 3 months old. Expression was correlated with the expression of another neurochemical marker of granule cells, the calcium binding protein, calbindin.