Cell proliferation in human ganglionic eminence and suppression after prematurity-associated haemorrhage

In premature infants, germinal matrix haemorrhage in the brain is a common occurrence. However, cell proliferation and fate determination in the normal human germinal matrix is poorly understood. Human ganglionic eminence samples were collected prospectively from autopsies of premature and term infants with no evidence of pathological process (n=78; dying at post-menstrual age 14-88 weeks). The ganglionic eminence was thickest at 20-26 weeks and involuted by 34-36 weeks. Proliferating cells, detected by Ki67 immunoreactivity, were abundant throughout the ganglionic eminence prior to 18 weeks, after which a sharp boundary between the dorsal and ventral ganglionic eminence appeared with reduced cell proliferation in the dorsal region. Ki67 immunoreactivity persisted in the majority of ventral cells until ∼28 weeks, after which time the proportion of proliferating cells dropped quickly. The expression of cell lineage markers (such as Olig2, SOX2, platelet-derived growth factor receptor alpha) showed partitioning at the microscopic level. The hypothesis that germinal matrix haemorrhage suppresses cell proliferation was then addressed. In comparison to controls, germinal matrix haemorrhage (n=47; born at post-menstrual age 18-34 weeks followed by survival of 0 h to 98 days) was associated with significantly decreased cell proliferation if survival was >12 h. The cell cycle arrest transcription factor p53 was transiently increased and the oligodendroglial lineage markers Olig2 and platelet-derived growth factor receptor alpha were decreased. Cell death was negligible. A low level of microglial activation was detected. Haemorrhage-associated suppression of cell proliferation in premature human infants could partially explain the reduced brain size and clinical effects in children who suffer germinal matrix haemorrhage after premature birth.

Apoptosis in human embryo development: 3. Fas-induced apoptosis in brain primary cultures

Fas (APO-1/CD95) is an important apoptotic mediator for both immune and nervous systems. In the present study, we have investigated the expression and function of Fas in human embryonic/fetal brain primary cultures from 12 human embryos and fetuses with gestational ages between 5 to 22 weeks. Anti-Fas fluorescent antibody was used for labeling of Fas positive cells and for quantitation of Fas expression in brain cultures. To demonstrate that Fas receptor is functional in human embryonic/fetal brain cells, anti-Human-Fas monoclonal antibody (0.5 microg/ml) was used to induce apoptosis in brain primary cultures. Apoptosis was investigated by flow-cytometry and fluorescent microscopy using TUNEL and annexin V labeling. Fas was found to be expressed in the embryonic/fetal human primary brain cultures, on neuronal and glial cells or their precursors, varying with gestational ages. Cross-linking of Fas induced apoptosis in brain cultures indicating that Fas receptor functions as a death receptor. We also showed that cell death triggered through Fas receptor was caspase dependent, hence it was blocked by a selective caspase-8 inhibitor (IETD-fmk). These results suggest that Fas is involved in neuronal apoptosis in the developing human brain.

Apoptosis in human embryo development: 2. Cerebellum

We analysed the spatial and temporal distribution of apoptosis in human cerebellum development, during embryonic and fetal periods. Cerebella excised from two human embryos (8 weeks old) and eight fetuses (12-22 weeks old), were paraffin embedded and serially sectioned. Apoptotic cells were identified by propidium iodide staining, and TUNEL. In addition, immunohistochemistry for suicide receptor Fas(APO-1/CD95) was performed. We determined the distribution and percentage of apoptotic cells as well as Fas(APO-1/CD95)-positive cells in different regions and stages of development. Apoptotic cells were seen in both proliferative zones and postmitotic regions along the migratory pathways as well as in the developing cerebellar cortex in all examined stages. The Fas(APO-1/CD95) immunoreactivity was present in all examined stages in a small population of apoptotic cells: either neuroblasts or differentiated cells in postmitotic zones. These findings suggest that apoptosis drives the selection of the cells which are committed to differentiate during the early stages of cerebellar development. The differences between apoptotic cells distribution and Fas receptor expression suggest that cell selection is driven by different apoptotic pathways.

Apoptosis in human embryo development: 1. Cerebral cortex

We investigated the apoptosis at the beginning of human cerebral cortex development, in the 6th week of embryogenesis, Carnegie stages 16 and 17. Attention was focused on the dorsal wall of the telencephalon to the ventricular zone of proliferation and to the postmitotic zone with beginning of neuronal migration. We identified apoptotic cells in tissue sections by propidium iodide staining, TUNEL and immunohistochemistry for Fas(APO-1/CD95). We determined the distribution and the percentage (reported to the propidium iodide stained nuclei) of apoptotic TUNEL-positive and Fas(APO-1/CD95)-positive cells. TUNEL-positive apoptotic cells in the proliferative zone were 20% in stage 16 and 60% in stage 17. TUNEL-positive apoptotic cells in the postmitotic zone were 8% in stage16 and 30% in stage 17. CD95-positive apoptotic cells in the proliferative zone were 5% in stage 16 and 2% in stage 17. There were no CD95-positive cells in the postmitotic zone. We evidentiated the presence of the suicide receptor Fas(APO-1/CD95) only on a small population of apoptotic neuroblasts in the proliferative zone. The differences between apoptotic distribution and receptors in early corticogenesis suggest that different apoptotic pathways drive the selection of neuronal populations.