Computerized three-dimensional reconstruction of the developing blood-brain barrier

The perivascular astroglial sheath provides a complete covering of the brain microvessels: an electron microscopic 3D reconstruction

The unravelling of the polarized distribution of AQP4 in perivascular astrocytic endfeet has revitalized the interest in the role of astrocytes in controlling water and ion exchange at the brain-blood interface. The importance of the endfeet is based on the premise that they constitute a complete coverage of the vessel wall. Despite a number of studies based on different microscopic techniques this question has yet to be resolved. We have made an electron microscopic 3D reconstruction of perivascular endfeet in CA1 (stratum moleculare) of rat hippocampus. The endfeet interdigitate and overlap, leaving no slits between them. Only in a few sites do processes--tentatively classified as processes of microglia--extend through the perivascular glial sheath to establish direct contact with the endothelial basal lamina. In contrast to the endfoot covering of the endothelial tube, the endfoot covering of the pericyte is incomplete, allowing neuropil elements to touch the basal lamina that enwraps this type of cell. The 3D reconstruction also revealed large bundles of mitochondria in the endfoot processes that came in close apposition to the perivascular endfoot membrane. Our data support the idea that in pathophysiological conditions, the perivascular astrocytic covering may control the exchange of water and solutes between blood and brain and that free diffusion is limited to narrow clefts between overlapping endfeet.

A practical three-dimensional reconstruction method to measure the volume of the sexually-dimorphic central nucleus of the medial preoptic area (MPOC) of the rat hypothalamus

Published estimates of the volume of the sexually-dimorphic central nucleus of the medial preoptic area (MPOC) have been quite variable both within and between laboratories. To obtain MPOC volume, most experimenters began with a two-dimensional (2-D) approach. They outlined the MPOC on each of several individual sections; then they added up the area contained on each section and multiplied the total by the section thickness. A 3-D reconstruction approach, although promising, has been somewhat impractical until recently due to the requirements for highly specialized software and massive computing support. Here, we describe the application of commercially-available PC-based software to measure MPOC volume by 3-D reconstruction. Male and female Sprague-Dawley rats, 24 or 50 days of age, were perfusion-fixed with 10% neutral phosphate-buffered formaldehyde. Following processing and embedding, a series of 20-microm sagittal paraffin sections were cut and mounted onto slides. After staining with cresyl violet, they were digitized using a microscope-mounted video camera connected to a frame-grabber in a Pentium-class computer (MCID-M5+). In addition to the MPOC, the anterior commissure, fornix, paraventricular nucleus, medial division of the bed nucleus of the stria terminalis, third ventricle and the bed nucleus of the anterior commissure were identified on the screen image and outlined using a computer mouse. These outlines were then aligned and rendered in 3-D with a solid overlay. The additional areas, such as anterior commissure, form landmarks within 3-D space to improve the accuracy with which the MPOC may be located and outlined. The reconstruction provides a striking illustration of the geometric relations between the structures of the anterior hypothalamus in the male and female rat. Moreover, the volumes determined from the overlays were reproducible between repeated studies in our laboratory. Our volume measurements confirm the sexual dimorphism previously reported for MPOC volumes, and provide a relatively quick, accurate and reliable protocol that should be useful in future experimental studies of environmental estrogenic compounds.

Immunohistochemical and ultrastructural characterization of cortical plate microvasculature in the human fetus telencephalon

The blood-brain barrier (BBB) differentiation was investigated by immunohistochemistry and electron microscopy in the radial microvasculature of the telencephalon cortical plate (CP) of 12- and 18-week human fetuses. The BBB-specific glucose transporter isoform 1 (GLUT1) is expressed in both stages, with a main localization on the ablumenal and lateral plasma membranes of the endothelial cells. The endothelial cells are welded by short junctions with fusion points of the plasma membranes at 12 weeks and by extensive tight junctions at 18 weeks. The basal lamina is discontinuous beneath the endothelium-pericyte layer at 12 weeks and splits into two continuous layers circumscribing the pericytes in the later stage. The expression of laminin, a basal lamina glycoprotein, is continuous already at 12 weeks. The CP microvessels are tightly surrounded by processes of glial cells. Immunodetection of the cytoskeletal filament proteins, vimentin (VIM), and glial fibrillary acidic protein (GFAP), demonstrates that at 12 weeks the perivascular glial processes are mostly represented by VIM-stained fibers of the radial glia. At 18 weeks, GFAP-stained radial glia fibers, processes of VIM-stained astroblasts, and GFAP-positive astrocytes also build the perivascular envelopes. The results indicate that the vessel differentiation is already under way in the human CP at the midgestational age and entails the establishment of some barrier devices. The early relationship between perivascular glia coverage formation and endothelial barrier maturation suggests that also immature astroglial cells are involved in the setting up of the BBB.

The pecten oculi of the chicken: a model system for vascular differentiation and barrier maturation

The pecten oculi is a convolute of blood vessels in the vitreous body of the avian eye. This structure is well known for more than a century, but its functions are still a matter of controversies. One of these functions must be the formation of a blood-retina barrier because there is no diffusion barrier for blood-borne compounds available between the pecten and the retina. Surprisingly, the blood-retina barrier characteristics of this organ have not been studied so far, although the pecten oculi may constitute a fascinating model of vascular differentiation and barrier maturation: Pectinate endothelial cells grow by angiogenesis from the ophthalmotemporal artery into the pecten primordium and consecutively gain barrier properties. The pectinate pigmented cells arise during development from retinal pigment epithelial cells and subsequently lose barrier properties. These inverse transdifferentiation processes may be triggered by the peculiar microenvironment in the vitreous body. In addition, the question is discussed whether the avascularity of the avian retina may be due to the specific metabolic activity of the pecten.

An immunohistochemical and morphometric study on astrocytes and microvasculature in the human cerebral cortex

In this study, astrocytes and microvessels of the human cerebral cortex were analysed morphometrically with the aim of acquiring quantitative information on the glio-vascular relationships, considered to be of great importance in the formation and functioning of the blood-brain barrier. Immunohistochemistry for the astrocytic marker, glial fibrillary acidic protein, was used with a computerized image analysis system. The brain tissue was embedded using the progressive lowering of temperature method, and the image analyser was applied to semithin sections subjected to immunogold-silver staining and viewed by epipolarization microscopy. The results show that, in the human cerebral cortex, astrocytes cover 11.4% of the cortex area and that their perivascular processes are nearly as extensive as the vascular bed (0.8% versus 1.72% of the cortex area). These processes form a virtually continuous sheath around the vascular walls, only 11% of the vessel perimeter lacking this astrocytic glia covering. The present results, compared with previous unpublished data obtained by conventional immunocytochemical procedures on wax sections, indicate that low-temperature methods combined with gold-silver immunolabelling on semithin sections significantly improve the detection of immunoreactivity and the performance of the image analyser.