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Abstract

We have explored two aspects of internal capsule development that have not been described previously, namely, the development of glia and of blood vessels. To these ends, we used antibodies to glial fibrillary acidic protein (GFAP) and to vimentin (to identify astrocytes and to radial glia) and Griffonia simplicifolia (lectin; to identify microglia and blood vessels). Further, we made intracardiac injections of Evans Blue to examine the permeability of this dye in the vessels of the internal capsule during neonatal development. Our results show that large numbers of radial glia, astrocytes and microglia are not labelled with these markers in the white matter of the internal capsule until about birth; very few are labelled earlier, during the critical stages of corticofugal and corticopetal axonal ingrowth (E15-E20). The large glial labelling in the internal capsule at birth is accompanied by a dense vascular innervation of the capsule; as with the glia, very few labelled patent vessels are seen earlier. After intracardiac injections of Evans Blue, we find that the blood vessels of the internal capsule are not particularly permeable to Evans Blue. At each age examined (P0, P5, P15), blood vessels are outlined very clearly and there is no diffuse haze of fluorescence within the extracellular space, which is indicative of a leaky vessel. There are three striking differences between the glial environment of the internal capsule and that of the adjacent thalamus. First, the internal capsule is never rich with radial glial fibres (vimentin- and GFAP-immunoreactive) during development (except at P0), whereas the thalamus has many radial fibres from very early development (E15-E17). Second, astrocytes (vimentin- and GFAP-immunoreactive) first become apparent in the internal capsule (E20-P0) well before they do in the thalamus (P15). Third, the internal capsule houses a large transient population of amoeboid microglia (P0-P22), whereas the thalamus does not; only ramified microglia are seen in the thalamus. In summary, our results indicate that all three types of glia in the internal capsule are associated closely with the vasculature, suggesting they may play a role in the development of the blood-brain barrier among the vessels in this white matter region of the forebrain.

The behavior and functions of embryonic microglia

“…First, blood vessels and blood circulation might regulate microglial migration in the developing brain. Microglial distribution in the brain parenchyma coincides with vascularization, which occurs at E10 in mice (Earle and Mitrofanis 1998 ). Consistent with this finding, another study reported that microglia reached the area surrounding the spinal cord via the developing vasculature and then proliferated (Rigato et al 2011 ).…”

“…The first way is through blood vessels. An early study reported that microglia colonize the embryonic brain in accordance with vascularization (Earle and Mitrofanis 1998 ). A study on microglial origins using Ncx1 −/− mice, in which heartbeat and blood circulation are impaired due to deficiencies in Na + /Ca 2+ exchangers, confirmed that microglial precursors could not invade the brain parenchyma, indicating that these cells enter the embryonic brain in a manner dependent on the establishment of blood circulation (Ginhoux et al 2010 ).…”

Developmental Associations between Neurovascularization and Microglia Colonization

“…These precursor cells were found in the vicinity of brain capillaries and characterized by a round or irregularly shaped cell body, with or without pseudopodia [98]. The colonization of the brain by microglia was observed to coincide with CNS vascularization during embryonic development [99,100]. In the fetal human brain microglia were located at highly vascularized sites at between 16-and 22-weeks gestation [101].…”