Brain mast cell relationship to neurovasculature during development.

Mast cells, derived from the hematopoietic stem cell, are present in the brain from birth. During development, mast cells occur in two locations, namely the pia and the brain parenchyma. The current hypothesis regarding their origin states that brain mast cells (or their precursors) enter the pia and access the thalamus by traveling along the abluminal wall of penetrating blood vessels. The population in the pia reaches a maximum at postnatal (PN) day 11, and declines rapidly thereafter. Chromatin fragmentation suggests that this cell loss is due to apoptosis. In contrast, the thalamic population expands from PN8 to reach adult levels at PN30. Stereological analysis demonstrates that mast cells home to blood vessels. More than 96% of mast cells are inside the blood-brain barrier, with ~90% contacting the blood vessel wall or its extracellular matrix. Mast cells express alpha4 integrins -- a potential mechanism for adhesion to the vascular wall. Despite the steady increase in the volume of microvasculature, at all ages studied, mast cells are preferentially located on large diameter vessels (>16 microm; possibly arteries), and contact only those maturing blood vessels that are ensheathed by astroglial processes. Mast cells not only home to large vessels but also maintain a preferential position at branch points, sites of vessel growth. This observation presents the possibility that mast cells participate in and/or regulate vasculature growth or differentiation. The biochemical and molecular signals that induce mast cell homing in the CNS is an area of active investigation.