The role of the blood-brain barrier in perfusion fixation of the brain for electron microscopy.

Regions of the brain vascularized by capillaries of the blood-brain barrier (BBB) type require a different fixative from regions which have capillaries of the endocrine type. Fixative with isotonic buffer gives excellent ultrastructural preservation in the BBB regions, but cause severe shrinkage of cells in the endocrine regions. This is evidently due to the difference in the permeability of the capillary walls to solutes in the fixative. In the BBB regions in less permeable capillaries do not allow outflow of osmotically active particles to a harmful extent, whereas in the endocrine regions osomotic imbalances are created between the intra- and extracellular compartments. The diffusion rate of the fixative and the final volume of the fixed brain depend on the balance between the intravascular and intersitial hydrostatic and oncotic pressures across the capillary wall during the perfusion fixation, as those pressures regulate the amount of perfusate that will enter the parenchyma. Generally, as high a perfusion pressure as possible is recommended to obtain effective wash-out of blood and rapid diffusion of fixative into the tissue. Addition of macromolecules (2% PVP, mol. wt. 40,000) into the fixative slightly improved the ultrastructural preservation in the BBB regions of the central nervous system.