Freeze-etching images of capillary endothelial pores in the liver, thyroid and adrenal of the mouse.

The capillary endothelial pores of perfusion-fixed and immersion-fixed liver, thyroid and adrenal of mice were observed using freeze-etching preparations. In liver sinusoids, endothelial pores are heterogeneous in size and range from 50-100 nm. When the pores are classified into three types according to their size, small pores up to 100 nm in diameter represent 69.8%, intermediate pores measuring 100-500 nm 27.6% and large pores exceeding 500nm 2.6% of all pores in number. The endothelial pores of the thyroid blood capillaries are fairly homogeneous in size (70-90 nm), corresponding to the small type pores in the liver. In both adrenal cortex and medulla, the capillary endothelial pores measure 40-50 nm in diameter, showing small and intermediate types. In the adrenal cortex the former occupy 80.6% of all pores and the latter 19.4%. No larger type pores are recognized in the adrenal gland. These findings are not affected by different fixation methods, either perfusion or immersion.