Which tissue is characterized by the presence of sinusoid capillaries to permit entry of blood cells into the circulation?

• A. Skeletal muscle
• B. Red bone marrow
• C. Cartilage
• D. Skin

Answer: (B)

Sinusoid capillaries are specialized vessels with a wide, irregular lumen and a discontinuous endothelium that allow whole cells to move between blood and surrounding tissues. Red bone marrow relies on this feature to release mature blood cells into the circulation. Hematopoietic cells develop in the marrow and must cross the endothelial lining to enter the bloodstream, so the leaky, spacious sinusoids provide the necessary passageways for this export of blood cells.

Other tissues lack this structure. Skeletal muscle and skin have continuous capillaries with tight junctions that regulate small-molecule exchange rather than permitting passage of whole cells. Cartilage is avascular, so it has no blood vessels for cells to travel through at all.