Which capillary type would be most associated with facilitating the passage of newly formed blood cells into circulation?

Which capillary type would be most associated with facilitating the passage of newly formed blood cells into circulation?

• A. Continuous capillaries
• B. Fenestrated capillaries
• C. Sinusoids
• D. Lymphatic capillaries

Answer: (C)

The ability of capillaries to let developing blood cells through is tied to their structure and permeability. Bone marrow uses sinusoid capillaries, which are discontinuous with wide gaps between endothelial cells and a broken or absent basement membrane in spots. This open architecture lets newly formed red and white blood cells, as well as platelets, slip from the marrow into the bloodstream. Specialized marrow macrophages can also help by clearing or remodeling cells as they pass. Other capillary types are more restrictive. Continuous capillaries have tight junctions and an intact basement membrane, limiting passage of large cells. Fenestrated capillaries have pores that allow rapid exchange of small molecules and some proteins but still restrict large cells. Lymphatic capillaries are designed to collect interstitial fluid and cells for transport to the lymphatic system, not to deliver newly formed blood cells into circulation.

The ability of capillaries to let developing blood cells through is tied to their structure and permeability. Bone marrow uses sinusoid capillaries, which are discontinuous with wide gaps between endothelial cells and a broken or absent basement membrane in spots. This open architecture lets newly formed red and white blood cells, as well as platelets, slip from the marrow into the bloodstream. Specialized marrow macrophages can also help by clearing or remodeling cells as they pass.

Other capillary types are more restrictive. Continuous capillaries have tight junctions and an intact basement membrane, limiting passage of large cells. Fenestrated capillaries have pores that allow rapid exchange of small molecules and some proteins but still restrict large cells. Lymphatic capillaries are designed to collect interstitial fluid and cells for transport to the lymphatic system, not to deliver newly formed blood cells into circulation.