Bone marrow: Adult

Less hemopoiesis occurs in yellow marrow than in red marrow. Nonetheless, each tissue can replace the other depending on physiological demands. Adipocytes within and outside the bone marrow are similar morphologically, but differ functionally. Adipocytes inside bone marrow modulate hemopoietic activity and those outside are associated with the nutritional status of the individual.

In addition to abundant adipocytes, the remainder of yellow bone marrow is composed of hemopoietic tissue. Thus, some blood cell formation occurs in this relatively inactive site. If more blood cells are required, hemopoietic stem cell activity increases in these areas. This results in an increase in the number of developing blood cells and a decrease in size of individual adipocytes. Adipocytes do not differentiate into blood cells.

This red bone marrow is composed mostly of hemopoietic tissue rather than adipocytes, in a proportion typical of adults in their thirties and forties. As individuals age, adipocytes replace hemopoietic tissue, resulting in a gradual increase in yellow bone marrow to about 70% or more in the elderly.

Sinusoids are large diameter, thin walled capillaries (discontinuous capillaries). After maturing in the hemopoietic tissue, red and white blood cells migrate into the sinusoids for dispersal throughout the body.

Megakaryocytes are large cells that give rise to platelets, which are critical for blood clotting.  Platelets are cell fragments shed from the megakaryocytes.

Bone spicules form the spongy bone in which most hemopoiesis occurs in normal adults.