Bone marrow, in the aggregate, is the second largest organ of the mammalian body. Marrow is the reticular connective tissue that fills the central cavity of bone perfused by blood from the nutrient artery. Blood is distributed radially by thin-walled arterioles to the blood sinusoids and is returned by small venules to a central vein. Review the compact bone of the cortex and the spicules representing spongy (cancellous) bone, which project into the central cavity of the bone. Marrow in cancellous bone is the soft material found in the spaces between the bone spicules. It consists of reticular connective tissue that forms a delicate meshwork within the marrow cavity, and it is permeated by numerous thin-walled blood vessels. Within the spaces of this tissue, the developing red and white blood cells are segregated into developing cell nests. Recall that while all cells of the blood originate from pluripotent stem cells in the bone marrow, the bone marrow also serves as the major site of maturation of B lymphocytes in most mammals studied to date. Bone marrow also contains blood, adipocytes and megakaryocytes (large basophilic cells that give rise to platelets).