BONE MARROW




 
Bone marrow is a soft pulpy tissue which is found in the marrow cavities of all bones (Fig. 4.11) and even in the larger Haversian canals of lamellar bone. It differs in composition in different bones and at different ages and occurs in two forms, yellow and red marrow. In old age the marrow of the cranial bones undergoes degeneration and is then termed gelatinous marrow.

Yellow marrow Yellow marrow consists of a framework of connective tissue which supports numerous blood vessels and cells, most of which are adipocytes. A small population of typical red marrow cells persists and may be reactivated when the demand for blood cells becomes sufficiently great.Red marrow Red marrow is found throughout the skeleton in the fetus and during the first years of life. After about the fifth year the red marrow, which represents actively haemopoietic tissue, is gradually replaced in the long bones by yellow marrow. The replacement starts earlier, and is generally more advanced, in the more distal bones. By 20–25 years of age, red marrow persists only in the vertebrae, sternum, ribs, clavicles, scapulae, pelvis, cranial bones and in the proximal ends of the femur and humerus. Red bone marrow consists of a network of loose connective tissue, the stroma, which supports clusters of haemopoietic cells (haemopoietic cords or islands) and a rich vascular supply in which large, thin-walled sinusoids are the main feature (Fig. 4.11). The vascular supply is derived from the nutrient artery to the bone, which ramifies in the bone marrow, and terminates in thin-walled arterioles from which the sinusoids arise. These, in turn, drain into disproportionately large veins. Lymphatic vessels are absent from bone marrow. The stroma contains a variable amount of fat, depending on age, site and the haematological status of the body, and small patches of lymphoid tissue are also present. Marrow thus consists of vascular and extravascular compartments, both enclosed within a bony framework from which they are separated by a thin layer of endosteal cells (p. 91).Stroma Stroma is composed of a delicate network of fine type III collagen (reticulin) fibres secreted by highly branched, specialized fibroblast-like cells (reticular cells) derived from embryonic mesenchyme. When haemopoiesis stops, as occurs in most limb bones in adult life, these cells (or closely related cells) become distended with lipid droplets, and fill the marrow with yellow fatty tissue (yellow marrow). If there is a later demand for haemopoiesis, the stellate stromal cells reappear. The stroma also contains numerous macrophages attached to extracellular matrix fibres. These cells actively phagocytose cellular debris created by haemopoietic development, especially the extruded nuclei of erythroblasts, remnants of megakaryocytes and cells which have failed the B lymphocyte selection process. Stromal cells play a major role in the control of haemopoietic cell differentiation, proliferation and maturation. Marrow sinusoids are lined by a single layer of endothelial cells, supported by reticulin on their basal surfaces. Although the endothelial cells are interconnected by tight junctions, their cytoplasm is extremely thin in places, and the underlying basal lamina is discontinuous. The passage of newly formed blood cells from the haemopoietic compartment into the bloodstream appears to occur through an interactive process with the endothelium, producing temporary apertures (large fenestrae) in their attenuated cytoplasm.