Red bone marrow consists of blood cells in different stages of development and a network of reticular cells and fibers that serve as a supporting framework for the developing blood cells and vessels. As an individual grows, the amount of red marrow does not increase proportionately with bone growth. In later stages of growth and in adults, when the rate of blood cell formation has diminished, the tissue in the marrow cavity consists mostly of fat cells; it is then called yellow marrow. In response to appropriate stimuli, such as extreme blood loss, yellow marrow can revert to red marrow. In the adult, red marrow is normally restricted to the spaces of spongy bone in a few locations such as the sternum and the iliac crest. Diagnostic bone marrow samples and marrow for transplantation are obtained from these sites.
Mature bone is composed of structural units called osteons (Haversian systems).
Mature bone is largely composed of cylindrical units called osteons or Haversian systems (Fig. 8.3). The osteons consist of concentric lamellae (sing., lamella) of bone matrix surrounding a central canal, the osteonal (Haversian) canal, which contains the vascular and nerve supply of the osteon. Canaliculi containing the processes of osteocytes are generally arranged in a radial pattern with respect to the canal (Plate 11, page 244). The system of canaliculi that opens to the osteonal canal also serves for the passage of substances between the osteocytes and blood vessels. Between the osteons are remnants of previous concentric lamellae  FOLDER 8.1
Clinical Correlation: Joint Diseases
Inflammation of the joints or arthritis can be caused by many factors and can produce varying degrees of pain and disability, from the pathologic response of articular carti-lage to injury. Simple trauma to a joint by a single incident or by re-peated insult can so damage the articular cartilage that it calcifies and begins to be replaced by bone. This process can lead to ankylosis (i.e., bony fusion in the joint and subsequent loss of motion). The foot and knee joints of run-ners and football players and hand and finger joints of stringed instrument players are especially vulnerable to this condition. Immune responses or infectious processes that localize in joints, as in rheumatoid arthritis or tuberculosis, can also damage the articular cartilages, producing both severe joint pain and gradual ankylosis. Surgery that replaces the damaged joint with a prosthetic joint can often relieve the pain and restore joint motion in seriously debilitated individ-uals. Another common cause of damage to articular carti-lages is the deposition of crystals of uric acid in the joints, particularly those of the toes and fingers. This condition is known as gouty arthritis or, more simply, gout. Gout has become more common because of the widespread use of thiazide diuretics in the treatment of hypertension. In genet-ically predisposed individuals, gout is the most common side effect of these drugs. Gout causes severe, unbear-able pain because of the sharp crystals in the joint. The irritation also causes the formation of calcareous deposits that deform the joint and limit its motion.
lamellae of bone