Bone Anatomy and Histology
The cellular components of bone consist of osteogenic precursor cells, osteoblasts, osteoclasts, osteocytes, and the hematopoietic elements of bone marrow.[10,22] Osteoprogenitor cells are present on all nonresorptive bone surfaces, and they make up the deep layer of the periosteum, which invests the outer surface of bone, and the endosteum, which lines the internal medullary surfaces. The periosteum is a tough, vascular layer of connective tissue that covers the bone but not its articulating surfaces. The thick outer layer, termed the "fibrous layer," consists of irregular, dense connective tissue. A thinner, poorly defined inner layer called the "osteogenic layer" is made up of osteogenic cells. The endosteum is a single layer of osteogenic cells lacking a fibrous component.
Osteoblasts are mature, metabolically active, bone-forming cells. They secrete osteoid, the unmineralized organic matrix that subsequently undergoes mineralization, giving the bone its strength and rigidity. As their bone-forming activity nears completion, some osteoblasts are converted into osteocytes whereas others remain on the periosteal or endosteal surfaces of bone as lining cells. Osteoblasts also play a role in the activation of bone resorption by osteoclasts.
Osteocytes are mature osteoblasts trapped within the bone matrix. From each osteocyte a network of cytoplasmic processes extends through cylindrical canaliculi to blood vessels and other osteocytes. These cells are involved in the control of extracellular concentration of calcium and phosphorus, as well as in adaptive remodeling behavior via cell-to-cell interactions in response to local environment.
Osteoclasts are multinucleated, bone-resorbing cells controlled by hormonal and cellular mechanisms. These cells function in groups termed "cutting cones" that attach to bare bone surfaces and, by releasing hydrolytic enzymes, dissolve the inorganic and organic matrices of bone and calcified cartilage. This process results in the formation of shallow erosive pits on the bone surface called Howship lacunae.
There are three primary types of bone: woven bone, cortical bone, and cancellous bone.[10,22] Woven bone is found during embryonic development, during fracture healing (callus formation), and in some pathological states such as hyperparathyroidism and Paget disease. It is composed of randomly arranged collagen bundles and irregularly shaped vascular spaces lined with osteoblasts. Woven bone is normally remodeled and replaced with cortical or cancellous bone.
Cortical bone, also called compact or lamellar bone, is remodeled from woven bone by means of vascular channels that invade the embryonic bone from its periosteal and endosteal surfaces. It forms the internal and external tables of flat bones and the external surfaces of long bones. The primary structural unit of cortical bone is an osteon, also known as a haversian system. Osteons consist of cylindrical shaped lamellar bone that surrounds longitudinally oriented vascular channels called haversian canals. Horizontally oriented canals (Volkmann canals) connect adjacent osteons. The mechanical strength of cortical bone depends on the tight packing of the osteons.
Cancellous bone (trabecular bone) lies between cortical bone surfaces and consists of a network of honeycombed interstices containing hematopoietic elements and bony trabeculae. The trabeculae are predominantly oriented perpendicular to external forces to provide structural support.[16,29] Cancellous bone is continually undergoing remodeling on the internal endosteal surfaces.
Neurosurg Focus. 2001;10(4) © 2001 American Association of Neurological Surgeons
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