BONE ORGANIZATION AND GROWTH
Bone growth and maintenance are complex processes that continue throughout our lives. Our skeletons must grow, mature, and repair at the macro- and microscopic levels even as we use them. An understanding of how bones grow and are organized is central to many of the analyzes that forensic anthropologists perform (see “In More Detail: Bone Growth”).
IN MORE DETAIL: BONE GROWTH Two types of bone growth characterize the human skeleton: endochondral and intramembranous. Endochondral bone growth starts with a “model” of a bone consisting of cartilage and centers of ossification (see Figure 1). From these centers, bone is produced and infiltrates the cartilage
FIGURE 1 Bone growth starts in centers of ossification and these spread out to meet each other.
IN MORE DETAIL: BONE GROWTH—cont’d
model, which itself continues to grow. The developing shaft of the bone is called the diaphysis and the ends are called epiphyses. The growing areas eventually meet and the bone knits together. Not all epiphyses unite at the same time and the sequence of union is important for estimating age at death for individuals younger than about 25 years. In intramembranous bone growth, instead of a cartilage model, the ossification occurs within a membrane and this occurs in many bones of the skull. Bone differs from cartilage by having its collagenous connective tissue matrix impregnated with inorganic salts (primarily calcium phosphate and lesser amounts of calcium carbonate, calcium fluoride, magnesium phosphate, and sodium chloride). The osteoblasts, which form the osseous tis sue, become encapsulated in lacunae but maintain contact with the vascular system via microscopic canaliculi. When they become encapsulated, they are referred to as osteocytes. A characteristic feature of a cross section of the shaft (diaphysis) of a long bone is its organization in concentric rings around a central canal containing a blood vessel. This is called a Haversian system (osteon). Between neighboring Haversian systems are nonconcentric lamellae, devoid of Haversian canals, termed interstitial lamellae. Vascular canals, called Volkmann’s canals, traverse the long axis of the bone; they are always at right angles to Haversian canals. Their function is to link vascular canals of adjacent Haversian systems with each other and with the periosteal and endosteal blood vessels of the bone. The outer perimeter of a long bone, beneath the osteogenic connective tissue (called periosteum), is composed of circumferential lamellae, which also lack Haversian canals. This thick-walled hollow shaft of compact bone (the diaphysis) contains bone marrow. At the distal ends of long bones, where Haversian systems are not found, the bone appears spongy and is therefore called cancellous, or spongy, bone. The spongy appearance is misleading because care ful examination of the architecture reveals a highly organized trabecular system providing maximal structural support with minimal density of bony tissue. The epiphyses at the ends of the diaphysis or shaft contain the spongy bone covered by a thin layer of compact bone. The cavities of the epiphyseal spongy bone are in contact with the bone marrow core of the diaphysis except during growth of long bones in young animals. Interposed between the epiphysis and the diaphysis is the cartilaginous epiphyseal plate. The epiphyseal plate is joined to the diaphysis by columns of cancellous bone; this region is known as the metaphysis. When bone is formed in and replaces a cartilaginous “model,” the process is termed endochondral ossification. Some parts of the skull develop from osteogenic mesenchymal connective tissue, however, without a cartilaginous “model” having been formed first. This is termed intramembranous ossification and these bones are called membrane bones. In both instances, three types of cells are associated with bone formation, growth, and maintenance: osteoblasts, osteocytes, and osteoclasts. The osteoblasts produce osseous tissue (bone), become embedded in the matrix they manufacture and are then renamed osteocytes, to reflect their change of status. They remain viable because they have access to the vascular supply via microscopic canaliculi through which cellular processes extend to receive nutrients and oxygen. Osteoclasts actively resorb and remodel bone as required for growth; these are giant, multinuclear, phagocytic, and osteolytic cells.
Bones consist of an outer layer of hard, smooth compact bone, also called cortical bone, pictured in Figure 8.5. The inner layer is an infrastructure of sponge-like bone called trabecular bone in long bones, which increases the structural strength of the bone without additional weight. In the very center of long bones is the medullary cavity, which contains marrow, a fatty material that also houses blood-generating tissues. In life, this composite architecture creates a very strong but resilient frame work for our bodies. The microstructure of bone is quite complex and organized, as shown in Figure 8.6. Specialized growth cells (osteoblasts) produce bone and deposit it in layers, even tually becoming encapsulated in a self-made chamber (lacuna; plural lacunae).
FIGURE 2 The outer portion of a bone is the compact or cortical bone and is very dense. The inner portion of a bone is trabecular bone, which is made up of a fine web work of thin, boney spines. The center of a long bone contains marrow, where blood is made.
FIGURE 3 Bone grows much in the way a brick wall is made and repaired. Bone is laid down by osteoblasts (bone-generating cells) and then, in response to the stresses it undergoes, is torn down by osteoclasts (bone-destroying cells, shown here) before being reworked by the osteoblasts. Bone may seem dead, but it is a very active tissue during life.
They maintain contact with the circulatory system and other bone cells through microscopic vascular channels through which cellular processes extend to receive nutrients and oxygen. When an osteoblast becomes fully encapsulated, it is referred to as an osteon. The third main type of bone cell, osteoclasts, actively breaks down and remodels bone as required for growth. When an osteocyte reaches the end of its productivity, it dies and the bone around is reworked and made available to new osteoblasts. In response to the stresses our activities place on our skeletons, the interaction between osteoblasts, osteocytes, and osteoclasts model and shape our bones. Because new osteons are formed by remodeling existing structures, bone has a patchwork appearance at the cellular level. Bone that lies between recently reworked bone is called interstitial bone; the amounts of new, reworked, and old bone provide an indication of how old someone is; we will see later how this can provide an estimate of age at death.
