Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Musculoskeletal System
Skeleton
Bone Development and Growth. Age-Related Changes in Bones

Three stages can be distinguished in The Development of the human Skeleton, as in other vertebrates: membranous, cartilaginous, and bony.

The evolutionary process of the skeleton in vertebrate phylogenesis—the initial Formation of the membranous skeleton, its replacement by Cartilage, and subsequently by bone—serves as the prototype for human skeletal development in ontogenesis.

In humans, Bone tissue appears during the 6th–8th week of intrauterine life. Bones are formed either directly from embryonic Connective Tissue—mesenchyme (membranous osteogenesis)—or on The basis of a cartilaginous model of the bone (cartilaginous osteogenesis). One type of supportive tissue, being less differentiated, is replaced by another possessing superior mechanical properties. The BONES OF THE cranial vault, facial bones, and part of the clavicle develop directly from embryonic connective tissue, bypassing the cartilage stage. Such bones are referred to as primary or dermal bones. During the development of these bones, one or several ossification centers appear within the young connective tissue (approximately at the center of the future bone). An ossification center consists of young bone-forming Cells, or osteoblasts, the number of which increases rapidly. Osteoblasts produce the intercellular matrix, in which calcium salts are subsequently deposited. The osteoblasts themselves transform into mature bone cells (osteocytes) and become entombed within the bone matrix. The surface layers of the connective tissue transform into the periosteum.

The bones of the trunk, limbs, and Base of the Skull develop on the basis of cartilage. The cartilage is externally covered by the perichondrium, whose inner layer, adjacent to the cartilaginous tissue, is osteogenic. Bone Formation proceeds from one or more ossification centers. The first center appears in the middle part of the cartilage during the 8th week of Embryogenesis and gradually extends outward until the entire bone is formed. Initially, the inner layer of the perichondrium produces young bone cells (osteoblasts) that are deposited on the cartilage surface (perichondral ossification). The perichondrium itself gradually transforms into the periosteum, while the newly formed bone cells are layered onto the preceding layers of bone tissue by apposition, forming a bony plate on the cartilage surface. Thus, through the periosteum, the bone grows in thickness (periosteal mode of bone formation). Concurrently, bone tissue begins to form within the cartilage. Connective tissue, growing into the cartilage along with Blood Vessels, gives rise to young bone cells arranged in cords alongside the breaking down cartilage. This mode of bone formation (within the cartilage) is termed endochondral.

Shortly before or after birth, ossification centers appear in the epiphyses, which had previously remained cartilaginous. These centers increase in size, and the cartilage is progressively replaced by bone tissue. A thin cartilaginous layer situated between the ossifying epiphysis and the bone diaphysis—the epiphyseal plate—carries out an osteogenic function throughout Postnatal ontogenesis until the bone reaches its definitive dimensions (at 18–25 years of age). By this time, the epiphyseal cartilage is replaced by bone tissue, the epiphysis fuses with the diaphysis, and the bone becomes a unified Structure. Owing to the osteogenic function of the epiphyseal cartilage, tubular bones grow in length.

The medullary cavity in tubular bones develops within the substance of the diaphysis as the newly formed bone undergoes resorption. Embryonic connective tissue growing into the bone gives rise to the Cytology/practical/86.html">Red Bone Marrow.