BONE DEVELOPMENT
14.3.1 Embryonic Bone Formation
a. Intramembranous Ossification: In intramem- branous ossification, bones develop directly from mesenchymal (undifferentiated) connective tissue.
This process occurs primarily in flat bones, such as the skull bones and certain facial bones. Mesenchymal cells differentiate into osteoblasts, which secrete osteoid (organic bone matrix) that subsequently mineralizes to form bone tissue.b. Endochondral Ossification: In endochondral ossification, bones develop from a hyaline cartilage precursor. This process occurs in most of the bones in the body, including long bones such as the femur and humerus. Chondrocytes (cartilage cells) within the cartilage model undergo hypertrophy and secrete factors that attract osteoblasts to the developing bone. Osteoblasts then deposit osteoid around the cartilage model, which eventually becomes mineralized, replacing the cartilage with bone tissue.
14.3.2 Bone Growth in Juveniles
a. Epiphyseal Growth Plate: During childhood and adolescence, long bones grow in length primarily through the activity of the epiphyseal growth plate, also known as the growth plate or physis. The growth plate is a layer of cartilage located near the ends of long bones. Chondrocytes within the growth plate undergo mitosis, leading to the proliferation of cartilage cells. As new cartilage cells are produced, older cells undergo hypertrophy and mineralization, eventually forming bone tissue. The growth plate closes when growth ceases, marking the end of longitudinal bone growth.
b. Closure of Growth Plates: Or Closure and Epiphyseal Line Formation - As individuals reach skeletal maturity, the epiphyseal growth plates close, signaling the end of longitudinal bone growth. Closure occurs when chondrocytes within the growth plate stop dividing and undergo apoptosis (programmed cell death). Once the growth plate is completely replaced by bone tissue, it leaves behind a thin line of compact bone known as the epiphyseal line or metaphysis.
14.4