endochondrally

Endochondrally refers to the process by which most bones form through endochondral ossification. In this pathway, a hyaline cartilage template develops first and is gradually replaced by bone tissue as development proceeds.

During fetal development, mesenchymal cells condense to create a cartilage model of the future bone. Chondrocytes within this template proliferate and enlarge, and the surrounding matrix becomes calcified. Blood vessels invade the cartilage, and osteogenic cells from the periosteum lay down bone as a primary ossification center forms in the diaphysis. After birth, secondary ossification centers appear in the epiphyses, and remodeling creates the marrow cavity. Growth plates, or epiphyseal plates, of cartilage persist at the ends of long bones to allow lengthwise growth until skeletal maturity, when these plates ossify and growth ceases.

In contrast, intramembranous ossification forms bones directly from mesenchyme without a cartilage intermediate, giving rise to many flat bones of the skull and parts of the clavicle. Endochondral ossification accounts for the formation of the majority of the axial and appendicular skeleton, including long bones, vertebrae, and base of skull elements.

Clinical relevance: disturbances in endochondral ossification can lead to skeletal dysplasias and abnormal bone growth. Achondroplasia, for example, results from impaired cartilage growth at growth plates, limiting bone elongation. Other nutritional, metabolic, or genetic conditions can also affect endochondral bone formation and skeletal development.