Ossification & the Epiphyseal Plate


Ossification in the Epiphyseal Plate

All bone tissue first forms embryonically as a tissue called primary bone (woven bone). Primary bone is an immature bone that has a distinctive woven appearance under the microscope. Primary bone is formed in the fetus through one of two distinct processes: intramembranous ossification or endochondral ossification. Intramembranous ossification begins directly in mesenchymal tissue. Mesenchymal cells differentiate directly into osteoblasts, which then deposit the bone matrix. Bone tissue formed in this way is known as intramembranous bone. Only a few bones form entirely through this process, such as some bones of the developing skull, face, and mandible.

Endochondral ossification, the more complex of the two processes, involves the development of a cartilagenous model, made of hyaline cartilage, from which immature bone develops. Embryonic mesenchymal cells differentiate into chondroblasts, which deposit a cartilagenous matrix, forming a preliminary structural model of the bone. The primary bone tissue of most developing human bones, including most bones of the developing axial and appendicular skeletons, are formed through endochondral ossification. These bones are called endochondral bones.

Nearly all the primary bone formed through these ossification processes is later replaced, through a process of bone remodeling, by secondary bone (lamellar bone). Secondary bone is mature bone. Its matrix is more mineralized than primary bone, and it has a lamellar (layered) rather than woven structure.

The growth of endochondral bones, which takes place through adolescence and early adulthood, depends on the presence of an area of hyaline cartilage within the bone organ. The lengthwise growth of long bones (bones of the arms and legs, hands amd feet, but not wrists and ankles) takes place through an endochondral bone-forming process that takes place in a cartilagenous structure called the epiphyseal plate. A study of the tissue structure of the epiphyseal plate reveals the endochondral ossification process.

 


Copyright © 2009 Stephen Gallik, Ph. D.