Chapter 64. Development of The Skeletal and Articular Systems: Cartilage and Bone Histogenesis - Review of Medical Embryology Book

Introduction: these systems develop from mesoderm. The somites differentiate into 2 parts: cells of the sclerotome give rise to bone, cartilage, and ligaments; those of the dermomyotome give rise to skeletal muscle. The mesodermal cells form mesenchyme (embryonic connective tissue) which can differentiate into fibroblasts, chondroblasts, and osteoblasts. In addition to somite mesenchyme, the splanchnic and somatic mesoderm also can form mesenchyme (some head mesenchyme even arises from neuroectoderm). Most bones first appear as condensations of mesenchymal cells which give rise to hyaline cartilage models that ossify via endochondral ossification. Others develop in mesenchyme by intramembranous bone formation
Cartilage histogenesis: cartilage is seen as mesenchymal condensations, at about week 5, where it is to develop. The cells proliferate, round up, and elastic or cartilaginous fibers are deposited in the intercellular substance (matrix). Three types are described: hyaline, fibrocartilage, and elastic cartilage, depending on matrix
Bone histogenesis: bone develops in 2 types of preexisting connective tissue, namely, in mesenchyme or cartilage

The mesenchyme condenses, becomes very vascular, and the cells differentiate into osteoblasts (bone-forming cells) which deposit an intercellular matrix
The matrix is calcified to form spicules of spongy bone
Some osteoblasts are trapped in the matrix to become osteocytes (bone cells) as successive layers of lamellae are deposited by other osteoblasts
The spicules thicken, fuse, and form plates of compact bon With internal reorganization, haversian systems develop
Between the plates of bone, the intervening bone stays spongy and the mesenchyme forms bone marrow
Both osteoblasts and osteoclasts continue to remodel the bone
Ossification begins at the end of the embryonic period

ENDOCHONDRAL OR INTRACARTILAGINOUS OSSIFICATION takes place in a preexisting cartilage model

In a long bone (, the femur), the primary ossification center is seen in the diaphysis or shaft (between ends of the bone). Here the cartilage increases in size (hypertrophies), the matrix is calcified, and the cells die

Concurrently, a thin layer of bone is laid down under the perichondrium around the diaphysis and will become the periosteum
Vascular connective tissue invades from the periosteum and breaks up the cartilag Some of the mesenchymal cells form hematopoietic cells of the bone marrow and others form osteoblasts which deposit bone matrix on the spicules of calcified cartilag The spicules are remodeled by osteoblasts and osteoc1asts, and the process continues toward both ends of the bones (epiphysis)
The bone grows in length at the diaphyseoepiphyseal junction where the cartilage cells proliferate by mitosis
The cartilage cells facing the diaphysis hypertrophy, the matrix is broken up into spicules by the vascular tissue from the marrow, and bone is deposited on the spicules

Resorption of bone keeps the bone mass relatively constant in length and enlarges the marrow cavity
At birth, the diaphyses are largely ossified, but most epiphyses are still cartilaginous