Endochondral ossification

Introduction Most of the fetus's bones, such as the limb bones, the trunk bones, and the skull base, are mainly formed by cartilage osteogenesis. This bone formation includes both the process of osteogenesis similar to intramembranous ossification, the continuous growth and degradation of cartilage, and the unique occurrence of cartilage tissue being replaced by bone tissue, and its occurrence, growth and alteration are interspersed. It is more complicated than intramembranous osteogenesis. There are many factors affecting bone growth, such as the expression of genetic genes and the role of hormones, such as nutrition and vitamin supply.

Cause

There are many factors affecting bone growth, such as the expression of genetic genes and the role of hormones, such as nutrition and vitamin supply.

an examination

Related inspection

Bone alkaline phosphatase CT examination

(1) Cartilage degeneration and primary ossification center formation: At the same time as the formation of the bone collar, the chondrocytes in the middle part of the cartilage form hypertrophy and secrete alkaline phosphatase, causing the surrounding cartilage matrix calcification and hypertrophic chondrocytes to degenerate and die. Leave a large cartilage lacuna. This change indicates that the primary ossification center is about to form in this area. At the beginning of the primary ossification center, the blood vessels, along with osteoclasts and mesenchyme, pass through the bones of the periosteum to enter the degenerative cartilage area, and the calcified cartilage matrix is absorbed by the osteoclast to form many The original backbone has a long axis parallel to the tunnel. The wall of the tunnel is the residual calcified cartilage matrix, and the cavity of the tunnel is the primary marrow cavity. The cavity is filled with mesenchymal bone cells and osteoblasts, as well as osteoclasts and developing hematopoietic tissues, collectively referred to as primary bone marrow. Subsequently, the osteoblasts are attached to the original bone marrow wall (i.e., the surface of the residual calcified cartilage matrix) to form bone tissue, and a transitional trabecular bone with a calcified cartilage matrix as a central axis surface and bone tissue is formed. The site of the transitional trabecular bone, the primary ossification center, begins.

(2) Bone marrow cavity formation and bone growth: The transitional trabecular bone of the primary ossification center is soon decomposed and absorbed by the osteoclasts, making many primary bone marrow cavities into a larger secondary bone marrow cavity. The inner surface of the collar is also gradually decomposed and absorbed by the osteoclast. This edge of the bone collar forms an osteogenesis process that is decomposed and absorbed, so that the bone is thickened while maintaining the proper thickness of the bone tissue, and the bone marrow cavity is laterally enlarged. As the cartilage tissue at both ends of the primary ossification center continues to grow, the cartilage adjacent to the bone marrow cavity is degraded, so that the ossification process of the primary ossification center continues from the middle of the backbone to both ends, and the medullary cavity also expands longitudinally.

(3) Secondary ossification center and osteophyte formation: The time of occurrence of the secondary ossification center varies from bone to bone, from pre-natal and late to months or years after birth. The site of the appearance is at the center of the cartilage at both ends of the backbone. The secondary ossification center is similar to the primary ossification center. However, ossification is carried out radially from the center to the periphery. Finally, most of the cartilage tissue is replaced by primary bone cancellous, and the two ends of the bone are transformed into early osteophytes. The thin layer of cartilage, the articular cartilage, is always retained on the surface of the ankle. A layer of cartilage of a certain thickness is also retained between the early epiphysis and the diaphysis, that is, the cartilage, called the epiphyseal plate. The epiphyseal chondrocytes continue to divide and proliferate and degenerate. Osteoclasts and osteoblasts continuously decompose and absorb the calcified cartilage matrix from the medullary cavity side, and form a transitional trabecular bone, so that the ossification continues to advance to both ends, and the long bones continue When grown to 17-20 years old, the tarsal plate stops growing and is replaced by trabecular bone, leaving a linear trace between the stem and the sputum of the long bone, called the epiphyseal line. Early growth and reconstruction of the epiphysis will eventually form an epiphyseal with a loose inner bone and a thin layer of bone on the surface.

(4) Formation and reconstruction of the backbone bone compact: the primary bone cancellous constituting the original backbone, the mesh between the trabeculae becomes smaller through the thickening of the trabecular bone, and gradually becomes the primary bone dense. There is no bone unit and interplate in the primary bone density, and there are no external or inner ring plates. Until the age of 1 year, the osteoclasts are decomposed and absorbed on the outer surface of the original bone dense surface, and gradually form a longitudinal groove that is concave toward the deep surface. The vascular and osteogenic cells of the epithelium enter the sulcus, and the osteogenic cells differentiate into osteoblasts for bone formation. The longitudinal groove is first closed into a tube, and then attached to the surface of the tube wall to form a concentric from the outside to the inside. Round array of Haval bone plates. The shaft always retains a vascular channel, the central tube. The remaining osteogenic cells in the tube are attached to the inner surface of the innermost Haval bone plate to become the endosteum.

Differential diagnosis

Differential diagnosis of endochondral ossification:

Cartilage formation: At the site where the long bone is about to occur, mesenchymal cells are dense and differentiate into osteogenic cells, which in turn differentiate into chondrocytes. The chondrocytes secrete the cartilage matrix, and the cells are also embedded therein to become cartilage tissue. The surrounding mesenchyme differentiates into the perichondrium, thus forming a piece of hyaline cartilage. Its shape is similar to the long bone to be formed and is called the cartilage model.

Periarticular ossification: refers to the formation of bone around the middle part of the cartilage. The process begins with the appearance of blood vessels in the perichondrium. Due to the sufficient supply of nutrients and oxygen, the deep bone cells of the perichondrium divide and differentiate into osteoblasts. Osteoblasts produce osteoid on the surface of the cartilage and are themselves embedded in the bone cells. The osteoid is subsequently calcified into a bone matrix, thus forming a thin layer of primary bone cancellous material surrounding the mid-cartilage. Therefore, the layered bone is like a collar, so the bone collar. The perichondrium on the surface of the collar was renamed the epithelium. The deep bone cells of the epithelium gradually differentiate into osteoblasts, adding new trabecular bone to the surface of the bone collar and its two ends, so that the primary bone cancellosis of the bone collar is gradually thickened and extends from the middle part of the cartilage to both ends. . With the development of the embryo, the osteoblasts in the primary cancellous bone of the bone collar continuously add bone tissue to the trabecular wall, and the mesh of the trabecular bone gradually becomes smaller. This process continues to make the primary bone mass gradually become the primary bone density.

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