65. Bone Histogenesis: Secondary Ossification Centers and Joint Development

  1. Secondary ossification centers
    1. THE SECONDARY CENTERS APPEAR, during the first years of postnatal life, in the epiphyses of the bone (at the ends of the bone)
      1. Epiphyseal cartilage cells hypertrophy, and vascular connective tissue invades the epiphysis
        1. The articular cartilage and the epiphyseal plate remain cartilaginous as ossification spreads in all directions
        2. After the epiphyses become ossified, growth takes place on the diaphyseal side of the epiphyseal plate only
        3. When growth is finally complete, the epiphyseal plate is replaced by spongy bone, the epiphyses and diaphyses fuse, and no further growth occurs
    2. THE EPIPHYSES AND DIAPHYSES usually fuse by the 20th year
    3. DIAMETER GROWTH is due to bone deposition at the periosteum and resorption on the medullary surface. Balanced deposition and resorption regulate the size of the bone and its marrow cavity
    4. IRREGULAR BONES develop by a similar process with ossification beginning in the center of the bones and spreading outward
  2. Joint development
    1. SYNOVIAL JOINTS (e.g., shoulder, elbow, knee): the mesenchyme between the developing bones differentiates
      1. The mesenchyme gives rise to capsular and other joint ligaments peripherally
      2. The mesenchyme disappears in the middle, between the bones, to form the joint cavity proper
      3. The mesenchyme lining the capsule and the articular surfaces forms the synovial membrane
    2. CARTILAGINOUS JOINTS (e.g., neurocentra1 and symphysis pubis): the mesenchyme between the developing bones forms hyaline cartilage or fibrocartilage. Hyaline cartilage covers the bones of the joints at their articular surfaces
    3. FIBROUS JOINTS (e.g., the skull sutures): the mesenchyme between the developing bones forms a dense fibrous tissue
  3. Skeletal malformations in general
    1. HYPERPITUITARISM causes the infant to grow at a very abnormally fast rate. This can result in gigantism (increased height and body proportion) or acromegaly (great enlargement of the hands, face, and feet). The condition can be caused by a pituitary gland tumor. Congenital infantile hyperpituitarism is rare
    2. HYPOTHYROIDISM AND CRETINISM: due to a deficiency of fetal thyroid hormone leading to mental deficiency, skeletal abnormalities, as well as auditory and neurologic deficiencies
      1. Cretinism is generally very rare these days
      2. Agenesis of the thyroid gland is one possible cause of cretinism
    3. ACHONDROPLASIA (hypoplastic chondrodystrophy): a common cause of dwarfism. The extremities are short due to a disturbance of endochondral ossification at the epiphyseal plates of long bones during fetal existence
      1. The trunk is usually of normal length
      2. The head may be slightly smaller
      3. It is transmitted as a Mendelian dominant character

bone histogenesis:  secondary ossification centers and joint development: image #1