Bone Growth, Remodeling, and Repair PDF

Lecture 19 Types of bone ,Bone growth, remodeling &repair Types of bone: Bone is organized in two histological forms: 1. Compact bone. 2-Cancellous bone Compact bone: Sites: -Shaft of long bones -The outer part of the epiphysis -The outer part of the shor...

Lecture 19 Types of bone ,Bone growth, remodeling &repair Types of bone: Bone is organized in two histological forms: 1. Compact bone. 2-Cancellous bone Compact bone: Sites: -Shaft of long bones -The outer part of the epiphysis -The outer part of the short and flat bones. Structure of compact bone (Fig.1):  The outer surface of compact bone is covered with periosteum and bone marrow cavity is lined with endosteum.  It consists of osteons or Haversian systems.  Each Haversian system is composed of a Haversian canal surrounded by regularly arranged concentric lamellae of bone. Between concentric lamellae, there are interstitial lamellae. Under the periosteum and endosteum the lamellae of bone are called the outer and inner circumferential lamellae respectively. 100  Fig.(1): Histolgical structure of compact bone Cancellous bone Sites: - In the center of flat bone and irregular bones. - At the end of long bones. Structure of cancellous bone (Fig.2)  Cancellous bone is composed of irregular trabeculae of bone separated by bone marrow spaces.  The bone trabeculae are composed of osteocytes and intercellular substance and covered with endosteum.  These are no Haversian systems. 101  Fig.(2):Cancellous bone Histogenesis of Bone (Bone formation): Bone can be formed in two ways: 1- Direct mineralization of matrix secreted by osteoblast (Intramembranous ossification). 2- Deposition of bone matrix on a preexisting cartilage matrix (Endochondral ossification). Intramembranous Ossification: (Fig.3) Site Most of the flat bones as skull, clavicle and most of cranial bone) Steps 1-A primary center of ossification appears in the mesenchymal C.T. membrane rich in blood supply. 2- Mesenchymal cells differentiation into osteoprogenitor cells that give osteoblasts. 3- Osteoblasts produce osteoid (bone matrix) and calcification follows, gradually it encircles osteoblasts and become osteocytes and the bone formed is of spongy type. 4-The connective tissue that remains among the bone walls is penetrated by growing blood vessels and the undifferentiated mesenchymal cells, giving rise to the bone marrow cells. 102 5-The portion of the connective tissue layers that covers the trabeculae gives rise to the endosteum and the periosteum. Fig. (3): Steps of intramembranous ossification Endochondral Ossification: (Fig. 4) Site Endochondral ossification takes place within a piece of hyaline cartilage which has a shape resembling a model of the bone to be formed. This type of ossification is principally responsible for the formation of short and long bones. Steps The cartilage model consists of two epiphyses and a diaphysis. 1-A primary center of ossification arises in the middle of the shaft of the cartilage model (diaphysis). 2- Initially, the first bone tissue appears as a hollow bone cylinder that surrounds the mid portion of the cartilage model forming the bone collar. 103  3- The cartilage undergoes a degenerative process and cell death resulting in the formation of calcified cartilage matrix. 4-The nutrient artery from the periosteum invade the calcified cartilage through holes made by osteoclasts in the bone collar. 5-The mesenchymal cells give osteoprogenitor cells that differentiate into osteoblasts forming bone plate and also give bone marrow cells then cancellous bone develops. 6-Later on secondary centers of ossification occur after birth at the epiphyses. The ossification continues of whole bone except at two sites where hyaline cartilage persists; one is articular cartilage covering the articular surface of bone. The second site is at the junction between the epiphysis and diaphysis called epiphyseal plate. Fig.(4) Endochondral bone growth Growth in length of bone: - (Fig. 5)  Before the adulthood, the interstitial growth of the cartilage of epiphyseal plate continues to increase the length of long bone until the age of 20 years in males and 17 years in females. After that age the epiphysel cartilage disappear (closure of epiphysel line) 104   The chondrocytes undergo changes similar to the early events in the primary center of endochondral ossification. Five zones become recognizable along their length. (1) Zone of resting cartilage. (2) Zone of proliferation: The chondrocytes divide several times and they are arranged in rows parallel to the longitudinal axis of the long bone. (3) Zone of Hypertrophy: The chondrocytes increase in size and become rich in alkaline phosphatase enzyme vesicle. (4) Zone of Calcification: Calcium salts are deposited (in the matrix) around the chondrocytes by the activity of phosphatase enzyme. The calcified matrix cuts off the nourishment, to the chondrocytes which die and degenerate leaving empty spaces. (5) Zone of Ossification: Blood capillaries and mesenchymal cells penetrate the spaces left after the destruction of chondrocytes. Then bone matrix and bone marrow cells develop. Notice that the bone tissue is formed at the site of cartilage. Thus cartilage is not transformed into bone but is rather replaced by bone. 105  Fig. (5) Stages of ossification in the epiphyseal plate  Bone remodeling and repair: (previously discussed) References  Junqueira's Basic Histology: Text and Atlas  Wheater's Functional Histology: A Text and Colour Atlas  Textbook of Histology: Gartner 106