Appositional growth is the increase in the diameter of bones by the addition of bone tissue at the surface of bones. Bone remodeling involves the processes of bone deposition by osteoblasts and bone resorption by osteoclasts.
Osteoblasts are the cells involved in bone deposition, the formation of new bone. They are connective tissue cells found at the surface of bone. They can be stimulated to proliferate and differentiate into osteocytes. Osteocytes are bone cells.
In endochondral ossification, bone develops by replacing hyaline cartilage. Activity in the epiphyseal plate enables bones to grow in length (this is interstitial growth). Appositional growth allows bones to grow in diameter. Remodeling occurs as bone is resorbed and replaced by new bone.
Bone deposition is one of the two main events of bone remodeling. It is the process of depositing new bone materials. Osteoblasts are the bone-forming cells that carry out bone deposition. They secrete an organic matrix that is rich in collagen protein.
Figure 6.32 – Periosteum and Endosteum: The periosteum forms the outer surface of bone, and the endosteum lines the medullary cavity. Flat bones, like those of the cranium, consist of a layer of diploë (spongy bone), covered on either side by a layer of compact bone (Figure 6.3.
There are 4 basic types of tissue: connective tissue, epithelial tissue, muscle tissue, and nervous tissue. Connective tissue supports other tissues and binds them together (bone, blood, and lymph tissues). Epithelial tissue provides a covering (skin, the linings of the various passages inside the body).
Each bone has two types of bone tissue to ensure strength: The dense, hard outer layer is called compact or cortical bone while the inner, less dense, lattice-like bone is called cancellous, trabecular or spongy bone that is surrounded by bone marrow.
There are two types of bone ossification, intramembranous and endochondral. Each of these processes begins with a mesenchymal tissue precursor, but how it transforms into bone differs.
Bone remodeling is the replacement of old bone tissue by new bone tissue. It involves the processes of bone deposition by osteoblasts and bone resorption by osteoclasts. Normal bone growth requires vitamins D, C, and A, plus minerals such as calcium, phosphorous, and magnesium.
The increase in diameter is called appositional growth. Osteoblasts in the periosteum form compact bone around the external bone surface.
During appositional growth, osteoclasts resorb old bone that lines the medullary cavity, while osteoblasts, via intramembranous ossification, produce new bone tissue beneath the periosteum.
Appositional growth is the process by which old bone that lines the medullary cavity is reabsorbed and new bone tissue is grown beneath the periosteum, increasing bone diameter.
Bone growth is either longitudinal or appositional. Longitudinal bone growth is the ossification of the bone that grows lengthwise until they stop growing. Appositional bone growth continues till death and is the growth of the bones in diameter or thickness.
Endochondral ossification is the mechanism responsible for the formation of all long bones of the axial skeleton (vertebrae and ribs) and the appendicular skeleton (limbs).
Endochondral ossification first takes place at the primary site at the center of the diaphysis, which allows formation of the two growth plates. The growth plates are ultimately responsible for the elongation of the long bones.
In intramembranous ossification, bone develops directly from sheets of mesenchymal connective tissue. In endochondral ossification, bone develops by replacing hyaline cartilage.
The direct conversion of mesenchymal tissue into bone is called intramembranous ossification. This process occurs primarily in the bones of the skull. In other cases, the mesenchymal cells differentiate into cartilage, and this cartilage is later replaced by bone.
Appositional growth occurs when chondroblasts secrete new matrix along existing surfaces and this causes the cartilage to expand and widen.
Examples in the human body
The following bones develop in humans via Intramembranous ossification: Flat bones of the face. Most of the bones of the skull. Clavicles.
Osteoblast and osteoclast are the two main cells participating in those progresses (Matsuo and Irie, 2008). Osteoclasts are responsible for aged bone resorption and osteoblasts are responsible for new bone formation (Matsuoka et al., 2014).
The periosteum is a membranous tissue that covers the surfaces of bones. It is an intricate structure composed of an outer fibrous layer that gives structural integrity and an inner cambium layer that possesses osteogenic potential.
Compact bone: also known as cortical bone, this hard-outer layer is strong and dense.
The dense connective tissue on a long bone is found surrounding the shaft of the bone but does not cover the ends of the bone. The connective tissue is known as the periosteum and is a dense layer of vascular connective tissue. Vascular means that is has blood vessels running through it.