Endochondral ossification, the process by which cartilage is replaced by bone, results in long, short, and irregular bones. Flat bones form as a result of intramembranous ossification, which occurs when bone forms inside sheets of connective tissue. Bone growth is more complex than simple elongation or expansion. Growth involves an increase in both size and number of cells with production of new collagen and mineralization of this matrix. Ossification begins with formation of a fibrous cap at the end of each bone segment. This cap is made up of collagen fibers that surround small blood vessels and some marrow cells. As the bone grows in length, the growing tip ruptures the cap and the blood vessels within the fracture zone are exposed to the surrounding fluid. This triggers the formation of new osteoblasts, which produce a protein called type I collagen that will become the organic matrix of the bone. The rupture also creates two free edges of bone that will eventually be covered by soft tissue.
The skeletal system develops over time because bones grow longer through extracellular matrix deposition by osteoblasts and resorption by osteoclasts. Osteoblasts secrete a special gel that contains calcium and phosphorus molecules. This material is important for maintaining healthy bone structure. When there is an imbalance between bone formation and breakdown, such as what happens with osteoporosis, the skeleton becomes less dense and fragile. Endochondral ossification is responsible for most of the growth in the human body during childhood and adolescence.
Long bones form during fetal development by endochondral ossification, whereas flat bones form via intramembranous ossification. Both procedures entail the aggregation of mesenchymal stem cells into mesenchymal condensations, which offer a chondrogenic milieu. The cells within these aggregates differentiate into osteoblasts and secrete new bone matrix, which calcifies after death to give rise to inert bone tissue.
The facial skeleton develops from ectoderm-derived tissues, specifically from the oral cavity, nasal cavity, and brain. These tissues fuse together to form the face, including the teeth and jaws. In addition, the ears are also derived from the embryonic ectoderm and they too will eventually be lost if no further use is made of them. The auditory apparatus is therefore not needed once the embryo has been born alive, and it will eventually degenerate if it is not used thereafter.
The skeletal system consists of many different types of bones that provide support for the body and connect one part to another. There are several ways that bones gain mass during prenatal development, depending on their position within the body and how much stress they are expected to bear. For example, bones such as the long bones (lower leg bones) and skull bones increase in size through endochondral ossification.
At the epiphyseal plate, bones expand in length by a process analogous to endochondral ossification. The cartilage in the epiphyseal plate area next to the epiphysis continues to expand during mitosis. The chondrocytes in the diaphysis area age and deteriorate. The bone cells that remain become osteoblasts which produce new bone tissue. The osteoblasts then die and are replaced by new ones from the stem cell population located in the bone marrow.
The bones of the skull and the spinal column do not expand in length. These bones are expanded laterally (widely) and vertically (high).
The bones of the face and jaw are expanded laterally (widely) and vertically (upright).
The bones of the chest and abdomen are expanded horizontally (chest/abdominal wall).
The bones of the limbs and hand are expanded laterally (widely) and vertically (tall).
The bones of the foot and leg are expanded laterally (widely) and vertically (upward).
Bones absorb water within their structure to increase their size. As they absorb more water, they become more flexible.
Endochondral ossification is the process by which hyaline cartilage is replaced by bone tissue. The majority of the bones in the skeleton are created in this way. These are known as endochondral bones. Future bones are first created as hyaline cartilage models during this process. Once complete, the model is left inside the body where it will develop into a bone.
Exostoses are benign tumors that can form on any bone in your body. They may be small bumps or large masses of hard tissue. The parts of the body most often affected by exostoses are the hands, feet, arms, legs, chest, and jaw. The tumor itself is made up of dead bone, new blood vessels, and connective tissue. A thin layer of fibrous tissue may also cover the outside of the mass.
Osteosarcoma is the most common type of cancer that starts in the cells that make up bone tissue. It usually occurs in adolescents between 10 and 20 years old. The main risk factor for developing osteosarcoma is having a family member with this disease. Other factors include exposure to radiation, excessive intake of vitamin A, alcohol use, and certain genetic disorders such as Paget's disease and fibrodysplasia ossificans progressiva.
Ewing's sarcoma is a type of cancer that develops from cells that make up bone tissue. It mostly affects children and young adults.