Bones make up the skeletal system of the human body and are responsible for somatic rigidity, storage of different micronutrients, housing bone marrow and producing red blood cells and the various forms of white blood cells, structural outline and movement.
Of the two hundred and six bones in an adult human body, there are several types that are grouped together due to their general features, such as shape, placement and additional properties.
What is a Bone?
A bone is a somatic structure that is comprised of calcified connective tissue. Ground substance and collagen fibers create a matrix that contains osteocytes. These cells are the most common cell found in mature bone and responsible for maintaining bone growth and density. Within the bone matrix both calcium and phosphate are abundantly stored strengthening and densifying the structure.
Each bone is connected with one or more bones and are united via a joint (only exception: hyoid bone). With the attached tendons and musculature, the skeleton acts as a lever that drives the force of movement. The inner core of bones (medulla) contains either red bone marrow (primary site of hematopoiesis) or is filled with yellow bone marrow filled with adipose tissue.
The main outcomes of bone development are endochondral and membranous forms. This particular characteristic along with the general shape of the bone are used to classify the skeletal system. The main shapes that are recognized include long, short, flat, sesamoid and irregular.
Types of Bone
These bones develop via endochondral ossification, a process in which the hyaline cartilage plate is slowly replaced. A shaft, or diaphysis, connects the two ends known as the epiphyses (plural for epiphysis). The marrow cavity is enclosed by the diaphysis which is thick, compact bone. The epiphysis is mainly spongy bone and is covered by a thin layer of compact bone; the articular ends participate in the joints. The metaphysis is situated on the border of the diaphysis and the epiphysis at the neck of the bone and is the place of growth during development. This group of bones includes the humerus, ulna, radius, fibula, tibia, femur, metacarpal bones and phalanges.
A thin external layer of compact bone covers vast spongy bone and marrow, making a shape that is more or less cuboid. The carpal bones and tarsal bones fall into this category.
Two layers of compact bone cover both spongy bone and bone marrow space, they grow by replacing connective tissue. Fibrocartilage covers their articular surfaces. This group is compiled of the skull bones, ribs, sternum and the scapulae.
A thin layer of compact bone covers a mass of mostly spongy bone. This group is not categorized by shape, but by bone content and includes bones of the skull, vertebrae and coxa.
Sesamoid bones are embedded within tendons. They are found at the end of long bones in the limbs, where the tendons cross, for example the patella bone in the knee. Sesamoid bones protect the tendons from excess wear by reducing friction.
Common bone diseases often affect the bone density, e.g. in young children due to malnutrition. For example, rickets is a bone deformity seen in young children who lack vitamin D. Their legs are disfigured and they have trouble walking. The damage is irreversible though surgery may help. Osteomalacia and osteoporosis are diseases seen mainly in adulthood.
Osteomalacia is the improper mineralisation of bone due to a lack of available calcium and phosphate. The bone density decreases and the bones become soft. Osteoporosis has been noted in all ages but mostly in postmenopausal and elderly women. A progressive decrease in bone density increases the risk of fracture. Patients who are on long-term steroid medication are in particular risk.
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