Bones make up the skeletal system of the human body. The adult human has two hundred and six bones. There are several types of bones that are grouped together due to their general features, such as shape, placement and additional properties. They are usually classified into five types of bones that include the flat, long, short, irregular, and sesamoid bones.

The human bones have a number of important functions in the body. Most importantly, they are responsible for somatic rigidity,structural outline, erect posture and movement (e.g. bipedal gait). Due to their rigidity, bones are the main 'protectors' of the internal organs and other structures found in the body.

This article will describe all theanatomical and important histological facts about the bones.

A bone is a somatic structure that is composed 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 (e.g. skull bones 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 bones are mainly classified into five types that include:

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.

Some examples of this type of bones include:

The short bones are usually as long as they are wide. They are usually found in the carpus of the hand and tarsus of the foot.

In the short bones, a thin external layer of compact bone covers vast spongy bone and marrow, making a shape that is more or less cuboid.

The main function of the short bones is to provide stability and some degree of movement.

Some examples of these bones are:

In flat bones, the 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 includes the following bones:

The prime function of flat bones is to protect internal organs such as the brain, heart, and pelvic organs. Also, due to their flat shape, these bones provide large areas for muscle attachments.

Due to their variable and irregular shape and structure, the irregular bones do not fit into any other category. In irregular bones, the thin layer of compact bone covers a mass of mostly spongy bone.

The complex shape of these bones help them to protect internal structures. For example, the irregular pelvic bones protect the contents of the pelvis.

Some examples of these types of bones include:

Sesamoid bones are embedded within tendons. These bones are usually small and oval-shaped.

The sesamoid bones are found at the end of long bones in the upper and lower limbs, where the tendons cross.

Some examples of the sesamoid bones are the patella bone in the kneeor the pisiform bone of the carpus.

The main function of the sesamoid bone is to protect the tendons from excess stress and wear byreducing friction.

Learn the basics of the skeletal system with this interactive quiz.

The bones mainly provide structural stability to the human body. Due to the development of the complex bony structures(e.g. spine) the humans are able to maintain erect posture, to walk on two feet (bipedal gait)and for all sorts of other activities not seen in animals.

Due to their rigid structure, bones are key in the protection of internal organs and other internal structures. Some bones protect other structures by reducing stress and friction (e.g. sesamoid bones) while some bones join together to form more complex structures to surround vital organs and protect them (e.g. skull, thoracic cage, pelvis).

Bones also harbor bone marrow which is crucial in production of blood cells in adults. In addition, the bone tissue can act as a storage for blood cells and minerals.

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 mineralization 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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Bones: Anatomy, function, types and clinical aspects | Kenhub