Bones

Bones make up the skeletal system of the human body and are responsible for somatic rigidity, storage of different micronutrients, and housing bone marrow. They also produce red blood cells and the various forms of white blood cells and provide 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.

Bone matrix - histological slide

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.

Bone marrow - histological slide

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
• irregular

Types of Bone

Long bones

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.

Humerus - ventral view

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
• phalanges

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 carpal bones and tarsal bones fall into this category.

Hamate bone - ventral view

Flat bones

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
• scapulae

Sternum - ventral view

Irregular bones

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 the

• bones of the skull
• vertebrae
• coxa

Vertebral column (Fourth cervical vertebra) - ventral view

Sesamoid bones

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.

Patella - lateral-right view