Bone structure

Bone structure

This scene explains the structure and functions of the bones.

Biology

Keywords

bone, scapula, humerus, bone tissue, osteon, periosteum, skeleton, blood cell, tubular bone, flat bone, spongy bone, hydroxyapatite, bone cavity

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Scenes

Bone types

The human skeleton performs a variety of functions which are vital for the body. It supports the body, protects the internal organs, produces blood cells, stores essential minerals, and helps in human locomotion. To perform all these different tasks, the bones have various shapes but a similar basic structure.

Bones can be classified into several groups by their shape. Long bones are elongated bones with a tubular midsection and enlarged ends. Flat bones are thin and resemble shovels or shells. Short bones are cube-like bones located very close to each other, so they allow limited movement. Irregular bones take on various forms, hence their name. Some of them have a cavity inside, filled with air.

In this scene, the humerus and the scapula will be used to present the structure of long bones and flat bones.

Structure of long bones

  • humerus
  • articular cartilage - It covers the articular surfaces of the bones. Cartilage is very durable, it is protected against friction by synovial fluid.
  • spongy bone (cancellous bone) - A porous type of bone. It is more flexible but weaker than compact bone. It is found at the end of long bones, near joints and in flat bones.
  • compact bone (cortical bone) - It comprises the exterior of the bones and accounts for 80% of the skeletal mass. Compact bone is much stronger than spongy bone. The smooth surface and white colour of bones can be attributed to the compact bone.
  • bone marrow - A semi-solid tissue that fills the spaces between the trabeculae of the spongy portion of bones, as well as the medullary cavities of long bones.
  • bone membrane (periosteum) - A connective tissue layer well-supplied with blood vessels and nerves. It plays an important role in increasing the width of bones as well as in the recovery of bones after injuries, by synthesizing new bone tissue.

Bones are surrounded by a layer of connective tissue with veins and nerves, called periosteum. This layer provides the bone tissue with the necessary nutrients. The bone itself consists of a layer of compact bone (cortical bone) and a layer of spongy bone (cancellous bone). Both these layers comprise cellular elements: osteocytes (mature bone cells), osteoblasts (cells that synthesize bone tissue) and osteoclasts (cells that break down bone tissue). These cells are embedded in an extracellular matrix. About 30% of this matrix consists of organic components and about 70% comprises minerals, mostly hydroxyapatite, a calcium phosphate mineral.
Compact bone is made up of lamellae (layers) arranged in concentric circles. The strength of the bones is attributed primarily to the compact bone. Spongy bone has a sponge-like appearance with numerous large spaces between the trabeculae.

The middle, tubular section of long bones contains a medullary cavity. Both the spaces between the trabeculae of spongy bone and the medullary cavity are filled with bone marrow. During infancy, only red bone marrow is present in the bones; it is responsible for the production of blood cells. After the age of 4-5, fatty tissue gradually accumulates in the bone marrow, transforming it into yellow bone marrow. The main function of yellow bone marrow is to store nutrients; it does not produce blood cells. During adulthood, red bone marrow is found only in the spaces between trabeculae of spongy bone.

Structure of flat bones

  • scapula

Unlike long bones, flat bones do not have a medullary cavity. The structure of flat bones resembles a sandwich in which a layer of spongy bone is enclosed between two layers of compact bone. The space between the trabeculae is filled with red bone marrow even in adulthood, therefore flat bones play a key role in the production of blood cells. Another important function of flat bones is the protection of some internal organs.

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Connective tissues

Connective tissues include loose and dense connective tissues, adipose tissue, blood, tendon and bone tissue.

Bones of the lower limbs

Bones of the lower limbs are connected to the trunk by the pelvis.

Bones of the upper limbs

Bones of the upper limbs form the pectoral girdle and the arms.

Human skeleton

Our body´s internal support structure to which skeletal muscles are attached.

Knee joint

The knee joint is made up by the femur, the tibia and the kneecap.

The skull and the spine

The two main parts of the central nervous system, the brain and the spinal cord are protected by the skull and the spinal column.

Types of bone articulations

Human bones are joined together by cartilaginous or synovial joints, sutures or they can fuse together.

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