Bones

Author: Alexandra Sieroslawska MD • Reviewer: Dimitrios Mytilinaios MD, PhD • Last reviewed: April 06, 2020

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. The bones are essential for the bipedal posture that humans have since the latest major evolutional update.

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.

Key facts
Structure	Cortical bone - outer layer Bone tissue (cancellous bone) - inner layers Medullary canal - contains either red (active) or yellow (inactive) bone marrow
Cellular components	Osteoblasts (bone forming cells), osteocytes (inactive osteoblasts), osteoclasts (cells that reabsorb the bone)
Tissue types	Compact bone (cortical bone) - made of osteons Spongy bone (cancellous bone) - made of bony plates (trabeculae)
Shapes	Long bones - humerus, ulna, radius, fibula, tibia, femur, metacarpal bones, phalanges Short bones - carpal and tarsal bones Flat bones - some of the skull bones, ribs, sternum, scapulae Irregular bones - vertebrae, coxa, some of the skull bones Sesamoid bones - patella
Development	Intramembranous ossification - flat bones: ossification centre formation -> calcification -> trabeculae formation -> periosteum formation Endochondral ossification - long bones: development of a cartilage model -> growth and development of the cartilage model -> development of the primary and secondary ossification centers -> forming of the articular cartilage and epiphyseal plates
Clinical relations	Rickets, osteomalacia, osteoporosis, tumors, fractures

All of the anatomical and important histological facts about the bones, together with the clinical relations, are going to be desrcibed in this article.

Contents

What is a bone?
Types of bone
Clinical aspects
Related diagrams and images

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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
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.

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:

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:

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

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)

Clinical aspects

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.

Bones

What is a bone?