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Bone

German: Knochen

1 Definition

Bone or bone tissue describes a notably dense type of connective and supporting tissue which forms the human skeleton. The human body consist of between 208 and 212 bones.

Teeth form their own tissue entity.

2 Background

All vertebrates support their body by their internal skeleton, which is made of a multitude of various bones. The particular bones look differently depending on their localization and their dynamic shaping by functional conditions. Their size varies between the auditory [[ossicle[[s, which amount of only a few milimeters, to the point of the massive human thigh bone (femur).

3 Classification

3.1 ...by development

The Classification of bones by their embryonal development divides bones into the following groups:

  • Dermal bones which are created by intramembranous ossification are formed from connective tissue. This group contains mainly the bones of the skull region, like the flat bones of the skull, mandible, maxilla and clavicles. In the process of ossification minor isles of bone merge into greater and more compact buildings
  • Enchondral bones which are created by enchondral ossification derive from prior cartilaginous tissue. They form the more profound parts of the human skeleton (for example the vertebral column and the bones upper and lower limbs).

3.2 ...by morphology

The Classification of bones by their morphology respectivly their arrangement of tissue divides the tissue into the groups:

  • Substantia spongiosa: Cancellous, also known as trabecular or spongy bone tissue which fills the interior of the bone and
  • Substantia compacta: Compact bone or cortical bone, which forms the cortex of the bone and is stronger than the spongy bone tissue

3.3 ...by shape

Bones are furthermore differentiated by their shape or by their special structure:

Bones which cannot be assigned in any of the categories named above are described as Ossa irregularia.

4 Anatomy

The outer surface of the bone is surrounded by a tight layer of connective tissue, the periosteum. Underneath, the cortical layer of the bone (corticalis) can be found, which consists of substantia compacta. Further inside, the substantia compacta is exchanged by a spongy framework of trabecular bone, the substantia spongiosa or just spongiosa. The cavity in between the trabecular bones forms the medullary cavity (cavum medullare) of the bone. It is lined with a vascular membrane, the endosteum. In the medullary cavity the bone marrow is stored. With increasing age in most bones the red blood-producing bone marrow is exchanged with yellow bone marrow (adipose tissue).

5 Histology

The bone tissue consists of a framework made of living bone cells (osteocytes) which are embedded in a extracellular mineralized organic matrix, the bone matrix. The osteocytes are connected by cellular extensions inside of osseous canals. An own vascular system supplies the osteocytes with nutrients and oxygen. The functional base unit of the bone which is arranged around one central blood vessel is called osteon.

5.1 Cells

There are three different types of cells inside the bone tissue:

  • Osteoblasts: Osteoblasts originate from less differentiated progenitor cells and produce the organic matrix of the bone, the osteoid, as well as alkaline phosphatase, which regulates the mineralisation of the bone.
  • Osteocytes: Osteocytes are mature bone cells, which derive from osteoblasts, which encased themselves with their produced matrix. They communicate via cellular extensions and serve the maintenance of the matrix and the homeostasis of calcium.
  • Osteoclasts: Osteoclasts are polynuculear giant cells, which develop from monocytic stemcell lineages. They are responsible for bone resorption and can be found in so-called resorption pits, which are referred to as Howship's lacunae.

5.2 Extracellular matrix

Regarding the spacial organisation of the extracellular matrix in between of the osteocytes, two types of bone can be differentiated:

  • Woven bone: Regarding the woven bone, collagene fibers of the bone matrix run crossed and are interwoven with each other.
  • Lamellar bone: Lamellar bone is characterized by collagene fibers which run aligned ina parallel way.

5.3 Bone formation

The process of bone formation can be described as ossification or osteogenesis. Bones can be formed by the human body in different ways. The histological classification distinguishes between:

6 Biochemics

The bone matrix consists of 60-70% anorganic minerals, 10-15% water and 20-25% organic substances. Regarding the mineral portion, the major part contains calcium salt in the form of Hydroxylapatite. The portion of organic substances is composed of mainly collagen type 1 and furthermore of proteogylcans and other proteins (e.g. osteonectin, osteocalcin and sialoprotein).

7 Physiology

7.1 Shaping

Bones are the base of shape and statics of the human body. To fullfill this important function, the bone tissue underlies a constantly and dynamically remodeling by reacting to impulses of stress and relief. By this process the firmness and stability of the bone can be adapted to the particular demand.

7.2 Movement

Along with muscles, tendons, ligaments and joints, bones are responsible for biomechanical properties of the human body. Bones serve the transfer and forwarding of impacting force.

7.3 Protection

Another function which is provided by the bones is the protecion of the more sensitive inner organs, e.g. the skull protects the brain, the osseous thorax protects heart and lungs. Furthermore the high densitiy of minerals provides a relative protection against radiation for the internal, very sensitive bone marrow.

7.4 Mineralmetabolism

Bone tissue serves the storage of mineral salts (mainly calcium salt), which can be relased on demand. Moreover it can regulate the pH-value of the blood by producing buffering substances. To a certain extent harmful heavy metal ions can be bonded and eliminated of the bloodstream.

7.5 Hematopoiesis

After finishing the fourth embryonal month (the beginning of the medullar phase) the bone marrow is the most important blood producing organ of the human. The location of Hematopoiesis is nearly exclusively the red bone marrow. Whereas in sucklings red bone marrow can be found all over the bone, it is concentrated to the flat and short bones in adults.

8 Clinical significance

Diseases of the skeletal system and of the bones are topics of osteology, orthopaedics and trauma surgery. However the play a role in other medical fields as well. Among others, some of the most important diseases contain:

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