Teeth form their own tissue entity.
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).
The Classification of bones by their embryonal development divides bones into the following groups:
The Classification of bones by their morphology respectivly their arrangement of tissue divides the tissue into the groups:
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.
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).
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.
There are three different types of cells inside the bone tissue:
Regarding the spacial organisation of the extracellular matrix in between of the osteocytes, two types of bone can be differentiated:
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:
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).
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.
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.
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.
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.
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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