Bone tissue is a particularly strong connective and supporting tissue. It forms the human skeleton. There are between 208 and 212 bones in the body that are made up of bone tissue.
What is bone tissue?
Bones are made up of different tissues. Bone tissue is what gives bones their stability. It belongs to the connective and supporting tissues and consists of bone cells, among other things. According to the spatial arrangement, bone tissue can be distinguished between woven bones and lamellar bones. When a bone breaks, it is called a fracture.
Anatomy and structure
Bone tissue is composed of bone cells embedded in a bone matrix. Bone cells are also called osteocytes. Osteocytes are mononuclear cells and arise from osteoblasts, which become embedded during bone development. Osteoblasts are cells that are responsible for bone formation. The bone matrix consists of 25% water, 30% organic matter and 45% inorganic matter. In turn, 95 % of the organic substances consist of type 1 collagen and 5 % of so-called proteoglycans. Proteoglycans are glycosylated glycoproteins that serve to stabilize the osteocytes. Non-collagenous proteins such as osteonectin, osteopontin or osteocalcin are also part of the organic bone matrix in small proportions. The collagen of the organic matrix forms tensile collagen fibrils. Hydroxyapatite crystals are attached to these. To a small extent, citrate molecules are also incorporated in the bone. Depending on how the collagen fibrils are spatially arranged, the bone is referred to as braided bone or lamellar bone. In braided bone, the bone cells are irregularly distributed. The collagen fibers are aligned in bundles. Braided bones are rather rare in the human body. They occur only in the petrous bone, in the ossicles, and at the edges of the cranial sutures. Lamellar bones consist of several layers. In these layers, the collagen fibrils are aligned in the same way.
Function and tasks
The bone tissue provides stability to the bone. Bones, in turn, provide stability throughout the body. At first glance, one would not suspect that this robust tissue is constantly undergoing remodeling processes. Mathematically, a person receives a completely new skeleton almost every seven years. These dynamic processes make bone incredibly adaptable. Bone tissue must be so adaptable because it is constantly exposed to new stresses. For example, bones become thicker through exercise or heavy weight. In contrast, they become thinner and weaker with lack of exercise and stress. In the case of bone defects (e.g. fractures), remodeling processes take place to a greater extent. The osteoclasts and osteoblasts are responsible for these formation and degradation processes. Old and superfluous bone tissue is dissolved by the osteoclasts. This creates a temporary gap in the bone ridges. The osteoblasts move in and fill this gap with new bone tissue. In healthy bone metabolism, there is a balance between bone formation and bone resorption. Osteoblasts and osteoclasts are in constant exchange with each other. For example, osteoblasts can produce substances that increase or slow down the activity of osteoclasts. If the cooperation between osteoclasts and osteoblasts is impeded, various diseases can develop.
Diseases
In osteoporosis, osteoclasts work more. The osteoblasts can no longer fill the resulting gaps with bone substance. The bones become porous. This is why osteoporosis is also popularly known as bone loss. The reduced bone density increases the risk of bone fracture. Osteoporosis can be divided into primary and secondary osteoporosis. Primary osteoporosis occurs without any identifiable causes. This form is mostly found in women of older age. After menopause, the risk of disease increases. Secondary osteoporosis is a concomitant disease in other diseases. Endocrine causes affect the hormonal system. Thus, secondary osteoporosis may occur in the context of Cushing’s syndrome or hyperparathyroidism. However, osteoporosis can also be caused by disturbances in bone metabolism. Such metabolic causes include homocysteinuria or diabetes mellitus. Many drugs also have side effects on the bone system.These drugs include, for example, glucocorticoids, heparin or laxatives. Osteoporosis also occurs in tumor diseases of the bone system. Initially, the disease is completely asymptomatic. Only in later stages of the disease do symptoms become noticeable. There is back pain, hunchback, loss of height and also increased bone fractures. Osteomalacia is also a disease that affects the bone tissue. Here the mineralization of the bone is disturbed. In children, osteomalacia is called rickets. In most cases, the disease is caused by vitamin D deficiency. Disturbances in vitamin D metabolism can also cause osteomalacia. The leading symptom of the bone disease is generalized bone pain. These are often misinterpreted as rheumatic complaints. The pain particularly affects the thorax, spine, and thighs. The x-ray findings are similar to the findings of osteoporosis. A fracture is said to occur when there is a complete or partial disruption of continuity of the bone tissue. This severance results in loss of bone stability. The symptoms of a bone fracture are called fracture signs. Uncertain fracture signs include pain, swelling, bruising, and limited motion. Sure fracture signs include axial misalignment of the bone, rubbing noises, abnormal mobility and, in the case of an open fracture, visible bone fragments. Especially in children, so-called greenstick fractures may occur. Bone growth is not yet complete at a younger age, so that the bone can react with an elastic deformation when a particular force is applied. The bone buckles, but without damaging the periosteum.
