the bone

The Bone
Specialized connective tissue composed of intercellular calcified material( bone matrix)and types of cell (osteocyte , osteoblast, osteoclast) .the bone covered and lined by periosteum & endosteum.
Function
Support for the body, protection of many organs such as crainl thorasic cavities , harbors cavities for bone marrow(where blood cells are formed) , homeostatic regulation of blood calcium , phosphate and other ions , and muscle attachment.
1-Bone matrix
Which consist inorganic matter represent about 60%of the dry weight of bone matrix(calcium ,phosphorous and hydroxyapatite)are especially abundant, but bicarbonate, citrate, magnesium, potassium and sodium)are also found. The organic matter( type I collagen proteoglycans , glycoproteins , growth factor & cytokines) and water. There are space within the matrix called Lacunae (housed the osteocyte) which are connected by little tunnels called canaliculi and this allow contact between osteocytes )which extend processes into canaliculi) and blood vesseles.









Bone cells
Osteoprogenitor cells: Mesenchyme stem cells that divide to produce osteoblasts . Are located in inner, cellular layer of periosteum . Assist in fracture repair .

Osteoblasts: Immature bone cells that secrete matrix compounds (osteogenesis).It lined up side by side resemble to simple epithelial tissue and have cuboidal or polygonal shape and basophilic cytoplasm when it active but it seem to be flatten if activity decline. Osteoblasts may change to osteocytes.









Osteocytes: Flat and almond shaped cell found in cavities (lacunae) between bone matrix layers (which formed during the manufacture of the new matrix osteoid), extend cytoplasmic processes through canaliculi for exchanges between osteocytes and blood capillaries (Because metabolites are unable to diffuse through the calcified matrix of bone).

Osteoclasts: Large multinucleated, branched, motile cells lie within (howship,s lacunae) and belong to the mononuclear phagocyte system. The surface of cells –facing matrix have projections –ruffled border (related to the activity) and release lysosomes into the extracellular space to phagocytic and remodeling. The activity of this cell is controlled by a cytokine called osteoclast-stimulating factor* and hormones .the osteoclast have receptor for calcitonin and thyroid hormone but not for parathyroid .


Bone Membranes
Periosteum : Double-layer cover the externale serfuce of bone,there function is protective , nutritional ,maintain supply of new osteoblast for repair or growth of bone and provide a route for circulatory and nervous supply. Periosteum consists of :
- The outer layer is dense connective tissue, with small blood vessels, collagen bundles, and fibroblasts
-The inner, cellular layer called (Osteoprogenitor cells) which divide by mitosis and differentiate into osteoblast .











Endosteum :
delicate membrane covering internal surfaces of bone. Composed of single layer of flattened Osteoprogenitor cells and very small amount of connective tissue.
Classification of bone
1-According to shape
a- long bones : longer in one dimension than other ,e.g. tibia ,metacarpals
b- Short bones : nearly equal in length and diameter e.g. carpal bones
c- Flat bones :thin and plate like e.g. bones of the skull and sternum .
d- Irregular bones :e.g. vertebrae ,sinuses.





Long bones: consist of
Epiphysis = two expanded ends
Diaphysis = shaft
Metaphysis = portion between the
diaphysis and epiphysis.
Inner portion of bones contain the
medullary cavity.


2-According to histological structure
a- Spongy bone (cancellous bone)
The structure of this type look like sponge, the tissue arranged as trabeculae or spicules ,does not have osteons .Trabeculae have no blood vessels the space in spongy bone contain bone marrow which is highly vascular.


b- Compact bones(dense)
Solid structure, found in the shafts of long bones. The collagen fiber and other component of the bone are arranged as lamellae which exhibit typical forms
1.Haversian system (osteon): structural unit of concentric lamellae of osteocyte and matrix surrounding central canal containing blood and nerve vessels loose connective tissue, and endosteum. Between successive lamellae are lacunae, each with one osteocyte, interconnected by canaliculi containing the cell.
.
2.Outer circumferential lamellae: located beneath the periosteum .
3.Inner circumferential lamellae : located around the marrow cavity.
4.Interstitial lamellae :irregularly shape lamellae lie between the two circumferential lamellae.
There are transverse channels connecting two Haversian canals called Volkmann canals.








Bone Marrow
The space between trabeculae is filled with marrow, which is highly vascular
-Red bone marrow : supplies nutrients to osteocytes in trabeculae , forms red and white blood cells
-Yellow bone marrow : yellow because it stores fat
Location of Hematopoietic Tissue (Red Marrow)
In infants :(Found in the medullary cavity and all areas of spongy bone )
In adults: (Found in the flat bones, and the head of the femur and humerus)
Bone Development : Human bones grow until about age 25
1- Osteogenesis: bone formation
2- Ossification: the process of replacing other tissues with bone
Osteogenesis and ossification lead to:
The formation of the bony skeleton in embryos, bone growth until early adulthood ,bone thickness, remodeling, and repair through life.
Bones development being in the embryo by two distinct processes
1- Intramembranous ossification : is the process by which mesenchymal tissue is directly replaced by bone without an intermediate cartilage step. It occurs most notably in the bones of the skull.
2- Endochondral Ossification: occurs in the embryo when mesoderm initially forms a hyaline cartilage model, which then develops a primary ossification center at the diaphysis .Later, secondary ossification center form at the epiphysis at each end of the bone . Bones that form via endochondral ossification include the humerus ,femur ,tibia and other long bones.

Growth in length of long bones
Occurs at the epiphyseal plate, which includes a number of zones as indicated below
a-the zone of reserve contains resting chondrocytes.
b-the zone of proliferation contains chondrocytes undergoing mitosis and forming isogenous groups.
c- the zone of hypertrophy contains hypertrophied chondrocytes, which secrete alkaline phosphatase to increase calcium and phosphate levels.
d- the zone of calcification contain dead chondrocytes and calcified cartilage matrix called spicules .
e- the zone of ossification contain Osteoprogenitor cells that congregate on spicules and differentiate into osteoblasts. Osteoblasts deposit bone on the surface of spicule to form a mixed spicule , which consists of calcified cartilage matrix and bone .



Growth in diameter of long bones
Occurs at the diaphysis by deposition of bone at the periphery (oppositional growth) as Osteoprogenitor cells within the periosteum differentiate into osteoblasts.

Bone Remodeling
Remodeling is the mechanism by which older bone is replaced by the coordinated action of bone (resorbing the osteoclasts and bone forming osteoblasts. This follows a specific sequence of events, and takes about three months in total to complete:
1- Activation - Osteoblasts induce osteoclasts to break down bone matrix. This process lasts for approximately 3 days.
2- Resorption - In resorption, the ruffled border of the osteoclast forms a sealing zone which isolates the area of bone erosion. Organic acids and lysosomal enzymes dissolve the mineral component and break down the organic matrix. This process occurs at approximately 14 days.
3- Reversal - Osteoblasts begin to replace osteoclasts at the site of bone turnover.
4- Formation - Osteoblasts begin to lay down new lamellar bone on top of old bone. In doing so, cement lines are created to mark the borders between old and new bone matrix. This can take up to 70 days to complete.

Hormonal mechanism
-rising blood ca +2 levels trigger the thyroid to release calcitonin
- calcitonin stimulates calcium salt deposit in bone
* parathyroid hormone binds to receptor in osteoblast this is a signal to produce a osteoclast stimulating factor which increase the number of osteoclast and release ca+ in to blood
Red bone marrow supplies nutrients to osteocytes in trabeculae forms red and white blood cells. Yellow bone marrow yellow because it stores fat, located in the skull,sternum,pelvis & the ends of long bones.
During remodeling of compact bone, osteoclasts act as a cutting cone tunneling into existing bone matrix. Behind the osteoclasts, a population of osteoblasts enters the newly formed tunnel and lines its walls. The osteoblasts secrete osteoid in a cyclic manner, producing layers of new matrix (lamellae) with cells (osteocytes) trapped in lacunae. The tunnel becomes constricted with multiple concentric layers of new matrix, and its lumen finally exists as only a narrow central canal with small blood vessels. The dashed lines in (a) indicate the levels of the structures shown in cross-section (b). An x-ray image (c) shows the different degrees of mineralization in osteons and in interstitial lamellae