Nada M.AL-Khafaji General histology // secondstage Lecturer
The Bone
Specialized hard connective tissue characterized by mineralized extracellular matrix .
Function
Support , protection , homeostatic regulation of blood calcium , blood cell formation and muscle attachment.
Histology Composed of
1-Bone matrix
Which consist of proteoglycans , glycoproteins ,hydroxyapatite (Caphosphatecrystals )& water. There arespace within the matrix called (Lacunae) which are connected by little tunnels called canaliculi and this allow contact between osteocytes which extend processes into canaliculi.
2-Fibers: consist of I collagen give the hardness to the bone.
3-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)have cuboidal or polygonal shape found in the endosteum. Osteoblasts may change to osteocytes and exclusively located at the surface of bone.
Osteocytes:Mature bone cells that maintain the bone matrix Located in lacunae (single lacuna), extend cytoplasmic processes through canaliculi for nutrition .it is maintain matrix. Maintain protein and mineral content of matrix help repair damaged bone.The death of osteocytes will lead to bone resorption.
Osteoclasts: Large multinucleated, branched, motile cells and belong to the mononuclear phagocyte system. The surface –facing matrix have projections –ruffled border (related to the activity) their cytoplasm is acidophilic with many lysosomes and many mitochondria, found in howship, slacunae. The activity of this cell is controlled by a cytokine called osteoclast-stimulating factor. The activity is stimulated by parathyroid while calcitonin hormone is act directly to osteoclast and inhibited the activity. This cells are release lysosomes into the extracellular space to phagocytic and remodeling.
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 skulland 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 the histology
The bone can be divided into two type
according to the structure:
a-Spongy bone (cancellous bone)
The structure of this type look like sponge,the tissue arranged as trabeculae or spicules,does not have osteonsTrabeculae have no blood vesselsthe space in spongy bone contain bone 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, located in the skull,sternum,pelvis & the ends of long bones.
b-Compact bones(dense)
A lamellar orcortical bone, solidstructure, found in the shafts of long bones. The structural unit are calledosteon (Haversian system), concentric lamellae of osteocyte and matrix (outer, inner &interstitial lamellae) and 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 Membranes
Periosteum – double-layered protective membrane ,covers all bones, except parts enclosed in joint capsules made up of:
-outer, fibrous layer (tissue)
-inner, cellular layer (Osteoprogenitor cells)
-collagen fibers of the periosteum extend directly into the bone (sharpey,s fibers)
-Contains osteoblasts responsible for bone growth in thickness.
Functions of Periosteum
1. Isolate bone from surrounding tissues
2. Provide a route for circulatory and nervous supply
3. Participate in bone growth and repair
Endosteum – delicate membrane covering internal
surfaces of bone, incomplete cellular layer:
1-linesthe marrow cavity
2-covers trabeculae of spongy bone
3-lines central canals
4-contains osteoblasts, Osteoprogenitor cells, and
osteoclasts
5- Is active in bone growth and repair
Blood vessels and nerves
Blood vessels and nerves are present in Haversian canals and Volkmann, s canals, they are absent in lacunae and canaliculi.
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, whichincludes a number of zonesas indicated below
a-the zone of reserve contains resting chondrocytes.
b-the zone ofproliferation contains chondrocytes undergoing mitosis and forming isogenous groups.
c-the zone ofhypertrophy 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) asOsteoprogenitor cells within the periosteum differentiate into osteoblasts.
Bone Remodeling
In adults, after growth has ceased, bone is formed by the osteoblasts only where it was previously resorbed by the osteoclasts. This follows a specific sequence of events, and takes about three months in total to complete:
1-Activation - In the process of 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, respectively. This process occurs at approximately 14 days.
3-Reversal - Over time, 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
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