the bone histology -bone matrix canaliculi · 2018. 3. 28. · histology composed of 1-bone matrix...

Nada M.AL-Khafaji General histology // second stage Lecturer

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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 (Ca phosphate crystals ) &

water. There are space 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


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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, s lacunae. 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.

osteocytes


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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 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 osteons Trabeculae have no blood vessels the 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.


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b- Compact bones(dense)

A lamellar or cortical bone, solid structure, found in the shafts of long bones. The

structural unit are called osteon (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.


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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-lines the 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.


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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 .


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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

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.


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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

the bone histology -bone matrix canaliculi · 2018. 3. 28. · histology composed of 1-bone matrix...

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