Histology of bone tissuepremed II

Bone- specialised connective tissue composed of intercellular calcified material

-Specialised connective tissue composed of intercellular calicified material (bone matrix).

Reservoir of calcium, phosphate and other ions that can be released or stored in a controlled fashion to maintain constant concentrations of these important ions in the body fluids.

Bone – General Feature

-shaft- diaphysis

-end epiphysis -junction of epiphysis and diaphysis – metaphysis

-central medullary cavity

-outercovering – periosteum

-inner covering - endosteum

Osteocytes and the lacunae:

-each lacuna contains one osteocyte

- lacunar space extends outwards as hair like projections – canaliculi

-canaliculi of adjacent lacunae communicate and eventually with the Haversian canal.

-osteocytes gives of cytoplasmic processes that occupy the canaliculi of the contained lacuna; cytoplasmic processes of the osteocytes communicate with each other through gap junctions.

Since metabolites are unable to diffuse through the calcified matrix of bone,the exchange between osteocytes and blood capillaries depend on communication through canaliculi, thin cylindrical spaces that perforate the matrix.

Cytoplasmic processes of osteocytes of adjacent make contact by gap junctions.

Spongy Bone

- cancellous bone: network of bony trabeculae separated by interconnecting spaces

- no Haversian system

Periosteum: fibrous covering of bone contain osteoprogenitor cells-

not present in articular surfaces

Attached to bone substance by Sharpey’s fibres

Sharpey’s fibres and attachment of muscle to bone

Endosteum lines all inner cavities within the bone and is composed of a single layer of flattened osteoprogenitor cells and very small amount of connective tissue.

The endosteum is therefore considerably thinner than the periosteum.

The principal functions of periosteum and endosteum are nutrition of osseus tissue and provision of continuos supply of new osteoblasts for repair or growth of bone.

A 5-year-old boy falls off his bike and fractures his humerus. He is taken to the emergency room, and the bone is set by one of the emergency room physicians. Which of the following is responsible for producing the majority of the new bone that will reunite the two fragments? A. Cancellous bone B. Cartilage C. Compact bone D. Marrow E. Periosteum

Forms of bone tissue

1.Embryonic bone / primary bone/woven bone/non-lamellar bone

2. Adult bone/ secondary bone /lamellar bone

This is the first bone to appear in embryonic development and in fracture repair or other repair processes.

Woven bone:

-formed during development and repair

-randomly arranged collagen fibre bundles

-more cellular; randomly arranged

-show intense staining with basic stain – abundant ground substance

-found in adult where remodeling occurs

-regular feature of alveolar socket

Adult bone: lamellar bone-regular arrangement of collagen fibres as sheets (lamellae)

-cells oriented with their long axis parallel to the long axis of the fibre bundles

-harder than the woven bone

-all mature bone are lamellar in type

-Osteoblasts are synthesizing collagen matrix,

Which forms a strand of matrix to trap cells,

As this occurs, the osteoblasts gradually

-Differentiate to become osteocytes.

Bone tissue – structure

-is a specialised connective tissue for support, movement, to provide area for muscle attachment, form protective case for vital organs, give form and shape to the body, form store house for minerals

-Components:

-organic: extracellular matrix

- ground substance : proteoglycans (chondroitin sulphate)

-fibres – collagen type I

-cells: -osteocytes (bone cells)

- osteoblasts (bone + germ) bone forming cells

-osteoclasts (bone + broken) for bone resorption

**inorganic components of bone also depends on the presence of viable osteoblasts

Formed bone

osteoid

osteoblasts

marrow

Bone formation by osteoblasts

Osteoclasts:

-large giant cell belong to mononuclear phagocyte system

-developed from bone marrow

-motile cells

-multinucleated

- lie in Howship’s lacuna on the surface of the bone

Osteoclast activity is controlled by local signaling factors and hormones. Osteoclasts have receptorsfor calcitonin, a thyroid hormone, but not for parathyroid hormone.

Osteoblasts activated by PTH produce a cytokine called osteoclast stimulating factor. Thus, activityof these two cells is coordinated and both are essential in bone remodeling.

presents ruffled borders (small cytoplasmic infoldings) on the surface adjoining the bone

-cytoplasm adjoining the ruffled border is clear and devoid of organelle but rich in actin filaments (zone of attachment of the osteoclasts to the bone surface)

--secrete collagenase that dissolves collagen fibres and liberate calcium from bone

-acidic microenvironment that is created along the ruffled border by the accumulation of protons; lysosomal enzymes liberated from lysosomes facilitates the dissolution of calcium which is taken by the cell and released into the blood stream.

Hormonal control of osteoclastic activity:

-osteoclasts have receptors for calcitonin (thyroid hormone)

-osteoblasts have receptors for paratharmone (parathyroid hormone)

- when activated by paratharmone,osteoblasts releases cytokine – osteoclasts stimulating factor

-osteoclasts are activated by cytokine and act on bone to liberate minerals

-*bone has 99% of the body calcium in it; liberation and storage of which depends on the blood content of it

-both osteoblasts and osteoclast play a balanced role in mineralisation and demineralisation of the bone to maintain blood calcium level – calcium homeostasis

-What is the role of calcitonin on osteoclasts?

- Causes for abnormal bone formation:

-1. Osteomalacia: soft bone due to defective mineralization leading to reduction in calcium / organic matrix ratio

- bone becomes transparent to X-rays

-I). newly formed bone is not calcified

-II). decalcification of already formed bone

- III). may lead to abnormal deposition of calcium in kidney and blood vessel

- a. dietary deficiency of calcium

- b. hyperparathyroidism

- c. Vitamin D deficiency – leading failure of calcium uptake by the intestine

Osteoporosis:

-decrease in bone mass due to insufficient bone formation or increased bone resorption by osteoclasts.

- calcium/ organic matrix ratio is normal but the overall mass of the bone is reduced.

Osteopetrosis: marble bone

- normal bone formation with absence of osteoclastic activity resulting in overgrowth and thickening of bone; may be fatal as blood formation by the bone marrow may be impaired.

Hormonal regulation of bone formation:

1. Paratharmone (for calcium mobilisation from bone) and calcitonin (inhibitory action on osteocalsts) for balanced mineralisation of bone and to maintain calcium homeostasis

2. Growth hormone (somatotropin) from pituitary stimulates the growth of the epiphyseal cartilage-(IGF-1)

- over secretion causes gigantism in children

- lack of secretion causes dwarfism in children

- over secretion in adult causes acromegaly – abnormal thickening

3. Thyroid hormone – deficiency of which leads to cretinism (dwarfism)

4. Sex hormone : in excess causes retardation of bone growth as epiphyseal cartilage is ossified quickly; if deficient will cause tall stature.

Children who are hypothyroid from birth are called “cretins”. showed by thyroid hormone deficiency in children, associated with dwarfism due to arrested physical development and also mental development.

SEX HORMONES:-Influences the time of appearance and development of ossification centers and accelerate the closure of epiphysis.-Precocious sexual maturity : Caused by sex hormone producing tumors, retards body growth since the epiphyseal cartilage is quickly replaced by bone(closure of epiphysis).

In hormonal deficiencies caused by castration or by abnormal development of gonads, the epiphyseal cartilage remains functional for a longer period of time, resulting in a tall stature.

Pathology:

1. Rickets – (in children) calcium deficiency- matrix does not calcify and deformation of already formed bone

2. Osteomalacia- soft bone (in adult)- decrease in amount of calcium per unit of bone matrix

3. Osteoporosis: reduction of both matrix and mineral level due to excessive resorption of bone ; develops as a consquence of immobilisation and postmenopausal women

4. Osteopetrosis: lack of osteoclastic activity resulting in defective resorption and so dense, heavy bone (marble bone) – genetic disorder which involves absence of ruffled borders.

Parathyroid hormone:Activates and increases the no of (osteoclasts),promoting resortion of the bone matrix with the consequent liberation of calcium.

Calcitonin hormone:-Sythesized mainly by the parafollicular cells of the thyroid gland,inhibits matrix resorption.-Calcitonin has an inhibitory effect on osteoclast activity.

Anaemia and frequent infections can result from depress blood cell formation.Since the concentration of calcium in tissues and blood must be kept constant ,nutritional deficiencies of calcium results in decalcification of bones; decalcified bones are more likely to fracture and are more transparent to x-rays.rickets:due to defc. Of calcium in children. A disease in which bone matrix does not calcify normally and the epiphyseal plate becomes distorted.

Pathology:

1. Rickets – (in children) calcium deficiency- matrix does not calcify and deformation of already formed bone

2. Osteomalacia- soft bone (in adult)- decrease in amount of calcium per unit of bone matrix

3. Osteoporosis: reduction of both matrix and mineral level due to excessive resorption of bone ; develops as a consquence of immobilisation and postmenopausal women

4. Osteopetrosis: lack of osteoclastic activity resulting in defective resorption and so dense, heavy bone (marble bone) – genetic disorder

pubic Symphysis

***HERNIATION OF THE INTERVERTEBRAL DISC*****