histo 10 bone tissue 1

8/3/2019 Histo 10 Bone Tissue 1

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BONE TISSUE (11 OCT 2011)

NABIHAH MOKHTAR ABD RAHMAN Page 1

Bone is one derivative of CT. The discussion on bones will follow the same fashion as in CT, just

like the previous lecture on cartilage. The mesodermal layer will give rise to mesonchymal cells

(multipotent cells that give rise to different types of CT mothercells just like fibroblast in soft CT,

chondroblast in cartilage ts and osteoblast in the bone ts).

After the osteoblast lay down the ECM, fibers etc, they will be called a cyte; osteocyte which isresponsible in maintainance of the ts. In previous lecture, cartilage ts is a semi-solid support ts consists

of ECM and collagen fibers (collagen type II = hyaline cartilage and elastic cartilage, collagen type I =

fibrocartilage). Osteocyte will secrete ECM around itself and they will be located within space-like called

lacunae.

The main features of bone ts:

1. Vascular with nerve supply2. Have full power of regeneration.

Bone ts is among the actively dividing cells and one of the generative tissues of the body. The bone

ts will continuously undergo remodeling

The functional aspects of the bone ts:

1. Solid support for the body2. Storage of minerals

The most important mineral deposited within the bone ts is the Ca. there is always an active and

dynamic circulation of Ca between bone, blood and GIT system.

3. Production of blood cells in the bone marrow(hemopoiesis: formation of blood cellular components)Bone marrow is a part of bone ts

4. Leverage of motionSkeletal muscle is originated and inserted into the skeletal system. The contraction of muscle willmove the bone and producing movement of the limbs.

5. ProtectionThis differs from the support function. Ie; brain is protected by the skull, the internal organs of

thoracic cavity are protected by the chest wall (ribs, sternum, vertebrae etc), the uterus protected

by the pelvic girdle.

Type of bone ts:

1. OsteoblastOsteoblast are derived from the osteoprogenitor cell/ stem cell which is a mesenchymal cell. Theyare present in the outer membrane covering the outer part of the bone called periosteum and the

inner membrane covering the inner part of the bone called endosteum.

The oestoblast is a very active cell presenting the features of an active cell ie: active RER, prominent

nucleus, GA etc.

Types of cells in regard of the cell cycle:

1. Actively dividing cell2. Facultative dividing cell3. Non-dividing cell

Cartilage ts is surrounded by a CT membrane

called perichondrium except in fibrocartilage.

Fibrocartilage is an articulation cartilage;

located between two hard tissues (the cartilage

itself is solid), thus the presence of membrane is

illogical.

Remodeling: breakdown and buildup of

bone ts throughout life


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They will lay down the ECM and forming the osteoid (a layer between the osteoblast and the bone

formed earlier; not yet calcified), the fibers and then located within the lacunae. From this point,

they are called osteocyte.

Osteoblast functions in new bone formation and also after fractures occur.

2. OsteocyteOsteocyte they are the mature bone cell which maintains the bone ts and located within the

lacunae. The osteoblast has transverse processes anastomose with the other transverse processes

of another osteoblast. They will form a sort of junction complexes that allow the communication

between the osteoblast to perform a function (they cannot perform their function individually). The

canaliculi are the channels that are created after the deposition of ECM on top of the transverse

processes. We have transverse processes on the osteoblast, after the osteoblast lay down the ECM,

these channels created on top of the transverse processes are called canaliculi (contain blood

vessels).

3. OsteoclastThey are multinuclei (up to 50 nuclei in 100 microns), very large cells. Osteoclast is derivative of

monocyte origin (the osteoclast is a phagocyte) that has powerful enzyme to break down collagen

fibers etc in the matrix. The osteoclast is located in the Howship lacunae (as if a ship decked on a

harbor).

The osteoclast function is bone resorption; digesting the dying bone

ts, thus responsible in remodeling. In order to digest the dying bone

ts, they move to the site where the dying bone ts is going to be

digested by moving their membranous extension to attach

themselves to that site which is called the attachment site (the site

for bone digestion). Under the microscope, the part of the osteocyte on the

attachment site looks like a ruffled border(like the edges of teeth-mamelon ^o^).

The activity of all osteoclast is very important for the process of remodeling. Unless

you remove the dead ts, you are not able to replace it with new bone ts.

As mentioned earlier, Ca circulates and the bone is the main storage of Ca. As an

example, the development of skeletal in the fetus will need Ca from the mother. In

case of insufficient level of Ca in the mother, the fetus will take the Ca from thebones of the mother and this will lead to the mother losing their teeth. So the Ca

circulates in a dynamic circulation; from the bone of the mother (remodeling site of

bone in her own body) to the developing bone of the fetus.

Q: What are the cells involved in the actual digestion of dying bone ts?A: Osteoclast.

Remodeling:

Stimulated byPTH (parathyroid gland)

Inhibited bycalcitonin (thyroid gland)

PTH is concerned

with the

circulation of Ca in

the body!

The dynamic

circulation of Ca in

the body is

stimulated by PTH

and inhibited by

calcitonin.


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Bone matrix:

The bone matrix consists of organic and inorganic part. The organic part of the bone

matrix is the collagen fiber (90%) which is the collagen type I. The inorganic part is the

inorganic salts ie: hydroxyapetite of Ca, Mg is going to be deposited in the bone

matrix. 75 % of the dry weight of the bone consists of the mineral salts. The GS(ground substance-GAGs, proteoglycans etc) is also present. When does the

calcification begin?

Bone calcification:

Initially we have the deposition of organic matter proceeded by the deposition of inorganic matter.

Calcification of bones begins after the deposition of the organic matter.

Molecular structure of bone:

The first bone ts formed is immature bone (primary bone) or woven bone.The difference between woven bone and mature bone is that the organic

matrix which is collagen is not lain down in lamellae in the woven bone; it will be like a meshwork. After

a while the collagen will form lamellae which run parallel to each other around the central canal and the

bone will be called lamellar bone (secondary bone).

Bone structure:

The diaphysis is the shaft or the body of the bone and the epiphysis is the white part at

each end of the articulation part. Metaphysis is located between the epiphysis and the

diaphysis. The cartilaginous joint called the epiphyseal growthplate which is essentially

made of hyaline cartilage can be found between the metaphysis and the epiphysis. Theepiphyseal growth plate is the site bone formation.

As an example, in the structure of femur, there are the head, neck etc.

Essentially, in long bone ie: the femur, the shaft consists of two types of bone

structure. The outer plate is the compact or cortical bone while the inner

plate is the spongy / cancellous / trabecular bone.

From the histological perspective, we have the spongy bone and the compact

bone. So the compact bone will form the outer plate of the bone. As usual the

osteoblast will sort of deposit in a matrix and form sheet-like called lamellae.

The dots along the lamellae are the osteocytes which are present within the

lacunae. We have the central canal (Harvesian canal) which is the main

channel that lies in / parallel to the long axis in the center of the bone. Around

it is the lamellae (arrangement of collagen fibers) and in mature bone the

collagen fibers form concentric layers around those Harvesian canal. An

osteon consists of a central Harvesian canal and its concentric layers /

Cartilage: its matrix

and soft CT are

essentially organic

compound (collagen).

Oestoid: The essential

and functional

component of the

bone ts

In tooth sockets, the immature

bone is present. Any work done

eg: dental extraction, fracture

wills not occur.


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lamellae. The Harvesian canals differ in size from each other. The Harvesian canal contains 2 or more

capillaries and some nerve fibers. The general arrangement for the compact bone is having the central

Harvesian canal containing the capillaries and nerve fibers and collagen that form concentric layers

surrounding the canal. The perforating canals (Volkmann canals) provide microvasculature for the

central canals of the osteons and connecting the adjacent osteons horizontally / obliquely.

The outer part of the bone is covered with dense CT called periosteum and endosteum in the inside.

The stem cells can be found along the endosteum and periosteum. Periosteum is a membrane that

provides isolation from surrounding structure, the roots for circulative system (enter and leave) and aids

in broken bone repair (osteoprogenitor cells located within the periosteum). From the pathology

persepective, for a fracture to heal properly the layer of periosteum should be lined edge to edge.

Compact bone has Harvesian system. The trabecular bone (some may be lamellated) but they do not

have the Harvesian system. They simply have their osteocytes in lacunae within a matrix. Between the

trabecule bones, we have the bone marrow (progenitor of different type of blood cells).

Cartilage

-previous week-

1. The chondroblast will lay down the ECM and present in lacunae.2. Main characteristics of cartilage ts:

Avascular(no nerve supply) Surrounded byperichondrium except the fibrocartilage Considered theprecursor of bones in the body (most of the skeleton part in the fetus initially is

formed of cartilage which later converted to bone ts).

Apparently occur / arranged in pairs. As the matrix is being lain down, they are still capable ofundergoing one or two cell divisions.

3. The type of growth in immature cartilage occurs in two types. Interstitial growth is from within eg: a cartilage plate may have the growth in its center. Appositional growth is from the edges / sides. The mature cartilage can only grow in the appositional way. So if there is any injury to an articular cartilage especially in the center, the repair is by the

deposition of fibrous ts. As it is an articular cartilage, when there is deposition offibrous ts /

minerals, the elderly will often complain on the cracking sound of their knee.

histo 10 bone tissue 1

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