CHAPTER 6“Bones Tissue”

1. Functions of the skeletal system

2. Classification of bones based on shape

3. General features of bone

4. Bone cells and matrix of bone

5. Compact and spongy bone

6. Bone marrows

7. Bone development

8. Overview of bone growth and remodeling

SKELETAL SYSTEM:

Defined: Includes all of the bones of the human body (total of 206), and their associated cartilages and joints.

Functions:1. Support – supporting framework for body

2. Protection – protects vital organs (brain and thoracic cavity)

3. Movement / levers- allows movement and flexion as well as levers for different movements

4. Mineral storage- calcium and phosphate

5. Hematopoiesis- principal site for blood cell formation in red marrow of flat bones (e.g. sacrum, sternum, etc.)

6. Electrolyte balance of calcium and phosphorus

7. Acid/Base balance buffers the blood calcium phosphate

8. Detoxification by absorbing heavy minerals

Classification based on the “shape”

• Long bones – bones that are longer than they are wide Ex. Femur, tibia, fibula, ulna and humerus

• Short bones – bones that are shaped like a cube; there is also a special class of short bones called sesamoid bones. Ex. Tarsal, carpal, and patella.

• Flat bones – bones that are thin and flat. Ex.Scapula, skull, ribs, and sternum.

• Irregular bones – bones that do not fit in any of the prior categories because they have irregular shapes. Ex. Vertebrae and hip bones.

Shapes of bones

“GROSS” structure of a typical bone

• Articular cartilage: Consists of Hyaline cartilage covering the end of the bone surface where it articulates with another bone, (e.g. femur and tibia, humerus and scapula). Fibrocartilage makes up the menisci of the knee joints.

• Epiphyses: The end of the bone. One at each end of long bones.

• Epiphyseal line: Remnant of the cartilaginous “growth plate” or epiphyseal plate.

Typical bone structure

Bone structure

Periosteum: Tough outer connective tissue covering on bone. Consists of 2 layers; outside is dense irregular CT and deeper layer lined with osteoblast and osteoclast cells. It is richly supplied by blood vessels and nerves and secured to bone by Sharpey’s fibers.

Endosteum: connective tissue covering on inside of bone cavities. Is osteogenic in that it contains osteoblasts and osteoclasts.

Typical bone structure

Bone structure continued

Diaphysis: The shaft of the bone between the two epiphyses. Contains the medullary cavity and is filled with yellow marrow in adults.

Sharpey’s fibers: Bundles of collagenous fibers that tightly attach the periosteum to bony matrix.

Types of Bones cells

Bone is formed and metabolized by specific cells

and is in constant state of remodeling.

• Osteoclasts: Bone destroying cells

“C” means chewing

2. Osteoblasts: Bone generating cells

“B” means building

3. Osteocytes: Mature bone cells, spider shaped and maintain bone tissue

Bone Cells

Bone matrix

• The matrix of bone is made up of organic and inorganic matter.

• The organic portion is of collagen fibers and various proteoglycans, glycosaminoglycans and glycoproteins.

• The inorganic portion is calcium phosphate salts “hydroxyapetite” and calcium carbonate

• The combination of these makes for a bone that is very strong and yet flexible.

Chemical Composition of Bone

• In addition to bone cells, the majority of compact and spongy bone is composed of inorganic molecules.

• These molecules are called hydroxyapatitie and are made of calcium and phosphate.

• The combination of these form a cement like material that gives bone its hardness and strength. In combination with collagen fibers that form the matrix of bones and allows for elasticity and flexibility.

Compact bone termsOsteon/Haversian System: structural unit of

compact bone. Oriented parallel to shaft and forming a group of hollow tubes through which an artery, vein and nerve pass into and through bone.

Lacunae: small cavities (halo’s) containing osteocyte

Osteocyte: true bone cell, spider shaped and found in lacunae at the junctions of the lamellae

Lamellae: layers of the collagen fiber matrix with each layer going in opposite direction to the adjacent layer.

Compact bone termsLamellae may be concentric (forming rings like a tree)or circumferential (encircling the entire bone structure).

Canaliculi: Hair like canales that connect each lacunaeand in turn connect to the Central canal. Remove wastesand bring nutrients into osteocytes

Volkman’s canal/perforating canal: Canals runningperpendicular to the Haversian canals, but connecting tothem. They bring in the artery, vein and nerves to thebone structure.

Spongy Bone

• Spongy bone composes the inner portion of the bone lining the marrow cavity. It has a honeycomb appearance of trabeculae and spicules. Although it looks poorly organized it is designed to withstand the specific stresses put on each bone because of their trabeculae.

• Trabeculae are tiny bone struts or plates that form very strong support structure for the spongy bones. They are irregularly arranged lamellae and osteocytes, but contain no osteons per se as it receives it nutrients from the marrow tissue.

Spongy bone histology

Bone marrow

• Yellow marrow is found in medullary cavity of long bones and is not hematopoietic in adults. Yellow marrow replaces red marrow as we mature and is made up mainly of fat.

• Red marrow is found in the axial skeleton and girdles and in the epiphyses of the femur and humerus and is very active hematopoietically.

Classification based on bone formation

• During embryonic development a “blue-print” for each of our bones is formed from fibrous membranes and/or hyaline cartilage.

• Cartilage is replaced (the cartilage does NOT magically become bone) by bone in one of two ways: 1). Endochondral ossification and 2). Intramembranous ossification

Bone development

• Ossification or osteogenesis

- is the process of forming new bone

• Two methods of ossification:1. Endochondral ossification

2. Intramembranous ossification

Endochondral ossification• The process by which bone is formed from hyaline

cartilage

• Most bones in the body are formed by this method (including the vertebrae, pelvic bones and limb bones).

• Consists of 3 sites of ossification.1. The primary ossification center2. The metaphysis3. The secondary ossification center

Endochondral ossification

• During the first 8 weeks of fetal development, hyaline cartilage forms a model of future bone formation. The center (diaphysis) of the cartilage is “primary ossification site” which contains numerous chondrocytes in lacunae. Surrounding the cartilage model is an outer layer chondrocytes called the “perichondrium” which differentiate into “osteoblast cells” and begin to lay down a bony collar around the site.

• Once the collar is formed the perichondrium becomes periosteum.

Endochondral ossification

Endochondral ossification

• Buds of connective tissue grow from the periosteum into the caritlage and transform the primary ossification site into the primary marrow space. This space is lined with spongy bone.

• The metaphysis forms between the marrow space and the cartilaginous epiphyseal end.-It is considered as a transitional zone where cartilage is formed into bone at each end of the epiphysis. The metaphysis is just beneath the epiphyseal plate where bone growth continues until after adulthood.

The metaphysis

• Consists of five distinct zones:

1. Zone of reserve cartilage

2. Zone of cell proliferation

3. Zone of cell hypertrophy

4. Zone of calcification

5. Zone of bone deposition

The secondary ossification center

• Begins at the time of birth.

• Forms in the epiphysis and develops similarly to the primary ossification center.

• The bone formed in the secondary ossification site persists as spongy bone and growth occurs beneath the outer covering of hyaline cartilage which persists as articular cartilage within the joint cavity on each end of the epiphysis.

Intramembranous ossification

• Bone formed by replacing a fibrous membrane and not from cartilage. Ex. Skull and clavicle

• Basic Overview of Process– during the first 8 wks of embryonic development,

fiberous membranes (CT) form in the areas of future flat bones

– beginning around 8 wks, an “ossification center” forms in the membrane. This center is composed of osteoblasts.

– the osteoblasts begin to secrete hydroxy apetite– the internal spongy bone forms– the external compact bone forms

Bone Growth

• Once the cartilage models of embryonic development are replaced by bone, they must continue to grow through infancy, childhood and adolescence. – Increased length: bones continue to lengthen

because hyaline cartilage remaining in the epiphyseal plates continues to grow. As adulthood approaches, this cartilage becomes less active and is eventually replaced by bone.

Bone growth

Appositional growth - Increased Width: bones continue to widen as osteoblasts form more layers of bone around the outside and osteoclasts break down some of the bony matrix inside.

• Why would bone need to be broken down inside as it grows outside?

Control of Bone Growth

• Bones increase in length and width because of the influence of minerals, vitamins, and hormones in the body.

• Calcium and phosphate are necessary for calcification

• Vitamins A, C and D promote bone growth.

• The specific hormones which affect growth are growth hormone (GH), thyroid hormone (T3 and T4), and the sex steroids (estrogen and tesstosterone).

BONE REMODELING

• Even though the bones in an adult do not continue to grow as described above, they are constantly being remodeled. This means that bone is always being broken down by osteoclasts and reformed by osteoblasts (really no different from remodeling ones home).

• Each week we turn over about 5% of our bone mass.

Control of Remodeling

Two major factors influence remodeling.

• Calcium levels - our bodies need a homeostatic level of calcium in the blood for all cells to function properly.

• Mechanical stress - the varied activities of life puts different stresses on each bone as we age, which requires slight adjustments to compensate for these stresses.