STRUCTURE AND FUNCTIONS OF SKELETAL SYSTEM

OSTEOLOGY-2

Gross structureof an adult long bone

• Gross examination of the longitudinal and transverse section of the long bone shows the following features.

• 1. Shaft: From without inwards it is composed of periosteum, cortex and medullary cavity.

• (a) Periosteum is a thick fibrocellular membrane covering the surface of the bone.It is madeup of an outer fibrous and inner cellular layer. The inner cellular layer is osteogenic in nature. Periosteum is united to

• The underlying bone by collagen fibers, called sharpey’s fibres. This union is particularly strong over the attachments of tendons and ligaments. At the articular margins the periosteum is continuous with the capsule of the joint. The periosteal arteries nourish the outer part of the underlying cortex also. Periosteum has a very rich sensery innervation which makes it the most sensitive part of the

• Bone.• (b) Cortex is made up of compact bone which gives

it the desired strength to withstand all possible mechanical strains.

• (Medullary cavity is filled with red or yellow bone marrow. At birth the marrow is red everywhere with widespread active hemopoiesis. As the age advances, the red marrow at many places atrophies and is replaced by yellow, fatty marrow with no

• Power of hemopoiesis. The red marrow persists in cancellous ends of long bones. In the sternum,ribs iliac crest, vertabrae and skull bones the red marrow is found through life.

• 2. The two ends of the long bones are made up of the cancellous bone covered with hyaline cartilage.

Parts of a growing long bone

• A typical long bone ossifies in three parts, the two ends from secondary centres, and the intervening shaft from a primary centre. Before ossification is complete the following parts of a bone can be defined.

• 1. Epiphysis• The ends and tips of a bone which ossify from

secondary centre are called epiphysis. These are of the following types.

Types of epiphysis

• (a) Pressure epiphysis: pressure epiphysis develops at the articular ends of long bones. These epiphysis take part in transmission of weight and develop due to pressure of an adjascent articulating bone. Examples are head of the femur, lower end of radius, etc.

• (b) Traction epiphysis: Traction epiphysis is nonarticular and does not take part in transmission of weight. It always provides attachment to one or more tendons which

• Exert a traction on the epiphysis. The traction epiphysis ossify later than the pressure epiphysis. Examples: trochanters of femur and tubercles of humerus.

• Atavistic epiphysis: atavistic epiphysis is phylogenetically a separate bone which in man becomes fused to another bone. Examples: coracoid process of scapula and os trigonum or lateral tubercle of talus.

• Abberant epiphysis: these are unusually present,e.g; epiphysis of the head of the first metacarpal and the base of other metacarpal bones.

• Composite epiphysis: Some parts of bones are formed from multiple secondary centres;one of them is pressure epiphysis while others are traction epiphysis. All these centres coalesce before uniting with the primary centre, e.g; upper ends of humerus, femur ,ribs and vertebrae.

Functions of skeletal system1. Support. The skeleton serves as the structural framework for the body by supporting

soft tissues and providing attachment points for the tendons of most skeletal muscles.

2. Protection. The skeleton protects the most important internal organs from injury. For example, cranial bones protect the brain, vertebrae (backbones) protect the spinal cord, and the rib cage protects the heart and lungs.

3. Assistance in movement. Most skeletal muscles attach to bones; when they contract, they pull on bones to produce movement.

4. Mineral homeostasis (storage and release). Bone tissue stores several minerals, especially calcium and phosphorus, which contribute to the strength of bone. Bone tissue stores about 99% of the body’s calcium. On demand, bone releases minerals into the blood to maintain critical mineral balances (homeostasis) and to distribute the minerals to other parts of the body.

5. Blood cell production. Within certain bones, a connective tissue called red bone marrow produces red blood cells, white blood cells, and platelets, a process called hemopoiesis. Red bone marrow consists of developing blood cells, adipocytes, fibroblasts, and macrophages within a network of reticular fibers. It is present in developing bones of the fetus and in some adult bones, such as the hip bones, ribs, breastbone, vertebrae (backbones), skull, and ends of the bones of the arm and thigh.

6. Triglyceride storage. Yellow bone marrow consists mainly of adipose cells, which store triglycerides. The stored triglycerides are a potential chemical energy reserve. In a newborn, all bone marrow is red and is involved in hemopoiesis. With increasing age, much of the bone marrow changes from red to yellow.

7. Respiration and sound transmission: ribs have some protective function for the thoracic viscera, but respiration would have been tedious if not impossible without ribs.

• Muscle attached to skull are mandatory for chewing. • Ossicles small bones inside the middle ear cavity are important factor for sound

transmission from external ear to internal ear.• Speech, a hallmark of human race is impossible without skeletal element to which

speech muscle s are attached. 8. Body defense WBC are important to combat bacterial infections and prepare

antibodies against various antigens. These cells are also produced in the bone marrow.

• Reticulo-endothelial system consist of wide spread phagocytic cells incorporated in the walls of the capillaries in many organs e.g. liver spleen, etc. and bone marrow possesses sizeable part such cells in its sinusoids thus contributing to first defense line of the body

Uses made of dead bones1. Height Determination2. Age determination3. Sex determination4. Race determination5. Anthropometery 6. Dating of human Fossilsa) Carbon Datingb) Potassium Dating

STRUCTURE OF BONE:2. The diaphysis is the bone’s shaft or body—the long, cylindrical, main

portion of the bone.The diaphysis ossifies in a primary centre.Microscopically, the diaphysis differs from the shaft of adult long bones in having no true haversian system and its surface presents collagenous bundles of fibres which are continuous with periosteum. Thus microscopically it is a fibrous bone.

3. The metaphyses are the regions between the diaphysis and the epiphyses.In a growing bone, each metaphysis contains an epiphyseal (growth) plate, a layer of hyaline cartilage that allows the diaphysis of the bone to grow in length. When a bone ceases to grow in length at about ages 18–21, the cartilage in the epiphyseal plate is replaced by bone; the resulting bony structure is known as the epiphyseal line.

4. The articular cartilage is a thin layer of hyaline cartilage covering the part of the epiphysis where the bone forms an articulation (joint) with another bone. Articular cartilage reduces friction and absorbs shock at freely movable joints.

IMPORTANCE OF METAPHYSIS

• 1.Growth activities are predominantly marked in this area.

• 2.Most of the muscles of the joint are inserted in this area.

• 3.This is the most highly vascular area of the long bone

STRUCTURE OF BONE contd5. The periosteum surrounds the external bone surface wherever it is

not covered by articular cartilage. It is composed of an outer fibrous layer of dense irregular connective tissue and an inner osteogenic layer that consists of cells. Some of the cells of the periosteum enable bone to grow in thickness, but not in length. The periosteum also protects the bone, assists in fracture repair, helps nourish bone tissue, and serves as an attachment point for ligaments and tendons. It is attached to the underlying bone through perforating (Sharpey’s) fibers, thick bundles of collagen fibers that extend from the periosteum into the extracellular bone matrix.

6. The medullary cavity or marrow cavity is a hollow, cylindrical space within the diaphysis that contains fatty yellow bone marrow in adults.

7. The endosteum is a thin membrane that lines the internal bone surface facing the medullary cavity. It contains a single layer of cells and a small amount of connective tissue.

COMPOSITION OF BONE

Like other connective tissues, bone, or osseous tissue, contains an abundant extracellular matrix that surrounds widely separated cells. The extracellular matrix is about 25% water, 25% collagen fibers, and 50% crystallized mineral salts. The most abundant mineral salt is calcium phosphate [Ca3(PO4)2]. It combines with another mineral salt, calcium hydroxide [Ca(OH)2], to form crystals of hydroxyapatite [Ca10(PO4)6 (OH)2].

• As the crystals form, they combine with still other mineral salts, such as calcium carbonate (CaCO3), and ions such as magnesium, fluoride, potassium, and sulfate. As these mineral salts are deposited in the framework formed by the collagen fibers of the extracellular matrix, they crystallize and the tissue hardens. This process, called calcification, is initiated by bone-building cells called osteoblasts (described shortly).

• The process requires the presence of collagen fibers. Mineral salts first begin to crystallize in the microscopic spaces between collagen fibers. After the spaces are filled, mineral crystals accumulate around the collagen fibers.

Cells of Bone tissue

Four types of cells are present in bone tissue:• Osteogenic cells, • Osteoblasts, • Osteocytes, and • Osteoclasts

• 1. Osteogenic cells are unspecialized stem cells derived from mesenchyme, the tissue from which almost all connective tissues are formed. They are the only bone cells to undergo cell division; the resulting cells develop into osteoblasts. Osteogenic cells are found along the inner portion of the periosteum, in the endosteum, and in the canals within bone that contain blood vessels.

• 2. Osteoblasts are bone-building cells. They synthesize and secrete collagen fibers and other organic components needed to build the extracellular matrix of bone tissue, and they initiate calcification. As osteoblasts surround themselves with extracellular matrix, they become trapped in their secretions and become osteocytes. (Note: The ending -blast in the name of a bone cell or any other connective tissue cell means that the cell secretes extracellular matrix.)

3. Osteocytes mature bone cells, are the main cells in bone tissue and maintain its daily metabolism, such as the exchange of nutrients and wastes with the blood. Like osteoblasts, osteocytes do not undergo cell division.

• 4. Osteoclasts are huge cells derived from the fusion of as many as 50 granulocytes-macrophage precursor cells. they are concentrated in the endosteum. On the side of the cell that faces the bone surface, the osteoclast’s plasma membrane is deeply folded into a ruffled border. Here the cell releases powerful lysosomal enzymes and acids that digest the protein and mineral components of the underlying bone matrix. This breakdown of bone extracellular matrix, termed resorption is part of the normal development, maintenance, and repair of bone. In response to certain hormones, osteoclasts help regulate blood calcium level. They are also target cells for drug therapy used to treat osteoporosis.

• Bone is not completely solid but has many small spaces between its cells and extracellular matrix components. Some spaces serve as channels for blood vessels that supply bone cells with nutrients. Other spaces act as storage areas for red bone marrow. Depending on the size and distribution of the spaces, the regions of a bone may be categorized as compact or spongy. Overall, about 80% of the skeleton is compact bone and 20% is spongy bone.