Basics of skeletal system

The Skeletal System

Topics to be covered Functions of Bone and Skeletal System Structure of Bone Histology of Bone Tissue Blood and Nerve Supply of Bone Bone Formation Bone’s Role in Calcium Homeostasis Aging and Bone Tissue

Functions of Bone and Skeletal System4. Mineral Homeostasis:

Calcium balance: contribute to bone strength; stores 99% of body’s calcium;

Phosphorus balance: stored in bones and releases it on demand of the body

Histology of Bone Tissue Four types of cells are present in bone tissue

Osteogenic cells Undergo cell division; the resulting cells develop

into osteoblasts Osteoblasts

Bone-building cells Synthesize extracellular matrix of bone tissue

Osteocytes Mature bone cells, maintains bone tissues Exchange nutrients and wastes with the blood

Osteoclasts Release enzymes that digest the mineral

components of bone matrix (resorption) Regulate blood calcium level

Osteogenic cell

Osteoblast

Osteocytes

Osteoclast

Bone Formation The process by which bone forms is called

ossification or osteogenesis

Bone formation occurs in four situations:1. Formation of bone in an embryo2. Growth of bones until adulthood3. Remodeling of bone4. Repair of fractures

1. Formation bone

1. Development of the ossification center. At site where bone develops specific chemical

messages cause mesenchymal cells to aggregate and differentiate, first into osteogenic cells then osteoblasts.

1

Blood capillary

Ossification center

Mesenchymal cell

Osteoblast

Development of ossification center

Collagen fiber

2. Calcification: Within a few days calcium and other mineral salts

are deposited and extracellular matrix becomes calcified.

1. Formation bone

Calcification2

Osteocyte in lacuna

Canaliculus

Osteoblast

Newly calcified bonematrix

3. Formation of trabeculae: Extracellular matrix develops into trabeculae that

fuse with each other to form spongy bone Blood vessels grow into the spaces Connective tissue associated with blood vessels

differentiates into red marrow

1. Formation bone

Mesenchymecondenses

Blood vessel

Spongy bonetrabeculae

Osteoblast

Formation of trabeculae3

1. Intramembranous ossification4. Development of the periosteum:

In conjunction with formation of trabeculae, the mesenchyme condenses at the periphery

Eventually a thin layer of compact bone replaces the spongy bone, but spongy bone remains in the center

Much of the new bone is remodeled as the bone is transformed into adult shape and size

Periosteum

Spongy bone tissue

Compact bone tissue

Development of the periosteum4

Bone Growth During Infancy, Childhood and Adolescence

Growth in Length The growth in length of long bones

involves four major events:

1. Growth of cartilage on the epiphyseal plate

2. Replacement of cartilage by bone tissue in the epiphyseal plate

3. The activity of the epiphyseal plate is the way bone can increase in length

4. At adulthood, the epiphyseal plates close and bone replaces all the cartilage leaving a bony structure called the epiphyseal line

Bone Growth During Infancy, Childhood and Adolescence

Growth in Thickness Bones grow in thickness at the outer surface

Remodeling of Bone Bone forms before birth and continually renews

itself The ongoing replacement of old bone tissue by new

bone tissue Old bone is continually destroyed and new bone is

formed in its place throughout an individual’s life

Factors Affecting Bone Growth and Bone Remodeling

Normal bone metabolism depends on several factors Minerals

Large amounts of calcium and phosphorus and smaller amounts of magnesium, fluoride, and manganese are required for bone growth and remodeling

Vitamins Vitamin D helps build bone by increasing the absorption

of calcium from foods in the gastrointestinal tract into the blood

Vitamins K are also needed for synthesis of bone proteins

Bone’s Role in Calcium Homeostasis Bone is the body’s major calcium reservoir

Levels of calcium in the blood are maintained by controlling the rates of calcium resorption from bone into blood and of calcium deposition from blood into bone

Both nerve and muscle cells depend on calcium ions (Ca2+) to function properly

Blood clotting also requires Ca2+

Many enzymes require Ca2+ as a cofactor

Bone’s Role in Calcium Homeostasis Actions that help elevate blood Ca2+ level

Parathyroid hormone (PTH) regulates Ca2+ exchange between blood and bone tissue PTH increases the number and activity of osteoclasts PTH acts on the kidneys to decrease loss of Ca2+ in the

urine PTH stimulates formation of calcitriol a hormone

that promotes absorption of calcium from foods in the gastrointestinal tract

Bone’s Role in Calcium Homeostasis Actions that work to decrease blood Ca2+ level

The thyroid gland secretes calcitonin (CT) which inhibits activity of osteoclasts

The result is that CT promotes bone formation and decreases blood Ca2+ level

Accelerates uptake of calcium and phosphate into bone matrix

Exercise and Bone Tissue Bone tissue alters its strength in response to

changes in mechanical stress Under stress, bone tissue becomes stronger through

deposition of mineral salts and production of collagen fibers by osteoblasts

Unstressed bones diminishes because of the loss of bone minerals and decreased numbers of collagen fibers

The main mechanical stresses on bone are those that result from the pull of skeletal muscles and the pull of gravity

Weight-bearing activities help build and retain bone mass

Aging and Bone Tissue The level of sex hormones diminishes during

middle age, especially in women after menopause

A decrease in bone mass occurs Bone resorption by osteoclasts outpaces bone

deposition by osteoblasts

Female bones generally are smaller and less massive than males Loss of bone mass in old age has a greater adverse effect

in females

Aging and Bone Tissue There are two principal effects of aging on bone tissue:

1) Loss of bone mass Results from the loss of calcium from bone matrix The loss of calcium from bones is one of the symptoms in osteoporosis

2) Brittleness Results from a decreased rate of protein synthesis Collagen fibers gives bone its tensile strength The loss of tensile strength causes the bones to become very brittle and

susceptible to fracture