Skeletal System

Chapter 7

Bio160

Functions of Bone

• Supports Soft Tissue

• Attachment of skeletal muscles via tendons; when muscles contract, movement results.

• Protection of vital organs such as central nervous system housed in cranial cavity and vertebral column

Functions of Bone

• Hematopoiesis - manufacture of blood cells in red bone marrow (in adults; proximal epiphysis of humerus and femur; ribs, sternum, clavicle, hip bones, vertebrae, skull)

• Storage of triglycerides

• Reservoir of minerals such as calcium and phosphorus

Microscopic Structure of Bone

• Compact Bone

By weight, bone has 25% water, 25% collagen fibers and 50% calcium phosphate

– Collagen fibers produced by osteoblasts, bone-forming cells

– Calcium salts deposit along collagen fibers and cement osteoblasts into a small chamber, a lacuna

Microscopic Structure of Bone

Once osteoblasts become trapped in lacuna, they are known as osteocytes

Cytoplasmic extensions from osteocytes are located in canaliculi, small tubes through the material outside the cell, the extracellular matrix

– Nutrients reach osteocytes via canaliculi and cytoplasmic extensions from Haversian canal

Microscopic Structure of Bone

Haversian canal – longitudinal tube that forms the center of microscopic bone structure

Matrix arranged in concentric rings around Haversian canal called lamella

Haversian canal and surrounding lamella is called Haversian system (osteon)

• Spongy Bone

Bone Formation=Ossification

• Embryonic skeleton composed of fibrous membrane and hyaline cartilage in shape of bones

• Ossification begins 6th week of embryonic life

Bone Formation=Ossification

• Ossification replaces preexisting CT with bone

• Intramembranous formation

occurs in flat bones of skull, part of clavicle

begins at 6 weeks of embryonic life

Osteoblasts secrete collagen within a preexisting fibrous membrane

Bone Formation=Ossification

Spongy bone develops inside the membrane

Surface layers are eventually reconstructed into compact bone because osteoblasts on surface reconstruct bone (much of newly formed bone will be destroyed and reformed)

Bone Formation=Ossification

• Endochondral formation

gives rise to all other bones

hyaline cartilage model is first formed which is replaced by bone

begins at 8 weeks of life

Bone Formation=Ossification

Bony collar – Outside of cartilage model becomes calcified cutting off nutrient supply to tissue underneath

– Cartilage cells begin to die and the interior of the bone breaks down and disappears

Bone Formation=Ossification

Primary ossification center - blood vessels invade the center of the bone, where the cartilage is dying, carrying osteoblasts

– Osteoblasts replace dying cartilage with spongy bone, which eventually is remodeled into compact bone

Bone Formation=Ossification

– The hollow interior of a long bone is formed by another bone cell called an osteoclast

– Osteoclasts secrete acids and enzymes that dissolve the extracellular matrix of bone

Bone Formation=Ossification

Secondary ossification center - more blood vessels enter the ends of a bone, bringing with them osteoblasts which produce spongy bone

– generally secondary centers do not begin until after birth

Bone Formation=Ossification

after the secondary centers have formed, bone tissue completely replaces cartilage except in 2 regions:

– articular cartilage (prevents damage from bone to bone contact within joints)

– epiphyseal plate = metaphysis = growth plate

Bone Growth• Growth in length

Epiphyseal plate

Epiphyseal cartilage stops dividing and is replaced by bone at puberty with surge of hormones; what remains is the epiphyseal line

Bone Growth• Growth in diameter

osteoclasts inside the bone destroy bone lining the hallow cavity

osteoblasts on surface of bone add new tissue around outer surface of bone

Bone Growth• Reconstruction: remodeling is part of bone

maintenance; old bone is constantly reworked.

Remodeling is replacement of old bone tissue by new bone tissue like skin.

Initially bone is spongy and is replaced on outside of bones with compact bone.

Distal end of femur is replaced every 4 months.

Bone Growth

Value and need for Reconstruction

– Old bone weakened by degeneration of organic matrix must be replaced

– Constant exchange of Ca2+ (bones store 99% of Ca2+)

Bone Growth

– Bone adjusts to mechanical stress; thickens under stress

bones of athletes are heavier

movement of teeth in orthodontics involves reshaping of alveoli by stress applied with braces

Bone Growth• Hormones and vitamins that regulate

growth and remodeling of bone

Growth hormone and thyroxin - normal bone growth in young people

Sex hormones - osteoblasts have receptors for sex hormones

Bone Growth

– osteoporosis - with decrease of sex hormones with advancing age, porous bones may result; more common in females because of menopause when estrogen production essentially halts

adequate diet which may include Ca2+, exercise, and estrogen replacement may be indicated for prevention of osteoporosis in females

Bone Growth

Parathyroid hormone - increases osteoclast activity, therefore increases Ca2+ in blood

Calcitonin- (from thyroid) increases osteoblast activity, therefore accelerating deposit of Ca2+ into bone; blood Ca2+ levels decrease

Vitamin D - sun converts cholesterol derivative into Vitamin D in skin

Bone Growth

– Vitamin D is needed to absorb Ca2+ from intestine (Vitamin D aids in synthesis of a carrier protein molecule that is needed to transport Ca2+)

Vitamin C - promotes synthesis of collagen