Chapter 7 - Bones and Skeletal Tissue

Functions of the Skeleton

1. Support — limb, bones, and vertebrae support body organs

2. Protection — of brain, spinal cord, heat, lungs, and more

3. Movement — limb movements, breathing

4. Electrolyte balance — calcium and phosphate levels

5. Acid-base balance — buffers blood against large pH changes by altering phosphate and carbonate salt levels

6. Blood formation — (hematopoiesis) in marrow cavities

7. Hormone secretion — bone cells secrete hormones that affect action of insulin and moderate the stress response

Long bones (Humerus)

Longer than they are wide

Short bones (Talus)

Cube-shaped bones

Sesamoid Bones

Location of (short) cube-shaped bones

In wrist and ankle

Location of (short) sesamoid bones

Within tendons, e.g., patella

Flat bones (Sternum)

Thin, flat, slightly curved

Location of flat bones

Skull, scapula, sternum

Irregular Bones (Vertebra)

Complicated shapes

Location of irregular bones

Vertebrae and pelvis

Projections: Site for MM/ligament attachments

tubercle, tuberosity, trochanter, crest, process, spine

Articulations (joints)

head, facet, condyle

Passages

Canal (meatus), fissure, foramen

Depressions

fossa, sulcus (groove)

Compact bone (10 years)

Dense outer layer

Spongy (cancellous) bone (3-4 years)

Honey comb of trabeculae, more porous like a sponge

Periosteum

Outer fibrous layer, Inner osteogenic layer

Inner osteogenic layer (periosteum)

osteoblasts (bone-forming cells), osteoclasts (bone-destroying cells), osteogenic cells (stem cells)

Endosteum

Delicate membrane on internal surfaces of bone also contains osteoblasts and osteoclasts

Structure of short, irregular, and flat bones

1. Periosteum - covered compact bone on the outside

2. Endosteum - covered spongy bone within lattice of bone silvers (spicules) and plates (trabeculae)

Location of Hematopoietic Tissue (Red Marrow)

1. in nearly every bone in a child

2. in adults, found in skull, vertebrae, ribs, sternum, part of pelvic girdle, and proximal heads of humerus and femur

Osteogenic (osteoprogenitor) cells

Stem cells in periosteum and endosteum that give rise to osteoblasts

Osteoblasts

Bone-forming cells, secrete bone matrix

Osteocytes

Mature bone cells, monitor/maintain matrix

Osteoclasts

Cells that break down (resorb) bone matrix

Haversian system or osteon

Structural Unit

Lamellae

Concentric rings

1. Weight bearing

2. column like matrix tubes

Central (Haversian) canal

Contains blood vessels and nerves

Perforating (Volkmann’s) canals

1. at right angles to the central canal

2. connects blood vessels and nerves of the periosteum and central canal

Lacunae

Small cavities that contain osteocytes

Canaliculi

Hair-like canals that connect lacunae to each other and the central canal

Trabeculae in spongy bone

• Align along lines of stress

• no osteons

• contain irregularly arranged lamellae, osteocytes, and canaliculi

• capillaries in endosteum supply nutrients

Microscopic anatomy of compact bone

• Haversian system or osteon

• Perforating (Volkmann’s) canals

• Lacunae

• Canaliculi

Chemical composition of bone - of osseous tissue matrix

Organic matter, Inorganic matter

Organic matter

Synthesized by osteoblasts

Matrix

Collagen and carbohydrate-protein complexes, such as glycoaminoglycans, preoteoglycans, and glycoproteins

Inorganic Matter

Mineral Component — 85% hydroxyapatite (mineral salt), 10% calcium carbonate, many inorganic ions

Osteogenesis (ossification)

Bone tissue formation

Stages of Osteogenesis

1. initial formation — in embryo and fetus

2. growth — from infancy through adolescence

3. remodeling — of bone throughout life

4. repair — of fractures

Two types of ossification

Intramembranous ossifcation and Endochondral ossification

Intramembranous ossifcation

• membrane bone develops from fibrous membrane

• forms flat bones, e.g.m clavicles and cranial bones

Endochondral ossification

• cartilage (endochondral) bone forms by replacing hyaline cartilage

• uses hyaline cartilage models

• required breakdown of hyaline cartilage prior to ossification

• forms most of the rest of the skeleton below skull except claviclespo

Postnatal Bone Growth

1. Interstitial growth

2. Appositional growth

Interstitial growth

Higher length of long bones

Appositional growth

1. Higher thickness and remodeling of all bones by osteoblasts and osteoclasts on bone surfaces

2. Bone matrix secreted under periosteum

Growth Hormone

Stimulates epiphyseal plate activity (from pituitary gland)

Thyroid hormone

Modulates activity of growth hormone

Testosterone and estrogens

• promote adolescent growth spurts

• end growth by inducing epiphyseal plate closure (18 y/o females; 21 y/o for males)

Hormonal Control of Blood Ca2+

Primarily controlled by parathyroid hormone —> Blood Ca2+ levels —> Parathyroid glands release PTH —> PTH stimulates osteoclasts to degrade bone matrix and release Ca2+ —> higher blood Ca2+ levels

What controls continual remodeling of bone?

1. Hormonal mechanisms that maintain calcium homeostasis in the blood

2. Mechanical and gravitational forces

Wolff’s Law

A bone grows or remodels in response to forces or demands placed upon it

Observations supporting Wolff’s Law

• Handedness (right or left handed) results in bone of one upper limb being thicker and stronger

• curved bones are thickest where they are most likely to buckle

• trabeculae form along lines of stress

• large, bony projections occur where heavy, active muscles attach

• increased stress or decreased stress i.e. astronauts

Fracture classification

• displaced or non-displaced

• open or closed

• comminuted — bone fragments

• spiral - excessive twisting force

• compression - common in osteoporosis

• depressed

• greenstick

Stages in the healing of the bone fracture

1. hematoma

2. fibrocatilaginous callus (soft callus)

3. bony callus formation (hard callus)

4. bone remodeling

Hematoma

mass of clotted blood forms — site becomes swollen, painful, and inflamed

Fibrocartilaginous callus (soft callus)

• Fibroblasts secrete collagen to connect bone ends

• osteoblasts begin forming spongy bone in 1 week

Bony Callus formation (hard callus)

osteoblasts create bony collar to unite broken ends. firm union in 8 weeks.

Bone remodeling

In response to mechanical stresses, fragments removed by osteoclasts, osteoblasts lay down spongy bone and then converted to compact

Osteoporosis (osteopenia)

• loss of bone mass — bone resorption outpaces deposit

• spongy bone of spine and neck of femur become most susceptible to fracture

Risk factors of osteoporosis

post menopausal older women, small frame, white women of Asian and European origin, smoking, sedentary

How to detect osteoporosis

Bone densitometry, or DEXA scan that scans the spine and hips with low dose x-rays

Treatment and prevention of osteoporosis

treatment: drugs that stimulate bone deposition or slow rate of resorption

preventative methods: wegiht bearing exercise throughout life, even old age

Sesamoid Bones

Help improve leverage