Bones are alive

• Living bones:– Form– Grow– Repair– Remodel– Move at joints (with help from

muscles)

Skeletal System – Normal Function

Skeletal System

Normal Function

Support

Protection

Movement

Mineral Storage

Blood Cell Production

Bone Types

Long Bone – Macroscopic Anatomy

Epiphysis (end)

• covered with hyaline cartilage

• compact bone forms exterior

• spongy bone forms interior

• contains epiphyseal line/plate

Diaphysis (middle)

• compact bone forms exterior

• center composed of the medullary cavity containing yellow or red marrow. Marrow helps produce red blood cells.

Fig 7.1

Long Bone – Macroscopic Anatomy

Fig 7.2

Microscopic Anatomy

• Osteon = circular structure organized around blood vessels

• Osteocyte = bone cells

• Bone matrix = hard calcium-based material between cells

Fig 7.3

Tissue Types – Compact BoneFunctions

Hard and provides stability, framework, protection

Structure Matrix has hard mineral structure, based on calcium Cells and matrix are arranged in regular, concentric

pattern

Compact Bone

Tissue Types – Spongy BoneFunctions

Lightens total bone weight and provides space for bone marrow

Structure Matrix is softer mineral, less organized and less of it Empty spaces between matrix structure are filled with bone marrow

Spongy (cancellous) Bone

Spongy Bone

Tissue Types – Hyaline CartilageFunctions

Provides soft and smooth covering to end of bone (articular surface)

Helps provide smooth and easy movement between bones (at joints)

Provides the starting material for new bone growth

Structure Matrix is gelatinous with fibers

and is evenly distributed between the cells

Cells (chrondocytes) are found in pairs within capsules called lacunae

Hyaline Cartilage

Bone Cells

OSTEOCYTES = “bone” “cells”OSTEOBLASTS

“bone builders” which depositcalcium into bones

OSTEOCLASTS “bone crushers” which break down bone to release calcium whenNeeded

BONE IS NEVER AT REST = homeostasis!!!

Bone PhysiologyLiving bones perform the following processes:• Formation– Starts as an embryo, continues through fetal

development and after birth• Growth– Occurs from before birth through maturity

• Remodel– Old matrix is removed and replaced with new

matrix• Repair– Damaged bones will heal and return to normal

structure

Bone Physiology – Formation

Bone Physiology – Formation• Fetal bones are made of hyaline cartilage• As cartilage cells die, they are replaced

with spongy bone in diaphysis• After birth, spongy bone invades diaphysis• Compact bone replaces spongy bone in

diaphysis• Hyaline cartilage remains on epiphyseal

surface and in growth plate

Fig 7.5

Epiphyseal plate -cartilage band indicates ossification is occurring and spongy bone is being deposited

X Ray in Newborn

Bone Physiology - Repair• Hemotoma, a mass of

clotted blood, forms at the fracture site.

• A fibrous callus forms, and cartilage matrix is secreted.

• Spongy bone forms to replace cartilage.

• Bone remodeling occurs to remove excess material and replace compact bone.

Bone Physiology - Remodeling Bone matrix is inorganic

and breaks down over time

Specific bone cells (osteoclasts) remove old, broken matrix

Other bone cells (osteoblasts) re-make/deposit new matrix

The entire skeleton is re-modeled every 7-10 years

Remodeling slows with age, elderly people have thinner bones and are more susceptible to broken bones as a result

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

Osteoporosis

• Released by cells of the thyroid gland in response to high blood [Ca2+].

• Calcitonin acts to “tone down” blood calcium levels.• Calcitonin causes decreased osteoclast activity which results in

decreased break down of bone matrix and decreased calcium being released into the blood.

• Calcitonin also stimulates osteoblast activity which means calcium will be taken from the blood and deposited as bone matrix.

Notice the thyroid follicles on the right. The arrow indicates a C cell

Calcitonin

Parathyroid Hormone• Released by the cells of the

parathyroid gland in response to low blood [Ca2+].Causes blood [Ca2+] to increase.

• PTH will bind to osteoblasts and this will cause 2 things to occur:

• The osteoblasts will decrease their activity and they will release a chemical known as osteoclast-stimulating factor.

• Osteoclast-stimulating factor will increase osteoclast activity.

• Osteoclasts will begin breaking down bone in order to increase Ca 2+ into the blood.