Bones are alive Living bones: Form Grow Repair Remodel Move at joints (with help from muscles)

Skeletal System – Normal Function Movement Protection Normal Function Support 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) 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 Bone Functions 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 Bone Functions 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 Cartilage Functions 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” BONE IS NEVER AT REST = OSTEOBLASTS “bone builders” which deposit calcium into bones OSTEOCLASTS “bone crushers” which break down bone to release calcium when Needed BONE IS NEVER AT REST = homeostasis!!!

Bone Physiology Living 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

Calcitonin 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

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.