Unit E (Skeletal System)

Regions of a Long Bone

Diaphysis
Medullary Cavity
Epiphysis
Metaphysis

Diaphysis

The shaft of a long bone
Made up of comapct bone

Medullary Cavity

Hollow, cylindrical space within the diaphysis
Contains bone marrow (red in children and yellow in adults)

Epiphysis

The end part of a long bone
Thin layer of compact bone with an inner region of spongy bone

Metaphysis

Region of mature bone between the diaphysis and epiphysis
Contains the epiphyseal plate (growth plate)

Membranes of a Long Bone

Periosteum
Endosteum

Periosteum

Outer fibrous membrane of dense irregular connective tissue that covers the outer surface of the bone
Attachment site for ligaments (links bone to bone)
Attachment site for tendons (links muscle to bone)

Endosteum

Covers all internal surfaces of bone within Medullary Cavity
Contains osteoprogenitor cells, osteoblasts, and osteoclasts
Incomplete membrane

Red Bone Marrow

Hemopoietic (blood cell forming)
Located in children in spongy bone and medullary cavity of long bones
Located in adults in areas of axial skeleton

Yellow Bone Marrow

A product of red bone marrow degeneration as children mature
Fatty substance
May convert back to red bone marrow

What is the purpose of the incomplete membrane in the Endosteum?

Maximizes exchange; allows diffusion of nutrients and wastes
Does not block remodeling; Osteoclasts and Osteoblasts can access the bone membrane to perform functions

Functions of Skeletal System

Support + Protection (protects organs and tissues)
Movement (system of levels)
Hemopoiesis (blood cell production)
Storage of mineral and energy reserves (lipids stored in yellow bone marrow)

Long Bones

Greater in length than in width
Most common bone shape
Found in upper and lower limbs

Short Bones

Length nearly equal to width
(Carpal Bones + Tarsals)

Flat Bones

Flat
Provide surfaces for muscle attachment
(Scapulae + Sternum + Ribs)

Irregular Bones

Does not fit into any category
(Vertebrae + Ossa Coaxe)

What happens to the epiphyseal plate after age 25?

Epiphyseal plate becomes epiphyseal line in adult.
Growth stops

Osteoprogenitor Cells

Bone stem cells that differentiate into osteoblasts

Osteoblasts

Form from Osteoprogenitor Stem Cells
High energy and activity
Builds bone matrix by secreting Osteoid (soft, not calcified yet)
Matures into osteocytes

Osteocytes

Mature bone cells derived from Osteoblasts
Less energy and activity (loss of bone-forming activity)
Maintains the bone matrix
Located in lacunae
Responds to stress on bone

Osteoclasts

Derived from fused bone marrow cells
Large multinucleated cells
Break down bone tissue
Bone resorption; secretes acids and enzymes that dissolve bone matrix (both organic part and inorganic part)

Bone Matrix Parts

Organic Part
Inorganic Part

For what reason would yellow bone marrow convert back to red bone marrow?

In response to increased demand for blood cell production that existing red marrow cannot meet.
(Blood loss + High altitude living)

What type of connective tissue is found in bone?

Osseous Connective Tissue
(A type of structural adult connective tissue)

What are the four cell tyoes found in Osseous Connective Tissue?

Osteoprogenitor Stem Cells
Osteoblasts
Osteocytes
Osteoclasts

How do Osteoclasts break down bone?

Proteolytic enzymes released from lysosomes within osteoclasts helps to destroy organic part of matrix

Calcium and phosphate are dissolved by hydrochloric acid; helps to destroy inorganic part of matrix

Why do Osteoclasts break down bone?

To remove old tissue, repair damage, and release essential minerals into the bloodstream.

Organic Part of Bone Matrix

Contributes to flexibility and tensile strength
Contains collagen fibers, ground substance, proteins

Inorganic Part of Bone Matrix

Contributes to rigidity and hardens bone matrix
Contains calcium and phosphate

What contributes to strong bone matrix?

Fluoride

Spongy Bone

(Trabecular Bone, Cancellous Bone)
Open lattice of rods and plates of bones
Bone marrow fills spaces
No Osteons

Compact Bone

Basic functional and structural unit of mature bone tissue
Composed of Osteons

Components of Osteon

Central Canal
Concentric Lamellae
Osteoctyes
Canaliculi

Central Canal

Cylidrical channel at center of osteon
Blood vessels and nerves run through channel

Concentric Lamellae (Lacunae)

Surround central canal
Rings of bone connective tissue
Contributes to bone strength

Canaliculi

Tiny, interconnecting channels in bone connective tissue
Connects to lacunae and the central canal
Allows exchange of nutrients, gases, wastes

Ossification

Formation and development of bone connective tissue
Bone forms on cartilaginous matrix
Starts in 6th-8th week of gestation until 25 years old
At birth, most of cartilage has been replaced by bone
Some will be replaced throughout childhood

What are the two mechanisms for Ossification?

Intramembranous Ossification
Endochondrial Ossificaton

Intramembranous Ossification

Bone develops directly from undifferentiated messenchyme

Endochondrial Ossification

Bone develops by replacing hyaline cartilage; cartilage does not become bone
Typically takes longer to complete than intramembranous ossification

What bones does Intramembraneous Ossification produce?

Flat bones of skull
Some of the facial bones
Mandible
Central part of the clavicle

What bones does Endochondrial Ossification produce?

Produces most bones of the skeleton
Bones of upper and lower limbs, pelvis, vertebrae, ends of the clavicle

What are the Mechanisms of Bone Growth

Intersitial Growth
Appositional Growth

Intersitial Growth

Bone growth in length
Occurs in proliferating and hypertrophic cartilage
Pushes zone of resting cartilage toward epiphysis
Flexible hylaine cartilage permits growth and is replaced by bone

Appositional Growth

Bone growth in diameter or thickness
Occurs within the periosteum
Bone matrix is deposited in layers parallel to the surface
Increase in lamellae results in increased diameter

What are the Zones of the Epiphyseal Plate?

Zone 1: Zone of resting cartilage
Zone 2: Zone of proliferating cartilage
Zone 3: Zone of hypertrophic cartilage
Zone 4: Zone of calcified cartilage
Zone 5: Zone of ossification

Zone of Resting Cartilage

Closest to the epiphysis
Small Chondrocytes, scattered, not dividing
Normal hyaline cartilage matrix
Secures epiphysis to epiphyseal plate

Zone of Proliferating Cartilage

Below resting zone
Flattened, stacked chondrocytes, rapid mitosis
Newly secreted cartilage matrix
Adds new cartilage, pushes epiphysis away, lengthening bone

Zone fo Hypertrophic Cartilage

Middle of Plate
Chondrocytes stop dividing, mature, become enlarged in columns
Walls of lacunae become thin
Chondrocytes enlarge and prepare matrix for calcification

Zone of Calcified Cartilage

Next to diaphysis side
Chondrocytes undergo apoptosis, composed of two to three layers of chondrocytes
Minerals deposited between columns of lacunae, matrix becomes calcified

Zone of Ossification

Closest to diaphysis
Osteoprogenitor stem cells and capillaries invade, no chondrocytes left
Remaining cartilage coated with osteoid
Replaced by spongy bone at diaphysis

Bone Remodeling

Occurs at periosteal and endosteal surfaces of a bone
Distal part of femur replaced every 4 to 6 months
Diaphysis of femur not completely replaced over a lifetime
20% of skeleton replaced yearly

What minerals are used for bone remodeling?

Calcium and Phosphate are used to make matrix
Magnesium is a cofactor for proper enzyme function

What vitamins are used for bone remodeling?

Vitamin A controls osteoclast and osteoblast activity
Vitamin C maintains intracellular matrix
Vitamin D helps absorb calcium from the intestines and helps save calcium in the kidneys instead of passing it out in the urine

What is the Piezoelectric Effect in Bone Remodeling?

Bone generates small electrical charges when mechanically stressed

Osteocytes sense these signals and direct osteoblasts to build bone where stress is high and osteoclasts to remove bone where stress is low

This ensures bone strength matches mechanical demand

What is caused by a lack of mechanical stress?

Decreased bone mass
Reduced collagen formation
Demineralization
Decreased strength of unstressed bone in an immobilized fracture

What hormones are involved in Remodeling?

CT (Calcitonin)
PTH (Parathyroid Hormone)
Growth Hormone (Somatotropin)
Thyroid Hormone
Glucocorticoids
Serotonin
Estrogen and Testosterone

CT (Calcitonin) in Bone Remodeling

Made in thyroid
Decreases osteoclast activity
Decreases amount of bone broken down
Decreases amount of calcium entering the blood stream

PTH (Parathyroid Hormone) in Bone Remodeling

Made in parathyroids
Increases osteoclast activity
Increases amount of bone being broken down
Increases amount of calcoum moving from bone into blood

Growth Hormone (Somatotropin) in Bone Remodeling

Made by the anterior pituitary gland
Stimulates growth of cartilage in epiphyseal plate
Stimulates the liver to produce somatomedin (insulin-like growth factors)

Thyroid Hormone in Bone Remodeling

Secreted by the thyroid gland
Influences the basal metabolic rate of bone cells
Regulates normal activity at epiphyseal plates

Glucocorticoids in Bone Remodeling

Released from adrenal cortex
Regulate blood glucose level
High amounts increase bone loss and decrease bone formation, leading to osteoporosis
Impairs growth at epiphyseal plate in children, (glucocorticoids in asthma medication)

Serotonin in Bone Remodeling

Role in rate and regulation of normal bone remodeling
If levels are too high osteoprogenitor stem cells are prevented from differentiation into osteoblasts
Could be linked to low bone density disorders (Osteoporosis)

Estrogen and Testosterone in Bone Remodeling

Secreted in large amounts at puberty
Dramatically accelerates bone growth
Increases rate of cartilage growth and bone formation in epiphyseal plate
Bone formation rate is greater than cartilage growth

Aging Females in Bone Remodeling

After 30 years, there is accelerated loss of bone mass
Reaches a maximum in the late 40s (around the time of menopause)
By 70 years, there is a loss of 30% of original bone mass

Caused by decreased estrogen levels, leading to increased bone resorption and decreased formation because Estrogen stimulates osteoblasts

Agining Males in Bone Remodeling

Bone loss is much slower
Doesn’t begin until early 60s
Because testosterone levels decline gradually

Fracture Names

Partial
Complete
Open (Compound)
Closed (Simple)
Communuted
Greenstick
Spiral
Transverse
Impacted
Displaced
Non-Displaced
Pathologic
Stress

Partial Fracture

Breaks part through the bone

Complete Fracture

Breaks all the way through the bone

Open (Compund) Fracture

Breaks through the skin

Closed (Simple) Fracture

Remains internal

Comminuted Fracture

Complete break with fragments between the main pieces

Greenstick Fracture

Incomplete fracture, one side breaks while the other bends

Spiral Fracture

Twisting breaks bone

Transverse Fracture

Break is perpendicular to the long axis

Impacted Fracture

Complete break with one piece driven into the other

Displaced Fracture

Out of anatomical position

Non-displaced Fracture

Retains position

Pathologic Fracture

Break resulting from disease stateS

Stress Fracture

Mechanical stress cracks bone

Diseases in the Skeletal System

Ricketts
Osteomalacia
Paget’s Disease
Osteomyelitis

Ricketts

Bones stay soft and epiphyseal plate never becomes epiphyseal line
No calcium is absorbed in the intestines
Legs bow out from weight of body

Osteomalacia

Demineralization of bone
Cannot absorb fats
Vitamin D is fat or lipid soluble, therefore if you cannot take in fats, you cannot take in Vitamin D

Paget’s Disease

Irregular thickening of the bones
Results from an acceleration of the remodeling process
Usually occurs in people over 50 years

Osteomyelitis

Staphlococus aureus infection (Staph infection)
Bacteria enter the bone through the bloodstream or from a nearby infection, causing inflammation and pain