Bones and Skeletal Tissues

Dr. AndersonGCIT

Cartilage

• Skeletal Cartilage – softer than bone, precedes bones in human embryos

– Surrounded by perichondrium, which:• resists expansion of the

cartilage and • feeds the cells in the cartilage

matrix via its blood vessels

Cartilage Growth

• Appositional Growth – new tissue is laid down on the outside of the existing structure– Cartilage grows from the outside

• Interstitial Growth – new tissue is created within the matrix of existing cartilage– Cartilage grows from within

Bones!

• 206 in humans

• Axial Skeleton – skull, vertebral column and ribcage

• Appendicular skeleton – long bones in limbs, shoulder and pelvic girdles, and digits

General Bone Classification

• Long Bones – longer than wide– All limb bones sans the patella

• Short Bones – a roughly cubic or rounded– Carpals and tarsals

• Flat Bones – Thin, flattened, sometimes curved– Ribs, sternum, etc.

• Irregular Bones – Vertebrae, hip bones, etc.

Bone Function

• Support• Protection• Movement• Mineral and growth factor storage• Blood cell formation (marrow)• Fat storage

Gross Anatomy of Bones

• Bone markings– Bulges (heads, trochanters, etc.)– Depressions (fossae, sinuses, foramina, grooves)

• Textures (in cross section)– Compact bone– Spongy bone

Example - Long Bone Structure

• Diaphysis (shaft)

• Medullary cavity (contains marrow)

• Epiphyses (ends)– Immediately preceded by epiphyseal line

• Periosteum – surrounds the bone and contains bone-building cells (osteoblasts) and bone-destroying cells (osteoclasts)

Epiphyses often lined with hyaline cartilage at articulation points

Spongy bone present interior to compact bone

Periosteum & Endosteum• Periosteum - Double-layered membrane that

surrounds bone– Richly supplied with blood vessels and nerves that

enter the bone itself through the nutrient foramina

• Endosteum – covers internal bone surfaces (trabeculae in spongy bone and canals through compact bone)

Other Bones

• Short, Flat & Irregular Bones– No epiphyses or diaphyses

• Do contain spongy bone, compact bone and marrow– In flat bones, spongy bone is called the diploe

Red vs. Yellow Marrow

• Yellow marrow = stored fat

• Red Marrow = hematopoetic (blood producing) tissue– Located in most bones in children– In adults, most active areas are in the head of the

humerus and femur, and in the flat bones

Microscopic Anatomy

• Four Cell Types

• Osteogenic Cells – Stem cells that form osteoblasts

• Osteoclasts – bone-destroying cells

• Osteocytes -

Structural Unit of Bone• Osteon (Haversian

System) – a tube or cylinder of bone matrix

• Detailed structure can be found on pg. 181 in the text

Osteon

Made of concentric rings of bone surrounding a Haversian Canal (contains blood vessels)

Between each layer (lamella) are the osteocytes (in lacunae)

Connecting adjacent osteocytes are canaliculi

Interstitial Lamellae

• Lie between proper osteons to fill gaps– May have been an osteon that was “remodeled”

during growth from injury

• Circumferential Lamellae– Form the outer layers of the diaphyses of long

bones

Spongy Bone

• No osteons, however still provide support against stress

• Nutrients arrive from blood in the endosteum

Chemical Composition of Bone

• Organic Components – cells and osteoid, the organic part of the bone matrix– Osteoid is composed of collagen and glycoproteins,

which are made by osteoblasts– Collagen fibers can form sacrificial bonds – weak

bonds that break easily under stress and prevent the entire bone from fracturing

– Bones also made of salts (CaPO4 crystals) which are the inorganic component of bones that provide hardness

Bone Development (Ossification)

• Occurs during:– Initial Growth (child to adult size)

• Video http://www.youtube.com/watch?v=Plvd7eslg-Q

– Repair (Broken Bones)

Ossification Types

• Intramembranous Ossification – (flat bones) when bone develops from a fibrous membrane– Pg. 182 in textbook

• Endochondrial Ossification – Hyaline cartilage is replaced with bone by osteocytes– Pg. 183 in text

Bone Development (Intramembranous Ossification)

Bone Development (Endochondrial Ossification)

Class Assignment

• How do bones become ossified from birth?

• Make sure to include– Important structures– Prenatal to adult (stable) bone

Post-natal Bone Growth

• Chondrocytes proximal to the epiphyses grow fast, and away from the diaphysis

• Chondrocytes die, and the matrix is eventually replaced by new bone by invading osteocytes and calcified (the calcification zone)

• In adulthood, the diaphysis and epiphyses fuse, largely ending longitudinal growth

Growth in Diameter (Thickness)

• To deal with the increased forces from lengthening, bones need to gain thickness

• Bones change in length to thickness ratio as they grow

• Osteoclasts break down bone to remodel the shape, while osteoblasts add to bone thickness (appositional growth)

• http://www.youtube.com/watch?v=X6E5Rz9tOKE

http://www.google.com/url?sa=i&rct=j&q=robert%20wadlow&source=images&cd=&cad=rja&docid=ebl1Zsa-yYUC5M&tbnid=c-ht_WLtNvNLCM:&ved=0CAUQjRw&url=http://en.wikipedia.org/wiki/Robert_Wadlow&ei=UphuUarEAY2a8gSgvIH4CA&psig=AFQjCNF9I8tRUSxqNDU0LvGBqyvtRlnCtw&ust=1366288845729749

Hormonal Regulation of Bone Growth

• Growth Hormone (HGH) released by pituitary gland – Increase growth rate of long bones

• Thyroid Hormone (TH) – Increase long bone growth rate

• Androgens (Testosterone, Estrogen Progesterone)– Cause sex-specific changes to skeleton

Bone Remodeling and Repair

• Bone remodeling occurs under the periosteum or endosteum– 5-7% of our bone mass is replaced every

week!

• Factors affecting how bone is remodeled include– Stress– Nutrition– Age– Trauma

Bone Deposit and Resorption• Deposit – (by osteoblasts) occurs wherever bone is injured

and extra strength is required

• Resorption – osteoclasts “dissolve” bone by the release of enzymes and hydrochloric acid– Once liberated, these substances enter the blood stream where

they can either be used (Ca) or eliminated (dead cell fragments)

Response to Mechanical Stress

• Bones will adapt to the stress placed upon them (Wolff’s Law), – E.g. -Handedness : larger bones in

dominant limbs– Trochanters – form where the

largest muscles attach– Bone mass is also loss when

physical stresses are absent, e.g. astronauts in space, the disabled, tooth loss

Bone Repair

• Because of the crystalline nature of the inorganic matrix of bone, bones are subject to fractures

Terms to Describe Fractures• Displaced/non-displaced – epiphyses are either

aligned or unaligned

Non-displaced Displaced

• Complete fracture – bone not broken through• Incomplete – bone broken through completely

Terms to Describe Fractures

Complete Incomplete

• Linear – break is parallel to the long axis of the bone

• Transverse – bone breaks across short axis

Terms to Describe Fractures

Linear Transverse

Terms to Describe Fractures• Compound (Open) – bone penetrates the skin• Simple (Closed)- bone does not penetrate the skin

Compound (Open) Simple (Closed)

Bone Repair – The Break

Almost immediately after injury, a hematoma forms from broken blood vessels in the periosteum, endosteum and Haversian canals. Bone cells die and pain ensues.

The broken area is perfused with newly synthesized capillaries and WBC’s clean up dead cellular debris.

Bone reconstruction occurs via osteoblast and osteoclast migration to the area of damage.

Bone Repair – Callus (Soft) Formation

Bone Repair – Callus (Hard) Formation

• New scaffold of bone (trabeculae) form at the site

• This continues until a bony callus is formed roughly two months later

Bone Repair - Bone Remodeling

• Bony callus is remodeled (osteoclasts and osteoblasts) and eventually converted into compact bone on the outside surfaces

Bone Diseases• Osteomalacia – bones

not sufficiently mineralized– Poor nutrition leads to

limited mineralization– “Rickets” in children –

can lead to deformities in growing bone

Osteoporosis• When bone resorption

outpaces bone deposit

• Most common in post-menopausal women, as estrogen suppresses osteoclast activity

• May be combated with – Hormone replacement– Exercise– Good nutrition

Paget’s Disease

• Haphazard bone formation

• Osteoblast outpaces osteoclast activity

• Bone is deposited where is should not be