Bone function and_formation_2009

Bone Function and Formation

Chapter 7 (Part 1)

Parts of a Long Bone

Long bones are defined as any bone that is longer than it is wide.

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The expanded bulge at each end of a long bone is called the epiphysis The epiphysis articulates with other bones Each long bone has a proximal epiphysis

and a distal epiphysis The epiphyseal surface that articulates with

other bones is covered in articular cartilage


The long shaft of the bone is called the diaphysis




Bones are covered in a thin, tough layer called the periosteum. The periosteum covers the entire bone,

except where there is articular cartilage It is firmly attached to the bone and is

continuous with ligaments and tendons connecting to the bone.




Periostium


The wall of the diaphysis is composed mostly of compact bone Very tightly packed

bone cells No spaces in the

extracellular matrix.




In the center of the diaphysis, is a tube called the medullary cavity.

Special layer of cells called the endosteum produce marrow inside the cavity.



Marrow

Bone marrow comes in two different types Red marrow produces red blood cells,

white blood cells, and platelets Yellow marrow stores fat inside the bone As we age, red marrow is replaced with

yellow marrow The two can be converted from one to the

other, as needed.


The epiphysis is composed mostly of spongy bone. Lots of space

between bone cells Only a very thin layer

of compact bone covers the epiphysis

Helps to reduce the bone’s weight



Microscopic Anatomy

Bone is composed of cells called osteocytes (os- = bone) Found in small chambers called lacunae

Surround osteocytesMade of hard extracellular material (bone

matrix) Calcium carbonate Calcium phosphate

Microscopic Anatomy

Bone matrix is deposited in concentric circles called lamellae (singular: lamella)



Microscopic Anatomy

The lamellae surround a central canal

Every central canal contains a blood vessel that supplies nutrients to the osteocytes in the surrounding lamellae



Microscopic Anatomy

Osteocytes have cytoplasmic extensions called canaliculi

Used to help osteocytes obtain nutrients & communicate with surrounding osteocytes



Bone Formation

During development, tissue that will become bone starts off as cartilage

There are two basic ways bone can form: Intramembranous Bone Endochondral Bone

Intramembranous Bone

These bones develop into thin, sheet-like bones like those found in the cranium

Originates between two sheet-like layers of connective tissue

Bone forming cells called osteoblasts lay down spongy bone near the center between the membranes

Cells of the membrane form the periosteum Cells of the periosteum produce osteoblasts

that form compact bone to surround the spongy bone.

Endochondral Bone

Most of the bones are endochondral1. Blob of cartilage, shaped somewhat

like the bone they will become, is what we start with

Endochondral Bone

2. Near the middle of the long bone (diaphysis), the cartilage breaks down.

3. Osteoblasts invade the space left by cartilage breaking down, forming spongy bone This is called the primary

ossification center 4. Osteoblasts lay down a

periosteum to surround the diaphysis

Endochondral Bone

5. After a time of growth and ossification in the diaphysis, the cartilage in the epiphyses begins to break down

6. Osteoblasts invade the areas in the epiphyses Create a secondary ossificaiton center

Endochondral Bone

Endochondral Bone

As the primary and secondary ossification centers grow toward each other, they form an epiphyseal plate

AKA: Growth Plate



Endochondral Bone

As toddlers and teenagers grow, bones lengthen by laying down new bone matrix from the epiphyseal plate

Damage = bad!



Homeostasis of Bones

Throughout life, old bone tissue is broken down and “digested” by osteoclasts

New bone tissue is formed by osteoblasts.

Bone Function

Support the bodyProtect internal organsAllow for attachment of muscles that

help up moveProduce blood cellsStore inorganic salts

Protection

The skull protects the brain, eyes, nasal & oral passages

The ribs protect the heart, lungs, windpipe, and esophagus

Pelvis (hips) protect internal reproductive organs (female) and the urinary bladder.

Movement

Bones (and their associated muscles) act as levers in the body

Levers have three parts: A long, rigid rod or bar (the bone) A pivot point (a joint) Something that supplies a force to move

the rod (muscle)

Blood Cell Formation

Blood cells form in the spleen and liver during prenatal development

Later, they form in the red marrow found in the medullary cavities of long bones.

Hematopoiesis



Storage of Inorganic Salts

This is another homeostatic mechanism You know how much I love to test you on

theseThe bones store a reserve of calcium

salts, which are vital to metabolic processes and muscle contraction

The glands involved in this HM are the thyroid and parathyroid


If blood calcium levels get too low: Parathyroid (receptor) releases parathyroid

hormone to stimulate osteoclast activity. Osteoclasts (effectors) break down bone

tissue, releasing calcium saltsIf blood calcium levels get too high:

Thyroid gland releases calcitonin, which activates osteoblasts

Osteoblasts store calcium by forming new bone tissue.

Check page 132

Osteoperosis

Caused by increased osteoclast and decreased osteoblast activity.

Bone tissue is broken down and replaced with fatty tissue

Bones break easilyMost common in fair-skinned females

after menopause Some research suggests it is due to a

decline in Vitamin D

Osteoperosis



Osteoperosis

Preventing the disease: Avoid smoking and excess alcohol Exercise (especially early in life) Bone mass usually begins to dwindle in the

mid-30s. As you near this age, begin to take in extra calcium (milk or supplements)

Be sure to get plenty of Vitamin D Take supplements Spend 10-15 minutes in direct sunlight everyday.

Your skin makes Vitamin D using UV light.

Bone function and_formation_2009

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