Bones

Functions of bones

Support, protection, anchorage, electrolyte balance, acid-base balance, blood cell formation, fat storage, and hormone production

Long Bone

Bones that are longer than they are wide

Contains both compact and spongy bones

Articular cartilage

Covers the joint ends of long bones

Diaphysis

Shaft of a long bone

Epiphysis

Ends of long bone

Medullary cavity

Inside the epiphysis and contains yellow bone marrow in adults

Periosteum

A highly vascular membrane that covers the outside of long bones, except the joint surfaces

Flat Bone

Bones that are pretty wide and flat

Inner layer of flat bones

Spongy bone

Outer layer of flat bones

Compact bone

Dipole

The layer between the compact and spongy bone of a flat bone that contains both red and yellow bone marrow

Hematopoietic tissue

Red bone marrow that contains many immune cells

Found around the trabeculae of a long bone and the dipole of flat bones

Commonly extracted from the iliac crest of the pelvis

Trabeculae

Tiny framework structures that help to reduce mechanical stress and absorb shock

EPO

Stimulates bone marrow to produce more red blood cells leading to better oxygenation and thicker blood

Yellow bone marrow

A soft, fatty tissue found in the medullary cavities of adult long bones

Infants bones are composed only of yellow bone marrow, but it slowly transitions to be mostly red

Osteogenic cells

Mitotically active stem cells found in the periosteum and the endosteum that transitions into osteoblasts

Tip: genic → genetics → stem cells

Osteoblasts

Bone forming cells that secrete bone matrix called osteoid, but is not meiotically active. Transitions into osteocytes

Tip: blasts → B → builds

Osteocytes

Mature bone cells in the lucunae that act as sensors for remodeling in homeostasis. They are connected by gap junctions

Tip: Osteocytes → mature cites → mature cells

Osteoclasts

A multi nucleated cell that breaks down bone

Tip: clast → class → breakdown → breaks down

Bone formation

1. Osteoblasts produce osteoid, which is a collagen matrix

2. Bone deposition starts, meaning the bones begin to calcify

3. Minerals form crystals called hydroxyapatite’s, which harden the matrix

4. New bone slowly becomes mature and fully functional

Hydroxyapatite’

Crystals that make bone hard

Bone breakdown/removal

1. Osteoclasts attach to a section of the bone

2. The osteoclasts release acids and enzymes that dissolve the hydroxyapatites

3. The broken down materials are released back into the bloodstream to be reused

Compact bone

The dense, hard outer layer of bone that provides strength, support, and protection and is organized into osteons

Osteons

Circular structural units of rings around a concentric lamellae

Central canal

Provides a blood supply to compact bone and has a long axis

Perferating canal

Comes in at 90% and intersects with the central canal of compact bone

Interstitial lamellae

Lamellae of a compact bone that is not organized into an osteon

Circumferential lamellae

Located just under the periosteum

Spongy bone

The porous, lightweight inner bone tissue made of trabeculae that reduces bone weight, houses bone marrow, a

Canaliculi

Canals that serve as communication and nutrient networks for spongy bone

Connects the lacunae together and transfers nutrients and waste though gap junctions

Osteogenesis Imperfecta

Brittle bone disease that causes bones to easily break due to a genetic mutation that causes the body to NOT produce enough collagen

Inorganic components of bone

Loss of these minerals results in rubbery and flexible bones

Example: Calcium, magnesium, or sodium

Organic components of bone

Loss of these materials results in brittle bones that break easily, resulting in disorders such as osteogenesis imperfecta

Example: Collagen

Ossification

The formation of bone that begins to develop during week 6 in utero

Can be either endochondral or intramebranous

Mesenchymal stem cells

Less specialized osteoprogenitor cells that can develop into many different types of cells

Endochondral ossification

When a bone develops by first replacing hyaline cartilage, starting from a central point

CARTILAGE DOES NOT BECOME BONE, it has to be removed first

Epiphyseal plate

The growth plate is an area of flexible cartilage found at the ends of bones that produces new bone tissue to continuously allow bones to lengthen during childhood and puberty

Intramembranous ossification

When a bone develops from a fibrous membrane

Appositional growth

Bone grwoth that results in an increase in thickness that occurs though intramembranous ossification

Postnatal growth of bones

1. Zone of reserve cartilage

2. Zone of cell proliferation

3. Zone of hypertrophy

4. Zone of calcification

5. Zone of bone deposition

Zone of reserve cartilage

The first zone or level where hyaline cartilage is resting

Zone of cell proliferation

The second zone or level where chondrocytes are multiplying and lining up in rows of small flattened lacunae

Zone of Hypertrophy

The third zone or level where mitosis stops and enlargement of chondocytes beings, causing a thickening of the lacuna walls

Zone of calcification

The fourth zone or level where the cartilage matrix is temporarily calcified between columns of lacunae

Zone of bone deposition

The fifth zone or level where the lacuna walls get broken down, which leaves channels open. Chondrocytes die and bone deposition by osteoblasts begin to form the trabeculae of spongy bone.

Thyroid hormone

Accelerates growth before birth

Growth hormone (GH)

Accelerates growth after brith

GHRH

A hormone that casues GH to be relased

Pituitary dwarfism

Not enough GH during childhood

Gigantism

Excess GH during childhood before the closure of the growth plates that results in being extra tall

Acromegaly

Excess GH secretion after linear growth of bones is no longer possible that causes an overgrowth, thickening of hands, feet, and face

Sex hormones

Estrogen and testosteron

Promote osteoblast activity beginning at puberty that causes a growth spurt and closure of the growth plates

Calcium

A mineral stored in bones that helps with neuron communication, muscle contraction, blood clotting, and exocytosis

Hypocalcemia

Not enough blood calcium that causes the body to become oversensitive resulting in muscle spasms

Hypercalcemia

Excess blood calcium that causes the body to become less sensitive resulting in weakness and slowness

Parathyroid hormone (PTH)

A hormone released from the parathyroid gland when calcium levels are low to inhibit collagen synthesis by osteoblasts, promote calcium reabsorption, and promote the final step of vitamin D synthesis

Vitamin D

Increases calcium levels by increasing calcium absorption by the small intestines, meaning the calcium in bones does not get broken down

Calcitonin

A hormone released from the thyroid gland when blood calcium levels rise to high to inhibit osteoclasts and stimulate osteoblasts

Hormonal control

Determines if and when remodeling occurs in response to changing blood calcium levels

Mechanical stress

Determines where remodeling occurs

Wolffs Law

Bone grows or remodels in response to demand (load or pull)

Osteoporosis

A severe loss of mass and brittle bones due to a loss in organic material

Affects the spongy bone the most

Treatments aim to slow down the progression, but cannot stop or reverse damage

Kyphosis

A deformity of the spine due to vertebral bone loss that causes a hump

Estrogen

Maintains bone density in women by inhibiting resorbtion by osteoblasts

Testostorone

Maintains bone density in men by inhibiting resorbtion by osteoblasts