Anatomy Lecture Ch. 7 - Bone Tissues

Osteology

the study of bone

• Bones and teeth are the most durable remains of a body

• Living skeleton is made of dynamic tissues, full of cells, nerves, and blood vessels

  Continually remodels itself and interacts with other organ systems of the body

  • bones are in a constant state of fluctuation

Skeletal System

composed of bones, cartilages, and ligaments

• Cartilage is the forerunner of most bones

  • Covers many joint surfaces of mature bone

  • bones start out as hyaline cartilage

• ligaments —hold bones together at joints

  • dense regular tissue in ligaments

• tendons —attach muscle to bone

7 Functions of the Skeleton

1. Support —limb bones and vertebrae support body; jaw bones support teeth; some bones support viscera

2. Protection —of brain, spinal cord, heart, lungs, and more

3. Movement —limb movements, breathing, and other movements depend on bone

4. Electrolyte balance —calcium and phosphate levels

• stored in EC matrix of the bone

5. Acid–base balance —buffers blood against large pH changes by altering phosphate and carbonate salt levels

6. helps us resist changes in the pH of our bloodstream

7. Blood formation —red bone marrow is the chief producer of blood cells

Bone (osseous tissue)

connective tissue with the matrix hardened by calcium phosphate and other minerals

• Individual bones (organs) consist of bone tissue, bone marrow, cartilage, adipose tissue, nervous tissue, and fibrous connective tissue

Mineralization (calcification)

the hardening process of bones

Long Bones

• longer than wide

• has rigid levers acted upon by muscles

  • crucial for movement

• only long bones have the marrow cavity

Short Bones

• Approximately equal in length and width

• Glide across one another in multiple directions

• completely filled with spongy bone

Flat Bones

• Thin, curved plates

• protects soft organs

• completely filled with spongy bone tissue

• Sandwich-like construction

  • Two layers of compact bone enclosing a middle layer of spongy bone called the diploe

  • absorbs shock

Irregular Bones

Elaborate shapes that do not fit into other categories

Compact Bone

dense outer shell of bone

• skeleton is ¾ compact bone

• completely filled with minerals

• people who are physically active or weigh more have more compact bone tissue

• has concentric lamellae that surround a Central Canal

• Perforating canals —transverse or diagonal passages in bone that allows blood vessels and nerves to pass through

Periosteum

external sheath covering most of bone

• Outer fibrous layer of collagen

  • Some fibers are continuous with tendons

  • Perforating fibers—penetrate into bone matrix

• Inner osteogenic layer of bone-forming cells

  • Important to bone growth and healing of fractures

Osteogenic Cells

stem cells found in the endosteum and inner layer of periosteum

• Arise from embryonic mesenchyme

• Multiply continuously and give rise to most other bone cell types

Osteoblasts

bone-forming cells

• Forms a single layer of cells under endosteum and periosteum

• Synthesizes soft organic matter of matrix which then hardens by mineral deposition

• Stress stimulates osteogenic cells to multiply rapidly and increase the number of osteoblasts which reinforces the bone

Osteocytes

former osteoblasts that have become trapped in the matrix they deposited

• osteocyte is the maintenance cell of the bone tissue

• Act as strain sensors —when stressed, they produce biochemical signals that regulate bone remodeling (shape and density changes that are adaptive)

Osteoclasts

bone-dissolving cells found on the bone surface

• the cleaning crew

  • removes EC matrix

  • if not enough calcium, osteoclasts dissolves some of the EC matrix and puts the calcium into the blood stream

• Very large cells formed from fusion of several stem cells

• Have multiple nuclei in each cell

• Cells often reside in resorption bays (pits in bone surface)

• Dissolving bone is part of bone remodeling

• osteoclasts raise and decrease calcium to maintain homeostasis

The Matrix of Osseous (Bone) Tissue

Matrix of osseous tissue is about one-third organic and two-thirds inorganic matter

Organic matter—synthesized by osteoblasts

• Collagen, carbohydrate–protein complexes, such as glycosaminoglycans, proteoglycans, and glycoproteins

Inorganic matter

• hydroxyapatite (crystallized calcium phosphate salt)

• calcium carbonate

• Other minerals (fluoride, sodium, potassium, magnesium)

Mineral portion gives stiffness

Polymer (protein) gives some flexibility

Spongy (Cancellous) Bone

loosely organized bone tissue

• Found in the center of ends and center of shafts of long bones and in the middle of nearly all other bones

• Covered by more durable compact bone

• Spongy bone consists of:

  • Lattice of bone covered with endosteum

  • Thin plates of bone called trabeculae

  • Spaces are filled with red bone marrow

• has few osteons and no central canals

  • All osteocytes are close to the bone marrow

• Provides strength with minimal weight

• Trabeculae develop along bone’s lines of stress

Bone Marrow

soft tissue occupying marrow cavities of long bones and small spaces of spongy bone

• red and yellow bone marrow

Red Bone Marrow (myeloid tissue)

Contains hemopoietic tissue—produces blood cells

• In nearly every bone in a child

• In adults, this is found in the skull, vertebrae, ribs, sternum, part of pelvic girdle, and proximal heads of humerus and femur

• red marrow burns through a lot of ATP

Yellow Bone Marrow

Fatty marrow that does not produce blood

• found in adults

• Can transform back to red marrow in the event of chronic anemia

• fat storage

Ossification or Osteogenesis

the formation of bone

• In the human fetus and infant, bone develops by two methods

  • intramembranous ossification

  • endochondral ossification

• Ossification continues throughout life with the growth and remodeling of bones

  • Bones grow in two directions

    • Length

    • Width

Intramembranous Ossification

Produces flat bones from connective tissue

• Thickens long bones throughout life

Endochondral Ossification

how we make primarily long bones

• exclusive to youth

  1. During infancy and childhood, the epiphyses fill with spongy bone

  2. Cartilage is limited to the articular cartilage and epiphyseal plate

  3. Epiphyseal plate persists through childhood and adolescence

     • Serves as a growth zone for bone elongation

  4. by late teens to early 20s, all remaining cartilage in the epiphyseal plate is generally consumed

  5. Gap between epiphyses and diaphysis closes

  6. Primary and secondary marrow cavities unite into a single cavity

  7. Bone can no longer grow in length

Bone Elongation

Bone elongation is a result of cartilage growth within the epiphyseal plate

• length comes from epiphyseal plate

• the growth from within is called interstitial growth

  • width comes from interstitial growth

• Epiphyses close when cartilage is gone

  • epiphyseal line of spongy bone marks site of former epiphyseal plate

  • Lengthwise growth is finished

• Occurs at different ages in different bones

• the cell type that is active first in long bone growth is a chondroblast

Epiphyseal Plate

cartilage transitions to bone

• Functions as growth zone where bone elongates

• Has hyaline cartilage in the middle with transition zones on each side where cartilage is replaced by bone

• Metaphysis is zone of transition facing the marrow cavity

Achondroplastic Dwarfism

Long bones stop growing in childhood

• Normal torso, short limbs

• Failure of cartilage growth in metaphysis

• Spontaneous mutation produces mutant dominant allele

• bone ossification happens early bc epiphyseal plate has defects

Pituitary Dwarfism

Lack of growth hormone

• Normal proportions with short stature

• pituitary dwarfism is easy to treat with growth hormones

Appositional Growth

the continual growth in diameter and thickness of bones (increasing width)

• occurs at bones surface in the periosteum

• is a form of intramembranous ossification

• How it works:

  • Osteoblasts of inner periosteum deposit osteoid tissue

  • osteoblast becomes trapped as tissue calcifies

  • it lays down the matrix in layers parallel to the surface

    • Forms circumferential lamellae

  • Osteoclasts of endosteum enlarge the marrow cavity

Bone Remodeling

we remodel our bones throughout life (about 10% of our skeleton per year)

• Repairs microfractures, releases minerals into blood, and reshapes bones in response to use and disuse

• Remodeling is a collaborative and precise action of osteoblasts and osteoclasts

Wolff’s Law of Bone Remodeling

the architecture of bone is determined by mechanical stresses placed on it

• Bony processes grow larger in response to mechanical stress

A mature bone remains…

metabolically active

• has a large influence over the rest of the body by exchanging minerals with tissue fluid

Mineral Deposition (mineralization)

process in which calcium, phosphate, and other ions are taken from the blood and deposited in to the bone (to harden bone)

• Osteoblasts produce collagen fibers that spiral the length of the osteon

• the collagen fibers become encrusted with minerals

Mineral Resorption

process of dissolving bone and releasing minerals into blood

• Performed by osteoclasts at ruffled border (HCl at the border)

• HCl dissolves bone minerals

• Acid protease enzyme eats the collagen

Phosphate in the body

involved in bone structure, DNA, RNA, ATP, phospholipids, and pH buffers

Calcium in the body

involved in bone structure, neuron communication, muscle contraction, initiates blood clotting, and exocytosis

• majority of the calcium in our body is stored in our bones

• About 18% of skeletal calcium is exchanged with blood each year

• there is calcium in blood plasma that can diffuse across capillary walls and tissues

Minerals are stored…

in the skeleton and withdrawn when they are needed for other purposes

Hypocalcemia

not enough calcium in the blood

• Changes membrane potentials and causes overly excitable nervous system and tetany (muscle spasms)

• Caused by vitamin D deficiency, diarrhea, thyroid tumors, and under-active parathyroid glands

• Pregnancy and lactation cause an increased risk of hypocalcemia

Hypercalcemia

too much calcium in the blood

• Makes ion channels less responsive and thus nerve and muscle are less excitable

• Can cause emotional disturbance, muscle weakness, sluggish reflexes, cardiac arrest, or kidney stone formation

• this rarely occurs

Calcium Homeostasis

Calcium homeostasis depends on a balance between dietary intake, urinary and fecal losses, and exchanges between osseous tissue

• Calcium homeostasis is regulated by three hormones:

  • Calcitriol

  • Calcitonin

  • Parathyroid hormone (PTH)

• loss of calcium homeostasis disrupts functions of other organ systems, especially electrically active tissues

Calcitriol

a hormone that is the most active form of vitamin D and raises blood calcium level

• it increases calcium absorption using the small intestine

  • digestive absorption is the primary method to absorb calcium into bloodstream

• It also increases calcium resorption from the skeleton by stimulating osteoblasts to release a chemical that stimulates production of more osteoclasts

• It weakly promotes kidney reabsorption of calcium ions, so there is less lost in urine

• synthesized from UV light, then gets processed through the skin, liver, and then to the kidney to get calcitriol

• necessary for bone deposition—helping provide enough calcium and phosphate

• Inadequate calcitriol results in abnormal softness of bones in children (rickets) and in adults (osteomalacia)

Calcitonin

a hormone secreted by C cells (clear cells) of the thyroid gland when blood calcium levels rise too high

• Lowers blood calcium concentration in 2 ways:

  • inhibits osteoclasts thereby reducing bone resorption

  • stimulates osteoblasts to deposit calcium into bone

• is important in children, but has a weak effect in adults

  • Osteoclasts are more active in children due to faster remodeling

• helps inhibit bone loss in pregnant and lactating women

Negative feedback loop to correct hypercalcemia:

Parathyroid Hormone (PTH)

a hormone secreted by parathyroid glands on the posterior surface of the thyroid

• PTH is released when calcium levels are low in the blood

• PTH raises calcium blood level by 4 mechanisms:

  • Increasing osteoclast population and bone resorption

  • Promotes calcium reabsorption by kidneys, so less lost in urine

  • Promotes the final step of calcitriol synthesis in the kidneys, enhancing calcium-raising effect of calcitriol

  • Inhibits collagen synthesis by osteoblasts, inhibiting bone deposition

Negative feedback loop to correct hypocalcemia:

Phosphate Homeostasis

• most of our phosphate is stored in our bones

• we have phosphate in our blood plasma, but not as much as there is calcium

• Phosphate levels are not regulated as tightly as calcium levels

• Calcitriol raises phosphate levels by promoting its absorption by the small intestine

• PTH lowers blood phosphate levels by promoting its urinary excretion

Bone Growth in Puberty

Bone growth is especially rapid in puberty and adolescence

• Surges of growth hormone, estrogen, and testosterone occur and promote ossification (bone building)

• These hormones stimulate the multiplication of osteogenic cells, matrix deposition by osteoblasts, and chondrocyte multiplication and hypertrophy in metaphyses

• Girls grow faster than boys and reach full height earlier because estrogen has a stronger effect than testosterone on bone growth

• Males grow for a longer time and also taller

Anabolic Steroids

cause growth to stop

• Epiphyseal plate “closes” prematurely

• Results in abnormally short adult stature

Orthopedics

branch of medicine dealing with prevention and correction of injuries and disorders of bones, joints, and muscles

• Includes the design of artificial joints and limbs and the treatment of athletic injuries

Stress Fracture

a break caused by abnormal trauma to a bone

• ex) in a fall

Pathological Fracture

break in a bone weakened by disease (such as bone cancer or osteoporosis)

• Usually caused by a stress that would not break a healthy bone

Non-Displaced Fracture

the fragments of the fracture are still aligned

Displaced Fracture

fracture with shifted fragments

• bone needs to be set before healing

Comminuted Fracture

fracture with more than 2 pieces

Greenstick Fracture

a partial fracture

• mostly seen only in young children

Compound Fracture

broken bone sticks out of the skin

Steps to Healing a Fracture

1. Hematoma formation

2. Soft callus formation

3. Hard callus formation

4. Bone Remodeling

Hematoma Formation

The hematoma is converted to granulation tissue by the invasion of cells and blood capillaries

• periosteum helps keep the blood clot in place

Soft Callus Formation

the deposition (deposit) of collagen and fibrocartilage converts granulation tissue to a soft callus

Hard Callus Formation

Osteoblasts deposit a temporary bony collar around the fracture to unite the broken pieces while ossification (bone building) occurs

Bone Remodeling

Small bone fragments are removed by osteoclasts, while osteoblasts deposit spongy bone and then convert it to compact bone

• where bone has been broken, compact bone replaces it

• your bone becomes stronger after breaking it

Closed Reduction

procedure where bone fragments are manipulated into their normal positions without surgery

Open Reduction

involves surgical exposure of the bone and the use of plates, screws, or pins to realign the fragments

Traction

the act of applying a controlled pulling force to a broken bone or body part to realign it into proper position and stabilize it, often using weights and pulleys

• used often for healing children’s femoral fractures

Osteoporosis

Severe loss of bone density

• the most common bone disease

• Bones lose mass and become brittle due to loss of organic matrix and minerals

  • brittle bones from osteoporosis fracture and heal slowly due to decreased protein synthesis

• Affects spongy bone the most since it is the most metabolically active

• Subject to pathological fractures of hip, wrist, and vertebral column

• the loss is severe enough to compromise a person’s physical activity and health

Kyphosis

a deformity of spine due to vertebral bone loss

• aka widow’s hump

• Complications of loss of mobility are pneumonia and thrombosis

Estrogen

hormone that maintains bone density in both sexes

• inhibits resorption by osteoclasts

Who is most at risk for osteoporosis?

Postmenopausal white women at greatest risk

• Ovaries cease to secrete estrogen

• White women begin to lose bone mass as early as age 35

• can also be seen in young female athletes with very low body fat

Risk factors:

• race, age, gender, smoking, diabetes mellitus, diets poor which are poor in: calcium, protein, vitamins C and D

Treatments for Osteoporosis

• Estrogen replacement therapy (ERT)

  • slows bone resorption, but increases risk of breast cancer, stroke, and heart disease

• Drugs Fosamax and Actonel

  • destroy osteoclasts

• PTH injection

  • slows bone loss if given daily

• prevention - the best treatment, includes exercise and good diet

Osteopenia

loss of bone mass