Microanatomy of Skeletal System Quiz

Child vs.adult skeleton

• Epiphyseal plate vs. line

• Flexible and cartilage rich vs. fully mineralized and denser bone

• 270-300 bones vs. 206

Long bones

• Longer than wide

• Has epiphyses

• Ex. Arm and leg bones

Short bones

• Small and boxy

• Ex. Carpals and tarsals

Sesamoid bones

• Small and flat

• Develop in tendons

• Ex. Patella

• Location and number varies b/w individuals

Flat bones

• Thin and roughly parallel

• Protect and provide surface area for muscle attachment

• Ex. Cranial bones, sternum, ribs, scapulae

Irregular bones

• Complex shapes

• Ex. Vertebrae, pelvis, facial bones

Sutural bones

• Irregular bones b/w cranial bones

• Number, size and shape vary

Functions of skeletal system

• Structural support

• Framework for attachment of soft tissues

• Storage of minerals

  • Calcium and phosphate concentrations in blood must be mantained

    • Calcium salts act as mineral reserve

• Adipose tissue stores lipids

• Hematopoeisis

• Protects tissues

• Leverage (change magnitude and direction of skeletal muscle forces)

Hematopoiesis/ hemopoiesis

Blood cell production

• All types of blood cells produced in red bone marrow

Parts of long bone

• Epiphysis

  • Spongy bone

  • Compact bone

  • Articular cartilage

• Metaphysis

• Diaphysis

  • Medullary cavity

    • Red and yellow marrow

• Periosteum and endosteum

• Blood vessels

  • Blood supply important for growth and maintenance

  • Supply osetons w/ blood

  • Nutrient forarm provides access to cavity

Epiphyses

• At each end of long bone

• Consists mostly of spongy bone

Diaphysis

• Shaft

• Long, tubular

• Contains medullary cavity

Metaphysis

Connects epiphysis to shaft

• Contains epiphyseal plate/ line

Spongy bone

• Mostly in epiphysis

• Made of trabeculae, creating open network

  • Red bone marrow b/w trabeculae

• Holey, NOT soft

  • Spaces caused by osteoclasts breaking down before osteoblasts build back up

• No blood vessels in matrix

  • Nutrients reach osteons through canaliculi open to trabeculae surfaces

• Resists force from various directions

• Distributes body weight and reduces weight of skeleton

• Innermost layer

• Made of same stuff as compact bone

Compact bone

• AKA cortical bone

• Outer layer of bone

• Strong and organized

• Made of tightly packed osteons

• Protection and stress resistance

Articular cartilage

• Made of hyaline cartilage

• Covers articulating parts of epiphyses

• Avascular

Marrow cavity

• AKA medullary cavity

• In diaphysis

• Contains yellow marrow

• Usually lined w/ endosteum

Yellow marrow

• Adipose tissue

  • Energy reserve

• Mostly in medullary cavity

Red marrow

• Involved in hemopoiesis

• Mostly in spongy bone

  • Ends of long bones and flat bones

Periosteum

• Lines outside of bones (peri- around; osteum- bone)

  • wraps superficial layer of compact bone

  • Not where articular cartilage is

  • Isolates bone from surrounding tissue

• Provides blood and nervous supply route

• Promotes new bone production

  • Needs lots of blood vessels

  • Makes new tissue, supplying osteogenic cells

  • Part of appositional growth

Endosteum

• Lines inner surface of bone, medullary cavity of long bones and surfaces of trabeculae of spongy bone

• Used to be periosteum

• Makes new tissue, supplying osteogenic cells, promoting bone production

Osteocytes

• Osteo- bone; cyte- cell

• Mature cells

  • Can’t divide

• Maintain protein and mineral content of matrix

  • Dissolve matrix → release minerals into blood

  • Rebuild matrix → deposit mineral crystals from blood

• Occupy lacunae

  • Separated by lamellae

  • Connected by canaliculi

Osteoblasts

• ”Build;” produce new bony matrix

  • Osteogenesis/ ossification

  • Begins w/ release of proteins to produce osteoid

  • Assists in calcium salt deposit to convert osteoid to bone

• Surrounded by bony matrix → becomes osteocyte

Osteogenic cells

• Stem (Mesenchymal) cells that produce cells that differentiate into osteoblasts

• Fracture repair

• In inner lining of periosteum, lining endosteum in marrow cavity and lining passageways w/ blood vessels

Osteoclasts

• Remove and remodel bone matrix

  • Release acids and enzymes to dissolve matrix to release stored minerals into blood

  • Osteolysis

  • “Crush;” “break down”

Osteogenesis

• AKA ossification

• Production of bone by osteoblasts

Osteolysis

active resorption (dissolving/ breaking down) of bone matrix by osteoclasts

Osteon

• Make up compact bone

• Organized into lamellae around central canal

  • Connected by perforating canals

• Contain osteocytes in lacunae, blood vessels in central canal, and canaliculi

Lacunae

Pockets between lamellae that contain osteocytes

Canaliculi

Small channel; delivery system, connecting lacunae with each other and the central canal

Circumferential lamellae

Layers of bone matrix wrapping around bone’s entire circumference (outer and inner surfaces)

• One of bone layers lying beneath periosteum and endosteum

Interstitial lamellae

Layers of bone matrix between osteons

Concentric lamellae

Layers making up an osteon

Trabeculae

• Make up spongy bone

• Create open network

• Red bone marrow b/w them

• Contain lamellae and osteocytes and lined by endosteum

Effects of collagen in the bone matrix

• Provide flexibility

  • Taken out → brittle

• 1/3 bone weight

• Protein

Effects of calcium salts in the bone matrix

• Gives rigidity

  • Taken out → flexible/ soft

  • Lack of calcium → rickets

• Most abundant mineral in body w/ 99% in skeleton

Importance of calcium bank deposits and withdrawls

Maintain homeostasis so physiological functions can occur

• 8.5- 11 mg/dL needed

Physiological functions of calcium

Blood clotting/ coagulation, neural communication and muscle contraction

Role of intestines, kidneys and bones in calcium homeostasis

• Intestines absorb calcium from food

• Kidneys release calcium in urine

• Bones erode or build up bone matrix, releasing or taking up calcium from blood

Calcitonin

• Secreted from thyroid gland

• Decreases blood calcium levels

  • Increases deposits of calcium into bone, lowering blood calcium

  • Bones: Osteoclasts inhibited; calcium deposited in matrix

  • Intestines: Calcium absorption decreases w/ decreasing PTH and calcitriol

  • Kidneys: Inhibits calcitriol release and calcium reabsorption

PTH

• Parathyroid hormone

  • Secreted from parathyroid glands

• Stimulates organs to release more calcium in blood when calcium is low

  • Bones: Osteoclasts erode matrix, releasing stored calcium ions

  • Intestines: Calcitriol causes calcium absorption to increase

  • Kidneys: Increased calcitrol release → stimulates calcium reabsorption

Calcitriol

Form of vitamin D that increases calcium in blood

• Increase intestine absorption

• Stimulate kidney reabsorption

Osteoporotic bone

• disease that makes bones weak, brittle, and more likely to break

• bone breakdown exceeds bone formation → loss of bone mineral density

• Excessively “porous”

Appositional bone growth

• Increases bone diameter

• Additional circumferential lamellae deposited and bone diameter continues to increase

  • Collagen fibers from surrounding tendons/ joints/ ligaments cemented into circumferential lamellae

    • Osteogenic cells differentiate into osteoblasts that add bone matrix under periosteum

      • Endosteum also involved

    • Strong attachment (bone break before snapping fibers)

  • Trapped osteoblasts become osteocytes

  • Deeper lamellae recycled and replaced by osteons

• Bone matrix removed by osteoclasts to enlarge medullary cavity, then bone deposited by superficial osteoblasts

Endochondral ossification

“W/in cartliage”

• Starts as hyaline cartilage in embryo and gradually replaced by bone

• Growth in diameter and length

• Forms long bones

• Hormonal changes stimulate bone growth

  • Osteoblasts produce bone faster than chondrocytes produce cartilage

  • Epiphyseal closure: Epiphyseal cartilage narrows until disappears

1. Cartilage model enlarges: Chondrocytes in diaphysis enlarge, then die/ disintegrate, leaving cavities in cartilage

2. Blood vessels grow around edge

   1. Perichondral cells convert to osteoblasts, which form superficial bone layer

3. Blood vessles penetrate cartilage

   1. Fibroblasts → osteoblasts

   2. Spongy bone production at POC

   3. Bone formation spreads toward both ends

4. Remodeling: Medullary cavity created, diaphysis thickens, cartilage near epiphyses replaced by shaft and bone grows and length and diameter

5. Capillaries and osteoblasts migrate to epiphyses → SOC

6. Epiphyses fill w/ spongy bone

7. Bone grows in length around epiphyseal cartilage

   1. Chondrocytes produce cartilage near epiphyses, osteoblasts replace cartilage w/ bone near diaphysis

   2. Epiphyses pushed away by continued cartilage production

Primary ossification center

• In diaphysis

• Where chondrocytes near future medullary cavity enlarge

  • Becomes medullary cavity

• Spongy bone production

Secondary ossification center

• In epiphyses

  • Fill w/ spongy bones

  • After medullary cavity formed capillaries and osteoblasts migrate to epiphyses to create this

Intramembranous ossification

“ w/ in membrane”

• Stem cells differentiate into osteoblasts in connective tissue

• Mostly create flat bones

• Ex. Skull, low jaw, collarbone, patella, etc.

• Beings in 8th week of embryonic development w/ ossification visible at 10 weeks

1. Stem cells secrete osteoid matrix → osteoblasts

2. Bone grows out in spicules (small struts) → osteoblasts trapped in pockets and mature into osteocytes

3. Blood vessles enter and spicules meet and fuse → vessels trapped in developing bone

4. Continued osteoblasts activity near blood vessel → spongy bone w/ interwoven blood vessels

5. Remodeling around vessels produces osteons in compact bone: Connective tissue around bone becomes fibrous periosteum, while osteoblasts near surface becomes cellular periosteum

Epiphyseal plate vs. line

plate is layer of cartilage in children and adolescents, allowing long bones to grow in length, while line is bony remnant of plate in adults after growth has stopped

Bone remodeling

lifelong process where mature bone tissue broken down and new bone formed, maintaining bone strength and structure

• Always osteoclast and osteoblast activity

• Surfing of gravity stimulates osteoblasts → more movement= live more independent later