Introduction to the Skeletal System

• There are 206 bones in the human body, divided into two groups.

Axial Skeleton

• Forms the body's long axis.
• Includes the skull, vertebral column, and rib cage.

Appendicular Skeleton

• Composed of the appendages (upper and lower extremities).
• Attaches to the axial skeleton.
• Bone is osseous tissue.

Bone Characteristics

• Bone includes connective tissue, nerve tissue, cartilage, muscle, and epithelial tissue.
• Classified as organs.
• Highly vascular with a rich nerve supply.
• An active tissue, with 5% replaced every week.
• 99% of the body’s calcium is found in bone.
• Passageways for blood vessels, nerves, and lymph channels are present in bone.
• Calcium+ is deposited around the ground substance and collagen.

Embryonic Bone

• Begins as hyaline cartilage.
• Hardens to form adult bone through ossification.

Cartilage

• Avascular and lacks a nerve supply.
• Heals poorly and slowly.
• Semi-solid pliable gel.
• The matrix contains chondroitin sulfate.
• All types contain chondrocytes and collagen fibers.
• Chondrocytes form from chondroblasts.

Hyaline Cartilage

• Most abundant type of cartilage.
• Covers the ends of long bones as a shock absorber.
• Connects ribs to the sternum.

Elastic Cartilage

• Exhibits extreme flexibility.
• Contains fibroblasts and elastin.
• Located in the external ear.

Fibrocartilage

• Contains strong and dense collagen fibers.
• Able to withstand heavy pressure.
• Found in the knee and spinal discs.

Cartilage Growth

• Facilitated by chondrocytes.
• Appositional growth increases width.
• Interstitial growth increases length.

Bone Function

• Provides support as the body's framework.
• Offers protection to internal organs.
• Facilitates movement by providing attachment points for skeletal muscles.
• Acts as a mineral storage reservoir for calcium and phosphorus.
• Involved in blood formation (occurs in marrow, both red and white).
• Responsible for triglyceride storage (occurs in yellow bone marrow).

Bone Structure

• 30% of bone is composed of osteoid, which is uncalcified bone matrix made of collagen.
• 70% of bone is composed of calcium/phosphate, also known as hydroxyapatite crystals.
• Hydroxyapatite and osteoid together form a cement-like material.
• Lack of calcium results in weak and soft bones.

Bone Cells

• Made from mesenchyme.
• Osteoblasts secrete bone matrix (osteoid) to form new bone.
• Osteocytes are mature cells.
• Osteoclasts break down bone.
• Osteogenic cells are involved in trauma response.

Haversian System

• Osteon is the long, cylinder-shaped unit of bone.
• Osteons are densely packed together.
• Lamellae form circular rings, providing a hard/dense texture.
• Contain a central Haversian canal.
• Haversian canals are connected via Volkmann's canals.
• Lamellae are layers of circular rings.
• Lacunae are empty cavities where osteocytes reside, with one lacuna per osteocyte.

Bone Membranes

• Periosteum is a pearly white, thin connective tissue membrane, containing blood vessels and nerves.
• Found in all areas, except epiphyses.
• Contains osteoblasts and osteoclasts.
• Endosteum is a vascular connective tissue layer inside the marrow cavity.
• Bone marrow is the hemopoietic tissue

Bone Marrow

• Red marrow produces erythrocytes.
• White marrow produces leukocytes.
• Yellow marrow is for fat storage only.

Bone Markings

• Articulations are joints.
• Projections are attachment points for muscles/ligaments.
• Foramen are holes for blood vessels and nerves.

Types of Bone

• Compact bone provides a hard, smooth external covering found everywhere.
• The structural unit of compact bone is the osteon, which acts as a tiny weight-bearing pillar.
• Spongy bone, also known as cancellous bone, lacks osteons and provides a honeycomb-like internal covering.
• The honeycomb pattern is formed by thin columns called trabeculae.
• Trabeculae give strength to bone. In summary, trabeculae are thin, while osteons are thick.

Bone Classifications

• Bones are classified by shape, not size.
• Long bones are longer than they are wide, including fingers even though they are small.
• Epiphyses are the ends of long bones, covered with hyaline cartilage for protection.
• Hyaline reduces friction providing a gliding surface and contain the epiphyseal plate (growth plate).
• The plate produces osteocytes=growth allowing bone to grow lengthwise during childhood and adolescence.
• Diaphysis is the tubular shaft, wrapped in periosteum and highly vascular.
• The bone marrow cavity is located in the middle of the diaphysis.

Other Types of Bones

• These contain periosteum, compact, and spongy bone.
• There is no shaft or epiphyses so they contains a growth plate and no marrow cavity

Short Bones

• Cube-like in shape (e.g., wrist and ankle bones)

Flat Bones

• Thin, flat, and curved (e.g., sternum/breastbone, scapula/shoulder blades, ribs, most skull bones)

Sesamoid Bones

• Shaped like a sesame seed (e.g., patella)

Irregular Bones

• Complex shapes that do not fit into other categories (e.g., vertebrae)

Bone Formation (Osteogenesis)

• Bones harden during osteogenesis, known as ossification.
• Intramembranous ossification occurs in utero (womb) to adolescence, without cartilage involvement, where embryonic tissue (mesenchyme) turns into osteoblasts.
• Mesenchyme turns into Osteoblasts which turn into Osteocytes
• The human skeleton is formed in the fetus during week 8.
• Endochondral ossification occurs from adolescence (bone growth) throughout life (fracture repair), where hyaline cartilage is replaced by osteoblasts which leads to long bone growth (e.g., humerus, radius, ulna) and increases the length of a growing bone.
• Full ossification is the final step.

Bone Growth

• Occurs at the epiphyseal plate.
• Osteoblasts mature into osteocytes, occurring from epiphysis to diaphysis.
• Appositional occurs throughout life and increases bone width/diameter.
• Interstitial growth which increases bone length, stops after epiphyseal plate closure.

Hormones of Bone Growth

• Regulate blood calcium levels
• Calcitonin sends calcium into bone for storage when not needed.
• PTH (Parathyroid Hormone) takes calcium out of bone into blood.
• Calcitriol, made from Vitamin D3, is produced in the kidneys, has a direct effect on bones, and stimulates the intestines to absorb adequate calcium and phosphorus.
• Alkaline Phosphatase holds calcium in bone (enzyme), so AP is manufactured by osteoblasts, and a common blood test ordered by physicians.
• Estrogen preserves bone and stimulates the formation of new bone and inhibits osteoclast breakdown of bone.
• Testosterone is converted into Estrogen in all men, during which fat cells perform this conversion, which stimulates the formation of new bone and inhibits osteoclast breakdown of bone.

Bone Repair

• Occurs throughout life where osteoclasts eat or reabsorb dead/damaged bone cells and osteoblasts replace the dead bone cell.
• Bone remodeling is a balance between osteoblast and osteoclast activity.
• Mineral deposit, mineralization by osteoblasts, where calcium and phosphate are taken from blood and deposited in bone.
• Mineral reabsorption, bone breakdown by osteoclasts, dissolving Hydroxyapatite into minerals which can occur normally or during a disease process.
• Wolff's Law is that bone is added when there is a demand for it and bone is removed when there is no demand (use it or lose it).

Bone Fractures

• Greenstick fractures: bone fractures incompletely, one side breaks, the other side bends.
• Spiral fractures: spiraling damage causes by a twisting force common sports fracture.
• Transverse fractures occur at a 90-degree angle to the bone and are complete.
• Fissured fractures: incomplete longitudinal break.
• Comminuted fractures: complete with more than 2 fragments, excessive damage, common in the elderly.
• Compression fractures: bone is crushed and flattened, common vertebral fracture (spine).
• Depressed fractures: fractured bone is depressed inward, common skull fracture
• Epiphyseal fractures: epiphysis separates from the diaphysis along the growth plate can seriously impair future bone growth.

Fracture Intervention

• Open fractures involve broken skin.
• Closed reduction realigns of the bones are by a physician manually moving the fractured ends back together.
• Open reduction is Surgical Re-alignment involving securing the fractured ends with pins/wires/screws.
• Casting uses fiberglass casting is used to immobilize and allow healing with and a 6-8 week healing time.
• Can be much longer in the elderly due to poor circulation or in heavy weight bearing bones.

Fracture Repair

• Hematoma Formation is the first step where blood vessels tear and hemorrhage causing clotted blood and inflammation/pain.
• Fibrocartilaginous Callus Formation is that next step when soft callus begins to form, new capillary beds form, and osteoblasts/osteoclasts migrate into the site; spongy bone formation begins during this phase.
• Bony Callus Formation is the next step whwen hard callus begins to form, new bone trabeculae form, beginning 3-4 weeks post fracture.
• Bone Remodeling is the final step, where callus is remodeled to resemble the original bone.

Diseases of Bone

• Osteomalacia is when bones are not adequately mineralized, osteoid is produced, but calcium is not deposited where the bones are soft & brittle and causes by either a Calcium or Vitamin D deficiency.
• Rickets, osteomalacia in children leads to deformities of the pelvis, skull, and ribs.
• Paget's Disease is the malfunction in bone remodeling, excessive bone breakdown, which results in pain, bone weakness, and deformity
• Osteopenia is when the bone mineral density is reduced to lower than normal levels and reversible with medicine/diet.
• Osteoporosis is when bones become porous & brittle and occurs when reabsorption outpaces bone deposit commonly seen in postmenopausal females due to a drop off in Estrogen production.
• Is not seen in males due to Testosterone production with the best prevention= adequate intake of Calcium/Mg, Vitamin D, & regular weight bearing exercise

Menopause

• Problematic difference in men vs women with with an average age of 51 to begin where stoppage of reproductive cycles, no ovulation, change in female secondary sex characteristics, dramatic decrease in Estrogen production, zero in some women, significant bone loss in post-menopausal women.
• Majority of men are not affected by Osteopenia/Osteoporosis and men can produce life-long TESTOSTERONE.

Male vs Female Skeleton

• Men have larger & stronger bones and Testosterone controls size of bone.
• Females have stronger & wider pelvic bones, where where the mean pelvis=wine glass and the women pelvis=champagne glass
• Women do not go through Menopause but men do.