Skeletal System: Structure, Function, and Homeostasis

Unit 2: Support and Movement - Skeletal System

Learning Outcomes

  • List the functions of the skeletal system.
  • Classify bones.
  • Identify the parts of a long bone.
  • Describe bone cells.
  • Compare compact and spongy bone.
  • Classify fractures.
  • Discuss calcium and homeostasis.
  • Define skeletal terminology and apply these terms to principles bone markings.
  • Identify the bones and their markings in a variety of views for both the appendicular and axial skeletons.
  • Define the curvature of the vertebrae and alternative curves.
  • Understand skeletal system terms from pages 521-522 of the reference list.

Additional Terms

  • -clast: Suffix often indicating breaking down (e.g., osteoclast).
  • Condyle: A rounded articular projection.
  • Cost/o: Pertaining to the ribs.
  • Cox/o: Pertaining to the hip.
  • Crani/o: Pertaining to the skull.
  • Foramen: An opening, hole, or passage, especially in a bone.
  • Fossa: A shallow depression or hollow area in a bone.
  • -itis: Suffix indicating inflammation.
  • Mega-: Prefix meaning large.
  • -oid: Suffix meaning resembling or like.
  • Ped/o: Pertaining to the foot or child.
  • -physis: Suffix meaning growth.
  • Pod/o: Pertaining to the foot.
  • Xiph/o: Pertaining to the xiphoid process.
  • Zyg/o: Pertaining to a yoke or pair (e.g., zygoma).

Functions of the Skeletal System

  • Support: Provides a framework for the body and attachment points for most muscles.
  • Movement: Serves as levers for muscles to pull on, generating movement.
  • Protection: Protects internal organs (e.g., skull protects brain, rib cage protects heart and lungs).
  • Mineral Homeostasis: Bone tissue stores several minerals, especially calcium and phosphorus, releasing them into the bloodstream as needed to maintain mineral balances.
  • Blood Cell Production (Hematopoiesis):
    • Occurs in all bones of a fetus.
    • In adults, hematopoiesis primarily occurs in the red bone marrow found within spongy bone.

Overview of the Skeletal System

  • The human skeletal system consists of 206 bones.
  • It is divided into two main parts:
    • Axial Skeleton: Comprises 80 bones, forming the central axis of the body.
    • Appendicular Skeleton: Comprises 126 bones, forming the limbs and their girdles.

Axial Skeleton

Consists of 80 bones including:

  • Skull
    • Cranium: 8 bones
    • Face: 14 bones
  • Hyoid: 1 bone (supports the tongue)
  • Auditory Ossicles: 6 bones (three in each ear)
  • Vertebral Column: 26 bones (vertebrae)
  • Thorax
    • Sternum: 1 bone (breastbone)
    • Ribs: 24 bones (12 pairs)

Appendicular Skeleton

Consists of 126 bones including:

  • Pectoral Girdles (shoulder)
    • Clavicle (collarbone): 2 bones
    • Scapula (shoulder blade): 2 bones
  • Upper Limbs (arm, forearm, hand)
    • Humerus: 2 bones (upper arm)
    • Ulna: 2 bones (forearm)
    • Radius: 2 bones (forearm)
    • Carpals (wrist bones): 16 bones (8 per wrist)
    • Metacarpals (palm bones): 10 bones (5 per hand)
    • Phalanges (finger bones): 28 bones (14 per hand)
  • Pelvic Girdle (hip)
    • Hip or pelvic bone: 2 bones (each composed of ilium, ischium, and pubis)
  • Lower Limbs (thigh, leg, foot)
    • Femur (thigh bone): 2 bones
    • Patella (kneecap): 2 bones
    • Fibula (lower leg): 2 bones
    • Tibia (shin bone): 2 bones
    • Tarsals (ankle bones): 14 bones (7 per ankle)
    • Metatarsals (foot bones): 10 bones (5 per foot)
    • Phalanges (toe bones): 28 bones (14 per foot)

Bone Cells

  • Osteoblasts:
    • Produce bone matrix (collagen and other organic components).
    • Take up minerals (like calcium and phosphate) from the blood to deposit them into the matrix, leading to calcification.
    • Crucial for bone growth and repair.
  • Osteocytes:
    • Mature bone cells derived from osteoblasts that have become trapped within the calcified matrix.
    • Primary function is the maintenance of bone tissue (e.g., nutrient and waste exchange).
  • Osteoclasts:
    • Large cells that break down (resorb) bone tissue.
    • Secrete enzymes and acids to dissolve the bone matrix, releasing minerals (e.g., calcium) into the bloodstream.
    • Essential for bone remodeling, repair, and mineral homeostasis.

Bone Histology

Bone tissue consists of:

  • Dense Matrix:
    • Composed of calcium salts and collagen fibers.
    • Collagen fibers: Provide strength and flexibility to the bone.
    • Water: Component of the matrix.
    • Crystallized minerals: Primarily calcium phosphate (hydroxyapatite) makes bones hard and provides their compressive strength. Also contains some phosphate and magnesium.
  • Bone Marrow:
    • Soft tissue located within the bone cavities.
    • Red bone marrow is responsible for hematopoiesis.
  • Epithelial Tissue: Lines bone cavities and blood vessels within bone.
  • Blood Vessels and Nerves: Supply nutrients and innervation to bone tissue.

Types of Bone

Compact (Cortical) Bone

  • Highly organized into repeating structural units called osteons or Haversian systems.
  • Forms the dense outer layer of all bones, making up the bulk of the diaphysis (shaft) of long bones.
  • Provides protection and support and helps bones resist external stressors (e.g., weight, movement).

Spongy (Trabecular) Bone

  • Less organized than compact bone, characterized by a lattice-like network of bony plates called trabeculae.
  • Found in short, flat, and irregular bones, and in the epiphyses (ends) of long bones.
  • Lighter in weight than compact bone.
  • The spaces within the trabeculae are filled with red bone marrow, where hematopoiesis (the manufacture of blood cells) occurs.

Compact Bone Structure (Osteons)

  • Osteons (Haversian systems): Repeating cylindrical structural units.
  • Central (Haversian) Canal: A canal running longitudinally through the center of an osteon, containing blood vessels and nerves.
  • Perforating (Volkmann's) Canals: Canals that run perpendicularly to the central canals, connecting blood vessels and nerves of the periosteum and medullary cavity to the central canals.
  • Concentric Lamellae: Circular layers of calcified matrix that surround the central canal within an osteon, forming its bulk.
  • Lacunae: Small spaces or cavities located between the lamellae, which house osteocytes.
  • Canaliculi: Tiny canals radiating out from the lacunae, forming a network that connects lacunae to each other and to the central canal, allowing nutrients and wastes to be exchanged with osteocytes.

Classification of Bones

Bones are classified based on their shape:

  • Long bones: Longer than they are wide, with a shaft and two ends (e.g., femur, humerus).
  • Short bones: Cube-shaped, nearly equal in length and width (e.g., carpals, tarsals).
  • Flat bones: Thin, flattened, and often curved (e.g., parietal bone of skull, sternum, scapula).
  • Irregular bones: Have complex shapes that do not fit into the other categories (e.g., vertebrae, hip bones).
  • Sesamoid bones: Small, round bones embedded within tendons, typically found in joints of the knees, hands, and feet (e.g., patella).

Structure of a Typical Long Bone

Regions

  • Diaphysis: The shaft or main elongated portion of a long bone. Its hollow center contains the medullary cavity.
  • Epiphyses: The proximal and distal ends of a long bone. They are typically wider than the diaphysis and consist of spongy bone covered with a thin layer of compact bone.
  • Metaphyses: The regions in a mature bone where the diaphysis joins the epiphyses. In a growing bone, this region includes the epiphyseal (growth) plate/cartilage.
  • Medullary Cavity: The hollow cylindrical space within the diaphysis, containing yellow bone marrow (fat) in adults.

Tissues of Long Bone

  • Articular Cartilage: A layer of hyaline cartilage covering the epiphyses where the bone forms a joint. Reduces friction and absorbs shock.
  • Periosteum: A tough, dense irregular connective tissue membrane covering the outer surface of bone wherever articular cartilage is not present.
    • Contains bone-forming cells (osteogenic cells) that differentiate into osteoblasts, contributing to bone growth in width.
    • Provides attachment points for ligaments and tendons.
    • Rich in nerves and blood vessels.
  • Endosteum: A thin membrane that lines the medullary cavity and covers the trabeculae of spongy bone. Contains osteoblasts and osteoclasts, important for bone remodeling.

Ossification (Bone Formation/Growth)

  • Definition: The process of bone growth or bone formation.
  • Two Types of Ossification:
    • Intramembranous Ossification: Bone forms directly within mesenchymal (fibrous connective tissue) membranes, characteristic of flat bones of the skull and clavicles.
    • Endochondral Ossification: Bone forms by replacing a cartilaginous model. Most bones of the body, especially long bones, form this way.

Factors Affecting Bone Growth

  • Diet: Adequate intake of:
    • Minerals: Calcium, phosphorus, and magnesium are crucial for bone matrix formation.
    • Vitamins: Vitamin A (stimulates osteoblast activity), Vitamin C (needed for collagen synthesis), and Vitamin D (enhances calcium absorption from the gut).
  • Exercise: Weight-bearing exercise increases mechanical stress on bones, which stimulates osteoblast activity and increases bone density. It also increases the production of growth hormone (hGH).
  • Hormones: Various hormones play critical roles in regulating bone growth throughout life, particularly during infancy, childhood, and puberty.

Hormones Involved in Regulating Bone Growth and Remodeling

  • Parathyroid Hormone (PTH):
    • Secreted by the parathyroid glands when blood calcium levels are low.
    • Stimulates osteoclast activity, leading to bone resorption and the release of calcium into the blood, thus increasing blood calcium levels.
  • Calcitonin:
    • Secreted by the thyroid gland when blood calcium levels are high.
    • Inhibits osteoclast activity, thereby decreasing bone resorption and promoting the uptake of calcium into bone, lowering blood calcium levels.
  • Growth Hormone (hGH):
    • Stimulates bone growth primarily by stimulating the production of Insulin-like Growth Factors (IGFs).
  • Insulin-like Growth Factors (IGFs):
    • Produced by the liver and bone tissue in response to hGH.
    • Directly stimulate osteoblast activity, protein synthesis, and cell proliferation, leading to bone growth.
  • Thyroid Hormones (T3 and T4):
    • Promote bone growth by stimulating osteoblast activity and overall metabolism.
  • Insulin:
    • Plays a role in stimulating the uptake of circulating calcium and its incorporation into the bone matrix.
  • Sex Hormones (Estrogen and Testosterone):
    • Stimulate osteoblast activity and promote the growth spurt during puberty.
    • Contribute to the epiphyseal plate closure, halting longitudinal bone growth.

Calcium Homeostasis

  • Importance of Calcium: Calcium is vital for numerous physiological processes, including muscle contraction, nerve impulse transmission, blood clotting, and enzyme activity.
  • Regulation: The body maintains calcium homeostasis through a feedback system involving hormones that adjust bone cell activities to keep blood calcium levels within a healthy range.
    • Low Blood Calcium: Detected by parathyroid glands, which secrete Parathyroid Hormone (PTH). PTH increases osteoclast activity, causing calcium to leave bones and enter the bloodstream.
    • High Blood Calcium: Detected by the thyroid gland, which secretes Calcitonin. Calcitonin inhibits osteoclasts, thereby decreasing the amount of calcium released from bones into the blood.

Bone Formation (Ossification) Occurs in Four Situations

  1. Initial Formation in an Embryo: The development of bone tissue during fetal development.
  2. Growth of Bones: From infancy until adult size is reached, involving longitudinal growth at epiphyseal plates and appositional growth (increase in width).
  3. Remodeling of Bone: The continuous replacement of old bone tissue with new bone tissue by coordinated action of osteoclasts and osteoblasts, adapting bone to stress and maintaining mineral homeostasis.
  4. Repair of Fractures: The process by which damaged bone tissue self-repairs after a break.

Bone Fractures

  • Fracture: Any break in a bone.
  • Types of Fractures:
    • Partial: An incomplete break across the bone, such as a crack.
    • Complete: A complete break across the bone, resulting in two or more separate pieces.
    • Closed (Simple): The fractured bone does not break through the skin.
    • Open (Compound): The broken ends of the bone protrude through the skin, increasing risk of infection.
    • Greenstick: A partial fracture in which one side of the bone is broken and the other side bends (common in children).
    • Transverse: A fracture straight across the bone's long axis.
    • Spiral: The break spirals around the bone, often caused by twisting force.
    • Comminuted: The bone shatters into three or more pieces.

Repair of a Fracture Steps

  1. Hematoma Formation: Blood vessels rupture, and a blood clot (hematoma) forms at the fracture site.
  2. Phagocytosis: Phagocytes (e.g., macrophages) remove any dead bone tissue and debris from the fracture site.
  3. Fibrocartilaginous Callus Formation: Chondroblasts form a soft fibrocartilage callus that bridges the broken ends of the bone, creating a temporary splint.
  4. Bony Callus Formation: Osteoblasts convert the fibrocartilage callus into a hard, bony callus made of spongy bone tissue.
  5. Bone Remodeling: Over several months, the bony callus is remodeled. Dead portions of bone are absorbed by osteoclasts, and the spongy bone of the callus is gradually converted to compact bone, returning the bone to its original shape and strength, if possible.

Skeletal Terminology Reference

Projections (Bumps or Extensions)

  • Process: Any prominent projection or bump on a bone.
  • Ramus: An extension of a bone that forms an angle with the rest of the structure.
  • Trochanter: A very large, rough projection; found only on the femur.
  • Tuberosity: A large, rounded (but often rough) projection.
  • Tubercle: A small, rounded projection.
  • Crest: A prominent ridge or border.
  • Line: A low ridge, much less prominent than a crest.
  • Spine: A pointed, slender process.

Articular Portions (Surfaces for Joints)

  • Head: A prominent, rounded articular end of an epiphysis, typically separated from the shaft by a narrower region (neck).
  • Neck: A narrow connection between the epiphysis (head) and diaphysis (shaft) of a bone.
  • Condyle: A smooth, rounded articular process.
  • Trochlea: A smooth, grooved articular process, shaped like a pulley.
  • Facet: A small, flat articular surface.

Openings/Depressions (Holes or Indentations)

  • Fossa: A shallow depression or hollow area in or on a bone.
  • Foramen: A rounded passageway through bone for blood vessels and/or nerves.
  • Canal (or Meatus): A large passageway or channel through bone.
  • Fissure: An elongate, cleft-like opening or slit between adjacent parts of bones, through which blood vessels and nerves may pass.
  • Sinus (or Antrum): A chamber or air-filled cavity within a bone (e.g., paranasal sinuses).