Skeletal System: Structure, Function, and Development
Skeletal System Components
Technically includes bones, cartilages, tendons, and ligaments.
Bones are the main organs of the system.
Adults typically have 206 bones.
Exceptions: Wormian bones (extra bones due to individual variation, covered in lab) and polydactyly (extra fingers or toes), which is a dominant but uncommon gene.
A bone (organ) includes bone tissue (osseous tissue), dense regular collagenous tissue, dense irregular connective tissue, and bone marrow.
Terminology: "Osseous" means bone (e.g., 'chondro' means cartilage, 'osteo' means bone).
Functions of the Skeletal System
Protection: Guards vital organs (e.g., skull protects the brain, sternum protects the heart and lungs, pelvis protects reproductive organs).
Mineral Storage: Stores calcium and phosphorus, critical for many body functions beyond just bone health. The body will leach minerals from bones if dietary intake is insufficient.
Important for acid-base balance, as these minerals are electrolytes.
Blood Cell Formation (hematopoiesis): Occurs in the red bone marrow, located in spongy bone, where all blood cells are produced.
Fat Storage: Stored in yellow bone marrow, which also decreases the overall weight of the bone.
Movement: Provides attachment sites for most (but not all) skeletal muscles, enabling movement when muscles contract.
Support: Forms the body's structural framework.
Bone Classification based on Shape
Long Bones: Longer than they are wide. Size doesn't matter (can be very small, like phalanges in fingers/toes). Examples: Humerus, radius, ulna, tibia, fibula, metacarpals, phalanges.
Short Bones: Roughly as long as they are wide, or cube-shaped. Examples: Wrist (carpals) and ankle (tarsals) bones.
Flat Bones: Thin and broad. Examples: Skull bones, sternum, ribs, pelvis, scapula.
Irregular Bones: Have complex shapes that don't fit other categories. Examples: Vertebrae, facial bones.
Sesamoid Bones: Small, oval-shaped bones embedded within tendons. Easiest example: Patella (kneecap). Few others exist.
Structure of a Long Bone
Periosteum: Outer covering of the bone (not found on articular surfaces). Made of dense irregular connective tissue. Brings blood vessels and nerves to the bone. Held in place by perforating fibers (collagen fibers) that penetrate the bone matrix.
Diaphysis: The shaft or central part of the long bone.
Medullary (Marrow) Cavity: Housed within the diaphysis, lined by the endosteum (innermost lining).
Epiphyses: The ends of a long bone. Some epiphyses are filled with red marrow.
Articular Cartilage: Hyaline cartilage covering the surfaces of the epiphyses where bones form a joint (articulation).
Compact Bone: Hard, dense outer layer of bone. Functions to resist linear compression and twisting forces (e.g., gravity, rotational movements).
Spongy (Cancellous) Bone: Inner, lattice-like bone framework. Resists forces in many directions. Houses bone marrow and decreases bone weight due to its less dense structure.
Epiphyseal Lines: Remnants of growth plates (epiphyseal plates), which are made of hyaline cartilage. These plates are where bones grow in length during childhood and adolescence. They are located in the epiphyses, not the diaphysis or at the very ends.
Blood and Nerve Supply to Bone
Bones are well-supplied with blood vessels and many sensory nerve fibers, which is why bone fractures are painful.
Short, Flat, Irregular, and Sesamoid Bones: Receive blood supply primarily from vessels in the periosteum.
Long Bones: Receive blood supply from the periosteum and nutrient arteries.
Nutrient arteries enter the diaphysis through a small hole called the nutrient foramen.
Comparison to Cartilage: The rich blood supply in bone explains why bone heals much better than cartilage, which notoriously has poor blood supply.
Bone Marrow Transplant and Stem Cell Donation
Purpose: Treat conditions like leukemia, sickle cell anemia, and aplastic anemia, where bone marrow is dysfunctional.
Traditional Transplant (Bone Marrow Harvest): Involves inserting a needle into the pelvic bone (e.g., ileum) of a matching donor to withdraw up to 2 quarts of red marrow. The recipient's marrow is destroyed via chemotherapy and radiation, then donor marrow is administered intravenously.
Risks: High susceptibility to infection (due to destroyed immune system), transplant rejection.
If successful, new blood cells can be produced in 2-4 weeks.
Peripheral Blood Stem Cell Donation (Newer Alternative): A drug is injected into the donor to stimulate the release of stem cells from the bone marrow into the bloodstream. These cells are then removed from the donor's blood (similar to plasma donation) and used for transplantation. This method is less invasive for the donor.
Extracellular Matrix of Bone (ECM)
Important concept: The composition of the bone's ECM is crucial for its function.
Inorganic Matrix: Makes up about 65% of the bone's total weight.
Composed mainly of calcium salts and phosphorus, combined to form hydroxyapatite crystals (pronounced like appetite).
Also includes bicarbonate, potassium, magnesium, and sodium.
Purpose: Resists compression, giving bone its hardness.
Organic Matrix (Osteoid): Makes up about 35% of the bone's total weight.
Primarily composed of protein, mostly collagen.
Also includes bone-specific proteins like osteocalcin.
Collagen is produced by bone cells (osteoblasts) and forms the initial scaffolding of the bone, determining its shape, to which the inorganic minerals then attach.
Purpose: Resists twisting (torsion) and tensile (pulling or stretching) forces, giving bone its flexibility and elasticity.
Demonstration of Properties:
Removing the organic matrix (collagen) leaves only the inorganic salts, causing the bone to crumble and become brittle.
Removing the inorganic matrix (calcium salts) leaves only the collagen, making the bone flexible and pliable (can be twisted).
Bone Cells
Unlike cartilage, bone has multiple cell types.
Osteogenic Cells: Stem cells that differentiate into osteoblasts.
Osteoblasts: "Bone builders." These cuboid to columnar cells are found in the periosteum and endosteum. They perform bone deposition (making bone) by secreting the organic matrix (collagen) and assisting in the formation of the inorganic matrix (hydroxyapatite crystals). They are active on bone surfaces.
Osteocytes: Mature bone cells, originally osteoblasts that have become trapped within the calcified matrix. They reside in small cavities called lacunae. Osteocytes maintain the extracellular matrix and monitor bone health, secreting chemicals to recruit osteoblasts if damage or stress is detected. They have cytoplasmic extensions that pass through small canals called canaliculi, allowing them to communicate and sense pressure/tension.
Tension/Stress on Bone: Mechanical stress (e.g., running, weightlifting) is beneficial for bone, stimulating remodeling and strengthening, as long as it's not excessive (like a stress fracture).
Osteoclasts: "Bone breakers down." Large, multinucleated cells with ruffled borders, derived from bone marrow cells. They are responsible for bone resorption (breaking down bone and its extracellular matrix, ECM). They achieve this by secreting:
Hydrogen ions (acid): To dissolve the inorganic matrix.
Enzymes: To break down the organic matrix (collagen).
Osteogenesis Imperfecta (Discussion Point)
A disease characterized by defective collagen in the organic matrix of bone.
Effect: The bones are brittle, flimsy, and weak, breaking easily because the collagen framework is essential for resistance to twisting and tensile forces and for the proper attachment of the hard inorganic matrix. Often leads to individuals using wheelchairs.
Ossification / Osteogenesis (Bone Formation)
The process of making bone.
Continues through childhood, with most bones completing ossification by age 7, though some continue longer.
Primary (Woven) Bone: Immature bone formed first. Consists of irregularly arranged collagen bundles, many osteocytes, and relatively little inorganic matrix. It's quickly formed but not as strong.
Secondary (Lamellar) Bone: Mature, strong bone that replaces primary bone over time. Characterized by fully formed lamellae (concentric rings), organized parallel collagen bundles (which greatly increases its strength, similar to dense regular connective tissue), and a higher percentage of inorganic matrix.
Two Forms of Ossification (You must know the steps in order)
1. Intramembranous Ossification:
Bones are built on a model of embryonic connective tissue called a mesenchymal membrane.
Spongy bone forms first.
Occurs in most flat bones (e.g., skull bones, clavicles).
Steps:
Osteoblasts develop in the primary ossification center from mesenchymal cells (mesenchymal cells differentiate into osteogenic cells, then osteoblasts).
Osteoblasts secrete organic matrix, which then calcifies, trapping the osteoblasts (which become osteocytes).
Osteoblasts lay down trabeculae (forming spongy bone). Some surrounding mesenchyme differentiates into the periosteum, and some vascular tissue becomes bone marrow.
Osteoblasts and periosteum lay down early compact bone on the outer surfaces; the matrix is remodeled to form immature compact bone. Fontanels (soft spots) in the newborn skull remain incompletely calcified, allowing for rapid brain growth. These are areas of cartilage between the developing skull bones.
2. Endochondral Ossification:
Bones are built on a model of hyaline cartilage.
Outer compact bone forms first.
Occurs in most bones of the body, including most long bones.
Long bones contain both primary ossification centers (in the diaphysis) and secondary ossification centers (in the epiphyses).
Begins during the fetal period for most bones, but some (e.g., wrist, ankle bones) ossify later.
On X-rays, cartilage appears blank because it doesn't calcify, making it look like bones are missing in young children.
Terminology: "Perichondrium" is the outer covering of cartilage (like periosteum for bone). "Chondroblasts" make cartilage.
Note: Ossification completed around age 18 in females and sometimes in the early 20s in males.
Steps:
Chondroblasts in the perichondrium differentiate into osteoblasts. Chemical signals trigger this change from chondroblast to osteogenic cells, then osteoblasts.
Bone begins to ossify from the outside concurrently with internal calcification.
(a) Osteoblasts build a bone collar on the bone's external surface.
(b) Internal cartilage begins to calcify, which kills the chondrocytes within the cartilage.
In the primary ossification center, osteoclasts resorb the calcified cartilage, and osteoblasts replace it with early spongy bone. The medullary cavity begins to form/enlarge. Secondary ossification centers (in the epiphyses) also begin to ossify.
As the medullary cavity enlarges, the remaining cartilage is replaced by bone. Cartilage remains only in the epiphyseal plates (until growth in bone length ceases) and as articular cartilage for life.
Osteoporosis
A bone disease caused by an inadequate inorganic matrix, leading to brittle bones that fracture easily and heal slowly.
Causes:
Calcium ion and Vitamin D deficiency.
Lack of protective estrogen in postmenopausal women (estrogen levels drop severely, removing its protective effect).
Genetic factors.
Underlying diseases of the skin, digestive, or urinary systems.
Certain medications can also cause osteoporosis.
Susceptibility: Spongy bone is more susceptible than compact bone because it is not as strong to begin with. Areas rich in spongy bone, such as the ends of long bones and especially the bodies of the vertebrae, are commonly affected.
Effects: Can lead to compression fractures (especially in vertebrae) and kyphosis (a hunched back).