Chapter 7: Skeletal System
Major Functions of Bone Tissue
Support: Provides a structural framework for the body by supporting soft tissues.
Attachment: Serves as attachment points for tendons of skeletal muscles.
Protection: Provides a protective case for vital organs, including the brain and spinal cord.
Levers for Movement: Skeletal muscles pull on bones when they contract, producing movement.
Mineral Storage: Acts as a reservoir for minerals, specifically calcium and phosphate.
Calcium: Necessary for muscle contraction and blood clotting.
Phosphate: Needed for the synthesis of adenosine triphosphate (ATP) and nucleotides.
The skeleton releases and stores these minerals as needed by the body.
Hematopoiesis: The process of blood cell formation, which occurs within the red marrow cavities of certain bones.
Triglyceride (Energy) Storage: Fat is stored in the form of yellow bone marrow.
Cellular Components of Bone
Terminological Foundations:
The suffix "-blast" denotes an immature cell that secretes a matrix (e.g., osteoblast, chondroblast).
The suffix "-cyte" denotes a mature cell that maintains a matrix (e.g., osteocyte, chondrocyte).
Osteogenic Cells:
These are stem cells derived from mesenchyme.
They are the only bone cells capable of undergoing cell division.
They are located in the periosteum and endosteum.
Function: Give rise to osteoblasts.
Osteoblasts:
Developing from osteoprogenitor cells, these are "bone-forming" cells.
They produce the organic portion of the bone matrix known as osteoid.
As the matrix becomes calcified, the osteoblasts become trapped and differentiate into osteocytes.
Osteocytes:
These are mature bone cells derived from osteoblasts and are the most abundant cell type in bone.
They maintain contact with one another through gap junctions.
They function as mechanosensory cells, playing a critical role in regulating and maintaining normal skeletal and mineral homeostasis.
Osteoclasts:
Derived from the monocyte/macrophage lineage (phagocytic cells) rather than osteogenic cells.
They are large, multinucleated cells responsible for resorption, the process of breaking down bone matrix.
They are concentrated in the endosteum.
Ruffled Border: This is a folded plasma membrane facing the bone matrix used to create isolated, acidic environments required for resorption.
Gross Anatomy and Microscopic Structure of Bone
Compact Bone (Cortical Bone):
Found on the exterior of bones and makes up the bulk of the diaphysis in long bones.
It is the strongest form of bone tissue.
Osteon: The fundamental unit of compact bone.
Lacuna: Tiny spaces that house osteocytes.
Canaliculus: Tiny, hair-like channels connecting one lacuna to another, providing pathways for nutrients and waste passage between osteocytes.
Lamella: Thin, concentric layers of bone matrix that provide protection, support, and resistance to stresses from weight and movement.
Central Canal (Haversian Canal): A cylindrical channel at the center of each osteon containing blood and nerve supply.
Perforating Canal (Volkmann’s Canal): Transverse or diagonal canals that connect blood and nerve supplies between adjacent central canals.
Spongy Bone:
Always located inside the bone, typically at the epiphyses of long bones.
Composed of trabeculae ("little beams") oriented along lines of stress.
Spaces are filled with red or yellow bone marrow.
It is lighter than compact bone, allowing for more rapid movement when pulled by muscles.
Structural Organization of Long, Short, Flat, and Irregular Bones
Long Bone Parts:
Diaphysis: The shaft of the bone.
Epiphyses: The ends of the bone.
Proximal Epiphysis: The end closest to the trunk.
Distal Epiphysis: The end farthest from the trunk.
Metaphysis: The region between the diaphysis and epiphysis containing the epiphyseal plate (growth plate) in children (made of hyaline cartilage) or the epiphyseal line in adults (ossified plate).
Articular Cartilage: Hyaline cartilage covering both epiphyses to reduce friction and absorb shock in joints.
Medullary Cavity: A hollow cylindrical space within the diaphysis. It contains red bone marrow in children and transitions to yellow bone marrow in adults.
Short, Irregular, and Flat Bones:
These consist of thin plates of periosteum-covered compact bone on the exterior and endosteum-covered spongy bone (called diploë) on the interior.
They lack a diaphysis, epiphyses, and a medullary cavity.
They contain either red or yellow bone marrow between trabeculae.
Coverings and Linings:
Periosteum: Connective tissue surrounding the diaphysis.
Outer (fibrous) layer: Dense irregular connective tissue for protection.
Inner (cellular) layer: Contains osteoprogenitor cells, osteoblasts, and osteoclasts to build/break bone on the exterior.
Endosteum: A thin membrane lining the medullary cavity and trabeculae, containing osteoprogenitor cells, osteoblasts, and osteoclasts to build/break bone on the interior.
Bone Marrow and Blood/Nerve Supply
Red Marrow: Reticular connective tissue and developing blood cells.
In children: Found in spongy bone of most bones and the medullary cavity.
In adults: Limited to the flat bones of the skull, vertebrae, ribs, sternum, os coxae (hip bones), and the proximal heads of the humerus and femur.
Yellow Marrow: Fatty marrow that no longer produces blood cells.
In cases of severe or chronic anemia, yellow marrow can transform back into red marrow to resume hemopoietic function.
Vascularization and Innervation:
Bone is highly vascularized, particularly in spongy bone.
Nutrient Foramen: An opening through which one nutrient artery enters and one nutrient vein exits to supply nutrients and remove waste.
Nerves (mainly sensory) accompany these blood vessels to innervate the periosteum, endosteum, and medullary cavity to signal skeletal injury.
Chemical Composition of the Bone Matrix
Organic Matrix ():
Consists of osteoid, which is unmineralized.
Components: Collagen fibers, semisolid ground substance (proteoglycans), and glycoproteins.
Function: Provides tensile strength, flexibility, and resistance to stretching/twisting.
Inorganic Matrix ():
Consists of crystallized mineral salts called hydroxyapatites.
Chemical Formula:
Includes ions such as sodium, magnesium, fluoride, and sulfate.
Function: Responsible for bone hardness and resistance to compression.
Bone Development and Growth
Ossification (Osteogenesis) Situations:
Embryological/fetal development.
Growth before adulthood.
Bone remodeling.
Fracture healing.
Intramembranous Ossification:
Begins at week 8 of embryonic development.
Produces flat bones of the skull (e.g., frontal bone), facial bones (e.g., zygomatic, maxilla), mandible, and clavicle.
Process: Mesenchymal cells form ossification centers; osteoblasts secrete osteoid; osteoid calcifies; osteocytes are trapped; woven bone (immature/unorganized) and periosteum form; lamellar bone replaces woven bone.
Endochondral Ossification:
Uses a hyaline cartilage model and produces most bones in the body.
Process:
Hyaline cartilage model develops.
Cartilage calcifies and a periosteal bone collar forms.
Primary ossification center forms in the diaphysis; osteoclasts form the medullary cavity.
Secondary ossification centers form in the epiphyses.
Bone replaces cartilage except for articular cartilage and the epiphyseal plate.
Epiphyseal plates eventually ossify into epiphyseal lines ( years of age).
Bone Growth Mechanisms:
Interstitial Growth: Increase in bone length occurring at the epiphyseal plate via cartilage proliferation followed by ossification.
Appositional Growth: Increase in bone thickness. Osteoblasts beneath the periosteum secrete matrix (circumferential lamellae) while osteoclasts in the endosteum remove bone to widen the medullary cavity.
Bone Remodeling and Nutritional Factors
Remodeling Units: Adjacent osteoblasts (deposition) and osteoclasts (resorption) work at periosteal and endosteal surfaces.
Alkaline Phosphatase: An enzyme essential for the mineralization of bone during deposition.
Resorption Details: Osteoclasts secrete lysosomal enzymes to digest organic matrix and hydrochloric acid () to convert calcium salts into soluble forms for release into the blood.
Nutrients Required:
Calcium: Forms hydroxyapatite crystals.
Vitamin D: Necessary for calcium absorption.
Vitamin K: Supports bone mineralization.
Magnesium and Fluoride: Structural components.
Vitamin C: Required for collagen synthesis.
Vitamin A: Stimulates osteoblast activity.
Omega-3 Fatty Acids: Reduces inflammation interfering with osteoblast function.
Regulation of Bone Density and Calcium Levels
Mechanical Stress (Wolff’s Law): Bone grows or remodels in response to the demands placed on it. Osteocytes detect stress and signal osteoblasts to increase osteoid synthesis and mineralization.
Hormonal Influences:
Growth Hormone: Stimulates insulin-like growth factors (IGFs) in the liver to promote osteoblast activity and cell division at the epiphyseal plate (interstitial) and periosteum (appositional).
Testosterone and Estrogens: Increase osteoblast activity during puberty; eventually induce epiphyseal plate closure.
Calcium Homeostasis:
of calcium is stored in bones/teeth.
Average daily loss: .
Normal Serum Calcium: .
Ionized Calcium (Free Calcium): (the most active form).
Hormonal Regulation of Calcium:
Hypocalcemia: Low blood calcium levels signal the parathyroid glands to release Parathyroid Hormone (PTH). PTH increases osteoclast activity to release calcium into the blood. Also stimulates the kidneys to produce Calcitriol (Vitamin D) to increase intestinal calcium absorption.
Hypercalcemia: High blood calcium levels signal the thyroid to release Calcitonin. Calcitonin inhibits osteoclasts and stimulates osteoblasts to accelerate calcium deposition into the bone.
Bone Fractures and the Repair Process
Fracture Classifications:
Stress Fracture: Thin break from increased physical activity.
Pathologic Fracture: Weakening by disease.
Simple Fracture: Does not penetrate the skin.
Compound Fracture: One or both ends pierce the skin.
Reduction Methods:
Closed Reduction: Bone is manipulated into position without surgery.
Open Reduction: Surgery is required to reset the bone, often using pins or plates.
Stages of Fracture Healing:
Fracture Hematoma Formation: Blood vessels tear, causing a hemorrhage and a clot (hematoma). The site becomes swollen and inflamed; macrophages manage repair.
Fibrocartilaginous (Soft) Callus Formation: Capillaries grow into the tissue; fibroblasts produce collagen fibers to connect ends. (Approx. 5 weeks).
Bony (Hard) Callus Formation: Osteoblasts produce spongy bone to replace the soft callus. (Lasts approx. months).
Bone Remodeling: The bony callus is remodeled to match the original unbroken structure.
Questions & Discussion
Which type of bone cell builds up bone? Which type of bone cell breaks down bone?
Osteoblasts build up bone; Osteoclasts break down bone.
Which type of bone connective tissue is found on the outside of bones? Which is found on the inside? Which is filled with bone marrow?
Compact bone is on the outside; Spongy bone is on the inside and is filled with marrow.
Which type of bone marrow is involved in the development of blood cells?
Red bone marrow.
The section of hyaline cartilage at the metaphyses of long bones that allow for longitudinal bone growth is called what? What does it become once growing is complete?
Epiphyseal plate; it becomes the epiphyseal line.
What is the process by which bone grows in width or thickness?
Appositional growth.
True or false: bone is a static structure. Once it is formed, it remains consistent throughout its lifetime.
False. Bone is a dynamic organ constantly being remodeled.
Which hormone is released when someone has hypocalcemia? Which hormone is released when someone has hypercalcemia?
Hypocalcemia triggers PTH; Hypercalcemia triggers calcitonin.
A patient comes in with a compound bone fracture. The doctors perform surgery to reset the position of the bone and secure it with pins. Did the bone penetrate the skin? What type of reduction did the doctors perform?
Yes, the bone penetrated the skin because it was a compound fracture. They performed an open reduction because surgery was required.
What are the four steps involved in the repair of a simple fracture?
1. Fracture hematoma forms; 2. Fibrocartilaginous (soft) callus forms; 3. Hard (bony) callus forms; 4. Bone remodeling.