Chapter 6: The Skeletal System

Chapter 6: The Skeletal System

Overview of the Skeletal System

• Definition: The skeletal system comprises bones and their associated connective tissues, including:

  • Tendons

  • Ligaments

  • Cartilage

Anatomy of Bones

• Parts of a Typical Long Bone:

  • Proximal Epiphysis: The end of the bone nearest to the trunk of the body.

  • Distal Epiphysis: The end of the bone furthest from the trunk.

  • Metaphysis: A region between the diaphysis and the epiphysis where growth occurs (epiphyseal plate).

  • Diaphysis: The shaft or central part of a long bone.

  • Articular Cartilage: A layer of hyaline cartilage covering the epiphyses to reduce friction.

  • Compact Bone: Dense, solid bone that forms the outer layer.

  • Spongy Bone: Cancellous bone that is lighter and porous, found in the interior.

  • Medullary Cavity: Hollow space within the diaphysis containing yellow bone marrow.

  • Red Bone Marrow: Produces blood cells, primarily found in spongy bone.

  • Yellow Bone Marrow: Stores energy in the form of fat.

  • Endosteum: Lining of the medullary cavity.

  • Periosteum: A dense layer of vascular connective tissue enveloping the bones except at the surfaces of the joints.

  • Nutrient Artery: Supplies blood to the bone.

The Plan for Chapter 6

• Definitions: Cartilages, bone tissue patterns, and the difference between compact vs. spongy bone.

• Bone Shapes and Structures: Identification and features of five major bone shapes.

• Bone Formation: Discussion on intramembranous and endochondral ossification processes.

• Bone Growth: Distinction between appositional versus interstitial growth and remodeling.

• Hormonal Influences: How hormones affect bone growth and maintenance including:

  • Growth Hormone

  • Thyroid hormone

  • Estrogen

  • Testosterone

  • Calcitonin

  • Parathyroid Hormone

• Growth and Maintenance Factors: All factors impacting bone health.

• Disorders: Conditions like dwarfism, osteoporosis, rickets.

Functions of the Skeletal System

• Primary Functions:

  • Provides a rigid support framework.

  • Facilitates movement as muscles pull on bones.

  • Offers protection for soft internal organs.

  • Stores minerals in the bone extracellular matrix (e.g., calcium, phosphorus).

  • Stores energy as adipose tissue in yellow bone marrow.

  • Produces blood cells (hemopoiesis)

Skeletal Cartilages

• Characteristics: Common early in development and during childhood, typically replaced by bone later, except in joints.

• Types of Cartilages:

  • Hyaline Cartilage: Provides support with some flexibility (e.g., in joints).

  • Fibrocartilage: Tough and durable, provides support and absorbs shock (e.g., intervertebral discs).

  • Elastic Cartilage: Flexible structure (e.g., ear).

• Growth of Cartilage:

  • Appositional Growth: Growth in diameter.

  • Interstitial Growth: Growth in length.

Bone Classification

• Types of Bones Based on Shape:

  • Long Bones: Longer than they are wide; examples include femur and humerus; primary function is leverage.

  • Short Bones: Approximately equal in length, width, and thickness; examples include carpals and tarsals; provide stability and support while allowing motion.

  • Flat Bones: Thin and curved; provide protection; examples include sternum, ribs, and cranial bones.

  • Irregular Bones: Complex shapes; examples include vertebrae and facial bones.

  • Sesamoid Bones: Small and round, embedded in tendons; example includes patella.

Anatomy of a Typical Bone

• Typical bone structure includes:

  • Periosteum: Dense connective tissue surrounding bone.

  • Compact Bone: Forms the outer layer of bones.

  • Spongy Bone: Interior structure providing strength to bones.

  • Medullary Cavity: Central cavity housing the marrow.

  • Articular Cartilage: Covers the ends of bones where they articulate.

Bone Histology

• Components of Bone Matrix:

  • Inorganic Matrix: Predominantly composed of calcium phosphate, which provides hardness.

  • Organic Matrix: Composed of collagen fibers, glycoproteins, and proteoglycans that hold water.

Bone Cells

• Types of Bone Cells:

  • Osteogenic Cells: Stem cells differentiating into osteoblasts.

  • Osteoblasts: Responsible for bone formation and secrete bone matrix.

  • Osteocytes: Maintain the bone matrix; reside within lacunae and communicate via canaliculi.

  • Osteoclasts: Derived from macrophages, responsible for bone resorption through secretion of acids and enzymes.

Types of Bone

• Compact Bone: Solid and strong, defined by its structural unit, the osteon.

• Cancellous Bone (Spongy Bone): Contains numerous branching bony plates with irregular spaces, providing structural strength with reduced weight.

Osteon Structure

• Osteon Features:

  • Composed of concentric rings called lamellae.

  • Contains a central canal that houses blood vessels and nerves.

  • The orientation of collagen fibers contributes to bone strength.

  • Canaliculi: Small channels allowing for nutrient and waste exchange between osteocytes and blood supply.

Bone Structure Variations

• Structure of Different Bones:

  • Short, Flat, Irregular, and Sesamoid Bones: Covered by periosteum, with a structure analogous to long bones; featuring two outer layers of compact bone with a middle layer of spongy bone (diploë).

  • Blood and Nerve Supply: Bones receive a rich blood supply and sensory nerve fibers, primarily through the periosteum and nutrient arteries.

Bone Ossification

• Definitions:

  • Osteogenesis/Ossification: The formation of bone, occurring during:

  • Embryonic development

  • Childhood

  • Distinct processes of ossification include:

  1. Intramembranous Ossification: Formation of bone within connective tissue membranes, primarily responsible for most bones in the skull and the clavicles.

  2. Endochondral Ossification: Occurs within a cartilage model, resulting in the formation of most bones in the skeleton.

     • Process includes:

       • Formation of bone collar around the shaft.

       • Cavitation of the hyaline cartilage shaft.

       • Formation of spongy bone in internal cavities.

       • Formation of the medullary cavity in long bones; secondary ossification centers develop in epiphyses without a medullary cavity.

Bone Growth

• Types of Bone Growth:

  • Appositional Growth: Increase in bone width; involves new bone formation on existing bone.

  • Interstitial Growth: Increase in bone length through growth of the cartilage present at the epiphyseal plate, leading to eventual replacement by bone.

• Growth Zone Structure in Long Bones:

  • Various zones including proliferation, hypertrophy, and ossification layers of the epiphyseal plate contributing to bone lengthening.

Hormonal Regulation of Bone Growth

• Hormonal Influences: Bone growth is regulated by various hormones:

  • Growth Hormone: Secreted by the anterior pituitary gland; enhances protein synthesis and stimulates cell division in the epiphyseal plate and osteogenic cells.

  • Thyroid Hormone: Regulates growth hormone’s activity.

  • Sex Hormones (Testosterone and Estrogen):

  • Testosterone: Increases appositional growth; accelerates closure of the epiphyseal plates.

  • Estrogen: Stimulates longitudinal growth and inhibits osteoclast activity; rapidly accelerates epiphyseal closure compared to testosterone.

Calcium Homeostasis and Bone Health

• Hormones and Calcium Regulation:

  • Parathyroid Hormone (PTH): Released in response to low blood calcium levels, stimulates osteoclast activation to release calcium from bones, enhances kidney retention, and increases intestinal absorption of calcium (in conjunction with Vitamin D).

  • Calcitonin: Released when blood calcium levels are high; inhibits osteoclasts and promotes calcium storage in bones.

• Feedback Loops for Calcium Regulation: Dynamic process that maintains calcium within homeostatic ranges through a negative feedback mechanism.

Bone Homeostasis and Remodeling

• Bone Remodeling: A continuous process correlated with bone growth and repair; involves a balance between osteoclast activity (bone resorption) and osteoblast activity (bone formation).

  • Factors Influencing Bone Remodeling:

  • Mechanical stress, nutrient availability, hormones, and aging.

  • Bone Repair: Process involving four key steps:

  1. Blood clot formation.

  2. Fibrous callus formation.

  3. Cancellous (spongy) bone formation by osteoblasts within the callus.

  4. Remodeling resulting in compact and cancellous bone reformation.

Disorders of the Skeletal System

• Common Disorders:

  • Dwarfism: Can be due to

  • Pituitary issues (e.g., insufficient growth hormone).

  • Achondroplastic dwarfism (genetic factors leading to abnormalities in cartilage development).

  • Osteomalacia/Rickets: Resulting from vitamin D deficiency, leading to softening of bones.

  • Osteoporosis: Characterized by reduced bone density, making bones more susceptible to fracturing.