BIOS251: The Skeletal System

The Skeletal System

Overview

  • The skeletal system is made up of bones and their cartilages.

  • Bone is an organ composed of diverse tissues including bone, cartilage, dense connective tissue, adipose tissue, and nervous tissue.

  • The entire framework of bones is essential for body structure and function.

Functions of the Skeletal System

  • Support: Provides a structural framework to the body.

  • Protection: Safeguards vital organs such as the brain and heart.

  • Movement: Facilitates body movement in conjunction with muscles.

  • Mineral Homeostasis: Stores and releases calcium and phosphorus as needed.

  • Blood Cell Production (Hemopoiesis): Participates in the production of blood cells, mainly in red bone marrow.

  • Triglyceride Storage: Stores fatty tissue in the adipose cells of yellow marrow.

Structure of Bone

  • A long bone contains several key components:
      - Diaphysis: The shaft of the bone.
      - Epiphyses: The two ends of the bone, located at the joints.
      - Metaphyses: The region between the diaphysis and epiphysis.
      - Articular Cartilage: Cartilage covering both epiphyses to reduce friction.
      - Periosteum: A dense connective tissue surrounding the diaphysis.
      - Medullary Cavity: The hollow space within the diaphysis that contains marrow.
      - Endosteum: A thin membrane lining the medullary cavity.

Histology of Bone

  • There are four types of bone cells:
      - Osteoprogenitor Cells: Stem cells that can differentiate into other types of bone cells.
      - Osteoblasts: Cells responsible for bone formation by secreting bone matrix.
      - Osteocytes: Mature bone cells that maintain bone tissue.
      - Osteoclasts: Cells that resorb bone tissue and release calcium into the bloodstream.

Types of Bone

  • Compact Bone: Provides strength and support; organized into structural units called osteons.

  • Spongy Bone: Lightweight with a mesh-like structure, allowing tissue support. Contains red bone marrow necessary for blood cell production.

Anatomy of Bone - Compact Bone

  • Osteons: Functional units of compact bone, with osteocytes found in spaces called lacunae.

  • Lacunae are arranged in concentric circles around the central canal, which contains blood vessels, lymphatic vessels, and nerves.

  • Canaliculi: Small channels connecting lacunae, allowing nutrient exchange from the central canal to osteocytes.

Anatomy of Bone - Spongy Bone

  • Spongy bone has a non-organized appearance with osteocytes located in trabeculae (thin plates of bone).

  • Stress Distribution: Plates follow lines of stress, providing strength but remaining lightweight.

  • Red bone marrow within spaces produces blood cells; more prevalent in infants, less so in adults.

Blood and Nerve Supply of Bone

  • Periosteal Arteries: Enter the diaphysis via Volkmann’s canals alongside nerves and periosteal veins.

  • Nutrient Artery: Enters the diaphysis through a nutrient foramen, and nutrient veins exit through the same canal.

  • Epiphyses and metaphyses have their own vascular supply.

Bone Marrow

  • Red Marrow (Myeloid Tissue): Occupies spaces in spongy bone and is involved in blood cell production; present in nearly every bone in children, restricted in adults.

  • Yellow Marrow: Fatty tissue present in adults that can revert to red marrow during chronic anemia.

Bone Formation

  • Ossification (Osteogenesis): The process of bone formation, which occurs in several instances:
      - During fetal and embryonic development.
      - When bones grow prior to adulthood.
      - Through bone remodeling.
      - When fractures heal.

Methods of Bone Development

  1. Intramembranous Ossification:
       - Produces flat bones of the skull, clavicle, and parts of the mandible.
       - Continues throughout life, thickening long bones.

  2. Endochondral Ossification:
       - Involves the replacement of cartilage with bone in developing limbs.
       - Progressively forms the long bones of the body.

Processes of Endochondral Ossification

  • Development of a Cartilaginous Framework: Initial cartilage model formed.

  • Primary Ossification Center: Bone begins to replace cartilage.

  • Formation of Medullary Cavity: Replacement and calcification continue, hollowing out the center.

  • Articular Cartilage Development: Cartilage remains at the joints, while bone continues growing.

  • Epiphyseal Plate Closure: By late teens to early 20s, epiphyseal plates are consumed, halting lengthwise growth.

Bone Growth and Remodeling

  • Ossification occurs throughout life; bones grow in both length and width.

Widening and Thickening

  • Appositional Growth: Growth at the bone surface leads to increases in diameter and thickness.

  • Osteoblasts add new tissue while osteoclasts carve out the medullary cavity, resulting in balanced growth.

Bone Remodeling

  • Bone remodeling involves the continuous resorption and deposition of bone, with approximately 10% of the skeleton being remodeled annually.

  • Wolff’s Law: States that the architectural style of bone is influenced by mechanical stresses.

  • The interaction between osteoblasts and osteoclasts is crucial for maintaining bone health and strength.

Factors Influencing Bone Remodeling

  • Vitamins: Essential vitamins include A, C, K, B12, D.

  • Minerals: Primarily calcium and phosphorus.

  • Hormones:
      - Insulin-like growth factors (IGFs)
      - Sex hormones
      - Parathyroid hormone (PTH)
      - Calcitriol
      - Calcitonin

Bone's Role in Calcium Homeostasis

  • Bones store 99% of the body's calcium.

  • Parathyroid Hormone (PTH): Released when calcium levels drop; stimulates osteoclasts to increase bone resorption and release calcium.

  • PTH also instigates renal production of calcitriol for greater intestinal absorption of calcium.

Fracture and Repair of Bone

  • A fracture is defined as a break in a bone with various types:

Healing Phases and Steps

  1. Reactive Phase: Early inflammatory response.

  2. Reparative Phase: Creation of fibrocartilaginous then bony callus.

  3. Bone Remodeling Phase: Final shape and integrity restoration of the bone.

Steps of Healing

  • Hematoma Formation: Blood clot forms at the fracture site.

  • Fibrous Callus Formation: Fibrocartilage begins to bridge the fracture.

  • Bony Callus Formation: Replacement of fibrocartilage with new bone.

  • Bone Remodeling: Final adjustments to bone shape and structural integrity.

Aging and Bone Tissue

  • During childhood and adolescence, bone formation exceeds resorption.

  • In adulthood, the rates stabilize.

  • In older individuals, particularly post-menopausal women, bone resorption surpasses formation, resulting in reduced mass.

Rickets and Osteomalacia

  • Rickets: Condition in children due to vitamin D deficiency.
      - Leads to softening of bones and bowing during growth.

  • Osteomalacia: Adult version of rickets with softening of bones.
      - Symptoms include pain upon weight bearing.

Osteoporosis

  • A condition where bone resorption exceeds deposition leading to weakened bones.

Causes

  • Lack of sex hormones, exercise, calcium, and vitamin D.

Symptoms

  • Most pronounced in the spongy bone of the spine; occurs often in post-menopausal women.

  • Minor injuries can cause significant fractures.

Treatment Options

  • Calcium and Vitamin D supplementation.

  • Hormone replacement therapy.

  • Increasing weight-bearing exercises.

Axial Skeleton Anatomy

  • Comprises 80 bones including the skull, auditory ossicles, ribs, sternum, and vertebrae.

  • It protects vital organs and supports calcium storage and release.

Comparison with Appendicular Skeleton

  • Appendicular skeleton consists of 126 bones: upper and lower limbs, and girdles connecting limbs to the axial skeleton.

Bone Types and Classification

  • There are five main shapes of bones:
      - Long: Greater length than width.
      - Short: Cube-like.
      - Flat: Thin, flat layers.
      - Irregular: Complex shapes.
      - Sesamoid: Seed-like.

Bone Surface Markings

  • Adapted structures for functions, categorized into:
      - Depressions & Openings: Allow passage of soft tissues and form joints.
      - Processes: Projections for joint formation and attachment points.

Skull Anatomy

  • Comprises 22 bones (not including ear bones).

  • Cranial Bones: Frontal (1), Parietal (2), Sphenoid (1), Ethmoid (1), Temporal (2), Occipital (1).

Unique Features of the Skull

  • Sutures: Immovable joints aiding in skull construction.

  • Fontanelles: Soft spots in infants where sutures meet.

  • Paranasal Sinuses: Lighter the skull, moisten and clean inhaled air, and function as sound chambers.

Facial Bones

  • Comprise 14 bones: Nasal (2), Lacrimal (2), Inferior nasal concha (2), Maxilla (2), Mandible (1), Palatine (2), Zygomatic (2), Vomer (1).

Unique Features of Mandible

  • The largest and strongest facial bone, and the only moveable bone of the skull apart from auditory ossicles.

Hyoid Bone

  • Unique in that it does not articulate with any other bone and supports the tongue and neck muscles.

Vertebral Column

  • Comprises 26 vertebrae divided into five regions, protecting the spinal cord.

Intervertebral Discs

  • Located between vertebrae, composed of fibrocartilage and help absorb shocks.

Distinct Features by Region

  • Cervical Vertebrae (C1-C7): Identify with bifid spinous processes and transverse foramina.

  • Thoracic Vertebrae (T1-T12): Characterized by downward-pointing spinous processes and articulation facets for ribs.

  • Lumbar Vertebrae (L1-L5): Largest vertebrae without specialized features.

Sacrum and Coccyx

  • Fusion of five vertebrae forms the sacrum; four fuse to form the coccyx (tailbone).

The Thorax

  • Include the region encircling the thoracic cavity, involving the sternum, ribs, and costal cartilages.

Sternum Anatomy

  • Composed of three segments: manubrium, body, xiphoid process.

Ribs Structure

  • Composed of 12 pairs with types classified as:
      - True Ribs: First 7 pairs connecting directly to the sternum.
      - False Ribs: Next 5 pairs indirectly connecting to the sternum.
      - Floating Ribs: Last 2 pairs that do not connect to the sternum.

Disorders of the Skeletal System

  • Conditions such as herniated discs, scoliosis, hyperlordosis, and hyperkyphosis can arise affecting bone and overall skeletal health.

Spina Bifida

  • A congenital defect where laminae of the vertebrae don't develop properly, ranging in severity.

The Appendicular Skeleton

  • Incorporates 126 bones covering upper and lower limbs alongside the pelvic girdle.

Pectoral Girdle

  • Includes clavicle and scapula, facilitating upper limb attachment to the axial skeleton.

Upper Limb Structure

  • Comprised of the humerus, radius, ulna, carpal bones, metacarpals, and phalanges arranged similarly to the foot.

Pelvic Girdle

  • Composed of two hip bones (os coxa) formed from ilium, ischium, and pubis, articulating with the sacrum and supporting the pelvis for childbirth.

Lower Limb Anatomy

  • Includes femur, patella, tibia, fibula, tarsals, metatarsals, and phalanges, each with specific articulating features to support body weight and allow for mobility.