Skeletal System: Osseous Tissue Structure and Axial Skeleton

Skeletal System Overview

Introduction to the Skeletal System

The skeletal system is a complex framework of bones and connective tissues that provides structural support, protection, and movement for the human body. It is divided into two main divisions: the axial skeleton and the appendicular skeleton.

• Axial Skeleton: Consists of the skull, vertebral column, and thoracic cage (ribs and sternum). It forms the central axis of the body and protects vital organs such as the brain, spinal cord, and thoracic organs.

• Appendicular Skeleton: Includes the bones of the limbs and the girdles (pectoral and pelvic) that attach them to the axial skeleton. It is responsible for movement and manipulation of the environment.

Functions of the Skeletal System

• Support: Provides a rigid framework that supports the body and cradles soft organs.

• Protection: Encloses and protects vital organs (e.g., skull protects the brain, rib cage protects the heart and lungs).

• Movement: Serves as levers for muscles to act upon, enabling movement.

• Mineral Storage: Reservoir for minerals, especially calcium and phosphate.

• Blood Cell Production: Houses bone marrow, which produces blood cells (hematopoiesis).

• Energy Storage: Yellow bone marrow stores fat as an energy reserve.

Organ/Component	Primary Functions
Bones, Cartilages, and Joints	Support and protect soft tissues; store minerals; provide leverage for movement
Ligaments	Connect bone to bone, bone to cartilage, or cartilage to cartilage
Bone Marrow	Site of blood cell production (red marrow); storage of energy reserves in fat cells (yellow marrow)

Osseous Tissue Structure

Bone as a Dynamic Tissue

Bone is a living, dynamic tissue that constantly remodels itself in response to mechanical stress and metabolic needs. It is composed of specialized cells and a mineralized extracellular matrix.

• Osteoblasts: Bone-forming cells that secrete the bone matrix.

• Osteocytes: Mature bone cells that maintain the bone matrix.

• Osteoclasts: Large cells that resorb or break down bone matrix.

Example: During growth and repair, osteoblasts build new bone while osteoclasts remove old or damaged bone, maintaining bone strength and mineral balance.

Bone Development & Ossification

Bone formation, or ossification, occurs through two main processes: intramembranous and endochondral ossification.

• Intramembranous Ossification: Direct formation of bone from mesenchymal tissue, producing flat bones such as those of the skull.

• Endochondral Ossification: Replacement of hyaline cartilage with bone, forming most bones of the body, including long bones.

Primary Ossification Centers: The first area of a bone to start ossifying, usually in the diaphysis (shaft) of long bones.

Secondary Ossification Centers: Develop in the epiphyses (ends) of long bones after birth.

Stages of Endochondral Ossification

1. Cartilage model develops.

2. Blood vessels invade, bringing osteoblasts.

3. Primary ossification center forms in the diaphysis.

4. Secondary ossification centers form in the epiphyses.

5. Epiphyseal (growth) plate remains until bone growth is complete.

Bone Growth

• Interstitial Growth: Lengthening of bones at the epiphyseal plate.

• Appositional Growth: Increase in bone thickness by addition of new bone tissue at the surface.

Example: Long bones grow in length during childhood and adolescence through interstitial growth, while appositional growth increases bone diameter throughout life.

Microscopic Structure of Bone

Compact Bone

Compact bone is dense and forms the outer layer of bones. Its structural unit is the osteon (Haversian system).

• Osteon: Cylindrical structure consisting of concentric lamellae (layers) of bone matrix surrounding a central (Haversian) canal containing blood vessels and nerves.

• Lamellae: Layers of bone matrix; can be concentric (around osteons), interstitial (between osteons), or circumferential (around the bone's outer surface).

• Volkmann's Canals: Perforating canals that connect adjacent osteons and allow passage of blood vessels and nerves.

Spongy Bone

Spongy (cancellous) bone is found at the ends of long bones and inside flat bones. It consists of a network of trabeculae (thin columns and plates) that provide structural support and house bone marrow.

• Trabeculae: Lattice-like structures that align along lines of stress, providing strength without excessive weight.

• Red Bone Marrow: Located within the spaces of spongy bone; site of hematopoiesis.

Regulation of Bone Development and Growth

Hormonal and Nutritional Regulation

• Calcium and Phosphate: Essential minerals for bone mineralization.

• Vitamin D (Calciferol): Promotes calcium absorption from the gut.

• Parathyroid Hormone (PTH): Increases blood calcium by stimulating bone resorption.

• Calcitonin: Lowers blood calcium by inhibiting bone resorption.

• Growth Hormone, Thyroxine, Estrogen, Testosterone: Stimulate bone growth and maintenance.

Aging and the Skeletal System

Changes with Age

• Osteoblast Activity Declines: Bone formation slows with age.

• Osteoclast Activity Maintains or Increases: Leads to bone loss (osteopenia, osteoporosis).

• Hormonal Changes: Decreased estrogen/testosterone accelerates bone loss, especially in postmenopausal women.

Classification of Skeletal Elements

Types of Bones

• Long Bones: Longer than they are wide (e.g., femur, humerus).

• Short Bones: Nearly equal in length and width (e.g., carpals, tarsals).

• Flat Bones: Thin and broad (e.g., skull, ribs, sternum).

• Irregular Bones: Complex shapes (e.g., vertebrae, pelvis).

• Sutural (Wormian) Bones: Small bones within cranial sutures.

• Sesamoid Bones: Develop within tendons (e.g., patella).

Bone Markings

• Projections: Sites of muscle and ligament attachment (e.g., tuberosity, crest, trochanter).

• Depressions and Openings: Allow passage of blood vessels and nerves (e.g., foramen, fossa, groove).

Axial Skeleton: Skull and Vertebral Column

Major Sutures of the Skull

• Lambdoid Suture: Between parietal and occipital bones.

• Coronal Suture: Between frontal and parietal bones.

• Squamous Suture: Between parietal and temporal bones.

• Frontonasal Suture: Between frontal and nasal bones.

Fontanelles: Soft spots in the infant skull where bones have not yet fused, allowing for brain growth and passage through the birth canal.

Skull Landmarks

• Foramen: Openings for nerves and blood vessels (e.g., foramen magnum, jugular foramen).

• Processes: Projections for muscle attachment (e.g., mastoid process, styloid process).

• Fossa: Depressions that accommodate other structures (e.g., mandibular fossa).

Orbit and Nasal Complex

• Orbit: Formed by frontal, sphenoid, zygomatic, maxillary, palatine, lacrimal, and ethmoid bones; houses the eye.

• Nasal Complex: Includes nasal bones, ethmoid, vomer, inferior nasal conchae, and maxillae; contains paranasal sinuses.

• Paranasal Sinuses: Air-filled spaces that lighten the skull, secrete mucus, and resonate sound.

Hyoid Bone

• Hyoid Bone: U-shaped bone in the neck that supports the tongue and is not directly attached to other bones.

Vertebral Column

Structure and Function

• Vertebral Column: Supports the head and trunk, protects the spinal cord, and provides attachment for ribs and muscles.

• Regions: Cervical (7), thoracic (12), lumbar (5), sacral (5 fused), coccygeal (4 fused).

• Curvatures: Cervical and lumbar (concave posteriorly), thoracic and sacral (convex posteriorly); help maintain balance and absorb shock.

Specialized Vertebrae

• Atlas (C1): Supports the skull, allows nodding motion.

• Axis (C2): Has the dens (odontoid process) for rotation of the head.

Thoracic Cage

Structure and Function

• Thoracic Cage: Consists of the ribs, sternum, and thoracic vertebrae; protects the heart and lungs.

• Ribs: 12 pairs; true ribs (1-7) attach directly to sternum, false ribs (8-12) do not; floating ribs (11-12) have no anterior attachment.

• Costal Cartilage: Connects ribs to sternum, allowing flexibility.

Articulations

• Head of Rib: Articulates with vertebral bodies.

• Neck and Tubercle: Articulate with transverse processes of vertebrae.

• Costal Groove: Protects intercostal nerves and blood vessels.

Additional info: The notes above are expanded and clarified for academic completeness, with inferred details on bone cell types, ossification processes, and anatomical terminology to ensure a self-contained study guide.