Skeletal System: Axial & Appendicular Skeleton – Structure, Histology, and Classification

Skeletal System Overview

Introduction to the Skeletal System

The skeletal system provides the structural framework for the human body, supports movement, protects internal organs, stores minerals, and houses bone marrow for blood cell production. It is divided into two main parts: the axial skeleton and the appendicular skeleton.

• Axial Skeleton: Consists of the skull, vertebral column, and thoracic cage.

• Appendicular Skeleton: Includes the shoulder and hip bones, as well as the bones of the upper and lower limbs.

Cartilage Histology and Distribution

Types and Locations of Cartilage in Adults

Cartilage is a specialized connective tissue found throughout the body, providing support and flexibility. There are three main types of cartilage, each with distinct histological features and locations.

• Hyaline Cartilage: Most common type; found in the nose, respiratory tract, articular surfaces of joints, costal cartilages, and fetal skeleton. Appears glassy under the microscope due to fine collagen fibers.

• Fibrocartilage: Contains thick bundles of collagen fibers, providing strength and resistance to compression. Found in intervertebral discs, pubic symphysis, and menisci of the knee.

• Elastic Cartilage: Contains elastic fibers, allowing flexibility. Found in the external ear and epiglottis.

Functions of Cartilage:

• Supports soft tissues

• Provides a smooth, gliding surface for joint movement

• Serves as a model for bone formation during development

Cartilage Type	Main Locations	Key Features
Hyaline	Nose, trachea, articular joints, costal cartilage	Glassy matrix, fine collagen fibers
Fibrocartilage	Intervertebral discs, pubic symphysis, menisci	Thick collagen bundles, high tensile strength
Elastic	External ear, epiglottis	Elastic fibers, flexible

Classification and Anatomy of Bones

Bone Classification by Shape

Bones are classified based on their shapes, which relate to their functions and locations in the body.

• Long Bones: Greater length than width; found in limbs (e.g., femur, humerus).

• Short Bones: Nearly equal length and width; found in wrists (carpals) and ankles (tarsals).

• Flat Bones: Thin, flat surfaces; found in the skull, sternum, and ribs.

• Irregular Bones: Complex shapes; found in vertebrae and some facial bones.

Gross Anatomy of a Long Bone

Long bones have a characteristic structure that supports their function in movement and weight-bearing.

• Diaphysis: The shaft; provides leverage and weight support.

• Epiphyses: The expanded ends; contain spongy bone and red marrow.

• Metaphysis: Region between diaphysis and epiphysis; contains the epiphyseal plate (growth plate) in children or epiphyseal line in adults.

• Articular Cartilage: Thin layer of hyaline cartilage covering joint surfaces; reduces friction and absorbs shock.

• Medullary Cavity: Hollow space within the diaphysis; contains yellow bone marrow in adults.

Microscopic Anatomy of Bone

Compact vs. Spongy Bone

Bones are composed of two types of osseous tissue: compact bone and spongy bone, each with distinct structures and functions.

• Compact Bone: Dense and solid; forms the outer layer of bones. Organized into osteons (Haversian systems) with concentric lamellae surrounding central canals containing blood vessels and nerves.

• Spongy Bone (Cancellous Bone): Porous, lattice-like structure (trabeculae); found in the interior of bones, especially at the ends of long bones and in flat bones. Contains red bone marrow.

Key Structures in Compact Bone:

• Osteon: Structural unit of compact bone.

• Central (Haversian) Canal: Contains blood vessels and nerves.

• Lacunae: Small spaces housing osteocytes (bone cells).

• Canaliculi: Tiny channels connecting lacunae for nutrient and waste exchange.

Bone Markings

Types and Functions of Bone Markings

Bone markings are surface features that serve as attachment sites for muscles, passageways for nerves and blood vessels, and points of articulation with other bones.

• Foramen: Round or oval opening through a bone (e.g., foramen magnum).

• Process: Any bony prominence (e.g., mastoid process).

• Facet: Smooth, flat surface for articulation (e.g., articular facet of vertebrae).

• Crest: Prominent ridge (e.g., iliac crest).

• Condyle: Rounded articular projection (e.g., femoral condyle).

Axial Skeleton

Skull

The skull protects the brain and forms the structure of the face. It is divided into cranial and facial bones.

• Cranial Bones: Enclose the brain (e.g., frontal, parietal, temporal, occipital).

• Facial Bones: Form the face (e.g., maxilla, zygomatic, mandible).

• Sutures: Immovable joints between skull bones (e.g., sagittal, coronal, lambdoid).

Paranasal Sinuses: Air-filled spaces in certain skull bones; lighten the skull, humidify air, and serve as resonating chambers for voice.

Vertebral Column

The vertebral column supports the body, protects the spinal cord, and provides attachment points for ribs and muscles. It consists of 26 bones: 24 vertebrae, the sacrum, and the coccyx.

• Cervical Vertebrae (7): Smallest, found in the neck. The atlas (C1) supports the skull; the axis (C2) allows head rotation.

• Thoracic Vertebrae (12): Articulate with ribs; have long spinous processes.

• Lumbar Vertebrae (5): Largest, support lower back.

• Sacrum: Five fused vertebrae; forms the posterior wall of the pelvis.

• Coccyx: Four fused vertebrae; forms the tailbone.

General Vertebral Anatomy:

• Body: Weight-bearing region.

• Vertebral Arch: Forms the vertebral foramen (spinal cord passage).

• Processes: Spinous (posterior), transverse (lateral), and articular (superior/inferior) for muscle attachment and articulation.

• Intervertebral Discs: Pads of fibrocartilage between vertebral bodies; absorb shock.

Thoracic Cage

The thoracic cage protects vital organs and supports the upper body. It consists of the sternum, ribs, and thoracic vertebrae.

• Sternum: Flat bone at the front of the chest.

• Ribs (12 pairs):

  • True Ribs (1-7): Attach directly to the sternum via costal cartilage.

  • False Ribs (8-12): Do not attach directly to the sternum.

  • Floating Ribs (11-12): Subset of false ribs; do not attach to the sternum at all.

• Functions: Protects heart, lungs, and major vessels; provides attachment for muscles involved in respiration.

Appendicular Skeleton

Overview

The appendicular skeleton includes the bones of the limbs and the girdles (shoulder and pelvic) that attach them to the axial skeleton. It enables movement and manipulation of the environment.

• Pectoral Girdle: Clavicle and scapula; attaches upper limbs to the trunk.

• Upper Limbs: Humerus, radius, ulna, carpals, metacarpals, phalanges.

• Pelvic Girdle: Hip bones (ilium, ischium, pubis); attaches lower limbs to the trunk.

• Lower Limbs: Femur, patella, tibia, fibula, tarsals, metatarsals, phalanges.

Example: The femur is the longest bone in the body and is classified as a long bone. It articulates proximally with the hip bone and distally with the tibia and patella.

Additional info: The provided images and notes are consistent with standard introductory college-level Anatomy & Physiology content, focusing on the structure, classification, and function of the skeletal system.