Skeletal System: Structure, Organization, and Bone Types

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Skeletal System Overview

Levels of Organization in the Human Body

The human body is organized into hierarchical levels, each contributing to overall function and health. The skeletal system is one of several organ systems that work together to maintain life.

Organism Level: All organ systems function together for survival.
Organ System Level: Includes systems such as skeletal, muscular, nervous, cardiovascular, etc.
Organ Level: Organs are composed of multiple tissue types; for example, the heart contains muscle, connective, and nervous tissues.
Tissue Level: Groups of similar cells performing specific functions, such as cardiac muscle tissue.
Cellular Level: Cells are the basic units of life; heart muscle cells interlock to form tissue.
Chemical Level: Atoms and molecules form the structural and functional basis of cells.

Levels of organization in the human body

Bone Tissue Types

Compact Bone vs. Spongy Bone

Bones are composed of two main types of tissue, each with distinct structural and functional properties.

Compact Bone (Dense Bone): Forms the outer surface of bones; provides strength where stress is applied in one direction.
Spongy Bone (Cancellous Bone): Located inside compact bone; lightweight and porous, reducing bone weight and housing marrow.

Structure of flat bone showing spongy bone

Structure of Bones

Long Bone Anatomy

Long bones, such as the femur, have a distinct anatomy that supports movement and growth.

Diaphysis: The shaft; composed mainly of compact bone.
Epiphysis: The ends; contain spongy bone and are covered with articular cartilage for joint movement.
Metaphysis: The region between diaphysis and epiphysis; contains the growth plate (epiphyseal plate in youth, epiphyseal line in adults).
Periosteum: Tough membrane covering bone; outer fibrous layer for muscle/ligament attachment, inner cellular layer produces osteoblasts.

Long bone anatomy diagram Long bone structure with medullary cavity and periosteum

Flat Bone Anatomy

Flat bones, such as those in the skull, have a unique structure.

External Table: Outer layer of compact bone.
Diploë: Middle layer of spongy bone containing red marrow.
Internal Table: Inner layer of compact bone.
No marrow cavity: Red marrow in diploë produces blood cells.

Structure of flat bone showing diploë

Internal Structure of Long Bones

Medullary (Marrow) Cavity: Hollow interior lined with spongy bone; contains yellow marrow (fat storage) and red marrow (blood cell production).
Endosteum: Membrane lining the cavity; contains osteoclasts for bone remodeling.

Bone Cells and Histology

Types of Bone Cells

Bone tissue is maintained and remodeled by specialized cells.

Osteoblasts: Build bone matrix; active in growth and repair.
Osteocytes: Mature bone cells; maintain bone matrix.
Osteoclasts: Break down bone matrix for remodeling and mineral release.
Osteogenic Cells: Stem cells that differentiate into osteoblasts.

Types of bone cells: osteocyte, osteoblast, osteogenic cell, osteoclast

Compact Bone Histology

Compact bone is organized into structural units called osteons (Haversian systems).

Osteon: Column-like structure composed of concentric lamellae (rings of calcified matrix).
Lacunae: Small spaces containing osteocytes.
Canaliculi: Tiny channels for nutrient and waste exchange between osteocytes.
Central Canal: Contains blood vessels and nerves.
Perforating Canals: Connect central canals to periosteum and other osteons.

Structure of an osteon in compact bone Diagrammatic view of osteons and lamellae in compact bone Microscopic view of compact bone showing osteons

Spongy Bone Histology

Spongy bone lacks osteons and instead forms a lattice of trabeculae.

Trabeculae: Bony struts made of lamellae, intersected by canaliculi.
Spaces between trabeculae: Filled with red marrow, site of blood cell production.
Thin outer compact bone layer: Always seals spongy bone.

Organization of osteons and lamellae in compact bone and trabeculae in spongy bone

Organization of the Skeleton

Axial vs. Appendicular Skeleton

The adult skeleton consists of 206 bones, divided into two main parts.

Axial Skeleton (80 bones): Skull, vertebral column, sternum, ribs, hyoid bone; supports the central axis of the body.
Appendicular Skeleton (126 bones): Pectoral girdles, upper limbs, pelvic girdle, lower limbs; facilitates movement.

Anterior and posterior views of the skeleton Anterior view of the skeleton Posterior view of the skeleton

Classification of Bones by Shape

Bone Shapes and Examples

Bones are classified by their shapes, which relate to their functions.

Long Bones: Greater in length than width; found in arms, forearms, thighs, legs (e.g., humerus, femur).
Short Bones: Nearly equal in length and width; found in wrists (carpals) and ankles (tarsals).
Flat Bones: Thin, plate-like; found in skull, ribs, sternum.
Irregular Bones: Complex shapes; found in vertebrae, some facial bones.
Sutural Bones: Small bones in skull sutures; number varies per person.
Sesamoid Bones: Form in tendons; example: patella.

Classification of bone shapes Long bone example: humerus Short bone example: carpal bones Flat bone example: parietal bone Sutural bone example Sesamoid bone example: patella Irregular bone example: vertebra

Bone Markings

Categories of Bone Markings

Bone markings are anatomical features on bone surfaces, grouped into five categories.

General Structures: Body, head, neck.
Attachment Sites: Tuberosity, crest, spine; where tendons and ligaments attach.
Articulation Surfaces: Condyle, facet, trochlea; where bones meet.
Depressions: Fossa, sulcus; indentations for nerves or muscles.
Openings: Foramen, canal, fissure; passage for nerves and blood vessels.

Introduction to bone markings

Summary Table: Bone Types and Examples

Bone Type	Description	Example
Long Bone	Longer than wide	Humerus, femur
Short Bone	Nearly equal length and width	Carpals, tarsals
Flat Bone	Thin, plate-like	Parietal bone, ribs
Irregular Bone	Complex shape	Vertebra
Sutural Bone	Small bones in skull sutures	Wormian bones
Sesamoid Bone	Form in tendons	Patella

Key Equations and Concepts

Bone Growth and Remodeling

Bone growth occurs at the epiphyseal plate, and remodeling is regulated by osteoblasts and osteoclasts.

Bone Matrix Composition: Mainly collagen fibers and hydroxyapatite (calcium phosphate).
Remodeling Equation:

Blood Cell Production

Red marrow in spongy bone is the site of hematopoiesis (blood cell formation).

Hematopoiesis Equation:

Conclusion

The skeletal system is a complex organ system essential for support, movement, protection, blood cell production, and mineral storage. Understanding bone structure, types, and markings is fundamental for anatomy and physiology studies.