The skeletal system

Objectives of this section:

• Summarize the key roles of the skeletal system in maintaining homeostasis.
• Identify and explain the various bone types and classifications, providing relevant examples.
• Describe the primary bones that make up the axial and appendicular skeletons in the human body.
• Name and explain the functions of the three main bone cell types, and provide an overview of the bone matrix.

Functions of the Skeletal System

Functions of the skeletal system:

• Enables body movement
• Provides structural support — ligaments connect and stabilize bones
• Shields vital internal organs
• Serves as attachment points for muscles
• Acts as a storage site for essential minerals such as calcium and phosphorus, as well as fat
• Facilitates the production of blood cells

Types of Bones:

The adult human skeleton comprises 213 bones, not including sesamoid bones (Driscoll, 2006). It is divided into the appendicular skeleton, which contains 126 bones, the axial skeleton with 74 bones, and the auditory ossicles, which consist of six small bones. Throughout life, bones continuously undergo modelling to adapt to biomechanical changes, and remodelling to replace old or micro damaged bone with new, stronger tissue, maintaining overall bone strength.

Figure 1.1 Classifications of Bones: Bones are classified according to their shape (Biga et al., 2019)

Bone is primarily composed of collagen fibres and an inorganic mineral phase made up of small crystals. In vivo (within the living body), bone contains approximately 10–20% water. Of its dry weight, around 60–70% consists of mineral content, primarily hydroxyapatite (the main structural component of tooth enamel and bone mineral that provides hardness). The remainder is largely collagen, with small amounts of other proteins and inorganic salts.

Collagen, the body’s most abundant fibrous protein, features a triple-helical structure. Specific sites along these collagen fibres act as nucleation points for the deposition of mineral crystals, facilitating the organized formation of bone mineral.

The mineral phase of bone is commonly approximated by the formula for hydroxyapatite (HA), Ca₁₀(PO₄)₆(OH)₂, which has a calcium-to-phosphate (Ca:P) ratio of 1.67. However, actual bone mineral exhibits Ca:P ratios ranging from 1.37 to 1.87. This variation arises from its more complex composition, which includes additional ions such as carbonate, silicon, and zinc.

Cartilage, like bone, is also based on collagen but incorporates large protein-polysaccharide complexes that form a gel matrix interwoven with collagen fibres. Articular (or hyaline) cartilage covers the load-bearing surfaces of movable joints. It functions as a linear viscoelastic solid and is characterized by an extremely low coefficient of friction (typically < 0.01). This lubricating property is largely due to the presence of synovial fluid, which is expelled under compressive loading to reduce joint friction.

Bones are made of two tissue types:

• Compact bone: also known as cortical bone, this hard-outer layer is strong and dense
• Cancellous bone: also known as trabecular bone, this spongy inner layer network of trabeculae is lighter and less dense than cortical bone

Figure 1.2 Bone structure (National Cancer Institute, 2025)

Bones vary in their proportions of two types of bone tissue:
1. Compact bone

• Osteon – the structural unit of compact bone
• Lamellae – column-like matrix tubes composed of collagen and crystals of bone salts
• Central canal – (Haversian canal) canal containing blood vessels and nerves

2. Cancellous (spongy) bone

• No osteons
• Lamellae as trabeculae:
  – Arches, rods, plates of bone
  – Branching network of bony tissue
  – Strong in many directions
  – Red marrow (blood forming) space

Types of bones:

1. Long Bones
Function: Long bones are designed for support and mobility. They enable large movements and act as levers, especially in the limbs. They also play a key role in the production of red blood cells in the bone marrow.

Examples: Humerus, Femur, Phalanges, Radius, Ulna, Tibia, Fibula, Metacarpals, Metatarsals

Structure of a Long Bone:

Figure 1.3 Anatomy of a Long Bone (Medicine LibreTexts, 2022)

2. Short Bones
Function: Short bones are cube-like in shape and provide stability while allowing some motion. They help absorb shock and enable fine, precise movements.

Structure: Composed mainly of spongy bone enclosed by a thin layer of compact bone

Examples: Carpals (wrist bones), Tarsals (ankle bones), Patellae (also classified as sesamoid in some contexts)

3. Flat Bones
Function: Flat bones provide broad surfaces for muscle attachment and protect internal organs, such as the brain, heart, and lungs.

Figure 1.4 Anatomy of a Flat Bone (Medicine LibreTexts, 2022) 

Examples: Skull (including frontal, parietal, and occipital bones), Sternum, Ribs, Scapula

4. Irregular Bones
Function: Irregular bones have complex shapes and serve various roles including protection, support, and muscle attachment.

Examples: Vertebrae (protect the spinal cord), Pelvic bones (support body weight and protect reproductive organs)

Some classify the scapula here due to its irregular shape

5. Sesamoid Bones
Function: Sesamoid bones are small, round bones embedded within tendons. They reduce friction, enhance leverage, and protect tendons from wear and tear.

Examples: Patella (kneecap) – the largest sesamoid bone

Other small sesamoids can be found in the hands and feet (e.g., near the big toe and thumb)

Additional Notes on Bone Marrow and Blood Cell Production

Red bone marrow, primarily found in spongy bone (especially in flat and long bones), is responsible for producing red blood cells, white blood cells, and platelets.

In adults, active red marrow is mainly located in the pelvis, sternum, ribs, vertebrae, and ends of long bones.

Yellow bone marrow, found in the medullary cavities of long bones, primarily stores fat but can convert to red marrow under certain conditions (e.g., severe blood loss).

Skeleton is divided into two Divisions:

• Axial Skeleton – “central skeleton” (skull, vertebrae, sternum, and ribs), ~80 bones
• Appendicular Skeleton – “girdles and appendages”, ~126 bones 

Figure 1.5 Axial and Appendicular Skeleton (Biga, 2019)

The major bones of the skull include the following:

Figure 1.6 (Boundless, 2025)

An Introduction to the Appendicular Skeleton

Bones are made from:

• Protein fibre makes up the matrix
• Collagen
• Minerals especially calcium

The matrix is a web like structure or healthy bones which gives it strength, ensures it’s light and have a small amount of flexibility. Bone marrow is in the middle of bones and generates blood cells.

Bone (Osseous) Tissue: tissue that gives strength and structure to bones. Bone is made up of compact tissue (the hard, outer layer) and cancellous tissue (the spongy, inner layer that contains red marrow). Osseous tissue is maintained by bone-forming cells called osteoblasts and cells that break down bone called osteoclasts. Bones also contain blood vessels, nerves, proteins, vitamins, and minerals. Also called bone tissue (www.cancer.gov, 2011).

Matrix Minerals: bone mineral is present in two forms in the skeleton. Hydroxyapatite crystals, represented by the formula Ca₁₀(PO₄)₆(OH)₂, are the major forms and occur in mature bone. 

Two thirds of bone matrix is calcium phosphate, Ca₃(PO₄)₂, reacts with calcium hydroxide, Ca(OH)₂ to form crystals of hydroxyapatite, Ca₁₀(PO₄)₆(OH)₂ which incorporates other calcium salts and ions

Matrix Proteins – one third of bone matrix is protein fibres (collagen).

Cells in Bone:

Osteoprogenitor cells – precursors to osteoblasts
Osteoblasts – bone-forming cells
Responsible for osteogenesis (new bone)
Source of collagen, calcium salts
Osteocytes – mature bone cells between lamellae
Osteoclasts – bone-destroying cells, break down bone matrix for remodelling and release of calcium
Source of acid, enzymes for osteolysis
Calcium homeostasis

Types of Bone Cells:

Figure 1.7 Bone cell types: Table listing the function and location of the four types of bone cells (Biology LibreTexts, 2018)

Homeostasis

Figure 1.8 As a result of bone resorption, chemokines embedded in bone are released and activated to recruit mesenchymal progenitors while type I collagen and no collagenous proteins are de-mineralized and become available to interact with the recruited cells (Feng, 2009)

Types of Joints

A joint is the point where two bones meet. Joints can be classified by their structure (histological) or function. Structural classification is based on the main connective tissue type—fibrous, cartilaginous, or synovial. Functional classification depends on how much movement the joint allows: synarthrosis (no movement), amphiarthrosis (limited movement), and diarthrosis (free movement). These classifications are closely linked: fibrous joints are typically synarthroses, cartilaginous joints are amphiarthroses, and synovial joints are diarthroses.

Figure 1.9 (Ortho, 2016)

Figure 1.10 (Ortho, 2016)

Types of Cartilage

Cartilage – what does it do? It is part of a joint where two bones meet and allows movement to occur. They can also form structures that provide protection for the body organs as well as a shock absorber.

The skull is not one bone but many and provides protection 
  
The ribs, sternum and the spine form the rib cage, which protects what?

The human body contains three types of cartilage, each with a similar composition but varying proportions of components, giving them distinct properties and locations.

Hyaline Cartilage

Hyaline cartilage is the most common type. Named after the Greek word hyalos (meaning glass), it has a translucent, bluish-white, glossy appearance. It is typically 2–4 mm thick, as all cartilage is avascular and relies on diffusion for nutrients. Hyaline cartilage forms the early skeletal framework in embryos and remains present in adults in the ribs, joints, nose, larynx, and trachea.
Its collagen fibers (mostly type II) are extremely thin and blend with the matrix, making them nearly invisible under a microscope.

Fibrocartilage

Fibrocartilage is tough and flexible, found where strong support is needed—such as where tendons and ligaments attach to bone, the pubic symphysis, menisci, sternoclavicular joint, and intervertebral discs (annulus fibrosus).
It contains densely packed collagen fibers (types I and II) arranged in parallel bundles, giving it a white appearance and limited elasticity. Fibrocartilage lacks a perichondrium and provides excellent resistance to compression and shear forces.

Elastic Cartilage

Elastic cartilage is found in structures that require both shape retention and flexibility, such as the external ear, Eustachian tube, and epiglottis.
Composed of elastic fibers and type II collagen, it is yellowish in color and less orderly in structure than fibrocartilage. Its primary role is to provide resilience and maintain form after deformation.

Ligaments

Ligaments are soft-tissue components of the musculoskeletal system, classified alongside tendons and fasciae as connective tissues.

Their key differences lie in what they connect:

• Ligaments connect bone to bone and help stabilize joints and organs.
• Tendons connect muscle to bone, enabling movement.
• Fasciae connect muscle to muscle, providing support and structure.

Tendons

Tendons and ligaments are both types of connective tissue, but they serve different purposes in the body. Tendons connect muscles to bones, allowing for movement, while ligaments connect bones to other bones, providing stability to joints.

A tendon is a tough band of fibrous connective tissue that usually connects muscle to bone and is capable of withstanding tension. Tendons are similar to ligaments as they are all made of collagen. Tendons and muscles work together and can only exert a pulling force.

 

Bone Types and Classification

Bone Tissue Types

Tissue Type	Key Features	Components
Compact Bone	Dense, strong outer layer of bones	Osteons, Lamellae, Haversian Canal
Cancellous (Spongy) Bone	Light, porous inner layer that reduces bone weight and contains marrow	Trabeculae, Red Bone Marrow

Bone Shapes

Bone Type	Description	Examples	Key Structures (if applicable)
Long Bones	Longer than they are wide; support weight and enable movement	Humerus, Femur	Diaphysis, Epiphysis, Metaphysis
Short Bones	Cube-shaped; provide stability with limited movement	Carpals, Tarsals	–
Flat Bones	Thin and often curved; protect organs and offer muscle attachment	Ribs, Scapula	–
Irregular Bones	Complex shapes that don’t fit other categories	Vertebrae, Pelvis	–
Sesamoid Bones	Small, round bones within tendons; reduce friction and stress	Patella, Talus	–

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