Human Body Systems: Skeletal, Muscular, and Integumentary Anatomy

Posted on Jun 10, 2025 in Environmental Biology

The Skeletal System: Structure and Function

Functions of the Skeletal System

Support and shape to the body
Protection of internal organs
Movement in union with muscles
Storage of minerals (calcium, phosphorus) and lipids
Blood cell production

The skeletal system accounts for approximately 20% of the body’s weight and includes bones, cartilage, tendons, and ligaments.

Bones of the Human Skeleton

There are 206 bones in the adult human skeleton, divided into two main parts:

Axial Skeleton (80 bones)
Includes bones in the skull, vertebrae, ribs, sternum, and the hyoid bone (a U-shaped bone in the neck that supports the tongue).
Appendicular Skeleton (126 bones)
Comprises the upper and lower extremities plus two girdles (pectoral and pelvic).

Types of Bones

Long Bones: Longer than they are wide; have a shaft and two ends (e.g., bones of arms and legs, except wrist, ankle, and patella).
Short Bones: Roughly cube-shaped (e.g., ankle and wrist bones).
Sesamoid Bones: Short bones found within tendons (e.g., patella).
Flat Bones: Thin, flat, and often curved (e.g., sternum, scapulae, ribs, and most skull bones).
Irregular Bones: Odd shapes that do not fit into other classes (e.g., hip bones and vertebrae).

Vertebrae (Backbone or Spine)

The vertebral column, also known as the backbone or spine, consists of 33 bones:

Cervical (7): Characterized by transverse foramina and bifid spinous processes. Includes the vertebral prominens.
- Atlas (1st vertebra): Supports the head.
- Axis (2nd vertebra): Pivots to turn the head.
Thoracic (12): Have long spinous processes and rib facets.
Lumbar (5): Feature large bodies, thick, short spinous processes.
Sacrum (5 fused): Five fused vertebrae.
Coccyx (4 fused): Four fused vertebrae.

Synovial Joints

Synovial joints allow for various types of movement:

Ball and Socket: Allows for a complete range of motion.
Pivot: One bone pivots in the arch of another (e.g., Axis/Atlas, and proximal radioulnar joint).
Saddle: Two-directional movement (e.g., between the thumb and trapezium carpal bone).
Hinge: Like a door hinge – allows bending and extending (e.g., elbow, knee, finger joints).
Ellipsoid (Condyloid): Allows side-to-side and back-and-forth movement (e.g., radius end into carpal bones).
Plane or Gliding: Least movable – allows side-to-side movement only (e.g., intercarpal and intertarsal joints, between vertebrae).

Cartilage

Cartilage is mostly water, contains no blood vessels or nerves, is tough and resilient, and heals poorly. New cartilage forms from chondroblasts.

Growth of Cartilage

Appositional Growth: Cells in the perichondrium secrete matrix against the external face of existing cartilage.
Interstitial Growth: Lacunae-bound chondrocytes inside the cartilage divide and secrete new matrix, expanding the cartilage from within.

Types of Cartilage

Hyaline Cartilage: Fine collagen fiber matrix; the most abundant type. Found in articular (movable joint) cartilages, costal cartilages (connecting ribs to sternum), respiratory cartilages (in larynx and upper respiratory passageways), and nasal cartilages.
Elastic Cartilage: Similar to hyaline cartilage, but with more elastic fibers (very flexible). Found in the external ear and epiglottis (larynx covering).
Fibrocartilage: Rows of chondrocytes with thick collagen fibers; highly compressible with great tensile strength. Found in the menisci of the knee, intervertebral discs, and pubic symphysis.

Cellular Structure of a Long Bone

Compact Bone: The hard outer layer of bones, composed of compact bone tissue with minimal gaps and spaces. This tissue gives bones their smooth, white, and solid appearance, accounting for 80% of an adult skeleton’s total bone mass. Also known as dense bone or cortical bone.
Spongy Bone: Fills the interior of the organ, composed of a network of rod- and plate-like elements that make the overall organ lighter and allow room for blood vessels and marrow. Accounts for the remaining 20% of total bone mass, but has nearly ten times the surface area of compact bone.

Types of Bone Cells

Osteoblasts: Bone-forming cells that synthesize and secrete unmineralized ground substance, found in areas of high metabolism within the bone.
Osteocytes: Mature bone cells derived from osteoblasts that have formed bone tissue around themselves. These cells maintain healthy bone tissue by secreting enzymes and controlling bone mineral content; they also regulate calcium release from bone tissue into the blood.
Bone Lining Cells: Formed from osteoblasts along the surface of most adult bones. Thought to regulate the movement of calcium and phosphate into and out of the bone.
Osteogenic Cells: Respond to traumas, such as fractures, by giving rise to bone-forming and bone-destroying cells.
Osteoclasts: Bone-absorbing cells; large cells that break down bone tissue, important for growth, healing, and remodeling.

Red and Yellow Bone Marrow

The formation of blood cells (hematopoiesis) primarily occurs in the red marrow of bones. In infants, red marrow is found in bone cavities. With age, it is largely replaced by yellow marrow for fat storage. In adults, red marrow is limited to the spongy bone in the skull, ribs, sternum, clavicles, vertebrae, and pelvis. Red marrow functions in the formation of red blood cells, white blood cells, and blood platelets.

Microscopic Structure of Compact Bone

Haversian System (Osteon): The structural unit of compact bone.
Lamellae: Weight-bearing, column-like matrix tubes composed mainly of collagen.
Haversian (Central) Canal: Central channel containing blood vessels and nerves.
Volkmann’s Canals: Channels lying at right angles to the central canal, connecting the blood and nerve supply of the periosteum to that of the Haversian canal.
Osteocytes: Mature bone cells.
Lacunae: Small cavities in bone that contain osteocytes.
Canaliculi: Hair-like canals that connect lacunae to each other and the central canal.

Structure of Long Bone

Diaphysis: The main part of the long bone; a tubular shaft that forms the axis. Composed of compact bone surrounding the medullary cavity, which contains yellow bone marrow (fat).
Epiphyses: Expanded ends of long bones. The exterior is compact bone, and the interior is spongy bone. The joint surface is covered with articular (hyaline) cartilage. The epiphyseal line separates the diaphysis from the epiphyses.

Bone Membranes of Long Bone

Periosteum: A double-layered protective membrane. The outer fibrous layer is dense regular connective tissue (CT). The inner osteogenic layer is composed of osteoblasts and osteoclasts. Richly supplied with nerve fibers, blood, and lymphatic vessels (entering the bone via nutrient foramina). Secured to underlying bone by Sharpey’s fibers.
Endosteum: A delicate membrane covering the internal surfaces of bone.

Structure of Short, Irregular, and Flat Bones

These bones consist of thin plates of periosteum-covered compact bone on the outside with endosteum-covered spongy bone (diploë) on the inside. They have no diaphysis or epiphysis and contain bone marrow between the trabeculae.

Bone Markings

Bone markings are bulges, depressions, and holes that serve as: sites of attachment for muscles, ligaments, and tendons; joint surfaces; and conduits for blood vessels and nerves.

Projections for Muscle, Tendon, and Ligament Attachment

Tubercle: Small rounded projection.
Tuberosity: Rounded projection.
Crest: Narrow, prominent ridge of bone.
Trochanter: Large, blunt, irregular surface.
Line: Narrow ridge of bone.
Epicondyle: Raised area above a condyle.
Spine: Sharp, slender projection.
Process: Any bony prominence.

Projections That Help Form Joints

Head: Bony expansion carried on a narrow neck.
Facet: Smooth, nearly flat articular surface.
Condyle: Rounded articular projection.
Ramus: Arm-like bar of bone.

Depressions and Openings in Bones

Meatus: Canal-like passageway.
Sinus: Cavity within a bone.
Fossa: Shallow, basin-like depression.
Groove: Furrow.
Fissure: Narrow, slit-like opening.
Foramen: Round or oval opening through a bone.

Bone Fracture Terminology

Nondisplaced: Bone ends retain their normal position.
Displaced: Bone ends are out of normal alignment.
Complete: Bone is broken all the way through.
Incomplete: Bone is not broken all the way through.
Compound (Open): Bone ends penetrate the skin.
Simple (Closed): Bone ends do not penetrate the skin.

Common Types of Fractures

Linear: The fracture is parallel to the long axis of the bone.
Transverse: The fracture is perpendicular to the long axis of the bone.
Oblique: Diagonal breaks across the bone.
Comminuted: Bone fragments into three or more pieces; common in the elderly.
Spiral: Ragged break when bone is excessively twisted; common sports injury.
Avulsion: Pieces of the bone have been pulled apart.
Impacted: Opposite of avulsion fractures – a piece of bone is pushed down into another piece of bone.
Fissure: Cracks in the bone.
Depressed: Broken bone portion pressed inward; typical skull fracture.
Greenstick: Incomplete fracture – one side of the bone breaks and the other side bends; common in children.
Compression: Bone is crushed; common in porous bones.
Epiphyseal: Epiphysis separates from diaphysis along the epiphyseal line; occurs where cartilage cells are dying.

Bone Repair Process

Injury: Broken blood vessels, hematoma formation.
Invasion of Blood Vessels & Cells: (2-3 days)
Fibroblast Development: (1 week)
Chondroblast Development
Callus Formation: (4 weeks)
Remodeling with Osteoclasts: (8 weeks)

Skeletal System Injuries

Sprains: Ligaments reinforcing a joint are stretched or torn. Partially torn ligaments slowly repair themselves, while completely torn ligaments require prompt surgical repair.
Cartilage Injuries: The ‘snap and pop’ of overstressed cartilage; common aerobics injury. Repaired with arthroscopic surgery.
Dislocations: Occur when bones are forced out of alignment, usually accompanied by sprains, inflammation, and joint immobilization. Caused by serious falls and are common sports injuries. Subluxation refers to a partial dislocation of a joint.

Skeletal Disorders

Spinal Stenosis: Narrowing of the spinal column.
Achondroplasia: Defect in the formation of cartilage at the epiphysis of long bones (dwarfing).
Juvenile Rheumatoid Arthritis: Chronic inflammatory disease involving the joints or other organs in children under 16.
Ankylosing Spondylitis: Immobility of a joint in the spine.
Osteosarcoma: Malignant sarcoma of bone.
Osteoporosis: Loss of bone mass throughout the skeleton, predisposing individuals to fractures.
Disc Herniation: Rupture of the soft tissue separating two vertebral bones into the spinal canal.
Scoliosis: A lateral curvature of the spine.

The Muscular System: Movement and Metabolism

Muscle Function

Muscles perform several vital functions:

Stabilizing joints
Maintaining posture
Producing movement
Moving substances within the body
Stabilizing body position and regulating organ volume
Producing heat (muscle contraction generates 85% of the body’s heat)

Characteristics of Muscle Tissue

Excitability: Ability to receive and respond to stimuli.
Contractility: Ability to shorten and thicken.
Extensibility: Ability to stretch.
Elasticity: Ability to return to its original shape after contraction or extension.

Types of Muscle Tissue

Skeletal Muscle: Attached to bone, moves the whole body. Nucleus: Multiple/peripheral. Control: Voluntary. Striations: Yes. Cell Shape: Cylindrical.
Smooth Muscle: Found in hollow organs, glands, and blood vessels. Function: Compression of tubes and ducts. Nucleus: Single/central. Control: Involuntary. Striations: No. Cell Shape: Spindle-shaped.
Cardiac Muscle: Located in the heart. Function: Heart contraction to propel blood. Nucleus: Central and single. Control: Involuntary. Striations: Yes. Cell Shape: Branched.

Muscle Function and Similarities

Skeletal muscles are responsible for all locomotion. Smooth muscle helps maintain blood pressure and squeezes or propels substances (e.g., food, feces) through organs. Cardiac muscle is responsible for propelling blood through the body.

Skeletal and smooth muscle cells are elongated and are called muscle fibers. Muscle contraction depends on two kinds of myofilaments: actin and myosin. Muscle terminology is similar: Sarcolemma (muscle plasma membrane) and Sarcoplasm (cytoplasm of a muscle cell).

Skeletal Muscles

There are nearly 650 muscles attached to the skeleton. They work in pairs: one muscle moves the bone in one direction, and the other moves it back again. Most muscles extend from one bone across a joint to another, with one bone being more stationary than the other during a given movement. Muscle movement bends the skeleton at movable joints. Muscles are firmly anchored to bone by tendons, which are made of dense fibrous connective tissue shaped like heavy cords. Though very strong and securely attached to muscle, tendons may be injured. The attachment to the more stationary bone by a tendon closest to the body (or muscle head, or proximal end) is the origin, and the attachment to the more movable bone by a tendon at the distal end is the insertion. During movement, the origin remains stationary, and the insertion moves. The force producing the bending is always a pull of contraction. Reversing the direction is produced by the contraction of a different set of muscles. As one group of muscles contracts, the other group stretches, and then they reverse actions. Muscle contractions can be short, single contractions or longer ones.

Muscle Attachments

Muscles span joints and are attached to bone in at least two places. When muscles contract, the movable bone (the muscle’s insertion) moves toward the immovable bone (the muscle’s origin). Muscles attach:

Directly: The epimysium of the muscle is fused to the periosteum of a bone.
Indirectly: Connective tissue (CT) wrappings extend beyond the muscle as a tendon or aponeurosis.

Skeletal Muscle Anatomy

Nerve and Blood Supply

Each skeletal muscle is served by one nerve, an artery, and one or more veins. Each skeletal muscle fiber is supplied with a nerve ending that controls contraction. Contracting fibers require continuous delivery of oxygen and nutrients via arteries, and wastes must be removed via veins. Each muscle is a discrete organ composed of muscle tissue, blood vessels, nerve fibers, and connective tissue. Each muscle has thousands of muscle fibers in a bundle running from origin to insertion, bound together by connective tissue through which run blood vessels and nerves. Each muscle fiber contains many nuclei, an extensive endoplasmic reticulum (sarcoplasmic reticulum), many thick and thin myofibrils running lengthwise the entire length of the fiber, and many mitochondria for energy.

Connective Tissue Wrappings

The three connective tissue wrappings are:

Epimysium: An overcoat of dense regular connective tissue (CT) that surrounds the entire muscle.
Perimysium: Fibrous CT that surrounds groups of muscle fibers called fascicles.
Endomysium: A fine sheath of CT composed of reticular fibers surrounding each muscle fiber.

The Sarcomere

The basic functional unit of the muscle fiber is the sarcomere, which consists of thick filaments with myosin protein molecules and thin filaments with actin protein molecules, plus smaller amounts of troponin and tropomyosin. When viewed under the microscope, they appear as striations of dark A bands and light I bands. The A bands are bisected by the H zone, with the M line (or band) running through the center of this H zone. The I bands are bisected by the Z disk (or line). A sarcomere consists of the array of thick and thin filaments between two Z disks.

Sliding-Filament Model

In the thick filaments, myosin molecules contain a globular subunit, the myosin head, which has binding sites for the actin molecules of the thin filaments and ATP. Activating the muscle fiber causes the myosin heads to bind to actin molecules, pulling the thin filament a short distance past the thick filaments. The linkages break and reform (using ATP energy) further along the thick filaments. Thus, the thin filaments are pulled past the thick filaments in a ratchet-like action. No shortening, thickening, or folding of individual filaments occurs. As the muscle contracts, the width of the I bands and H zones decrease, causing the Z disks to come closer together, but there is no change in the width of the A band because the thick filaments do not move. As the muscle relaxes or stretches, the width of the I bands separates as the thin filaments move apart, but the thick filaments still do not move.

The Motor Unit: Nerve-Muscle Functional Unit

A motor unit is a motor neuron and all the muscle fibers it supplies. The number of muscle fibers per motor unit can vary from four to several hundred. Muscles that control fine movements (e.g., fingers, eyes) have small motor units. Large weight-bearing muscles (e.g., thighs, hips) have large motor units. Muscle fibers from a motor unit are spread throughout the muscle; therefore, contraction of a single motor unit causes weak contraction of the entire muscle.

Sequential Events of Contraction

Cross-bridge Attachment: Myosin cross-bridge attaches to the actin filament.
Working (Power) Stroke: Myosin head pivots and pulls the actin filament toward the M line.
Cross-bridge Detachment: ATP attaches to the myosin head, and the cross-bridge detaches.
“Cocking” of the Myosin Head: Energy from ATP hydrolysis cocks the myosin head into the high-energy state.

Regulation of Contraction

In order to contract, a skeletal muscle must:

Be stimulated by a nerve ending.
Propagate an electrical current (action potential) along its sarcolemma.
Have a rise in intracellular Ca2+ levels, which is the final trigger for contraction.

Linking the electrical signal to the contraction is known as excitation-contraction coupling.

Nerve Stimulus of Skeletal Muscle

Skeletal muscles are stimulated by motor neurons of the somatic nervous system. Axons of these neurons travel in nerves to muscle cells. Axons of motor neurons branch profusely as they enter muscles. Each axonal branch forms a neuromuscular junction with a single muscle fiber.

Muscle Fatigue and Oxygen Debt

Muscle fatigue is a state of physiological inability to contract. Muscle fatigue occurs when:

ATP production fails to keep pace with ATP use.
There is a relative deficit of ATP, causing contractures.
Lactic acid accumulates in the muscle.
Ionic imbalances are present.

Vigorous exercise causes dramatic changes in muscle chemistry. For a muscle to return to a resting state:

Oxygen reserves must be replenished.
Lactic acid must be converted to pyruvic acid.
Glycogen stores must be replaced.
ATP and creatine phosphate (CP) reserves must be resynthesized.

Oxygen debt is the extra amount of O2 needed for these restorative processes.

Heat Production During Muscle Activity

Only 40% of the energy released in muscle activity is useful as work; the remaining 60% is given off as heat. Dangerous heat levels are prevented by radiation of heat from the skin and sweating.

Muscle and Tendon Injuries

Strains: Injuries from overexertion or trauma involving stretching or tearing of muscle fibers. They are often accompanied by pain and inflammation of the muscle and tendon. If the injury is near a joint and involves a ligament, it is called a sprain.
Cramps: Painful muscle spasms or involuntary twitches.
Stress-induced Muscle Tension: May cause back pain and headaches.

Muscular Disorders

Poliomyelitis: Viral infection of the nerves that control skeletal muscle movement.
Muscular Dystrophies: Most commonly caused by a mutation of the gene for the protein dystrophin, which helps in attaching and organizing the filaments in the sarcomere. Duchenne Muscular Dystrophy and Becker Muscular Dystrophy are the two most common types. The gene for dystrophin is on the X chromosome, making the disorder sex-linked. Muscle function is impaired.
Myasthenia Gravis: Autoimmune disease affecting the neuromuscular junction. Patients have smaller end-plate potentials due to antibodies being directed against the receptors. It affects the ability of the impulse to cause muscle contraction. Administering an inhibitor of acetylcholinesterase can temporarily restore contractility.
Tetanus: A serious bacterial disease that affects the nervous system, leading to painful muscle contractions, particularly of the jaw and neck muscles. Tetanus can interfere with breathing and, ultimately, threaten life. It is commonly known as ‘lockjaw.’

Age-Related Changes in Muscles

With age, connective tissue increases and muscle fibers decrease; muscles become less strong and more sinewy. By age 80, 50% of muscle mass is lost (sarcopenia). Regular exercise can reverse sarcopenia. Aging of the cardiovascular system affects every organ in the body; atherosclerosis may block distal arteries, leading to intermittent claudication and causing severe pain in leg muscles.

Benefits of Exercise

Exercise helps muscles become more effective and efficient. Tendons will become thicker and able to withstand greater force. High-intensity exercise for short durations produces strength, size, and power gains in muscles, while low-intensity exercise for long durations provides endurance benefits. Trained muscles have better tone, or a state of readiness to respond. Exercise promotes good posture, enabling muscles to work effectively and helping prevent injury. During exercise, muscle cells use more oxygen and produce increased amounts of carbon dioxide. The lungs and heart have to work harder to supply the extra oxygen and remove the carbon dioxide. Heart rate also increases to transport oxygenated blood to the muscles. Muscle cell respiration increases, meaning more oxygen is used up and carbon dioxide levels rise. The brain also signals the heart to beat faster so that more blood is pumped to the lungs for gaseous exchange. This results in more oxygenated blood reaching the muscles and more carbon dioxide being removed.

The Integumentary System: Skin, Hair, and Glands

Functions of the Integumentary System

The integumentary system consists of the skin, hair, nails, the subcutaneous tissue below the skin, and assorted glands. Its functions include protection against injury and infection, regulation of body temperature, sensory perception, regulation of water loss, and chemical synthesis.

Skin Protection

The skin covers and protects the entire body against injury and infection.

Physical Barriers
The continuity of the skin and the hardness of keratinized cells. Due to the skin’s physical characteristics, such as keratinized cells and the waterproofing properties of glycolipids, keratin helps waterproof the skin and protects from abrasions and bacteria. Glycolipids prevent the diffusion of water and water-soluble substances between cells, and continuity prevents bacterial invasion. Substances able to penetrate the skin include:
- Lipid-soluble substances (e.g., oxygen, carbon dioxide, steroids, and fat-soluble vitamins)
- Oleoresins of certain plants (e.g., poison ivy and poison oak)
- Organic solvents (e.g., acetone, dry cleaning fluid, and paint thinner)
- Salts of heavy metals (e.g., lead, mercury, and nickel)
- Topical medications (e.g., motion sickness patches)
Chemical Barriers
Skin secretions and melanin. Skin secretions such as sebum, human defensins (antimicrobial peptides), and the acid mantle of the skin retard bacterial growth and/or kill them. Melanin provides protection from UV damage. The acid mantle and low pH of sebum slow bacterial growth on the skin surface. Human defensin is a natural antibiotic. Cathelicidins are proteins that prevent Streptococcus A infection in wounded skin. Melanin is a chemical pigment that prevents UV damage.
Biological Barriers
Langerhans’ cells, macrophages, and DNA. Langerhans’ cells (members of the dendritic cell family, residing in the basal and suprabasal layers of the epidermis and in the epithelia of the respiratory, digestive, and urogenital tracts) specialize in antigen presentation and belong to the skin immune system. Langerhans’ cells in the epidermis present antigens to lymphocytes. Dermal macrophages (the second line of defense) attack bacteria and viruses that have penetrated the epidermis. Langerhans cells and macrophages present in the skin help activate the body’s immune system. DNA structure: The electrons in DNA absorb UV radiation and convert it to heat.

Temperature Regulation

Temperature regulation involves the production of copious amounts of sweat to dissipate heat. When body temperature rises and is hotter than the external environment, blood vessels in the dermal area dilate, and sweat glands are stimulated into activity. Evaporation of sweat from the skin’s surface helps dissipate heat from the body. Conversely, constriction of dermal blood vessels helps retain heat. When it is cold outside, the dermal blood vessels constrict and pull blood away from the skin, keeping it close to the body core to protect crucial internal organs.

Cutaneous Sensations

Cutaneous sensations are detected by cutaneous sensory receptors:

Meissner’s Corpuscles: Light touch.
Merkel Discs: Light touch.
Pacinian Receptors: Lie in the deeper dermis/hypodermis and detect deep pressure contacts.
Hair Root Plexus: Sensations from movement of hairs; hair follicle receptors detect movement across the surface of the skin.
Bare Nerve Endings: Detect painful stimuli (chemicals, heat, cold).

Excretion and Absorption

Elimination of nitrogen-containing wastes (ammonia, urea, uric acid), sodium chloride, and water. The skin also regulates water loss.

Metabolic Functions of Skin

Synthesis of Vitamin D: Increases calcium absorption in the body. Vitamin D is a fat-soluble vitamin that may be absorbed from the intestines or produced by the skin when exposed to ultraviolet light (particularly sunlight). It is converted to its active form by the body in two steps, occurring first in the liver and completed in the kidneys. In its active form, vitamin D acts as a hormone to regulate calcium absorption from the intestine and to regulate levels of calcium and phosphate in the bones. Vitamin D deficiency causes Rickets (when the body is deficient in vitamin D, it is unable to properly regulate calcium and phosphate levels). If blood levels of these minerals become low, other body hormones may stimulate the release of calcium and phosphate from the bones into the bloodstream.
Chemical Conversion: Chemical conversion of many substances.
Blood Reservoir: Preferential shunting of blood as needed.

Body Membranes

Membranes are thin, sheet-like structures that protect parts of the body.

Serous Membranes: Line body cavities that have no opening to the outside; secrete a watery fluid called serous fluid that lubricates surfaces.
Mucous Membranes: Line cavities and tubes that open to the outside.
Synovial Membranes: Form the inner lining of joint cavities; secrete a thick fluid called synovial fluid.
Cutaneous Membrane: Also known as skin.

Characteristics of Skin

The integument covers the entire body and is the largest and heaviest organ (approximately 2 square meters and 16% of body mass). It is composed of the epidermis and dermis, is pliable yet durable, and has a thickness ranging from 1.5 to 6.0 mm.

Types of Skin

Thin Skin: 1-2 mm thick on most of the body (0.5 mm in eyelids). It is hairy, covers all parts of the body except the palms and soles, has a thin epidermis, lacks stratum lucidum and dermal papillae, and has more sebaceous glands but fewer sweat glands and sensory receptors than thick skin.
Thick Skin: Up to 6 mm thick on the palms of hands and soles of feet. It is hairless, covers the palms and soles, has a thick epidermis and a distinct stratum lucidum. Epidermal ridges are present due to well-developed, numerous dermal papillae. It lacks sebaceous glands but has more sweat glands and more densely packed sensory receptors.

Types of Cells in Epidermis

Keratinocytes: 90% of epidermal cells; keratinized, containing keratin (a fibrous protein). They protect and waterproof the skin.
Melanocytes: 8% of epidermal cells; produce melanin, contributing to skin color and absorbing UV light.
Langerhans Cells: Arise from red bone marrow and migrate to the epidermis; constitute a small portion of epidermal cells. They participate in immune responses and are easily damaged by UV light.
Merkel Cells: Least numerous epidermal cells, found in the deepest layer of the epidermis. Along with tactile discs, they function in the sensation of touch.

Layers of Epidermis

Stratum Corneum: 25-30 layers of dead, flat keratinocytes, shed continuously and replaced by cells from deeper strata. Serves as a water, microbe, and injury barrier.
Stratum Lucidum: Present only in thick skin; 3-5 layers of clear, flat, dead keratinocytes with densely packed intermediate filaments and thick plasma membranes.
Stratum Granulosum: Located above the stratum spinosum; 3-5 layers of flattened keratinocytes undergoing apoptosis. Organelles begin to disintegrate, and cells become nonliving. Marks the transition between deeper metabolically active strata and the dead cells of the superficial strata. Contains lamellar granules, which secrete a lipid-rich secretion that acts as a water sealant.
Stratum Spinosum: Located above the stratum basale; 8-10 layers of keratinocytes. Some cells retain their ability for cell division. Cells have spine-like projections (bundles of filaments of the cytoskeleton) that tightly join cells to each other, providing skin with both strength and flexibility.
Stratum Basale (Stratum Germinativum): The deepest layer of the epidermis, where new cells are formed. A single row of cuboidal or columnar keratinocytes.

A major difference is that the dermis in males is much thicker than in females, whereas the epidermis and hypodermis are thicker in females, resulting in total skin that is 40% thicker in males.

Growth of Epidermis

Newly formed cells in the stratum basale undergo keratinization as they are pushed to the surface. They accumulate more keratin during the process, then undergo apoptosis, eventually sloughing off and being replaced. This process takes about 4 weeks. The rate of cell division in the stratum basale increases during injury.

The Dermis

The dermis is the second deepest part of the skin, where blood vessels, nerves, glands, and hair follicles are embedded. It is composed mainly of connective tissues (collagen and elastic fibers). Collagen fibers make up 70% of the dermis and provide structural toughness and strength. Elastin fibers are loosely arranged in all directions and give elasticity to the skin. The dermis has two layers: the Papillary Layer and the Reticular Layer.

Papillary Layer: The superficial portion of the dermis, consisting of areolar connective tissue containing elastic fibers. Its surface area is increased due to projections called dermal papillae, which contain capillaries or tactile receptors. Epidermal ridges conform to the dermal papillae.
Reticular Layer: The deeper portion of the dermis, consisting of dense irregular connective tissue containing collagen and elastic fibers. It provides skin with strength and elasticity and contains hair follicles, nerves, sebaceous, and sudoriferous glands.

The Hypodermis (Subcutaneous Layer)

The hypodermis (subcutaneous layer) attaches the skin to underlying organs and tissues. It is not technically part of the skin but lies below the dermis. It contains connective tissue and adipose tissues (subcutaneous fat) for insulation. Infants and the elderly have less of this tissue than adults and are therefore more sensitive to cold.

Skin Appearances and Diagnostic Clues

The epidermis appears translucent when there is little melanin or carotene. White skin appears pink to red depending on the amount and oxygen content of blood moving in the capillaries of the dermis. Albinism is an inherited trait where a person cannot produce melanin. Individuals with albinism have melanocytes but are unable to make tyrosinase (the enzyme that initiates melanin production), so melanin is missing in their hair, eyes, and skin.

Skin Color as Diagnostic Clues for Medical Conditions

Cyanosis: Skin appears bluish because hemoglobin is depleted of oxygen (e.g., in someone who has stopped breathing).
Jaundice: Buildup of bilirubin (yellow pigment) in the blood gives a yellowish appearance to the eyes and skin, indicating liver disease. Bilirubin is produced when old red blood cells are broken down by the body. Normally, it is processed in the liver and then deposited in the intestine to be excreted in stool.
Erythema: Engorgement of capillaries in the dermis, indicating skin injury, infection, heat exposure, inflammation, allergies, emotional state, or hypertension.
Pallor: Paleness, indicating emotional state, anemia, or low blood pressure.
Bronzing: Associated with Addison’s disease (adrenal cortex dysfunction), leading to too much melanin/melanocyte activity.
Bruising (Hematoma): Escaped blood has clotted, forming hematomas. May indicate a deficiency in Vitamin C or hemophilia.
Leathery Skin: Caused by overexposure, leading to clumping of elastin fibers, a depressed immune system, and potential DNA alteration causing skin cancer.
Photosensitivity: Sensitivity to substances like antibiotics and antihistamines.

Skin Color Factors and Pigments

Skin color is influenced by genetic factors, environmental factors, and blood volume. Three pigments are responsible for skin color: melanin, carotene, and hemoglobin.

Melanin: Located mostly in the epidermis. The number of melanocytes is about the same in all races; differences in skin color are due to the amount of pigment that melanocytes produce and disperse to keratinocytes. Freckles and liver spots are caused by the accumulation of melanin in patches. Melanocytes synthesize melanin from an amino acid called tyrosine, along with an enzyme called tyrosinase. All this occurs in the melanosome, an organelle in the melanocyte. Two types of melanin exist: eumelanin (brownish-black) and pheomelanin (reddish-yellow). Fair-skinned people tend to have more pheomelanin, while dark-skinned people have more eumelanin.

Environmental Factors Affecting Skin Color

Environmental factors, such as UV light, increase enzymatic activity in the melanosomes, leading to increased melanin production. A tan is achieved because the amount of melanin has increased, as well as the darkness of the melanin. (Eumelanin provides protection from UV exposure, while pheomelanin tends to break down with too much UV exposure.) While melanin provides protection from UV radiation, prolonged exposure may still cause skin cancer.

Carotene: A yellow-orange pigment (from the word ‘carrot’). It is a precursor for Vitamin A, which is used to make pigments needed for vision. Found in the stratum corneum and fatty areas of the dermis and hypodermis layer.
Hemoglobin: The oxygen-carrying pigment in red blood cells.

Skin Markings

The skin is marked by many lines, creases, and ridges.

Friction Ridges: Markings on fingertips characteristic of primates, allowing us to manipulate objects more easily. Fingerprints are friction ridge skin impressions.
Flexion Lines: Found on flexor surfaces of digits, palms, wrists, elbows, etc. The skin is tightly bound to deep fascia at these points.
Freckles: Flat, melanized patches that vary with heredity or exposure to sun.
Moles: Elevated patches of melanized skin, often with hair; mostly harmless, sometimes referred to as beauty marks.

Aging Effects on Skin

Beginning in our 20s, the effects of aging become visible in the skin:

Stem Cell Activity Declines: Skin thins, and repair becomes difficult.
Epidermal Dendritic Cells Decrease: Reduced immune response.
Vitamin D3 Production Declines: Calcium absorption declines, leading to brittle bones.
Glandular Activity Declines: Skin dries, and the body can overheat.
Blood Supply to Dermis Declines: Tendency to feel cold.
Hair Follicles Die or Produce Thinner Hair.
Dermis Thins and Becomes Less Elastic: Leading to wrinkles.
Sex Characteristics Fade: Fat deposits spread out, and hair patterns change.

Genetically programmed chronological aging causes biochemical changes in collagen connective tissues that give skin its firmness and elasticity. The genetic program for each person is different, so the loss of skin firmness and elasticity occurs at different rates and times in individuals. As skin becomes less elastic, it also becomes drier. Underlying fat padding begins to disappear. With the loss of underlying support by fat padding and connective tissues, the skin begins to sag. It looks less supple, and wrinkles form. Skin may be itchy with increased dryness, and cuts may heal more slowly.

Derivatives of Skin

During embryonic development, thousands of small groups of epidermal cells from the stratum basale push down into the dermis to form hair follicles and glands.

Skin Receptors

Your skin and deeper tissues contain millions of sensory receptors. Most of your touch receptors sit close to your skin’s surface.

Light Touch: Meissner’s corpuscles are enclosed in a capsule of connective tissue, react to light touch, and are located in the skin of your palms, soles, lips, eyelids, external genitals, and nipples. These areas of your body are particularly sensitive.
Heavy Pressure: Pacinian corpuscles sense pressure and vibration changes deep in your skin. Every square centimeter of your skin contains around 14 pressure receptors.
Pain: Skin receptors register pain. Pain receptors are the most numerous; each square centimeter of your skin contains around 200 pain receptors.
Temperature: Skin receptors register warmth and cold. Each square centimeter of your skin contains 6 receptors for cold and 1 receptor for warmth. Cold receptors start to perceive cold sensations when the surface of the skin drops below 95°F. They are most stimulated when the surface of the skin is at 77°F and are no longer stimulated when the surface of the skin drops below 41°F. This is why your feet or hands start to go numb when submerged in icy water for a long period. Hot receptors start to perceive hot sensations when the surface of the skin rises above 86°F and are most stimulated at 113°F. Beyond 113°F, pain receptors take over to avoid damage to the skin and underlying tissues. Thermoreceptors are found all over the body, but cold receptors are found in greater density than heat receptors – most of the time, our environment is colder than our body temperature. The highest concentration of thermoreceptors can be found in the face and ears, which is why your nose and ears always get colder faster than the rest of your body on a chilly winter day.

Anatomy of Hair Follicle

Shaft: The portion of hair that projects from the skin surface. Straight hair has a round shaft; curly hair is oval.
Root: The portion of hair deep to the shaft, penetrating the dermis. It has three layers:
- Medulla: Contains pigment granules and air spaces.
- Cortex: The middle layer; in dark hair, it contains pigment; in gray or white hair, it contains air bubbles.
- Cuticle: The outer layer, composed of heavily keratinized cells that lie like shingles.

Base of the Hair Follicle

Bulb: Houses the papilla, which contains the blood vessels that nourish the growing hair follicle.
Matrix: Responsible for hair growth and produces new hair.
Arrector Pili: Smooth muscles that extend from the dermis to the side of the hair follicle. Hair grows at an angle to the skin surface. Arrector pili muscles contract and pull hair straight, causing goosebumps.
Hair Root Plexus: Dendrites of neurons sensitive to touch.

Important Features and Texture of Hair

Roughly 5 million hairs cover the body of an average individual, with about 100,000 on the scalp. Almost every part of the body is covered with hair, except the palms of hands, soles of feet, sides of fingers and toes, lips, and parts of the genitals. Hair shafts differ in size, shape, and color. In the eyebrows, they are short and stiff, while on the scalp, they are longer and more flexible. Over the rest of the body, they are fine and nearly invisible. Oval-shaped hair shafts produce wavy hair, flat or ribbon-like hair shafts produce curly or kinky hair, and round hair shafts produce straight hair.

Skin Glands

Sudoriferous (Sweat) Glands
3-4 million glands in the body, emptying onto the skin through pores or into hair follicles. Two main types:
1. Eccrine Sweat Glands: Secrete cooling sweat directly onto the skin. They begin to function soon after birth. Sweat is composed of 98% water and 2% dissolved salts and nitrogenous wastes (e.g., urea and uric acid). They help regulate body temperature and aid in waste removal.
2. Apocrine Sweat Glands: Stimulated during emotional stress/excitement. Secrete into hair follicles and begin to function at puberty. Their secretions are slightly more viscous than eccrine secretions, composed of the same components as eccrine sweat plus lipids and proteins. Often referred to as ‘cold sweat.’
Sebaceous (Oil) Glands
Mostly connected to hair follicles and embedded in the dermis over most of the body. Absent in the palms and soles, they vary in size, shape, and number in other areas. They secrete an oily substance called sebum, which lubricates the hair and skin. Sebum is a mixture of fats, cholesterol, proteins, inorganic salts, and pheromones. It coats the surface of hair, prevents excessive evaporation of water from the skin, keeps skin soft and pliable, and inhibits the growth of some bacteria. Sebaceous gland activity increases with puberty due to male and female hormone activity. Accumulation of sebum in the ducts leads to white pimples; if the sebum darkens, blackheads form. Acne is an inflammation of sebaceous gland ducts.
Ceruminous Glands
Modified sweat glands of the external ear that produce earwax. They open directly onto the surface of the external auditory canal (ear canal) or into ducts of sebaceous glands. Earwax is a combination of secretions from ceruminous and sebaceous glands. Earwax and hair combine to provide a sticky barrier against foreign items.

Hair Growth Cycle

Hair follicles grow in repeated cycles, consisting of three phases:

Anagen Phase (Growth Phase)
Approximately 85% of all hairs are in this phase at any one time. The Anagen phase can vary from two to six years. Hair grows approximately 10 cm per year, and any individual hair is unlikely to grow more than one meter long. Each hair on your body grows from its own individual hair follicle. Inside the follicle, new hair cells form at the root of the hair shaft. As the cells form, they push older cells out of the follicle. As they are pushed out, the cells die and become the hair we see. A follicle will produce new cells for a certain period, depending on its location on your body.
Catagen Phase (Transitional Phase)
At the end of the Anagen Phase, hair enters the Catagen Phase, which lasts about one or two weeks. During this phase, the hair follicle shrinks to about 1/6 of its normal length. The lower part is destroyed, and the dermal papilla breaks away to rest below.
Telogen Phase (Resting Phase)
Normally lasts about 5-6 weeks. During this time, the hair does not grow but stays attached to the follicle while the dermal papilla remains in a resting phase below. Approximately 10-15% of all hairs are in this phase at any one time. When the hair follicle enters the Resting Phase, the hair shaft breaks, so the existing hair falls out, and a new hair takes its place. Therefore, the length of time that the hair can spend growing during the growth phase controls its maximum length. The cells that make the hairs on your arms are programmed to stop growing every couple of months, so arm hair stays short. Hair follicles on your head, however, are programmed to let hair grow for years at a time, allowing it to grow very long. Animals that shed have hair follicles that synchronize their rest phase so that all follicles enter the rest phase at once. Some factors that affect the rate of hair growth and replacement are illness, diet, stress, gender, radiation therapy, and medication. At the end of the Telogen phase, the hair follicle re-enters the Anagen Phase. The dermal papilla and the base of the follicle join together again, and a new hair begins to form. If the old hair has not already been shed, the new hair pushes the old one out, and the growth cycle starts all over again.

Skin Imbalances and Disorders

The skin can develop over 1000 different ailments. The most common skin disorders result from allergies or infections; less common are burns and skin cancer.

Skin Lesions

Skin lesions are any measurable variation from the normal structure of the skin.

Elevated Lesions: Cast a shadow outside the edges (e.g., warts, plaque, blister).
Flat Lesions: Do not cast a shadow (e.g., scab, elevated lesion with pus, hive).
Depressed Lesions: Cast a shadow within their edges (e.g., lacerations, ulcers, fissures).

Skin Infections

Viral: E.g., cold sores (herpes simplex, especially around lips and oral mucosa); Warts (benign neoplasms caused by papillomavirus (HPV)).
Fungal: E.g., athlete’s foot, Tinea.
Bacterial: E.g., boils and carbuncles (inflammation of hair follicles and sebaceous glands, especially on the face or dorsal side of the neck); Impetigo (Streptococcus infection).

Contact Dermatitis

Contact dermatitis is a condition in which the skin becomes red, sore, or inflamed after direct contact with a substance. There are two kinds of contact dermatitis: irritant or allergic.

Irritant Dermatitis: The most common type, caused by contact with acids, alkaline materials (such as soaps and detergents), fabric softeners, solvents, or other chemicals. The reaction usually looks like a burn.

Treatments for Contact Dermatitis

Treatments include:

Washing with lots of water to remove any traces of the irritant that may remain on the skin.
Avoiding further exposure to known irritants or allergens.
Anti-itch (antipruritic) or drying lotions may be recommended to reduce other symptoms.
Corticosteroid skin creams or ointments may reduce inflammation.
Corticosteroid pills or a corticosteroid shot from the doctor may be needed in severe cases.

Genetic Skin Diseases

Psoriasis: A chronic, non-infectious skin disease where the skin becomes dry and scaly, often with pustules and many varieties. The cycle of skin cell production increases by 3-4 times normal, and the stratum corneum thickens as dead cells accumulate. It seems to have a genetic component and is often triggered by trauma, infection, hormonal changes, or stress.
Vitiligo: An autoimmune pigmentation disorder where melanocytes in the epidermis are destroyed (e.g., as seen in Michael Jackson).

Burns

First-Degree Burns: Skin is inflamed and red; the surface layer of skin may be shed.
Second-Degree Burns: Deeper injury; blisters form as fluid builds up beneath outer layers of the epidermis.
Third-Degree Burns: Full thickness of skin is destroyed, sometimes even subcutaneous tissues, resulting in ulcerating wounds. Typically results in catastrophic fluid loss (dehydration, electrolyte imbalances), high susceptibility to infections, and slow recovery (from cells of hair follicles if they survive; otherwise, must heal from wound margins). May require autografts, cadaver skin, or pig skin. Prognosis may depend on the extent of damage.
Fourth-Degree Burns: Additionally involve injury to deeper tissues, such as muscle or bone.

Skin Cancer

Cells have a built-in mechanism called contact inhibition, where healthy cells stop growing when they come into contact with one another. In damaged cells, contact inhibition is lost, and therefore the cells continue to grow until they start piling up. Excessive or chronic exposure to UV radiation, X-rays, radiation, chemicals, or physical trauma are predisposing factors to cancer. Most forms progress slowly and are easily treated, but a few are deadly.

Basal Cell Carcinoma: Least malignant and most common (78% of all skin cancers). Cells in the stratum basale cannot form keratin and lose the boundary layer between the epidermis and dermis, resulting in tissue erosion and ulceration. 99% of these cancers are fully cured.
Squamous Cell Carcinoma: Accounts for 20% of all skin cancers. Cancer of the cells in the stratum spinosum, arising from squamous cells of the epidermis, usually induced by sun exposure. Cells grow rapidly and can invade lymphatic tissues. Appears as a hardened, small red growth. Spreads rapidly if not removed. Good chance of recovery if detected and treated early.
Malignant Melanoma: Cancer of pigment-producing cells (melanocytes).