Skin & Membranes – Human Anatomy and Physiology, Chapter 7

Integumentary System – Scope & Components

The integumentary system is the body's largest organ, acting as a protective barrier between your inside and the outside world. It includes your skin, hair, nails, and various glands like sweat and oil glands. You can think of it as your body's outer shield, constantly renewing itself.

Crash-course references (Pt 11 and Pt 22) provide additional information, highlighting how the system has evolved, its role in health and disease, and how it connects to other body systems.

Key parts:
• Skin
• Hair
• Nails
• Sweat glands (eccrine & apocrine)
• Oil glands

Macroscopic Architecture of Skin

Your skin is made of three main layers, like a three-story building, working together.

1. Epidermis – This is the outermost, thin layer. It doesn't have its own blood supply and is mainly made of tough, protective cells.

2. Dermis – This is the thick, middle layer. It's rich in blood vessels and strong, stretchy fibers, giving skin its strength.

3. Hypodermis (superficial fascia) – This fatty layer is under the dermis. While not technically part of the skin, it helps anchor the skin, cushions you, and keeps you warm.

Epidermis – Cellular Constituents & Functional Morphology

The epidermis is a busy layer constantly rebuilding itself, mostly with protective cells. Four main types of cells in this layer handle protection, color, immune defense, and touch sensation.

• Keratinocytes – These are the main cells. They make a tough protein called keratin, which forms a protective barrier against harm. They are constantly born in the bottom layer and move up, shedding every 2525–4545 days. They stick together tightly.

• Melanocytes – These cells produce melanin, the pigment that gives your skin its color and protects your DNA from harmful UV rays. Everyone has roughly the same number of melanocytes; skin color differences come from how active these cells are.

• Dendritic (Langerhans) cells – These are immune cells that gobble up invaders (like bacteria) and show them to other immune cells, starting your body's defense.

• Tactile (Merkel) cells – These special cells at the bottom of the epidermis are connected to nerves and help you feel light touches.

Epidermal Stratification

The epidermis has different layers, forming either thin skin (4 layers) or thick skin (5 layers), depending on whether it has a clear layer called the stratum lucidum.

1. Stratum basale (germinativum) – This is the deepest, single layer of active cells that constantly divide to make new skin cells. Melanocytes (color cells) are also found here.

2. Stratum spinosum – This layer has several cell rows that look prickly because of their strong connections. It's also where you'll find most of the immune (dendritic) cells.

3. Stratum granulosum – In these 1–5 layers, cells start to flatten, lose their nuclei, and fill with granules that help make keratin and a water-resistant sealant. Cells above this layer die because they no longer get nutrients.

4. Stratum lucidum (only on palms/soles) – This transparent layer, 2–3 cells thick, consists of flat, dead cells and is found only in thick skin, helping it resist abrasion.

5. Stratum corneum – This is the top, thickest layer, made of 20–30 layers of flat, dead, tough cells (corneocytes). It's like a strong, waterproof shield that protects against injuries, chemicals, and germs.

Dermis – Structural Matrix

This is a strong layer of connective tissue that makes your skin flexible and tough. It contains blood vessels and nerves.

• It has cells like fibroblasts (which make fibers), macrophages (immune cells), and sometimes mast cells and leukocytes (white blood cells).

• Hair follicles, oil glands, and sweat glands are embedded within it.

• The jelly-like substance between cells is what gets used to make leather.

Papillary Layer

• This is the thin, upper part of the dermis, made of loose connective tissue with a network of tiny blood vessels.

• Dermal papillae are small, finger-like projections that extend up into the epidermis. In thick skin, these create patterns like fingerprints, increasing surface area for nutrient exchange and improving grip.

Reticular Layer

• This makes up about 80 % of the dermis and is made of densely packed, strong collagen fibers (for toughness) and elastic fibers (for stretchiness).

• When skin is stretched too much (like during pregnancy or rapid weight gain), these collagen fibers can tear, leading to striae (stretch marks).

• The cutaneous (dermal vascular) plexus is a network of blood vessels located at the border of the dermis and hypodermis. It's crucial for controlling body temperature and storing blood.

Cutaneous Pigmentation

Your skin color comes from three main pigments:

1. Melanin – This is the only pigment your skin makes naturally. It ranges from reddish-yellow to brown-black. Sun exposure increases melanin production, making your skin darker. Freckles and moles are formed by melanin clusters.

2. Carotene – This is a yellow-orange pigment that comes from your diet (like carrots). It stores in the top layer of skin and the hypodermis, especially noticeable in your palms and soles. It's also involved in making vitamin A.

3. Hemoglobin – The oxygen-carrying part of red blood cells in the tiny blood vessels of your dermis gives light-colored skin a pinkish tint because the epidermis is clear enough to see through.

Hair (Pili) – Anatomy & Physiology

Hair is a thread-like structure made of hard keratin, growing from a pocket (follicle) in the dermis.

• Functions: helps detect bugs, insulates the head, protects from UV rays, and filters dust from eyes (eyelashes) and nose.

• It's not found on palms, soles, lips, nipples, and certain genital areas.

Morphological Regions

• Shaft – The part of the hair that extends outside the skin, where keratin hardening is complete.

• Root – The part of the hair embedded in the skin, where keratin hardening is still happening.

Cross-sectional Layers

1. Medulla – The soft, central core of the hair shaft, containing air spaces.

2. Cortex – The middle layer, made of flattened cells arranged in concentric circles.

3. Cuticle – The outermost layer of overlapping cells, like shingles on a roof. Damage to this layer causes split ends.

Hair color comes from melanocytes in the hair follicle. Red hair has a specific melanin called pheomelanin. Gray or white hair occurs when melanin production stops and air bubbles get trapped in the hair.

Follicle Microanatomy

• It starts as an indentation of the epidermis that reaches down into the dermis, forming the hair bulb.

• Hair papilla – A small bit of connective tissue inside the hair bulb that has blood vessels to feed the growing hair.

• Hair matrix – The active area of cells at the bottom of the hair bulb that divide to produce new hair cells, pushing the hair upward as it grows and hardens.

• Root hair plexus – A network of sensory nerves around the hair follicle that makes your hair sensitive to touch.

• Arrector pili – A tiny smooth muscle attached to the hair follicle. When it contracts (like when you're cold or scared), it pulls the hair upright, causing "goose bumps."

Growth Dynamics & Types

• Vellus hair – Fine, light-colored, short hair, often found on the bodies of children and adult females.

• Terminal hair – Coarse, longer, and usually darker hair, found on the scalp, eyebrows, and later in armpits, pubic area, and male facial hair.

• Hair follicles go through periods of active growth (around 2 mm per week for about 4 years) and resting phases. About 90 hairs are typically lost per day.

• Alopecia (hair loss) after age 40 often involves terminal hair being replaced by vellus hair.

Nails – Keratinised Protective Plates

Nails are like hard shields at the ends of your fingers and toes, protecting them and helping you pick up small objects.

They have a free edge, the main nail plate, and a root. The nail bed is the skin underneath, and the nail matrix (which produces the nail) is under the white crescent shape called the lunule. The eponychium (cuticle) and hyponychium (skin under the free edge) seal the nail. Changes in nail color or shape can sometimes indicate underlying health problems.

Cutaneous Exocrine Glands

Eccrine (Merocrine) Sweat Glands

• There are about 3 million of these, found densely on palms, soles, and forehead.

• These are simple coiled tubes that release a watery sweat (mainly 99 % water, plus salts, vitamin C, antibodies, and some waste products).

• They are primarily for cooling the body: normal sweating is about 500 mL/day (insensible), while heavy sweating (up to 12 L) occurs during heat stress. They are controlled by the nervous system using acetylcholine.

Apocrine Sweat Glands

• These are larger glands located deeper in the dermis/hypodermis, usually emptying into hair follicles in areas like the armpits and groin.

• They produce a thicker, milky sweat that contains proteins and lipids. When bacteria break this down, it creates body odor.

• They become active at puberty and are thought to play a role in producing pheromones.

• Modified versions include ceruminous glands (producing earwax) and mammary glands (producing milk).

Sebaceous (Oil) Glands

• These glands are found everywhere except palms and soles and are usually associated with hair follicles. They become active at puberty, stimulated by male hormones (androgens).

• Sebum is their oily secretion, made of lipids and cell debris. It softens hair and skin, prevents water loss from the skin, and has germ-killing properties.

Comparative Summary (Table 5.1)

This table would compare the function, type of secretion, how it's released, where it exits, and where it's distributed for eccrine, apocrine, and sebaceous glands.

Physiologic Functions of Skin

Your skin performs many vital jobs:

1. Protection – It's a multi-layered shield:

   • Chemical protection: produces an acidic layer and antimicrobial substances, and melanin blocks UV rays.

   • Physical protection: the tough stratum corneum (like bricks and mortar) provides a strong barrier.

   • Biological protection: immune cells like dendritic cells and macrophages in the skin guard against invaders.

2. Thermoregulation – It helps control body temperature by sweating and adjusting blood flow in the dermis.

3. Cutaneous Sensation – It contains various nerve endings and receptors that allow you to feel touch, pressure, pain, and temperature.

4. Metabolic – It's involved in making vitamin D when exposed to sunlight, activating steroids, producing enzymes, and detoxifying some substances.

5. Blood Reservoir – Up to 5 % of your body's total blood volume can be stored in the dermis.

6. Excretion – It's a minor way your body gets rid of small amounts of waste products like nitrogenous wastes, salt, and water.

Lesions – Taxonomy & Clinical Significance

Skin lesions are simply changes in skin appearance. They are categorized by how they look:

• Elevated: like a small bump (papule), a larger flat-topped bump (plaque), a small fluid-filled blister (vesicle), a pus-filled blister (pustule), a dried scab (crust), or a temporary swollen area (wheal).

• Flat: a discolored spot that isn't raised (macule).

• Depressed: a scrape (excoriation), an open sore (ulcer), or a crack (fissure). When skin is damaged, it starts to heal: bleeding stops, new skin cells grow in from the edges, fibroblasts lay down new tissue, and a scar forms.

Burns – Depth Classification & Rule of Nines

Burns are classified by how deep they go into the skin:

• First-degree: Only the top layer (epidermis) is affected, causing redness but no blisters. It's like a sunburn.

• Second-degree: The epidermis and part of the dermis are damaged, causing redness, pain, and blistering.

• Third-degree: The entire epidermis and dermis are destroyed. The area might look pale, charred, or leathery, and initially, there's no pain because nerve endings are destroyed.

• Fourth-degree: Extends deep into muscle and bone.

• The Rule of Nines is a quick way to estimate how much of the body surface is burned for adults. It divides the body into 11 sections, each representing 9 % of the total body surface area, plus 1 % for the genitals.

Infections & Parasitic Dermatoses

• Impetigo – A highly contagious skin infection by bacteria (staph/strep) causing red sores, usually on the face, that easily rupture and form a yellowish crust.

• Tinea – Fungal infections, commonly known as ringworm, that can affect various parts of the body (e.g., athlete's foot, jock itch).

• Warts – Small, rough growths on the skin caused by the Human Papillomavirus (HPV).

• Boils (furuncles) – Painful, pus-filled lumps caused by a bacterial (staph) infection of a hair follicle.

• Scabies – An intensely itchy skin condition caused by tiny mites that burrow into the skin.

Vascular & Inflammatory Conditions

• Decubitus ulcers – Also known as bedsores, these are skin injuries caused by prolonged pressure on the skin, leading to lack of blood flow and tissue death.

• Urticaria (hives) – Itchy, red, raised welts on the skin that appear suddenly, often due to an allergic reaction, causing blood vessels to leak fluid.

• Scleroderma – A rare disease that causes the skin and connective tissues to harden and tighten, either in localized areas or systemically throughout the body.

• Psoriasis – A chronic autoimmune disease where skin cells grow too quickly, leading to thick, red, scaly patches.

• Eczema (atopic/irritant/contact dermatitis) – A general term for various skin inflammations, causing redness, itching, and sometimes blistering and crusting. Examples include allergic reactions or irritation from chemicals.

Cutaneous Oncology

Skin cancers:

• Squamous cell carcinoma – A common type of skin cancer that looks like a red, scaly, crusty nodule. It can spread (metastasize) if not treated.

• Basal cell carcinoma – The most common and least dangerous type of skin cancer. It often appears as a pearly, cratered bump. It rarely spreads because it grows slowly.

• Malignant melanoma – The most serious type of skin cancer, originating from melanocytes. Look for the ABCDE warning signs: Asymmetry (one half doesn't match the other), Border (edges are irregular), Color (uneven color or multiple colors), Diameter (larger than 6 mm), Evolution (changes in size, shape, or color).

• Kaposi sarcoma – A type of cancer that causes purple-brown skin lesions, often seen in people with weakened immune systems (e.g., AIDS) and caused by HHV-8 virus. The main cause of most skin cancers remains exposure to ultraviolet (UV) radiation.

Integrative & Ethical Considerations

Understanding the skin is important beyond just dermatology. It's crucial in treating injuries (like in burn units), preventing cancer (reducing UV exposure), understanding hormone effects (vitamin D), and even psychology (how skin conditions like hair loss or psoriasis affect self-image). Ethical patient care involves being sensitive to body image concerns, ensuring everyone has access to sun protection education, and carefully using treatments that affect skin health (like certain medications).

Real-World Relevance & Prognostic Metrics

• People with high occupational UV exposure (e.g., outdoor workers) and those who tan frequently have a higher risk of skin cancer. Public health efforts aim to reduce these avoidable risks.

• The Rule of Nines is used to calculate the fluid replacement needed for burn patients (e.g., using the Parkland formula).

• The rise in antibiotic resistance in skin infections like impetigo highlights the need for careful use of antibiotics.

• Advances in medical science now allow for engineered skin grafts that mimic natural skin, based on our understanding of how skin layers interact.