Understanding Skin: Anatomy, Function, and Common Conditions

Posted on Mar 15, 2025 in Environmental Biology

Understanding Your Skin: Anatomy, Function, and Conditions

Your skin is the largest organ in your body, performing several vital functions:

  • Protection from injury and infection
  • Temperature regulation
  • Sensory reception
  • Regulation of water loss
  • Vitamin D synthesis

Epidermal Cells

The epidermis, the outermost layer of skin, contains several types of cells:

  • Keratinocytes (90%): Protect and waterproof the skin.
  • Melanocytes (8%): Produce melanin, protecting the skin by absorbing UV light.
  • Langerhans Cells: Participate in immune responses; easily damaged by UV light.
  • Merkel Cells: Function in the sensation of touch with tactile discs.

Layers of the Epidermis

The epidermis is composed of distinct layers:

  • Stratum Corneum: 20-25 layers of dead, flat keratinocytes that provide a barrier against water loss, microbes, and injury.
  • Stratum Lucidum: 3-5 layers of dead, flat, clear keratinocytes found only in the palms and soles.
  • Stratum Granulosum: 3-5 layers of flattened keratinocytes undergoing apoptosis, secreting a lipid-rich water sealant.
  • Stratum Spinosum: 8-10 layers of keratinocytes, providing flexibility and strength.
  • Stratum Basale: A single row of vertically arranged keratinocytes, also known as the stratum germinativum.

Life of a Cell

Cells formed in the stratum basale undergo keratinization as they are pushed to the surface, accumulating keratin. They then undergo apoptosis and eventually slough off, a process that takes about a month.

Dermis

The dermis is primarily made of connective tissue and contains hair follicles, nerves, glands, and blood vessels. Collagen fibers make up 70% of the dermis, while elastin fibers provide elasticity. The dermis has two layers:

  • Papillary Layer: Loose connective tissue containing elastic fibers.
  • Reticular Layer: Dense irregular connective tissue containing collagen and elastic fibers, as well as hair follicles, nerves, and sebaceous (oil) and sudoriferous (sweat) glands.

Hypodermis

The hypodermis is not part of the skin but attaches it to underlying organs and tissues. It contains connective tissue and body fat for insulation.

Skin Appearances

  • Little melanin or carotene makes the epidermis appear translucent.
  • White skin appears pink to red depending on blood flow and oxygen content in the dermis.
  • Albinism is an inherited trait causing a lack of melanin production.

Medical Conditions Affecting Skin

  • Cyanotic: Bluish skin due to lack of oxygen.
  • Jaundice: Yellowish skin and eyes due to bilirubin buildup, indicating liver disease.
  • Erythema: Redness due to capillary engorgement, indicating injury, infection, or inflammation.
  • Pallor: Paleness due to emotional state, low blood pressure, or anemia.
  • Bronzing: Associated with Addison’s disease.
  • Bruising: Clotted blood from escaped vessels, possibly indicating vitamin C deficiency or hemophilia.

Skin Color

Skin color is determined by three pigments:

  1. Melanin: Eumelanin (brown-black) and pheomelanin (red-yellow).
  2. Carotene: A yellow-orange pigment and precursor to vitamin A.
  3. Hemoglobin: Oxygen-carrying pigment in red blood cells.

Skin Markings

  • Friction ridges (fingerprints)
  • Flexion lines
  • Freckles
  • Moles

Aging and the Skin

Skin aging begins in the 20s, leading to:

  • Thinner skin
  • Slower repair
  • Decreased vitamin D3 production
  • Increased cold sensitivity
  • Wrinkles
  • Reduced immune response
  • Dry skin
  • Increased risk of overheating
  • Thinner hair
  • Loss of underlying fat padding

Hair

  • Shaft: The visible portion of hair (round for straight hair, oval for curly hair).
  • Root: The portion of hair within the skin.

Hair Layers

  • Medulla: The innermost layer containing pigment granules and air bubbles.
  • Cortex: The second layer, containing pigment in dark hair and air bubbles in gray or white hair.
  • Cuticle: The outermost layer of heavily keratinized cells.

Base of Hair Follicle

  • Bulb: Houses the papilla, which contains blood vessels.
  • Matrix: Responsible for hair growth.
  • Arrector Pili: Smooth muscle causing goosebumps.
  • Hair Root Plexus: Dendrites sensitive to touch.

Important Notes

  • The average body has roughly 5 million hairs, with about 100,000 on the scalp.
  • Hair covers almost every part of the body except the palms, soles, sides of fingers and toes, and lips.

Hair Growth Cycle

  • Anagen (Growth): Lasts 2-6 years.
  • Catagen (Transition): Lasts 1-2 weeks.
  • Telogen (Resting): Lasts 5-6 weeks.

Sudoriferous (Sweat) Glands

There are 3-4 million sweat glands in the body:

  • Eccrine Sweat Glands: Secrete cooling sweat directly onto the skin, functioning after birth.
  • Apocrine Sweat Glands: Secrete into hair follicles, stimulated by emotional stress, and begin functioning at puberty.

Sebaceous (Oil) Glands

  • Connected to hair follicles, secreting sebum to lubricate hair and skin.
  • Absent in palms and soles.
  • Secretion increases with puberty.
  • Accumulation of sebum can lead to white pimples or blackheads.
  • Acne is inflammation of sebaceous gland ducts.

Ceruminous Glands

Modified sweat glands in the external ear that produce earwax, providing a barrier against foreign items.

Nails

Made of tightly packed, hard, keratinized epidermal cells:

  • Nail Body: Visible portion of the nail.
  • Free Edge: Part extending past the digit.
  • Nail Root: Portion buried in the skin.
  • Lunula: Crescent-shaped area.
  • Hyponychium: Secures the nail to the fingertip.
  • Eponychium (Cuticle): Narrow band of epidermis.

Functions of Nails

  • Grasping and manipulating objects
  • Protecting the ends of digits
  • Scratching

Skin Imbalances and Conditions

The skin can develop over 1000 different ailments. Common disorders result from allergies or infections.

Skin Lesions

  • Elevated Lesions: Warts, plaques, blisters.
  • Flat Lesions: Scabs, elevated lesions with pus, hives.
  • Depressed Lesions: Lacerations, ulcers, fissures.

Infections

  • Viral: Cold sores, herpes simplex, warts (HPV).
  • Fungal: Athlete’s foot, Tinea.
  • Bacterial: Boils, carbuncles, impetigo (Streptococcus).

Contact Dermatitis

Inflammation after direct contact with a substance, either irritants or allergens.

Genetic Diseases

  • Psoriasis: Chronic, noninfectious skin disease with dry, scaly skin.
  • Vitiligo: Autoimmune disorder where melanocytes are destroyed.

Burns

Categorized by degree of penetration:

  • 1st Degree: Redness and inflammation.
  • 2nd Degree: Blisters.
  • 3rd Degree: Full thickness destruction, requiring grafts.
  • 4th Degree: Injury to deeper tissues like muscle or bone.

The “Rule of 9’s” estimates burn damage based on body surface area.

Skin Cancer

Caused by loss of contact inhibition in damaged cells, often due to UV radiation.

  • Basal Cell Carcinoma: Most common, easily cured.
  • Squamous Cell Carcinoma: Arises from stratum spinosum, can spread rapidly.
  • Malignant Melanoma: Cancer of melanocytes, deadly, requires early detection.