Skin Derivatives and Pathology: Structure, Function, and Clinical Relevance

Lecture 14: Skin Derivatives and Pathology

Overview

This lecture covers the structure and function of the main epidermal derivatives—hair, glands, and nails—as well as common skin pathologies such as lacerations, burns, and skin cancers. Understanding these structures and their associated disorders is essential for comprehending the protective and regulatory roles of the integumentary system.

Accessory Structures of the Skin

Introduction to Accessory Structures

Accessory structures, also known as skin derivatives, are specialized structures derived from the epidermis. They include hair, nails, and glands. These structures contribute to the skin's protective, sensory, and regulatory functions.

• Hair: Provides protection, thermoregulation, and sensation.

• Nails: Protect the distal phalanges and enhance fine touch.

• Glands: Include sweat and sebaceous glands, which aid in thermoregulation and lubrication.

Hair (Pili)

Functions of Hair

• Thermoregulation: Hair helps to retain heat by trapping a layer of air close to the skin.

• Protection: Eyelashes and eyebrows protect the eyes from debris; scalp hair protects against UV radiation.

• Sensation: Hair follicles are associated with nerve endings, allowing detection of light touch.

Structure of Hair

Hair is a filamentous structure derived from the epidermis and composed of keratinized stratified squamous epithelial cells. The amount and type of melanin in the hair determines its color.

• Hair Shaft: The visible part above the skin surface, composed of dead keratinocytes.

• Hair Root: Located below the skin surface, embedded in the dermis within the hair follicle.

• Hair Follicle: A sheath of epithelial and connective tissue that surrounds the root.

• Hair Bulb: The base of the root, containing the hair matrix where keratinocyte mitosis occurs.

• Hair Papilla: A region of connective tissue containing blood vessels that nourish the growing hair.

• Arrector Pili Muscle: A small band of smooth muscle attached to the follicle; contraction causes "goose bumps."

Transverse Section of Hair

• Medulla: Central core, present only in thick hairs (e.g., scalp).

• Cortex: Middle layer, provides strength and color.

• Cuticle: Outermost single layer of overlapping keratinocytes.

Types and Conditions of Hair

• Lanugo: Fine, non-pigmented hair covering the fetus, usually shed before birth.

• Alopecia: General term for hair loss or baldness, which can affect both sexes.

Nails

Structure and Function of Nails

Nails are hard, keratinized structures derived from the epidermis, protecting the tips of fingers and toes and aiding in manipulation of small objects.

• Nail Plate: The main visible part, divided into the nail body (exposed) and nail root (under the skin).

• Nail Matrix: The proximal part of the nail root containing actively dividing cells responsible for nail growth.

• Eponychium (Cuticle): A fold of stratum corneum that protects the area between the skin and the nail plate.

• Hyponychium: The area under the free edge of the nail, attaching the nail to the fingertip.

• Lunula: The crescent-shaped, whitish area at the base of the nail, due to thickened underlying matrix.

Skin Glands

Types of Skin Glands

The skin contains several types of glands, each with specialized functions:

• Sudoriferous (Sweat) Glands: Produce sweat for thermoregulation and excretion.

• Sebaceous (Oil) Glands: Secrete sebum to lubricate and waterproof the skin and hair.

Sudoriferous (Sweat) Glands

• Eccrine Sweat Glands: Most numerous; simple tubular glands found throughout the body, especially on palms, soles, and forehead. They secrete a watery sweat (99% water, electrolytes, lactic acid, antimicrobial compounds) directly onto the skin surface via sweat pores. Main function is thermoregulation.

• Apocrine Sweat Glands: Located in axillae (armpits), anal area, and areolae. Secrete a thicker, protein-rich sweat into hair follicles. Sweat is metabolized by skin bacteria, producing body odor. Become active at puberty and are regulated by sex hormones.

• Ceruminous Glands: Modified apocrine glands in the external ear canal. Secrete cerumen (ear wax) into hair follicles, which lubricates the tympanic membrane and traps particles.

• Mammary Glands: Specialized sweat glands that produce milk, containing proteins, lipids, sugars, and other substances for infant nutrition.

Sebaceous Glands

• Branched glands with clusters of secretory acini cells, usually associated with hair follicles.

• Secrete sebum, a waxy, oily mixture of lipids, via holocrine secretion (cells accumulate sebum and rupture to release contents).

• Sebum lubricates and waterproofs skin and hair, and has antimicrobial properties.

• Most abundant on face and scalp; absent on palms and soles.

• Secretion increases after puberty due to hormonal influence.

Skin Pathologies

Common Skin Injuries and Disorders

The skin is susceptible to various injuries and diseases, including lacerations, burns, and cancers.

Lacerations

• Deep cuts or tears in the skin that may require closure with sutures or stitches.

Burns

• Caused by heat, cold, electricity, chemicals, or radiation.

• Classified by depth and extent of tissue damage:

Degree	Layers Affected	Symptoms	Prognosis
First Degree	Only epidermis	Erythema, minor pain, no blisters	No permanent damage
Second Degree	Epidermis and part of dermis	Significant pain, blistering, possible scarring	Requires medical attention
Third Degree	Epidermis, dermis, hypodermis, possibly deeper tissues	Major tissue damage, dehydration, swelling, risk of infection	May require skin grafting

• Rule of Nines: Used to estimate the percentage of body surface area affected by burns. The body is divided into 11 areas, each representing 9% (or multiples thereof) of the total body surface area.

Skin Cancer

Skin cancer results from mutations that cause uncontrolled cell division. Tumors may be benign or malignant, with the latter capable of metastasis (spreading to other tissues).

• Basal Cell Carcinoma: Most common type; affects keratinocytes in the stratum basale. Appears as a "cratered center" lesion. Rarely metastasizes; treated by surgical removal.

• Squamous Cell Carcinoma: Second most common; affects keratinocytes in the stratum spinosum. Forms scaly plaques that may bleed. Common on head and neck; requires surgical removal.

• Malignant Melanoma: Most dangerous; arises from melanocytes. May invade dermal blood vessels and metastasize. Requires surgical excision and possibly chemotherapy or radiation.

ABCDE Rule for Malignant Melanoma

• Asymmetry: Two sides of the lesion do not match.

• Border irregularity: Edges are ragged or blurred.

• Color: Varies (may be blue-black or have multiple colors).

• Diameter: Greater than 6 mm.

• Evolving: Changing in size, shape, or color.

Additional info: The lecture also references the importance of early detection and treatment of skin cancers, as well as the role of the integumentary system in overall health and disease prevention.