Chapter 5: The Integumentary System – Structure and Function

Introduction to the Integumentary System

Overview

The integumentary system is a complex organ system that includes the skin and its accessory structures, such as hair, nails, and glands, as well as associated blood vessels, muscles, and nerves. It serves as the body's primary barrier to the external environment and plays a vital role in protection, sensation, and regulation.

• Dermatology: The medical specialty focused on diagnosing and treating disorders of the integumentary system.

• Accessory structures include hair, nails, and glands.

Structure of the Skin

General Properties

The skin, also known as the cutaneous membrane, is the largest organ of the human body by surface area and weight.

• Surface area: ~2 square meters (22 square feet)

• Weight: 4.5–5 kg (10–11 lb), about 16% of total body weight

• Thickness: 0.5–4 mm

  • Thinnest on the eyelids

  • Thickest on the heels

  • Average thickness: 1–2 mm

Layers of the Skin

The skin consists of two major layers, with an additional underlying layer:

• Epidermis: Outer, thinner layer composed of epithelial tissue

• Dermis: Inner, thicker layer made of connective tissue

• Subcutaneous (subQ) layer / Hypodermis: Not part of the skin proper, but attaches skin to underlying tissues and organs

Sectional Anatomy of the Skin

The skin is organized into distinct regions, each with specialized structures and functions. The following sectional view highlights key components:

• Epidermal ridges: Increase grip and form fingerprints

• Hair shaft: Projects above the skin surface

• Dermal papillae: Increase surface area for exchange between epidermis and dermis

• Blood vessels: Supply nutrients and aid in thermoregulation

• Sweat and sebaceous glands: Involved in secretion and protection

• Nerve endings: Detect touch, pressure, pain, and temperature

Additional info: The provided diagram illustrates the relationship between the epidermis, dermis (papillary and reticular regions), subcutaneous layer, and accessory structures such as hair follicles, glands, and blood vessels.

Cell Types and Layers of the Epidermis

Major Cell Types

• Keratinocytes: Make up ~90% of epidermal cells; produce keratin, a tough, fibrous protein that waterproofs and protects the skin.

• Melanocytes: Produce melanin pigment, which protects against ultraviolet (UV) radiation.

• Langerhans cells: Specialized white blood cells involved in immune responses.

• Merkel cells: Associated with tactile discs; function in detecting touch sensations.

Layers of the Epidermis

The epidermis contains four major layers in thin skin and five in thick skin:

• Stratum basale (germinativum): Deepest layer; site of continuous cell division.

• Stratum spinosum: 8–10 layers of keratinocytes.

• Stratum granulosum: Contains keratohyalin and lamellar granules.

• Stratum lucidum: Present only in thick skin (fingertips, palms, soles).

• Stratum corneum: Many sublayers of dead keratinocytes (corneocytes); cells are continuously shed and replaced.

Keratinization is the process by which cells accumulate keratin as they move from the deepest layer to the surface. Excess shedding of keratinized cells from the scalp results in dandruff.

Dermis

Structure and Regions

The dermis is a connective tissue layer beneath the epidermis, providing strength and elasticity to the skin.

• Papillary region: Superficial; consists of areolar connective tissue, thin collagen and elastic fibers, dermal papillae, touch corpuscles, and free nerve endings.

• Reticular region: Deeper; consists of dense irregular connective tissue, collagen and elastic fibers, hair follicles, nerves, sebaceous (oil) glands, and sudoriferous (sweat) glands.

Striae (stretch marks) can appear if the skin is stretched excessively.

Dermal Features

• Lines of cleavage (tension lines): Indicate predominant direction of collagen fibers.

• Epidermal ridges: Reflect contours of underlying dermal papillae; form fingerprints and footprints, increasing grip by enhancing friction.

• Dermatoglyphics: Study of epidermal ridge patterns.

Structural Basis of Skin Color

Pigments Affecting Skin Color

• Melanin: Yellow-red or brown-black pigment produced by melanocytes in the epidermis; absorbs UV radiation.

• Carotene: Yellow-orange pigment found in the stratum corneum, dermis, and subcutaneous layer.

• Hemoglobin: Red pigment in erythrocytes (red blood cells) flowing through dermal capillaries.

Skin color differences are due to the amount of pigment produced, not the number of melanocytes.

• Nevus (mole): Benign localized overgrowth of melanocytes.

• Albinism: Inherited inability to produce melanin.

• Vitiligo: Partial or complete loss of melanocytes from patches of skin.

Subcutaneous Layer (Hypodermis)

Functions

The subcutaneous layer is not part of the skin but attaches it to underlying tissues and organs. It contains adipose tissue and lamellated (Pacinian) corpuscles that detect external pressure.

Accessory Structures of the Skin

Hair

• Composed of dead, keratinized epidermal cells.

• Consists of a shaft (above skin surface), root (penetrates dermis), hair follicle, epithelial root sheath, and dermal root sheath.

• Functions: Protection, reduction of heat loss, sensing light touch.

• Types: Lanugo, vellus hairs, terminal hairs.

• Hair color is determined by the amount and type of melanin.

Skin Glands

• Sebaceous (oil) glands: Connected to hair follicles; secrete sebum to prevent dehydration and inhibit bacterial growth.

• Sudoriferous (sweat) glands: Two types—eccrine and apocrine.

Sudoriferous (Sweat) Glands

• Eccrine (merocrine) glands: Numerous; help cool the body by evaporation and eliminate small amounts of waste.

• Apocrine glands: Located mainly in the axilla, groin, areolae, and bearded facial regions; ducts open into hair follicles; secrete during emotional stress and sexual excitement.

Ceruminous Glands

• Modified sweat glands in the ear canal; produce cerumen (earwax) with sebaceous glands, forming a barrier against foreign bodies.

Nails

• Composed of hard, keratinized epidermal cells over the dorsal surfaces of fingers and toes.

• Each nail consists of a free edge, nail body (plate) with a whitish lunula at its base, and a nail root embedded in a fold of skin.

Types of Skin

Classification

• Thin (hairy) skin: Covers all body regions except palms, palmar surfaces of digits, and soles.

• Thick (hairless) skin: Covers palms, palmar surfaces of digits, and soles.

Functions of the Skin

Major Functions

• Regulation of body temperature

• Blood reservoir

• Protection

• Cutaneous sensations

• Excretion and absorption

• Synthesis of vitamin D

Wound Healing

Epidermal Wound Healing

Involves regeneration of the epidermis after minor injuries.

Deep Wound Healing

Involves repair of both the epidermis and dermis, typically following more severe injuries. Includes inflammatory, migratory, proliferative, and maturation phases.

Development and Aging of the Integumentary System

Development

• The epidermis develops from the ectoderm.

• Nails, hair, and skin glands are epidermal derivatives.

• The fetal epidermis is protected by a fatty substance called vernix caseosa.

• The dermis develops from the mesoderm.

Aging

• Wrinkling and decreased immune responsiveness

• Dehydration and cracking of the skin

• Decreased sweat production

• Decreased numbers of functional melanocytes, resulting in gray hair and atypical skin pigmentation

• Loss of subcutaneous fat

• General decrease in skin thickness

• Increased susceptibility to pathological conditions

• Growth of hair and nails decreases; nails may become more brittle

Summary Table: Layers of the Skin

Layer	Main Tissue Type	Key Features
Epidermis	Epithelial tissue	Keratinocytes, melanocytes, Langerhans cells, Merkel cells; multiple layers
Dermis	Connective tissue	Papillary and reticular regions; collagen and elastic fibers; glands, hair follicles, nerves
Subcutaneous (Hypodermis)	Adipose and areolar tissue	Attaches skin to underlying tissues; contains Pacinian corpuscles

Summary Table: Types of Skin

Type	Location	Features
Thin (hairy) skin	Most body regions	Contains hair follicles, sebaceous glands, fewer sweat glands
Thick (hairless) skin	Palms, soles, palmar surfaces of digits	No hair follicles or sebaceous glands; more sweat glands; stratum lucidum present