The Integumentary System: Structure and Function of Skin

The Integumentary System

Overview

The integumentary system is composed of the skin and its associated structures, including hair, nails, and glands. It serves as the body's primary barrier against environmental hazards and plays a vital role in protection, sensation, and regulation.

• Main components: Skin, hair, nails, sweat glands, sebaceous glands

• Functions: Protection, sensation, thermoregulation, excretion, vitamin D synthesis

Layers of the Skin

The skin consists of two main layers: the epidermis and the dermis, with a subcutaneous layer beneath.

• Epidermis: Superficial region; consists of epithelial tissue and is avascular

• Dermis: Underlies the epidermis; mostly dense connective tissue and is vascular

• Subcutaneous (hypodermis): Mostly adipose tissue; absorbs shock, insulates, and anchors skin to underlying structures (mainly muscle fascia)

Epidermis

Structure and Cell Types

The epidermis is made up of keratinized stratified squamous epithelium and contains four main cell types:

• Keratinoctyes: Produce keratin, the protein that gives skin its protective properties; connected by desmosomes; major cell type of the epidermis; millions slough off daily; new epidermis every 25–45 days; thickening leads to callus formation

• Melanocytes: Spider-shaped cells in the deepest epidermis; produce pigment melanin, which protects keratinocyte nuclei from UV damage

• Dendritic (Langerhans) cells: Star-shaped macrophages that patrol deep epidermis; key activators of immune system; arise from bone marrow

• Tactile (Merkel) cells: Sensory receptors for touch; located at the epidermal-dermal junction

Layers of the Epidermis (Strata)

The epidermis is organized into four or five distinct layers, depending on skin thickness:

• Stratum basale (basal layer): Deepest layer; attached to dermis; consists of a single row of stem cells; cells divide and move toward the surface; also contains melanocytes and occasional Merkel cells

• Stratum spinosum (prickly layer): Several cell layers thick; contains intermediate filaments and desmosomes; keratinocytes appear spiky; scattered dendritic cells

• Stratum granulosum (granular layer): Four to six cell layers; cells flatten, organelles disintegrate; keratohyaline and lamellar granules present; cells die as they move upward

• Stratum lucidum (clear layer): Only in thick skin (palms, soles); thin, translucent band of dead keratinocytes

• Stratum corneum (horny layer): 20–30 rows of dead, anucleated keratinized cells; protects deeper cells, prevents water loss, and acts as a barrier against abrasion and penetration

Table: Layers of the Epidermis

Layer	Location	Main Features
Stratum basale	Deepest	Stem cells, melanocytes, Merkel cells
Stratum spinosum	Above basale	Desmosomes, dendritic cells, spiky keratinocytes
Stratum granulosum	Middle	Keratohyaline and lamellar granules, cell death
Stratum lucidum	Thick skin only	Translucent, dead keratinocytes
Stratum corneum	Superficial	Dead, keratinized cells, barrier function

Dermis

Structure and Function

The dermis is a strong, flexible connective tissue layer beneath the epidermis. It contains fibroblasts, macrophages, mast cells, and white blood cells. The dermis binds the body together and contains nerves, blood vessels, and lymphatic vessels, as well as hair follicles, oil glands, and sweat glands.

• Papillary layer: Superficial; areolar connective tissue; contains capillaries, nerve endings, and touch receptors; forms dermal papillae that create friction ridges (fingerprints)

• Reticular layer: Deep; dense fibrous connective tissue; collagen fibers provide strength and resilience; contains cutaneous plexus (blood vessels), adipose cells, and cleavage lines

Table: Layers of the Dermis

Layer	Main Features
Papillary	Areolar tissue, dermal papillae, capillaries, touch receptors
Reticular	Dense connective tissue, collagen fibers, cutaneous plexus, adipose cells

Functions of the Skin

• Protection: Physical, chemical, and biological barrier

• Thermoregulation: Sweat glands and blood vessels regulate temperature

• Sensation: Sensory receptors detect touch, pain, and temperature

• Metabolic functions: Synthesis of vitamin D

• Excretion: Removal of wastes via sweat

Skin Color

Pigments and Clinical Relevance

Skin color is determined by three main pigments: melanin, carotene, and hemoglobin.

• Melanin: Produced by melanocytes; protects against UV damage; color varies from reddish yellow to brownish black

• Carotene: Yellow to orange pigment; most obvious in palms and soles; can be converted to vitamin A

• Hemoglobin: Pinkish hue in fair skin due to lower melanin levels

Table: Skin Color and Clinical Indications

Color Change	Possible Cause
Cyanosis (blue)	Low oxygenation of hemoglobin
Pallor (pale)	Anemia, low blood pressure, fear, anger
Erythema (redness)	Fever, hypertension, inflammation, allergy
Jaundice (yellow)	Liver disorders
Bruises (black/blue marks)	Clotted blood beneath skin (ecchymoses or hematomas)
Brown/black "necklace" or bruises	Insulin resistance, elevated blood glucose

Clinical Considerations

• Homeostatic Imbalance 5.1: Extreme stretching of skin can cause dermal tears (striae or stretch marks); acute, short-term trauma can cause blisters

• Homeostatic Imbalance 5.2: Excessive sun exposure damages skin, destroys folic acid, and increases risk of skin cancer; photosensitivity can be triggered by drugs or perfumes

• Homeostatic Imbalance 5.3: Alterations in skin color can indicate disease (cyanosis, pallor, erythema, jaundice, bruises)

Key Terms and Definitions

• Keratinoctyes: Cells that produce keratin

• Melanocytes: Cells that produce melanin

• Desmosomes: Cell junctions that bind cells together

• Callus: Thickened skin due to friction

• Striae: Stretch marks

• Ecchymoses: Bruises or hematomas

Formulas and Equations

• Keratinocyte turnover rate: $\text{Epidermal renewal time} = 25\text{ to }45\text{ days}$

Additional info: The notes have been expanded to include definitions, tables, and clinical context for a comprehensive review of the integumentary system suitable for college-level Anatomy & Physiology students.