Structure and Function of the Integumentary System

Integument Structure

Overview of the Integumentary System

The integumentary system is composed primarily of the skin and its associated structures, serving as the body's first line of defense and playing a vital role in protection, sensation, and regulation. The skin itself consists of multiple layers, each with distinct tissue types and functions.

• Skin is the largest organ of the body, accounting for about 7% of total body weight.

• It provides a protective barrier against mechanical injury, pathogens, and water loss.

• Major components include the epidermis, dermis, and subcutaneous tissue (hypodermis).

Layers of the Skin

Epidermis

The epidermis is the outermost layer of the skin, providing a protective shield for the body. It is composed of stratified squamous epithelial tissue and is avascular (lacking blood vessels).

• Stratified squamous epithelium consists of multiple layers of flat cells, offering protection against abrasion.

• Receives nutrients via diffusion from the underlying dermis.

Dermis

The dermis lies beneath the epidermis and forms the bulk of the skin. It is primarily made up of dense connective tissue and is richly vascularized.

• Contains fibroblasts, macrophages, and occasional mast cells and white blood cells.

• Houses nerves, blood vessels, lymphatic vessels, hair follicles, oil glands, and sweat glands.

Subcutaneous Tissue (Hypodermis or Superficial Fascia)

The subcutaneous tissue, also known as the hypodermis or superficial fascia, is not technically part of the skin but shares some functions and is considered part of the integumentary system.

• Composed mostly of adipose tissue with some areolar connective tissue.

• Functions include:

  • Anchoring the skin loosely to underlying structures, mainly muscles.

  • Providing shock absorption and insulation.

Structure of the Epidermis

Major Epidermal Cell Types

The epidermis contains several specialized cell types, each contributing to the skin's protective and sensory functions.

• Keratinocytes: Produce fibrous keratin, a protein that gives skin its strength and waterproof properties. These are the most abundant cells in the epidermis. Keratinocytes are continuously produced and slough off, renewing the epidermis every 25-45 days.

• Melanocytes: Spider-shaped cells located in the deepest layer of the epidermis. They produce the pigment melanin, which protects against ultraviolet (UV) radiation. Melanin production increases with sun exposure, and skin color differences are due to the amount and type of melanin.

• Additional info: Other important epidermal cells include Langerhans (dendritic) cells, which are immune cells, and Merkel (tactile) cells, which are involved in sensation.

Structure of the Dermis

Layers of the Dermis

The dermis is divided into two main layers, each with distinct structural and functional properties.

• Papillary Dermis: The thin, superficial layer composed of areolar connective tissue with loose, interlacing collagen and elastic fibers. Contains dermal papillae, which are fingerlike projections that extend into the epidermis. These papillae contain capillary loops, free nerve endings (pain receptors), and touch receptors (Meissner's corpuscles).

• Reticular Dermis: The deeper, thicker layer made of dense irregular connective tissue. Contains many elastic fibers for stretch-recoil properties and collagen fibers for strength and resiliency. The dermal vascular plexus is a network of blood vessels between the reticular layer and subcutaneous tissue, helping to bind water and keep skin hydrated.

Appendages of the Skin

Hair

Hair (pili) are flexible strands of dead, keratinized cells produced by hair follicles. Hair contains hard keratin, which is tougher and more durable than the soft keratin found in skin.

• Root: The portion within the scalp where keratinization is ongoing.

• Shaft: The portion that extends above the scalp where keratinization is complete.

• Functions include protection, sensation, and regulation of body temperature.

Sweat Glands

Sweat glands are distributed over most skin surfaces and play a key role in thermoregulation. There are about 3 million sweat glands per person, and they are absent from the nipples and parts of the external genitalia.

• Contain myoepithelial cells that contract upon nervous system stimulation to force sweat into ducts.

Types of Sweat Glands

• Eccrine Sweat Glands: The most numerous type, abundant on palms, soles, and forehead. Ducts connect to pores that open on the skin surface. Function in thermoregulation and are regulated by the sympathetic nervous system. Secrete sweat that is 99% water, with salts, metabolic wastes (urea, uric acid, ammonia), and dermcidin (a microbe-killing peptide).

• Apocrine Sweat Glands: Located mainly in axillary and anogenital areas. About 2,000 large glands are found deep in the dermis and subcutaneous tissue. Secrete viscous, milky or yellowish sweat containing fatty substances and proteins. Bacteria break down this sweat, leading to body odor. Ducts empty into hair follicles in these regions.

Sebaceous Glands

Sebaceous glands are found all over the body except on the palms and soles, where the skin is thickest. Larger glands are present on the face, neck, and chest.

• Secrete sebum, an oily secretion of lipids, mostly into hair follicles.

• Functions include lubricating the skin, deterring water loss, and killing surface bacteria.

Summary Table: Major Skin Glands

The following table summarizes the main types of skin glands, their locations, secretions, and functions.

Gland Type	Location	Secretion	Main Function
Eccrine Sweat Gland	Most skin surfaces (palms, soles, forehead)	Watery sweat (99% water, salts, wastes)	Thermoregulation
Apocrine Sweat Gland	Axillary and anogenital areas	Viscous sweat (fatty substances, proteins)	Body odor (after bacterial breakdown)
Sebaceous Gland	All skin except palms and soles	Sebum (oily lipids)	Lubrication, water loss prevention, antibacterial

Key Terms and Concepts

• Keratin: A fibrous protein providing strength and waterproofing to skin and hair.

• Melanin: A pigment produced by melanocytes, responsible for skin color and UV protection.

• Dermal Papillae: Projections in the papillary dermis that increase surface area for exchange and contain sensory receptors.

• Thermoregulation: The process by which sweat glands help maintain body temperature.