Integumentary System Structure, Function and Wound Healing

Function of the Skin

Protection = interface between internal organs and external environment - 'First line of defense'

Thermoregulation

Immune functions

Sensation

Water regulation

3 Layers of the Skin

epidermis

dermis

subcutaneous layer (hypodermis)

Epidermis Function

protection from epidermal water loss, protect against bacterial invasion and physical stresses like shear friction and toxic irritants

Dermis Function

provides tensile strength, support, moisture retention,

and blood and oxygenation to the skin

Subcutaneous Layer (Hypodermis) Function

attaches to underlying structures, thermal insulation,

body shape, mechanical shock absorber

Cells of the Epidermis

Keratinocytes - form protective keratin

Melanocytes - pigment-producing cells

Merkel Cells - light touch sensation

Langerhans Cells - immune reactions

5 Layers of Epidermis

Stratum corneum

Stratum lucidum

Stratum granulosum

Stratum spinosum

Stratum basale

What is the function of dermal papillae of the dermis?

They minimize separation between dermis and epidermis to resist shear, form fingerprints, and house nociceptors, thermoreceptors, and capillaries.

What characterizes the reticular dermis?

It's thick and elastic, with collagen bundles running parallel to the body's surface.

What does the subcutaneous layer contain, and how thick can it be?

Adipose and connective tissue, plus blood vessels, nerves, and lymphatic tissue; up to 3cm thick on the abdomen (more with obesity).

What is the function of eccrine sweat glands?

To cool the body.

What is the function of apocrine sweat glands?

They drain sweat into a hair follicle and are responsible for body odor.

What do sebaceous glands produce and why?

Sebum, an oil that assists in skin protection.

What are the four phases of wound healing and their timing?

Hemostasis (injury to day 4), Inflammation (day 4-6), Proliferation (day 4-14), Remodeling (day 8 to 1 year).

What is the goal of the hemostasis phase, and how is it achieved?

To stop the bleeding: platelets aggregate to form a plug, thrombin converts fibrinogen to fibrin (a temporary matrix), and fibrinolysis later breaks down the clot.

What are the three key leukocytes in the inflammatory phase and their roles?

Neutrophils kill bacteria; macrophages clean debris and secrete growth factors; lymphocytes monitor for infection and control inflammation/proliferation.

What is granulation tissue and what does it provide?

A gel-like matrix of collagen, hyaluronic acid, and fibronectin with a bright red budding appearance; it provides a scaffold for epithelial migration.

How does re-epithelialization occur?

Keratinocytes migrate from adjacent epidermis via a leap frog model, forming a layer 4-6 cells thick through contact inhibition.

What is the difference between angiogenesis and vasculogenesis?

Angiogenesis forms new blood vessels from existing ones; vasculogenesis forms new blood cells from endothelial progenitor cells.

What factors affect wound contraction, and what is hypercontraction?

Contraction depends on hydration, myofibroblasts, and absence of eschar. Hypercontraction is when wound edges come together prematurely, risking tunneling.

What happens during the remodeling phase?

Collagen synthesis and lysis occur, fibers reorient, and disorganized Type III collagen converts to Type I (can take years); scar strength increases but never reaches 100 percent.

What is the timeline for scar tensile strength recovery?

1 week: 3 percent, 3 weeks: 30 percent, 3 months: 80 percent.

What distinguishes primary from secondary intention healing?

Primary intention has approximated wound edges (e.g., sutures) with minimal inflammation and proliferation; secondary intention occurs when edges can't approximate, healing via granulation, contraction, and epithelialization (e.g., infected open wounds).

What is the difference between partial and full thickness wounds?

Partial thickness involves the epidermis and none or part of the dermis; full thickness involves the epidermis and the entire dermis.

What is eschar?

Thick, adherent, leathery necrotic tissue that is black and hard when mature.

What is the difference between transudate, exudate, and purulent drainage?

Transudate is watery and clear, exudate is thicker and can be yellow, and purulent drainage contains pus.

What is epibole?

A rolled wound edge where the epithelium rolls under itself, preventing healing.

What's the difference between undermining, tunneling, and fistula?

Undermining is excavation beneath the skin surface; tunneling is a point of deep penetration (sinus tract); a fistula is a sinus tract that connects to an organ.

Why does aging increase dehiscence risk?

Aging causes delayed wound contraction, decreased tensile strength, reduced collagen cross-linking, and decreased epithelialization, increasing dehiscence risk 2 to 3 times in patients over 60.

What is indurated tissue?

hardened tissue, usually by extensive edema

What is macerated tissue?

usually wet, whitish, exposed to too much moisture

What is beefy tissue?

healthy, red granulation bed - what we want