Module 5 Notes: Integumentary System

Overview of the Integumentary System (Module 5.1)

• Skin accounts for $10-15\%$ of total body weight; it is the largest organ in the body. It is more than an outer covering; a complex organ with many homeostasis-related functions. (Figure 5.1 referenced)
• The skin is called the cutaneous membrane and has two main components:
  • Epidermis – superficial layer of keratinized stratified squamous epithelium resting on a basement membrane
  • Dermis – deep to the epidermis and basement membrane; consists of loose connective tissue and dense irregular connective tissue
• The hypodermis (not part of skin) lies deep to the dermis and anchors skin to underlying structures; it is also called superficial fascia or subcutaneous fat and has an abundant blood supply.
• Accessory structures embedded in the cutaneous membrane include: sweat glands, sebaceous glands, hair, and nails.
• Skin contains sensory receptors and arrector pili muscles (small bands of smooth muscle associated with hair).
• The epidermis is avascular and relies on diffusion of oxygen and nutrients from blood vessels in the dermis; this is an example of the Gradients Core Principle and limits epidermal thickness. About $50\%$ of epidermal cells are too far from adequate blood supply to sustain life; the superficial layers consist largely of dead cells.
• The hypodermis anchors the skin to deeper structures (muscle and bone) and is rich in adipose tissue and loose connective tissue with a robust blood supply.

Skin Structure

• Accessory structures within the skin: sweat glands, sebaceous glands, hair, nails.
• The skin houses sensory receptors and arrector pili muscles that influence hair.
• Epidermis is avascular and depends on dermal diffusion for nutrients and oxygen.
• Hypodermis (superficial fascia) lies beneath the dermis; not part of skin but essential for anchoring and vascular supply.

The Epidermis

• The epidermis is the most superficial layer and is mainly composed of keratinocytes, which make up about $95\%$ of epidermal cells.
• Two features strengthen the epidermis against mechanical trauma:
  • Keratin manufacture (keratin is a tough fibrous protein)
  • Desmosomes link keratinocytes to one another, increasing cohesion and resilience (Structure-Function Core Principle)
• The epidermis is organized into five structurally distinct strata (layers): $5$ layers
  1) Stratum basale (stratum germinativum) – single layer of stem cells resting on the basement membrane; closest cells to the dermal blood supply; highly metabolically and mitotically active; involved in vitamin D synthesis and replacement of dead keratinocytes from more superficial layers
  2) Stratum spinosum – thickest layer; lies on the basale; still near blood supply; metabolically and mitotically active
  3) Stratum granulosum – 3–5 layers with prominent cytoplasmic granules filled with keratin bundles and lipid-based substances; lipids provide waterproofing; contributes to cell isolation and death in this layer and above
  4) Stratum lucidum – a narrow layer of clear, dead keratinocytes found only in thick skin
  5) Stratum corneum – outermost layer; several layers of dead flattened keratinocytes with thickened plasma membranes; filled with keratin bundles; cells are ultimately sloughed off as desmosomes loosen
• Thick skin vs thin skin:
  • Thick skin (~paper towel thickness) contains all five epidermal layers and a very thick stratum corneum; lacks hair follicles but contains many sweat glands
  • Thin skin (~printer paper thickness) has four layers (lucidum missing) and contains numerous hairs, sweat glands, and sebaceous glands
• Callus – extra layers of stratum corneum formed in response to repetitive pressure
• Keratinocytes are the main cell type; about $95\%$ of epidermal cells are keratinocytes

Other Cells of the Epidermis

• Dendritic (Langerhans) cells – located in stratum spinosum; phagocytes of the immune system that protect skin and deeper tissues from pathogens
• Merkel cells – oval cells in the stratum basale; associated with small neurons in the dermis; detect light touch and shapes/textures; concentrated in highly tactile areas (fingertips, lips, base of hairs)
• Melanocytes – located in the stratum basale; produce melanin, the pigment ranging in skin color from orange-red to brown-black

Thick and Thin Skin

• Epidermal thickness varies to match function (Structure-Function Core Principle)
• Thick skin is found on palms and soles; high mechanical stress areas; contains all five epidermal layers and many sweat glands; no hair follicles
• Thin skin covers most of the body; contains hair follicles, sweat glands, sebaceous glands; lacks stratum lucidum
• Callus formation occurs in both thick and thin skin in response to repetitive pressure

The Dermis

• The dermis is a highly vascular layer deep to the epidermis; it provides blood supply to the epidermis, contains sensory receptors, and anchors epidermis in place
• It is composed of two distinct connective tissue layers: the papillary layer and the reticular layer
• Papillary layer – the thinner, superficial layer made of loose connective tissue; contains special collagen fibers at the dermal-epidermal junction that anchor the epidermis to the dermis
• Dermal papillae – tiny projections at the surface of the papillary layer that contact the epidermis; contain capillary loops for diffusion of oxygen and nutrients into the avascular epidermis; Meissner (tactile) corpuscles are also found here and respond to light touch
• Reticular layer – deeper, thicker layer of dense irregular connective tissue; contains irregularly arranged collagen bundles that strengthen the dermis and elastic fibers that allow it to recoil after stretching
• The reticular layer is rich in proteoglycans that draw water into the ground substance, helping keep the skin firm and hydrated
• Lamellated (Pacinian) corpuscles – sensory receptors in the reticular layer that respond mainly to pressure and vibration
• The dermis houses blood vessels, sweat glands, hair follicles, sebaceous glands, and adipose tissue
• Fig. 5.5 illustrates the structure of the dermis highlighting the papillary and reticular layers

Skin Markings

• Skin markings are small visible lines in the epidermis caused by interactions between the dermis and epidermis; most visible in thick skin on palmar and plantar surfaces
• Dermal ridges are prominent in areas with thick collagen bundles; they indent the overlying epidermis to create epidermal ridges, which form characteristic fingerprint patterns (loops, arches, whorls); sweat pores open along ridges, leaving fingerprints
• Epidermal ridges are visible in multiple imaging methods (LM/SEM) and are genetically determined and unique to each person
• Reticular layer also creates tension lines (cleavage lines) and flexure lines; gaps between collagen bundles create indentations in the epidermis
• Cleavage lines and flexure lines influence surgical incisions: incisions along tension lines heal faster with less scarring; perpendicular incisions to tension lines heal more slowly and may scar more
• Figure 5.7 demonstrates the surgical relevance of tension lines

Accessory Structures of the Integument: Hair, Nails, and Glands

Hair (Pili)

• Hair and nails are derived from epithelial cells; hair is distributed over most of the body except for thick skin areas, lips, and parts of external genitalia
• Structure of a hair includes the shaft, root, and follicle; the hair follicle includes epithelial root sheath and dermal root sheath; the arrector pili muscle attaches to the hair
• The hair bulb, hair matrix, and hair papilla are involved in hair growth; melanocytes contribute to hair color
• Hair shaft structure includes cuticle, cortex, and medulla

Nails

• Nails are hard accessory structures at the ends of digits; composed of stratified squamous epithelium filled with hard keratin
• Nail plate is the visible part; rests on the nail bed
• Nail body is the visible portion of the nail plate; nail root lies under the skin; the nail matrix contains actively dividing cells

Glands

• Skin contains two basic gland types, both derived from epidermis but located deeper in the dermis:
  • Sweat (sudoriferous) glands that produce sweat
  • Sebaceous glands that produce oily sebum

Skin Cancer

• Cancer is a common disease caused by mutations in DNA that disrupt cell cycle control; unchecked cell division forms tumors that can metastasize via blood or lymphatic vessels, altering the function of invaded organs
• Three skin cancers are linked to UV radiation exposure; other risk factors include exposure to carcinogens and various forms of radiation
• Basal cell carcinoma (BCC) – most common skin cancer; arises from keratinocytes in the stratum basale; usually UV-exposed skin; presents as a nodule with a central crater; rarely metastasizes; typically treated with surgical removal
• Squamous cell carcinoma (SCC) – second most common; cancer of keratinocytes in the stratum spinosum; presents as scaly plaques that may ulcerate or bleed; higher metastasis risk than BCC; surgical removal remains effective
• Malignant melanoma – cancer of melanocytes; early detection is critical due to high metastasis tendency; cancerous melanocytes extend into the dermis and access dermal blood vessels, enabling spread via the bloodstream; treatment includes surgical removal and possibly radiation or chemotherapy; prognosis depends on tumor size, depth in the dermis, and metastasis status
• ABCDE rule for distinguishing malignant melanoma from benign moles:
  • A: Asymmetry of shape (two sides do not match)
  • B: Border irregularity
  • C: Color, usually blue-black or multiple colors
  • D: Diameter typically larger than $6\text{ mm}$
  • E: Evolving (changing) shape and size
• Figure 5.14c summarizes the three main forms of skin cancer