Integumentary System (Skin)

Layers of the Epidermis

The epidermis is the outermost layer of the skin, composed of several distinct strata, each with unique cellular characteristics and functions.

• Stratum corneum: Thick, anucleate, flaking "plates"; serves as the barrier layer of the skin.

• Stratum lucidum: Present only in thick skin; clear layer.

• Stratum granulosum: Dark "grainy" band; contains keratohyalin granules responsible for waterproofing.

• Stratum spinosum: Characterized by "spiny" desmosomes between polygonal cells.

• Stratum basale: Single row of cells on basement membrane; contains mitotic stem cells (germ layer).

Dermis and Associated Structures

The dermis lies beneath the epidermis and provides structural support, nourishment, and houses various accessory structures.

• Dermis: Composed of dense irregular connective tissue (CT) with swirled collagen fibers; contains blood vessels and nerves just beneath the epidermis.

• Hypodermis: Contains adipose tissue (not true skin proper); stores fat and provides insulation.

• Accessory structures: Includes hair follicles, sebaceous glands (oil glands), sweat glands (ducts opening into follicles), and merocrine glands (sweat glands).

Tissue Types to Identify

• Epidermis lining: Keratinated stratified squamous epithelium (not cuboidal, squamous simple, or transitional).

• Dermis composition: Dense irregular CT (not epithelium or cartilage).

Functions & Common Decoys

• Outermost barrier: Stratum corneum (not basale).

• Waterproofing: Localized to stratum granulosum (keratohyalin granules).

5-Second Identification Drills

• Thick, flaky, anucleate layer: Corneum

• Dark, grainy band just deep to corneum: Granulosum

• Swirled collagen with vessels/nerve profiles: Dermis (dense irregular CT)

Histology — Epithelia & Key Connective Tissue

Types of Epithelia

Epithelial tissues are classified by cell shape and layering, each serving specific functions in various organs.

• Simple squamous: Flat "tile" cells; function in diffusion; found in alveoli of lungs.

• Simple cuboidal: Cube cells with round central nuclei; found in ducts/tubules; function in secretion/absorption.

• Simple columnar: Tall cells; may contain goblet cells; found in GI tract.

• Stratified squamous (keratinized): Many layers; flat apical cells; provides protection (epidermis).

• Transitional: Dome-topped surface cells; stretches; found in urinary bladder.

Connective Tissue Focus Points

• Dermis: Dense irregular CT (multidirectional collagen).

Common Traps

• Alveoli are simple squamous, not cuboidal.

• Outer skin surface is stratified squamous (keratinized), not transitional.

Skeleton Divisions & Landmarking

Axial vs. Appendicular Skeleton

The skeleton is divided into axial and appendicular components, each containing specific bones and structures.

• Axial skeleton: Skull, vertebral column, sternum, and ribs. (Sternum is axial.)

• Appendicular skeleton: Scapula, clavicle, upper and lower limbs (humerus is upper limb).

Skull — Bones & High-Yield Landmarks

• Cheekbone: Zygomatic (do not confuse with maxilla/temporal/mandible).

• Zygomatic arch: Formed by zygomatic and temporal bones.

• Forehead bone: Frontal.

• Sella turcica (pituitary seat): Located in sphenoid (not occipital).

• Foramen magnum (spinal cord passage): Located in occipital.

• Upper jaw: Maxilla; Lower jaw: Mandible.

Vertebral Column & Thoracic Cage

• Sacrum: Composed of 5 fused vertebrae.

• Spinous process: Palpable posterior spike; name this projection.

• Vertebral foramen: Houses spinal cord; intervertebral foramen (between vertebrae) is the nerve exit.

• Thoracic cage: Sternum + ribs are axial (place correctly).

Limbs & Key Articulations

• Knee joint partner: Femur + tibia (not fibula).

• Tarsal bearing tibial weight: Talus (ankle).

• Phalanges (fingers/toes): Classified as long bones.

Rapid "No-Surprises" Checklist

• Name the epidermal layer that divides: Stratum basale (site of mitosis).

• Point to the waterproofing layer: Stratum granulosum (keratohyalin).

• Call the outer barrier: Stratum corneum.

• Identify dermis tissue: Dense irregular CT.

• Skin functions: Includes sensory and vitamin D production (not blood production).

Master-Level ID Tips (What Students Actually See)

On Epidermis Slides

• Look for anucleate plates up top (corneum), a thin dark belt of granules (granulosum), then the basal germ layer at the base.

On Dermis Views

• If you see thick, swirled collagen with vessels and nerve profiles, say dense irregular CT—and note this sits directly below the epidermis.

On Respiratory Histology

• Alveoli in lung tissue are simple squamous. Anything cuboidal/columnar here is a decoy.

On Skull Models

• Zygomatic arch = zygomatic + temporal.

• Sella turcica = sphenoid.

• Foramen magnum = occipital.

On Vertebrae

• Posterior spike you can palpate? Spinous process. Between-vertebra hole for nerves? Intervertebral foramen.

Summary Table: Key Skin Layers and Functions

Key Equations and Terms

• Keratinized stratified squamous epithelium: Multiple layers of flat cells with surface keratinization.

• Dense irregular connective tissue: Collagen fibers arranged in multiple directions for strength.

• Equation for Diffusion (Fick's Law):

Where is the diffusion flux, is the diffusion coefficient, and is the concentration gradient.

Additional info: The study guide covers foundational identification and function of skin layers, key histological tissue types, and major skeletal landmarks, which are essential for introductory Anatomy & Physiology courses.