5-26-26 Skin

Integumentary System

• The integumentary system is the first system of the body being studied.

• It is identified as the largest organ on the human body.

• The study of the skin involves looking at layers under the microscope and understanding the specific anatomy of the scan.

The Three Layers of the Skin

• The skin is composed of $3$ different layers:     * Epidermis: This is the top layer of the skin.     * Dermis: This is the middle layer, located beneath the epidermis.     * Hypodermis: This is the bottom layer, also known as the subcutaneous layer.

Anatomy and Strata of the Epidermis

• The epidermis is the superficial layer that sloughs off every day.

• Humans shed up to $500,000,000$ cells a day. These cells are left on desks, in clothes, and in mattresses.

• The epidermis is avascular, meaning it contains no blood vessels. A true paper cut that does not bleed is only shallow enough to affect the epidermis.

• Hair appears to come from the epidermis, but its root and follicle are located deeper in the skin.

• Sweat pores are located on the epidermis.

• The epidermis consists of $5$ distinct layers (strata), which must be known in the correct order from superficial to deep:     1. Stratum Corneum: The first, most superficial layer consists of dead cells that slough off. This is the layer removed during exfoliation.     2. Stratum Lucidum: The second layer.     3. Stratum Granulosum: The third layer ($g-r-a-n-u-l-o-s-u-m$).     4. Stratum Spinosum: The fourth layer.     5. Stratum Basale: The fifth and deepest layer, known as the basement layer ($b-a-s-a-l-e$).

• Cell Turnover Process: Skin turns over from the stratum basale. Cells from the stratum basale turn into the stratum spinosum, then into the stratum granulosum, then into the stratum lucidum, and finally into the stratum corneum before being shed.

• Memory Acronym for Epidermal Layers: "Come Let's Get Sunburned" (Corneum, Lucidum, Granulosum, Spinosum, Basale).

• Skin Pigmentation: Melanocytes, which give skin its pigment (melanin), are located in the stratum basale (basement layer).

• Histology: The epidermis is composed of stratified squamous epithelium.     * Stratified: Means multiple layers.     * Squamous: Means flat, scale-like cells (comparable to the shape of a fried egg or "squish cells").

The Dermis: Structure and Functions

• The dermis is located beneath the epidermis and contains complex structures including blood vessels, hair follicles, and glands.

• Tissue Type: The dermis is primarily composed of dense irregular connective tissue.

• Dermal Layers:     * Papillary Layer: The upper part of the dermis that attaches to the epidermis.     * Reticular Layer: The deeper part of the dermis, which makes up about half of the total dermal thickness.

• Dermal Papilla: These are egg-crate-shaped structures that anchor the epidermis to the dermis. They contain capillaries (the smallest extensions of the arterial network).

• Blood Supply: Unlike the epidermis, the dermis is vascular. If a cut bleeds, it has reached the dermis where blood capillaries are located.

• Plexus Systems:     * Subpapillary Plexus: A network of nerves and blood vessels located near the papillary layer.     * Cutaneous Plexus: A deeper network representing an anastomosis where arteries (oxygenated, red) cross over to become veins (deoxygenated, blue or purple).

• Nerves, Arteries, and Veins (VAN): These three structures often travel together in a "VAN" bundle.

The Hypodermis (Subcutaneous Layer)

• The hypodermis is located below the dermis.

• Function: It provides insulation for the body.

• Composition: It consists of subcutaneous fat (adipose tissue). Adipose tissue is typically represented in yellow in anatomical models and appears yellow in the body.

• Classification: Technically, the hypodermis is not considered part of the skin; the dermis and epidermis comprise the skin itself.

• Clinical Note: The "hypodermic needle" gets its name because it must pass through or to the hypodermis layer for injections. Cutting down to the "whitening" refers to reaching this layer where blood vessels are not as immediately densely packed as in the dermis, meaning it may take a minute for blood to pool.

• Components: Contains cutaneous blood vessels (veins and arteries), adipose tissue, and motor nerve fibers.

Glandular Structures of the Skin

• Sebaceous Glands:     * Surrounding the hair follicle.     * Produces Sebum ($s-e-b-u-m$), the body's natural natural oil or lubricant.     * Keeps skin lubricated; without it, skin becomes dry, cracks, and may leak.

• Sweat Glands:     * Eccrine (Merocrine) Glands: The most common type found all over the skin. These are used when the body is too warm and needs to cool off through perspiration.     * Apocrine Glands (Pseudoriferous): Usually attached to a hair follicle. These produce pheromones and are found in specific areas like the armpits and groin. These secretions have different properties that create attraction or hormonal signals.

Nerve Receptors and Sensation

• Sensory Information: Sensory nerve fibers carry information in (e.g., touching a hot stove), while motor nerve fibers send responses out (e.g., moving the hand).

• Tactile Corpuscle (Meissner's): A touch receptor for light touch.

• Pacinian Corpuscle (Lamellar/Romella): A pressure receptor for deeper pressure sensation.

• Free Nerve Endings: Located near the surface to pick up various sensations.

• Hair Follicle Receptors: Sensory receptors attached to the hair follicle that detect when a hair is moved or pulled.

Detailed Anatomy of the Hair and Follicle

• Hair Shaft: The part of the hair that sticks out above the epidermis.

• Hair Root: The part of the hair located below the surface; if you tweeze a hair, you pull it from the root.

• Hair Bulb: The base of the hair follicle.

• Hair Papilla: Where blood vessels (veins and arteries) connect to the hair to provide essential nutrients and nourishment from the blood.

• Arrector Pili (Pillow Erector) Muscle: The muscle attached to the hair follicle that allows hair to stand up. When the body is cold, this muscle contracts to create a "heating blanket" of hair to warm the body.

• Follicle Layers:     * External root sheath.     * Internal root sheath.     * Henle layer (on the outskirts of the external root sheath).     * Huxley layer (transcript refers to a layer right across the HEMI layer).     * Hair cuticle, cortex, and medulla.

Histology and Cell Types

• Keratinocytes: Dead keratinocytes make up the stratum corneum.

• Langerhans Cell: Found in the stratum spinosum.

• Lamellar Granules: Make up the stratum granulosum.

• Melanocytes: Located in the stratum basale to provide pigment.

• Adipose Tissue: Found in the hypodermis.

• Dense Irregular Connective Tissue: Found in the dermis.

• Stratified Squamous Epithelium: Found in the epidermis.

Questions & Discussion

• Question: What is the specific acronym for the epidermis layers?     * Response: "Come Let's Get Sunburned" (Corneum, Lucidum, Granulosum, Spinosum, Basale). You cannot write the acronym on a quiz, but you can write it on the side as a tool to remember the correct terms.

• Question: Can you repeat the spelling of the second layer of the epidermis?     * Response: It is Lucidum, $l-u-c-i-d-u-m$.

• Question: What kind of injury is common for the epidermis?     * Response: A sunburn (first-degree burn).

• Question: What determines the degree of a burn?     * Response: Degrees refer to how many layers the burn penetrates (epidermis, dermis, or hypodermis).

• Question: What is the difference between specific sweat glands?     * Response: Eccrine (Merocrine) is for regular cooling; Apocrine is for pheromones and is attached to hair.

• Question: Discussion on Career Paths (Sonography vs. Nursing):     * One student is pursuing Nursing to be a pediatrician and work at a cancer center hospital to make an impact on kids.     * Another student is pursuing Sonography, specifically OBGYN Sonography, to scan pregnant women.     * The discussion noted that sonography can cause carpal tunnel or repetitive stress injuries in the wrist due to the scanning angles while patients are laying down.     * The student noted that while they perform the scan, the doctor is the one who delivers specific medical news to the patient.