Skin, Hair, Glands, and Thermoregulation — Comprehensive Study Notes

Quick anatomical review: the epidermis and dermis (very brief recap)

• Epidermis is composed of stratified squamous epithelium.

• Layers (from basal to surface):

  • Stratum basale (basal layer): site of cell division; contains stem-like cells.

  • Stratum spinosum: metabolically active; keratin production occurs here.

  • Stratum granulosum: cells begin to die; organelles shrink; granules form.

  • Stratum lucidum: clear layer (present in thick skin).

  • Stratum corneum: outermost layer; dead keratin-filled cells.

• Melanocytes and pigmentation:

  • Melanocytes produce melanin and transfer pigment to neighboring cells; pigmentation influences skin tone.

  • Pigment types:

  • Eumelanin (darker brown/black tones)

  • Pheomelanin (reddish/yellow tones)

• Intercellular connections:

  • Desmosomes connect epidermal cells; gaps between cells are maintained by robust adhesion.

  • Tight junctions would prevent passage between cells; here, desmosomes predominate at cell–cell interfaces.

• Dermis structure:

  • Papillary layer: mostly connective tissue; dermal papillae increase surface area for diffusion.

  • Reticular layer: irregular dense connective tissue; provides strength.

• Hypodermis (subcutaneous): primarily adipose tissue; site of subcutaneous injections.

• Hair follicle context: hair is an accessory organ that is continuous with the epidermis; hair shaft grows from the follicle.

Hair and pigmentation details

• Hair shared features with stratum corneum: dead cells and keratin; hair is somewhat waterproof due to keratin and oils.

• Hair color is determined by pigment granules produced by melanocytes in the follicle; colors vary with the balance of eumelanin and pheomelanin.

• Erector pili muscle: smooth muscle attached to each hair follicle; when contracted, hair stands on end (goosebumps).

  • This can provide insulation in some contexts but humans have limited hair to trap significant air.

• Areas with no hair (thick skin): palms and soles lack hair and have a thicker stratum corneum and a prominent stratum lucidum.

• Functions of hair: protection (e.g., nose hairs, eyelashes) and insulation to some extent.

Hair loss disorders (examples given in lecture)

• Androgenic alopecia (male-pattern baldness): associated with androgens (e.g., testosterone); more common in males; prevalence cited: 35{,}000{,}000 men and 20{,}000{,}000 women (approximate figures discussed).

• Alopecia areata: autoimmune condition; antibodies attack hair follicles; affected individuals exist in population estimates (e.g., 2{,}500{,}000 as mentioned in the transcript).

• Celebrity examples referenced in class:

  • John Travolta cited for androgenic alopecia.

  • A garbled reference to another celebrity (transcript mentions “Jacob can get Smith” which appears to be a misstatement).

Sebaceous glands and sebum

• Sebaceous glands are associated with hair follicles.

• Secretion type: holocrine secretion; whole cells disintegrate to release sebum.

• Sebum composition and function:

  • Oily substance that keeps hair soft and pliable and helps with movement.

  • Over-washing (e.g., daily shampooing) can remove too much sebum, leading to drier, more brittle hair.

• Visual cue: sebaceous glands appear as bulbous pink structures adjacent to hair follicles in diagrams.

Pseudoriferous (sweat) glands: eccrine vs apocrine

• Two main types:

  • Eccrine (merocrine) sweat glands: most common; duct opens onto skin surface; secretion mode is merocrine (secrete product only, no cytoplasm loss).

  • Primary role: thermoregulation via cooling by evaporation of sweat on the skin surface.

  • Apocrine sweat glands: associated with hair follicles (e.g., in axillary and pubic regions); activated at puberty; secretion involves loss of part of the cytoplasm (apocrine secretion).

  • Secretion is often odoriferous due to bacterial breakdown of the secretions and is involved in pheromone-like signaling in some contexts.

• Duct and structural notes:

  • Both glands originate as glandular epithelium embedded in the dermis.

  • The duct of merocrine/eccrine glands opens directly to the surface; apocrine glands open into hair follicle channel.

  • Ducts of apocrine glands are held together by tight junctions to prevent leakage along the duct.

• Sebaceous association with holocrine secretion is separate from sweat glands.

In-depth look at sebaceous/apocrine/eccrine relationships

• Sebaceous gland (associated with hair follicle): holocrine secretion → sebum → coats hair shaft; keeps hair soft and pliable.

• Eccrine sweat glands (merocrine secretion): thermoregulation via evaporative cooling; ducts open to skin surface; gland cells connected by tight junctions to prevent leakage of sweat into dermis.

• Apocrine sweat glands: associated with hair follicles; active at puberty; produce odor-containing sweat; ducts open into hair follicle canal; secretion involves loss of apical cytoplasm.

• Key terms to distinguish on exams: identify secretion mode (merocrine/eccrine vs apocrine vs holocrine) and duct/opening location (skin surface vs hair follicle).

Temperature regulation and homeostasis (skin’s role)

• Physical means of heat transfer (thermoregulation):

  • Radiation: heat transfer without direct contact (heat radiates from the body to surroundings).

  • Conduction: requires contact (e.g., sitting on a chair that absorbs heat from your body).

  • Convection: requires air movement (wind/breeze carries heat away from the body).

  • Evaporation: evaporation of sweat; cooling only happens when sweat transitions from liquid to gas, which requires energy absorption.

• Physiological means to regulate heat:

  • Altering blood flow in dermal vessels (vasodilation to lose heat; vasoconstriction to conserve heat).

  • Skeletal muscle activity (shivering) to generate heat.

  • Sweat gland activity and hair movement (goosebumps) to modulate heat loss and insulation.

• Negative feedback loop for body temperature:

  • Stimulus: body temperature deviates from set point.

  • Sensor/Receptor: thermoreceptors detect change.

  • Control center: hypothalamus compares to set point (usually 37^ ext{°C}).

  • Effectors: cutaneous blood vessels (vasodilation/vasoconstriction) and sweat glands; skeletal muscles for shivering when needed.

  • Response: return toward set point; this is negative feedback (opposes the deviation).

• Summary of effectors and responses:

  • If temperature rises: vasodilation increases skin blood flow; more heat radiates; sweat produced and evaporates for cooling.

  • If temperature falls: vasoconstriction reduces heat loss; sweating is inhibited; shivering generates heat.

Hyperthermia, heat-related illnesses, and hypothermia

• Hyperthermia states:

  • Heat exhaustion: sweating profusely; nausea or mild confusion; management includes seeking shade and hydrating.

  • Heat stroke: sweating may stop; requires urgent medical attention as body’s cooling mechanisms fail; hospitalization often needed.

• Cellular membrane stability at high temperatures:

  • Elevated temperatures increase membrane fluidity; potential gaps can allow harmful entities (e.g., bacteria) to cross membranes, increasing risk of sepsis.

• Hypothermia:

  • Prolonged exposure to cold; shivering may stop as body loses the ability to regulate; cognitive impairment may occur, leading to poor decision making (e.g., removing clothes when already cold).

  • Diffusion slows with low temperatures because molecular motion decreases; diffusion of oxygen and CO2 across membranes can be hindered, affecting cellular respiration.

• Note on diffusion and temperature:

  • Diffusion rate decreases as temperature decreases; at very low temperatures, poor diffusion impairs nutrient/waste exchange.

• Population-level caveat mentioned in class:

  • The lecture ties these concepts to everyday skin function and homeostasis; ensure you understand how the skin contributes to maintaining core temperature across various conditions.

Quick connections to foundational principles

• Negative feedback in physiology (homeostasis): body temperature is a classic example.

• Structure–function relationships in skin:

  • Epidermis provides barrier; dermis provides support and houses glands/follicles; hypodermis stores energy and provides insulation.

• Diffusion and osmosis concepts tied to epidermal/dermal function (e.g., keratinization, melanin transport, fluid balance).

• Microbiology and host defense: hyperthermia can influence membrane integrity and susceptibility to infection; sepsis risk if membranes become compromised.

Formulaic references and numbers to remember

• Exam format: 80 points of multiple choice, 20 points of short answer.

• Population examples mentioned: 35{,}000{,}000 men and 20{,}000{,}000 women with certain hair loss conditions.

• Set point for healthy human body temperature: 37^\circ ext{C} (commonly used in class to illustrate negative feedback).

• Temperature-related physics: no explicit equations were given, but the concepts include rates and balances (radiation, conduction, convection, evaporation) that relate to energy transfer and heat loss.

Key terms to study and quick definitions

• Epidermis, dermis, hypodermis

• Stratum basale, spinosum, granulosum, lucidum, corneum

• Desmosomes vs tight junctions

• Melanin, eumelanin, pheomelanin

• Melanocytes

• Hair follicle, hair shaft, arrector pili muscle

• Sebaceous gland, sebum, holocrine secretion

• Eccrine (merocrine) sweat gland, apocrine sweat gland, apocrine secretion, duct opening locations

• Thermoregulation, vasodilation, vasoconstriction, shivering

• Negative feedback, set point, hypothalamus

• Hyperthermia (heat exhaustion, heat stroke), hypothermia, diffusion changes with temperature

Quick study tips drawn from the lecture

• When studying, create connections between anatomy (where glands secrete) and physiology (what heat transfer method is used for cooling).

• Be comfortable distinguishing secretion styles and their functional outcomes (merocrine/eccrine vs apocrine vs holocrine).

• Practice explaining why hair and sebaceous secretions matter for skin/hair health and appearance.

• Memorize key numeric references (exam format, population numbers) as they were highlighted in the lecture.

• Understand the practical implications of temperature regulation for real-world health (recognizing heat illness risk and first-aid steps).