lecture 3

The skin is composed of three main layers, each with distinct structures and functions.

Type of tissue: Stratified squamous epithelial tissue (keratinized)
Functions:
- Protection
- Water resistance
- Barrier to pathogens
Major cells:
1. Keratinocytes: Produce keratin for strength and protection.
2. Melanocytes: Produce melanin, which contributes to skin pigmentation.
3. Langerhans cells: Play a role in immune defense.
4. Merkel cells: Function as sensory receptors for touch.
Layers of the epidermis (superficial → deep):
1. Stratum corneum: Composed of dead, keratinized cells that serve as a protective barrier.
2. Stratum lucidum: Present only in thick skin (e.g., palms and soles); described as the “clear layer.”
3. Stratum granulosum: Site where keratin formation begins; known as the “grainy layer.”
4. Stratum spinosum: Provides strength and flexibility; referred to as the “spiny layer.”
5. Stratum basale: Involves cell division; contains melanocytes and basal cells.
- Key point: New skin cells are formed in the stratum basale and migrate upward, undergoing a process of dying as they reach the surface.

Type of tissue: Connective tissue (composed of collagen and elastic fibers)
Functions:
- Provides strength and flexibility.
- Nourishes the epidermis.
Two components of the dermis:
- Papillary layer: Loose connective tissue containing capillaries and sensory neurons.
- Reticular layer: Dense irregular connective tissue that houses hair follicles, sweat glands, and nerves.

Type of tissue: Loose connective tissue and adipose (fat) tissue.
Functions:
- Anchors skin to underlying organs.
- Cushions and insulates tissue.
- Acts as an energy storage site.
Note: The hypodermis is technically not part of the skin but works closely with it.

Function	Description	Example
Protection	Serves as a barrier against pathogens, UV rays, and dehydration.	Keratin, sebum, and melanin protect cells.
Sensation	Detects touch, pain, pressure, and temperature.	Receptors located within dermis and epidermis.
Thermoregulation	Maintains body temperature.	Sweating (cooling effect), dilation and constriction of blood vessels.
Excretion	Removes waste products through sweat.	Excreted materials include salts, water, and small amounts of urea.
Vitamin D synthesis	UV light converts precursors present in the skin into vitamin D.	Essential for calcium absorption.
Storage	Stores elements such as fat for cellular energy, water, and insulation.	Hypodermal adipose tissue.
Immunity	Immune cells in the skin provide protection against microbial pathogens.	Langerhans cells and antimicrobial secretion.

Summary: The skin acts as the body's primary barrier and plays a crucial role in maintaining homeostasis through various regulatory and protective functions.

What is MC1R?
- MC1R (Melanocortin 1 Receptor): A gene responsible for regulating the type and amount of melanin produced by melanocytes.
Types of melanin:
- Eumelanin: Brown or black pigment that provides a high level of UV protection.
- Pheomelanin: Red or yellow pigment that offers less UV protection.
Reasons for the prevalence of mutations in light-pigmented populations:
- Dark-skinned populations (typically located near the equator):
- Experience high levels of UV radiation, creating a strong selective pressure to maintain functional MC1R for eumelanin production (which protects against UV damage).
- Mutations that reduce eumelanin production are detrimental and are thus selected against.
- Light-skinned populations (generally situated further from the equator):
- Encounter lower levels of UV exposure, leading to a diminished necessity for high melanin protection.
- Beneficial mutations in MC1R accumulated as they had neutral effects or advantages, allowing more UV absorption which is favorable for vitamin D production.
- Outcome over time:
- Genetic drift and adaptation have resulted in increased variation and a higher incidence of loss-of-function MC1R mutations in lighter-skinned populations.
Key takeaway: MC1R mutations exhibit adaptive significance in low-UV environments, facilitating vitamin D synthesis despite lower UV intensity exposures.

Aspect	High Melanin (Dark Skin)	Low Melanin (Light Skin)
UV Protection	Excellent protection from UV radiation; lower risk of DNA damage.	Less protection resulting in higher risks of sunburn and DNA mutations.
Vitamin D Synthesis	Reduced production due to UV blocking.	Increased production; more efficient at synthesizing vitamin D.
Risk of Skin Cancer	Lower risk of skin cancer.	Higher risk of skin cancer.
Folate Preservation	Provides better protection against folate destruction by UV radiation.	Higher UV exposure can lead to decreased folate levels.
Evolutionary Fitness (by region)	Advantageous in equatorial regions where UV radiation is high.	Advantageous in northern latitudes where UV levels are low.

Summary of Trade-Offs:
- High melanin: Offers protection for DNA and folate levels but limits vitamin D synthesis.
- Low melanin: Promotes enhanced vitamin D production but increases the risk of DNA damage due to UV exposure.
Conclusion: Natural selection balances UV exposure, melanin levels, and vitamin D needs based on geographic location throughout human evolution.

Skin layers: epidermis → dermis → hypodermis.
Integumentary functions: protection, sensation, thermoregulation, vitamin D synthesis.
MC1R mutations: More common in light-skinned populations due to reduced UV pressure.
Melanin trade-off: Higher melanin provides protection; lower melanin enhances vitamin D production—both are tied to human adaptation and homeostasis.