Pigment in the skin is located in the basal layer of the epidermis, the deepest layer of the skin's outer layer:

Melanocytes, specialized cells that produce melanin, are found in the basal layer. Melanin is the pigment that gives skin its color and protects it from the sun. The amount of melanin in the skin, and the ratio of eumelanin (brown–black) to pheomelanin (yellow–red), determine skin pigmentation.

Melanin is stored in melanosomes, which accumulate in the basal layer. The melanin spreads to nearby skin cells, and is eventually removed when epidermal cells shed.
Sun exposure causes melanocytes to produce more melanin to protect the skin from UV rays, resulting in a suntan. Other factors that can cause hyperpigmentation include physical damage to the skin, such as inflammation, scarring, or some light or laser skin treatments.

They are both proteins found in connective tissues, but they have different functions and structures.

Elastin

A stretchy protein that gives tissues the ability to stretch and recoil. Elastin is found in tissues that need to be stretchy, like the lungs, bladder, and large blood vessels. It's also present in smaller amounts in the skin and ear cartilage.

Collagen

The most abundant protein in the body, providing structure, strength, and support. Collagen is the main fibrous protein in the body, and is present in connective, fibrous, and supportive tissues.
Elastin and collagen work together to keep skin firm and supple. Collagen provides structure, while elastin allows the skin to stretch and bounce back. As people age, collagen breaks down and elastin weakens, which can lead to wrinkles, fine lines, and leathery skin.

Neocollagenesis occurs in the human body as a natural component of wound re- pair. The formation of new collagen in scars results from an inflammatory response to injury. This inflammation produces increased fibroblast stimulation and collagen deposits.
Dermal neocollagenesis is typically the main reason of visible skin improvement after different aesthetic treatment such as laser.

Photodamaged skin, also known as sun-damaged or photoaged skin, is skin that has been permanently damaged by exposure to ultraviolet (UV) radiation from the sun or tanning beds. Photodamage can cause a number of changes to the skin, including:

• Wrinkles: Fine and coarse wrinkles can develop in the skin
• Pigmentation changes: Age spots, liver spots, freckles, and other pigmentation changes can occur
• Roughness: The skin may feel rough and uneven
• Broken capillaries: Also known as spider veins, these can appear around the nose and chest
• Redness and blotchiness: The skin may appear red and blotchy
• Loss of skin tone: The skin may lose elasticity and firmness

Photodamage can occur in areas of the body that are frequently exposed to the sun, such as the face, ears, arms, legs, and back of the hands.

Photodamage is different from chronological aging, which is caused by age and genetics. Photodamage is caused by external factors, and is therefore generally controllable

Dry skin is a skin type, while dehydrated skin is a condition that can affect anyone:

Dry skin

A skin type where the skin doesn't produce enough oil to stay moisturized. Dry skin can appear red and flaky, and may feel tight or itchy. Environmental factors like weather can make symptoms worse, especially in the winter. To treat dry skin, you can use oil-based products and moisturizers made for dry skin.

Dehydrated skin

A condition where the skin doesn't have enough water in the top layer. Dehydrated skin can appear dull and flat, and may feel tight, itchy, or uncomfortable. Dehydrated skin can be caused by external factors like weather, or lifestyle factors like not drinking enough water. To treat dehydrated skin, you can drink more water and use water-based products.

To test if your skin is dehydrated, you can pinch the skin on your cheek and see how quickly it returns to normal. If it takes a while to go back down, your skin is likely dehydrated.

Cortisol, a hormone released in response to stress, can worsen rosacea symptoms and trigger flare-ups:

Inflammation

Cortisol is inflammatory and can make it harder for the skin to heal. When cortisol levels are high, the skin's sebaceous glands produce more oil, which can lead to clogged pores, inflammation, and acne.

Skin barrier

High levels of cortisol can break down collagen and elastin, which can impair the skin's barrier function.

Stress cycle

Rosacea symptoms like swelling and burning can cause stress, which can lead to more pronounced breakouts.
Other factors that can contribute to rosacea include:

• Hormonal imbalances, such as those that occur around perimenopause
• Sun exposure
• Hot drinks
• Exercise

To reduce the impact of stress on rosacea, you can try: Maintaining a skin care regimen, Eating healthy meals, Drinking enough water, and Practicing stress management techniques

Stress and cortisol are closely related because cortisol is the primary hormone released by the body in response to stress: 

How stress triggers cortisol release

When you perceive a threat, the hypothalamus in your brain signals the pituitary gland to produce a hormone that tells the adrenal glands to release cortisol. 

How cortisol helps with the stress response

Cortisol helps your body respond to stress by: 

• Increasing blood sugar (glucose) for energy 
• Enhancing the brain's use of glucose 
• Increasing the availability of substances that repair tissues 
• Slowing nonessential or harmful functions 
• Changing immune system responses 
• Suppressing the digestive, reproductive, and growth systems 

How cortisol levels return to normal

In a healthy stress response, cortisol levels rise quickly and then return to normal after the stress has passed. However, chronic stress can cause cortisol levels to remain high even when there's no immediate stress. This is called dysregulated cortisol secretion and can take weeks to return to normal.

Cortisol and stress can affect the skin in many ways, including:

Inflammation: Stress triggers the release of cortisol, which can increase inflammation in the body. This inflammation can prevent acne from healing and can lead to flare-ups of skin conditions like eczema, rosacea, or psoriasis. 

Skin barrier function: Cortisol can impair the skin's barrier function, making it more susceptible to environmental stressors. 

Oil production and Acne: Cortisol can increase oil production, which can clog pores and lead to breakouts. 

Hair loss: Cortisol can trigger hair loss. 

Other symptoms: Stress can also lead to hives, itchiness, and impaired wound healing. 

Hormonal acne during pregnancy is caused by increased levels of hormones, such as progesterone, which stimulate the skin's oil glands. This leads to excess oil production, which can clog pores and cause pimples.

Other factors that can contribute to pregnancy acne include:

• Immune system changes: Pregnancy can alter the skin's balance, making it more prone to breakouts
• Stress: Stress can contribute to pregnancy acne
• Diet: Diet can contribute to pregnancy acne
• Genetics: Genetics can contribute to pregnancy acne
• Wearing a face mask: The friction from a face mask can irritate the skin, and the humidity trapped under the mask can trap oil and bacteria in pores
• Sun exposure: Too much sun can cause blotches during pregnancy

After pregnanc

After pregnancy, the body doesn't need as many of these hormones, so levels drop. This drop in hormones causes the body to produce more sebum, an oily substance that can clog pores and lead to pimples.

Breastfeeding

Breastfeeding increases hormone levels, so it can take longer for acne to go away for women who breastfeed

Skin pigmentation can be categorized in a number of ways, including:

Hyperpigmentation
An overproduction of melanin that results in dark spots or patches on the skin. This can be caused by sun exposure, inflammation, hormonal changes, or acne scars.

Hypopigmentation
A reduction or absence of melanin that results in lighter patches of skin. This can be caused by conditions such as vitiligo, albinism, or excessive UV exposure.

Epidermal pigmentation
Usually close to the surface of the skin and induced by excessive sun exposure. Examples include freckles, solar lentigines, and cafe-au-lait macules.

Dermal pigmentation
Typically found in the dermal layers. Examples include hori's macules and nevus of ota.

Mixed dermal/epidermal pigmentation
A mixed type that traverses through both the superficial and deeper layers of the skin. Melasma is an example of this.

Other types of pigmentation include:

Post inflammatory hyperpigmentation
Irregular, darkly pigmented macules or patches caused by trauma or inflammation

Solar lentigines
Well-circumscribed macules that are light yellow to dark brown in color and caused by acute or chronic ultraviolet exposure

Ephelides
Sharply defined macules that are red or tan to light brown in color and caused by sun exposure in susceptible individuals

Acne is a skin condition that can appear as pimples, whiteheads, and blackheads on the face, chest, and back. Dermatologists classify acne into four grades based on the type and severity of the lesions:

• Grade 1 (mild): Mostly whiteheads and blackheads, with some papules and pustules
• Grade 2 (moderate): More widespread whiteheads and blackheads, with many papules and pustules
• Grade 3 (moderately severe): Numerous papules and pustules, along with occasionally inflamed nodules
• Grade 4 (severe): Numerous large, painful and inflamed pustules and nodules

Acne can be caused by a number of factors, including:

• Hormone level changes during puberty, menstruation, or pregnancy
• Starting or stopping birth control pills
• Certain medicines, such as corticosteroids, lithium, and barbiturates
• Oil and grease from the scalp, mineral or cooking oil, and certain cosmetics
• Diet

The main difference between a papule and a pustule is that a papule is a solid, inflamed bump that doesn't contain pus, while a pustule is a papule that has become infected and filled with pus:

Papule
A small, solid, raised bump that's a symptom of inflammatory acne. Papules are usually caused by clogged pores from excess oil and skin cells, or by bacteria, stress, or a diet high in sugar.

Pustule
A papule that has become infected and filled with pus, usually appearing as a white or yellow-colored bump with a red ring around it. Pustules are also known as whiteheads. They can be caused by clogged hair follicles, hormonal imbalances, or allergic reactions.
Papules can develop into pustules when the body's immune system sends white blood cells to fight an infection. The white blood cells die and collect inside the papule, forming pus

Melasma is a common skin condition that causes brown or gray-brown patches or freckle-like spots on the face:
Melasma occurs when the skin's pigment-producing cells, called melanocytes, produce too much melanin. This can be caused by a number of factors, including:

Hormonal changes: Pregnancy and hormonal birth control pills are common triggers.

Sun exposure: Ultraviolet (UV) radiation from the sun, as well as visible light from screens and lights, can contribute to melasma.

Genetics: A genetic predisposition may play a role, and identical twins have been reported to develop melasma.

Melasma is also known as chloasma or the "mask of pregnancy". It's usually harmless and fades after a few months for some people. However, it can be difficult to treat and can cause significant emotional distress

Duration
Melasma is a chronic disorder, meaning it can last for three months or more. It can go away on its own, or it may be permanent.

Triggers
Melasma can be triggered by sunlight, birth control pills, and stress. It often returns when you spend time outdoors without protecting your skin from the sun.

Treatment
There is no one best treatment for melasma, but treatments can help fade the discoloration. Treatments include topical creams, chemical peels, laser treatments, microneedling, and microdermabrasion.

Cell turnover is the natural process of replacing old skin cells with new ones. Dead skin cells are shed from the skin's surface, exposing new cells that are produced in a deeper layer. This process keeps skin looking smooth, radiant, and youthful.

Factors that affect cell turnover
Age, sun exposure, skin disorders, stress, hormones, and professional procedures like chemical peels or laser resurfacing can all influence cell turnover.

Skin regeneration rate
The rate of skin regeneration peaks at night, between midnight and 6 AM.

Cycle time
The average person's cycle from the birth of a skin cell to its sloughing off at the skin's surface takes approximately 28 to 40 days.

Benefits of healthy cell turnover
A healthy skin cell turnover cycle can:

• Make skin appear shinier, smoother, and clear of imperfections
• Reduce the risk of developing acne
  

How to increase cell turnover
You can try:

• Using products with retinol, which can enhance skin cell turnover
• Using glycolic and lactic acids (AHAs) to break down and remove dead skin
• Using chemical peels or micro-dart patches

It's important to stop using active ingredients, like retinol, alpha hydroxy acids (AHAs), and beta hydroxy acids (BHAs), a week before a facial to avoid skin irritation and discomfort:

Skin sensitivity
These ingredients can make skin more sensitive, which can increase the risk of adverse reactions during a facial.

Inflammation
Stopping active ingredients can help prevent inflammation after your facial.

Skin renewal
If you're using active ingredients for the first time, your skin may go through a purge, which can cause flakiness or red bumps.

You should also avoid other harsh skincare products, like scrubs and exfoliators, in the week before your facial.

There are several types of retinol, including:

• Retinyl esters: A beginner over-the-counter (OTC) retinoid
• Retinol: An intermediate OTC retinoid
• Retinaldehyde: An advanced intermediate OTC retinoid
• Adapalene: An advanced OTC retinoid
• Tretinoin: An intermediate prescription retinoid

Other topical retinoids include: Alitretinoin (Panretin®) and Bexarotene (Targretin®).
The potency of a retinol product depends on the percentage of the ingredient. Non-prescription retinoids are usually safer for sensitive skin because they have gentler formulations.