Under normal conditions, melanosomes cover the upper part of the keratinocytes and protect them from genetic damage. One melanocyte supplies melanin to thirty-six keratinocytes according to signals from the keratinocytes. They also regulate melanin production and replication of melanocytes. People have different skin colors mainly because their melanocytes produce different amount and kinds of melanin. The genetic mechanism behind human skin color is mainly regulated by the enzyme tyrosinase, which creates the color of the skin, eyes, and hair shades. Differences in skin color are also attributed to differences in size and distribution of melanosomes in the skin. Melanocytes produce two types of melanin. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms. Most are derived from the amino acid tyrosine. Eumelanin is found in hair, areola, and skin, and the hair colors gray, black, blond, and brown. In humans, it is more abundant in people with dark skin. Pheomelanin, a pink to red hue is found in particularly large quantities in red hair, the lips, nipples, glans of the penis, and vagina.Both the amount and type of melanin produced is controlled by a number of genes that operate under incomplete dominance. One copy of each of the various genes is inherited from each parent. Each gene can come in several alleles, resulting in the great variety of human skin tones. Melanin controls the amount of ultraviolet (UV) radiation from the sun that penetrates the skin by absorption.
Brimonidine is a medication used to treat open-angle glaucoma, ocular hypertension, and rosacea. In rosacea it improves the redness. It is used as eye drops or applied to the skin.Common side effects when used in the eyes include itchiness, redness, and a dry mouth. Common side effects when used on the skin include redness, burning, and headaches. More significant side effects may include allergic reactions and low blood pressure. Use in pregnancy appears to be safe. When applied to the eye it works by decreasing the amount of aqueous humor made while increasing the amount that drains from the eye. When applied to the skin it works by causing blood vessels to contract.Brimonidine was patented in 1972 and came into medical use in 1996. It is available as a generic medication. In 2019, it was the 217th most commonly prescribed medication in the United States, with more than 2 million prescriptions. Medical uses
Brimonidine is indicated for the lowering of intraocular pressure in patients with open-angle glaucoma or ocular hypertension. It is also the active ingredient of brimonidine/timolol along with timolol maleate. A 2017 Cochrane review found insufficient evidence to determine if brimonidine slows optic nerve damage.In 2013, the FDA approved topical application of brimonidine 0.33% gel for persistent facial redness of rosacea. Mechanism of action
Brimonidine is an α2 adrenergic agonist.α2 agonists, through the activation of a G protein-coupled receptor, inhibit the activity of adenylate cyclase. This reduces cAMP and hence aqueous humour production by the ciliary body.