Amelanism (also known as amelanosis) is a pigmentation abnormality characterized by the lack of

pigments A pigment is a powder used to add or alter color or change visual appearance. Pigments are completely or nearly solubility, insoluble and reactivity (chemistry), chemically unreactive in water or another medium; in contrast, dyes are colored sub ...

called

melanin Melanin (; ) is a family of biomolecules organized as oligomers or polymers, which among other functions provide the pigments of many organisms. Melanin pigments are produced in a specialized group of cells known as melanocytes. There are ...

s, commonly associated with a genetic loss of

tyrosinase Tyrosinase is an oxidase that is the rate-limiting enzyme for controlling the production of melanin. The enzyme is mainly involved in two distinct reactions of melanin synthesis otherwise known as the Raper–Mason pathway. Firstly, the hydroxy ...

function. Amelanism can affect

fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...

amphibian Amphibians are ectothermic, anamniote, anamniotic, tetrapod, four-limbed vertebrate animals that constitute the class (biology), class Amphibia. In its broadest sense, it is a paraphyletic group encompassing all Tetrapod, tetrapods, but excl ...

reptile Reptiles, as commonly defined, are a group of tetrapods with an ectothermic metabolism and Amniotic egg, amniotic development. Living traditional reptiles comprise four Order (biology), orders: Testudines, Crocodilia, Squamata, and Rhynchocepha ...

bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...

s, and

mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...

s including humans. The appearance of an amelanistic animal depends on the remaining non-melanin pigments. The opposite of amelanism is

melanism Melanism is the congenital excess of melanin in an organism resulting in dark pigment. Pseudomelanism, also called abundism, is another variant of pigmentation, identifiable by dark spots or enlarged stripes, which cover a large part of the bod ...

, a higher percentage of melanin. A similar condition,

albinism Albinism is the congenital absence of melanin in an animal or plant resulting in white hair, feathers, scales and skin and reddish pink or blue eyes. Individuals with the condition are referred to as albinos. Varied use and interpretation of ...

, is a hereditary condition characterised in animals by the absence of pigment in the eyes, skin, hair, scales, feathers or cuticle. This results in an all white animal, usually with pink or red eyes.

Melanins and melanin production

Melanin Melanin (; ) is a family of biomolecules organized as oligomers or polymers, which among other functions provide the pigments of many organisms. Melanin pigments are produced in a specialized group of cells known as melanocytes. There are ...

is a compound found in

plant Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...

animal Animals are multicellular, eukaryotic organisms in the Biology, biological Kingdom (biology), kingdom Animalia (). With few exceptions, animals heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, ...

s, and

protist A protist ( ) or protoctist is any eukaryotic organism that is not an animal, land plant, or fungus. Protists do not form a natural group, or clade, but are a paraphyletic grouping of all descendants of the last eukaryotic common ancest ...

s, and is derived from the

amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...

tyrosine -Tyrosine or tyrosine (symbol Tyr or Y) or 4-hydroxyphenylalanine is one of the 20 standard amino acids that are used by cells to synthesize proteins. It is a conditionally essential amino acid with a polar side group. The word "tyrosine" is ...

. Melanin is a photoprotectant, absorbing the DNA-damaging

ultraviolet Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...

radiation of the sun.

Vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...

s have melanin in their

skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other animal coverings, such as the arthropod exoskeleton, have different ...

and

hair Hair is a protein filament that grows from follicles found in the dermis. Hair is one of the defining characteristics of mammals. The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and ...

feather Feathers are epidermal growths that form a distinctive outer covering, or plumage, on both avian (bird) and some non-avian dinosaurs and other archosaurs. They are the most complex integumentary structures found in vertebrates and an exa ...

s, or

scales Scale or scales may refer to: Mathematics * Scale (descriptive set theory), an object defined on a set of points * Scale (ratio), the ratio of a linear dimension of a model to the corresponding dimension of the original * Scale factor, a number ...

. They also have two layers of pigmented tissue in the eye: the stroma, at the front of the iris, and the iris pigment epithelium, a thin but critical layer of pigmented cells at the back of the iris. Melanin is also present in the

inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...

, and is important for the early development of the

auditory system The auditory system is the sensory system for the sense of hearing. It includes both the ear, sensory organs (the ears) and the auditory parts of the sensory system. System overview The outer ear funnels sound vibrations to the eardrum, incre ...

. Melanin is also found in parts of the

brain The brain is an organ (biology), organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head (cephalization), usually near organs for ...

and

adrenal gland The adrenal glands (also known as suprarenal glands) are endocrine glands that produce a variety of hormones including adrenaline and the steroids aldosterone and cortisol. They are found above the kidneys. Each gland has an outer adrenal corte ...

Melanins are produced in

organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as Organ (anatomy), organs are to th ...

s called

melanosome A melanosome is an organelle found in animal cells and is the site for synthesis, storage and transport of melanin, the most common light-absorbing pigment found in the animal kingdom. Melanosomes are responsible for color and photoprotectio ...

s. The production of melanins is called

melanogenesis Melanocytes are melanin-producing neural crest-derived cells located in the bottom layer (the stratum basale) of the skin's epidermis, the middle layer of the eye (the uvea), the inner ear, vaginal epithelium, meninges, bones, and heart foun ...

. Melanosomes are found in specialized pigment cells called

melanocyte Melanocytes are melanin-producing neural-crest, neural crest-derived cell (biology), cells located in the bottom layer (the stratum basale) of the skin's epidermis (skin), epidermis, the middle layer of the eye (the uvea), the inner ear, vagina ...

s, but may also be engulfed by other cells, which are then called melanophages. Hair acquires pigment from melanocytes in the root bulb, which deposit melanosomes into the growing hair structure. A critical step in the production of melanins is the

catalysis Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...

by an

enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...

called

, producing dopaquinone. Dopaquinone may become eumelanin, or phaeomelanin. Eumelanin, meaning ''true black'', is a dense compound that absorbs most wavelengths of light, and appears black or brown as a result. Phaeomelanin, meaning ''rufous-black'', is characterized by the presence of

sulfur Sulfur ( American spelling and the preferred IUPAC name) or sulphur ( Commonwealth spelling) is a chemical element; it has symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms ...

-containing

cysteine Cysteine (; symbol Cys or C) is a semiessential proteinogenic amino acid with the chemical formula, formula . The thiol side chain in cysteine enables the formation of Disulfide, disulfide bonds, and often participates in enzymatic reactions as ...

, and it appears reddish to yellowish as a result. Melanosomes containing eumelanin are eumelanosomes, while those containing phaeomelanin are phaeomelanosomes.

Melanocyte-stimulating hormone The melanocyte-stimulating hormones, known collectively as MSH, also known as melanotropins or intermedins, are a family of peptide hormones and neuropeptides consisting of α-melanocyte-stimulating hormone (α-MSH), β-melanocyte-stimulating ...

(MSH) binds to the

Melanocortin 1 receptor The melanocortin 1 receptor (MC1R), also known as melanocyte-stimulating hormone receptor (MSHR), melanin-activating peptide receptor, or melanotropin receptor, is a G protein–coupled receptor that binds to a class of pituitary peptide hormon ...

(MC1R) and commits melanocytes to the production of eumelanin. In the absence of this signal, melanocytes produce phaeomelanin. Another chemical,

Agouti signalling peptide Agouti-signaling protein is a protein that in humans is encoded by the ASIP gene. It is responsible for the distribution of melanin pigment in mammals. Agouti interacts with the melanocortin 1 receptor to determine whether the melanocyte (pigment ...

(ASP), can attach itself to MC1R and interfere with MSH/MC1R signalling. In many mammals, variation in the level of ASP switches melanocytes between eumelanin and phaeomelanin production, resulting in coloured patterns. Lightmatter lab mice

Melanocytes, and the parallel melanophores found in fishes, amphibians, and reptiles, are derived from a strip of tissue in the

embryo An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sp ...

called the

neural crest The neural crest is a ridge-like structure that is formed transiently between the epidermal ectoderm and neural plate during vertebrate development. Neural crest cells originate from this structure through the epithelial-mesenchymal transition, ...

Stem cell In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can change into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type of cell ...

s in the neural crest give rise to the cells of the

autonomic nervous system The autonomic nervous system (ANS), sometimes called the visceral nervous system and formerly the vegetative nervous system, is a division of the nervous system that operates viscera, internal organs, smooth muscle and glands. The autonomic nervo ...

, supportive elements of the skeleton such as

chondrocyte Chondrocytes (, ) are the only cells found in healthy cartilage. They produce and maintain the cartilaginous matrix, which consists mainly of collagen and proteoglycans. Although the word '' chondroblast'' is commonly used to describe an immatu ...

s, cells of the

endocrine system The endocrine system is a messenger system in an organism comprising feedback loops of hormones that are released by internal glands directly into the circulatory system and that target and regulate distant Organ (biology), organs. In vertebrat ...

, and melanocytes. This strip of tissue is found along the dorsal midline of the

, and

multipotent Cell potency is a cell's ability to differentiate into other cell types. The more cell types a cell can differentiate into, the greater its potency. Potency is also described as the gene activation potential within a cell, which like a continuum ...

cells migrate down along the sides of the embryo, or through

germ layer A germ layer is a primary layer of cell (biology), cells that forms during embryonic development. The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans (animals that are sister taxa to the sponges) produce tw ...

s, to their ultimate destinations. Melanocyte stem cells are called melanoblasts. Conditions associated with abnormalities in the migration of melanoblasts are known collectively as piebaldism. Pigment cells of the iris pigment epithelium have a separate embryological origin.Robins, Ashley H. (1991) pg. 75 Piebaldism and amelanism are distinct conditions.

In mammals

The only pigments that mammals produce are melanins. For a mammal to be unable to chemically manufacture melanin renders it completely pigmentless. This condition is more commonly called

. Amelanistic mammals have white hair, pink skin, and eyes that have a pink, red, or violet appearance. Reddish eyes are due to the lack of pigment in the iris pigment epithelium. When the stroma is unpigmented but the iris pigment epithelium is not, mammalian eyes appear blue. Melanin in the pigment epithelium is critical for visual acuity and contrast. Loss of melanogenesis function is linked to the

gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

that encodes

. Certain

allele An allele is a variant of the sequence of nucleotides at a particular location, or Locus (genetics), locus, on a DNA molecule. Alleles can differ at a single position through Single-nucleotide polymorphism, single nucleotide polymorphisms (SNP), ...

s of this gene, ''TYR'', at the ''Color'' locus, cause oculocutaneous albinism type 1 in humans and the familiar red-eyed albino conditions in

mice A mouse (: mice) is a small rodent. Characteristically, mice are known to have a pointed snout, small rounded ears, a body-length scaly tail, and a high breeding rate. The best known mouse species is the common house mouse (''Mus musculus' ...

and other mammals. Afra 007

In other vertebrates

Other vertebrates, such as fishes, amphibians, reptiles and birds, produce a variety of non-melanin

. Disruption of melanin production does not affect the production of these pigments. Non-melanin pigments in other vertebrates are produced by cells called

chromatophore Chromatophores are cells that produce color, of which many types are pigment-containing cells, or groups of cells, found in a wide range of animals including amphibians, fish, reptiles, crustaceans and cephalopod A cephalopod is any member o ...

s. Within this categorization, xanthophores are cells that contain primarily yellowish pteridines, while erythrophores contain primarily orangish

carotenoid Carotenoids () are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, archaea, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, corn, tomatoes, cana ...

s. Some species also possess iridophores or leucophores, which do not contain true pigments, but light-reflective structures that give iridescence. An extremely uncommon type of chromatophore, the cyanophore, produces a very vivid blue pigment. Amelanism in fishes, amphibians, reptiles and birds has the same genetic

etiology Etiology (; alternatively spelled aetiology or ætiology) is the study of causation or origination. The word is derived from the Greek word ''()'', meaning "giving a reason for" (). More completely, etiology is the study of the causes, origins ...

as in mammals: loss of

function. However, due to the presence of other pigments, other amelanistic vertebrates are seldom white and red-eyed like amelanistic mammals. Nymphicus hollandicus2

Aeumelanism

Melanocytes depend on the

(MC1R) to signal the production of eumelanin. Loss of melanocortin 1 receptor function or high activity of the MC1R-antagonist,

, can cause the widespread absence of eumelanin. Loss of MC1R function, a recessive trait, has been observed in many species. In humans, various

mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, ...

s of the ''MC1R'' gene result in

red hair Red hair, also known as ginger hair, is a human hair color found in 2–6% of people of northern Europe, Northern or Northwestern European ancestry and lesser frequency in other populations. It is most common in individuals Zygosity#Homozy ...

blond hair Blond () or blonde (), also referred to as fair hair, is a human hair color characterized by low levels of eumelanin, the dark pigment. The resultant visible hue depends on various factors, but always has some yellowish color. The color can be ...

, fair skin, and susceptibility to sundamaged skin and

melanoma Melanoma is the most dangerous type of skin cancer; it develops from the melanin-producing cells known as melanocytes. It typically occurs in the skin, but may rarely occur in the mouth, intestines, or eye (uveal melanoma). In very rare case ...

.Online Mendelian Inheritance in Man, OMIM (TM). Johns Hopkins University, Baltimore, MD. MIM Number: : :. World Wide Web URL: https://www.ncbi.nlm.nih.gov/omim/ Aeumelanic hair coats, associated with mutations of the ''MC1R'' gene, have also been identified in mice, cattle, dogs, and horses. These coat colors are called "yellow" in mice and dogs, "red" in cattle and

chestnut The chestnuts are the deciduous trees and shrubs in the genus ''Castanea'', in the beech family Fagaceae. The name also refers to the edible nuts they produce. They are native to temperate regions of the Northern Hemisphere. Description ...

in horses. The loss of eumelanin in the coat is, in these species, harmless. The distinction between aeumelanism and hyperphaeomelanism – over abundance of phaeomelanin – is semantic. Brown and black horse

Aphaeomelanism

Aphaeomelanism is the abnormal absence of phaeomelanin from the

integumentary system The integumentary system is the set of organs forming the outermost layer of an animal's body. It comprises the skin and its appendages, which act as a physical barrier between the external environment and the internal environment that it serves ...

and/or eyes. Phaeomelanin is produced by melanocytes in the absence of melanocortin 1 receptor. This absence is mediated by agouti signalling protein, which antagonizes melanocortin 1 receptor. Loss of function of agouti signalling protein can permit unmediated eumelanin production, producing a uniformly black-to-brown coat color. This condition can be observed in dogs, cats,
Online Mendelian Inheritance in Man, OMIM (TM). Johns Hopkins University, Baltimore, MD. MIM Number: : :. World Wide Web URL: https://www.ncbi.nlm.nih.gov/omim/ and horses. The appearance of mammals with recessive agouti mutations is typically dense black. As with aeumelanism, the difference between lack of phaeomelanin and abundance of eumelanin is one of words. Some agouti alleles in mice are associated with health defects, but this is not the case in dogs, cats, or horses.

References