Melasma
The dark spot you see on your face is the last step in a chain that began long before.
A long-form, illustrated walk through the pigmentation pathway — the same pathway that, when it refuses to stop, becomes melasma.
To remove melasma, first understand what builds it.
A dark patch is the surface of something much deeper. Eighteen steps run beneath the skin before a single pigmented cell ever reaches your eye. Each step is a switch. Each switch is precise. Each switch is something a treatment can address — but only once you can name what it does.
This is the pathway. Read it once. Then the patch on your face will no longer feel arbitrary.
The ground beneath the patch — where pigment is born.
Your skin is built in three layers. Pigment lives at the very bottom of the top one.
The epidermis is the only floor you see. Below it sits the dermis, then the hypodermis. Everything you are about to read happens at the boundary between the first two — a single row of cells called the stratum basale.
Pigment does not float through your skin. It is made in one place. It travels in one direction. It is what your eye sees last.
A single melanocyte reaches thirty-six neighbors.
One melanocyte sits in the basal layer. From its body, long branches called dendrites extend outward, touching roughly thirty-six surrounding keratinocytes.
This is the epidermal melanin unit. One factory. Many addresses. Whatever pigment the factory makes will be delivered to all of them.
One photon. Nine signals. The chain that builds a dark spot.
A single UV photon damages a keratinocyte's DNA.
UV light enters the epidermis and strikes a keratinocyte. Inside, the DNA is damaged. The cell responds with a distress signal — a small protein called α-MSH.
The chain has begun. Not in the melanocyte. In the cell next door.
α-MSH travels downward through the layers.
The α-MSH released by the damaged keratinocyte does not stay put. It moves down through the epidermis toward the basal layer, toward the melanocyte waiting below.
Signal becomes message. Message becomes destination.
α-MSH binds MC1R on the melanocyte's surface.
On the melanocyte's outer membrane sits a receptor called MC1R. α-MSH locks into it. The factory has received its first instruction.
Nothing visible has happened yet. Inside, everything is about to.
MC1R fires up adenylyl cyclase. cAMP floods the cell.
The activated MC1R triggers adenylyl cyclase, an enzyme on the inside of the membrane. Adenylyl cyclase begins producing cAMP — a small messenger molecule that will carry the signal deeper.
One bound receptor. Many cAMP molecules. The signal is being amplified.
cAMP fills the cytoplasm.
cAMP molecules now diffuse throughout the melanocyte's interior. The cell is being told — at scale — that pigment is needed.
This is the volume knob of the entire pathway. Turn it up here, and everything downstream amplifies.
cAMP activates Protein Kinase A.
cAMP binds to and activates Protein Kinase A (PKA). PKA is an enzyme that adds phosphate groups to other proteins — the molecular equivalent of throwing a switch.
The next switch it will throw determines whether the pigment genes turn on at all.
PKA phosphorylates CREB. CREB wakes up.
PKA travels into the nucleus and phosphorylates a transcription factor called CREB. A phosphorylated CREB is an active CREB — and an active CREB can switch on specific genes.
The chain is about to enter the genome.
CREB switches on MITF — the master of pigment.
Active CREB binds to the promoter of the MITF gene. MITF is transcribed. MITF protein appears.
MITF is the master regulator of pigmentation. Everything that comes next happens because MITF is now in the room.
MITF activates Tyrosinase, TRP-1, and TRP-2.
MITF turns on the three enzymes that actually make melanin: Tyrosinase, TRP-1, and TRP-2. The construction crew has arrived.
Tyrosinase is the rate-limiting enzyme — the one most pigment-correcting treatments target.
From one UV photon to one master transcription factor. The signal is now committed. What follows is the building of the pigment itself.
The pigment itself — built, packed, prepared.
Tyrosinase makes melanin inside melanosomes.
Inside the melanocyte sit small organelles called melanosomes. Tyrosinase, along with TRP-1 and TRP-2, converts the amino acid tyrosine into melanin — directly inside each melanosome.
The pigment is now built. But it is still inside the factory. It has not yet been delivered.
The delivery — pigment leaves the source.
Melanosomes travel the dendrites outward.
Once filled with melanin, melanosomes are loaded onto the cytoskeleton and carried outward along the melanocyte's dendrites. They move from the cell body toward the tips of every branch.
The factory is shipping. Each branch is a delivery route.
One factory. Thirty-six addresses.
Every melanocyte's dendrites end at roughly thirty-six neighboring keratinocytes. Each melanosome is destined for one of them.
This is why a single signaling event can pigment a wide area: the geometry was built that way.
Keratinocytes absorb the melanosomes.
At the tip of each dendrite, the melanosome crosses the cell boundary and enters the neighboring keratinocyte. The exact mechanism is debated — membrane fusion, filopodial transfer, phagocytosis — but the outcome is the same.
Once inside the keratinocyte, the pigment belongs to a regular skin cell. The factory has finished its work. The pigment now lives in the cells your eye will eventually see.
A closer look at the moment of transfer.
Zoom in. The melanosomes line up along the dendrite. They cross the boundary. They settle inside the keratinocyte.
Each keratinocyte now carries a share of the pigment its melanocyte made. The chain is no longer chemical. It is now spatial.
The climb upward — pigment reaches the surface.
Keratinocytes carry pigment up.
Loaded with melanin, the keratinocytes migrate upward through the epidermis — basale, spinosum, granulosum, corneum — over the span of roughly twenty-eight days.
By the time they reach the top of your skin, the pigment they carry is what your eye finally sees. The chain that began with one photon is now visible.
Why your body built this chain — in the first place.
Melanin forms a cap above the DNA.
Inside each keratinocyte, the melanosomes do not scatter. They form a small cap above the nucleus, exactly where the DNA lives. When the next UV photon arrives, the cap absorbs it.
This is the evolutionary purpose of the entire chain. Melanin exists to protect your genome. Your skin is doing what it was built to do.
So why does melasma happen?
The chain you just read is supposed to run, then stop. In melasma, it does not stop. Three forces keep it on — and once you know them, the patch on your face becomes legible.
Chronic UV
Daily, year-round sun exposure keeps α-MSH releasing and MC1R activating. The chain never resets. The factory runs constantly.
Hormonal shift
Estrogen and progesterone — from pregnancy, oral contraceptives, hormone therapy — directly upregulate MITF and Tyrosinase. The chain fires without UV.
Heat & inflammation
Visible light, infrared heat, and skin inflammation activate the same pathway. This is why melasma darkens on cloudy days, in hot kitchens, after irritation.
This page describes the normal pigmentation pathway and the conditions associated with melasma. It is an educational overview. It is not medical advice. For diagnosis or treatment, consult a licensed dermatologist.