What is a melanocyte, and where is it?

The melanocyte is the cell that makes your skin's pigment. It lives in the deepest layer of the epidermis — the stratum basale.

[[ IMG_1 — Skin layers showing the melanocyte in the stratum basale ]]

Although melanocytes are few in number, each one feeds pigment to about 36 surrounding keratinocytes through its branches, called dendrites. Together, this group is known as the "epidermal melanin unit."

What is a keratinocyte?

Keratinocytes are the most abundant cells in the epidermis — about 90–95% of it. They build the skin barrier and slowly rise toward the surface. They don't make pigment themselves; they receive it from the melanocyte.

Melanin is your skin's natural sunscreen

Melanin isn't just color — it's protection. Inside each skin cell, melanin gathers into tiny "caps" that sit over the cell's nucleus like a parasol, absorbing incoming UV before it can damage the DNA. The more sun the skin senses, the more melanin it makes. That's exactly why we tan.

[[ IMG_CAP — Melanin protective caps absorb UV → DNA protected ]]

How melasma forms

Pigment, called melanin, is made inside the melanocyte in tiny packets called melanosomes. These travel outward and are delivered to the keratinocytes. When the process speeds up too much, excess pigment builds up — and that becomes the visible patch of melasma.

[[ IMG_2 — Melanosomes transferred to keratinocytes ]]

What activates the melanocyte to produce melanin?

There's no single cause. Several pathways — or "signals" — can switch the melanocyte on. In pregnancy melasma, 5 to 8 of these typically combine at once.

[[ IMG_3 — 9 Ranked Trigger Pathways ]]

The pathways, ranked

Pathway 1 — The sun route (MC1R → MITF)

When UV light damages a keratinocyte, a chain reaction reaches the melanocyte and ends in pigment. Step by step:

[[ IMG_S1 — UV damages keratinocyte → releases α-MSH ]]

Step 1

UV hits the skin

A UV photon damages the keratinocyte's DNA. In response, it releases a signal called α-MSH.

[[ IMG_S2 — α-MSH binds MC1R ]]

Step 2

α-MSH binds MC1R

The signal travels to the melanocyte and locks onto its MC1R receptor on the surface.

[[ IMG_S3 — cAMP rises → activates PKA ]]

Step 3

cAMP & PKA fire up

MC1R switches on adenylyl cyclase; cAMP rises inside the cell and activates the enzyme PKA.

[[ IMG_S4 — PKA → CREB → MITF ]]

Step 4

MITF is switched on

PKA activates CREB, which switches on MITF — the master switch for pigment production.

[[ IMG_S5 — MITF activates tyrosinase, TRP-1, TRP-2 ]]

Step 5

Pigment genes turn on

MITF enters the nucleus and activates the genes for tyrosinase, TRP-1 and TRP-2 — the pigment-building enzymes.

[[ IMG_S6 — Tyrosinase makes melanin in melanosomes ]]

Step 6

Melanin is made

Tyrosinase builds melanin inside melanosomes, which darken and pass to the keratinocytes.

Pathway 2 — The hormonal route (pregnancy)

Pregnancy hormones reach the same master switch by a different door. Step by step:

[[ IMG_H1 — Pregnancy surge of estrogen + progesterone ]]

Step 1

Hormones surge

During pregnancy, the placenta drives estrogen and progesterone 10–100× above normal.

[[ IMG_H2 — Estrogen binds ERα/ERβ, progesterone binds PR ]]

Step 2

Hormones bind receptors

Estrogen binds the ERα/ERβ receptors and progesterone binds PR on the melanocyte.

[[ IMG_H3 — Complexes enter the nucleus ]]

Step 3

Into the nucleus

The hormone-receptor complexes move into the nucleus and begin activating gene transcription.

[[ IMG_H4 — Activates MITF ]]

Step 4

Same master switch: MITF

They activate MITF — the exact same pigment master switch the sun pathway uses.

[[ IMG_H5 — Excess melanin → melasma ]]

Step 5

Excess melanin → melasma

MITF ramps up tyrosinase, the melanocyte makes excess melanin, and it transfers to keratinocytes as visible melasma.