Melanogenesis
Melanogenesis is the enzymatic synthesis of melanin inside melanosomes and the subsequent transfer of those pigment granules to surrounding keratinocytes, where they shield nuclear DNA.
Melanogenesis begins with tyrosine inside a lysosome-derived organelle, the melanosome. Tyrosinase hydroxylates tyrosine to L-DOPA and oxidises L-DOPA to dopaquinone, the rate-limiting pair of steps; downstream branching decides whether the product is eumelanin or, where cysteine is available, pheomelanin. Mature melanosomes then travel along melanocyte dendrites to keratinocytes, which cap the pigment over the nucleus on the sun-facing side.
The unit of interest is not the melanocyte alone. One melanocyte serves roughly thirty to forty keratinocytes, and melanocyte density is broadly similar across human populations; visible differences come from melanosome number, size, melanisation and whether granules stay clustered or disperse. Ultraviolet drives the process indirectly, through keratinocyte p53 induction of proopiomelanocortin and release of alpha-MSH onto neighbouring melanocytes, which is why tanning appears over days.
Having four separable stages, transcription, enzyme activity, transfer and epidermal turnover, matters when reading a depigmenting claim, because agents act at different points and are not interchangeable. It also sets the clock for a trial: pigment already in the upper epidermis leaves only as those keratinocytes are shed, so a study shorter than one or two turnover cycles cannot show the full effect of blocking new synthesis.
Where this goes wrong is the leap from enzyme assay to skin. A compound that inhibits isolated tyrosinase has cleared the easiest hurdle; it must still survive formulation, cross the stratum corneum, reach the melanosome and act for weeks. An ingredient list naming a documented inhibitor speaks to none of that, and effects seen in culture at micromolar concentrations rarely survive dilution to parts per million in a cream.