- For the geological term, see melanosome (geology)
Cells which produce melanosomes are called melanocytes, whereas cells which have merely engulfed the melanosomes are called melanophages.
Melanosomes are bound by a lipid membrane and are generally rounded, sausage-like or cigar-like in shape.
The shape is constant for a given species and cell type.
Synthesis of melanin
Before it contains much pigment (sufficient to be seen on light microscopy), it is known as a pre-melanosome.
Dysfunction or absence of the melanin-synthesising enzymes leads to various patterns of albinism.
In some melanocytes, the melanosomes remain static within the cell. In other types of melanocyte, the cell can extend its surface as long pseudopodia, carrying melanosomes away from the centre of the cell and increasing the cell's effectiveness in absorbing light.
For example, this happens slowly in dermal melanocytes in responsive to ultraviolet light, as well as production of new melanosomes and increased 'donation' of melanosomes to adjacent keratinocytes, the normal skin surface cells.
Collectively these changes are responsible for 'tanning' after exposure to UV or sunlight.
In many species of fish, amphibians, crustaceans and reptiles, melanosomes can be highly mobile within the cell in response to hormonal (or sometimes neural) control, and this leads to visible changes in colour that are used for behavioural signalling.
Melanosomes are believed to template melanin polymerization by way of amyloidogenesis of the protein pMel, which is present in abundant quantities in melanosomes.
- Fowler, et al. PLoS Biol. 2005 Nov 29;4(1)
- Histology image: 08103loa – Histology Learning System at Boston University - "Integument: pigmented skin"