What Is a Melanosome and What Does It Do?

A melanosome is an organelle inside animal cells that produces and stores melanin, the pigment giving color to skin, hair, and eyes. These structures are synthesized within specific cells called melanocytes, found in the skin and eyes. The primary function of melanosomes is to serve as the site for melanin synthesis, which directly influences an individual’s coloration.

Melanosome Structure and Maturation

Melanosomes form within melanocytes, originating from early endosomal compartments. Their development is an organized process that occurs in four stages. In Stage I, the melanosome is an empty, spherical vesicle with an early internal fibrillar matrix. During Stage II, the organelle elongates into a sausage-like shape, and the internal fibrils become more structured, forming a scaffold for pigment deposition.

As the melanosome enters Stage III, melanin synthesis begins, and the dark pigment is deposited onto the internal matrix. Finally, in Stage IV, the melanosome is a fully mature, electron-dense granule with no visible internal structure because it is completely packed with pigment.

The Role in Producing Pigment

Inside the melanosome, a complex biochemical process transforms the amino acid tyrosine into melanin. This organelle functions as a self-contained factory, keeping the components of melanin synthesis separate from the rest of the cell. The specific type of melanin produced determines the resulting color.

Eumelanin is a dark pigment that produces brown and black hues and is the most common form in humans. Pheomelanin, on the other hand, is responsible for reddish and yellow tones, such as those in red hair. The final color of an individual’s skin, hair, and eyes is determined by the ratio of these two melanin types and the total amount of melanin produced.

Transfer and Cellular Protection

Once fully mature, melanosomes are transferred from the melanocyte to neighboring cells. In the skin, these recipient cells are primarily keratinocytes, the main cells of the epidermis. The melanosomes are transported through long, branching extensions of the melanocyte to the surrounding keratinocytes.

Upon entering the keratinocytes, the melanosomes are arranged in a cap-like formation over the cell’s nucleus. This “supranuclear cap” acts as a shield, absorbing and scattering incoming ultraviolet (UV) radiation from the sun. By doing so, this protects the cell’s DNA from UV-induced damage that can lead to mutations.

Influence on Pigmentation Diversity

The diversity in human skin color is not due to the number of melanocytes, but rather the activity of their melanosomes. Variations in the number, size, and distribution of melanosomes within keratinocytes account for the spectrum of skin tones. For instance, individuals with darker skin have larger and more numerous melanosomes rich in eumelanin.

In contrast, in lighter skin, melanosomes are smaller, less numerous, and grouped in clusters within the keratinocytes. In darker skin, the larger melanosomes are distributed more individually, providing more uniform UV protection. When there are disruptions in the production or function of melanosomes, it can lead to pigmentation disorders, such as albinism, which is characterized by a significant reduction or complete absence of melanin pigment.