Summer is drawing to an end and it seems to be cramming all the sunshine in whatever time there is left. While we are all well aware of the importance of avoiding excess exposure to harmful UV rays, most of us end up getting a bit of a suntan during the summer months. Even as a melanoma scientist, I have to admit I am partial to getting a bit of colour to contrast the greenish tinge acquired through working indoors throughout the winter. Most people know about melanin, the brownish pigment that gives human skin its colour. Not many people, however, know about the intricate process through which melanin gives people a suntan. It’s a delightful example of how cells are complex beasts that interact with each other through an incredibly fine-tuned dance to function as a whole organism.
Our tale of tanning begins in the skin. The human skin is made of a bottom, goey layer known as the dermis and a top layer of tough skin cells known as the epidermis. In turn, the epidermis is made of several layers of skin cells. Cells at the top are the hardest as they are filled with keratin, a hard fibrous protein that gives the skin its resistant qualities. These cells are constantly shedding off through routine tear-and-wear. Shod cells are replaced by the lower layers, which progressively become less vital and fill up with keratin, becoming tougher and tougher. Cells at the bottom of the pile divide, creating new cells that will form the top layer of the skin.
Buried amidst the bottom layer of skin cells are melanocytes, the cells in charge of producing and managing melanin. Melanocytes sometimes form little clusters, which we know as moles. Sometimes, a tumour arises in a single melanocyte or in a mole, which is known as melanoma. However, most of the time melanocytes are happy to protect it from harmful sun rays. While the main body of a melanocyte sits at the bottom layer of the skin, these cells have a series of thin protrusions that spread upwards life the branches of a leafless tree. These branches are called dendrites and are weaved through skin cells in the middle layer of the skin to distribute melanin throughout the skin.
Melanocytes produce melanin within little cigar-shaped pouches known as melanosomes inside the core body of the cell. When the skin exposed to UV radiation, these melanin-filled vesicles are slowly transported along the dendrites throughout the skin. Once they reach skin cells, the little bullets of melanin are “donated” from the melanocyte to the skin cell. In fact, skin cells eat up, or “engulf” the very tip of the branch-like protrusions of the melanocytes that contain the melanosomes. Once the little melanin globules are incorporated into the skin cell, they are transported back into the centre of the cell. What is really interesting to the cell biology geek is how melanosomes go in opposite directions in the melanocyte (in-out) and in the skin cell (out-in).
Skin cells, melanocytes and all other cells in the human body contain a scaffold of rigid protein filaments which are collectively known as the cytoskeleton – literally, the skeleton of the cell. Just like a bone skeleton, the cytoskeleton mediates all movement in the cell. Cellular organs, or “organelles” and nutrients are transported up and down the cytoskeleton like tram carriages on the rail line. In fact, melanosomes are also transported in melanocytes and skin cells along the cytoskeletal “rails”. Things move up and down the cytoskeleton thanks to two sets of motors: “kinesin” motors move from the centre of the cell to its periphery and “dynein” motors move from the periphery of the cell to the centre. So melanosomes are carried by kinesin motors in the melanocytes along the dendrites and the hop over on the opposite direction motor, dynein, to go back toward the centre of the cell in the skin cells.
Written by: Gaia Cantelli