The epidermis is the thin layer at the surface that varies in thickness from 0.05 mm on the eyelids to 1.5 mm on the palms of the feet. The top of the epidermis is called the cornified layer, and it contains thickened dead squamous cells. This part of the skin is very important in protecting our skin from environmental damage, like radiation from the sun and pressure from sharp objects.
Below the epidermis is the dermis where there are blood capillaries, hair follicles, sweat glands, sebaceous gland, nerve endings, fat cells and a range of proteins that provide support and keep the skin flexible.
We shed 1.5 grams of skin cells every day, and to compensate for this loss of cells, we have cells that are busily dividing. These dividing cells are found at the base of the epidermis in the Malpighian layer – they are called basal cells and are found in the basal layer. Also in the Malpighian layer are melanocytes, which produce the pigment melanin that gives skin its tan colour, and another type of cell that has spiny projections, which give its name to the spinous layer.
Basal cells are continually dividing by mitosis, and the new cells formed push up older cells to the surface. As they are being pushed up, they are developing into different types of cells that do not divide. In the granular layer, these cells mature to form cells containing more and more protein keratin granules. Cells of this layer also can become cancerous.
As the cells are pushed up to the surface, they get filled up with keratin granules, grow bigger and flatter, become dehydrated, die and fuse together into layers of tough durable material that continues to migrate to the skin surface. They finally reach the cornified layer, which is made of up 10–30 thin layers of dead keratinocytes that are continually being shed.
Young people take about 28–30 days for their keratinocytes to appear on the surface and be shed, while older people’s shedding cycles take longer (45–50 days).
Melanocytes that are found in the basal layer produce the skin pigment called melanin, which gives skin its tan or brown colour and protects the deeper layers of the skin from the harmful effects of the sun. Sun exposure causes these melanocytes to increase production of melanin and you get a suntan. Patches of melanin in the skin cause birthmarks and age spots. Melanoma occurs when melanocytes become malignant.
Nature of Science
The Malpighian layer is named after Italian doctor Marcello Malpighi (1628–1694). He was a pioneer in using the microscope and has been described as a founder of microscopic anatomy. Most of his research results, from the universities of Pisa and Messina, were published by the Royal Society of England, and he became the first Italian to be recognised by this learned scientific society in 1668.
It takes some time for melanocytes to produce melanin in reaction to ultraviolet radiation in sunlight, which is why you don’t get tanned instantly. Europeans are different from most other races because they don’t produce melanin continuously. The number of melanocytes is roughly the same within and between races, and colour differences are due to the amount of melanin produced and the kind of pigments produced. For example, eumelanin is a pigment that produces a brown colour, and phaeomelanin is a pigment that produces a yellow and red colour. Red heads produce more phaeomelanin and less eumelanin.