The skin makes up around 16% of total body weight, making it the largest organ in the body - although it’s hard to imagine it as a single organ. The skin along with its accessory structures--like oil and sweat glands--makes up the integumentary system. The integumentary system protects the body from infections, helps regulate body temperature, and contains nerve receptors that detect pain, sensation, and pressure.
Now, the skin is divided into three layers--the epidermis, dermis, and hypodermis. The epidermis forms the thin outermost layer of skin. Underneath, is the thicker dermis layer that contains the nerves and blood vessels. And finally, there’s the hypodermis which is made of fat and connective tissue that anchors the skin to the underlying muscle.
The epidermis itself is made of multiple layers of developing keratinocytes - which are flat pancake-shaped cells that are named for the keratin protein that they’re filled with. Keratin is a fibrous protein that allows keratinocytes to protect themselves from getting destroyed when you rub your hands through the sand at the beach. Keratinocytes also make and secrete glycolipids, glyco meaning part sugar and lipid meaning part fat. Glycolipids help to prevent water from easily seeping into and out of the body. Keratinocytes start their life at the lowest layer of the epidermis called the stratum basale, or basal layer, which is made of a single layer of stem cells that continually divide and produce new keratinocytes. These new keratinocytes then migrate upwards to form the other layers of the epidermis. The stratum basale also contains another group of cells - melanocytes, which secrete a protein pigment, or coloring substance, called melanin. Melanin is actually a broad term that constitutes several types of melanin found in people of differing skin color. These subtypes of melanin range in color from black to reddish yellow and their relative quantity define a person’s skin color. When keratinocytes are exposed to the sun, they send a chemical signal to the melanocytes, which stimulates the melanocytes into making more melanin. The melanocytes move the melanin into small sacs called melanosomes, and these get taken up by newly formed keratinocytes. Melanin then acts as a natural sunscreen, because its protein structure disspitates, or scatters, UVB light--which if left unchecked can damage the DNA in the skin cells and lead to skin cancer. Darker types of melanin and greater quantities of of this kind of melanin are produced by individuals living close to the equator because they typically get more sun exposure. However, it’s a fine balance because UVB light helps us generate vitamin D, which is an important regulator of calcium absorption. Keratinocytes contain cholesterol precursor molecules that are activated by UVB into Vitamin D.
As keratinocytes in the stratum basale mature and lose the ability to divide, they migrate into the next layer, called the stratum spinosum which is about 8 to 10 cell layers thick. Keratinocytes in the stratum spinosum layer have tiny proteins on the membrane that look like tiny spines; these help the cells adhere to one another. The stratum spinosum layer also has dendritic cells lurking around - these are star-shaped immune cells, that are constantly patrolling - looking for invading microbes.
The next layer up is the stratum granulosum which is 3 to 5 cell layers thick. Keratinocytes in this layer begin the process of keratinization, which is the process where the keratinocytes flatten out and die, and in the process they create the epidermal skin barrier. To do this, keratinocytes in the stratum granulosum layer produce large amounts of keratin precursor proteins and glycolipid which remain within granules called keratohyalin granules and lamellar granules, respectively. Keratohyalin granules eventually start to aggregate and cross-link forming enormous bundles of keratin within the keratinocyte. Lamellar granules, on the other hand, get secreted and stick to the outer cell surface. It forms a sort of cement between the cells, making them more resistant to external forces and water loss. Over time, the intracellular organelles disintegrate so the cells flatten out and die.
Keratinization leads to development of the stratum lucidum layer which is 2 to 3 cell layers thick of translucent, dead keratinocytes that have secreted most of their lamellar granules. The stratum lucidum is only found in thick skin like on the palms and soles of the feet, because those are the areas that need extra protection. The stratum lucidum is absent in thin skin, which covers the rest of the body, and the other layers are thinner.