Skin

Understand the functions of the skin

There are lots of functions of the skin, most of which you are likely already familiar with. Firstly, skin protects you from all kinds of things, like harmful bacteria and abrasion. Secondly, there are many blood vessels underlying the skin that can be dilated or constricted to help regulate body temperature. There are also receptors in the skin that can help us sense the world. Synthesis of some stuff, like vitamin D, also occurs in the skin.

Identify the basic layers in skin
Describe the structure of the different layers in skin

There are three main layers of skin: epidermis, dermis, and hypodermis (a.k.a. "subcutaneous layer").

The epidermis is keratinised stratified squamous epithelium that can be divided up into four layers (or five, if you're talking about the thick skin on the palms of your hands and soles of your feet). The deepest layer, the stratum basale, is where stem cells divide, but there are also melanocytes (which secrete melanin) and Merkel cells (cells that sense light touch). Next up is the stratum spinosum, consisting of keratinocytes joined together by desmosomes. The next layer is the stratum granulosum, made up of layers of flattened cells and granules. The next layer, the stratum lucidum, is only present in thick skin. Finally, the most superficial layer is the stratum corneum, where all the cells are full of keratin and have died due to being too far away from blood vessels.

The layer below the epidermis is called the dermis. It is made up of connective tissue, often dense irregular connective tissue. It also contains blood vessels, nerves, and cells, such as fibroblasts and macrophages. The dermis also contains "epidermal derivatives" (structures that were derived from the epidermis but moved downwards), including receptors and glands, as well as hair follicles. Most of these structures have basement membranes to separate ectodermal-derived and mesoderm-derived tissues. Here are some more details about these structures that you should know:

• Hair- Associated with sebaceous glands (secrete sebum, an oily substance), erector pili muscles (smooth muscle that raises the hairs), and sensory nerves. The number of hairs are about the same for everybody, but hair can differ in terms of thickness and colour.
• Sebaceous glands- Glands that secrete an oily substance called sebum. Tend to associate with hair follicles.
• Ceruminous glands- Special sebaceous glands that are found in the ear canal. Secrete cerumen (ear wax).
• Sweat glands- Secrete sweat. Look like little knots.

Finally, below the dermis is the hypodermis, which is mostly fat. Beneath the fat, there is connective tissue, which connects the fat to tissues beneath it, allowing the skin to move freely.

Discuss the difference in skin between different areas

As I mentioned earlier, the skin on the palms of your hands and soles of your feet is thick, and thick skin also has a stratum lucidum layer in the epidermis. The rest of the body is covered in thin skin, which lacks this stratum lucidum layer.