Biology: Review of the Skin - for Those Seeking Entry into Medical School

The MCAT is the exam taken by those seeking entry into medical school. Two years ago, I took this long and taxing exam. I earned a score of 41 out of 45. I wanted to share some information with those of you who will be taking this exam. The following information is what I felt I needed to know to answer questions regarding the human organ, the skin, in the Biological Sciences section of the exam. In writing this, I assume you have knowledge of certain terms, gained from your biology classes.

The skin is the largest organ in the body. The role of the skin is to protect the body from pathogens, to prevent excessive water evaporation, and to regulate the body's temperature.

There are three layers that comprise the human skin. The "epidermis" is the outermost layer of skin. Underneath the epidermis is the "dermis." Underneath the dermis is the "hypodermis," which is also known as "subcutaneous tissue."

The epidermis, or top layer of skin, is composed of stratified squamos epithelial cells. "Stratified" means that there are layers of squamos epithelial cells. The deepest part of the epidermis is called the stratum basale. Cells of the stratum basale undergo mitosis. When a cell in the stratum basale divides, one of the daughter cells moves outward. This cell soon dies and is pushed outward farther as mitosis in the stratum basale continues. Eventually, that cell is sloughed off from the surface of the body.

The many layers of epithelial cell provide part of the skin's protective structure. Another part of the epidermis' protective structure is found in dead squamos cells. When stratified squamos cells die, they become surrounded by keratin. Keratin is a strong, hydrophobic protein. As keratin is hydrophobic, it helps to make the skin waterproof.

Furthermore, the epithelial cells of the epidermis contain melanin. Melanin is a pigment produced by melanocytes. Melanocytes are specialized cells in the epidermis. Melanin absorbs ultraviolet light, preventing tissue damage by UV light.

Different types of cells embedded in connective tissue matrix make up the dermis. The dermis contains blood vessels that bring nutrients to the dermis and epidermis. The dermis also contains sensory receptors. These sensory receptors convey information to the central nervous systems regarding touch, pain, pressure, etc. The dermis also contains sebaceous glands (oil glands), sudoriferous glands (sweat glands), and hair follicles.

In humans, the skin functions in regulating body temperature. The main way the human body generates heat is through metabolism. The skin insulates the body in order to conserve the heat generated via the body's metabolic processes. The layer of insulating fat in the subcutaneous tissue helps insulate that heat. Cutaneous vasoconstriction is the constriction of blood vessels in the dermis. Cutaneous vasoconstriction prevents heat loss by conduction. When there is excess heat, cutaneous dilation leads to heat loss via conduction. Also, sweating allows heat loss via evaporation.