The skin is comprised of three basic layers—the subcutaneous layer, the dermis, and the epidermis.

The subcutaneous layer of the skin, also known as the subcutis, makes up the very bottom layer of the skin and is basically a layer of fat that provides cushioning between the upper layers of skin and the muscle layer that lies below. Blood vessels and lymph vessels are present here, as are pressure sensitive nerves called lamellated corpuscles. The dermis is the layer of skin just above the subcutaneous layer. It is the most active and live layer of the skin, and is often called the “true skin.” The dermis, like the subcutaneous layer, contains many blood and lymph vessels as well as nerves that detect cold, heat, and pain. It also contains both major skin glands: the sebaceous glands, which secrete sebum—a fatty complex that flows through the follicles and lubricates the surface of the skin—and the sudoriferous glands that secrete sweat to help regulate body temperature.

There are two parts to the dermis. The reticular dermis (or dermal reticulum) sits just above the subcutaneous layer, and comprises the largest part of the dermis.

The reticular dermis contains the amazing network of collagen and elastin fibers, both protein structures that help the skin retain its shape, firmness, and provide elasticity, or the ability that allows skin to stretch and return to its original form. Collagen fibers are the most prevalent protein in the skin, comprising 70 percent of the skin’s dry weight—the weight of the skin after water has been removed.

Collagen is responsible for the firmness of the skin. Elastin helps the skin stretch and “snap back” to its normal state, and makes up significantly less of the skin’s dry weight, about 2 percent.

Fibroblasts, specialized cells in the reticular dermis are responsible for originating both collagen and elastin fibrils. The collagen and elastin fibers weave throughout the reticular der-mis, and are immersed in ground substance, a jelly-like fluid that serves as matrix filler in the dermal reticulum, filling the spaces between fibrils. The ground substance, along with fat, also helps surround and protect blood arteries and veins, lymph vessels, and nerves present in the reticular layer.

Ground substance is made from polysaccharide car-bohydrate chains, known as glycosaminoglycans. Hyaluronic acid, a strong hydrating molecule that holds up to 1,000 times its own weight in water, is one of these polysaccharides. Hyaluronic acid is frequently used in hydrating moisturizers for the skin; however, hyaluronic acid is too large of a molecule to penetrate the skin. It is used in skin care products to help retain moisture on the surface of the skin.

Hyaluronic acid is also used in medical inject-able fillers such as Restylane or Juvederm to help reduce the appearance of wrinkles or depressed acne scars. The hair follicle, also known as the pilosebaceous unit (also called pilosebaceous apparatus), is the structure out of which hairs grow.

The follicle structure begins at the base of the reticular dermis, where it is nourished by blood vessels. The papilla is a structure at the bottom of follicle from which hair grows. The terms follicle and pore are often used interchangeably by both estheticians and clients; however, pore generally refers to the appearance of the follicle opening when looking at the surface of the skin. The opening to the follicle at the skin surface is known as an ostium (plural: ostia).