The skin has three principal layers: the epidermis, the dermis, and the subcutaneous fat layer. Associated with these layers are the adnexal structures, including the hair follicles, sebaceous glands, and the sweat glands. Other structures penetrating the skin are nerves, blood vessels, lymphatic vessels, and some smooth muscle cells around the hair follicles.
The Epidermis
Cell Layers of the Epidermis
The outer region of the skin, the epidermis, is approximately 1/10 of one millimeter thick (approximately the thickness of three to four book pages), although it can be much thicker in certain areas, such as the palms of the hands and soles of the feet.
The epidermis is composed of cells organized into layers, from the top down: the horny layer (stratum corneum), granular layer, and basal layer. The major purpose of the epidermis is to form the stratum corneum, a layer of dead but functionally important cells that acts as a barrier between the body and the environment. With a functioning stratum corneum, microorganisms, water, and particles cannot get in and body fluids cannot get out.
Cells of the Epidermis: Keratinocytes
Keratinocytes make up approximately 80% to 90% of the cells in the epidermis. After being formed in the deepest layer of the epidermis, the basal layer, through mitosis, these important cells begin their journey upward toward the skin surface. In the process, they change from nucleated, dividing cells to non-nucleated, non-living cells that are regularly exfoliated from the skin surface. The dead cells of the stratum corneum are known as corneocytes.
One Keratinocyte Maturation Cycle
Skin proliferation is normally tightly controlled so that the production of keratinocytes in the basal layer balances the loss from the stratum corneum through exfoliation. It normally takes about 40 days (about six (6) weeks) from the time a newly formed cell leaves the basal layer until it is exfoliated. Those six (6) weeks are an important concept and are referred to in this manual as one (1) keratinocyte maturation cycle or one (1) skin cycle in the Obagi Skin Health Restoration program.
Keratin
The stratum corneum, the hair, and the nails contain large amounts of the fibrous protein, keratin that is made by keratinocytes. Keratin helps to make the skin impermeable and reduces the penetration of ultraviolet light into deeper levels of the skin. Keratin also plays a major role in neutralizing the TCA in the Blue Peel to produce the frost.
Blood Vessels and Nerve Endings
There are no blood vessels in the epidermis. Nutrients must come in and waste products go out by diffusion from the papillae of the dermis. The skin is richly supplied with nerve endings that detect pain, itch, temperature, pressure, and vibration.
During a Blue Peel procedure the blood vessels and blood flow will create the pink background of the frost at the papillary dermis level. The frost without pink (no blood flow) indicates the peel has reached the immediate upper reticular dermis (IRD) level (more information on IRD in Section 3).
The Epidermal-Melanocyte Unit
In normal skin, melanin synthesizing cells, known as melanocytes, are found only in the basal layer of the epidermis and adnexal structures where they are interspersed among the basal keratinocytes. Melanocytes have finger-like projections that put them in contact with keratinocytes to form an “epidermal-melanocyte unit.”
Melanocytes and Melanosomes
Inside the melanocyte, organelles known as melanosomes produce granules of melanin pigment, the substance that determines skin color.
Melanin granules then move into the finger-like projections and are transferred to the surrounding keratinocytes. The role of melanin is to surround the keratinocyte nucleus and protect its DNA from mutations such as ultraviolet radiation. However, many factors can make the melanocyte not function properly and produce a wide variety of pigmentation disorders.