The epidermis is divided into four layers, starting from bottom to top: the basal cell layer, stratum spinosum, stratum granulosum, and stratum corneum. The basal cell layer contains basal cells which divide and differentiate into other cells in the epidermis, and melanocytes, the cells that make melanin which gives skin its color. The stratum spinosum lies above the basal cell layer and is made of keratinocytes, cells that make the protein keratin. Keratin is an important component of the stratum corneum as well as hair and nails. Cells in the stratum granulosum are flattened and contain dark granules that are expelled and provide the “cement” that holds cells together in the overlying stratum corneum.
This uppermost layer of the epidermis is actually made of tightly- packed layers of dead cells filled with keratin that form the major physical barrier for the skin. The stratum corneum is thicker in areas like the palms and soles that withstand more daily wear and tear than other parts of the body. The epidermis also contains Langerhans cells, which act as part of the skin’s defense against infection. The dermal- epidermal junction is where the epidermis meets the dermis. The basement membrane zone serves as the “glue” between these two layers.
The dermis is divided into the upper papillary dermis and the lower reticular dermis. The structural components of the dermis include collagen, elastic fibers, and ground substance. Nerves and blood vessels also course through the dermis. Skin appendages are the eccrine and apocrine sweat glands, hair follicles, sebaceous glands, and nails. Except for nails, all the skin appendages are located in the dermis.
Eccrine sweat glands release sweat from eccrine glands, acting as part of the body’s cooling process. Sweat is produced in a coiled tubule in the dermis and is transported by a sweat duct through the epidermis to be secreted. The entire body surface has 2-3 million eccrine sweat glands and can produce up to 10 L of sweat per day.
In humans, apocrine sweat glands serve no known function and are regarded as vestigial glands perhaps useful to our ancestors. They are located mainly in the underarm and genital areas. Like eccrine sweat, apocrine sweat is also produced in coiled tubules in the dermis, but the apocrine duct drains sweat into a hair follicle from which it reaches the skins surface. Contrary to popular belief, the sweat from apocrine glands is odorless. The action of normal skin bacteria on excreted apocrine sweat is responsible for body odor.
Hair follicles are made of keratin, the same substance that forms nails and the top layer of the epidermis (stratum corneum). Different cells located in the root of the hair make keratin and melanin, which gives hair its color. Humans have two types of hair: vellus (light and fine) and terminal (dark and thick). A sebaceous gland secretes an oily substance called sebum that drains into the canal of a hair follicle to reach the surface of the skin. Together, a hair follicle and its associated sebaceous gland are called a pilosebaceous unit. Hair follicles are distributed everywhere on the body except the palms and soles. In humans, hair is largely decorative, but it also serves a protective function. Eyebrows and eyelashes protect the eyes from dust and sun, while nasal hairs block out foreign bodies from your nose. Scalp hair provides some temperature insulation.
The sebaceous glands
produce an oily substance called sebum. They are most prominent in the skin of the scalp, face, and upper trunk and are absent from the palms and soles. As part of the pilosebaceous unit, sebaceous glands secrete sebum that drains into the follicular canal and eventually onto the surface of the skin. Sebaceous glands increase in size and produce more sebum in response to increased hormone levels, specifically androgen, during adolescence. They play an important role in the development of acne.
The subcutaneous layer lies between the dermis and the underlying fascia covering muscle. This layer is made of
groups of adipocytes (fat cells) that are separated by fibrous septa. It serves three functions: to insulate the body from cold, to absorb trauma and cushion deeper tissues, and to act as storage for the body’s reserve fuel.
Nails are the only skin appendages that are not located in the dermis but instead are located at the ends of fingers and toes. They are used to grasp and pinch objects and, like hair, have a growing decorative function in our society. The nail plate is made of dead keratin, which forms a hard protective structure about 0.3-0.65 mm thick. Keratin is formed in the nail matrix by dividing epidermal cells. The nail bed is the epithelial layer that is tightly attached to the bottom of the nail plate. The blood vessels of the nail bed give nails their pink color. The proximal nail fold, or cuticle, protects the base of the nail from infection- causing organisms. Nails grow at an average rate of 0.1 mm per day, and toenails grow slower than fingernails.
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