6.3: Secondary Structures of the Skin

Secondary Structures of the Skin

Above: Illustrated diagram of the skin and underlying hypodermis with all of its associated secondary structures including hair, sweat glands, and sebaceous glands.

Hair

Hair is a keratinous filament growing out of the epidermis. It is primarily made of dead, keratinized cells. Strands of hair originate in an epidermal penetration of the dermis called the hair follicle. The hair shaft is the part of the hair not anchored to the follicle, and much of this is exposed at the skin’s surface. The rest of the hair, which is anchored in the follicle, lies below the surface of the skin and is referred to as the hair root. The hair root ends deep in the dermis at the hair bulb, and includes a layer of mitotically active basal cells called the hair matrix. The hair bulb surrounds the hair papilla, which is made of connective tissue and contains blood capillaries and nerve endings from the dermis.

Above: Illustration of a hair and associated structures and tissues.

Hair fibers have a structure consisting of several layers, starting from the outside:

• cuticle: outer covering, which consists of several layers of flat, thin cells laid out overlapping one another as roof shingles
• cortex: which contains the keratin bundles in cell structures that remain roughly rod-like. The highly structural and organized cortex, or second of three layers of the hair, is the primary source of mechanical strength and water uptake. The cortex contains melanin, which colors the fiber based on the number, distribution and types of melanin granules. The shape of the follicle determines the shape of the cortex, and the shape of the fiber is related to how straight or curly the hair is. People with straight hair have round hair fibers. Oval and other shaped fibers are generally more wavy or curly.
• medulla: a disorganized and open area at the fiber's center. The innermost region of the hair.

Above: Microscopic cross section of skin and hair. Tissue is magnified by 40x.

Hair growth begins inside the hair follicle. The only "living" portion of the hair is found in the follicle. The hair that is visible is the hair shaft, which exhibits no biochemical activity and is considered "dead". The base of a hair's root (the "bulb") contains the cells that produce the hair shaft. Other structures of the hair follicle include the oil-producing sebaceous gland which lubricates the hair and the arrector pili muscles, which are responsible for causing hairs to stand up. In humans with little body hair, the effect results in goose bumps.

Sweat Glands

There are two types of sweat glands, each secreting slightly different products:

Eccrine Sweat Glands

The eccrine sweat gland is a type of gland that produces a hypotonic sweat for thermoregulation. These glands are found all over the skin's surface, bus are especially abundant on the palms of the hand, the soles of the feet, and the forehead. They are coiled glands lying deep in the dermis, with the duct rising to a pore on the skin surface, where sweat is released. This type of sweat, released by exocytosis, is hypotonic and composed mostly of water, with some salt, antibodies, traces of metabolic waste, and dermicidin, an antimicrobial peptide.

Above: Illustration of skin cross section showing an eccrine sweat gland in the dermis and its duct traveling through the dermis and epidermis to a pore on the skin surface.

Above: Microscopic image of skin cross section showing eccrine sweat glands and ducts. Tissue is magnified by 100x.

Apocrine Sweat Glands

An apocrine sweat gland is usually associated with hair follicles in densely hairy areas, such as armpits and genital regions. Apocrine sweat glands are larger than eccrine sweat glands and lie deeper in the dermis, sometimes even reaching the hypodermis, with the duct normally emptying into the hair follicle. In additional to water and salts, apocrine sweat includes organic compounds that make the sweat thicker and subject to bacterial decomposition and subsequent smell. The release of this sweat is under both nervous and hormonal control, and plays a role in the poorly understood human pheromone response. Most commercial antiperspirants use an aluminum-based compound as their primary active ingredient to stop sweat. When the antiperspirant enters the sweat gland, the aluminum-based compounds precipitate due to a change in pH and form a physical block in the duct, which prevents sweat from coming out of the pore.

Sebaceous Glands

A sebaceous gland is a type of oil gland that is found all over the body and helps to lubricate and waterproof the skin and hair. Most sebaceous glands are associated with hair follicles. They generate and excrete sebum, a mixture of lipids, onto the skin surface, thereby naturally lubricating the dry and dead layer of keratinized cells on the stratum corneum keeping it pliable. The fatty acids of sebum also have antibacterial properties, and prevent water loss from thus kin in low-humidity environments. The secretion of sebum is stimulated by hormones, many of which do not become active until puberty. Thus, sebaceous glands are relatively inactive during childhood.

Above: Microscopic image of a sebaceous gland and its duct releasing sebum (oil secretion) onto a hair. Tissue is magnified by 100x.

Attributions

• "Anatomy and Physiology" by J. Gordon Betts et al., OpenStax is licensed under CC BY 4.0
• "Anatomy and Physiology I Lab" by Victoria Vidal is licensed under CC BY 4.0
• "BIOL 250 Human Anatomy Lab Manual SU 19" by Yancy Aquino, Skyline College is licensed under CC BY-NC-SA 4.0
• "Digital Histology" by Department of Anatomy and Neurobiology and the Office of Faculty Affairs, Virginia Commonwealth University School of Medicine and the ALT Lab at Virginia Commonwealth University is licensed under CC BY 4.0