Accessory Structures of the Skin

Accessory Structures of the Skin

• Hair, nails, sweat glands, and sebaceous glands are accessory structures of the skin.
• These structures develop from the epidermis and extend into the dermis and hypodermis.

Hair

• Hair is a keratinous filament growing out of the epidermis.
• Mainly made of dead, keratinized cells.
• Hair follicles are epidermal penetrations of the dermis where hair strands originate.
• The hair shaft is the part of the hair above the skin surface.
• The hair root is the part of the hair anchored in the follicle, below the skin surface.
• The hair bulb is the end of the hair root in the dermis, containing the hair matrix.
• The hair matrix is a layer of actively dividing basal cells.
• The hair bulb surrounds the hair papilla, composed of connective tissue with blood capillaries and nerve endings.
• The hair shaft is composed of three layers: medulla, cortex, and cuticle.
• The medulla is the central core, surrounded by the cortex, a layer of compressed, keratinized cells.
• The cuticle is the outermost layer of hard, keratinized cells.
• Hair texture depends on the shape and structure of the cortex and medulla.
• Hair growth begins with the production of keratinocytes by the hair bulb.
• New cells push the hair shaft up through the follicle towards the surface.
• Keratinization is completed as cells reach the skin surface.
• Hair is completely dead and composed of keratin, lacking sensation.
• Cutting or shaving hair only affects the shaft.
• Hair growth occurs in three phases: anagen, catagen, and telogen.
• The anagen phase is the active growth phase, lasting from 2 to 7 years.
• During the catagen phase, the hair follicle transitions to rest, lasting 2 to 3 weeks.
• The telogen phase is a resting phase lasting 2 to 4 months, without new growth.
• A new anagen phase follows the telogen phase, with new hair production pushing out the old.
• Hair grows at a rate of 0.3 millimeters per day during the anagen phase.
• Hair loss occurs when shedding exceeds replacement, due to hormonal changes, diet, aging, or hormone influence.

Nails

• The nail bed is a specialized structure of the epidermis at the tips of fingers and toes.
• The nail body forms atop the nail bed, protecting the tips of fingers and toes.
• The nail body is composed of densely packed dead keratinocytes.
• The nail body forms at the nail root, containing the nail matrix, a proliferating cell layer.
• The lateral nail fold overlaps the nail, anchoring the nail body.
• The cuticle is the nail fold meeting the proximal end of the nail body.
• The nail bed is highly vascular, making the nail appear pink, except for the lunula.
• The lunula is a crescent-shaped region at the nail base, where a thick epithelial layer covers the nail matrix.
• The hyponychium is the thickened layer of stratum corneum beneath the free edge of the nail.

Sweat Glands

• Sweat glands develop from epidermal projections into the dermis.
• Sweat glands are classified as merocrine glands, secreting through ducts without affecting the gland cells.
• There are two types of sweat glands: eccrine and apocrine.
• Eccrine sweat glands produce hypotonic sweat for thermoregulation.
• Eccrine glands are found on the skin surface, especially on palms, soles, and foreheads.
• Eccrine sweat glands are coiled glands in the dermis, with ducts leading to pores on the skin surface.
• Eccrine sweat is primarily water with salt, antibodies, metabolic waste, and dermcidin.
• Eccrine glands play a crucial role in thermoregulation and homeostasis.
• Apocrine sweat glands are associated with hair follicles in hairy areas like armpits and genitalia.
• Apocrine glands are larger than eccrine glands, lying deeper in the dermis or hypodermis.
• Apocrine sweat includes organic compounds, making it thicker and prone to odor upon bacterial decomposition.
• Apocrine sweat release is controlled by the nervous system and hormones, potentially playing a role in pheromones.
• Antiperspirants use aluminum-based compounds to block sweat ducts and reduce sweat production.