Accessory Structures of the Skin

Nails have a variety of functions.

1. They protect the distal end of the digits. 2. They provide support and counterpressure to the palmar surface of the fingers to enhance touch perception and manipulation. 3. They allow us to grasp and manipulate small objects, and they can be used to scratch and groom the body in various ways.

External Root Sheath

A downward continuation of the epidermis.

Hair Matrix

A germinal layer of cells found in the bulb. The hair matrix cells arise from the stratum basale, the site of cell division. Hence, hair matrix cells are responsible for the growth of existing hairs, and they reproduce new hairs when old hairs are shed. This replacement process occurs within the same follicle. Hair matrix cells also give rise to the cells of the internal root sheath.

Eponychium or Cuticle

A narrow band of epidermis that extends from and adheres to the margin (lateral border) of the nail wall. It occupies the proximal border of the nail and consist of stratum corneum.

Papilla of the Hair

A nipple-shaped indentation found in the bulb which contains areolar connective tissue and many blood vessels that nourish the growing hair follicle.

Hyponychium

A thickened region of stratum corneum beneath the free edge which secures the nail to the fingertip.

Epithelial Root Sheath

Composed of an external root sheath and an internal root sheath.

Anatomy of a Hair

Each hair is composed of columns of dead, keratinized, epidermal cells bonded together by extracellular proteins.

Emotional Sweating or a Cold Sweat

Eccrine sweat glands also release sweat in response to an emotional stress such as fear or embarrassment. This type of sweating is referred to as emotional sweating or a cold sweat. In contrast to the thermoregulatory sweating, emotional sweating first occurs on the palms, soles, and axillae and then spreads to other areas of the body.

Skin Glands

Glands are epithelial cells that secrete a substance. Several kinds of exocrine glands are associated with the skin: sebaceous (oil) glands, sudoriferous (sweat) glands, and ceruminous glands.

Accessory Structures of the Skin

Hair, skin glands, and nails develop from the embryonic epidermis.

Hair

Hairs, or pili, are present on most skin surfaces except the palms, palmar surfaces of the fingers, the soles, and plantar surfaces of the feet.

Ceruminous Sweat Glands

Modified sweat glands in the external ear produce a waxy lubricating secretion. The secretory portions of ceruminous sweat glands lie in the subcutaneous layer, deep to sebaceous glands. Their excretory ducts open either directly onto the surface of the external auditory canal (ear canal) or into ducts of sebaceous glands. The combined secretion of the ceruminous and sebaceous glands is a yellowish material called cerumen.

Free Edge

Part of the nail body that may extend past the distal end of the digit. The free edge is white because there are no underlying capillaries.

Nails

Plates of tightly packed, hard, dead, keratinized epidermal cells that form a clear, solid covering over the dorsal surfaces of the distal portions of the digits. Each nail consists of a nail body, a free edge, and a nail root.

Internal Root Sheath

Produced by the matrix and forms a cellular tubular sheath of epithelium between the external root sheath and the hair.

Sebum

Sebaceous glands secrete an oily substance called sum, a mixture of triglycerides, cholesterol, proteins, and inorganic salts. Sebum coats the surface of hairs and helps keep them from drying and becoming brittle. Sebum also prevents excessive evaporation of water from the skin, keeps the skin soft and pliable, and inhibits the growth of some (but not all) bacteria.

Sebaceous Glands or Oil Glands

Simple, branched acinar (rounded) glands. With few exceptions, they are connected to hair follicles. The secreting portion of a sebaceous gland lies in the dermis and usually opens into the neck of a hair follicle. In some locations, such as the lips, glans penis, labia minora, and tarsal glands of the eyelids, sebaceous glands open directly onto the surface of the skin. Absent in the palms and soles, sebaceous glands are small in most areas of the trunk and limbs, but large in the skin of the breasts, face, neck, and superior chest.

Apocrine Sweat Glands

Simple, coiled tubular glands but have larger ducts and lumens than eccrine glands. They are found mainly in the skin of the axilla (armpit), groin, areolae (pigmented areas round the nipples) of the breasts, and bearded regions of the face in adult males. The secretory portion of these sweat glands is located in the lower dermis or upper subcutaneous layer, and the excretory duct opens into hair follicles. In contrast to eccrine sweat glands, apocrine sweat glands are not active during thermoregulatory sweating and, therefore, do not play a role in thermoregulation.

Eccrine Sweat Glands

Simple, coiled tubular sweat glands that are much more common than apocrine sweat glands. They are distributed throughout the skin of most regions of the body, especially in the skin of the forehead, palms, and soles. Eccrine sweat glands are not present, however, in the margins of the lips, nail beds of the fingers and toes, glans penis, glans clitoris, labia minora, or eardrums. The secretory portion of eccrine sweat glands is mostly located in the deep dermis (sometimes in the upper subcutaneous layer). The excretory dict projects through the dermis and epidermis and ends as a pore at the surface of the epidermis.

Hair Root Plexus

Surrounding each hair follicle are dendrites of neurons that form a hair root plexus. The hair root plexus generates nerve impulses if the hair shafts are moved.

Hair Follicle

Surrounding the root of the hair is the hair follicle, which is made up of an external root sheath and an internal root sheath, together referred to as an epithelial root sheath.

Insensible Perspiration

Sweat that evaporates from the skin before it is perceived as moisture.

Sensible Perspiration

Sweat that is excreted in larger amounts and is seen as moisture on the skin.

Nail Matrix

The portion of the epithelium proximal to the nail root. The superficial nail matrix cells divide mitotically to produce new nail cells. The growth rate of nails is determined by the rate of mitosis in matrix cells, which is influenced by factors such as a person's age, health, and nutritional status. Nail growth also varies according to the season, the time of day, and environmental temperature. The growth rate is somewhat slower in toenails. The longer the digit the faster the nail grows.

Bulb

The base of each hair follicle and its surrounding dermal root sheath is an onion-shaped structure.

Cerumen

The combined secretion of the ceruminous and sebaceous glands is a yellowish material called cerumen. Cerumen, together with hairs in the external auditory canal, provides a sticky barrier that impedes the entrance of foreign bodies and insects. Cerumen also waterproofs the canal and prevents bacteria and fungi from entering cells.

Cuticle

The cuticle of the hair, the outermost layer, consists of a single layer of thin, flat cells that are the most heavily keratinized. Cuticle cells on the shaft are arranged like shingles on the side of a house, with their free edges pointing toward the end of the hair.

Dermal Root Sheath

The dense dermis surrounding the hair follicle.

Thermoregulation

The homeostatic regulation of body temperature.

Medulla

The inner medulla, which may be lacking in thinner hair, is composed of two or three rows of irregularly shaped cells that contain large amounts of pigment granules in dark hair, small amounts of pigment granules in gray hair, and a lack of pigment granules and the presence of air bubbles in white hair.

Cortex

The middle cortex forms the major part of the shaft and consists of elongated cells.

Nail Root

The portion of the nail that is buried ion a fold of skin.

Root

The root is the portion of the hair deep to the shaft that penetrates into the dermis, and sometimes into the subcutaneous layer.

The shaft and root of the hair.

The shaft and root of the hair both consist of three concentric layers of cells: medulla, cortex, and cuticle of the hair.

Shaft

The shaft is the superficial portion of the hair, which projects above the surface of the skin.

Nail Bed

The skin below the nail plate that extends from the lunula to the hyponychium. The epidermis of the nail bed lacks a stratum granulosum.

Arrector Pili

The smooth muscle. It extends from the superficial dermis of the skin to the dermal root sheath around the side of the hair follicle. In its normal position, hair emerges at a less than ninety degree angle to the surface of the skin. Under physiological or emotion stress, such as cold or fright, autonomic nerve endings stimulate the arrector pili muscles to contract, which pulls the hair shafts perpendicular to the skin surface. This action causes "goose bumps" or "gooseflesh" because the skin around the shaft forms slight elevations.

Nah Body (plate)

The visible portion of the nail. It is comparable to the stratum corneum of the epidermis of the skin, with the exception that its flattened, keratinized cells fill with a harder type of keratin and the cells are not shed. Below the nail body is a region of epithelium and a deeper layer of dermis. Most of the nail body appears pink because of blood flowing through the capillaries in the underlying dermis.

Lunula

The whitish, crescent-shaped area of the proximal end of the nail bed. It appears whitish because the vascular tissue underneath does not show through due to a thickened region of epithelium in the area.

Sudoriferous Glands or Sweat Glands

There are three million to four million sudoriferous glands, or sweat glands in the body. The cells of these glands release sweat, or perspiration, into hair follicles or onto the skin surface through pores. Sweat glands are divided into two main types, eccrine and apocrine, based on their structure and type of secretion.

Thermoregulatory Sweating

This role of eccrine sweat glands in helping the body to achieve thermoregulation. During thermoregulatory sweating, sweat first forms on the forehead and scalp and then extends to the rest of the body, forming last on the palms and soles.