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DERIVATIVES OF SKIN

Derivatives of skin include hair, sebaceous glands, sweat glands, and nails (see Graphic 11-2). These structures originate from epidermal downgrowths into the dermis and hypodermis, while maintaining their connection to the outside.

•Each hair is composed of a shaft of cornified cells and a root contained within a hair follicle

is associated with a sebaceous gland that secretes an oily sebum into the neck of the hair follicle.

A small bundle of smooth muscle cells, the arrector pili muscle, attaches to the hair follicle and, cradling the sebaceous gland, inserts into the superficial aspects of the skin.

•Sweat glands do not develop in association with hair follicles.These are simple, coiled, tubular glands whose secretory units produce sweat, which is delivered to the surface of the skin by long ducts.

Myoepithelial cells surround the secretory portion of these glands.

•Nails are cornified structures on the distal phalanx of each finger and toe. These horny plates lie on a nail bed and are bounded laterally by a nail wall.

The cuticle (eponychium) lies over the lunula, an opaque, crescent-shaped area of the nail plate.

The hyponychium is located beneath the free edge of the nail plate.

Itching (Pruritis)

The sensation of itching is accompanied by an instinctive, almost irrepressible urge to scratch. There are many different causes of itching, some as simple as a fly walking on one’s skin and moving the hair follicles, or as serious as debilitating systemic conditions such as kidney failure or liver disease. If the itching is accompanied by a rash, then the probable cause is not the kidney or the liver. Parasitic infestations (mites, scabies, etc.), insect bites, plant toxins (such as poison oak and poison ivy), and drug allergies are usually accompanied by a rash and require medical intervention. If the itching is long-term, the patient should seek the assistance of a physician. Pregnancy and cold, dry weather may also be contributing factors to itching.

Psoriasis Vulgaris

Psoriasis vulgaris is a commonly occurring condition characterized by reddish patchy lesions on the skin with grayish sheen, located especially around joints, sacral region, the navel, and the scalp. This condition is produced by increased proliferation of keratinocytes and an acceleration of the cell cycle, resulting in an accumulation of cells in the stratum corneum but with an absence of a stratum granulosum and, frequently, the presence of lymphocytic infiltrates in the papillary layer. The condition is cyclic and is of unknown etiology.

This photomicrograph is of a patient suffering from psoriasis vulgaris. Note that the stratum spinosum and stratum corneum are thickened and that the stratum granulosum is absent. The papillary layer of the dermis displays an infiltration by lymphocytes. (Reprinted with permission from Mills SE, Carter D, et al., eds. Sternberg’s Diagnostic Surgical Pathology, 5th ed., Philadelphia, Lippincott, Williams & Wilkins, 2010,. p. 6.)

Erythema Multiforme

Patches of elevated red skin, frequently resembling a target, displaying a symmetrical distribution over the face and extremities, that occurs periodically indicate the disorder erythema multiforme. It is most frequently due

to herpes simplex infection. The condition is not usually accompanied by itching, although painful lesions (blisters) on the lips and buccal cavity are common occurrences. Usually the condition resolves itself, but in more severe cases, medical intervention is indicated.

Warts

Warts are benign epidermal growths on the skin caused by papilloma viral infection of the keratinocytes. Warts are common in young children, in young adults, and in immunosuppressed patients.

Vitiligo

A condition in which the skin has patches of white areas due to the lack of pigmentation is known as vitiligo. The melanocytes of the affected region are destroyed in an autoimmune response. The condition may appear suddenly after a physical injury or as a consequence of sunburn. If the area affected has hair, as the hair grows it will be white. Although there are no physical consequences to vitiligo, there may be psychological sequelae.

Malignancies of Skin

The three most common malignancies of skin are basal cell carcinoma, squamous cell carcinoma, and malignant melanoma.

Basal cell carcinoma, the most common human malignancy, develops in the stratum basale from

damage caused by ultraviolet radiation. The foremost type of basal cell carcinoma is the nodulocystic type where small hyperchromatic cells form spherical nodules that are separated from the surrounding connective tissue elements of the dermis by narrow spaces. The most frequent site of basal cell carcinoma is on the nose, occurring as papules or nodules, which eventually craters. Surgery is usually 90% effective with no recurrence.

Squamous cell carcinoma, the second most frequent skin malignancy, is invasive and metastatic. Its probable etiology is environmental factors, such as ultraviolet radiation and x-irradiation, as well as a variety of chemical carcinogens, including arsenic. The carcinoma originates in cells of the stratum spinosum and appears clinically as a hyperkeratotic, scaly plaque with deep invasion of underlying tissues, often accompanied by bleeding. Surgery is the treatment of choice.

Malignant melanoma may be a life-threatening malignancy. It develops in the epidermis where melanocytes become mitotically active and form a dysplastic nevus. It may then enter a radial-growth phase where individual melanocytes invade the dermis, then enter the vertical growth phase where they begin to form tumors in the dermis, and eventually become a full-fledged, metastatic melanoma whose cells eventually enter the lymphatic and circulatory system to metastasize to other organ systems.

This photomicrograph is of a patient with basal cell carcinoma. Note that the lesion is composed of dark, dense basal cells that form rounded nodules that are separated from the dermal connective tissue by narrowed spaces. (Reprinted with permission from Mills SE, Carter D, et al., eds. Sternberg’s Diagnostic Surgical Pathology, 5th ed., Philadelphia, Lippincott, Williams & Wilkins, 2010. p. 49.)

This photomicrograph is of a patient suffering from malignant melanoma. Note that the melanocytes are invading the dermis in large numbers, indicating that the melanoma is in the vertical growth phase. (Reprinted with permission from Mills SE, Carter D, et al., eds. Sternberg’s Diagnostic Surgical Pathology, 5th ed., Philadelphia, Lippincott, Williams & Wilkins, 2010. p. 92.)

Derivatives Its and Skin • 1-11 GRAPHIC

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Epidermis

Stratum corneum

Stratum spinosum

Stratum basale

Dermis

Papillary layer Reticular layer

Eccrine sweat gland

Apocrine sweat gland

Root hair plexus

Hypodermis

(superficial fascia)

Meissner’s corpuscle

Hair shaft

Sebaceous (oil) gland

Arrector pili muscle

Eccrine sweat gland

Apocrine sweat gland

Hair follicle

Hair root

Pacinian corpuscle

Artery

Vein

Adipose tissue of hypodermis

Glands Sebaceous and Glands, Sweat Hair,• 2-11 GRAPHIC

Skin hick • T1-11 PLATE

CL

4 SB

N

S SG

PL

SS

SC

M

SS

SL

SB

SG

d SC

Skin hick • T1-11 PLATE

Skin hin • T2-11 PLATE

HF

E

D

d

swG

AP

B

FIGURE 1

SC

E

PL

IP

CF

N

AP

RL

BV

FIGURE 2

H

sG

SC

SS

sDR

CL

SB

CF D

FIGURE 3

Skin hin • T2-11 PLATE

Glands Sweat Structures, Associated and ollicles F Hair• 3-11 PLATE

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FIGURE 1. Hair follicle. l.s. Human. Paraffin section. ×132.

The terminal expansion of the hair follicle, known as the bulb, is composed of a connective tissue, papilla (P), enveloped by epithelially derived cells of the hair root (HR). The mitotic activity responsible for the growth of hair occurs in the matrix, from which several concentric sheaths of epithelial cells emerge to be surrounded by a connective tissue sheath (CTS). Color of hair is due to the intracellular pigment that accounts for the dark appearance of some cells (arrow).

FIGURE 3. Sebaceous gland. Human. Paraffin section. ×132.

Sebaceous glands (sG) are branched, acinar holocrine glands, which produce an oily sebum. The secretion of these glands is delivered into the lumen of a hair follicle (HF), with which sebaceous glands are associated. Basal cells (BC), located at the periphery of the gland, undergo mitotic activity to replenish the dead cells, which, in holocrine glands, become the secretory product. Note that as these cells accumulate sebum in their cytoplasm, they degenerate, as evidenced by the gradual pyknosis of their nuclei (N). Observe the arrector pili muscle (AP), which cradles the sebaceous glands.

FIGURE 2. Hair follicle. x.s. Human. Paraffin section. ×132.

Many of the layers comprising the growing hair follicle may be observed in these cross sections. The entire structure is surrounded by a connective tissue sheath (CTS), which is separated from the epithelially derived components by a specialized basement membrane, the inner glassy membrane (BM). The clear polyhedral cells compose the external root sheath (ERS), which surrounds the internal root sheath (IRS), whose cells become keratinized. At the neck of the hair follicle, where the ducts of the sebaceous glands enter, the internal root sheath disintegrates, providing a lumen into which sebum and apocrine sweat are discharged. The cuticle (Cu) and cortex (Co) constitute the highly keratinized components of the hair, whereas the medulla is not visible at this magnification. Note the presence of arrector pili muscle (AP).

FIGURE 4. Sweat gland. Monkey. Plastic section. ×132.

The simple, coiled, tubular eccrine gland is divided into two compartments: a secretory portion (s) and a duct (d). The secretory portion of the gland consists of a simple cuboidal epithelium, composed of dark and clear secretory cells (which cannot be distinguished from each other unless special procedures are utilized). Intercellular canaliculi are noted between clear cells, which are smaller than the lumen (L) of the gland. Ducts (d) may be recognized readily since they are darker staining and composed of stratified cuboidal epithelium. Insets a and b. Duct and secretory unit. Monkey. Plastic section. ×540. The duct is readily evident, since its lumen (L) is surrounded by two layers of cuboidal cells. Secretory cells (s) of the eccrine sweat gland are surrounded by darker-staining myoepithelial cells (My). Hair root, eccrine sweat gland, and sebaceous gland.

Co AP

CTS

BM

ERS

P

HR

s

s

N

d

b)

AP

sG

s

s

L

HF

d

a)

Glands Sweat Structures, Associated and ollicles F Hair• 3-11 PLATE

Corpuscles Meissner’s and Pacinian Nail,• 4-11 PLATE

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FIGURE 1. Fingernail. l.s. Paraffin section. ×14.

The nail is a highly keratinized structure that is located on the dorsal surface of the distal phalanx (Ph) of each finger and toe. The horny nail plate (NP) extends deep into the dermis, forming the nail root (NR). The epidermis of the distal phalanx forms a continuous fold, resulting in the eponychium (Ep) or cuticle, the nail bed (NB) underlying the nail plate, and the hyponychium (Hy). The epithelium (arrow) surrounding the nail root is responsible for the continuous elongation of the nail. The dermis (D) between the nail bed and the bone (Bo) of the distal phalanx is tightly secured to the fibrous periosteum (FP). Note that this is a developing finger, as evidenced by the presence of hyaline cartilage (HC) and endochondral osteogenesis (arrowheads).

FIGURE 3. Meissner’s corpuscle. Paraffin section. ×540.

Meissner’s corpuscles are oval, encapsulated mechanoreceptors lying in dermal ridges just deep to the stratum basale (SB). They are especially prominent in the genital areas, lips, fingertips, and soles of the feet. A connective tissue capsule (Ca) envelops the corpuscle. The nuclei (N) within the corpuscle belong to flattened (probably modified) Schwann cells, which are arranged horizontally in this structure. The afferent nerve fiber (NF) pierces the base of Meissner’s corpuscle, branches, and follows a tortuous course within the corpuscle.

FIGURE 2. Fingernail. x.s. Paraffin section. ×14.

The nail plate (NP) in cross section presents a convex appearance. On either side, it is bordered by a nail wall (NW), and the groove it occupies is referred to as the lateral nail groove (NG). The nail bed (NB) is analogous to four layers of the epidermis, whereas the nail plate represents the stratum corneum. The dermis (D), deep to the nail bed, is firmly attached to the fibrous periosteum (FP) of the bone (Bo) of the terminal phalanx. Observe that the fingertip is covered by thick skin whose stratum corneum (SC) is extremely well developed. The small, darkly staining structures in the dermis are sweat glands (swG).

FIGURE 4. Pacinian corpuscle. Paraffin section. ×132.

Pacinian corpuscles, located in the dermis and hypodermis, are mechanoreceptors. They are composed of a core with an inner (IC) and an outer (OC) region, as well as a capsule (Ca) that surrounds the core. The inner core invests the afferent nerve fiber (NF), which loses its myelin sheath soon after entering the corpuscle. The core cells are modified Schwann cells, whereas the components of the capsule are continuous with the endoneurium of the afferent nerve fiber. Pacinian corpuscles are readily recognizable in section since they resemble the cut surface of an onion. Observe the presence of an arrector pili muscle (AP) and profiles of ducts (d) of a sweat gland in the vicinity of, but not associated with, the pacinian corpuscle.

FIGURE 1

SB

Ca N

NF

FIGURE 3

Corpuscles Meissner’s and Pacinian Nail,• 4-11 PLATE

FIGURE 2

AP

d

IC Ca

OC

NF

FIGURE 4

FIGURE 1. Sweat gland. x.s. Human. Electron microscopy. ×5,040.

Tight junctions (arrows) occur at three locations in the secretory coil of human sweat glands: (1) between clear cells (C) separating the lumen of the intercellular canaliculus (arrowhead) and the basolateral intercellular space, (2) between

two dark cells (D) separating the main lumen and the lateral intercellular space, and (3) between a clear cell and a dark cell, separating the main lumen (L) and intercellular space. Note the presence of secretory granules (SG) and myoepithelial cell

(ME). (From Briggman JV, Bank HL, Bigelow JB, et al. Structure of the tight junctions of the human eccrine sweat gland. Am J Anat 1981;162:357–368.)