Cui Dongmei. Atlas of Histology: with functional and clinical correlations. 1st ed

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Figure 13-2. Overview of the layers of the epidermis.

The epidermis is composed of ﬁ ve cell layers. (1) The stratum basale is composed of a single layer of cuboidal or tall cuboidal cells,

melanocytes and Merkel cells which are also called Merkel cell neurite complexes or Merkel disks. Many of these cells are actually

stem cells; they divide continuously and migrate from the basal layer toward the surface and give rise to keratinocytes in the

other layers. (2) The stratum spinosum contains polygon-shaped keratinocytes with many tonoﬁ lament bundles in their cytoplasm.

These cells are interconnected with each other by desmosomes. Langerhans cells are often found in this layer. The stratum basale

and the stratum spinosum are the only layers with mitotically active cells, and, together they are also called the Malpighian layer.

(3) The stratum granulosum contains three to ﬁ ve layers of keratinocytes with ﬂ attened nuclei. The cytoplasm of the cells is ﬁ lled

with basophilic keratohyalin granules from which the name derives. The cytoplasm also contains lamellar granules, which can

release their contents into the intercellular spaces to help seal the skin, preventing water loss. This layer is more obvious in thick

skin; only a single cell layer is visible in thin skin. (4) The stratum lucidum is a very thin, clear layer that contains keratinocytes with

pycnotic nuclei. This layer is found only in thick skin. (5) The stratum corneum is the top layer of the epidermis. It contains many

layers of ﬂ attened cells ﬁ lled with mature keratin. These are dead cells with no nuclei or organelles. The cells in this layer, which are

constantly replaced by cells from deeper layers, form a barrier to prevent loss of water and entry of pathogens. This layer is much

thicker in thick skin than in thin skin.

SYNOPSIS 13-1 Functions of the Skin

Protection ■ of body from invasion of pathogens; prevention of tissue damage by toxic chemicals and ultraviolet light

Prevention

■ of dehydration and loss of body ﬂ uids (impermeable to water)

Regulation

■ of body temperature (production and excretion of sweat, vascular shunts)

Sensation

■ of touch, pain, temperature, pressure, and vibration; important for communication, dexterity, and injury

prevention

Immunological function

■ of Langerhans cells (antigen-presenting cells) present antigens to lymphocytes in the immune

responses (See Chapter 10, “Lymphoid System.”)

Production

■ of vitamin D from precursors under the effects of steroids and sunlight

5. Stratum

corneum

3. Stratum

granulosum

Vein

Artery

Nerve

2. Stratum

spinosum

Merkel cell (Merkel disk)

Langerhans cell

Horny cell (dead cell)

Flattened keratinocyte

Keratinocyte with

keratohyalin granules

Keratinocyte

Sensory nervous fiber

1. Stratum

basale

Melanocyte

Nerve fiber

4. Stratum

lucidum

Small blood vessels

Epidermis

Dermis

Epidermis

Dermis

Hypodermis

(subcutaneous layer)

Reticular

layer

Papillary

layer

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Organ Systems

Figure 13-3A. Thick skin, palm. H&E, 34; inset 116

The skin can be classiﬁ ed into thick skin and thin skin based on

the thickness of the epidermis. Thick skin has a thick epidermis

(400–600 μm) with ﬁ ve distinct cell layers. The stratum corneum

is extremely thick in this skin. Thick skin covers the palms of

the hands and soles of the feet. Thick skin has abundant eccrine

sweat glands and lacks hair follicles. The epidermis is a stratiﬁ ed

squamous epithelium. Because it is an avascular tissue (no direct

blood supply), nutrients are delivered to the tissue by ﬂ uid diffu-

sion from the dermis (connective tissue). The dermis is composed

of a superﬁ cial papillary layer, a layer of loose connective tissue,

and a deeper reticular layer, which is a thick layer of dense irregu-

lar connective tissue. This section shows thick epidermis containing

the duct of an eccrine sweat gland.

Duct of eccrine

sweat gland

Duct of eccrine

sweat gland

Duct of eccrine

sweat gland

Duct of eccrine

sweat gland

Stratum

corneum

Stratum

corneum

Epidermis

Reticular

layer

Reticular

layer

Papillary

layer

Papillary

layer

Dermis

Figure 13-3B. Layers of the epidermis, palm. H&E, 68;

insets 422

The epidermis of thick skin has ﬁ ve layers. (1) The stratum basale

contains a single layer of cuboidal/tall cuboidal cells (stem cells)

and sits upon the basement membrane. This layer forms a dividing

line between the epidermis and dermis (dotted line). (2) The stra-

tum spinosum contains 5 to 10 layers of polyhedral keratinocytes,

ﬂ attened toward the surface. These cells are also called prickle

cells. (3) The stratum granulosum contains three to ﬁ ve layers of

ﬂ attened keratinocytes ﬁ lled with keratohyaline granules, which

appear dark blue here. (4) The stratum lucidum is a very thin layer

containing extremely ﬂ attened and tightly packed keratinocytes

ﬁ lled with keratin ﬁ laments. Their nuclei are beginning to be elimi-

nated. (5) The stratum corneum is a layer of dead, nonnucleated

cells, which form the most superﬁ cial layer of the skin. Cells in this

layer are constantly sloughed off and replaced by new cells.

Dermal

papilla

Dermal

papilla

Epidermal

rete ridge

Epidermal

rete ridge

Stratum

corneum

Stratum

corneum

Stratum

lucidum

Stratum

lucidum

Stratum

granulosum

Stratum

granulosum

Stratum

spinosum

Stratum

spinosum

Stratum

basale

Stratum

basale

Papillary

layer

Papillary

layer

Dermis

Reticular

layer

Reticular

layer

Stratum

granulosum

Stratum

granulosum

Stratum

spinosum

Stratum

spinosum

Stratum

lucidum

Stratum

lucidum

Figure 13-3C. Dermal papilla, palm. H&E, 272; inset

192

The border between the epidermis and dermis (dotted white line) is

expanded into folds. The dermis is a connective tissue layer, which

contains blood vessels, nerves, and sensory receptors (free nerve

endings and Meissner corpuscles). The portion of the epidermis

that projects into the dermis is termed the epidermal rete ridge, and

the portion of the dermis that projects into the epidermis is called

the dermal papilla. This unique feature increases the contact area

between these two layers, preventing the epidermis from detaching

from the dermis. The dermal papilla contains loose connective tis-

sue that includes many capillaries, free nerve endings, and encapsu-

lated sensory receptors. Meissner corpuscles are shown here. The

nerve ﬁ bers cannot be seen in H&E stains; demonstration of nerve

ﬁ bers requires special stains (Fig. 13-1; see also Chapter 7, “Ner-

vous Tissue,” Fig. 7-8A,B). Meissner corpuscles are responsible for

discriminative touch and are more numerous in thick skin such

as at the tips of the ﬁ ngers. These receptors help us to distinguish

between, for example, different coins by touch alone.

Dermal

papilla

Dermal

papilla

Meissner

corpuscle

Meissner

corpuscle

Meissner

corpuscles

Meissner

corpuscles

Epidermal

rete ridge

Epidermal

rete ridge

Dermis

Thick Skin

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Thin Skin

Figure 13-4A. Thin skin, scalp. H&E, 25; inset 84

Thin skin covers the entire body surface except for the palms of

the hands and the soles of the feet. Thin skin has a thin epidermis,

largely because its stratum corneum is much reduced compared to

that of thick skin. In contrast to thick skin, thin skin contains hair

follicles and their associated sebaceous glands. This section shows

the epidermis and dermis of the skin and a deeper layer of sub-

cutaneous tissue called the hypodermis. The hypodermis is a layer

of loose connective tissue, which contains adipose tissue, nerves,

arteries, and veins. The nerves give off branches, which provide the

various types of sensory and autonomic nerve endings in the dermis.

Pacinian corpuscles, sensory receptors that respond to vibration

stimuli, are found in the hypodermis of both thin and thick skin.

They are found in many regions of the body but are more numerous

in the tips of the ﬁ ngers and toes than in other areas (Fig. 13-1). The

hypodermis serves as a transition layer, providing the dermis with a

ﬂ exible attachment to the underlying muscles and other structures.

Sebaceous

glands

Sebaceous

glands

Arrector

pili muscles

Arrector

pili muscles

Duct of the

sweat glands

Duct of the

sweat glands

Skin

Epidermis

Dermis

Hypodermis

Pacinian

corpuscle

Pacinian

corpuscle

Hair

follicles

Hair

follicles

Adipose

tissue

Adipose

tissue

Eccrine

sweat

glands

Eccrine

sweat

glands

Figure 13-4B. Thin skin. Elastic ﬁ ber stain, 142; inset 487

The epidermis of thin skin consists of four layers, including

the stratum basale, stratum spinosum, stratum granulosum, and

stratum corneum. (The stratum lucidum is absent in thin skin.)

The stratum granulosum is very thin, often only a single cell layer,

and it is not easily distinguished in thin skin. The stratum cor-

neum is thin but varies in thickness from region to region. This

section is stained with an elastic ﬁ ber stain, which shows the elas-

tic ﬁ bers in the dermis. These ﬁ bers become very ﬁ ne toward the

epidermis. The dermis contains type I collagen ﬁ bers and elas-

tic ﬁ bers, which give the skin ﬂ exibility and strength. The inset

shows a few very ﬁ ne ﬁ bers called oxytalan ﬁ bers. The elastic

ﬁ bers can be classiﬁ ed into three types based on their microﬁ -

bril and elastin content: (1) elastic ﬁ bers, the largest ﬁ bers, con-

taining predominantly elastin; (2) elaunin ﬁ bers, intermediate

in size, containing small amounts of amorphous elastin; and (3)

oxytalan ﬁ bers, the smallest ﬁ bers, containing only microﬁ brils.

Oxytalan

fibers

Oxytalan

fibers

Elastic fibers

Stratum basale

Elaunin fibers

Stratum

spinosum

Stratum corneum

Figure 13-4C. Stratum corneum, thin skin. Elastic ﬁ ber stain, 284

Fine grooves (sulci cutis) and elevated areas (cristae cutis) are the basis of

the varying surface contours characteristic of speciﬁ c areas of both thin skin

and thick skin. The orientation of the grooves varies from region to region.

Fingerprints (dermatoglyphics) are a good example of a skin pattern, which

is distinctive. The top layer of the epidermis, the stratum corneum, is com-

posed of several layers of ﬂ attened and corniﬁ ed keratinocytes. These cells

have no nuclei and are ﬁ lled with keratin, which helps to stabilize the cells

against physical stress. This layer of cells is constantly sloughed off and

replaced by differentiating cells from beneath. In this section, the extensive

spaces between the dead cells of the stratum corneum are artifacts of speci-

men preparation. Some of the cuboidal cells in the stratum basale are stem

cells capable of cell division. Some cells derived by division of the stem cells

remain in the stratum basale as stem cells and some begin differentiation

in the stratum spinosum. Keratinocytes undergo an orderly sequence of

differentiation (keratinization) and cell death (apoptosis) as they move up

toward the surface of the epidermis.

Collagen

fibers

Collagen

fibers

Cuboidal cells

in stratum basale

Cuboidal cells

in stratum basale

Elastic

fibers

Elastic

fibers

Capillary

Keratinocytes

in stratum

spinosum

Keratinocytes

in stratum

spinosum

Sulci cutis

Stratum corneum

Crista cutis

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Organ Systems

CLINICAL CORRELATION

Figure 13-5B.

Squamous Cell Carcinoma. H&E,

108

Squamous cell carcinoma (SCC) is the second most

common form of skin neoplasm. It originates from

keratinocytes of the epidermis. This carcinoma is

characterized by a slow-growing reddish or ulcerated

lesion with hard, raised borders. It is often found in

sun-exposed areas, with a high incidence in elderly

male Caucasians. Prolonged sun exposure, chronic

inﬂ

ammatory lesions, and genetic factors, especially

p53 tumor suppressor gene mutations, contribute to

the development of the disease. SCCs called actinic

keratoses commonly arise in premalignant lesions on

sun-damaged skin. Carcinoma cells have enlarged and

hyperchromatic nuclei with variable differentiation,

some lesions producing abundant keratin. Treatment

includes surgical excision, cryosurgery, electrosurgery,

radiation therapy, and topical treatment.

Eosinophilic

squamous

cell carcinoma

cells

Figure 13-5A. Stratum corneum of the epidermis, thin skin. EM, 8,065; inset 21,889

The stratum corneum is the ﬁ nal product of the proliferation and differentiation that take place in the deeper layers of the epidermis. The

dead cells (horny cells) of the stratum corneum have lost the usual organelles and have become ﬁ lled with mature keratin, a tough net-

work of keratin intermediate ﬁ laments that are cross-linked by the protein ﬁ laggrin. Keratin and the persisting desmosomes between cells

account for the mechanical strength of the epidermis. Contributing to the relative impermeability of the stratum corneum is a coating of

involucrin linked to the inner surfaces of the plasma membrane and the presence of lipid that has been secreted into the spaces between

cells. In this specimen of thin skin, the stratum granulosum is only one cell thick, and some of its characteristic features (keratohyalin gran-

ules and lamellar granules) are not clearly discernible. Bundles of tonoﬁ laments, melanin granules, and desmosomes can be distinguished.

Remaining

desmosomes

Remaining

desmosomes

Nucleus of stratum

spinosum cell

Cornified keratinocytes

Cornified envelope

Stratum granulosum

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Figure 13-6A. Keratinocytes in stratum spinosum, thin skin. EM, 7,097 (left); 12,390 (right)

The stratum spinosum gets its name from the many small processes that seem to join neighboring cells with one another. The basis for

these spines is obvious in this transmission electron micrograph. Each cell is joined to its neighbors by numerous maculae adherens

(desmosomes), and the spines reﬂ ect the persistence of these connections after some cell shrinkage has occurred during processing of

the tissue. The electron dense tuft of material evident on either side of each desmosome is a bundle of tonoﬁ laments anchored into

the attachment plaques at the cytoplasmic faces of the desmosomes. The desmosomes and tonoﬁ lament bundles are numerous and

more easily seen in the higher magniﬁ cation view in the right-hand panel. As indicated by the extensive euchromatin in the nuclei of

these cells, the keratinocytes are actively synthesizing proteins, most prominently subunits of keratin ﬁ laments, which will function

in establishing a tough, impermeable barrier layer at the surface of the skin.

Desmosomes

Keratin filaments

Keratin

filaments

Keratin

filaments

Desmosomes

Melanin

granules

Melanin

granules

Melanin

granules

Euchromatin

Heterochromatin

Keratinocyte

Keratinocytes in

stratum spinosum

CLINICAL CORRELATION

Figure 13-6B.

Basal Cell Carcinoma. H&E, 50

Basal cell carcinoma is the most common form of malig-

nant skin neoplasm. It originates from the basal layer

of epidermis and often occurs on sun-exposed areas.

Basal cell carcinoma rarely metastasizes and is usually

non–life-threatening if addressed early. Local invasion

may damage surrounding tissues causing cosmetic con-

cerns. Genetics and long-term exposure to ultraviolet

light and arsenical compounds contribute to the dis-

ease. Clinically, basal cell carcinoma appears as pearly

white nodules or waxy bumps on the face or neck with

telangiectatic blood vessels. Subtypes of basal cell car-

cinoma include nodular, superﬁ cial, pigmented, and

ﬁ brosing. Histologic features include a lobular growth

pattern of malignant basal cells with peripheral pali-

sading and retraction of lobules from the surrounding

stroma. Treatment includes surgical excision, cryosur-

gery, curettage, and electrodessication.

Peripheral

palisading

Lobe of basal cell

carcinoma with

squamous

diffentiation

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Organ Systems

Figure 13-7A. Special types of cells in the epidermis, thin skin.

H&E, 281

Keratinocytes make up the bulk of the epidermis, but there are

additional cell types: melanocytes, Merkel cells, and Langerhans

cells. All three types of cells have clear cytoplasm and are some-

times called clear cells. It is difﬁ cult to distinguish among them

in H&E-stained sections. Melanocytes and Merkel cells are both

located in the stratum basale where they are scattered among the

basal cuboidal cells. In contrast, Langerhans cells are typically

found in the stratum spinosum. The functions of the three cells

are quite different: (1) Melanocytes produce melanin granules and

insert them into keratinocytes. (2) Merkel cells are receptor cells,

that establish synaptic contacts with sensory nerve terminals;

they have cytoplasmic granules, which contain neurotransmitters

(Fig. 13-2). (3) Langerhans cells are monocyte derivatives, which

play an important role in capturing antigens and presenting them

to lymphocytes, thereby participating in the cutaneous immune

response. Langerhans cells make no desmosome junctions with

neighbors; indeed, their function requires that they be motile so

that they can transport any captured antigens from the epidermis

to lymph nodes deep to the skin.

Langerhans

cells

Langerhans

cells

Melanocytes

Figure 13-7B. Melanocyte. EM, 8,129

Melanocytes, along with Merkel cells, are the two clear cell types found in the stratum basale of the epidermis. The two cells cannot

be distinguished in conventionally prepared sections for light microscopy, but each has distinctive ultrastructural features. The mel-

anocyte generates melanin granules and injects them into nearby keratinocytes. This is a continuous process because keratinocytes

are constantly replaced as they differentiate and move toward the surface. The melanin protects the cells, particularly the nucleus,

from the mutagenic effects of ultraviolet irradiation. As shown here, the melanocyte contacts the basal lamina of the epidermis, but

it does not establish desmosome junctions with the keratinocytes.

Melanocyte

processes

Melanin granules

Nuclei of keratinocytes

Basal lamina of epidermis

Collagen

fibrils in

the dermis

Nucleus of the

melanocyte

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CLINICAL CORRELATIONS

Figure 13-8A.

Malignant Melanoma. H&E, 198.

Melanoma is an aggressive malignant skin neoplasm

originating from melanocytes of the skin. It is charac-

terized by signiﬁ cant morphologic diversity, with skin

lesions of irregular shapes and various degrees of pig-

mentation. Melanoma metastasizes via the lymphatic

system. Malignant melanoma is less common than basal

cell or SCCs, but it causes the majority of deaths from

skin cancer. Genetic factors and sun exposure contribute

to the development of the disease. The most common

forms of melanoma include superﬁ cial spreading mela-

noma and nodular malignant melanoma. Melanoma

cells contain large nuclei with irregular contours, often

with prominent nucleoli. Treatment includes surgical

excision, chemotherapy, radiation therapy, and immu-

notherapy.

Figure 13-8B.

Melanocytic Nevus. H&E, 22

Melanocytic nevi, commonly referred to as “moles,”

may be congenital or acquired, and are composed of

melanocytes in nests at the dermo-epidermo junction, in

the dermis, or both. If the nevus cells are restricted to the

dermis, the lesion is referred to as a dermal melanocytic

nevus. If the nevus cells are present only at the dermo-

epidermo junction, the lesion is referred to as a junctional

nevus. If the nevus cells are present in both locations, the

term compound nevus is used. Moles typically appear as

raised tan to brown soft lesions on sun-exposed or sun-

restricted skin. Dysplastic nevi, which may transform

to malignant melanoma, are typically larger than most

nevi, may have irregular borders, and pigmentary varia-

tion. Microscopically

, dysplastic nevi may show larger

junctional nests that fuse to adjacent nests and cytologic

atypia. This image shows a dermal melanocytic nevus

with no evidence of dysplasia or atypia.

Melanoma

(tumor) cells

Nests of

nevus cells

Type of Skin Epidermis Hair/Hair

Follicles

Glands Sensory

Receptors

Location/

Distribution

Special Features

Thick skin Five layers;

thick stratum

corneum;

thick stratum

granulosum

No Lack of

sebaceous

glands;

more eccrine

sweat glands

More receptors Palms of the

hand and soles

of the feet

Thick epidermis:

thick stratum

corneum; stratum

lucidum present;

several cell layers of

stratum granulosum

Thin skin Four layers; no

stratum lucidum;

single layer of

or no stratum

granulosum

Present in most

areas (except a

few places, such

as lips, labia

minora, and

glans penis)

Many

sebaceous

glands;

fewer

eccrine

sweat glands

Fewer receptors Entire body

except thick

skin areas

Thin epidermis: thin

stratum corneum;

stratum lucidum

absent; one layer

or no stratum

granulosum

TABLE 13-1 Comparison of Thick and Thin Skin

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UNIT 3

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Organ Systems

Accessory Structures

Figure 13-9A. Sebaceous gland, thin skin (scalp). H&E, 35;

inset 66

Sebaceous glands are found in thin skin, usually associated with hair

follicles. They are most numerous in the skin of the scalp and face.

Sebaceous glands are classiﬁ ed as simple branched acinar glands (see

Chapter 3, “Epithelium and Glands”). The secretory cells are lipid-

producing cells, arranged into several acini, which open into a short

duct. Usually, the ducts of sebaceous glands empty their oily secre-

tion, called sebum, into a hair follicle (Fig. 13-1); however, the ducts

sometimes open directly onto the surface of the skin. Sebaceous glands

release their products by holocrine secretion, that is, by the disintegra-

tion of entire cells. Sebum lubricates the skin and coats and protects

hair shafts from becoming brittle. The inset shows an acinus of a seba-

ceous gland and a nearby arrector pili muscle. Arrector pili muscles

are bundles of sympathetically innervated smooth muscle cells that

span between hair follicles and the papillary layer of the dermis. They

contract to stand the hair up in response to cold or fear (Fig. 13-1).

Sebaceous gland

duct

Sebaceous gland

duct

Hair

follicle

Hair

follicle

Sebaceous glands

Arrector pili

muscle

Arrector pili

muscle

Figure 13-9B. Eccrine sweat gland, thin skin (scalp). H&E, 248

Eccrine (merocrine) sweat glands can be found in both thin and thick

skin over most of the body. They are more numerous in the palms

and soles. Eccrine sweat glands produce a clear watery product called

sweat. These glands are simple glands in which the secretory cells are

arranged into coiled tubules. These glands have long unbranched, but

coiled, ducts, which are lined by two layers of cuboidal cells and open

directly onto the surface of the skin. Sweat consists mainly of water

(99%), some ions (K

, Na

, and Cl

−

), waste, and metabolic prod-

ucts. Releasing sweat onto the surface of the skin helps adjust body

temperature as well as aiding in the excretion of metabolic wastes.

The secretory units of eccrine sweat glands contain three cell types.

(1) Dark cells are pyramid-shaped cells containing dark secretory

granules. These cells lie toward the lumen of the tubule. (2) Clear

cells are also pyramid shaped and have no secretory granules, but

have ultrastructural features of ion pumping cells. They are located

toward the basement membrane. (3) Myoepithelial cells are not

secretory cells. They are spindle-shaped contractile cells, which help

to push secretory products into and along the lumen.

Eccrine sweat glands

Duct of eccrine

sweat gland

Duct of eccrine

sweat gland

Eccrine sweat glands

Figure 13-9C. Apocrine sweat gland, labia. H&E, 248

Apocrine sweat glands are simple coiled tubular glands like the

eccrine sweat glands, but their lumens are larger (about 10 times

larger than those of the eccrine sweat glands) and their ducts empty

into the superﬁ cial regions of the hair follicles. The secretory cells of

the apocrine glands release their products by shedding part of their

apical cytoplasm; this is called apocrine secretion. The tubules of the

glands are lined by cuboidal or columnar epithelial cells, depending

on the secretory stage. These glands are inﬂ uenced by hormones and

start to function at puberty. They are also called sexual scent glands.

They are restricted in location to some speciﬁ c regions of thin skin,

such as the axilla, the areola (nipple), and the perianal and genital

areas. Their product is a viscous, thick, milky ﬂ uid that contains pro-

tein, ammonia, lipids, and carbohydrates. These secretory ﬂ uids are

odorless when they are released but may have an axillary body odor

after degradation by bacteria.

Secretory cells

(cuboidal cells)

Secretory cells

(cuboidal cells)

Lumen of the

apocrine

sweat gland

Lumen of the

apocrine

sweat gland

Secretion

inside lumen

Secretion

inside lumen

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CLINICAL CORRELATION

Figure 13-10C.

Androgenetic Alopecia. H&E, 50.

Androgenetic alopecia is the most common form of hair

loss, affecting 30% to 40% of the adult population.

Males and females have a similar incidence in developing

this type of hair loss, characterized by varying degrees of

partial hair thinning from the vertex and frontal areas of

the scalp. In females, it rarely leads to total baldness. In

males, the cause is both genetic and androgen dependent.

Patients usually have higher levels of 5-alpha-reductase

and androgen receptors. 5-alpha-reductase increases

production of dihydrotestosterone, which binds to the

androgen receptors in susceptible follicles to trigger the

genes to miniaturize the follicles and weaken hair growth.

reatment options include the oral medication ﬁ nasteride

and topical solutions of minoxidil. This cross section

of scalp tissue shows variation in hair follicle size with

miniaturized follicles and the absence of inﬂ ammation.

Miniaturized

follicle

D. Cui /T. Yang

Hair medulla

Connective

tissue sheath

Hair cuticle

Inner root

sheath

Outer root

sheath

Hair cortex

Blood vessel

Hair medulla

Hair cuticle

inner root sheath

Inner root sheath

outer root sheath

Outer root sheath

Hair cortex

Connective

tissue sheath

Connective

tissue sheath

Figure 13-10A. Hair follicle. H&E, 95

A cross section of a hair follicle is shown on the left and a photomicrograph of a cross section of a hair follicle from the scalp on

the right. The structures of the hair follicle containing a hair shaft include (from inside to outside) the hair medulla (thin core of the

hair shaft), the hair cortex (keratinized cells surrounding the medulla), the hair cuticle (outermost layer of the hair shaft), the inner

root sheath (cellular sheath that extends from the hair bulb and surrounds and grows along with the hair), the outer root sheath

(a cellular sheath which is a continuation of the epidermis), and the connective tissue sheath (dermal root sheath).

Figure 13-10B. Hair follicles, thin skin (scalp). H&E, 78; inset

172

Hair follicles are the structures that produce the hair and maintain

hair growth. They are cellular structures extending from the epider-

mis into the dermis or hypodermis. The basal region of the hair fol-

licle forms a balloon-shaped structure called the hair bulb, which is

composed of the hair root and the dermal papilla. The hair root con-

tains melanocytes and a group of epithelial cells called the matrix

or germinal matrix. These cells are capable of cell division and give

rise to the inner root sheath and to the hair. The epithelial cells form

a cap around the dermal papilla (hair papilla). The dermal papilla

contains capillaries and nerve ﬁ bers that supply the hair follicle. The

interaction between the hair bulb and dermal papilla induces hair fol-

licle differentiation and the growth of the hair. The photomicrograph

shows a longitudinal section of hair follicles. The inset shows melanin

granules, which give color to the hair. The melanin granules are pro-

duced by melanocytes in the hair bulb.

Inner root

sheath

Inner root

sheath

Outer root

sheath

Outer root

sheath

Matrix

Melanin

granules

Melanin

granules

Connective

tissue sheath

Connective

tissue sheath

Hair root

Dermal papilla

(hair papilla)

Dermal papilla

(hair papilla)

Hair

bulb

Hair

bulb

CUI_Chap13.indd 253 6/2/2010 8:21:25 AM

254

UNIT 3

■

Organ Systems

CLINICAL CORRELATION

Figure 13-11C.

Molluscum Contagiosum. H&E, 53

Molluscum contagiosum is a viral skin infection, caused

by the molluscum contagiosum virus, a member of the

poxvirus family

. The disease is characterized by ﬂ esh-

colored, dome-shaped, pearly papules with a dimpled

center. Lesions are typically 1 to 5 mm in diameter

and common on the trunk, arms, and legs. The disease

is common in childhood, and usually self-limited in

immunocompetent patients. In adults, the disease usu-

ally indicates cellular immunodeﬁ ciency. The papules

are usually nonpainful, but may itch or be complicated

by secondary infection. The diagnosis is mainly based

on the clinical appearance of the lesions. This slide

shows lobules of keratinocytes with large eosinophilic

intracytoplasmic inclusions called molluscum bodies

within the stratum granulosum and stratum corneum.

The treatment may include laser therapies, cryotherapy,

or curettage, or there may be no treatment at all.

Molluscum

bodies

Figure 13-11A. Nail, ﬁ nger. H&E, 17

The nail is a translucent, hard, keratinized

sheet resting on the tip of each digit. It includes

many components: (1) the nail plate, the nail

itself, which is hard keratin; (2) the nail root,

also called the nail matrix, seen as the lunula

in the living state; (3) the nail bed, a layer of

epidermis beneath the nail plate; (4) the epony-

chium, also called the nail cuticle, which is the

junction zone between the skin of the ﬁ nger

and the nail plate and which forms a protec-

tive seal; (5) the perionychium (nail wall), the

skin that surrounds the edge of the nail; and

(6) the hyponychium, the junction seal between

the nail plate and the skin of the ﬁ ngertip. All

of the sealed areas at the edges of the nail plate

protect the delicate nail matrix and nail bed

from dehydration and infection.

Nail

lunula

Nail

lunula

Eponychium

Nail root

(matrix)

Finger tip

(thick skin)

Nail bed

Nail

plate

Hyponychium

Eponychium

(nail cuticle)

Thin skin

Nail

groove

Perionychium

Nail

matrix

Nail

plate

Figure 13-11B. Nail root (matrix) and nail bed. H&E, 69

The nail root is a cellular layer and is also called the matrix or

germinal matrix. It contains many layers of epithelial cells, which

are responsible for the production of the nail plate. These cells

proliferate and become ﬂ attened and highly keratinized and are

pushed forward by newly formed cells. As they differentiate, the

cells ﬁ nally lose color and shape and become part of the nail plate.

The nail plate is similar to the hair shaft, but the pattern of kera-

tin formation is different. The nail bed (equivalent to the epider-

mis) rests under the nail plate. The nail bed extends from the nail

matrix to the hyponychium.

Nail

matrix

Nail

matrix

Nail plate

Eponychium

Nail bed

Normally, the nail bed is smooth and allows for healthy nail

growth and a smooth appearance. If a nail bed is infected by

bacteria or fungus, the nail bed becomes rough, and an accumu-

lation of organic waste materials can react with the nail plate

and cause the nail to become thickened and distorted.

CUI_Chap13.indd 254 6/2/2010 8:21:30 AM