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SKIN

Dr. Emad I Shaqoura, M.D

M.Sc. Anatomy

Faculty of Medicine, IUG

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Skin

The skin is the largest single organ of the body, typically accounting for 15% - 20% of total body weight.
It is composed of the:

Epidermis: an epithelial layer of ectodermal origin.
Dermis: a layer of connective tissue of mesodermal origin.

Based on the comparative thickness of the epidermis:

Thick skin (400 - 1400 µm).
Thin skin (75 - 150 µm).

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FIGURE 18-1

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Skin

At the junction between the dermis and epidermis, projections called dermal papillae interdigitate with invaginating epidermal ridges to strengthen adhesion of the two layers.

Peg & socket.
Ridges & grooves (palms & soles).

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Skin

Epidermal derivatives:

Hairs.
Nails.
Sebaceous glands.
Sweat glands.

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Skin

Beneath the dermis lies the hypodermis or subcutaneous tissue.
It is a loose connective tissue that may contain a pad of adipose cells.
The hypodermis binds skin loosely to the subjacent tissues and corresponds to the superficial fascia of gross anatomy.

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Functions of the Skin

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Protective

Sensory

Thermoregulatory

Metabolic

Sexual signaling

Dermatoglyphics

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Medical Application

Friction blisters are lymph-filled spaces created between the epidermis and dermis of thick skin by excessive rubbing, as with ill-fitting shoes or hard use of the hands.
If continued, such activity produces protective thickening and hardening of the outer cornified epidermal layers, seen as corns and calluses.

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Epidermis

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Epidermis

The epidermis consists mainly of a stratified squamous keratinized epithelium composed of cells called keratinocytes.

Stratum Basale.
Stratum Spinosum.
Stratum Granulosum.
Stratum Lucidum.
Stratum Corneum.

There are also three much less abundant epidermal cell types:

Pigment-producing melanocytes.
Antigen-presenting langerhans cells.
Tactile epithelial cells called Merkel cells.

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FIGURE 18-2

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FIGURE 18-3

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Stratum Basale

The stratum basale consists of a single layer of basophilic columnar or cuboidal cells resting on the basement membrane at the dermal-epidermal junction.
Desmosomes bind the cells of this layer together in their lateral and upper surfaces.
Hemidesmosomes, found in the basal plasmalemma, help bind these cells to the basal lamina.

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Stratum Basale

Characterized by intense mitotic activity and is responsible for constant renewal of epidermal cells.
All cells in the stratum basale contain intermediate keratin filaments about 10 nm in diameter.
As the cells progress upward, the number of filaments increases until they represent half the total protein in the stratum corneum.

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Stratum Spinosum

The spinous layer is the thickest layer, especially in the epidermal ridges.
It consists of polyhedral cells having central nuclei with nucleoli and cytoplasm actively synthesizing keratins.
Just above the basal layer, some cells may still divide and this combined zone is sometimes called the stratum germinativum.
The keratin filaments assemble here into visible bundles called tonofibrils that converge and terminate at the numerous desmosomes holding the cell layers together.

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Stratum Spinosum

The cells extend slightly around the tonofibrils on both sides of each desmosome (and the extensions elongate if the cells shrink slightly during histologic processing), leading to the appearance of many short “spines” or prickles at the cell surfaces.
The epidermis of thick skin subject to continuous friction and pressure (such as the foot soles) has a thicker stratum spinosum with more abundant tonofibrils and desmosomes.

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FIGURE 18-4

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Medical Application

In adults, one-third of all cancers originate in the skin.
Most of these derive from cells of the basal or spinous layers, producing, respectively, basal cell carcinomas and squamous cell carcinomas.
Fortunately, both types of tumors can be diagnosed and excised early and consequently are rarely lethal.
Skin cancer shows an increased incidence in fair-skinned individuals residing in regions with high amounts of solar radiation.

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Stratum Granulosum

The granular layer consists of 3-5 layers of flattened cells, now undergoing the terminal differentiation process of keratinization.
Their cytoplasm is filled with intensely basophilic masses called keratohyaline granules.
These are dense, non–membrane-bound masses of filaggrin and other proteins associated with the keratins of tonofibrils, linking them further into large cytoplasmic structures.

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FIGURE 18-2

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FIGURE 18-5

Figure 18–5 Stratum granulosum and stratum lucidum: thick skin.

In keratinocytes moving upward from the stratum spinosum (S), differentiation proceeds with the cells becoming filled with numerous large, amorphous masses of protein called keratohyaline granules.

Cells with these basophilic granules make up the stratum granulosum (G), where keratin filaments are cross-linked with filaggrin and other proteins from these granules to produce tight bundles filling the cytoplasm and flattening the cells. Smaller organelles called lamellar granules undergo exocytosis in this layer, secreting a lipid-rich layer around the cells which makes the epidermis impermeable to water. Together, the lipid envelope and the keratin-filled cells determine most of the physical properties of the epidermis.

The cells leaving the stratum granulosum, still bound together by desmosomes, undergo terminal differentiation and in thick skin appear as a dense, thin layer called the stratum lucidum (L). Here proteins are dispersed through the tonofibril bundles, giving this layer a regular, “clear” appearance. In the most superficial stratum corneum (C), the cells have lost nuclei and cytoplasm, consisting only of flattened, keratinized structures called squames bound by hydrophobic, lipid-rich intercellular cement. At the surface they are worn away (thick skin) or flake off (thin skin). X560. H&E.

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Stratum Granulosum

Characteristic ultrastructural features in cells of the granular layer are the membranous, Golgi-derived lamellar granules, small ovoid structures with many lamellae containing various lipids.
The lamellar granules undergo exocytosis, producing a lipid-rich, impermeable layer around the cells.
This material forms a major part of the skin’s barrier against water loss.
Together, keratinization and production of the lipid-rich layer also have a crucial sealing effect in skin, forming the barrier to penetration by most foreign materials.

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Stratum Lucidum

The stratum lucidum is found only in thick skin.
It consists of a thin, translucent layer of flattened eosinophilic keratinocytes held together by desmosomes.
Nuclei and organelles have been lost, and the cytoplasm consists almost exclusively of packed keratin filaments embedded in an electron-dense matrix.

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FIGURE 18-5

Figure 18–5 Stratum granulosum and stratum lucidum: thick skin.

In keratinocytes moving upward from the stratum spinosum (S), differentiation proceeds with the cells becoming filled with numerous large, amorphous masses of protein called keratohyaline granules.

Cells with these basophilic granules make up the stratum granulosum (G), where keratin filaments are cross-linked with filaggrin and other proteins from these granules to produce tight bundles filling the cytoplasm and flattening the cells. Smaller organelles called lamellar granules undergo exocytosis in this layer, secreting a lipid-rich layer around the cells which makes the epidermis impermeable to water. Together, the lipid envelope and the keratin-filled cells determine most of the physical properties of the epidermis.

The cells leaving the stratum granulosum, still bound together by desmosomes, undergo terminal differentiation and in thick skin appear as a dense, thin layer called the stratum lucidum (L). Here proteins are dispersed through the tonofibril bundles, giving this layer a regular, “clear” appearance. In the most superficial stratum corneum (C), the cells have lost nuclei and cytoplasm, consisting only of flattened, keratinized structures called squames bound by hydrophobic, lipid-rich intercellular cement. At the surface they are worn away (thick skin) or flake off (thin skin). X560. H&E.

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Stratum Corneum

The stratum corneum consists of 15-20 layers of squamous, keratinized cells filled with filamentous keratins.
Keratin filaments contain at least six different polypeptides, synthesized during cell differentiation in the immature layers.
As they form, keratin tonofibrils become heavily massed with filaggrin and other proteins in keratohyaline granules.
By the end of keratinization, the cells contain only amorphous, fibrillar proteins with plasma membranes surrounded by the lipid-rich layer.
These fully keratinized or cornified cells called squames are continuously shed at the epidermal surface as the desmosomes and lipid-rich cell envelopes break down.

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FIGURE 18-5

Figure 18–5 Stratum granulosum and stratum lucidum: thick skin.

In keratinocytes moving upward from the stratum spinosum (S), differentiation proceeds with the cells becoming filled with numerous large, amorphous masses of protein called keratohyaline granules.

Cells with these basophilic granules make up the stratum granulosum (G), where keratin filaments are cross-linked with filaggrin and other proteins from these granules to produce tight bundles filling the cytoplasm and flattening the cells. Smaller organelles called lamellar granules undergo exocytosis in this layer, secreting a lipid-rich layer around the cells which makes the epidermis impermeable to water. Together, the lipid envelope and the keratin-filled cells determine most of the physical properties of the epidermis.

The cells leaving the stratum granulosum, still bound together by desmosomes, undergo terminal differentiation and in thick skin appear as a dense, thin layer called the stratum lucidum (L). Here proteins are dispersed through the tonofibril bundles, giving this layer a regular, “clear” appearance. In the most superficial stratum corneum (C), the cells have lost nuclei and cytoplasm, consisting only of flattened, keratinized structures called squames bound by hydrophobic, lipid-rich intercellular cement. At the surface they are worn away (thick skin) or flake off (thin skin). X560. H&E.

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Medical Application

In the chronic skin condition called psoriasis, keratinocytes are typically produced and differentiate at accelerated rates, causing at least slight thickening of the epidermal layers and increased keratinization and desquamation.
Psoriasis is caused by overactive T lymphocytes that trigger an autoimmune reaction in the skin, which can also lead to inflammation with redness, irritation, itching, and scaling, with a defective skin barrier.

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Melanocytes

The color of the skin is the result of the keratinocytes’ content of melanin and carotene and the number of blood vessels in the dermis.
Eumelanins are brown or black pigments produced by the melanocyte, found among the cells of the basal layer and in hair follicles.
The similar pigment found in red hair is called pheomelanin.
Melanocytes are neural crest derivatives that migrate into the epidermis’ stratum basale, where eventually one melanocyte accumulates for every 5-6 basal keratinocytes�(600-1200/mm2 of skin).

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Melanocytes

They have pale-staining, rounded cell bodies attached by hemidesmosomes to the basal lamina, but lacking attachments to the neighboring keratinocytes.
Several long irregular cytoplasmic extensions from each melanocyte cell body penetrate the epidermis, running between the cells of the basal and spinous layers and terminating in invaginations of 5-10 keratinocytes.
Ultrastructurally, a melanocyte has numerous small mitochondria, short cisternae of RER, and a well-developed Golgi apparatus.

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FIGURE 18-6

Figure 18–6 Melanocytes.

(a) Micrograph shows melanocytes (M) in the epidermal basal layer which synthesize melanin granules and transfer them into neighboring keratinocytes of the basal and spinous layers. Typically melanocytes are pale-staining cells on the basement membrane, with lower total melanin content than the keratinocytes. X400. H&E.

(b) Diagram of a melanocyte shows the irregular cytoplasmic processes between neighboring keratinocytes for transfer of melanin to those cells.

(c) Ultrastructurally, a melanocyte is located on the basal lamina (BL) and has well-developed Golgi complexes (G) producing the vesicles in which melanin is synthesized. As they fill, these vesicles become melanin granules (MG), which accumulate at the tips of the dendritic cytoplasmic extensions (CE) before transfer to keratinocytes (K). X14,000.

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Melanocytes

The first step in melanin synthesis is catalyzed by tyrosinase, a transmembrane enzyme in Golgi-derived vesicles.
Tyrosinase activity converts tyrosine into 3,4-dihydroxyphenylalanine (DOPA), which is then further transformed and polymerized into the different forms of melanin.
Melanin pigment is linked to a matrix of structural proteins and accumulates in the vesicles until they form mature elliptical granules about 1 μm long called melanosomes.
Melanosomes are then transported via kinesin to the tips of the cytoplasmic extensions.

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FIGURE 18-7

Figure 18–7 Melanosome formation.

The diagram of a melanocyte shows the main features of melanin formation. The granules containing melanin mature through four stages that are characterized ultrastructurally, as shown on the right. Tyrosinase is synthesized in the rough ER, processed through the Golgi apparatus, and accumulates in vesicles that also have a fine granular matrix of other proteins (stage I melanosomes). Melanin synthesis begins in the ovoid stage II melanosomes, in which the matrix has been organized into parallel filaments on which polymerized melanin is deposited and accumulates in stage III. A mature melanin granule (stage IV) has lost tyrosinase and other activities and has the internal matrix completely filled with melanin. The mature granules are ellipsoid, approximately 0.5 by 1 μm in size, and visible by light microscopy.

Melanin granules are transported to the tips of the processes of melanocyte and are then transferred to the neighboring keratinocytes of the basal and spinous layers. In keratinocytes the melanin granules are transported to a region near the nucleus, where they accumulate as a supranuclear cap shading the DNA against the harmful effects of UV radiation.

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Melanocytes

The neighboring keratinocytes phagocytose the tips of these dendrites, take in the melanosomes, and transport them by dynein toward their nuclei.
The melanosomes accumulate within keratinocytes as a supra-nuclear cap that prior to keratinization absorbs and scatters�sunlight, protecting DNA of the living cells from the ionizing, mutagenic effects of UV radiation.
Although melanocytes produce melanosomes, the keratinocytes are the melanin depot and contain more of this pigment than the cells that make it.
One melanocyte plus the keratinocytes into which it transfers melanosomes make up an epidermal-melanin unit.

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Melanocytes

The density of such units in skin is similar in all individuals.
Melanocytes of people living near the equator, produce melanin granules more rapidly and accumulate them more abundantly in keratinocytes than those living in regions with much less sunlight such as northern Europe.
Darkening of the skin, or tanning, after exposure to solar radiation is a two-step process:

A physicochemical reaction darkens preexisting melanin.
Paracrine factors secreted by keratinocytes experiencing increased UV radiation accelerate melanin synthesis and its accumulation in the epidermis.

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Medical Application

Albinism is a congenital disorder producing skin hypopigmentation due to a defect in tyrosinase or some other component of the melanin-producing pathway.
An acquired condition called vitiligo involves skin depigmentation, often only in affected patches, due to the loss or decreased activity of melanocytes.
The causes of melanocyte loss are not clear, but they may include environmental, genetic, or autoimmune conditions.

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Medical Application

Melanocytes can normally proliferate in skin to produce moles, or benign melanocytic nevi of various types.
Changes in the size or appearance of moles are sometimes indicative of dysplasia that can progress further to malignant melanoma.
Dividing rapidly, malignantly transformed melanocytes often penetrate the basal lamina, enter the dermis, and metastasize by invading blood and lymphatic vessels.

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Langerhans Cells

Langerhans cells are APCs, which are most clearly seen in the spinous layer, representing 2% - 8% of the epidermal cells.
Cytoplasmic processes extend from these dendritic cells between keratinocytes of all the layers, forming a dense network in the epidermis.
Langerhans cells, along with epidermal lymphocytes and other APCs in the dermis, make up a major component of the skin’s adaptive immunity.
Various epidermal features participate in both innate and adaptive immunity, providing an important immunologic component to the skin’s overall protective function.

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FIGURE 18-8

Figure 18–8 Langerhans cells.

Langerhans cells are dendritic APCs of the epidermis where they comprise an important defense against pathogens and environmental insults. Like other APCs, they develop in the bone marrow, move into the blood circulation, and finally migrate into stratified squamous epithelia where they are difficult to identify in routinely stained sections. (a) Section of immunostained skin shows Langerhans cells (yellow) abundant in hair follicles (F), where many microorganisms live, and throughout the epidermis (E). Keratin of the epidermis and follicles is stained green. X40. Antibodies against langerin/CD207 and keratin.

(b) Face-on view of an epidermal sheet stained using the same antibody showing the network of Langerhans cells among the other epidermal cells, which detects invading microorganisms. After sampling the invaders’ antigens, Langerhans cells leave the epidermis and travel to the nearest lymph node to elicit lymphocytes that can mount a collective immune response. X200. Anti-langerin/CD207.

(Reproduced, with permission, from Romani N et al. Acta Path Micro Immunol Scandinavica. 2003;111:725.)

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Merkel Cells

Merkel cells, or epithelial tactile cells, are sensitive mechanoreceptors essential for light touch sensation.
Joined by desmosomes to keratinocytes of the basal epidermal layer, Merkel cells resemble the surrounding cells but with few, if any, melanosomes.
They are abundant in highly sensitive skin like that of fingertips�and at the bases of some hair follicles.
Merkel cells originate from the same stem cells as keratinocytes and are characterized by small, Golgi-derived dense-core neurosecretory granules containing peptides.
The basolateral surfaces of the cells contact expanded terminal discs of unmyelinated sensory fibers penetrating the basal lamina.

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FIGURE 18-9

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Medical Application

Merkel cells are of clinical importance because Merkel cell carcinomas, though uncommon, are very aggressive and difficult to treat.
Merkel cell carcinoma is 40 times less common than malignant melanoma but has twice the mortality of that disease.

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Dermis & SC Tissue

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Dermis

The dermis is the layer of connective tissue that supports the epidermis and binds it to the subcutaneous tissue.
The thickness of the dermis varies with the region of the body and reaches its maximum of 4 mm on the back.
The surface of the dermis is very irregular, especially in skin subject to frequent pressure, where they reinforce the dermal-epidermal junction.

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Dermis

A basement membrane always occurs between the stratum basale and the dermis, and follows the contour of the interdigitations between these layers.
This membrane is a composite structure consisting of the basal lamina and the reticular lamina, and can usually be seen with the light microscope.
Nutrients for keratinocytes diffuse into the avascular epidermis from the dermal vasculature through this basement membrane.

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Medical Application

Abnormalities of the dermal-epidermal junction can lead to one type of blistering disorder (bullous pemphigoid).

Another type of blistering disorder (pemphigus) is caused by autoimmune damage to intercellular junctions between keratinocytes.

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Dermis

The dermis has two sublayers with indistinct boundaries:
The thin papillary layer:

Includes the dermal papillae.
It consists of loose C.T, with types I and III collagen fibers.
From this layer, anchoring fibrils of type VII collagen insert into the basal lamina, helping to bind the dermis to the epidermis.

The thick underlying reticular layer:

It consists of dense irregular C.T (mainly bundles of type I collagen), with more fibers and fewer cells than the papillary layer.
A network of elastic fibers is present, providing elasticity to the skin.
Between the fibers are abundant proteoglycans rich in dermatan sulfate.

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FIGURE 18-10

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Dermis

Both dermal regions contain a rich network of blood and lymphatic vessels.
Nutritive vessels form two major plexuses:

Subpapillary plexus: lies between the papillary and reticular dermal layers, from which capillary branches extend into the dermal papillae and form a rich, nutritive network just below the epidermis.
A deep plexus: with larger blood and lymphatic vessels lies near the interface of the dermis and the subcutaneous layer.

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FIGURE 18-1

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Dermis

In addition to the nutritive function, dermal vasculature has a thermoregulatory function, which involves numerous arteriovenous anastomoses located between the two major plexuses.
The shunts decrease blood flow in the papillary layer to minimize heat loss in cold conditions and increase this flow to facilitate heat loss when it is hot, thus helping maintain a constant body temperature.
Lymphatic vessels begin in the dermal papillae and converge to form two plexuses located with the blood vessels.

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Dermis

The dermis is also richly innervated.
Sensory afferent nerve fibers form a network in the papillary dermis and around hair follicles, ending at epithelial and dermal receptors.
Autonomic effector nerves to dermal sweat glands and smooth muscle fibers in the skin of some areas are postganglionic fibers of sympathetic ganglia; no parasympathetic innervation is present.

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Subcutaneous Tissue

The subcutaneous layer consists of loose connective tissue that binds the skin loosely to the subjacent organs, making it possible for the skin to slide over them.
This layer, also called the hypodermis or superficial fascia, contains adipocytes that vary in number in different body regions and vary in size according to nutritional state.
The extensive vascular supply at the subcutaneous layer promotes rapid uptake of insulin or drugs injected into this tissue.

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Dermis & Hypodermis

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Sensory Receptors

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Sensory Receptors

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Unencapsulated

Encapsulated

Merkel cells.
Free nerve endings.
Root hair plexus.

Meissner corpuscles.
Pacinian corpuscles.
Krause end bulb.
Ruffini corpuscles.

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Unencapsulated Receptors

The Merkel cells, each associated with expanded nerve endings, which function as tonic receptors for sustained light touch and for sensing an object’s texture.
Free nerve endings in the papillary dermis and extending into lower epidermal layers, which respond primarily to high and low temperatures, pain, and itching, but also function as tactile receptors.
Root hair plexuses, a web of sensory fibers surrounding the bases of hair follicles in the reticular dermis that detects movements of the hairs.

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FIGURE 18-11

Figure 18–11 Tactile receptors.

Skin contains several types of sensory receptors, with or without capsules of collagen and modified Schwann cells. Most are difficult to see in routine preparations. In the epidermis are free nerve endings and tactile discs of nerve fibers associated with Merkel cells in the basal layer. Both have unencapsulated nerve fibers, as does the root hair plexus around the bases of hair follicles in the dermis. They detect light touch or movement of hair, although epidermal free nerve endings also detect pain and temperature extremes.

More complex, encapsulated tactile receptors are located in the dermis and hypodermis, and include Meissner corpuscles for light touch, lamellated (pacinian) corpuscles detecting pressure and high-frequency vibration, Krause end bulbs for low-frequency vibrations/movements, and Ruffini corpuscles detecting tissue distortion. The latter two receptors are less widely distributed in skin and less commonly seen.

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Encapsulated Receptors

The encapsulated receptors are all phasic mechanoreceptors, responding rapidly to stimuli on the skin.
Meissner corpuscles:
They are elliptical structures, 30 to 75 μm X 50 to 150 μm, consisting of sensory axons winding among flattened Schwann cells arranged perpendicular to the epidermis in the dermal papillae.
They initiate impulses when light touch or low-frequency stimuli against skin temporarily deform their shape.
They are numerous in the fingertips, palms, and soles but decline slowly in number with age.

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FIGURE 18-11

Figure 18–11 Tactile receptors.

Skin contains several types of sensory receptors, with or without capsules of collagen and modified Schwann cells. Most are difficult to see in routine preparations. In the epidermis are free nerve endings and tactile discs of nerve fibers associated with Merkel cells in the basal layer. Both have unencapsulated nerve fibers, as does the root hair plexus around the bases of hair follicles in the dermis. They detect light touch or movement of hair, although epidermal free nerve endings also detect pain and temperature extremes.

More complex, encapsulated tactile receptors are located in the dermis and hypodermis, and include Meissner corpuscles for light touch, lamellated (pacinian) corpuscles detecting pressure and high-frequency vibration, Krause end bulbs for low-frequency vibrations/movements, and Ruffini corpuscles detecting tissue distortion. The latter two receptors are less widely distributed in skin and less commonly seen.

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Meissner (Tactile) Corpuscle

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Medical Application

The density of tactile Meissner corpuscles in skin can be determined approximately by two-point discrimination tests.
This test indicates that the number of tactile corpuscles in skin normally declines during adult life.
Loss of tactile corpuscles or reduction in their activity can also be detected in scleroderma and certain other connective tissue disorders that lead to sclerosis (hardening) of the dermis and tightening of the skin.

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Encapsulated Receptors

Lamellated (Pacinian) corpuscles:
They are large oval structures, approximately 0.5 mm by 1 mm, found deep in the reticular dermis and hypodermis, with an outer�capsule and 15-50 thin, concentric lamellae of flattened Schwann cells and collagen surrounding a highly branched, unmyelinated axon.
Lamellated corpuscles are specialized for sensing coarse touch, pressure (sustained touch), and vibrations.
Pacinian corpuscles are also found in the connective tissue of organs, including the wall of the rectum and urinary bladder, where they also produce the sensation of pressure when the surrounding tissue is distorted.

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FIGURE 18-11

Figure 18–11 Tactile receptors.

Skin contains several types of sensory receptors, with or without capsules of collagen and modified Schwann cells. Most are difficult to see in routine preparations. In the epidermis are free nerve endings and tactile discs of nerve fibers associated with Merkel cells in the basal layer. Both have unencapsulated nerve fibers, as does the root hair plexus around the bases of hair follicles in the dermis. They detect light touch or movement of hair, although epidermal free nerve endings also detect pain and temperature extremes.

More complex, encapsulated tactile receptors are located in the dermis and hypodermis, and include Meissner corpuscles for light touch, lamellated (pacinian) corpuscles detecting pressure and high-frequency vibration, Krause end bulbs for low-frequency vibrations/movements, and Ruffini corpuscles detecting tissue distortion. The latter two receptors are less widely distributed in skin and less commonly seen.

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FIGURE 18-12

Figure 18–12 Meissner and lamellated (pacinian) corpuscles.

Micrographs show the two most commonly seen sensory receptors of skin. (a) Meissner tactile corpuscles (TC) are specialized to detect light touch and are frequently located in dermal papillae (DP), partially surrounded by epidermis (E). They are elliptical, approximately 150 ìm long, with an outer capsule (from the perineurium) and thin, stacked inner layers of modified Schwann cells, around which course nerve fibers. X400. H&E.

(b) Lamellated (pacinian) corpuscles (PC) detect coarse touch or pressure and are large oval structures, frequently 1 mm in length, found among adipose tissue (A) deep in the reticular dermis or in the subcutaneous tissue. Here the outer connective tissue capsule surrounds 15-50 thin, concentric layers of modified Schwann cells, each separated by slightly viscous interstitial fluid. Several axons enter one end of the corpuscle and lie in the cylindrical, inner core of the structure. Movement or pressure of this corpuscle from any direction displaces the inner core, leading to a nerve impulse. X40. H&E.

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Encapsulated Receptors

Krause end bulbs are simpler encapsulated, ovoid structures, with extremely thin, collagenous capsules penetrated by a sensory fiber. They are found primarily in the skin of the penis and clitoris where they sense low frequency vibrations.

Ruffini corpuscles have collagenous, fusiform capsules anchored firmly to the surrounding connective tissue, with sensory axons stimulated by stretch (tension) or twisting (torque) in the skin.

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Epidermal Appendages

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Hair

Hairs are elongated keratinized structures that form within epidermal invaginations, the hair follicles.
The color, size, shape, and texture of hairs vary according to age, genetic background, and region of the body.
All skin has at least minimal hair except the glabrous skin of the palms, soles, lips, glans penis, clitoris, and labia minora.
Hairs grow discontinuously, with periods of growth followed by periods of rest, and this growth does not occur synchronously in all regions of the body or even in the same area.

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Hair

The growing hair follicle has a terminal dilation called a hair bulb.
A dermal papilla inserts into the base of the hair bulb and contains a capillary network required to sustain the hair follicle.
Keratinocytes continuous with those of the basal epidermis cover the dermal papilla.
These cells form the matrix of the elongating hair root; the part of a hair extending beyond the skin surface is the hair shaft.
The keratinocytes of the hair bulb are generally similar to�those in the basal and spinous layers of epidermis.
They divide rapidly and then undergo keratinization, melanin accumulation, and terminal differentiation.

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FIGURE 18-13

Figure 18–13 Hair.

All types of body hair have a similar composition and form in hair follicles derived from the epidermis but extending deep into the dermis. (a) The diagram shows major parts of a hair and its follicle, including vascularized, nutritive hair dermal papilla and the arrector pili muscle that pulls the hair erect.

(b) A longitudinal section of a hair root and bulb shows the matrix, medulla and cortex in the root and the surrounding epithelial and connective tissue sheaths. Cells of the hair bulb matrix proliferate, take up melanin granules, and undergo keratinization to differentiate as the three concentric layers of the hair. X70. H&E.

(c) The outermost layer of the hair is the thin cuticle, composed of shingle-like cells, shown in this SEM of a hair shaft emerging at the stratum corneum. X260.

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Hair

A section in most thick hairs (hair root) is composed of:

A central medulla: large, vacuolated, and moderately keratinized cells.
The cortex: heavily keratinized, densely packed cells around the medulla.
The cuticle: the most peripheral cells that form a thin layer of heavily keratinized, squamous cells covering the cortex.

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FIGURE 18-14

Figure 18–14 Layers of a hair and its follicle.

(a) The base of a hair follicle sectioned obliquely shows the vascularized dermal papilla (DP) continuous with the surrounding connective tissue sheath (CTS). The papilla is enclosed by the deepest part of the epithelial sheath, which is continuous with both the internal root sheath (IRS) and external root sheath (ERS). Both of these layers are in turn continuous with the stratified epidermis. Just outside the ERS is the glassy membrane (G) that is continuous with the basement membrane of the epidermis. The epithelial cells (keratinocytes) around the papilla proliferate and differentiate as the root of the hair itself. Above the papilla, only the cortex (CO) of the hair is clearly seen in this section. X140. H&E.

(b) A hair root sectioned more transversely shows the same layers of the follicular sheath, but the layers of the hair root are now seen to include the medulla (M), cortex (CO), and cuticle (CU). Other labels are like those of part (a). X140. H&E.

(c) SEM of a similar specimen gives a different perspective on these layers, including the shingle-like nature of the thin cuticle surface (CU), and the small blood vessel (BV) and collagen bundles (CB) near the surrounding connective tissue sheath (CTS). Other labels are like those of (a). X2600.

(Figure 18–14c, with permission, from Kessel RG, Kardon RH. Tissues and Organs: A Text-Atlas of Scanning Electron Microscopy. San Francisco, CA: W.H. Freeman & Co.; 1979.)

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Hair

The outermost cells of the hair bulb are continuous with the epithelial root sheath, in which two layers can be recognized:
The internal root sheath: completely surrounds the initial part of the hair root but degenerates above the level of the attached sebaceous glands.
The external root sheath: covers the internal sheath and extends all the way to the epidermis, where it is continuous with the basal and spinous layers.
The glassy membrane: is the thickened basement membrane separating the hair follicle from the dermis.
The surrounding dermis forms a connective tissue sheath.

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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).

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Hair

The arrector pili muscle, a small bundle of smooth muscle cells, extends from the midpoint of the fibrous sheath to the dermal papillary layer.
Contraction of these muscles pulls the hair shaft to a more erect position, usually when it is cold in an effort to trap a layer of warm air near the skin.
In regions where hair is fine, contraction of arrector pili muscles is seen to produce tiny bumps on the skin surface (“goose bumps”) where each contracting muscle distorts the attached dermis.

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FIGURE 18-13

Figure 18–13 Hair.

All types of body hair have a similar composition and form in hair follicles derived from the epidermis but extending deep into the dermis. (a) The diagram shows major parts of a hair and its follicle, including vascularized, nutritive hair dermal papilla and the arrector pili muscle that pulls the hair erect.

(b) A longitudinal section of a hair root and bulb shows the matrix, medulla and cortex in the root and the surrounding epithelial and connective tissue sheaths. Cells of the hair bulb matrix proliferate, take up melanin granules, and undergo keratinization to differentiate as the three concentric layers of the hair. X70. H&E.

(c) The outermost layer of the hair is the thin cuticle, composed of shingle-like cells, shown in this SEM of a hair shaft emerging at the stratum corneum. X260.

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Hair Growth Cycle

The hair growth cycle has three major phases:
Anagen: a generally long period of mitotic activity and growth.
Catagen: a brief period of arrested growth and regression of the�hair bulb.
Telogen: a final long period of inactivity during which the hair may be shed.
At the beginning of the next anagen phase, epidermal stem cells located in small bulge of the external root sheath near the arrector pili muscle produce progenitor cells for the matrix of a new hair bulb.
Hair growth on the face and pubis is strongly influenced by sex hormones, especially androgens, and begins at puberty.

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Medical Application

Loss of hair to produce baldness or alopecia results from a complex combination of genetic and hormonal factors that is not well understood.
Arresting mitotic activity in the hair matrix during cancer chemotherapy disrupts both the function and the structural integrity of hair follicles and usually leads to rapid, reversible alopecia.

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Nails

The nails are hard plates of keratin on the dorsal surface of each distal phalanx.
The proximal part of the nail is the nail root and is covered by a fold of skin, from which the epidermal stratum corneum extends as the cuticle, or eponychium.
The nail plate is bound to a bed of epidermis, the nail bed, which contains only the basal and spinous epidermal layers.
The nail root forms from the nail matrix in which cells divide, move distally, and become keratinized in a process somewhat similar to hair formation but without keratohyaline granules.

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FIGURE 18-15

Figure 18–15 Nails.

Nails are hard, keratinized derivatives formed in a process similar to that of the stratum corneum and hair. (a) Surface view of a finger shows the nail’s major parts, including the crescent-shaped white area called the lunula, which derives its color from the opaque nail matrix and immature nail plate below it.

(b) A diagrammatic sagittal section includes major internal details of the growing nail and the hyponychium where the free end of the nail plate is bound to epidermis.

(c) A sagittal section from a finger shows the proximal nail fold (PNF) and its epidermal extension, the eponychium (E) or cuticle. The nail root (NR), the most proximal region of the nail plate (NP), is formed like the hair root by a matrix of proliferating, differentiating keratinocytes. These cells make up the dorsal nail matrix (DNM) and ventral nail matrix (VNM), which contribute keratinized cells to the nail root. The mature nail plate remains attached to the nail bed (NB), which consists of basal and spinous epidermal layers over dermis (D), but is pushed forward on this bed by continuous growth in the nail matrix. X100. Mallory trichrome.

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Nails

The nail root matures and hardens as the nail plate.
Continuous growth in the matrix pushes the nail plate forward over the nail bed (which makes no contribution to the plate) at a rate of about 3 mm/mo for fingernails and 1 mm/mo for toenails.
The distal end of the plate becomes free of the nail bed at the epidermal fold called the hyponychium.
The nearly transparent nail plate provide a useful window on the amount of oxygen in the blood by showing the color of blood in the dermal vessels.

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FIGURE 18-15

Figure 18–15 Nails.

Nails are hard, keratinized derivatives formed in a process similar to that of the stratum corneum and hair. (a) Surface view of a finger shows the nail’s major parts, including the crescent-shaped white area called the lunula, which derives its color from the opaque nail matrix and immature nail plate below it.

(b) A diagrammatic sagittal section includes major internal details of the growing nail and the hyponychium where the free end of the nail plate is bound to epidermis.

(c) A sagittal section from a finger shows the proximal nail fold (PNF) and its epidermal extension, the eponychium (E) or cuticle. The nail root (NR), the most proximal region of the nail plate (NP), is formed like the hair root by a matrix of proliferating, differentiating keratinocytes. These cells make up the dorsal nail matrix (DNM) and ventral nail matrix (VNM), which contribute keratinized cells to the nail root. The mature nail plate remains attached to the nail bed (NB), which consists of basal and spinous epidermal layers over dermis (D), but is pushed forward on this bed by continuous growth in the nail matrix. X100. Mallory trichrome.

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Skin Glands

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Skin Glands

Sweat Glands

Eccrine Sweat Glands

Apocrine Sweat Glands

Sebaceous Glands

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FIGURE 18-16

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Sebaceous Glands

Sebaceous glands are embedded in the dermis over most of the body, except in the thick, glabrous skin of the palms and soles.
There is an average of about 100 such glands/cm² of skin, but the frequency increases to 400-900/cm² in the face and scalp.
Sebaceous glands are branched acinar glands with several acini converging at a short duct that usually empties into the upper portion of a hair follicle.
A hair follicle and its associated sebaceous glands make up a pilo-sebaceous unit.
The stem cell niche of the follicle’s bulge region also forms the progenitor cells of the associated sebaceous glands.

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FIGURE 18-17

Figure 18–17 Sebaceous glands.

Sebaceous glands secrete a complex, oily mixture of lipids called sebum into short ducts that in most areas open into hair follicles. Sebum production is the classic example of holocrine secretion, in which the entire cell dies and contributes to the secretory product. (a) A section of a pilosebaceous unit shows acini composed of large sebocytes (S), which undergo terminal differentiation by filling with small lipid droplets and then disintegrating near the ducts (D) opening at the hair (H) shaft. X122. H&E.

(b) A micrograph shows the gland’s capsule (C) and differentiates sebocytes (S) at higher magnification. Proliferation of the small progenitor cells just inside the capsule continuously forces sebum into the ducts; myoepithelial cells are not -present. X400. H&E.

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Sebaceous Glands

In certain hairless regions, such as the penis, clitoris, eyelids, and nipples, sebaceous ducts open directly onto the epidermal surface.
The acini of sebaceous glands are the classic example of holocrine secretion.
They have a basal layer of flattened epithelial cells on the basal lamina, which proliferate and are displaced centrally, undergoing terminal differentiation as large, lipid-producing sebocytes filled with small fat droplets.
Their nuclei shrink and undergo autophagy along with other organelles, and near the duct the cells disintegrate, releasing the lipids as the main secretory product.
This product, called sebum, gradually covers the surfaces of both�the epidermis and hair shaft.

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Sebaceous Glands

Sebum is a complex mixture of lipids that are hydrolyzed by bacterial enzymes after secretion.
Secretion from sebaceous glands increases greatly at puberty, stimulated primarily by testosterone in men and by ovarian and adrenal androgens in women.
Sebum helps maintain the stratum corneum and hair shaft and exerts weak antibacterial and antifungal properties.

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Medical Application

Acne vulgaris is an inflammatory disorder of the pilo-sebaceous unit, which can be expected to occur during adolescence.
It involves excessive keratinization within this unit and excess sebum production, both of which contribute to the blockage of ducts in the follicle.
Anaerobic bacteria, typically Propionibacterium acnes, grow in the accumulated sebum, leading to localized inflammation and neutrophil infiltration.
The resulting enlarged follicle is called a comedone.

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Sweat Glands

Sweat glands develop as long epidermal invaginations embedded in the dermis.

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Aspect	Eccrine Sweat Glands	Apocrine Sweat Glands
Distribution	Wide, esp. foot soles	Confined to axillary & perianal regions (depend on sex hormones)
Secretory Portion	Coiled with narrow lumen. More pale-staining than the ducts. Stratified cuboidal epithelium with 3 cell types (clear, dark & myoep.)	Have larger lumen. Simple cuboidal epithelium of acidophilic cells.
Ducts	Coiled with narrow lumen. 2 layers of acidophilic cells (mit). Open on the surface of epidermis.	Similar to eccrine glands. Open into the hair follicle.
Mode of secretion	Merocrine	Merocrine
N.S	Cholinergic fibers	Adrenergic fibers

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FIGURE 18-16

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FIGURE 18-18

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Eccrine Sweat Glands

The secretory part has stratified cuboidal epithelium with three cell types:
Pale-staining clear cells:

They produce the sweat.
They have abundant mitochondria and microvilli to provide large surface areas.
Interstitial fluid from the capillary-rich dermis around the gland is transported through the clear cells, either directly into the gland’s lumen or into intercellular canaliculi that open to the lumen.

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Eccrine Sweat Glands

Dark cells:

Filled with strongly eosinophilic granules.
Line most of the lumen and do not contact the basal lamina.
The granules undergo merocrine secretion to release a poorly understood mixture of glycoproteins with bactericidal activity.

Myoepithelial cells:

On the basal lamina.
Contract to move the watery secretion into the duct.

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FIGURE 18-19

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Eccrine Sweat Glands

The ducts of eccrine sweat glands have cell membranes rich in Na +, K +-ATPase.
These duct cells absorb Na+ ions from the secreted water to prevent excessive loss of this electrolyte.

Functions of Eccrine Sweat Glands:

They produce sweat (10 L/d) that quickly evaporates upon release, cooling the skin and the blood present there.
They act as auxiliary excretory organs, eliminating small amounts of nitrogenous waste and excess salts.

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Apocrine Sweat Glands

Their development is complete and they become functional at puberty.
They produce slightly viscous secretion that is initially odorless but may acquire a distinctive odor as a result of bacterial activity.
The production of pheromones by apocrine glands is well established in many mammals and is likely in humans, although in a reduced or vestigial capacity.

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Medical Application

The sweat of infants with cystic fibrosis (CF) is often salty and is commonly taken as indicative of this genetic disease.
CF patients have defects in a transmembrane conductance regulator (CFTr) of epithelial cells that lead to disruptive accumulations of thick mucus in the respiratory and digestive tracts.
Failure to remove salt from sweat is related to the same genetic defect.

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Skin Repair

Skin has a good capacity for repair, which is important in this exposed and easily damaged organ.
The process of cutaneous wound healing, whether initiated surgically or accidentally, involves several overlapping stages that vary in duration with the size of the wound.

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FIGURE 18-20

Figure 18–20 Major stages of cutaneous wound healing.

Skin repair occurs in overlapping stages shown here schematically. The process begins with blood quickly clotting at the wound site, releasing platelet-derived growth factors and other substances (1). Macrophages and neutrophils enter the wound as inflammation begins, and epithelial cells from the cut edges of the stratum basale begin to migrate beneath and through the blood clot (2).

Under the influence of growth factors and hydrolytic enzymes released in part from macrophages, fibroblasts proliferate and produce much new collagen to form “granulation tissue” containing many new, growing capillaries (3). The epidermis gradually reestablishes continuity over the wound site, but excessive collagen usually remains in the dermis as scar tissue (4).

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