Module 3.1

N. Swailes, Ph.D.

Required reading

Mescher AL, Junqueira’s Basic Text and Atlas, 12th Edition, Chapter 18: pp316-331

Learning objectives

1) Use your knowledge of the basic tissues to describe the histological organization of skin.

2) Identify the and discuss its embryological origin, organization and functions.

3) Identify the and hypodermis and discuss their embryological origins, organization and functions.

4) Examine a variety of skin adnexa and determine their function.

5) Think about the histological organization of skin the next time you are making an incision during Gross .

A: General organization

Skin is divided into 3 main regions, each providing a distinct role in the overall function of the skin.

1. Epidermis - is the most superficial layer of the skin - is derived from embryonic and is a specialized stratified squamous - forms first physical and immunological barrier to invasion of foreign substances into the body.

2. Dermis - is connective tissue derived from embryonic mesoderm - provides mechanical support and elasticity of the skin - contains immune cells involved in defense against foreign invaders passing through the epidermis

3. Hypodermis - is connective tissue derived from embryonic mesoderm - also known as subcutaneous connective tissue - recognized as the superficial in gross anatomy - stores adipose tissue for cushioning, insulation and energy. -

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PART B: Epidermis

The epidermis consists of four main cell type: the main cell is the stratified squamous epithelial cells called (A) , other cells are less abundant and include: (B) Melanocytes (pigment producing), (C) Langerhans cells (antigen presenting) and (D) Merkel cells (tactile cells).

A. Keratinocytes

From the dermis outwards the epidermis consists of four layers of keratinocytes (five in thick skin):

1. - layer of columnar ‘palisades’ on the at dermal-epidermal junction - attached to the by hemidesmosomes and to each other by desmosomes - undergo intense and are responsible for 5 1 the constant production of epidermal cells

2. 4* - the thickest part of the epidermis 3 - consists of slightly flattened cells that actively 1 synthesize filaments - the filaments radiate outward and converge on the desmosomes that anchor the cells to each other

3. - 3-5 layers of flattened cells undergoing terminal keratinization 2 - granular appearance due to: 1

i. Keratohyalin granules Contain the protein filaggrin that binds together keratin filaments in the cytoplasm.

ii. Lamellar (Odland) bodies Contain that when released by exocytosis surround the keratinocytes and 1 prevent water loss and serve a waterproofing 1 function.

4. * - an additional layer of pale staining keratinocytes only seen in thick skin (not illustrated here)

5. - 15-20 stacked layers of flattened non-nucleated keratinized cells filled with filamentous keratin. Melanocyte Keratinocytes Merkel cell Langerhans cell - continuously shed at the surface of the corneum

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B. Melanocytes

- neural crest derived cells that have migrated into the stratum basale of the epidermis - have characteristically long, irregular processes that branch into the epidermis between the keratinocytes of the stratum basale and spinosum - one melanocyte serves 5-6 basal keratinocytes forming a epidermal-melanin unit

Pigmentation Melanin is: - primarily responsible for skin color - synthesized by melanocytes and accumulates in vesicles called melanosomes - transported within melansomes to the end of melanocyte processes. - phagocytosed when keratinocytes phagocytose the melansome containing processes - is released from melansomes around the nuclei. One melanocyte plus the keratinocytes that ingest its melanin make up a epidermal-melanin unit. - absorbs and scatters sunlight to protect nuclear DNA from UV radiation.

Clinical Correlations Skin color Melanocytes of people with ancestral origins near the equator where the need for protection against UV light is greatest produce more melanosomes more rapidly and accumulate them in keratinocytes more abundantly resulting in dark skin.

Tanning

Tanning is the result of a darkening of pre-existing melanin and an increase in melanin synthesis and its transfer to surrounding keratinocytes in response to exposure to solar radiation

C. Merkel cells

Merkel cells are mechano-receptors derived from neural crest cells and located in the stratum basale. They are present in greatest numbers in areas of high tactile sensitivity. Their basolateral surfaces contact sensory nerve fibers that penetrate the basal lamina.

D. Langerhans cells

Langerhans cells are antigen-presenting cells. They are found in the stratum spinosum but send out long processes between the keratinocytes of all layers.

Due to this extensive coverage of the skin, they can can present any antigens that may invade the epidermis to surrounding T-lymphocytes triggering an immune response.

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PART C: Dermis and Hypodermis

A. Dermis

The dermis is the connective tissue component of the skin. It supports the epidermis and binds it to the hypodermis and contains the numerous blood vessels that supply the avascular epidermis with nutrients and are responsible for . If you examine the dermis carefully you will observe a number of features:

1. Dermal papillae - a series of dermal projections into the epidermis - more pronounced in regions of skin that regularly receive pressure (soles, etc).

2. Papillary dermis - a thin layer of loose irregular connective tissue below the basement membrane - it forms the dermal papillase - anchoring fibrils (Type VII collagen) in this layer insert into the basal lamina to hold the epidermis and dermis together.

3. Reticular dermis - a thick layer of dense irregular connective tissue immediately below the papillary dermis. - contains Type I collagen bundles which gives skin its strength against tensile forces - contains elastin fibers (seen only with special stains) that provide skin with its elasticity. - contains and follicles (see adnexa).

4. Vascular network - a deep vascular plexus lies between the reticular dermis and hypodermis - deep plexus supplies a superficial plexus that lies between papillary and reticular dermis - capillary loops extend from the superficial plexus into the dermal papillae - the plexuses, particularly the superficial plexus, vasoconstrict and vasodilate under cold and hot environment conditions to assist in thermoregulation

5. Meissner’s (tactile) corpuscles - elliptical structures located in the dermal papillae of the fingertips, palms and soles - they are encapsulated mechanoreceptors for light touch

B. Hypodermis

The hypodermis or layer is composed of loose irregular connective tissue and adipocytes ( cells). It is highly vascular and is therefore a prime site for administration of drugs (like insulin). If you examine the hypodermis carefully you may observe the following feature:

1. Pacinian (lamellated) corpuscles - large, oval structures deep in the reticular dermis or hypodermis - have concentric lamellae of flat Schwaan-type cells that surround an axon - are speicalized in the sensation of coarse touch, pressure (sustained touch) and vibrations

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PART D: Adnexa

A. Glands

1. Sebaceous glands

Location - widely distributed except palms and soles of feet - often associated with hair follcles

Structure - acinar

Secretion - sebaceous glands are glands - sebum is a lipid released by decomposition of secretory cells

Function - sebum helps maintain the stratum corneum and hair, it also has Sebaceous gands are antibacterial and antifungal properties branched acinar glands - a disturbance in the flow of sebum from these glands is one of the reasons for developing - a chronic of the sebaceous that obstructs sebum flow

2. Eccrine () sweat glands

Location - widely distributed in the skin - most abundant on the soles of the feet - ~3 million sweat glands in the body - produce around 1.8 liters of sweat per hour

Structure - coiled, - pale staining secretory cells located in hypodermis - dark staining portion difficult to locate but opens at skin surface

Secretion - sweat is produced by secretory cells that form the deep portion of the gland in the hypodermis. - myoepithelial cells contract in response to cholinergic stimulation by Eccrine sweat glands are sympathetic nerves to increase flow of secretion into the duct. coiled tubular glands.

Function - sweat is an effective means of temperature regulation, removing excess heat from the body by evaporation.

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3. sweat glands

Location - axillae, genital region, external auditory meatus, and of the .

Structure - coiled tubular glands - wider lumen than eccrine sweat glands - empty sweat into hair follicles not onto the skin surface - myoepithelial cells contract when stimulated by adrenergic sympathetic nerves to increase flow of secretion

Secretion - despite the gland name, this is a holocrine secretion - sweat produced is an odorless, milky secretion - when broken down by bacteria it causes

Function - function in humans is unknown - in other they function as scent glands Apocrine glands are coiled tubular glands.

B. Nails [Just for fun – This won’t be on the exam]

Nails are hard flexible plates of keratin on the dorsal surface of each distal phalanx. Identify the:

A. root proximal part of the nail, it is covered by the proximal nail fold.

B. Proximal nail fold fold of skin proximal to nail that ends in the .

C. Cuticle () the terminal layer of stratum corneum covering the nail root.

D. Nail plate the keratinized nail itself.

E. Nail bed a bed of epidermis to which the nail plate adheres.

F. Nail matrix cells of the matrix divide and become keratinized to produce the nail root which moves forward to form the nail plate during nail growth.

1 1 1

A B C 1

D

1 E

1

F

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C. Hair and hair follicles [Just for fun – This won’t be on the exam]

Hairs are elongated keratinized structures derived from infolds of the epidermis called hair follicles.

A. Hair bulb - the terminal dilation of the growing follicle. - lining keratinocytes are derived from the stratum basale and stratum spinosum - form a mass of cells called the - constantly divide and undergo keratinization to form the hair itself - contain melanocytes that produce the melanin responsible for hair color

B. Dermal papilla - protrudes into the base of the hair bulb - contains a capillary network required to sustain the

C. Internal and external root sheaths - internal is visible only below the level of the - external root sheath extends all the way to the epidermis where it is continuous with the stratum basale and stratum spinosum

D. Dermal sheath - layer of loose irregular connective tissue surrounding the hair follicle.

E. Sebaceous gland (see previous)

F. - smooth muscle extending from papillary dermis to the midpoint of the external root sheath. - contracts to erect the hair, usually when it is cold, to trap a layer of warm air close to the skin. The result is “goosebumps”.

E 1

F 1

D 1 C A 1 1

B 1

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