Dr. Nabil khouri

Integumentary System • Consists of:

1. Skin (Epidermis, Dermis & hypodermis)

2. Accessory structures

• Hair

• Nails

• Glands

Functions of the Integumentary System

• Protection – Chemical, physical, and mechanical barrier

• Body temperature

– Regulated by dilation (warming) and constriction (cooling) of dermal

vessels

• Sweet glands: Termo-regulation, increase secretions for body cooling

• Coetaneous sensation – Receptors cells to sense touch and pain

• Metabolic functions – Synthesis of vitamin D

• Blood reservoir – Skin blood vessels Reservoar : store up to 5% of the

body’s blood volume

• Excretion – Internal Balance, Control the amount of nitrogenous wastes

which are eliminated from the body in sweat – Ion balance.

• Absorption - One large door for medical usage – Application.

Skin Layer

Epidermis

Papillary layer

Dermis

Reticular layer

Hypodermis

Epidermis and Dermis

Dermal blood

vessels

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Epidermis • Stratified Squamous

Keratinized Epithelium

• Contains no blood vessels/avascular

• 4 types of cells

• 5 distinct strata (layers) of cells

Epidermal Cells

• Cell types

– Keratinocytes: Produce keratin for strength

– Melanocytes: Contribute to skin color (pigment melanin)

– Langerhans’ cells: Part of the immune system

– Merkel’s cells: Detect light touch and pressure

• Desquamate: Older cells slough off

• Keratinization: the prosses by which the cells die and produce an outer layer that resists abrasion and Impermeabil layer

Keratinocytes (90%)

Stem cell.

Produce keratin and

Connected to adjacent cells

by desmosomes and to basal

lamina by hemidesmosomes

Langerhan cells (2-3%)

– Originate from the bone marrow

– Provide immunity

– Their fine processes form a network between the cells of the epidermis.

– Function as phagocytosic cells.

– Langerhans cells may only be temporary residents of the skin

Epidermis Cells

Melanocytes (8 %)

• produces melanin pigment

• Send long fine processes between the other cells.

• The melanin is located in membrane-bound organelles

called melanosomes.

• Melanin is transferred to

other cells through cell

Processes

• Merkel cells (< 1%)

– Touch receptor cells.

– Found within the deepest layer

– Specialized cells for light touch and pressure

– Connected receptor with sensory neuron

Epidermis Cells

Epidermal Layers • Stratum Basale

– Deepest portion of epidermis and single layer

– High mitotic activity and cells become keratinized

• Stratum Spinosum

– Limited cell division

• Stratum Granulosum

– Superficial cell layer with nucleus and organelles that degenerate and dies

• Stratum Lucidum

– Thin, clear zone

• Stratum Corneum

– Most superficial and consists of cornfied cells

Epidermal Layers and Keratinization

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• Basal cells are the stem cells of the epidermis.

• The deepest epidermal layer firmly attached to the dermis

• Consists of a single layer arranged in one row of columnar or cuboidal cells with intermediate keratine filaments bound by desmosomes

• Rest on basement membrane.

• Made of the youngest keratinocytes with Mitotic activity - Cells undergo rapid and intense division (alternate name, stratum germinativum).

• The renewal of the human epidermis takes about 3 to 4 weeks.

Layers of the Epidermis: Stratum Basale

• Cells become irregularly polygonal (cuboidal to to slightly flattened cells) with central nucleus separated by translucent clefts

• Spines of cells meet end-to-end or side to-side and are attached to each other by desmosomes.

• Cells contain intermediate filaments (tonofilaments) attached to desmosomes

• Melanin granules and Langerhans cells are abundant in this layer.

Stratum Spinosum

Spines of cells

meet side to-

side

Desmosomes

• Three to five cell layers in which drastic changes in keratinocyte distribution and appearance occurs

– In thick skin, of a few layers of flattened cells.

– In thin skin, only one layer may be visible.

• The cytoplasm of the cells contains numerous fine grains, keratohyalin granules.

• The nuclei begin to degenerate in the outer part of the stratum granulosum

Stratum Granulosum (Granular Layer)

• Thin, transparent band superficial to the stratum granulosum.

• Consists of several layers of flattened dead cells.

• The faint nuclear outlines are visible in only a few of the cells.

• The stratum lucidum can usually not be identified in thin skin.

• Present only in thick skin

Stratum Lucidium (Clear Layer)

• Outermost layer of keratinized cells (15-

20 layers)

• cells are completely filled with keratin

filaments (horny cells) which are

embedded in a dense matrix of proteins.

• Accounts for three quarters of the

epidermal thickness

• Individual cells are difficult to observe

because

– (1) nuclei can no longer be identified

– (2) the cells are very flat and have

protein envelop

– (3) the space between the cells has

been filled with lipids, which cement

the cells together into a continuous

membrane.

Stratum Corneum

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Other important notes

• The red coloration of the skin are due to haemoglobin in

the red blood cells, which pass through the capillaries

beneath the epidermis.

• Yellow coloration of the skin are due to carotene, which

could accumulates in fat cells found in the dermis and

hypodermis.

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Thick & Thin skin

The dermis

• It is called "true skin”.

• The dermis is a CT layer composed of collagen (mostly type I), Reticular fibers in addition to elastin, and glycosaminoglycans.

• It is much thicker than the epidermis, comprising 80-90% of the total dermis and epidermis.

• It contains extensive vasculature, neurons, smooth muscle, and fibroblasts and skin apendeces.

• It is the most important mechanical barrier of skin.

• The network of elastic fibers function to support the epidermis and bind the skin to the deeper hypodermis.

Dermis

– Structural strength

– Two layers

• Papillary layer:

Superficial layer

underneath

epidermis - Loose

connective tissue

• Reticular layer:

Deeper layer -

Dense connective

tissue; stretch

marks (striae)

The Dermis “Papillary layer”

• The papillary layer is a relatively thin layer of loose connective tissue

that lies immediately beneath the epidermis.

• This layer consists of loose connective tissue, cell-rich connective

tissue, which fills a hollows finger–like projection at the deep surface

called dermal papillae.

• A key feature of this layer is the dermal ridges (dermal papillae) that

extend up in ridges into the overlying epidermis and interdigitate with

epidermal invaginations (epidermal ridges).

• A double row of dermal papillae in finger pads produces the ridged

fingerprints on fingertips.

– The dermal ridges contain Meissner’s corpuscles, encapsulated

nerve endings, and capillary loops that provide nutrients to the

avascular epidermis.

– A basal lamina supports the basal layer of the epidermis.

– Collagen and elastic fibers underlie the basal lamina.

– Fibroblasts, mast cells, and macrophages occupy the papillary

layer. Collagen fibres appear finer than in the reticular layer.

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The reticular layer • The reticular layer resides below the

papillary layer.

• The reticular layer appears dense and

contains fewer cells

• It is comprised of thick criss-crossing

collagen fibers aggregate into bundles.

These fibers form an interlacing

network with direction that is parallel to

the surface of the skin called cleavage

(Langer's) lines.

• Usually, their main orientation will

follow the "lines of greatest tension" in

the skin (Kraissl lines)

• Elastic fibers (irregular dense connective

tissue) together with relatively few cells

are present

Arteries and veins of the dermis

– Run through the hypodermis and branch upward to form plexuses

of anatomizing vessels.

– The coetaneous plexus resides at the junction of the hypodermis

and dermis, and the papillary plexus resides just beneath the

epidermis within the dermal papilla.

• This system provides nourishment to the dermis and by

diffusion to the epidermis, which is a-vascular.

• The vascular system functions in thermoregulation. Blood flow

is controlled by contraction of arterioles and venules they send

blood through the capillary bed for heat radiation.

• In some regions of skin, arterial-venous anatomizes, or

shunts, can send blood directly from the arterioles to venules

in order to reduce heat loss.

Receptors in Skin

Classification by Location

• Exteroceptors – sensitive to stimuli arising from outside

the body

– Located at or near body surfaces

– Include receptors for touch, pressure, pain, and

temperature

• Interoceptors – (visceroceptors) receive stimuli from

internal viscera

– Monitor a variety of stimuli

• Proprioceptors – monitor degree of stretch

– Located in kin covering musculoskeletal organs

Classification by Modality

• Mechanoreceptors – respond to mechanical forces

• Thermoreceptors – respond to temperature changes

• Chemoreceptors – respond to chemicals in solution

• Photoreceptors – respond to light – located in the eye

• Nociceptors – respond to harmful stimuli that result in

pain

• Divided into two groups

• Free nerve endings “unencapsulated”

• Encapsulated nerve endings

Neuronal Elements of the dermis

The skin is richly innervated

Motor nerve endings are directed to the glands, smooth muscle of blood

vessels, Erector pili muscle of the hair

– The dermis contains elements for touch, pain, itch, and temperature reception.

• Some receptors are free nerve endings.

• Other nerve endings associate with Merkel cells in the epidermis.

• Meissner’s corpuscles reside in the dermal papillae and function as mechanoreceptors in touch perception.

• Pacinian corpuscles are found deep in the dermis (and in the hypodermis) and function in pressure sensation.

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Encapsulated Receptors • Consist of one or more end fibers of

sensory neurons

• Enclosed in connective tissue

• Mechanoreceptors

• Include four main types

Meissner’s corpuscle

Encapsulated nerve ending in the dermal papilla of glabrous skin; light pressure and touch receptors sensitive to low frequency stimuli

Pacinian corpuscles

Axon looses myelin and passes as a single process into a concentrically layered capsule (combination of CT and Schwann cells).

Found deep in the dermis and in the hypodermis; sense the vibration and pressure

Un-Encapsulated Nerve Endings

Free nerve endings:

nerve terminal looses its myelin sheath at the epidermal/dermal junction and passes into the epidermis ending in the stratum granulosum; sense pain and temperature

Hair follicle receptors:

axon looses myelin sheath and bare axon coils around the shaft within the external root sheath; senses hair motion and its direction (mechanoreceptor)

Merkel cell endings:

axon looses its myelin and forms a disc like structure with the Merkel cell; mechanoreceptor

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Un-Encapsulated Nerve Endings

Hypodermis

• Skin rests on this, but not a part

• Also called

– Subcutaneous tissue

– Superficial fascia

• Consists of loose connective tissue

• Types of cells

– Fibroblasts

– Adipose (fat) cells

– Macrophages

• Subcutaneous fat

Epidermal appendages

• Hair

– Found everywhere on human body except palms, soles, lips, nipples, parts of external genitalia, and distal segments of fingers and toes

• Nails

• Glands

– Sebaceous or oil glands

– Sudoriferous or sweat glands

– Mammary glands

Hair (Pilus)

– is composed of dead epidermal cells that have undergone a modified epidermal keratinization including the expression of specific keratin proteins that are highly crosslinked by disulfide bonds.

– It is derived from hair folicles, which are epidermal invaginations that project into the dermis or hypodermis.

• Two types of hair are present on humans:

– Vellus (fine) hair, which is short, soft, fine, and pale

– Terminal (thick) hair, which is hard, large, coarse, long and dark.

» The number of hairs on all primates is similar, but most is vellus on humans and terminal on other primates.

Hair structure

• Composed of shaft and root

– Shaft protrudes above skin surface

– Root located below surface and base forms the hair bulb

• Pigmented by

melanocytes at

the base of the

hair

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The hair follicle

• undergoes a cycle of active and resting phases during which a new hair is started and then falls out, respectively.

• Melanocytes are scattered throughout the hair shaft. These impart color to the hair. With age, tyrosinase production decreases and the hair becomes gray.

• Has 3 concentric cell layers – Medulla: Central axis

– Cortex: Forms bulk of hair

– Cuticle: Forms hair surface

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Hair follicle

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Hair follicle

Hair Bulb • The root sheath extending from the epidermal surface into the

dermis

• Deep end is expanded forming a hair bulb

• A knot of sensory nerve endings (a root hair plexus) wraps

around each hair bulb

• Bending a hair stimulate these ending hence our hairs act as

Sensitive touch

receptors

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Hair bulb

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Structure of a Nail

• Scalelike modification of the epidermis on the distal, dorsal

surface of fingers and toes

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Glands Holocrine Seb.Gl

Merocrine Seb.Gl

Simple coiled tubular

glands secretion

released by

exocytosis, with no

loss of cytoplasm Sebaceous gland

SEBUM secretion released,

filling a dead cell

HOLOCRINE

MODES OF SECRETORY RELEASE

MEROCRINE / ECCRINE

released by exocytosis, with

a little loss of cytoplasm

rare

APOCRINE rare

Clear & Dark

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Skin glands

Glands classification Sebaceous Glands Eccrine Sweat Glands

Location ubiquitous except palms and soles,

usually open into hair follicles

ubiquitous, most numerous on palms

and soles, open onto skin surface

Mechanism of

Secretion holocrine merocrine

Gland Histology multiple, saclike acini, differentiating

squamous epithelium

tubular coils, simple cuboidal

epithelium

Duct very short, stratified squamous

epithelium

double layer of cuboidal cells, uptake of

ions

Composition of

Secretion

sebum (lipid - wax esters, squlaene,

fatty acids and triglycerides)

hypotonic salt solution, lactate, urea

dark cells - sialomucins

clear cells - water, salt

Secretion

Mechanism

spontaneous, aided by arrector pili

muscles

myoepithelial cells on gland, also

provide support

ECCRINE

SWEAT GLAND

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Apocrine sweat

glands

• are simple tubular glands that empty into hair follicles in axillary and anogenital regions.

• The secretion is a mixture of proteins, carbohydrates, and ferric ions that is odorless when secreted, but is acted on by commensal bacteria.

• They begin to function at puberty; but their function is unknown

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Sebaceous Glands

PRODUCE OILY SECRETION KNOWN AS SEBUM THAT SPREADS OUT ALONG THE SURFACE OF THE SKIN AND KEEPS THE KERATIN RICH EPIDERMIS FLEXIBLE AND WATERPROOF.