Anatomy and Physiology of Skin

Anatomy of skinByDr.Alaa NaifDec 07, 2020

Is the largest organ in the body

Function

Photoprotection from skin cancer and sunburn

Albinism

(No melanin)
are liable
to skin cancer and
sunburn

Barrier against microorganisms

Steven Johnson syndrome
(sloughing of skin)
caused by drugs
liable to infection and
septicemia

Thermoregulation

Ectodrmal dysplasia

(no sweat glands)
Liable to heat stroke in hot weather

Vit D synthesis

Immunological protection

Renal transplant recipients

Liable to skin cancer

Beauty organ

Sensory organ

Skin is composed of three layers:

(1) Epidermis
(2) Dermis
(3) subcutaneous fat (panniculus)


There is considerable regional variation in the relative thickness of these layers

The epidermis is thickest on the palms and soles and thinnest on eyelids

The dermis is thickest on the back

The subcutaneous fat is thickest on the abdomen and buttocks

Epidermis
Composed of:

(1) Keratinocytes

(2) Melanocytes
(3) Langerhans cells
(4) Merkel cells
(5) Skin appendages


Keratinocytes
Ectodermal origin

Produce filamentous protein(keratin) that form a surface coat(stratum corneum) and is the structural protein of hair and nail

Mutation of this protein cause many diseases

Mutation of keratinEpidermolysis bullosa

Keratinocytes are composed of many layers:

Basal layer(stratum basale)
Stratum lucidum (on thick skin only)
Prickle layer(stratum spinosum)
Granular layer(stratum granulosum)
Horny layer(stratum corneum)

The horny layer is nonviable cells and does not have nuclei

Parakeratosis is when the horny layer keep their nuclei as in psoriasis

Hyperkeratosis is when the horny layer get thicker as in corn

Granular layer has keratohyalin granules

Stem cells provide a reservoir for regeneration of the epidermis located the deepest portions of the basal layer and hair bulge


Keratinocytes play an active role in the immune function and secrete a wide array of cytokines and inflammatory mediator

Keratinocytes are attached

to each other by bridges called
desmosomes

Epidermal cell cycle

After the cells are produced by basal layer, they ascend and by reaching the surface they lose their nuclei and become corneocytes that are shed continuously as scales to be replace by new ones

This cycle take 4 week

This cycle is accelerated in ceratin diseases such as psoriasis

Melanocytes

Are dendritic cell and derived from the neural crest

Reside in the basal layer at a frequency of about 1 in every 10 basal keratinocytes.

Face and genitalia and sun damaged skin has a higher density of melanocytes

Increased melanocytes

Decreased melanocytes

Racial differences in skin color are not caused by differences in the number of melanocytes. It is caused by the number, size, and distribution of the melanosomes (pigment granules) within keratinocytes

Melanocytes in red-haired individuals produce more pheomelanin and in other skin type produce more eumelanin

Within keratinocytes, melanin typically forms a umbrella over the nucleus, functions principally as photoprotective

Langerhans Cells

Originte in bone marrow and found scattered among keratinocytes of the stratum spinosum

Cytoplasm contain Birbeck granules that appear as a tennis racquet

Responsible for the recognition, uptake, processing, and presentation of antigens to sensitized T lymphocytes (antigen presenting cells)

Merkel cell

Have neuroendocrine functions

Found in basal layer of epidermis

Skin appendages

Apocrine glands

Eccrine glands
Open into infundibulum of hair follicle
Open directly onto skin surface
Secretion is mediated by adrenergic innervation and androgen
Secretion is Mediated by cholinergic innervation
Found at axillae, areolae, anogenital region, external auditory canal and eyelids
Are found at all skin sites (mostly palms, soles and axilla)
Apocrine sweat is odorless until it reaches the skin surface, where it is altered by bacteria
(responsible for odor of body)
The play a role in cooling when it is hot (thermoreguation)

Hair Follicles

Infundibulum is the uppermost part, extend from the skin surface to opening of sebaceous gland


The isthmus is middle part, extend between opening of sebaceous gland and insertion of arrector pili muscle is isthmus

hair bulb is the lower part

The hair shaft is produced by matrix

(in hair bulb)

Hair goes through three phases:

Anagen: growth phase last for three years
Catagen: involution phase last or three weeks
Telogen: resting phase last for three months

Anagen length determine the length of hair

Sebaceous Glands
Sebaceous duct open into the infundibular portion of the hair follicle

They are distributed throughout all skin sites except the palms and soles

They are found in large numbers in face, scalp, upper chest, back and shoulder(seborreic areas)

They are always associated with hair follicles, except :

Eyelids (meibomian glands)
Buccal mucosa and the lip (Fordyce spots)
Penile mucosa (Tyson glands),
labia minora and Female areola (Montgomery tubercles)

Nails

Is composed of thick keratin

The nail plate is produced by nail matrix

Parts of nail:

Nail plate

Nail bed
Nail matrix
Nail bed
Cuticle

Dermis

The constituents of the dermis are mesodermal in origin except for nerves

Support the epidermis structurally and nutritionally(source of the nutrition to the skin)

Dermis is comoposed of:

Cells: fibroblast, mast cells, macrophages

Fibers: Collagen fibres type I and III constitute the bulk, other fibers are elastic and reticular


Ground substance: composed of glycosaminoglycan e.g. hyaluronic acid due to its high water binding capacity is responsible for hydration of skin