Anatomy of Skin

📍Skin is the largest organ system

📍Measures about 2.12 Sq m, Weighing 4.2kg

📍Hair, Sebaceous and Eccrine glands, Nails, Mucous membranes make important components with specialized physiological functions

Embryology of Skin

📍All constituents are derived from ectoderm and mesoderm

📍Ectoderm and mesoderm begin to proliferate and differentiate at 4th week of intrauterine life

📍The specialized structures of skin, teeth, hair, nails and glands begin to appear at this time

Layers of Skin

📍Epidermis:-

- stratum basale
- stratum spinosum
- stratum granulosum
- stratum corneum

📍Dermis: - 

-papillary dermis
-reticular dermis

📍Subcutaneous tissue

Epidermis

1)Stratum basale (Basal cell layer, Stratum germinatum, Germinative layer) :-

📍Cuboidal /columnar cells; large oval nuclei, dense basophilic cytoplasm

2)Stratum spinosum (Stratum spinosum, Spinous layer, Prickle cell layer) :-

📍5-10 layers of Polygonal cells with delicate spinous processes (desmosomes) connecting adjacent keratinocytes 

3)Stratum granulosum (Granular cell layer) :-

📍1-3 layers of Flattened diamond shaped cells filled with coarse basophilic keratohyaline granules

4)Stratum lucidum :-

📍Clear layer found in palms and soles in between stratum corneum and stratum granulosum

5)Stratum corneum :-
📍Flattened, anuclear, eosinophilic corneocytes;Dead layer shed during epidermal turnover

📍Epidermal turnover/ transit time :
Time taken for a cell to pass from basal layer to surface of skin is 52-75
days (normal skin)

📍Other Cells in Epidermis
1)Melanocyte
2)Langerhans cell
3)Merkel cell

Melanocytes

📍Neural crest derived Dendritic cells

📍Synthesize and secrete melanin containing organelles called melanosomes
📍Located in basal cell layer; 1:10 ratio
📍Epidermal Melanin Unit : A single melanocyte supplies melanosomes to 36 keratinocytes (1:36)
📍Melanosomes vary in distribution and size according to skin type.
However, the density of melanocytes in different races is the same.

📍Melanocytes is responsible for Melanogenesis
(Melanin formed through mediation of tyrosinase and DOPA from tyrosine)
Melanogenesis is controlled by :
Genetic factors
UVR
Hormones (e.g. MSH)

📍Melanocytes - Functions
Melanin:
Impart colour to skin and hair
Protect the skin from UV radiation
Biochemical neutralizer of toxic, free radical oxygen derivatives

Langerhans cells

📍Dendritic cells
📍Type of macrophage
📍Role in various immune processes like - allergic contact dermatitis ,immune  tolerance, surveillance against viral infections and neoplasia

Merkel cell

📍Non-Dendritic cells lying in or near basal layer or hair follicles
📍Neuritic cells
📍Fine touch receptors
📍Detect mechanical deformities of epidermis

Functions of Epidermis

📍Cornification
📍Barrier function
📍Permeability
📍Maintenance of fluid and electrolyte balance
📍Thermoregulation
📍Pigmentation
📍Immune function
📍Sensory receptor
📍Vitamin D Synthesis

Epidermal Appendages

📍Hair follicles
📍Sebaceous glands
📍Sweat glands
📍Eccrine glands
📍Apocrine glands
📍Apo-eccrine glands

Hair

📍Found over the entire surface of the body except palms, soles, glans penis, clitoris, labia minora, mucocutaneous junction and distal portions of the
fingers and toe.

📍Types of Hair
*Lanugo hair : Fine, soft hair in fetus
*Vellus hair : Fine, short, non-medulated hair over most parts of the body
*Terminal hair : Long, coarse, medulatd hair over scalp, beard, and body depending on age and gender

📍Hair parts in longitudinal section
*Infundibulum
*Isthmus
*Stem
*Bulb

📍Hair parts in Cross section
*Outer sheath
*Inner sheath
*Henle’s layer
*Huxley’s layer
*Cuticle
*Cortex
*Medulla

📍Hair Growth Cycle

Hair cycle consists of three phases:

1)Anagen : Phase of growth and activity, lasts for 2-10 years
About 90% of hair are in anagen at a given time

2)Catagen : Phase of transition, lasts for 1-3 weeks.
About 1% hair are in catagen

3)Telogen : Resting phase lasts for about 3 months.
About 10% hair are in telogen. Telogen hair is shed and anagen hair replaces it
Average hair loss is 100/day

Sebaceous Glands

📍Lipid producing holocrine, multi-lobed glands
📍Arise from the hair follicle at the junction of the infundibulum and the isthmus
📍Distributed all over the body except the palms and soles
📍Numerous, large and productive over the face and scalp
📍Stimulated by androgens and mature at puberty
📍Consists of lobules of epithelial cells that differentiate toward lipid producing cells in a centripetal manner
📍Enlarged, vacuolated cells in the center of the lobule disintegrate into an amorphous mass – the sebum
📍Major components of sebum : Triglycerides, wax esters, squalene,cholesterol esters, and cholesterol
📍Significant in:-Acne, Rosacea, Seborrheic dermatitis

Sweat Glands

📍Depending on mode of secretion these are –Eccrine sweat glands and Apocrine sweat glands. Apo-eccrine glands have features of both
📍All have 2 parts:-
*Secretary coil
*Duct

Eccrine Sweat Glands:-
📍These are present all over body except over the lips, external ear canal and labia minora
📍Most concentrated in the palms, soles and axillae
📍The secretary coil lies deep in the dermis that connects with the surface by a duct
📍Major role is in Thermoregulation
📍Control of sweating
📍Innervated by sympathetic cholinergic nerve fibers
📍The central control of sweating lies in preoptic hypothalmic sweat centre-
Temperature, Emotions, Hormones (antidiuretic hormone, aldosterone),Gustatory (hot spicy foods)

Apocrine Sweat Glands:-
📍These are mainly present in axillae, nipples, peri-umblical skin, perineum and genitalia
📍Modified apocrine glands are -Ceruminous glands (external ear canal),
📍Moll’s glands (eyelids) and Mammary glands
📍The secretary coil lies at junction of dermis and subcutis
📍The duct opens into the mid part of hair follicle just above the entrance of sebaceous gland
📍Innervated by adrenergic sympathetic nerve fibers
📍Vestigial sexual function and represent Scent glands

DERMIS

📍Constitute 15-20% of body weight
📍Papillary dermis - thin zone beneath epidermis
📍Reticular dermis - thick zone which extends from base of papillary dermis
to the surface of subcutaneous fat
📍The upward projection of dermis, the dermal papillae, strongly interdigitate with the downward , the rete pegs
📍3 components : Cells, Fibers and Ground substance.
📍Dermis - Structure
*Cellular contents-Fibroblasts, mast cells, histiocytes, Langerhans cells, lymphocytes and eosinophils
*Non-cellular connective tissue-Collagen (80%), elastic fibers (2%) and ground substance
(mucopolysaccharides, glycoproteins, chondroitin sulphate)
*Embedded nerves, blood vessels, lymph vessels, muscles and pilo sebaceous, apocrine and eccrine units
📍Dermis - Variation in thickness
Difference of thickness of the skin is dependent largely on dermal thickness, with the palms and soles being thickest (1.5 mm) and thinnest in the eyelids and post-auricular region (0.05 mm).
📍Males have thicker skin than females
📍Children and elderly have thinner skin than adults

Dermo-epidermal Junction
(Basement Membrane Zone)

📍Consists of:-
*Basal lamina
 *Lamina lucida
*Lamina densa
*Anchoring filaments
*Anchoring fibrils
*Dermal microfibril bundles

📍Functions:-
*Attachment of dermis to epidermis
*Mechanical support to epidermis
*Regulation of permeability for nutrients and exogenous substances
*Influence growth, differentiation and migration of basal keratinocytes

📍Clinical significance:-
Autoantibodies to proteins in the dermo-epidermal junction responsible for Bullous pemphigoid
Inherited defect in Epidermolysis bullosa

Nerves & Innervation of skin

📍Rich network of nerves with 2 types of sensory endings – ‘The Corpuscles (Mechano-receptors)’ and ‘Free nerve endings (Nociceptors)’

*Mechano-receptors
Light touch : Merkel cells of the epidermis, Meissner’s corpuscles in dermal papillae
Pressure : Pacinian corpuscles in deep dermis or subcutaneous tissue

*Nociceptors
-Pain and Itch : Transmitted through naked fine free nerve endings located in the basal layer of the epidermis close to the dermo-epidermal junction

-Temperature :
Krause bulbs detect cold, Ruffini end organs detect heat.

Heat, cold and proprioception also located in the superficial dermis
Adjacent dermatomes often overlap, important for local anesthesia

Blood & Lymphatic Supply of skin

📍Extensive subdermal and dermal plexuses

📍Dermal plexus: 2 horizontal plexuses connected by vertical communicating vessels
*Superficial horizontal plexus lies in the papillary dermis, feeding  arterioles/capillary loops in dermal papillae
*Deep horizontal plexus lies deep just above the subcutis, supplies to sweat gland and hair follicles

📍Cutaneous vasculature important in thermoregulation

📍Cutaneous lymphatics parallels the blood supply