Anatomy & physiology of skin
Skin StructureSkin is the single largest organ in the human body. It weighs an average of 4 kg and covers an area of 2 m2 Three distinct layers Epidermis: Composed of epithelial tissue Dermis: Composed of a combination of connective tissues Hypodermis: usually contains abundant fat.
Epidermis
It’s outermost layer of skin.It consists of many layers of closely packed cells. The most superficial of which are flattened and filled with keratins.It is a stratified squamous epithelium. Contains no blood vessels. It varies in thickness from less than 0.1 mm on the eyelids to nearly 1 mm on the palms and soles.Stratum Basale
the deepest layer, rests on a basement membrane, which attaches it to the dermis. It is a single layer of columnar cells. In normal skin only 30% of basal cells are preparing for division. Once basal cell leaves basal layer in humans, normal transit time to stratum corneum is at least 14 days, and transit through stratum corneum to desquamation requires 14 days, 28 days total.Stratum Spinosum
Consists of 8-10 layers of Keratinocytes. They are named for the spine-like appearance of the cell margins in histologic sections. As these cells differentiate and move upward through the epidermis, they become progressively flatter and develop organelles known as lamellar granules Composed of Keratinocytes attached to each other via desmosomes. Contains langerhans cells that aid in the immune system response.Stratum Granulosum
Stratum Granulosum: The middle layer of 3-5 layers of cells that help form keratin. Contains keratohyline granules that produce a secretion These make up the thick and tough peripheral protein coating of the horny envelope.Stratum Lucidum
Stratum Lucidum: Found only in the thick skin of the palms and soles of the feet Tends to be translucent.Stratum Corneum
Stratum Corneum: The outermost layer of the epidermis. Contains up to 30 layers of flattened dead cells (corneocytes) separated by lipids in the intercellular space. Provides protection. Continuously shed and replaced.Cells of the epiderms
Keratinocyte 80% Melanocyte 10% Langerhan’s cell 2-4% Merkle cellMelanocytes
Melanocytes are the only cells that can synthesize melanin. They migrate from the neural crest into the basal layer of the ectoderm.They are also found in hair bulbs, the retina and pia arachnoid.Each dendritic melanocyte associates with a number of keratinocytes, forming an ‘epidermal melanin unit’ . Their cytoplasm contains discrete organelles, the melanosomes, containing varying amounts of the pigment melanin injected into surrounding keratinocytes to provide them with pigmentation to help protect the skin against damaging ultraviolet radiation.Difference between White & dark individual
SameSame
Melanocyte number
Many
Few
Melansome number
Large
Small
Melanosome size
High
Low
Degree of melanization
Individually dispersed
Grouped in membrane coated complex
Distribution in keratinocyte
Langerhan’s cell Bone marrow –derived dendritic cell.Location in epidermis: St. sipnosumLangerhans cells constitute 2-4% of the total epidermal cell population.Characteristic finding on electron microscopy – Birbeck granules- tennis racket shaped bodies in the cell.Function: Antigen presenting cells, immune surveillance.Functionally impaired by ultraviolet radiation and topical steroid.Special stain: S100, CD1a
Merckel’s cell Shape: Non dendritic cell Site: MM, lip, digit & near hair Location in epidermis: St. Basalis Origin: Unknown Function: Transducer of fine touch
Cells of the epiderms
Merckle’s cell Langerhans cellMelanocyte
Few
Next common
Most common
Prevalence
-
2-4%
10%
Percentage
St.Basalis
St. Spinosum
St. basalis
Location in epidermis
Unknown
Bone marrow
Neural crest
Origin
Sensory function
Immune surveillance
Production of melanin
Function
MM, lip, digit
All skin
All skin
Type of skin
Basement Membrane Zone
Site of interaction between epidermis and dermis Three layers 1.) Lamina lucida, 2.) Lamina densa, 3.) Sub-lamina densa Hemidesmosomal adhesion complex: 1.) hemidesmosomal plaque 2.) anchoring filaments 3.) anchoring fibrilsDermis
Dermis: Composed primarily of connective tissue, mainly collagen, elastic fibers and ground substances. Divides into 2 Categories: Papillary Region: Superficial part that contains the dermal papillae and comprises the wavy boundary between the epidermis and dermis. Houses capillaries, nerve endings, and Meissner corpuscles (touch sensors). Reticular Region: Deeper region made up of dense irregular connective tissue. Contains many of the glands, nail roots, and root follicles.Hypodermis
Hypodermis: Known as the subcutaneous tissue or superficial fascia. The boundary of the dermis is indistinct, but it is clear that there is more areolar and adipose tissue than in the dermis. Primarily serves as a storage site for large blood vessels and fat. Houses Lamellated or Pacinian corpuscles (nerve endings sensitive to pressure).Adnexal structures
Sebaceous glands Sweat glands: Eccrine sweat glands Apocrine sweat glandsSebaceous glands
Most sebaceous glands develop embryologically from hair germs, but a few free glands arise from the epidermis. The glands themselves are multilobed and contain cells full of lipid, which are shed whole (holocrine secretion) during secretion. Sebum contains a mixture of triglycerides, fatty acids, wax esters, squalene and cholesterol. Sebum is discharged into the upper partof the hair follicle. It lubricates and waterproofs the skin, and protects it from drying. It is also mildly bactericidal and fungistatic. Androgenic hormones stimulate sebaceous gland activity.Eccrine sweat glands
There are 2–3 million sweat glands distributed all over the body surface but they are most numerous on the palms, soles and axillae.The tightly coiled glands lie deep in the dermis, and the emerging duct passes to the surface by penetrating the epidermis in a corkscrew fashion. Sweat is formed in the coiled gland by active secretion.Sweat glands have an important role in temperature control, the skin surface being cooled by evaporation. Three stimuli induce sweating: 1- Thermal sweating 2- Emotional sweating 3- Gustatory sweating The eccrine sweat glands are innervated by cholinergic fibres of the sympathetic nervous system.Apocrine sweat glands
Apocrine glands are limited to the axillae, nipples, periumbilical area, perineum and genitalia. The coiled tubular glands (larger than eccrine glands) lie deep in the dermis, and during sweating the luminal part of their cells is lost (decapitation secretion). Apocrine sweat passes via the duct into the mid-portion of the hair follicle. The action of bacteria on apocrine sweat is responsible for body odour. The glands are innervated by adrenergic fibres of the sympathetic nervous system.Sebaceous
Appocrine
Eccrine
Hairy skin, free in eyelid, lip, nipple, genetalia
Axilla, areola, Periumblical, genetalia
All skin
Distribution
Multilobulated
Coiled
Coiled
Acinar part
Short & wide
Straight
Straight
Duct shape
Hair follicle
Hair follicle
Skin surface
Duct opening
Holocrine
Decapitation
Merocrine
Excretion
Oily
Milky & odorless
Isotonic
Secretion
Androgen
Adrenalin
Acetyl choline
Stimulant
Waterproof, lubricant Anti-microbial.
Unknown ,Sex stimulant
Thermoregulation
Function
Epidermal appendages