SKIN Basic facts about skin Largest organ of

SKIN

Basic facts about skin • Largest organ of the body • Functions include 1. 2. 3. 4. Barrier Regulate body temperature and water loss. Sensory information Pigments protect from harmful light. 5. Protect against damage to tissues by chemicals, heat etc.

The 3 layers of skin • Epidermis – Stratified squamous keratinized epithelium – Grows continuously • Dermis – Connective tissue – Mechanical support • Hypodermis (Subcutaneous fascia) – Contains adipose tissue

Layers of skin

Layers of the Epidermis • From basal lamina to free surface: 1. stratum germinativum 2. stratum spinosum 3. stratum granulosum 4. stratum lucidum 5. stratum corneum Figure 5– 3

Epidermis

Layers of the Epidermis Figure 5. 2 b

stratum basale • Is the deepest layer of the epidermis (closest to the dermis). • It is found close to the dermal blood supply. • It consists of a single layer of columnar or cuboidal cells which rest on the basement membrane.

• Basal cells are the stem cells of the epidermis. Their mitotic activity replenishes the cells in more superficial layers as these are eventually shed from the epidermis which are called as keratinocytes. • So , this layer is also called as stratum germinativum or germinal layer.

stratum spinosum • The cells become irregularly polygonal. • The cells are attached to one another by desmosomes. • During routine preparation they retract from each other except at desmosomes so they appear to have numerous spines. that's why this layer is called as stratum spinosum. • Some mitosis can be seen in deeper cells of this layer so this layer also comes under germinative zone.

stratum granulosum • Consists, in thick skin, of a few layers of flattened cells which are characterized by presence of deeply staining granules in cytoplasm. • The granules in them consist of protein called keratohyalin. • The nuclei of cells in this layer are condensed and darkly stained (Pyknotic). • Only one layer may be visible in thin skin.

stratum lucidum • This layer is so called because it appear homogenous , the cell boundaries are extremely indistinct. • Traces of flattened nuclei can be seen in some cells. • The stratum lucidum usually cannot be identified in thin skin.

stratum corneum • Most superficial layer of skin. • This layer is acellular. • It is made up of flattened scale like elements (squames) containing keratin filaments embedded in proteins. • These squames are held together by a glue like materials containing lipids and carbohydrates. • Presence of lipids make it highly resistant to permeation by water. • Its thickness is greatest in thick skin.

• The superficial layers of skin are constantly shed of and are replaced by proliferation of cells in deeper layers. • Stratum corneum, lucidum and granulosum are collectively called as zone of keratinization or cornified zone.

The Dermis • Made up of connective tissue. • Richly innervated and vascularized. • Consists of 2 layers Papillary layer : - contain connective tissue of dermal papillae, these papillae are best developed in thick skin. Papillae contains capillary loops, and meissner’s corpuscles. .

Reticular layer: - consist of bundles of collagen fibers. It contain number of elastic fibers, intervals between fibers is occupied by adipose tissue. • It rest on superficial fascia through which it is attached to deeper structures.

Dermis

Sensory Structures of Dermis • Deep touch/pressure: Pacinian corpuscles • Light touch/pressure: Meisner’s corpuscles • Warm temperature: Free nerve endings • Cold temperature: Free nerve endings • Pain: Free nerve endings

Thin skin • Average 0. 08 mm thickness of epidermis. • All over the body except at palm and soles. • Hair present Thick skin • Up to 6 x thicker • Palm, soles etc. • hairless

Thick skin Stratum corneum Stratum lucidum Stratum granulosum Stratum spinosum Stratum germinativum Papillary layer of dermis

Cells of the Epidermis • Keratinocytes – produce the fibrous protein keratin • Melanocytes – produce the brown pigment melanin • Langerhans’ cells – epidermal macrophages that help activate the immune system • Merkel cells – function as touch receptors in association with sensory nerve endings

KERATINOCYTES • Basal layer contain keratinocytes which are derived from stem cells. • After enter into the stratum spinosum they undergo further mitosis these are called as intermediate stem cells. • Thereafter they does not divide.

Keratin formation • Basal cells of epidermis contain numerous cytokeratin filaments. • As basal cells moves into stratum spinosum the protein forming tonofibrils undergoes changes that convert them to keratin filaments. • When epidermal cells reach to stratum granulosum they synthesize keratohyalin granules.

• Keratin consist of keratin filaments embedded in keratohyalin. • Cells in granular layer also show membrane bound , circular granules that contain glycoproteins. These granules are called as keratosomes or lamellated bodies. • When these cells die the material in these granules is released and act as a glue that hold together flakes of keratin.

Skin color depends on : • blood circulation (red cells) • the pigments carotene and melanin • Melanin • Yellow-brown or black pigment. • Present in cells of basal layer and adjoining cells of stratum spinosum. • Produced by melanocytes in stratum germinativum or at the junction of epidermis and dermis.

• Melanocytes are derived from melanoblast that arise from neural crest cells. • Stored in transport vesicles (melanosomes) • Melanin granules formed in melanocyte transferred to keratinocytes through the processes of melanocytes. Because of the processes of melanocytes these are also called as dendritic cells.

Melanocyte

FLEXURE LINES These are certain permanent lines along which the skin folds during habitual movements chiefly flexion.

SKIN APPENDAGES • Hair • Glands (sweat glands and sebaceous glands) • Nails

Hair • Hair is distributed over the entire skin surface except • Palms, soles, and lips • Nipples and portions of the external genitalia • Parts of the hair • Shaft (visible part) • Cortex (outer layer)- acellular , made up of keratin. • Medulla (inner layer)- in thick hair it consist of cornified cells. • Cuticle- membrane covering hair surface formed by flattened cornified cells. These cornified elements contain melanin which is responsible for color of hair.

• Both in medulla and cortex air bubbles are present which are also responsible for color of hair. The amount of air present in hair increases with age and loss of pigment also. These are responsible for greying of hairs. • Root (embedded part) • Hair bulb (expanded lower end of root) • Hair papilla (bulb invaginated from below by dermis) • Hair follicle (tubular sheath which surrounds the root)

The wall of follicle is made up of three layers 1. Inner root sheath present in lower part of follicle. 2. Outer root sheath which is continuous with stratum spinosum. 3. Connective tissue sheath derived from dermis.

Hair Follicles

Hair within follicle dermis papilla

Accessory Structures of Hair • Arrector Pilli : • involuntary smooth muscle • causes hairs to stand up • produces “goose flesh” • Sebaceous glands : • lubricate the hair • control bacteria

Nails • Scale like modification of the epidermis on the distal, dorsal surface of fingers and toes. • Made of dead cells packed with keratin • Produced in a deep epidermal fold near the bone called the nail root

• Nail body • the visible portion of the nail • covers the nail bed • Lunula • the pale crescent at the base of the nail • cuticle

Structure of a Nail Figure 5. 4

Arrector Pili • Smooth muscle attaches to follicle • A sebaceous gland lies in the angle between hair follicle and arrector pili muscle. • contraction of muscle raises hairs (goose flesh) • This occurs during cold and emotional excitement. • Second effect of contraction of muscle is that sebaceous gland is pressed upon and its secretion are squeezed into hair follicle.

Sebaceous glands • Connected to hair follicles, but are found everywhere except palms and soles. • Secrete a waxy, oily substance (sebum) • Secretion increases at puberty

Sebaceous gland sebaceous gland duct hair follicle sebaceous gland

Sweat glands • Found everywhere • Excrete water, salt, metabolic Wastes • Function – cooling of the body

Structures of skin: sweat glands • Duct segment • less coiled, leads to epidermis • Stratified cuboidal epithelium • Secretory segment • In deep dermis or hypodermis • Secretory cells • Myoepithelial cells lie between secretory cells, contraction expels sweat

sweat glands and ducts

Developmental Aspects of the skin • Epidermis develops from ectoderm • Dermis and hypodermis develop from mesoderm

Applied anatomy Albinism: There is no melanin pigment , usually an inherited condition. Dermatitis: There is redness, swelling, itching & exudation.

Thank you