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 the Epidermis 1. 2. 3. 4. 5. Stratum germinativum Stratum spinosum Stratum granulosum Stratum lucidum Stratum corneum Figure 5– 3
Layers of the Epidermis Figure 5. 2 b
Stratum basale It is the deepest layer of the epidermis. It is found close to the dermal blood supply. It consists of a single layer of columnar cells. Basal cells are the stem cells of the epidermis. This layer is also called as stratum germinativum or germinal layer.
• Stratum spinosum • This consists of several layers of polyhedral cells. • They lie over the stratum basale. • They are called spinosum as the edges of cells look like spines.
Stratum granulosum • A few layers of flattened cells with 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 called so 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. • 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. • Papillae contains capillary loops and meissner’s corpuscles. .
Reticular layer: Consists of bundles of collagen fibers. It contain number of elastic fibers Intervals between fibers is occupied by adipose tissue. It rests 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. • 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 • simple cuboidal epithelium
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.
Thin skin
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