Skin Zhong Jie Li （李仲杰）, Ph. D School of Medicine,Zhejiang University

skin epidermis (dermis) hypodermis

General outline   skin is the largest organ of the human body, it constitutes about 16% of body weight  is composed of epidermis, dermis and its appendages  its total surface area is about m 2  the function of the skin: barrier and protection sensory reception excretion body temperature regulation absorbing some materials remarkable regenertive capacity

Thick skin Thin skin

1.Structure of skin ---epidermis ---dermis

1)Epidermis ---consist of keratinised cell and non-keratinised cell

① keratinised cell: ---from basal to surface, we can classify the cells into five layers Keratinised stratified squamous epithelium

stratum corneum （ C ） stratum lucidum （ L ） stratum granulosum （ G ） stratum spinosum （ S ） stratum basale （ B ） Epidermis

a.stratum basale ---structure: LM: -a layer of cuboidal or low columnar cell with a large, pale nuclei -basophilic cytoplasm EM: -free ribosome -keratin filament - desmosome ---function: mitotic activity and proliferation, for constant renewal of epidermal cells.

b. stratum spinosum ---structure: LM: 4-10 layers polygonal cell with large round nucleus -spinous processes -slight basophilic cytoplasm EM: -tonofibrils -lamellated granules: / nm membrane- coated /contain phospholipid and steroid -intercellular bridges- Desmosome

c. stratum granulosum ---structure: LM: 3-5 layers flattened cell -nuclei begin to degenerate-stained slightly -keratohyalin granules: basophilic EM: -keratohyalin granules: with tonofilament insert into them -lamellated granules: fused with cell membrane * keratohyalin + tonofilament = keratin

d. stratum lucidum ---structure: LM: -3-4 layers of cell appear homogeneous and transparent -no nucleus and organella -eosinophilic-keratohyalin -tonofilament embedded in homogeneous matrix lucidum

e. stratum corneum ---structure: LM: 15~20 layers horny cell ---died cell- no nucleus and organella ---eosinophilic ---keratin * desquamation: surface keratin will shed from outer surface due to abrasion corneum lucidum

② non-keratinised cell: a. melanocyte: ---structure: LM: -large cell with long branches -located among stratum basale cells EM: -risosome -RER -Golgi complexes tyrosine -melanosome(tyrosinase) →↓ melanin ↓ melanin granules

---function: responsible for skin color absorb ultraviolet light protect deep tissue

b. Langerhans cell ---structure: LM: -dark nucleus, light cytoplasms -among the spinous cell -dendritic-typed processes EM: -lysosome -Birbeck granule: /membrane-coated /characteristic racket-shape /15-30 nm long, 4 nm in D ---function: antigen presenting cell in skin involve in immune reaction

c. Merkel’s cell located in basal layer with short processes contain many dense- core granules chemical synapse: between Merkel’s cell and afferent N

---function: not very clear, may be sensory epithelial cell neuroendocrine cell

2) Dermis: located beneath the epidermis  Reticular layer:  Papillary layer: a thin layer of LCT which increase the total contact surface between the dermis and epidermis, contain capillary network and nerve ending (tactile corpuscle) under the papillary layer; consists of irregular DCT; contain large blood vessels, appendages of skin, nerves (lamellated corpuscle). The elastic network is responsible for the elasticity of the skin.

---papillary layer: dermal papillae- increase the junction between epi. and underlying CT papillary layer reticular layer

Meissner’s corpuscle

---reticular layer: DCT, contains rough F,CF, EF, RF large BV, LV lamellar corpuscle skin appendages: including sweat gland, sebaceous gland and hair

2. hypodermis: LCT and fat tissue

Skin appendages   hair  sebaceous gland  sweat gland

Skin appendages

Hair Hair is a thorny thread-like structure. Hair shaft is the part that extends above the surface and hair root is part that is embedded in the skin Hair resides in a hair follicle. Hairs differ in length, thickness and color according to their position on the body

papilla hair root hair bulb hair follicle Sturcture of Hair root

Hair root Hair follicle

Hair root

Sebaceous Glands Associated with the hair follicles Produce oily substance, which is act as a lubricant to make the skin soft and protects both skin and the hair from drying out. Oil released to outside via the hair follicle. If passage becomes blocked, cause acne.

Sebaceous glands Secretory portion: in the central : Large Polygonal shaped cells with many droplets fat in cytoplasm; the basal layer : small cuboidal shaped cells,dark staining. Production: sebum Arrector pili muscle

Arrector pili

Arrector pili – smooth muscle of hair this muscle contracts– “goose pimple”

Sweat Glands Sweat glands are distributed over most of the body. Simple tubular glands that transverse from the dermis to the surface of the epidermis. Merocrine sweat glands are found on most of the body and produce a thin watery solution and function in heat regulation. Apocrine sweat glands are found in axilla, mammary areolae, and circumanal region produce much thicker secretion that is odor producing. Often become activated at puberty

A sweat gland is divided into two parts: Secretory portion: is composed of pyramidal secretory cells Duct: lined by two layers of cuboidal epithelial cells Secretory portion duct