L10 - Skin PDF - Human Biology

- SKIN MALARVANI  Learning Objectives Describe the structure and functions of skin and its appendages Describe the microscopic structure of epidermis, dermis and hypodermis Differentiate between thick skin and thin skin Describe the structure, location and functions of...

- SKIN MALARVANI  Learning Objectives Describe the structure and functions of skin and its appendages Describe the microscopic structure of epidermis, dermis and hypodermis Differentiate between thick skin and thin skin Describe the structure, location and functions of sebaceous glands and sweat glands.  This system is composed of : Skin Hair Glands Nails The skin is the largest organ of the body Protective function : FUNCTIONS OF SKIN It is the first line of defense. It protects our body from infection, pathogens, and harmful UV irradiation. Sensory function: Free nerve endings on the skin are sensitive to pain, touch, heat and cold, resulting in either voluntary or reflex activities. Secretory function: Sweat help in temperature regulation and sebum makes skin smooth. Heat regulatory function: Sweating and cutaneous blood flow help in temperature regulation Excretory function: Through the secretion of glands of the skin – water, salt, fatty substances and urea are excreted. Synthetic function : Sun’s ultraviolet rays help in synthesis of natural vitamin D. skin can also manufacture melanin pigment. Water balance: Skin serve a useful means in regulating water balance of the body by perspiration.  Regulation of Body Temperature HOMEOSTASIS Problems in Temperature Regulation Hyperthermia – abnormally high body temperature (Heat stress and heat stroke) Hypothermia – abnormally low body temperature (frostbite) 7 The skin is divided into 3 regions, ❖ Epidermis, ❖ Dermis & ❖Hypodermis (subcutaneous connective tissue)  Skin layers **Epidermis The most superficial layer of the skin The first barrier of protection from the invasion of foreign substances The epidermis is subdivided into 5 layers or strata (1)Stratum basale Provides the germinal cells necessary for the regeneration of the layers of the epidermis. Separated from the dermis by a thin layer of basement membrane. After a mitotic division a newly formed cell will undergo a progressive maturation called keratinization as its migrates to the surface. (2)Stratum spinosum The cells that divide in the statum germinativum soon begin to accumulate many desmosomes on their outer surface which provide the characteristic prickles of the stratum spinosum (SS), which is often called the prickle-cell layer. (3)Stratum granulosum The progressive maturation of a keratinocyte is charcterized by the accumulation of keratin, called keratinization. The cells of the stratum granulosum (SGR) accumlate dense basophilic keratohyalin granules. These granules contain lipids, which along with the desmosomal connections, help to form a waterproof barrier that functions to prevent fluid loss from the body.  (4)Stratum Lucidum The stratum lucidum represents a transition from the stratum granulosum to the stratum corneum. Consists of densely packed flat cells filled with keratin. More pronounced in the areas of thickened skin like sole and palm. Lucid= clear (5)Stratum corneum The stratum corneum is the outermost layer of the epidermis, consisting of dead cells (corneocytes) that lack nuclei and organelles. Desquamation, the process of cell shedding from the surface of the stratum corneum, balances proliferating keratinocytes that form in the stratum basale. A B C A D F E  TYPES OF EPIDERMAL CELLS Stem cell  (1)-keratinocytes: They are responsible for keratin formation Formed of many layers that continuously shed and regenerate every 2-4 weeks They are arranged In many layers (2)-Melanocytes: Found inbetween cells of the basal layer Branched cells with centeral nuclei by EM contains organells for protein synthesizes (rER, Golgi, mitochondria &melanosomes). They form melanin by tyrosinase from tyrosine amino acid by converting it to dioxyphenyl alanine DOPA.  (3)- Langerhans cells: Langerhans cells are dendritic cells (antigen- presenting immune cells) of the skin. Found in upper layers of st.spinosum Have branched shape &central nuclei Represent 3-8%of epidermal Cells (4)-Merkel cells Found in basal cell layer They are modified epidermal cells Sensory nerve fibers form terminal disk under Merkels cells Function as touch receptors **Dermis The dermis is typically subdivided into two zones, Papillary layer - Contains blood vessels and Supply the avascular epidermis with nutrients Reticular layer - Glands and hair follicles are present in this layer Thermoregulation  Skin Appendages 1-Hair and Hair Follicles - Made of hard keratinized epithelial cells 2-Sweat Glands Eccrine or merocrine sweat glands Apocrine sweat glands 3-Sebaceous glands 4-Nails Associated hair structures: Arrector pili muscle Smooth muscle Pulls hairs upright when cold or frightened Sebaceous gland  Hair Color and Texture Brunette Blonde Red Gray and White Eumelanin Eumelanin & Pheomelanin Air in medulla and lack > Eumelanin of pigment in cortex Pheomelanin (2)-Sweat Glands Merocrine sweat gland Apocrine sweat gland ▪ Merocrine secretion ▪ Empty into hair follicle ▪ Location: armpits, groin, nipples ▪ Also called as Eccrine sweat gland ▪ Viscous, cloudy secretion → good ▪ Empty directly onto skin surface nutrient source for bacteria (odor !!) ▪ Location: most all over body (esp. ▪ Secretion begins at puberty and is stimulated during emotional distress abundant on palms & soles) (3)-Sebaceous glands (Holocrine glands) Sebum discharged mostly into hair follicles (lubrication & bactericidal)  Types of skin Thick skin Thin skin *4 layers *5 layers *Less Prominent * Prominent stratum corneum stratum corneum * Absence of * Presence of stratum lucidum stratum lucidum * Dominant and * Palms of the lines most of the hands and soles of body surface the feet * Thicker dermis * Thinner dermis * Presence of Hair * No hair and and sebaceous sebaceous glands glands THICK SKIN THIN SKIN References: 1. Agur, Anne M.R; Dalley, Arthur F. (2025) Grant Atlas of Anatomy, 16th edition. Lippincott Williams & Wilkins, United States of America. 2. Harold Ellis. (2018). Clinical Anatomy for Students and junior doctors, 14th ed. Wiley- Blackwell, Chichester, West Sussex, UK. 3. Sadler, T.W. (2024). Langman's Medical Embryology. 15th ed. Lippincott Williams & Wilkins, United States of America. 4. Moore, Keith L., Dalley, and Arthur F. Agur, A. M. R. (2024). Clinically oriented anatomy, 9th ed. Lippincott Williams & Wilkins, Philadelphia, London. 5. Snell, Richard S., (2019). Clinical anatomy by regions, 10th ed. Wolters Kluwer (Health)/Lippincott Williams & Wilkins, Philadelphia. 6. Victor P. Eroschenko, (2017). Di Fore’s Atlas of Histology with Functional Correlation. 13th ed. Wolters Kluwer, Philadelphia.