Basic Skin Histology The skin is divided into two main regions, the epidermis, and the dermis. The dermis is attached to an underlying hypodermis, also called subcutaneous connective tissue. Introduction to Skin Histology The skin is considered the largest organ of the body and has many different functions. By: Mohamed Antar Aziz Mohamed
**Epidermis Skin layers The most superficial layer of the skin The first barrier of protection from the invasion of foreign substances Keratinocyte The epidermis is subdivided into four layers or strata, the stratum germinativum (SG), the stratum spinosum(SS), the stratum granulosum(SGR), and the stratum corneum(SC)
(1)Stratum germinativum 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 is normally only well seen in thick epidermis and represents a transition from the stratum granulosum to the stratum corneum.
(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.
Types of skin Thin skinThick skin *5 layers * Prominent stratum corneum * Well developed stratum granulosum * Palms of the hands and soles of the feet * Thinner dermis * No hair and sebaceous glands *4 layers *less Prominent stratum corneum * Less developed stratum granulosum * Dominant and lines most of the body surface * Thicker dermis * hair and sebaceous glands
TYPES OF EPIDERMAL CELLS
(1)-keratinocytes: Formed of many layers that continuously shed and regenerate every 2-4 weeks They are responsible for keratin formation They are arranged In many layers
They form melanin by tyrosinase from tyrosine amino acid by converting it to dioxyphenyl alanine DOPA. (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).
(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 ¢ral nuclei Represent 3-8%of epid. Cells
(4)-Merkel cells Found in basal cell layer Function as touch receptors They are modified epidermal cells Sensory nerve fibers form terminal disk under Merkels cells
The dermis contains mostly fibroblasts which are responsible for secreting collagen, elastin and ground substance that give the support and elasticity of the skin. **Dermis Thermoregulation Supply the avascular epidermis with nutrients The dermis is typically subdivided into two zones, a papillary dermis and a reticular layer.
(1)-Papillary dermis The papillary dermis (PD) contains vascular networks that have two important functions. The first being to support the avascular epidermis with vital nutrients and secondly to provide a network for thermoregulation. The papillary dermis also contains the free sensory nerve endings and structures called Meissners corpuscles also called mechanreceptor which is responsible for light touch
(2)Reticular dermis The reticular layer of the dermis (RD) consists of mainly loose connective tissue The reticular layer of the dermis is important in giving the skin it overall strength and elasticity, as well as housing other important epithelial derived structures such as glands and hair follicles.
Components of the Dermis The dermis is composed of The dermis is composed of three major types of cell: Fibroblasts , Mast cells, and Adipocytes. Apart from these cells, the dermis is also composed of matrix components such as collagen (which provides strength), elastin (which provides elasticity), and extrafibrillar matrix, an extracellular gel-like substance primarily composed of glycosaminoglycans (most notably hyaluronan), proteoglycans, and glycoproteins) A Mast cell contains many granules rich in histamine and heparin. Although best known for their role in allergy and anaphylaxis, mast cells play an important protective role as well, being intimately involved in wound healing and defense against pathogens.
Fibroblast Fibroblast is a type of cell that synthesizes the extracellular matrix and collagen and plays a critical role in wound healing. Fibroblasts have a branched cytoplasm surrounding an elliptical, speckled nucleus having two or more nucleoli. Active fibroblasts can be recognized by their abundant rough endoplasmic reticulum. Inactive fibroblasts, which are also called fibrocytes , are smaller and spindle shaped. They have a reduced rough endoplasmic reticulum.
FUNCTIONS OF SKIN Protective function : 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 : Suns 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.
Skin Appendages 1-Hair Follicles and hair 2-Sweat Glands Eccrine or merocrine sweat glands Apocrine sweat glands 3-Sebaceous glands 4-Nails
(1)-Hair and hair Follicles Hair -Produced by hair follicle which are made of hard keratinized epithelial cells Melanocytes provide pigment for hair color Structure of Hair Follicle
Hair Anatomy Hair anatomy Central medulla Cortex surrounds medulla Cuticle on outside of cortex Most heavily keratinized
Associated hair structures Hair follicle Dermal and epidermal sheath surround hair root Arrector pili muscle Smooth muscle Pulls hairs upright when cold or frightened Sebaceous gland
(2)-Sweat Glands Empty into hair follicle Location: armpits, groin, nipples Viscous, cloudy secretion good nutrient source for bacteria (odor !!) Secretion may contain Pheromones Secretion begins at puberty and is stimulated during emotional distress Apocrine sweat gland Merocrine secretion Empty directly onto skin surface Location: most all over body (esp. abundant on palms & soles: ~ 500/cm2) Clear, watery secretion (99% H2O; rest NaCl + some waste products Eccrine sweat gland
4-Nail Nails Scale-like modifications of the epidermis Heavily keratinized Stratum basale extends beneath the nail bed Responsible for growth Lack of pigment makes them colorless Nail Anatomy Nail structures Free edge Body is the visible attached portion Root of nail embedded in skin Cuticle is the proximal nail fold that projects onto the nail body
Keratinocytes Culture in Vitro Dermal Fibroblast DMEM medium (high glucose) supplemented with 2 mM L- glutamine ,10% fetal bovine serum and and antibiotics (100 unit/ml penicillin, 100 g/ml streptomycin). Culture Medium: Subcultu