Basic Skin Structure Dr. Salman Bin Dayel, M.D College of Medicine Salman Bin Abdulaziz University

The largest organ The full-thickness 1.5- to 4.0-mm Surface area about 2 square meters) It weighs about over 5KG The Skin:

Functions Of The Skin:

3 Main layers:  Epidermis  The superficial portion of the skin  Dermis  The deeper layer of the skin  Primarily composed of connective tissue  Subcutaneous layer or hypodermis SKIN STRUCTURES:

 Stratified squamous epithelium  Avascular (contains no blood vessels)  4 types of cells  4 distinct layers of cells  Regional variation in the relative thickness  The epidermis is thickest on the palms and soles, measuring approximately 1.5 mm  It is very thin on the eyelid where it measures less than 0.1 mm EPIDERMIS

Complosed of 4 layers :  Stratum basale (germinativum)  Stratum spinosum  Stratum granulosum  Stratum corneum  In Palms and soles 5 th layer called Stratum Lucidum EPIDERMIS

Contains 4 types of cells: 1-keratinocytes (85% of cells) 2-Melanocytes 3-Langerhans cells 4-Merkel cells EPIDERMIS:

Stratum basale (germinativum)  Deepest single layer of epidermis  It contains mitotically active (stem cells)10%,  columnar-shaped keratinocytes  KCs attached to each other by desmosomes & to basement membrane via keratin filaments (K5 & K14) by hemidesmosomes  Melanocyte present in this layer

Basal layer

Stratum spinosum:  Several cells layers  Have cytoplasmic processes (spines)  Desmosomes connect bewteen keratinocytes

Stratum granulosum: 1 to 3 layers of fusiform shaped basophilic cells Containes granules (Keratohyalin granules) contain profilaggrin and Loricrin proteins.

Stratum corneum: * Superifical keratinized layer Cells * Almost filled with keratin * Flattened, non nucleate * Coated with extra-cellular lipids that form water barrier of skin * Provide Mechanical protection, barrier for water loss * Layer that varies most in thickness thick  palm & sole (20-30 cell layers thin  other area ( 3-4 cell layers)

EPIDERMAL CELLS:

* 80-85% of epidermis * Ectoderm in origin * For keratin formation * They are arranged In many layers that continuously shed * regenerate every 4 weeks * Psoriasis  4days KERATINOCYTES:

* Found in between cells of the basal layer & At the basal part of the hair follicles. * Origin  neural crest * Branched cells with centeral nuclei * They produce the Melanin to keratinocytes  color to the skin * No. of melanocytes for the same site is same in white and black skin. * Provide protection from UV light MELANOCYTES:

* Found in upper layers of st.spinosum * Have branched shape & central nuclei * Represent 3-8%of epid. Cells * Bone marrow in origin. * Phagocytic & antigen presenting cells (Immune response) LANGERHANS CELLS:

* Found in basal cell layer * Are modified epidermal cells * Sensory nerve fibers * form terminal disk under Merkels cells * Function as touch receptors MERKEL’S CELLS:

The Dermis

 Connective tissue layer  composed of collagen & elastic fibers, fibroblasts, macrophages & mast cells.  Contains hair follicles, glands, nerves & blood vessels  Two major regions of dermis  papillary region  reticular region The DERMIS:

Upper part of dermis Top 20% of dermis Contain finer collagen compared to reticular dermis Finger like projections are called dermal papillae anchors epidermis to dermis contains capillaries that feed epidermis contains Meissner’s corpuscles (touch) & free nerve endings for sensations of heat, cold, pain, tickle, and itch PAPILLARY DERMIS:

Meissner’s (Tactile) Corpuscle:

The thicker deep layer Dense irregular connective tissue Contains interlacing collagen and elastic fibers contain sweat glands, fat & hair follicles Provides strength, extensibility & elasticity to skin Contain many nerve receptors: Ruffini corpuscles  pressure touch. Pacinian corpuscles  vibratory pressure & touch RETICULAR DERMIS:

Basement membrane zone (BMZ)  I t forms the interface between the epidermis and the dermis.  It is continuous along the epidermis and skin appendages.  Its width ranges between nm.

It is divided into: 1- Plasma membrane and hemidesmosomes of basal keratinocytes. 2- Lamina lucida 3- Lamina densa 4- Sublamina densa fibrillar zone

Basement Membrane Zone: Main functions: 1- Attach the epidermis to the dermis. 2- Separate components of the epidermis and dermis. 3- Provide resistance against external shearing forces. 4- Maintain tissue architecture during remodeling and repair.

the layer between the dermis and the fascia The fat tissue acts as a cushion against external physical pressure, retain moisture. The thickness of the subcutaneous tissue depends on the body site, age and other factors Fat cells separated by the connective fibroid fat septum are called fat lobules. Subcutaneous fat tissue

 Subcutaneous plexus  Deep  Dermal-subcutis interface  Large blood vessels  Subpapillary plexus  superficial  Parallel to epidermis Vascular channels and nerves:

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