The skin

Normal structure of skin :-

The skin or the integument is the external organ that protects against mechanical trauma , UV light 💡& infection .

In addition,  the skin is concerned with thermoregulation,consevation,excretion of fluid & sensory perception .

 👆 -Cone shaped dermal papillae extend upward into epidermis forming peg-like ridge of the epidermis .

The histology  of  normal skin shows some variation in different parts of the body. 

In general,  it is composed of  two layers which are separated by irregular border :-
                               
               *Epidermis 

               *Dermis 

*Epidermis :- Composed of  five layers from base to surface .

1-Basal cell layer (stratum germinatum) :-

#consist of a single layer of keratinocytes that form the junction between the epidermis & dermis .

#nuclei are perpendicular to the epidermal basement membrane. 

#Other type of  dendritic cells in the basal layer are Langerhans cells (which are bone marrow -derived cells of mononuclear -phagocytes system . 

2-Prickle cell layer (Stratum spinosum, stratum malpighii) :-
#Composed of several layer of polygonal prickle cells or squamous cells. 

#layer become flate as they near the surface 

#These cells possess intracellular bridge 🌉 or tonofilaments. 

#Intracellular cytoplasmic tonofilaments contains PAS-positive material that is precursor of keratin .

3-Granular cell layer (Stratum granulosum) :-

#consist of one to three layers of flate cells containing keratohyaline basophilic granules (which are PAS-negative )

#Granular cells layer is much thicker in Palms and soles. 

4-Stratum lucidum :-

#present in exclusively in palms & soles as a thin homogeneous, eosinophilic, non-nucleate zone. 

5-Horny layer (Stratum corneum ):-

#Normally devoid of nuclei 

#Consists of eosinophilic layer of keratin. 

#Intraepidermal nerve endings are present in the form of  merkel cells which are touch receptors. 

*Dermis :- composed of fibrocollagenic tissue containing blood vessels, lymphatics & nerves. 

consists of two parts 

       *superficial pars papillaris or papillary dermis .

       *Deeper pars reticualris or reticular dermis .



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