- Human Skin Histology.

-Human skin is considered the largest organ of the body and form  about 16% of the body weight (1) with a surface area somewhat less than 1.5 m² (2).
-The skin form the external surface of the human body.At the orifices of the mouth,nose and the anal canal, the skin join the mucous membrane at the muco-cutaneous junction (3).

-Types of human skin (1):-

1. Thick skin (Non-Hairy) which has thick epidermis and found only in the palms and soles as they are the most sites subjected to abrasions and trauma and thick skin shows characteristic parallel ridges and grooves which are called "Finger prints".
2. Thin skin (Hairy) which has thin epidermis and covers the rest of the body.
   

-Histological structure of the skin :-

skin is composed of 3 layers

1. Epidermis
2. Dermis
3. Hypodermis (Subcutaneous fatty layer) 

1- Epidermis:-

• It is the outer superficial epithelial layer of skin.
• It is composed of Stratified squamous keratinized epithelium.
• The thickness of the epidermis varies in different types of skin. It is the thinnest on the eyelids and the thickest on the palms(0.8mm) and soles(1.4mm).
• The epidermis is ectodermal in origin.
  
• The epidermis is devoid of blood vessels and gets its nutrition through diffusion.
  
  
  
  

-It consists of :-

• Keratinocytes (85% of cells) which are responsible for formation of keratin,
• Dendertic cells which are (melanocyte and the Langerhans cell)
• Merkel cells which until recently, were not consistently identified as the third dendritic cell in human epidermis, because light microscopy and low power electron microscopy did not readily allow the differentiation of merkel cells from the other two dendritic cells of the epidermis.

-Keratinocytes are arranged in 5 layers from above downward:-

• Stratum basale (basal cell layer)(The stratum germinatum):- which is single layer of columnar cells with basal oval nuclei,cells of this layer are resting on basement membrane and attached to each other and with the overlying cells by inter-cellular bridges called "desmosomes" these basal cells continue to divide throughout the life so they are called mother cells of epidermis.
• Stratum spinosum (prickle cell layer):- formed of 5-7 layers of nucleated polygonal cells attached to each other by desmosomes which are like spines hence its name.
• Stratum granulosum (Granular cell layer):- formed of 2-3 layers of nucleated spindle shaped cells which 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.
• Stratum licidum (Clear layer):- only well seen in thick epidermis and represents a transition from the stratum granulosum to the stratum corneum and formed of dead clear non nucleated containing eleidin granules.
• Stratum corneum (horny layer):- formed of flattened non nucleated cells called "squames" condensed in linear manner and containing Keratin which derived from eleidin granules.These squames are continuously shed from the surface and replaced from the deeper layer by new ones.

-Melanocytes:- they form melanin from tyrosine under the effect of tyrosinase enzyme and lie just under and in between basal cells and in hair matrix.

-The Langerhans' cells:- present in Stratum spinosum,they act as macrophages.

-The Merkel's cell's:- present in Stratum basale and they act as receptors for touch sensation.

2- Dermis:-

• The dermis is typically subdivided into two zones, a papillary layer and a reticular layer.
• It is 15-40 more thicker than epidermis.
• Function:- act as frame work and supports for nerves,lymphatics,hair follicles,sweat glands,Sebaceous glands and blood vessels to supply the avascular epidermis with nutrients.
• The dermis contains mostly fibroblasts which are responsible for secreting collagen, elastin and ground substance that give the support and elasticity of the skin. Also present are immune cells that are involved in defense against foreign invaders passing through the epidermis.
• The dermis is mesodermal in origin.
• The papillary layer lies below and interdigitates with the epidermal rete ridges,papillary layer is formed of loose C.T with fine collagenous fibers type II,reticular and elastic fibers . and contains the free sensory nerve endings and structures called Meissner's corpuscles in highly sensitive areas.
• The reticular layer lies below papillary layer and is formed of  dense C.T with coarse collagenous fibers type I and some elastic fibers the fibers are irregularly arranged,this layer contain Pacinian corpuscles.

3- Hypodermis:-

- Contains adipose tissue and this layer is rich in adipose tissue except  in scrotum and eyelids.

- Skin has 4 appendages :-

• Hair.
• Nails.
• Sweat Glands.
• Sebaceous glands.

Finally, I found some videos on you tube about skin on anatomical models models used at Modesto Junior College to teach human anatomy through Dr bobrd channel.

Composite Skin Model-Introduction


Composite Skin Model - Epidermis and Dermis

Simple Skin Model - epidermis

Simple Skin Model - Dermis

 Dr Ibrahim

*References:-

(1)Principles of histology Part-II,Prof Dr Essam A.Elmohandes ,faculty of medicine,mansoura university,ch1,p.g1.
(2)Dermatology,AN ILLUSTRATED COLOURTEXT,David J. Gawkrodger,BASIC PRINCIPLES,Microanatomy of the skin,p.g2.
(3) Introduction to anatomy ,Anatomy and embryology department, Mansoura uni,p.g10.