Epidermis, dermis of skin, thick vs thin skin with clinical anatomy

 

Histology and clinical anatomy of epidermis

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Skin : It has two  layers 

1.      Epidermis

2.     Dermis

Sometimes, hypodermis consider as third layer of skin contain subcutaneous fat.

Epidermis

It  is the superficial layer of skin which is formed by  keratinized stratified squamous epithelium.

Cells of epidermis:  It is composed of four cell types:

1.      Keratinocytes,

2.     Melanocytes,

3.     Langerhans cells, and

4.     Merkel cells.

The keratinocytes are arranged in five layers, and the remaining three cell types are interspersed between them .

 Five layers of the epidermis: deep to superficial

1. Stratum Basale :

A single layer of cuboidal to columnar cells that stand on the basement membrane. This is a region of cell division.

It also contains melanocytes and Merkel cells.

2. Stratum Spinosum: it is composed by many layers of polyhedral prickle cells bearing intercellular bridges. Mitotic activity is also present.

It also contains Langerhans cells and processes of melanocytes.

3. Stratum Granulosum : it is composed by flattened cells which contain keratohyalin granules.

It is absent as a distinct layer in thin skin.

4. Stratum Lucidum A thin, translucent layer that is also absent in thin skin.

5. Stratum Corneum it is composed of squamous cells packed with keratin. Superfi cial squamous cells  are desquamated.

 

        Location of skin cells and functions

Difference between location, functions & development of different skin cells

Cells of skin	Location	Functions	Development
Keratinocytes	New skin cells develop at the bottom layer of your epidermis (stratum basale) and travel up through the other layers as they get older.	It forms barrier against environmental damage by heat, UV radiation, dehydration, pathogenic bacteria, fungi, parasites, and viruses.	Surface ectoderm
Melanocytes	Stratum basale	Melanocytes are well known for their role in skin pigmentation, and their ability to produce and distribute melanin has been studied extensively	Neural crest
Langerhans cells	Stratum spinosum	These cells act as the outermost guard of the cutaneous immune system and are likely to induce the first reactions against pathogens encountered via the skin	Fetal Langerhans cells from Primitive yolk sac in fetal life , in adult Langerhans cells  from fetal monocytes
Merkel cells	Stratum basale	It is also known as tactile epithelial cells, which is an  oval-shaped mechanoreceptors essential for light touch sensation	Surface ectoderm

 

SN: Keratinocytes, reach the outermost layer of your epidermis after about a month, where the skin cells shed from your body as new cells develop at the bottom layer.

Difference between thick and thin skin

	Thick skin	Thin skin
Stratum corneum (Cornifi ed cell layer) Composed of several layers of dead, anucleated, flattened keratinocytes (squamous cells) that are being sloughed from the surface.	As many as 50 layers of keratinocytes are located in the thickest skin (e.g., sole of the foot).	Only about five or so layers of keratinocytes (squames) comprise this layer in the thinnest skin (e.g., eyelids).
Stratum lucidum (Clear cell layer)	thin, well-defined layer formed by weakly stained keratinocytes which are filled with keratin  Organelles and nuclei are absent within this cell layer	Layer is absent but individual cells of the layer are probably present.
Stratum granulosum (Granular cell layer)	Only three to five thickness cell layers which contain  polygonal-shaped nucleated keratinocytes with a normal complement of organelles as well as keratohyalin and membrane-coating granules	Layer is absent but individual cells of the layer are probably present
Stratum spinosum (prickle cell layer)	This thickest layer is composed of mitotically active and maturing polygonal keratinocytes (prickle cells) that interdigitate with one another via projections (intercellular bridges) that are attached to each other by desmosomes. The cytoplasm is rich in tonofi laments, organelles, and membrane-coating granules. Langerhans cells are present in this layer.	This stratum is the same as in thick skin but the number of layers is reduced
Stratum basale (stratum germinativum)	This deepest stratum is composed of a single layer of mitotically active tall cuboidal keratinocytes that are in contact with the basal lamina. Keratinocytes of the more superfi cial strata originate from this layer and eventually migrate to the surface where they are sloughed. Melanocytes and Merkel cells are also present in this layer. This layer is the same in thin skin as in thick skin	This layer is the same in thin skin as in thick skin
Papillary layer	Papillary layer Is comprised of loose connective tissue containing capillary loops and terminals of mechanoreceptors. These dermal papillae interdigitate with the epidermal ridges of the epidermis. These interdigitations are very prominent in thick skin. The papillary layer is comprised of the same loose connective tissue as in thick skin. However, its volume is much reduced. The depth of the dermal/epidermal interdigitations is also greatly reduced	The papillary layer is comprised of the same loose connective tissue as in thick skin. However, its volume is much reduced. The depth of the dermal/epidermal interdigitations is also greatly reduced
Reticular layer	Reticular layer Is composed of dense irregular collagenous connective tissue containing the usual array of connective tissue elements, including cells, blood, and lymphatic vessels. Sweat glands and cutaneous nerves are also present and their branches extend into the papillary layer and into the epidermis. Same as in thick skin with the addition of. Sebaceous glands and hair follicles along with their arrector pili muscles are observed	Same as in thick skin with the addition of. Sebaceous glands and hair follicles along with their arrector pili muscles are observed

 

Albinism

 It is a congenital disorder characterized by the complete or partial absence of melanin pigmentation in the skin, hair,  iris of eye, and eyelashes.

 It is an autosomal recessive/X -linked disorder. 

 Although the melanocytes are present, but genetic mutation results in lack of tyrosinase enzyme, which catalyzes the production of melanin from tyrosine.

People with albinism have skin that is very sensitive to light and sun. 

This allows light to shine through the irises and makes the eyes extremely sensitive to bright light.

Vitiligo

 It is s a clinical condition with presence of spots without melanin in the skin, which occurs due to partial or complete absence of melanocytes.

 

 Dermis it  is a dense, irregular, collagenous connective tissue which has two layers: papillary and reticular. 

1. in papillary Layer, the dermal ridges (dermal papillae) and secondary dermal ridges interdigitate with the epidermal ridges (and interpapillary pegs) of the epidermis. Collagen fibers are slender in comparison with those of deeper layers of the dermis. Dermal ridges contain capillary loops and Meissner’s corpuscles. 

2. Reticular Layer it  is composed of coarse bundles of collagen fibers. 

 Incisions are preferably made parallel to cleavage/Langer’s lines.

The reticular layer  of dermis of skin contains bundles of collagen fibers which run in parallel rows. 

The direction of  bundles of collagen fibers  are responsible for cleavage/Langer’s line which run longitudinally in the limbs and circumferentially in the trunk and neck area.  Incisions  made parallel to cleave lines will cut fewer collagen fibers, but cut many collagen fibers if you give incision perpendicular to the cleavage lines. Parallel incision result the wound heals faster and does not lead to formation of ugly scar.