Integumentary System: Skin

 Integumentary System: Skin.

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Introduction:

• Integument means “Covering”, this system is made of structures which provide covering to the body.

• Skin along with subcutaneous tissue and accessory structures like hair, sweat glands etc. forms integument of the human body.

Skin:

• It is the largest organ of the body.

• It constitutes around 7% of the total body weight.

• Skin is made up of two layers;

• Epidermis.

• Dermis.

• 

Epidermis of Skin:

• The epidermis is made of stratified squamous keratinizing epithelial tissue.

• It is thickest on the palms and soles.

• It is avascular in nature i.e. there is no blood supply.

• Cells are very closely packed.

• It contains four types of cells as 

• Keratinocytes.

• Melanocytes.

• Langerhans Cells.

• Merkel Cells.

• The epidermis of skin is made up of Four or Five layers as;

• Stratum corneum.

• Stratum lucidum.

• Stratum granulosum.

• Stratum spinosum.

• Stratum basale.

Cells of the Epidermis of Skin:

1. Keratinocytes:

• Most abundant.

• Contain keratin protein.

• Synthesize a defensive substance called “Defensine”. 

2. Melanocytes:

• They are in less numbers as compared to Keratinocytes.

• They have large processes that connect to many keratinocytes.

• They synthesize a chemical “Melanin”  and inject it into attached keratinocytes.

• Melanin is a coloring pigment of reddish brown or yellowish red in color responsible for skin color.

• Melanin protects the cells from harmful UV radiations.

3. Langerhans Cells:

• These large cells are basically “Macrophages” formed in bone marrow and migrated towards the skin.

• They perform the function of defense.

4. Merkel Cells:

• These cells are found in the last layer of epidermis called Stratum basale.

• These cells are along with sensory nerves from Merkel discs.

• They perform functions of sense.

Layers of Epidermis of Skin:

• Layers of epidermis are called “Strata”.

• The thin skin contains Four layers;

• Stratum corneum.

• Stratum granulosum.

• Stratum spinosum.

• Stratum basale.

• While thick skin contains Five layers;

• Stratum corneum.

• Stratum lucidum.

• Stratum granulosum.

• Stratum spinosum.

• Stratum basale.

1. Stratum corneum:

• The outermost layer.

• Contains many layers of dead squamous keratinized epithelium cells.

• The cells that compose a large proportion are called “Corneocytes”.

• As cells contain keratin this layer provides waterproof property to the skin.

• It has a defensive function as bacteria and most of the harmful chemicals  cannot enter the unbroken layer.

2. Stratum lucidum:

• Only present in the thick areas of the skin like palms and soles. 

• It is made up of 4-5 layers of dead cells.

• It appears translucent under a microscope.

3. Stratum granulosum:

• It is present between Stratum lucidum and Stratum spinosum.

• The cells appearing in this layer triggers Apoptosis (Programmed Cell Death).

• They appear granular and start producing lamellar bodies that impart hydrophobicity to the skin.

• Cells contain a protein called “Keratothylline” that produces keratin from intermediate keratin filaments.

4. Stratum spinosum:

• It is made up of 8-10 layers of the cells.

• When observed under the microscope, cells show the presence of thorns like spines and hence the name is given “Spinosum”.

• It mainly contains keratinocytes and a few melanocytes, merkel cells and Langerhans cells.

5. Stratum basale:

• The deepest and last layer of epidermis.

• Also called Stratum germinativum.

• Made up of a single row of cuboidal or columnar keratinocytes (Some cells are stem cells).

• Melanocytes, Merkel cells along with discs are scattered in the layer.

• The cells are anchored to the basement membrane present between epidermis and dermis with help of hemidesmosomes.

• The cells show continuous cell divisions.

Dermis:

• It is a deeper layer of the skin and is present below the epidermis.

• Basically it is irregular strong connective tissue.

• It is vascular and has nerve supply.

• It shows the presence of collagen and elastin fibers.

• The cell component of dermis contains fibroblasts, mast cells, eosinophils etc.

• The matrix component is made up of Hyluronic acid and proteoglycans.

• Dermis is divided into two layers;

• Papillary Region.

• Reticular Region.

1. Papillary Region:

• It is a small layer and consists of one-fifth of the total layer’s thickness.

• Its surface area that protrudes in epidermis is finger shaped and called “Dermal papillae” hence the name “Papillary region”.

• Dermal papillae increases the surface area significantly.

• This layer consists of small collagen fibers and fine elastin fibers.

• It also shows the presence of blood vessels, touch receptors and free nerve endings.

2. Reticular Region:

• It has a net like structure and hence the name is given Reticular Region. 

• It contains thicker collagen fibers arranged in a net like formation, stronger elastin fibers and several fibroblasts along with many wandering cells like macrophages, mast cells, eosinophils etc.

• Adipocytes are also present in this region.

• Blood vessels, nerves, hair follicles, sebaceous (oil) glands, and sudoriferous (sweat) glands are found in the spaces between fibers.

• A smooth muscle called as “Arrector pili” is present alongside the hair follicle.

Commonly Asked Questions.

1. With help of a well labelled diagram explain the structure of skin.

2. Draw a well labelled diagram of LS of skin.

3. Write in short about various layers of the epidermis of skin.

4. Name different cells found in epidermis of the skin.