Integumentary System
Notes
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:
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:
Keratinocytes:
Most abundant.
Contain keratin protein.
Synthesize a defensive substance called as “Defensine”.
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.
Langerhans Cells:
These large cells are basically “Macrophages” formed in bone marrow and migrated towards the skin.
They perform function of defense.
Merkel Cells:
These cells are found in the last layer of epidermis called Stratum basale.
These cells are along with sensory nerve form Merkel discs.
They perform function 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.
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 defensive function as bacteria and most of the harmful chemicals cannot enter unbroken layer.
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 microscope.
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 start producing lamellar bodies that impart hydrophobicity to the skin.
Cells contain a protein called “Keratothylline” it produces keratin from intermediate keratin filaments.
Stratum spinosum:
It is made up of 8-10 layers of the cells.
When observed under the microscope cells shows presence of thorn like spines and hence name is given “Spinosum”.
It mainly contains keratinocytes and few melanocytes, merkel cells and Langerhans cells.
Stratum basale:
The deepest and last layer of epidermis.
Also called as Stratum germinativum.
Made up of 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 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
Papillary Region:
It is a small layer and consist of one-fifth of 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.
Reticular Region:
It has a net like structure and hence name is given Reticular Region.
It contains thicker collagen fibers arranged in 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 of hair follicle.
Accessory structures of the skin:
Skin shows presence of various accessory structures like,
Hair,
Sebaceous Glands,
Sudoriferous Glands (Sweat Glands).
Ceruminous glands.
Hair:
The hairs are formed by downgrowth of epidermal cells in the dermis or subcutaneous tissue.
Hair has three layers,
Cuticle.
Dermis,
Medula.
The base of the hair follicle is a cluster of cells called as “Papila of Bulb”.
The visible part of hair that comes out of the skin is called as “Shaft” while the part not visible to the eye and is present in the dermis is called the root of hair.
The cells of bulb as grows are pushed forward, as they go away from site of nourishment they become dead and keratinized.
The melanocytes present in hair bulb pushes “melanin” the coloring pigment in the hair cells, the inability of these cells to produce melanin replaces the coloring pigment with air bubbles resulting in white hair.
The reduced growth rate, reduce blood supply or destruction of hair bubble cells by immune response causes loss of hairs producing baldness condition is called as Alopecia.
Characteristics of the hair are controlled by genetics and hormonal effects.
Sebaceous Glands:
These consist of secretory epithelial cells derived from the same tissue as the hair follicles.
They secrete an oily substance, sebum, hair follicle only.
Present on all body parts except the palms and soles.
Sebaceous glands present on glans penis, labia minora, lips opens directly on skin.
Sebum keeps the hair soft, flexible and gives it a shiny appearance.
On the skin it provides waterproofing and acts as a bactericidal and fungicidal agent, preventing infection.
It also prevents drying and cracking of skin, especially on exposure to heat and sunshine.
The activity of these glands increases at puberty and is less at the extremes of age, rendering the skin of infants and older prone to the effects of excessive moisture.
Sudoriferous Glands (Sweat Glands):
Widely distributed throughout the skin and are most numerous in the palms of the hands, soles of the feet, axillae and groins.
They are formed from epithelial cells.
The two main types are as following;
Apocrine Sweat Glands.
Eccrine Sweat Glands.
Eccrine Sweat Glands:
They are numerous in number, and are present on the skin of almost all organs of the body except skin of glans penis, clitoris, margins of the lips.
They are numerous on forehead, palms and soles.
They pour their secretion i.e. sweat directly onto the skin surface through minute openings.
The sweat produced by eccrine sweat glands is a thin clear liquid containing water , salts, glucose, urea, uric acid etc.
The main function is thermoregulation.
They get activated soon after the birth of the individual.
Apocrine Sweat Glands:
They are concentrated in axial, areola and groin area.
They pour their secretion in the hair follicle.
The sweat produced by apocrine sweat glands is thick liquid containing the ingredients like eccrine sweat but additionally contains Lipids and Proteins.
When released the Proteins and lipids the sweat gets decomposed by bacterial action producing a typical body odor.
They get activated at puberty.
They are not involved in thermoregulation rather gets activated at emotional stress and sexual excitement.
4) Nails:
Nails are plates of tightly packed, hard, dead keratinized epidermal cells.
They are similar to horns, hoofs and scales in the animals.
Each nail consists of a nail body, a free edge, and a nail root.
Nails have a variety of functions:
Protection.
Support.
Grip to hold small objects.
Scratching and grooming.
5) Ceruminous Glands:
Modified sweat glands in the external ear, called ceruminous, produce a yellowish waxy lubricating secretion called “Cerumen”.
Cerumen provides a sticky barrier that prevents the entrance of foreign bodies, such as insects.
Cerumen also waterproofs the canal and keeps bacteria and fungi from entering cells.