presentation of developmental biology

8/6/2019 Presentation of Developmental Biology

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Topic : Hair FormationPresented by : Aila Manzoor


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Each strand of hair consists of three layers.

An innermost layer or medulla which is only present inlarge thick hairs.

The middle layer known as the cortex . The cortexprovides strength and both the color and the texture of hair.

The outermost layer is known as the cuticle . The cuticle isthin and colorless and serves as a protector of the cortex.


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The part of the hair we see above is dead, the actual living partis very small which is knows as root of the hair which exists inthe part known as the papilla. In the papilla, cells multiply andproduce a protein called keratin - this is responsible for thehardness of the hair.

The part of the hair which is within the skin, is enclosed by asleeve of tissue called the foll ic l e . Next to each follicle is asebaceous gland - the oil producing gland of our skin. This oilforms a natural protective film over the length of hair, keepingit soft, supple and look shiny.The chemical composition of hair is:

Carbon : 50.56%Hydrogen: 6.38%Nitrogen: 17.14%Sulphur : 5.00%

Oxygen : 20.85%


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Process of

hair formation


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o The mammalian hair follicle is a dynamic structure that generates ahair shaft through a tightly controlled cycle of growth, remodeling,and loss. Once a hair follicle is made, it can undergo many of thesecycles, continually making, growing, and losing the hair shaft.

o In mammals, the cycle of hair growth includes three stages:

anagen (follicle generation and hair production),

catagen (follicle regression), and

telogen (resting phase).


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Initial inductiveevents

The first signal is probably from the dermis, telling theepidermis to "make an appendage."Regions of epidermalcells proliferate and form local thickenings (placodes) of theepidermis.

The epidermal placodes then respond by sending a messageinto the mesenchyme, telling the mesenchyme cells to"aggregate beneath the epidermal placodes."

Once aggregated, these mesenchyme cells now form thedermal papilla . The papilla sends a second message tothe epidermis: "make a hair placode ."


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The epidermis responds to this signal by proliferatingfurther into the mesenchyme .

The mesenchyme to condense into the definitive dermalpapilla that will become surrounded by the proliferatingepidermal placode. The epidermal placode has nowbecome a primitive hair shaft.


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D ifferentiation of the hair shaft

The epidermal hair germ is surrounded by mesoderm. At itsbase is the dermal papilla. On its sides are less condensedmesenchyme, which, nevertheless, had been part of theoriginal dermal papilla.

Cells at the center of the follicle become the medulla(center) of the hair shaft, while slightly more marginal cellsbecome the cortex and cuticle of the hair shaft. Even moreperipheral cells, those cells differentiating at the base of the dermal papilla (near where it meets the tip of theectoderm), differentiate into the inner root sheath.

The regions of the hair peg become distinguishable bytheir cell adhesion molecules.


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The innermost region of the hair matrix, directly atop thedermal papilla is characterized by P cadherin, while theregion above that has both E and P cadherin. This will

become the keratogenic hair matrix. The outermost hair matrix contains E cadherin.

Melanin pigment from the neural crest melanoblast cellsenters into the hair shaft, mingling with the keratogenichair matrix cells.

The sebocytes form two bulges. The uppermost bulgebecomes the sebaceous gland, while the lower bulgebecomes the attachment site for the arrector pili muscles(those that "make your hair stand on end") and the

residence of the hair follicle stem cells.The innermost regions of the hair peg become the cortexand cuticle of the hair fiber, while the outer layers becomeparts of the inner root sheath that covers the hair shaft. Asa result of continued proliferation by the stem cells directlyover the dermal papilla, the hair fiber is pushed upwardswithin the follicle and it produces the hair keratins.


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The hair shaft then extends through the hair canal, anopening that is thought to be formed by the apoptosis of thecells at the center of the hair peg where the hair peg meets

the skin.The outer root sheath surrounds the hair follicle (much likea sleeve) and it is continuous with the epidermis. It has twoproliferation zones:

(1) in the bulb and(2) in the basal layer of the epidermis.

In the bulb, there are two layers. The outer layer of thecell is germinative and continuous with the epidermalbasal cells. Differentiation occurs by the horizontalmovements of cells from the basal layer of the outer rootsheath to the center of the follicle.


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This later stage of hair development is illustrated in Figure


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presentation of developmental biology

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