The Skin and its Appendages

The skin is derived from three diverse component:1.Surface Ectoderm: The epidermis is derives from the surface ectoderm. From single layer ectodermal cells proliferate to give rise to typical stratified squamouse epithelium.The superficial layer sheds off and get mixed with secretions of sebaceous glands to form a whitish sticky substance called vernix caseosa, which acts as a protective covering for newborn infant. Vernix caseosa is protective in function.

SKIN

Epidermal ridges develops between 3rd and 5th months of fetal age, this led to formation of characteristics pattern on the tips of fingers and toes. These patterns are genetically determined and are different for each person.

2. Neural cresti. Melanoblasts (dendritic cells) These are the cells of epidermis derived from

the neural crest cell. The cells synthesize melanin pigments responsible for skin and hair colour.

ii. Cells of Merkel Appears in epidermis between 8th and 12th

weeks of intrauterine life. They are associated with the sense of light

touch discrimination of shapes and textures.iii. Cells of Langerhans They are developed in bone marrow. In skin infection, the local Langerhans cells take

up and process microbial antigen to become fully functional antigen presenting cells.

3. The dermis The dermis is formed by condensation and differentiation of mesenchyme underlying the surface ectodermThis mesenchyme is believed to be derived from the dermatome of somite.Recently it has been held as follows:a.Only the dermis on the dorsal aspect of the head and the trunk arises from dermatomes.b.The dermis of the limbs and that on the lateral and ventral aspect of the trunk arises from the lateral plate mesoderm.c.The dermis over most of the head and over the anterior aspect of neck is derived from neural crest.

1. The line of junction between dermis and epidermis is at first straight.

2. Subsequently, the epidermis shows regularly spaced thickenings that project into the dermis.

3. The portion of dermis intervening between these projections form the dermal papillae.

4. Still later, surface elevations (epidermal ridges) are formed by further thickening of the epidermis in the same situation.

Stages in development of epidermsi

The nails develops from surface ectoderm. The ectoderm at the tip of each digits becomes

thickened to form a primary nail field. Subsequently this thickening migrates from the

tip of the digit onto its dorsal aspect. The cells in the most proximal part of the nail

field proliferate to form the root of nail. Here the cells of the germinal layer multiply to

form a thick layer of cells called the germinal matrix.

As the cells multiply they are transformed into the nail substance which corresponds to the stratum lucidum of the skin.

NAILS

They are also derived from surface ectoderm. At the site where a hair follicle is to form, the germinal

layer of the epidermis proliferates to form a cylindrical mass that grows down into dermis.

The lower end of this downgrowth becomes expanded and is invaginated by a condensation of mesoderm, which forms a papilla.

The hair itself is formed by proliferation of germinal cells overlying the papilla.

As the hair grows to the surface, the cells forming the wall of the downgrowth surrounds it and form the epithelial root sheath.

In addition dermal root sheath is formed from the surrounding mesenchymal cells.

A thin band of smooth muscle called arrector pili is formed from the dermal root sheath

HAIRS

A sebaceous gland is formed as a bud arising from ectodermal cells forming the wall of hair follicle.

Sebaceous glands

A sweat gland develops as a outgrowth from the epidermis.

The outgrowth is at first solid but is later canalized.

The lower end of the outgrowth becomes coiled and forms the secretory part of gland.

Sweat glands

1. Albinism Absence of pigment in skin, hair and eyes

occurs because melanocytes are unable to synthesize melanin.

Anomalies of Akin and its Appendages

2. AplasiaThe skin may fail to develop in certain regions.

3. DysplasiaThe skin may be in abnormal structure.

Nails maybe absent. Occasionally they may show over development.

Anonychia

Congenital alopecia- hairs may be absent over the scalpAtrichia- absence of hairs in any body part.Hypertrichosis- overgrowth of hairs.

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