Question

5. The skin possesses a number of amazing protective properties, and one of the most notable is skin coloration as a protection against U.V. irradiation. Briefly, describe the anatomical, biochemical and physical means by which the skin adapts to U.V. radiation. How does light (U.V.) potentially disrupt cellular function and how is melanin poised to protect against damage. Describe four features of melanin as it relates to interpersonal differences between individuals, and what impact does this have on rates of skin cancer. Describe at least one method used to associate U.V. irradiation with either skin damage or repair. (10 points)

Answer 1

Soln 1- The skin is the largest organ of the body. It is organized into two primary layers - epidermis and dermis which are made up of epithelial, mesenchymal, glandular and neurovascular components. The epidermal biology and physical characteristics of skin play an enormous role in resistance to UV radiation. The various anatomical, physical and biochemical means are as listed below-

1. Keratinocytes are the most abundant cells in the epidermis and are known for their expression of cytokeratins and formation of desmosomes and tight junctions with each other to form an effective physiochemical barrier.

2.The dermis derived from mesoderm contains abundant immune cells and fibroblasts which acts as an physiological barrier in the skin.

3.Besides the creation of highly physical barrier, the keratinocytes also accumulate melanin pigments as they mature and epidermal melanin functions to potently block UV penetration into the skin.

4.The amount and the type of epidermal melanin is the main factor that determines skin complexion and UV sensitivity. Melanin acts as a biochemical barrier because of its chemical forms.

Soln 2- UV radiation is classified as complete carcinogen because of its mutation and non specific damaging factors. It can both initiate and promote tumor. It causes skin cancer and many other environmemtally influenced skin disorders.

Longer UV wavelengths induce oxidative stress and protein denaturation whereas shorter UV wavelengths causes mainly DNA damage to cells altering cellular functions. They form pyrimidine dimers and 6-4 photoproducts which are bulky DNA lesions that halt RNA polymerase II elongation and are repaired enzymatically. Due to incomplete repair, damaged bases may be misinterpretated during replication that results in accumulation of cancer predisposing mutations.

The longer the exposure to UV light the more thymine dimers are formed in the DNA and the risk of an incorrect repair is increased. If the cellular processes are disrupted due to an incorrect repair then the cell cannot carry out its normal functions.

Melanin apart from.functioning as a broadnand UV absorbent, also has antioxidant and radical scavenging properties. Melanin has a shielding effect especially eumelanin because of its ability to serve as physical barrier that scatters UVR and as an absorbent filter that reduces the penetration of UV through the epidermis. Dark skin which contains more eumelanin than fair skin is better protected against UV induced damage.

Soln 3- Four properties of melanin-

a.Melanin is a large bio aggregate composed of subunits of different pigments formed by oxidation and cyclization of the amino acid tyrosine.

b. Melanin exists in two forms- Eumelanin, a dark pigment expressed abundantly in the skin of heavily pigmented individuals and Pheomelanin, a light colored sulfated pigment resulting from incorporation of cysteines into melanin precursors.

c.the production of melanin takes place in specific ovoid organelles known as melanosomes which are produced in dendritic melanocytes.

d. Melanin acts as a shield against UV radiation.

The differences in skin pigmentation results from the melanogenic activity, the type of melanin produced in size and numbers. Melanin in dark skin i.e. eumelanin is twice as effective than the white skin in inhibiting UVB radiation from penetrating. Highly pigmented skin is profoundly protected from carcinogenesis. In Lightly pigmented skin melanosomes are degraded and only persists as melanin dust. This reduction of melanosomes in the upper layer of epidermis is considered to be a major factor in carcinogenesis as it compromises the photoprotection of skin. Pheomelanin is responsible for this factor because it can generate hydrogen peroxide and superoxide anions that might cause mutations in melanocytes or other cells.