What is Sunburn?
Sunburn is an acute inflammatory response of the skin to excessive exposure to ultraviolet (UV) radiation, primarily from the sun. It manifests as erythema (redness), pain, and in severe cases, blistering. Histologically, sunburn is characterized by damage to the
epidermis and the underlying
dermis.
Histological Changes in the Epidermis
In the epidermis, the most superficial layer of the skin, UV radiation causes direct DNA damage in
keratinocytes and
melanocytes. This damage often leads to the formation of
thymine dimers, which can disrupt normal DNA replication and transcription. The
stratum corneum may become thickened as a protective response, while the
stratum spinosum and
stratum basale show signs of cellular apoptosis, also known as sunburn cells.
Dermal Response to Sunburn
The dermis, which lies beneath the epidermis, also undergoes significant changes. The UV radiation can induce an inflammatory response characterized by increased presence of
Langerhans cells and infiltration of
neutrophils and
lymphocytes. Blood vessels in the dermis may become dilated, contributing to the redness and swelling commonly associated with sunburn.
Role of Melanin
Melanin is a pigment produced by melanocytes in the epidermis that provides some protection against UV radiation by absorbing and dissipating the harmful rays. However, excessive UV exposure can overwhelm this protective mechanism, leading to DNA damage and subsequent sunburn.
Repair Mechanisms
The skin has several mechanisms to repair UV-induced damage.
Nucleotide excision repair is a critical process that removes thymine dimers and other DNA lesions. Cells may also activate
p53, a tumor suppressor protein that can induce cell cycle arrest and apoptosis to prevent the propagation of damaged cells.
Long-term Effects
Repeated sunburns can lead to chronic skin damage, increasing the risk for skin cancers such as
basal cell carcinoma,
squamous cell carcinoma, and
melanoma. Histologically, chronic UV exposure may result in
solar elastosis, where the elastic fibers in the dermis are damaged, leading to premature aging of the skin.
Prevention and Protection
Preventing sunburn involves minimizing UV exposure through the use of
sunscreen, wearing protective clothing, and avoiding the sun during peak hours. Sunscreens contain compounds that either absorb or reflect UV radiation, thereby reducing the amount that penetrates the skin.
