húðþekjan

Húðþekjan is the outermost layer of skin in animals, serving as a protective barrier against the environment. It is a stratified squamous epithelium, meaning it is composed of multiple layers of flat cells. The primary cell type in the epidermis is the keratinocyte, which produces keratin, a tough protein that gives the skin its strength and water-resistant properties.

The epidermis is avascular, meaning it contains no blood vessels. Nutrients and oxygen are supplied to the epidermal cells from the underlying dermis through diffusion. The epidermis is constantly regenerating, with new cells produced in the deepest layer, the stratum basale, and migrating outwards to the surface. As these cells move towards the surface, they flatten, die, and eventually shed. This process of keratinization is crucial for maintaining the integrity of the protective barrier.

The epidermis is further divided into several sub-layers, or strata. From deepest to most superficial, these are the stratum basale, stratum spinosum, stratum granulosum, stratum lucidum (found only in thick skin, such as on the palms of the hands and soles of the feet), and the stratum corneum. The stratum corneum, the outermost layer, consists of flattened, dead keratinocytes that form a tough, protective shield.

In addition to keratinocytes, the epidermis also contains other specialized cells, including melanocytes, which produce melanin pigment responsible for skin color and UV protection, and Langerhans cells, which play a role in the immune response. The epidermis protects against mechanical injury, UV radiation, and invasion by pathogens.