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Ichthyosis (also named fish scale disease) [1] is a family of genetic skin disorders characterized by dry, thickened, scaly skin. [2] The more than 20 types of ichthyosis range in severity of symptoms, outward appearance, underlying genetic cause and mode of inheritance (e.g., dominant, recessive, autosomal or X-linked). [3]
Ichthyosis vulgaris (also known as "autosomal dominant ichthyosis" [1] and "Ichthyosis simplex" [1]) is a skin disorder causing dry, scaly skin.It is the most common form, and one of the mildest forms, of ichthyosis, [2] [3]: 486 affecting around 1 in 250 people. [4]
[13] [9] Early oral retinoid therapy has been shown to soften scales and encourage desquamation. [19] After as little as two weeks of daily oral isotretinoin, fissures in the skin can heal, and plate-like scales can nearly resolve. Improvement in the eclabium and ectropion can also be seen in a matter of weeks.
In around 10% of cases the baby sheds this layer of skin and has normal skin for the rest of its life. [2] [5] This is known as self-healing collodion baby. The remaining 15% of cases are caused by a variety of diseases involving keratinization disorders. [5] Known causes of collodion baby include ichthyosis vulgaris and trichothiodystrophy. [3]
Skin breathing, known as cutaneous respiration, is common in fish and amphibians, and occur both in and out of water. In some animals waterproof barriers impede the exchange of gases through the skin. For example, keratin in human skin, the scales of reptiles, and modern proteinaceous fish scales impede the exchange of gases.
Scale forms on the skin surface in various disease settings, and is the result of abnormal desquamation. In pathologic desquamation, such as that seen in X-linked ichthyosis, the stratum corneum becomes thicker (hyperkeratosis), imparting a "dry" or scaly appearance to the skin, and instead of detaching as single cells, corneocytes are shed in clusters, which forms visible scales. [2]
Phyllolepida ("leaf scales") were flattened placoderms found throughout the world. Like other flattened placoderms they were bottom-dwelling predators that ambushed prey. Unlike other flattened placoderms, they were freshwater fish. Their armour was made of whole plates, rather than the numerous tubercles and scales of Petalichthyida.
The word skin originally only referred to dressed and tanned animal hide and the usual word for human skin was hide. Skin is a borrowing from Old Norse skinn "animal hide, fur", ultimately from the Proto-Indo-European root *sek-, meaning "to cut" (probably a reference to the fact that in those times animal hide was commonly cut off to be used as garment).