Many people have seen images of “Harlequin ichthyosis" or "Harlequin fetus" before, what with this being the internet, and the internet having a taste for what it sees as morbid. However, ichthyosis takes many forms other than that most severe condition, and is quite interesting on a genetic and cellular level.
Almost all forms of ichthyosis are congenital (inherited), though the mechanism of inheritance varies between forms - autosomal dominant, autosomal recessive, and X-linked inheritance patterns have all been found. There are also many different presentations of the conditions, ranging from what looks like severe dry skin, to spiny scales covering the body, to Harlequin-type ichthyosis. Historically, there was very little one could do to treat the condition, and many infants and children died from many forms other than just Harlequin-type, which is currently the only presentation that still has a significant mortality rate.
In all forms of ichthyosis, the keratin in the skin is disordered. Keratin is the group of proteins that form hair and nails, and make our skin tough enough to survive in the world. Without it, our skin would be incredibly easy to perforate, and we would have no water-retention abilities - all of our fluids that are needed to nourish the sub-dermal tissues would just evaporate as soon as they reached the part of the body exposed to the air. In places like your feet, or on calluses you might form from hard work, the keratin is thicker, and tougher to penetrate than the rest of the skin. However, the thicker keratin also leads to a significant loss of flexibility. In the palms and feet, that’s usually not a problem, but if you have that sort of excess keratin building up on other parts of the body, or building up for no reason? That’s called hyperkeratinosis, and in many of its manifestations (such as several forms of ichthyosis), it can be seriously debilitating without treatment.
Skin Diseases of Children. George Henry Fox, 1898.