Hypotrichosis is the term dermatologists use to describe a condition of no hair growth. Unlike alopecia, which describes hair loss where formerly there was hair growth, hypotrichosis describes a situation where there wasn't any hair growth in the first place. Hypotrichoses (plural) then are conditions that affect individuals right from birth and usually stay with them throughout their lives.
The majority of hypotrichoses are due to genetic aberrations or defects of embryonic development. There are hundreds of types of genetic hypotrichoses. Often, affected individuals have other physical or mental problems beyond a lack of hair. Conditions such as Graham-Little syndrome, Ofuji syndrome, cartilage-hair hypoplasia, Jeanselme and Rime hypotrichosis, Marie Unna hypotrichosis, and metaphyseal chondrodysplasia, among many others, can involve the symptom of hypotrichosis.
With the rapidly improving understanding of the human genome, our comprehension of why and how genetic defects cause hair loss and other symptoms is growing. But, while we may understand the genetics and the biochemistry behind hypotrichoses, treating them is very difficult. Most conditions involving hypotrichosis have no known treatment.
A few forms of hypotrichosis are worth mentioning, either because they are relatively common or because they are interesting in terms of understanding hair follicles.
Aplasia cutis congenita, or congenital aplasia, is a developmental defect where, for reasons not understood, the skin does not fully form as an embryo develops. A baby may be born with a patch of skin that is like an open wound or an ulcer. Often this defect occurs at the back of the scalp, at the center of the "whorl pattern" of hair growth. If the defect is small, the skin will scab over and the baby is left with a scar.
Sometimes this happens in the womb and all that can be seen at birth is a patch of scalp where there are no hair follicles. However, if a baby is born with a large congenital aplasia, it usually requires an operation to cut out the affected area and close up the skin. This is often done with some urgency, since the open wound is a site of potential hemorrhage and infection. The quicker the defect heals, the better.