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.
Triangular alopecia (alopecia triangularis) is a condition similar to congenital aplasia. It is usually apparent from birth and tends to affect a triangular patch of skin and hair above the temples. For unknown reasons, the skin fails to grow hair follicles in this area in a few people. While babies are not born with open ulcers as with aplasia cutis congenita, the long-term result is much the same -- a bald patch where hair does not grow. The affected area can be surgically removed or implanted with hair follicles taken from elsewhere on the scalp.
Congenital atrichia or papular atrichia is a unique condition in terms of the hair loss pattern. It was the first human hair loss disease researchers determined was caused by a single gene defect. Although the condition is generally regarded as a hypotrichosis, it is not strictly so. People with congenital atrichia can be born with a full head of hair like any normal baby. But in early childhood they lose all their hair, and it never regrows.
Normal hair follicles rely on chemical communication between two basic cells types: modified keratinocytes, which form the outer skin epithelium, and modified fibroblasts, called dermal papilla cells. These two cell groups must "talk" to each other through biochemical signals to ensure that hair growth and cycling occurs. The cells must stay in close contact with each other to keep the process going. One cell population cannot grow hair without receiving signals from the other cell population.
The mechanism of congenital atrichia is not fully understood, but it seems that as the hair follicles enter their first resting (telogen) state in early childhood, the two cell types get separated from each other. The epithelial cell component of the hair follicle retracts, as it should when hair follicles go into the resting phase of the hair cycle. Normally the dermal papilla cells would also go with the retracting epithelial cells, keeping in close contact, but in congenital atrichia they don't.
Instead the dermal papilla cells get left behind deep in the skin and too far away from the epithelial cells to talk to them. Without this communication a new anagen growth phase cannot occur and hair never grows again. While congenital atrichia is genetic and runs in families, it is a gene defect that can spontaneously develop in some embryos born to parents who do not have the condition. Some people diagnosed with alopecia universalis have congenital atrichia instead.