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.