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
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
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
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
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
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.