Front Matter

Blood–Brain Barrier 255

In two of the examples mentioned earlier, Rubella infection and Zika virus
infection of the fetus, the congenital anomalies are rarely observed after
20 weeks of gestation even though maternal viremia can be documented [41].
These observations point towards the presence of a functional BBB in a fetus
after 20 weeks of gestation.
Recent evidence confirms that the fetal brain develops tight junctions and
many of the transport mechanisms between the blood, brain, and cerebrospinal
fluid (CSF) during the very early stages of development. Some properties of
these barrier mechanisms and their susceptibility to disruption may lead to
brain damage and later neurological disorders.
Understanding the role of BBB mechanisms in normal brain development
and possible deleterious effects should these mechanisms be dysfunctional is
important from the clinical perspective of whether or not drugs or toxins,
once they cross the placenta, may have access to the vulnerable developing
brain. A reason given by regulatory bodies in the USA and the European
Union for caution in giving drugs to pregnant women or infants is the “imma­
turity” of the BBB [38].
There is no doubt that blood vessels in the developing fetal brain are signifi­
cantly more fragile than in an adult brain. These concepts were established
from experiments in which excessive volumes of fluid were injected into
fetuses [47]. A specific example of susceptibility of cerebral blood vessels in

Abluminal

membrane

Luminal

membrane

Blood Hg

Endothelial cell

Tight junction

Astrocyte

Basement

membrane

Pericyte

Astrocyte

Neuron

Brain
capillary

Figure 8.5 Cross section of a cerebral capillary. This is the structure of the blood–brain
barrier. The blood–brain barrier is not a distinct organ, but rather a functional concept.
Source: http://captain‐nitrogen.tumblr.com/post/4927485305/a‐fortress‐of‐intelligence.