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Chapter 2: Development of the Choroid Plexus
Amniotic fluid is present initially in the primitive
cerebral recesses of the human embryo, but then the ependymal cells and
eventually the growing choroid plexuses take over the production of
cerebrospinal fluid118. A
series of experiments on dogs performed by Walter Dandy in
In the normal adult about 500mls of CSF is produced daily240. The function of CSF is not fully understood, although
it is considered mainly to be a shock-absorber, it contains many nutrients,
particularly in embryonic and fetal development, and may be a carrier of
neuro-transmitting molecules.
2.1 Choroid Plexus Growth & Development
Choroid plexus (from the Latin chorion = coat, and plectere = to plait or braid) tissue is found
in the third, fourth and both lateral ventricles of the human brain. The choroid plexuses are developmentally
fetal structures, forming at the end of the embryonic period. The growth of the choroid plexus in the
ventricular system begins in the third ventricle, on the roof of which the
choroidal plate forms. In the lateral
ventricles, the first signs of an invagination of vascular mesenchyme
(connective tissue derived from the mesoderm germ layer) into the roof of the
ventricle occur at six to seven weeks (CRL = 17 - 19 mm). Anteriorly, this invagination (the area
epithilealis) is in contact with the choroidal plate of the third ventricle
(the paraphyseal arch) and it passes posteriorly and superiorly along a
curved area of indentation in the medial ventricular wall called the choroidal
fissure. Uniquely, in this region the
pia mater is in direct contact with the ependymal cells of the ventricle. Special cells, the choroid villi, begin to
appear on the plexus at around stages 20 and 21 (seven weeks) as the process of
differentiation of the choroid plexus from the fissure continues with a
finger-like process into the ventricle that moves around the fissure towards
the pes hippocampus in the temporal horn, where it terminates196.
Several stages of developing complexity in the structural
appearance of the lateral ventricular plexus were described by Shuangshoti and
Netsky173, based on cross-sections of embryonal and fetal
brains.
2.2 Stage I
At 7 - 9 weeks, the
choroid plexus of the lateral ventricles begins as a club-shaped structure with
a short stalk, as shown in Fig 1. The stroma (supportive structure) of the
plexus is loosely organised mesenchyme.
The shape of this club becomes more lobulated towards the end of this
period.
Fig 1. Section
of fetal brain, showing the choroid plexus during Stage 1. (From Shuangshoti
and Netski173.)
O’Rahilly and Müller143 describe how some of the
newly forming villi near the base of the plexus at this stage are “slender and
vesicular.”
2.3 Stage II
(9 -17 weeks), the plexus is
extremely large in relation to the size of the ventricle, occupying about
one third of its volume. Glycogen
appears abundantly in the cytoplasm of the endothelial cells. The stromal mesenchyme is extremely loose,
and secretes large amounts of mucin, a sticky substance.
The lobulations are becoming more and more pronounced, as demonstrated
in Fig 2. As a result, towards the
end of this period and into the next stage, many tubules are formed within
the lobulated plexus when the mesenchyme folds onto itself, creating blind
tracts that are usually filled with CSF.
Completely entrapped areas distend to form cysts, which grow with
CSF production to variable sizes. According
to Shuangshoti and Netski:
... the tubules we find are formed by the folding of the surface epithelium into the stroma during differentiation of the plexus The number of tubules increase at the same rate as the number of lobules or villi of the plexus, indicating that formation is closely related to the inter-lobular or inter-villous clefts. These tubules are formed when the tips of these clefts are pinched off. We find these tubules in all three major plexuses. Tubules in seven specimens are large enough to be designated as incipient neuroepithelial cysts.173
Fig 2. Section of fetal brain in early Stage II showing
increasing lobulation of the choroid plexus. (From Shuangshoti and Netski173.
Arrow point to choroid in the roof of the 3rd ventricle.)
2.4 Stage III
(17 - 29 weeks), the
relative size of the plexus is reducing but the lobulation continues to become
more complex, as demonstrated in Fig 3. Glycogen is less abundant in the
cytoplasm. During this stage, the amount
of loose stroma decreases as connective tissue fibres increase. Large numbers of small tubules are present,
but towards the end of this stage the cysts disappear as the relative amount of
mucin decreases. Fluid is released when
the mesenchymal elements become unstuck, perhaps with the assistance of
pressure from within the cyst.
Fig 3. Section
of choroid plexus showing complex lobulation and the intervillous spaces.
(From Shuangshoti and Netski173.)
2.5 Stage IV
At this stage (29 weeks on), the villi are finer and more
delicately frond-like. The plexus is smaller
compared to the ventricle. Many small
tubules persist, and the mesenchymal elements become scant as connective tissue
fibres predominate. Almost all cysts are
gone.
2.6 Conclusion
The increasingly complex structure of the choroid plexus during its growth in the second trimester with the development of many tubules into incipient cysts, provides an explanation for the origin of CPC in the brain of both normal and trisomic fetuses. The change in mucin production and the gradual reduction in larger tubules appears to explain the gradual resolution of these cysts.