Folding Of The Embryo
After the
formation of the three germ layers of the embryo during week 3 (see Chapter
14), the embryo remains a flat, oval sheet of cells with an amniotic cavity
above it and a yolk sac beneath. Differential growth of these embryonic and
extra‐embryonic cells causes the flat embryo to curve and fold at the head end,
the tail end and laterally. With this folding and rolling up the embryo begins
to take on the early shape of a body.
As the flat
embryo grows, its amniotic cavity grows, but the yolk sac does not. The
enlarging sheet of the embryo pushes out and over the rim of the yolk sac, and
is pulled around and underneath itself (Figure 20.1).
As the
cranial fold progresses, the buccopharyngeal (or oro-pharyngeal) membrane (see
Figure 14.2) moves around to the position of the future mouth, and the early
neural tube that will form the brain comes to lie cranially to it. A region of
cells that begin to form the heart tube (see Chapter 27) are also pulled around
and come to lie in the future thorax, caudal to the mouth.
At the
caudal end, folding brings the cloacal membrane (Figure 14.2) underneath the
embryo, and the connecting stalk around towards the future umbilical region of
the embryo’s abdo- men. With this movement the connecting stalk, the allantois
and the yolk sac are all brought close together (Figure 20.1). The connecting
stalk is the link between the embryo and the placenta. The yolk sac by this
stage (day 26) is linked to the early gastrointestinal tract by the vitelline
duct (see Chapter 33).
As the
embryo curls up longitudinally, it also rolls up across its width. The left and
right flanks of the embryonic disc extend and curl around underneath the
embryo, squeezing the sides of the yolk sac (see Figure 19.1).
The left and
right flanks meet, and the germ layers of either side meet and fuse. The
ectoderm of the left side meets the ectoderm of the right side forming a
continuous external surface for the embryo. Similarly, the mesodermal and
endodermal layers meet. The endoderm forms a tube that ends at the
buccopharyngeal and cloacal membranes, which also remains continuous with the
yolk sac. This is the lining of the gastrointestinal tract (see Chapter 33).
This meeting of the left and right flanks or folds of the embryo begins at the
cranial and caudal ends and continues towards the middle. By day 30 the yolks
sac’s connection to the gastrointestinal tract is squeezed by this growth, but
remains substantial (Figure 20.1).
As a result
of this folding, curving, rolling and pinching, the embryo has a ‘tube within a
tube’ body plan at the start of week 5. The outer tube is made of ectoderm, the
inner tube is endoderm, and in between lies mesoderm and the early body cavity
(also known as the coelom). This arrangement is common to many embryos, from
nematodes to humans, and marks a major trend in evolution.
Clinical relevance
Gastroschisis describes the herniation
of abdominal contents externally through the anterior abdominal wall. It is
usually detected before birth by ultrasound, and the defect often lies to one
side of the umbilicus. Gastroschisis may result from a failure of the anterior
body wall to form normally as described above. It can be treated after birth
surgically or by protecting the herniated bowel in an aseptic film and allowing
the intestine to return to the abdominal cavity slowly over time. Omphalocoele
is a different type of foetal herniation, in which the abdominal ts herniate
into the umbilicus and are therefore covered (see Chapter 35).
