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.
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).