The disc-like arrangement of the germ layers is converted into a recognizable vertebral embryo by folding in the fourth week of development. Folding is due to a differential rate in growth of the different parts of the embryo. The embryo is in a contained space so as it grows, it curves and ridges of tissues form. The embryonic disc grows rapidly particularly in length, because of the growth of the brain and tail, so it has to fold. Although this is a momentous stage of development, relatively little is known about it. The yolk sac does not grow and, as the outer rim of the endoderm is attached to the yolk sac, the embryo becomes convex. Folding occurs at the cephalic (head) and lateral regions on day 22 and at the caudal (tail) end of the embryo on day 23 (Fig. 9.8). The cephalic, lateral and caudal edges of the embryonic disc are brought into apposition and the layers fuse along the midline, which converts the endoderm into the gut tube. Initially the foregut and the hindgut fuse, leaving the midgut open to the yolk sac. The folds cause a constriction between the embryo and yolk sac. The yolk sac gives rise to the primitive gut. The amnion expands, enveloping the connecting stalk and neck of the yolk sac, forming the umbilical cord. Folding is precisely coordinated and is controlled. Failure at this point results in conditions such as omphalos and gastroschisis. The developmental pattern involves synchronized tissue communication and interaction. Adjacent tissues induce changes in the movement and behaviour of neighbouring cells. Signals integrating genetic and environmental influences control cell proliferation, migration and apoptosis (Bard and Weddon, 1996). These signals, which may be diffusible molecules or direct physical contact, direct the expression of particular genes in the responding cells. Although all cells have the same DNA in their nuclei, depending on the signal received, some will express certain genes but not others. So, for instance, a skin cell expresses the genes that control the behaviour of a skin cell because they are switched on by the signals skin cells receive. A liver cell has the same genes as the skin cell but expresses different genes.

Techniques and concepts used to study molecular genetics (how the genetic code is expressed) in bacteria and Drosophila can be applied to mammalian embryogenesis, including human development. The DNA in the nucleus sets up a basic body plan, which establishes the pattern of the early embryo. The genes that control the basic body plan are the same in very diverse species. A highly conserved region of about 180 base pairs of DNA, known as the homeobox, is found in the genes that regulate the craniocaudal (head–tail) axis of embryonic development in almost all species studied (Murtha et al., 1991). The homeobox encodes a protein domain called the homeodomain which can bind DNA specifically. Homeobox genes encode transcription factors which switch on cascades of other genes, for instance all the ones needed to make a particular body part. Other morphogenic agents, signals and growth factors activate the homeobox genes. Hox genes are a particular subgroup of homeobox genes which are found in a special gene cluster, the HOX cluster. Hox genes determine the patterning of the body axis. They direct the identity of particular body regions, determining where limbs and other body segments will develop in the fetus. Limb abnormalities such as polydactyly may result from abnormal Hox genes. There appears to be a series of three sequential steps in the conversion of the oval trilaminar embryonic disc into the cylindrical configuration with the endoderm on the inside, the ectoderm on the outside and the mesoderm in between (Fig. 9.9) (Carlson, 2008). These steps result in segmentation of the embryo. Gap genes subdivide the embryo into broad regional domains. Pair-rule genes are involved in the formation of individual body segments and segment-polarity genes control the anterior–posterior organization of each segment (De Robertis et al., 1990). As the embryo develops, the segmental plan becomes less evident; remnants can be seen in the arrangement of the backbone and ribs and in the organization of the spinal nerves.

Box 9.4 summarizes the events taking place during weeks 4–8.

Neurulation is the formation of the neural plate and neural folds and the closure of these folds to form the neural tube, which is the precursor of the brain and spinal cord. The neural tube is completed by the end of the fourth week. The developing notochord induces the overlying ectoderm to thicken forming the neural plate, a raised slipper-like plate of neuroepithelial cells. This will give rise to the central nervous system (brain and spinal cord) and other structures such as the retina. In the middle of the third week, the neural groove appears in the centre of the neural plate (Fig. 9.10). To each side of the groove are neural folds, which enlarge at the cranial end as the start of the developing brain. Marked development of the brain is a characteristic of embryonic development in primates; human brain growth exceeds that of other species, continuing into adulthood. At the end of the third week, the neural folds start to fuse forming the neural tube, which separates from the surface ectoderm. The neural crest cells, which detach from the lateral edges of the neural folds, give rise to the spinal ganglia and ganglia of the autonomic system as well as a number of other cell types (Box 9.8). The paraxial mesoderm, closest to the notochord and developing neural tube, differentiates to form prominent paired blocks of tissue, or somites. The first somites appear from day 20. There are about 30 pairs of somites by day 30 increasing to a total of 44 pairs, but the cranial ones begin differentiation as new somites are added at the caudal end. The somites differentiate into sclerotomes, myotomes and dermatomes, which give rise to the axial skeletal bones, skeletal muscles and the dermis of the skin, respectively. The number of somites indicates the age of the embryo. The limbs carry with them the nerves from the somites from which they developed. The somatic pattern of development is important in understanding referred pain (see Chapter 13).