Longitudinal Section

This set of illustrations represents the structures within the embryo around the beginning of the fourth week, before folding begins.  The large image to the left is a longitudinal section through the embryo, while the three screens to the right show cross sections through the future thoracic, middle, and caudal regions of the embryo.

At this point, the embryo is a trilaminar germ disc consisting of three layers: ectoderm, mesoderm, and endoderm.  These layers are distinct from, but continuous with, extraembryonic tissues.

Specifically the ectoderm of the embryonic disc is continuous with the amnioblasts of the amniotic membrane.  These two layers together bound the amniotic cavity, which contains amniotic fluid.  The endoderm of the embryonic disc is continuous with the extraembryonic endoderm of the yolk sac.  The intraembryonic mesoderm of the embryonic disc is continuous with the extraembryonic mesoderm of the yolk sac and amniotic membrane; these extraembryonic mesodermal tissues are continuous with the extraembryonic mesoderm of the connecting stalk and chorion of the developing placenta.  

The extraembryonic coelom, also called the chorionic cavity, is continuous with the intraembryonic coelom along the lateral edge of the embryo, where the lateral plate mesoderm has split into splanchnopleuric and somatopleuric layers.

Also note at this point that the developing heart and septum transversum, which will become the diaphragm, are developing within the mesoderm at the very cranial end of the embryo.

Authors typically describe embryonic folding by stating that some tissues or organs migrate relative to others.   For example, as you will see shortly, the heart and diaphragm will move caudally relative to the developing brain.  Note that, in reality, embryonic folding occurs because some tissues, particularly the developing central nervous system, are growing at a more rapid rate than others, such as the digestive tract.  This causes the apparent “movement” described in texts.

Before you being the animation showing folding, it may help to imagine what would happen if you took a needle, inserted it into the amniotic cavity, and injected fluid into that space.  Although this is not what exactly causes embryonic folding, you may find that this will go a long way toward helping you visualize what is occurring during this process.

As you can see, folding of the embryo essentially involves movement of the edges of the disc-shaped embryo ventrally toward the midline.  As the right and left lateral edges of the embryo meet, corresponding tissues fuse to their equivalent counterparts from the other side.  For example, ectoderm fuses with ectoderm to form a continuous epithelial layer that will later form the epidermis.  The endoderm fuses to form the gut tube, and splanchnopleuric and somatopleuric mesoderm tissue each fuse with their corresponding counterpart from the other side.

The end result is that the embryo is transformed from a flat disc with ectoderm on the dorsal surface and endoderm on the ventral surface into a tube-shaped organism with ectoderm on the outside, endoderm lining the gut tube on the inside, and mesoderm in between these two layers.

Also note that the cranial and caudal ends of the embryo migrate ventrally, and roll up toward the middle of the embryo.  This means that the lateral folding described above is initially complete at the cranial and caudal ends of the embryo, and progresses from both ends toward the future umbilical cord.

Cranial folding also moves the heart and the septum transversum from the cranial end of the embryo into the area of the future thorax.  This cranial folding also creates the blind-ending foregut, which is the cranial portion of the digestive tract lined with endoderm.

Caudal folding moves the connecting stalk to the ventral side of the embryo, where it contributes to a substantial portion of the connective tissue of the umbilical cord.  This caudal folding also creates another blind-ending portion of the gut tube, the hindgut.  This hindgut has an expanded caudal-most region referred to as the cloaca.  A small diverticulum of the cloaca, that allantois, projects into the umbilical cord.

As you observe the four-week old embryo that has completed most of its folding, take a moment to recall that folding is complete at the cranial and caudal ends of the embryo, but is incomplete where the developing umbilical cord is forming.  The junction of the embryo and the umbilical cord is referred to as the umbilical ring.  Here, tissues and spaces of the embryo are still continuous with extraembryonic tissues and spaces.

Specifically, the ectoderm forming the epidermis of the embryo is continuous with the amnioblasts of the amniotic membrane, which is now covering the connecting stalk to form the outer layer of the umbilical cord.  The endoderm of the embryonic gut tube has a constricted projection into the umbilical cord called the vitelline duct, which connects the midgut with the waning yolk sac.  Embryonic mesodermal tissues are still continuous with extraembryonic mesoderm of the connecting stalk, which is continuous with the extraembryonic mesoderm of the chorion.  The intraembryonic coelom, which will form the body cavities of the embryo, is still continuous with the extraembryonic coelom via a space within the umbilical cord.

Again, it should be emphasized that all of these continuities are located at the umbilical ring, and still exist in the developing embryo, even at term.  In a sense, embryonic folding is not complete until after birth, when the umbilical cord is cut and the epithelium of the skin heals.

As the embryo continues to develop, the amniotic cavity expands.  This expansion is complete when the extraembryonic mesoderm of the amniotic membrane becomes opposed to the extraembryonic mesoderm of the chorion.  In addition, note that the amniotic membrane wraps around the connecting stalk and vestigial yolk sac, forming the outer layer of the umbilical cord.  This effectively obliterates the extraembryonic coelom, also known as the chorionic cavity.  However, note that the amniotic membrane never fuses completely with the chorion or the connecting stalk.  Thus, the potential space of the extraembryonic coelom, which is continuous with the intraembryonic coelom at the umbilical ring, can be recreated, allowing viscera within the peritoneal cavity of the developing embryo to herniate into the umbilical cord, a condition called omphalocele.  After birth, increased abdominal pressure can breach the same weakness in the abdominal wall, creating an umbilical hernia.