Phases of Embryonic Development

1. Phases of Embryonic Development 1- Growth : it involves cell division and the elaboration of cell products. 2- Morphogenesis : During it, development of shape;size, features of the whole body. 3- Differentiation : During itmaturation of physiological processes Congenital anomalies are common during the 4th to 8th weeks, because the tissue & organs are differentiatingrapidly and their exposure to teratogens raise the incidence of these anomalies. Critical developmental events occur during the first 3 weeks. Such as development of the nervous & cardiovascular systems. The 4th to 8th weeks of development constitute most of the embryonic period. All major external & internal structures are established during these weeks. By the end of this organogenetic period( end of the 8th week ) the embryo has a distinctly human appearance.

2. Folding of the Embryo in Median Plane ( in longitudinal plane ) The head fold By the beginning of the 4th week, the neural folds in the cranial region have thickened to form the primordium of the brain. At the beginning, the developing brain projectsdorsally into the amniotic cavity. Later the developing forebrain grows cranially beyond the oropharyngeal membrane and overhangs the developing heart. So, the septum transversum ( transverse septum ); primordial heart;pericardial coelom and oropharyngeal membrane move onto the ventral surface of the embryo. During longitudinal folding, part of the endoderm of the yolk sac is incorporated into the embryo as the foregut ( primordium of pharynx, esophagus ).

3. The foregut lies between the brain & the heart and the oropharyngeal membrane separating the foregut from the stomodeum. After folding, the septum transversum lies caudal to the heart where it develops into the central tendon of the diaphragm.

4. The head fold also affects the arrangement of the embryonic coelom (primordium of body cavities ). Before folding, the coelom consists of a flattened, horseshoe- shaped cavity. After folding, the pericardial coelomlies ventral to the heart and cranial to the septum transversum. At this stage the intraembryonic colelomcommunicates widely on each side with extraembryonic coelom.

5. The Tail Fold ( in longitudinal plane ) Folding of the caudal end of the embryo results from growth of the distal part of the neural tube ( primordium of the spinal cord ). As the embryo grows, the caudal eminence ( tail region )projects over the cloacal membrane ( future anus ). During folding, part of the endodermalgerm layer is incorporated into the embryo as the hindgut ( primordium ofdescending colon ). The terminal part of the hindgut soon dilated to form the cloaca ( primordium of urinary bladder & rectum ). Before folding, the primitive streak lies cranial to the cloacal membrane. After folding, it lies caudal to it.

6. The connecting stalk (primordiumof the umbilical cord ) is nowattached to the ventral surface of the embryo. The allantois which is a diverticulum of the yolk sac is partially incorporated into the embryo.

7. Folding in Horizontal plane Folding of the sides of the embryo produces right and left lateral folds. Lateral folding is produced by the rapidlly growing spinal cord and somites. The primordia of the ventrolateral wall fold toward the median plane, rolling the edges of the embryonic discventrally and forming a roughly cylindrical embryo. As the abdominal walls form, part of the endoderm germ layer is incorporated into the embryoas the midgut ( primordium of small intestine ). Initially, there is a wide connection between the midgut & yolk sac. After lateral folding the connection is reduced to a yolk stalk.

8. The region of attachment of the amnion to the ventral surface of the embryo is also reduced to a relative narrow umbilical region As the umbilical cord forms from the connecting stalk, ventral fusion of the lateral folds reduces the region of communication between the intraembryonic & extra-embryonic coelomic cavities to a narrow communication. As the amniotic cavity expands and obliterates most of the extraembryonic coelom, the amnion forms the epithelial covering of the umbilical cord. Body foldingabnormalities are uncommon. Antenatal ultrasonography is used to diagnose these cases.