2DERIVATIVES OF ENDODERMAL GERM LAYER This germ layer covers the ventral surface of the embryo forms the roof of the yolk sac.GIT is the main organ system derived from the endodermal germ layer.17 days22 daysSagittal midline sections
3After the end of the third week, the three germinal layers begin to differentiate and transform so that the initially flat embryonic disk develops into a cylindrical structure like a "C". The folding and genesis of the abdominal wall permits a delimitation of the embryo
4FOLDING OF EMBRYO Leading to the formation of abdominal wall: CEPHALOCAUDAL FOLDING, MEDIAN PLANECAUSED BY RAPID LONGITUDINAL GROWTH OF CNSLATERAL FOLDING, HORIZONTAL PLANECAUSED BY RAPIDLY GROWING SOMITES
5Cont… CEPHALOCAUDAL FOLDING When the brain vesicles develop, the embryonic disc begins to bulge into the amniotic cavity and begins to fold cephalocaudally.Head fold and tail fold24 days28 days
6Head fold: The cranial region of the embryonic disc, from caudal to cranial contains: The buccopharyngeal membraneThe cardiogenic areaSeptum transversum
7With the 180° degree turn that results from the folding, the following occurs: The cranial rim of the disc folds under the cephalic neural plate. The buccopharyngeal membrane extends towards the lower front (mouth area) and the cardiogenic plate (which initially lay most cranially) into the thorax area. Between the heart and the yolk sac a mesenchymal bridge forms, the septum transversum.After this movement is completed, the brain lies the most cranially, followed by the mouth, heart, and diaphragm (septum transversum).
87 89Extraembryonic mesoderm Throat Septum transversumFuture prosencephalon Notochord Neural tube Pericardial cavity Cardiac tube Pharyngeal membrane Thus the cardiogenic area and the buccopharyngeal membrane are placed in the future thoracic and mouth regions
9Tail fold: The caudal rim of the embryonic disc contains: The cloacal membraneThe connecting stalk, with the slender allantois.The folding of the caudal end occurs after the cephalic folding and has the result that the body stalk comes closer to the yolk sac. Due to the large axial growth the caudal end of the embryonic disk (with the cloacal membrane) comes to lie under the original embryonic disk and thus shoves the allantois and the body stalk in the ventral direction, up to the yolk sac and merges with its stalk.
10Notochord Neural tube Amniotic cavityPrimitive streak Endoderm Cloacal membrane Allantois Body stalk Hind gut
11As a result of folding:A large portion of the endoderm-lined cavity is incorporated into the body of the embryo proper.Anterior part forms the foregut and tail region forms the hindgut.The midgut temporarily communicates with the yolk sac by way of broad stalk, the vitelline duct.
12In the 4th week buccopharyngeal membrane ruptures and an open communication is established between amniotic cavity and the primitive gut.
13Cont…Cloacal membrane breaks down in the 7th week to create the opening for the anus.The mid gut remains in communication with the yolk sac though vitelline duct
14LATERAL FOLDINGInitial flat embryonic disc also folds laterally and embryo obtains round appearanceAs a result of this folding:The ventral body wall is established except where yolk sac duct and connecting stalk are attachedWhen Vitelline duct is obliterated, only then does the midgut looses its connection with original endoderm lined cavity and obtain its free position in abdominal cavity
15Partial incorporation of allantois into body of embryo where it forms cloaca while its distal portion remain in connecting stalkBy 5th week Yolk sac duct (vitelline duct), allantois, and umbilical vessels are restricted to the region of umbilical ring
16In humans yolk sac is vestigial and in all probability has a nutritive role only in early stages of developmentIn second month it lies in chorionic cavity.
18CROWN RUMP LENGTH CORRELATED TO AGE IN WEEKS CRL (mm)AGE (WEEKS)5-8510-14617-22728-308
19EXTERNAL APPEARANCE DURING THE 2ND MONTH Increase in head size and formation of the limbs, face, ears, nose and eyes occur.By the beginning of the 5th week, forelimbs and hind limbs appear as paddle shaped buds. The forelimb buds are located dorsal to the pericardial swelling at the level of the C4 – T1 somitesHind limb buds appear at the level of the lumbar and upper sacral somites
20Terminal portions of buds flatten and circular constriction separates them from proximal more cylindrical segmentsSoon 4 radial grooves separates 5 slightly thicker areas appear on distal portion of budsThese grooves are called rays which appear in hand region first and shortly afterwards in foot as upper limb is slightly more advanced in development than lower limbWhile fingers and toes are formed 2nd constriction divides the proximal portion of buds into two segments and 3 characteristic features of adult extremity are recognizable
26BIRTH DEFECTSMost major organs and organ systems are formed during the 3rd to 8th week. This period is called the period of organogenesis. Stem cell populations are establishing each of the organ primordia, any insult from genetic and environmental influences can result in birth defects. The 3rd and 4th weeks are particularly vulnerable.