HUMAN EMBRYONIC PERIOD

ByDR. SAMINA ANJUM

DERIVATIVES 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.

Sagittal midline sections

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• After 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

FOLDING OF EMBRYO

Leading to the formation of abdominal wall:• CEPHALOCAUDAL FOLDING, MEDIAN PLANE

– CAUSED BY RAPID LONGITUDINAL GROWTH OF CNS

• LATERAL FOLDING, HORIZONTAL PLANE– CAUSED BY RAPIDLY GROWING SOMITES

Cont…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 fold

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• Head fold: The cranial region of the embryonic disc, from caudal to cranial contains:

• The buccopharyngeal membrane • The cardiogenic area • Septum transversum

• With 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). 

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Future prosencephalonNotochordNeural tubePericardial cavityCardiac tubePharyngeal membraneThus the cardiogenic area and the buccopharyngeal membrane are placed in the future thoracic and mouth regions

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Extraembryonic mesodermThroatSeptum transversum

Tail fold: The caudal rim of the embryonic disc contains: • The cloacal membrane • The 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. 

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NotochordNeural tubeAmniotic cavity Primitive streakEndodermCloacal membraneAllantoisBody stalkHind gut

As 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.

• In the 4th week buccopharyngeal membrane ruptures and an open communication is established between amniotic cavity and the primitive gut.

Cont…

• 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

• LATERAL FOLDING• Initial 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 attached• When 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

• Partial incorporation of allantois into body of embryo where it forms cloaca while its distal portion remain in connecting stalk

• By 5th week Yolk sac duct (vitelline duct), allantois, and umbilical vessels are restricted to the region of umbilical ring

• In humans yolk sac is vestigial and in all probability has a nutritive role only in early stages of development

• In second month it lies in chorionic cavity.

ENDODERM – DERIVATIVES• EPITHELIAL LINING OF:

– PRIMITIVE GUT– ALLANTOIS & VITTELINE DUCT– RESPIRATORY TRACT – URINARY BLADDER & URETHRA– TYMPANIC CAVITY & AUDITORY TUBE

• PARENCHYMA OF THYROID, PARATHYROIDS, LIVER & PANCREAS

• RETICULAR STROMA OF TONSILS & THYMUS

CROWN RUMP LENGTH CORRELATED TO AGE IN WEEKS

CRL (mm) AGE (WEEKS)

5-8 5

10-14 6

17-22 7

28-30 8

EXTERNAL 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 somites

• Hind limb buds appear at the level of the lumbar and upper sacral somites

• Terminal portions of buds flatten and circular constriction separates them from proximal more cylindrical segments

• Soon 4 radial grooves separates 5 slightly thicker areas appear on distal portion of buds

• These 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 limb

• While 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

EXTERNAL APPEARANCE OF EMBRYO 8TH WEEK

BIRTH DEFECTS

• Most 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.

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