Organogenesis

Embryonic Period (4th – 8th week)

Dr. Sherif Fahmy

Fate of Ectoderm

Dr. Sherif Fahmy

DEVELOPMENT OF NEURAL TUBE• Neural plate is median thickened area between

primitive node and prochordal membrane. Two strips separate neural plate from the rest of ectoderm which are called neural crest.

• Neural folds are raised margins of neural plate while depressed median region is called neural groove.

• Neural tube is formed by fusion between two neural folds in its middle and extends cranio-caudally. Cranial and caudal ends (neuropores) are the last to be closed.

Dr. Sherif Fahmy

Neural groove

Neural fold

Notochord

Fusing neural folds to form neural tube

Neural crest

EctodermEndoderm

Dr. Sherif Fahmy

Dr. Sherif Fahmy

Dr. Sherif Fahmy

Dr. Sherif Fahmy

Dr. Sherif Fahmy

Dr. Sherif Fahmy

Dr. Sherif Fahmy

Fate of the neural tube• The tube grows in the median region leading to

elongation of the embryonic disc in cranio-caudal direction.

• The cranial part of the tube dilates to form the brain vesicle while the caudal part forms the spinal cord.

• The brain vesicle divides by 2 constrictions into:– Forebrain: forms cerebral hemispheres and

diencephalone.– Midbrain: forms the midbrain (upper part of brain stem).– Hindbrain: forms medulla, pones and cerebellum.

Dr. Sherif Fahmy

Dr. Sherif Fahmy

Fate of neural crest• Ganglia: Sensory (of cranial and spinal

nerves), sympathetic and parasympathetic.• Cells: Chromaffin cells of supra-renal

medulla, Schwann cells and melanoblasts.• Others: Pia mater, arachnoid mater, enamel

of teeth, septa of the heart and some bones of the skull.

Dr. Sherif Fahmy

Other derivatives of ectoderm- Otic placodes form internal ear.- Lens placodes form lens of the eye.- Peripheral nerves.- Sensory epithelium in ear, nose, eye and

epidermis of skin.- Pituitary gland.- Anterior part of oral cavity and lower ½ of

anal canal.Dr. Sherif Fahmy

FoldingDr. Sherif Fahmy

FOLDING OF THE EMBRYO• It is the process by which the embryo becomes folded upon

itself.Time of folding: • At the end of 3rd week and completed at the end of 4th

week.Causes of folding:• Rapid increase of cranio-caudal length due to rapid growth

of neural tube and somites.• Rapid expansion of amniotic cavity.Types of folding:• Head and tail folds are folding of cranial and caudal parts of

the disc.• Lateral folds are folding of lateral parts of the disc.

Dr. Sherif Fahmy

Dr. Sherif Fahmy

Dr. Sherif Fahmy

Dr. Sherif Fahmy

Results of Folding

Dr. Sherif Fahmy

Dr. Sherif Fahmy

Dr. Sherif Fahmy

Embryonic disc with removed ectoderm

Cloacal membrane

Notochord

Paraxial mesoderm (somites)

Bucco-pharyngeal membrane

Cardiogenic area

Septum transversum

Peritoneal canal

Pericardium

Dr. Sherif Fahmy

Ectoderm

Mesoderm

Endoderm

Buccopharyngeal membrane

Cloacal membrane

Hindgut

Midgut

Foregut

Forebrain

Forebrain bulge

Pericardial bulge Vitelline duct AllantoisDefinitive yolk sac

Stomodeum

L.S. in folded embryoHeart

Dr. Sherif Fahmy

Peritoneal canals

Gut

Ventral mersentry

Dorsal mesentry

Dr. Sherif Fahmy

RESULTS OF FOLDING1-Cylindrical appearance: Transformation of emryonic disc to cylindrical shape.

2- Amniotic cavity: Before folding it lies dorsal to embryonic disc, after folding, it surrounds all aspects of the embryo.

3- Formation of definitive yolk sac: It is the part of yolk sac outside the embryo in the umbilical cord.4- Formation of primitive umbilical ring: It is a ventral defect in anterior abdominal wall that contains connecting stalk, allantois and vitello-intestinal duct

Dr. Sherif Fahmy

5-Formation of the gut: •It is formed from endodermal layer together with part of yolk sac. Foregut is formed in head fold with bucco-pharyngeal membrane closing its cranial end. Hindgut: is formed in tail fold and closed caudally by cloacal membrane. The caudal part is dilated and called cloaca which is connected ventrally to allantois. Midgut: is formed by lateral folds and present between foregut and hindgut. It is connected with defenitive yolk sac by vitelline duct.

6- Formation of stomodeum: Ectodermal depression between forebrain bulge and cardiac bulge.

Dr. Sherif Fahmy

7- Formation of mesenteries: Ventral and dorsal mesenteries are formed around gut.

8- Reversal of positions:-Heart and pericardium become cranial to septum transversum (before folding septum transversum is most cranial).

-Connecting stalk becomes ventral and more cranial inspite of being most caudal.

Dr. Sherif Fahmy

Somites After Folding

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Dr. Sherif Fahmy

Dr. Sherif Fahmy

Dr. Sherif Fahmy

Development of Endoderm(Page 30)

-Epithelium of digestive system, respiratory tract, most of urinary bladder and urethera, tympanic cavity and Eustachian tube.-Parenchyma of liver, pancreas, thymus, thyroid, parathyroid and palatine tonsils. Dr. Sherif Fahmy

Fetal Membranes

Dr. Sherif Fahmy

Fetal membranes:1- Chorion 2- Placenta.2- Amnion.3- Umbilical cord.4- Yolk sac.

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Chorion

Dr. Sherif Fahmy

It is the wall of chorionic vesicle.Time: Chorionic vesicle is formed at the 12th day by the formation of extra-embryonic mesoderm.Structure of chorion:1- Syncytiotrophoblast.2- Cytotrophoblast.3- Somatic extra-embryonic mesoderm.Chorionic velli:1- Primary.2- Secondary.3- Tertiary.

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Connecting stalk

Somatic mesoderm

Syncytio-trophoblast

Cyto-trophoblast

Chorion

Chorionic Vesicle

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Dr. Sherif Fahmy

Dr. Sherif Fahmy

Primary chorionic villus

Cyto-trophoblast

Syncytio-trophoblast

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Dr. Sherif Fahmy

Syncytio-trophoblast

Cyto-trophoblast

Somatic mesoderm

Secondary chorionic villus

Dr. Sherif Fahmy

Dr. Sherif Fahmy

Syncytio-trophoblast

Cyto-trophoblast

Mesoderm

Fetal blood vessels

Tertiary chorioniv villus

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Decidua basalis

Chorion frondosum

Chorionic plate

Chorion leave

Dr. Sherif Fahmy

Dr. Sherif Fahmy

1- Primary chorionic velli (start of 3rd week): cyncytiotropholblasts and cytotrophoblast.

2- Secondary chorionic velli (middle of 3rd week):

Cyncytiotrophoblast, cytotrophoblast and mesoderm (in the central core).

3- Tertiary chorionic velli (end of 3rd week): formation of fetal blood vessels in the mesoderm.

-Tertiary velli, opposite decidua basalis form side branches and called chorion frondosum while under decidua capsularis it will degenerates to form chorion leave.

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PLACENTA(Page 38)

Dr. Sherif Fahmy

Morphology of Placenta• It is the organ of exchange of materials between fetal

and maternal blood.• Shape: Disc like.• Surfaces:• -Fetal surface: It is covered with amnion and fetal blood

vessels. Umbilical cord is attached near the center of this surface.

• -Maternal surface: Shows 15 – 20 rounded elevations (cotyledons) with septa inbetween).

• Diameter: 15 -25 cm.• Thickness: About 3 cm.• Weight: About 500 – 600 gm• Site: At original implantation site which is upper part of

posterior wall of uterus.Dr. Sherif Fahmy

Dr. Sherif Fahmy

Cotyledon

Groove between cotyledons

Umbilical cord

Maternal surface

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Dr. Sherif Fahmy

Fetal surface covered with amnion

Umbilical cord

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Dr. Sherif Fahmy

Formation of Placenta

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Dr. Sherif Fahmy

Dr. Sherif Fahmy

Dr. Sherif Fahmy

Dr. Sherif Fahmy