Third week of development - PowerPoint Presentation

1. Third week of development Second stage –first semester Anatomy and histology and Embryology Department college of medicine – Diyala university Assist. Lec. Reham Saad Kadhum Master Degree in Applied Embryology

2. Objectives Formation of ectoderm, mesoderm and endoderm.Formation of notochord & cloacal membrane.Formation of allantois.Development of primary, secondary, and tertiary villi.

3. Tri laminar Germ Disc The most characteristic event occurring during the third week of gestation is gastrulation, Gastrulation, the process that estabhshes all three germ layers (ectoderm, mesoderm, and endoderm) in the embryo. Gastrulation begins with formation of the primitive streak on the surface of the epi- blast . Primitive streak : A narrow groove with slightly bulging regions on either side. occurs on the surface of the epiblast , it marks the beginning of Gastrulation , initially its ill defined , then it becomes clear by day 15, 16 and, The cephalic end of the streak, the primitive node, consists of a slightly elevated area surrounding the small primitive pitit shrinks and disappears, by the end of 4th week.

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5. Formation of Trilaminar germ Disc Cells of the epiblast migrate toward the primitive streak Upon arrival in the región of the streak, they become flask-shaped, detach from the epiblast, and slip beneath it . This inward movement is known as invagination Note : Cell migration and specification are controlled by fíbroblast growth factor 8 (FGF8), which is synthesized by streak cells themselves. Once the cells have invaginated, some displace the hypoblast, creating the embryonic endoderm,others come to lie between the epiblast and newly created endoderm to form mesoderm. Cells remaining in the epiblast then form ectoderm.

6. Note The epiblast, through the process of gastrulation, is the source of all of the germ layers, and cells in these layers will give rise to all of the tissues and organs in the embryo.

7. Oropharyngeal membrane:small tightly adherent region of ectoderm and endoderm cells at the cranial end of the embryonic disc.Cloacae membrane :Tightly adherent region of ectoderm and endoderm cells at the caudal end of the embryonic disc.

8. Formation of Notochord Prenotochordal cells invaginating in the primitive node move forward cranially in the midline until they reach the prechordal plateThese prenotochordal cells become intercalated in the hypoblast so that for a short time, the midline of the embryo consists of two cell layers that form the notochordal plate As the hypoblast is replaced by endoderm cells moving in at the streak, cells of the notochordal plate proliferate and detach from the endoderm. They then form a solid cord of cells, the definitive notochord Because elongation of the notochord is a dynamic process, the cranial end forms first, and caudal regions are added as the primitive streak assumes a more caudal position The notochord and prenotochordal cells extend cranially to the prechordal plate (an area just caudal to the oropharyngeal membrane) and caudally to the primitive pit. At the point where the pit forms an indentation in the epiblast, the neurenteric canal temporarily connects the amniotic and yolk sac cavities definitive notochord serves the bases of the axial skeleton.

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10. Fate map of migrating epiblast during gastrulation:1.First cells that migrate through the primitive node in cephalic direction, will form prechordal plate.2.Cells migrating through the cranial part of the primitive node become notochord.3. Cells migrating through the lateral edge of the primitive node and the cranial end of the primitive streak will form paraxial mesoderm.4. Cells migrating through the midstreak will form intermediate mesoderm. 5. Cells migrating through the caudal part of the streak will form lateral plate mesoderm.6. Cells migrating though most caudal part of the streak, contribute to extraembryonic mesoderm.

11. FURTHER DEVELOPMENT OF THE TROPHOBLASTBy the beginning of the third week, the trophoblast is characterized by primary villi that consist of a cytotrophoblastic core covered by a syncytial layer . During further development, mesodermal cells penetrate the core of primary villi and grow toward the decidua. The newly formed structure is known as a secondary villus By the end of the third week, mesodermal cells in the core of the villus begin to differentiate into blood cells and small blood vessels, forming the villous capillary system The villus is now known as a tertiary villus or a definitive placental villus.

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14. Capillaries in tertiary villi make contact with capillaries developing in the meso- derm of the chorionic píate and in the connecting stalk .These vessels, in turn, establish contact with the intraembryonic circulatory system, connecting the placenta and the embryo. Henee, when the heart begins to beat in the fourth week of development, the villous system is ready to supply the embryo proper with essential nutrients and oxygen.

15. The cytotrophoblast of the villi penetrates deeply into the overlong syncytium to reach the uterine stroma. Then these cells cover all the syncytium forming an outer Cytotrophoblast shell that strengthens the implantation of the placenta.

16. The chorionic cavity, meanwhile, becomes larger, and by the 19th or the 20th day, the embryo is attached to its trophoblastic shell by a narrow connecting stalk The connecting stalk later develops into the umbilical cord, which forms the connection between the placenta and embryo. Formation of allantois; a diverticulum from the posterior wall of yolk sac inside the connecting stalk during day 16.

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18. Clinical Notes Caudal dysgenesis ( Sirenomelia):Cause:Toxic insult such as high blood sugar, This will lead to insufficient mesoderm .Clinical picture: Hypoplasia and fusion of lower limb, vertebral abnormalities, renal agenesis, imperforated anus, abnormalities of genital organs.

19. Craniofacial defects:Cause:high doses of alcohol at this stage will kill cells located in the anterior midline of the germ disc , and this will lead to defect in midline craniofacial structures ( holoprosencephaly).Clinical pictures:Small forebrain, two lateral brain ventricle merge into one , eyes are close together .

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