Alaqah 2

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Alaqah 2

  1. 1. •ALAQAH-2 •Trilaminar Germ Disc •3rd week
  2. 2. • Introduction • Formation of the primitive streak • Genesis of the germ layers • Genesis of the notochord • Location of the epiblast cell target and the development of the primitive streak • The bilaminar membranes
  3. 3. • Evolution of the mesoblast – The paraxial mesoderm and the differentiation of the somites – The intermediate mesoblast – The lateral mesoblast • The intraembryonic coelom • Induction of the neural plate - neurulation
  4. 4. • During first week of development zygote multiplies and form morula, which contains inner cell mass in the center. • The morula changes into blastula or blastocyst during the same week and the inner cell mass has shifted to one-side, and is now called embryoblast.
  5. 5. • During second week of development the cells of the embryoblast differentiate into two layers, the epiblast, which forms the floor of the amniotic cavity and the hypoblast, which forms the roof of yolk sac. • The cells of each of the germ layers form a flat circular disc and together they are known as the bilaminar germ disc.
  6. 6. • In the median sagittal plane in the cephalic region the hypoblast disc shows a slight thickening forming an oval plate of large vesicular cells, the prochordal plate. It is a small circular or oval area of columnar cells in the hypoblast, which are firmly adherent to the overlying epiblast disc. • Prochordal plate together with overlying epiblast forms oropharyngeal (buccopharyngeal) membrane.
  7. 7. • Similarly in the caudal region the hypoblast cells show a slight thickening forming an oval plate of large vesicular cells. This is cloacal plate. • Together with overlying epiblast it is given the name cloacal membrane. • During 3rd week the developing embryo has three layers.
  8. 8. Gastrulation • It is the process by which the embryo acquires three germ layers. • During the third week mesoblast layer develops between the epiblast and hypoblast. • Day-16. The epiblast cells near primitive streak proliferate, and loose their connection with one another. • They also develop foot like processes called pseudopodia and start walking down through primitive groove. • This process of invagination and ingress is called gastrulation.
  9. 9. • The bilaminar germ disk differentiates itself further into a trilaminar embryo, in that the cells flow in over the primitive streak between the two already existing germinal layers and so form the third embryonic germinal layer (mesoblast/derm).
  10. 10. • This phenomenon is also termed epithelio-mesenchymal transition (gastrulation in lower vertebrates)
  11. 11. • The germ disc now becomes trilaminar and the three layers are now given new names. – Ectoderm (Superior layer) – Mesoderm (Middle layer) – Endoderm (Inferior layer) • The process by which the bilaminar germ disc is converted into trilaminar germ disc is called gastrulation and the developing structure is now called gastrula. • Gastrulation begins with formation of primitive streak.
  12. 12. PRIMITIVE STREAK • It is a thickened linear band of epiblast cells present in the midline in the caudal region on the superior surface of epiblast. • It starts developing by the beginning of 3rd week close to cloacal plate. • The cells of the epiblast multiply and move toward primitive streak and the streak elongates in cephalic direction.
  13. 13. 15-day developing embryo showing bilaminar germ disc.
  14. 14. • At this stage the embryonic disk is oval -shaped and the ectoblast is bathed in amniotic fluid.
  15. 15. • As the streak elongates by addition of cells, its cranial end thickens to form a primitive knot or node. • The primitive knot is a slightly elevated rounded area at the cranial end of the primitive streak. • Concurrently, a narrow groove and pit develop in the primitive streak and primitive knot, called primitive groove and primitive pit respectively. The groove and the pit are continuous with each other.
  16. 16. 1 2 3 4 5 6 7 8 9 NB Primitive groove Primitive pit Primitive node Oropharyngeal membrane Cardial plate Sectional edge of amniotic membrane Mesoderm Endoderm Future cloacal membrane 1+2+3 primitive streak • Embryonic disk viewed dorsally. The red arrows show schematically the migration directions of the epiblast cells to their points of final destination.
  17. 17. FORMATION OF ENDODERM • Day-15. The epiblast cells near primitive streak proliferate, and loose their connection with one another. • They also develop foot like processes called pseudopodia and ingress down through primitive groove. • These cells invade the hypoblast and replace its cells. • Eventually the hypoblast is completely replaced by a new crop of cells derived from epiblast. Now hypoblast is given the name endoderm.
  18. 18. • Day 16. As more and more cells of the epiblast multiply and migrate toward the primitive groove, they detach and fall through the groove on the underlying hypoblast as mesoblast cells. • Here these wandering mesoblast cells spread in lateral and cephalic direction and form a network of cells called mesoblast. • Gradually mesoblast cells become organized into a layer called intraembryonic mesoderm.
  19. 19. 1. Primitive groove 2. Epiblast 3. Extraembryonic mesoblast 4. Definitive endoblast 5. Invading epiblastic cells forming 6. the intraembryonic mesoblast 7. Hypoblast 8. Transverse section at the level of the primitive groove with the immigration of epiblast cells, which form the future mesoblast, as well as the endoblast, which replaces the hypoblast.
  20. 20. • This intra-embryonic layer of mesoderm gradually expands beyond the margins of the disc and establishes contact with the extraembryonic mesoderm covering the yolk sac and amnion. • Now the mesoblast cells are often called mesenchymal cells.
  21. 21. • At the same time, due to the migration of cells through the primitive node in the cranial direction, two further structures are formed: • the prechordal plate, which is located cranial to the primitive node • the notochordal process
  22. 22. • In the cephalic direction, mesenchymal cells pass on each side of the prochordal plate to meet each other in front of this plate where they form cardiogenic (heart forming) plate. • Similarly the mesoblast or mesenchymal cells migrate around cloacal plate and reach posterior to cloacal plate and join connecting stalk (the extraembryonic mesoderm).
  23. 23. FORMATION OF NOTOCHORD • The cells detaching and falling from primitive pit move straight forward and form a cord-like process, known as notochordal or head process (day-17). • Remember that notochordal process contains a canal continuous with primitive node. • The notochordal process extends cranially from the primitive node toward prochordal plate.
  24. 24. • Later on the floor of the notochordal process fuses with the underlying endoderm (day 18). • The fused regions degenerate and openings appear in the floor of the notochordal process, bringing the notochordal canal into communication with the yolk sac. • These openings rapidly join each other and the floor of the notochordal process disappears. • The remaining part of notochordal process forms a dome-shaped plate grooved from below, known as the notochordal plate.
  25. 25. • A small passage, the neurenteric canal temporarily communicates the amniotic cavity with the yolk sac (day-18). • The neurenteric canal appears due to the degeneration and disappearance of the floor of the notochordal process. It runs obliquely downward and forward from the primitive pit. • Beginning at the cranial end, the notochordal plate enfolds and separates from the endoderm to form a solid cord of cells, known as the notochord (day-19).
  26. 26. • Initially the embryonic disc is flat and essentially circular, but it soon becomes pear- shaped and than elongated slipper-shaped as the notochordal process grows and the notochord forms. • Expansion of the embryonic disc occurs mainly in the cranial region; the caudal end remains more or less unchanged at this stage of development.
  27. 27. • Initially the intraembryonic mesoderm forms a thin flattened sheet on both sides of midline. • By about day 17 the mesenchymal cells close to the midline proliferate and form thick bilateral bars of mesoderm along the sides of notochord, called paraxial mesoderm.
  28. 28. • Laterally toward the edges of the disc intraembryonic mesoderm remains thin flattened and is known as lateral plate mesoderm. • A number of small spaces appear in lateral plate mesoderm. These spaces soon coalesce and divide the lateral plate mesoderm into two layers: 1. Somatic or parietal mesoderm layer continuous with the somatic mesoderm covering the amnion. 2. Splanchnic or visceral mesoderm layer continuous with the splanchnic mesoderm covering the yolk sac. • The space between the two layers is known as intraembryonic coelom, which on each side of the embryonic disc, is continuous with extraembryonic coelom.
  29. 29. Between paraxial mesoderm and lateral plate mesoderm, the mesenchymal cells proliferate (day 20) and form another set of thick bilateral bars of mesoderm known as intermediate mesoderm.
  30. 30. ALLANTOIS • (Gr. Allantos, sausage) • The allantois, also called allanto-enteric diverticulum, appears on about day 16 as a relatively tiny, finger-like diverticulum. • It is an endodermal outgrowth from the posterior wall of the secondary yolk sac and grows into the mesoderm of the connecting stalk.
  31. 31. Day 18 developing embryo showing the formation of allantois.
  32. 32. • It serves as a storage place for the excretion products of renal system. (It has a respiratory function in embryos of reptiles, birds and some mammals). • Contribute in development of urinary bladder. • As the bladder enlarges, the allantois becomes the urachus connecting the apex of bladder with the umbilicus. • The allantois also induces the formation of the blood vascular system in the connecting stalk. If allantois does not develop, the blood vessels fail to develop in the connecting stalk. So the intraembryonic blood vascular system cannot establish communication with the extraembryonic blood vascular system or placenta or with maternal blood. Thus the embryo cannot survive.

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