Early-Development-in-Birds.pptx

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1. Early development in birds Group 3 Najito , Obnamia , Penolio , Pureza , Reyes, Sacil , Soriano, Tarrega , Valencia, Zara

2. CLEAVAGE

3. Cleavage in bird eggs Accessible all year Easily raised At any particular temperature, developmental stage can be accurately predicted. Large numbers of embryos can be obtained at the same stage. Chick embryo can be surgically manipulated Often served as a surrogate for human embryos.

4. Cleavage in bird eggs Fertilization occur in the oviduct before the albumen and the shell are secreted upon it. The egg is telocithal (like that of a fish) Eggs undergo discoidal meroblastic cleavage. Cleavage in bird eggs

5. First cleavage furrow appear centrally in the blastodisc Equatorial and vertical cleavages divide the blastoderm into 5-6 cell tissue thick Subgerminal cavity – space between blastoderm and yolk. It is created when blastoderm cell absorb fluid from the albumin and secrete it between themselves and the yolk. At this stage, deep cells in the center of the blastoderm shed and die, leaving the one cell thick area pellucida Discoidal meroblastic cleavage

6. Discoidal meroblastic cleavage Area pellucida forms most of the actual embryo. Area opaca – the peripheral ring of blastoderm cell that have not shed their deep cells Marginal zone – thin layer cell between area pellucida and area oraca Some of the marginal zone cells become very important in determining cell fate during early chick development. Discoidal meroblastic cleavage

7. Discoidal meroblastic cleavage Figure 1. Discoidal Meroblastic Cleavage

8. GASTRULATION in the Chick Embryo

9. The time a hen lay an egg, the blastoderm contains about 20, 000 cells. Most area pellucida cell remain at the surface, froming the epiblast Other pellucida cells delaminated and migrated individually into the subgerminal cavity to form the polyinvagination island (primary hypoblast) It is an archipelago of disconnected clusters containing 5-20 cells each. A sheet of cells from the posterior margin of the blastoderm (with Koller’s sickle ) migrates anteriorly to join the polyinvagination island, later forming the secondary hypoblast. Gastrulation – THE HYPOBLAST

10. Figure 3. The primary epiblast Gastrulation – THE HYPOBLAST Figure 4. Forming of secondary epiblast

11. Gastrulation – THE HYPOBLAST Two layered blastoderm ( epiblast and hypoblast) is jooned together at the margin of the orea opaca , and the space between then forms a blastocoel. The embryo entirely come from the e piblast Hypoblast cells form portion of external membranes (esp. the yolk sac and stalk) Yolk stalk link the yolk mass to the endodermal digestive tube. All 3 layers are formed from the epiblastic cells. Gastrulation – THE HYPOBLAST

12. Gastrulation – THE HYPOBLAST Figure 5 . Hypoblast cells migration from deep cells of the posterior region

13. Gastrulation – the primitive streak Primitive streak – the major structural characteristic of avian, reptilian and mammalian gastrulation. Gastrulation – THE HYPOBLAST Figure 6. Cell migration during gastrulation

14. The streak elongates toward the future head region. At the same time, the secondary hypoblast cells continue to migrate anterior to the posterior margin of the blastoderm . The streak extends 60-75% of the length of area pellucida . Gastrulation – the primitive streak Figure 7. Anterior and posterior view during gastrulation Figure 8. Formation of the notochord

15. The streak defines the axes of the embryo (extend from posterior to anterior , migrate cell from dorsal side to ventral side) Those close to the streak will be the medial structure, and farther will be the distal structures Gastrulation – the primitive streak Figure 9. Formation of the foregut and other structures

16. Gastrulation – the primitive streak As cell converge from the streak, a depression forms within the streak (called primitive groove ) It serves as the opening to the migrating cell into the blastocoel (analogous to amphibian blastopore) Primitve knot or Hensen’s node – regional thickening of cells at the anterior end of the primitive streak. It is the functional equivalent of the dorsal lip of the amphibian blastopore and the fish embryonic shield. Primitve pit – a funnel shape depression at the center of Hensen’s node . Gastrulation – the primitive streak

17. Gastrulation – the primitive streak As the streak form, epiblast cell begin to migrate through it and into the blastocoel. In the blastocoel, they migrate anteriorly, forming the foregut, head mesoderm, and notochord. Cell passing laterally of the streak forms the majority of endodermal and mesodermal tissues. Scatter factor – a 190-kDA protein thought to decompose the basal lamina and release cells into the embryo as cell enter the streak. Can convert epithelial sheets into mesenchymal cells in several ways. Involve in downregulation of E-cadherin expression and prevention of E-cadherin to function. Gastrulation – the primitive streak

18. Gastrulation – the primitive streak Figure 10. The primitive streak

19. First to migrate through Hensen’s node are destined to become the pharyngeal endoderm of the foregut. Inside the blastocoel, endodermal cell migrate anteriorly and displace hypoblast cell to the confined region of area pellicda anterior portion Germinal crescent – contain precursors of the germ cells which later migrate through the blood vessel to the gonads Gastrulation – endoderm and mesoderm formation

20. Next to enter through Hensen’s node move anteriorly but don’t move far ventrally as the destined foregut endodermal cells, rather they remain between the endoderm and epiblast to form the head mesenchyme and prechordal plate mesoderm. These cells all move anteriorly, pushing the epiblast to form the head process. The head of the avian embryo forms anterior ( rostral) to Hensen’s node. Next cell to migrate through Hensen’s node become chordamesoderm (notochord) cells Gastrulation – endoderm and mesoderm formation

21. Cells migrating inwardly through the lateral portion of the primitive streak. In the blastocoel, these cell separate into 2 layers. Layer 1 – deep layer join hypoblast along its midline and displace hypoblast cell to the sides. They give rise to all endodermal organs and most of the extraembryonic membranes (hypoblast forms the rest) Layer 2 – cell spread between the endoderm and epiblast, forming a loose layer. This generate the mesodermal portion of the embryo and extraembryonic membranes. Gastrulation – endoderm and mesoderm formation

22. Gastrulation – endoderm and mesoderm formation Figure 11. Close view of gastrulation

23. The primitive streak start to regress ( Hensen’s node move near the center of area pellucida ) As the node moves posteriorly the notochord is laid down, starting at the level of the future midbrain. The posterior notochord (after somite 17 in the chick) forms from the condensation of mesodermal tissue that has ingressed through the streak (not through Hensen’s node). This portion of the notochord extends posteriorly to form the tail of the embryo. Gastrulation – regression of the primitive streak

24. Hensen’s node regresses to its most posterior position, forming the anal region At this time, all presumptive endodermal and mesodermal cells have entered the embryo, and the epiblast is composed entirely of presumptive ectodermal cells. Avian (and mammalian) embryos exhibit a distinct anterior-to-posterior gradient development maturity. While cell of posterior portion of the embryo undergoes gastrulation, cell at anterior end starts to form organs. Gastrulation – regression of the primitive streak

25. Gastrulation – epiboly of the ectoderm Figure 12. Epiboly of ectoderm Figure 13. Notochord length vs. time

26. Ectodermal precursors proliferate while the presumptive mesodermal and endodermal cells are moving inwardly. Ectodermal cell migrate to surround the yolk by epiboly. ( took 4 days to complete) It involves the continuous production of new cellular material and the migration of the presumptive ectodermal cells along the underside of the vitelline envelope Only the cells of the outer edge of the orea opaca attach firmly to the vitelline envelope. Gastrulation – epiboly of the ectoderm

27. These cells are inherently different from the other blastoderm cells, as they can extend enormous (500 μm ) cytoplasmic processes onto the vitelline envelope. These elongated filopodia are believed to be the locomotor apparatus of these marginal cells, by which they pull the other ectodermal cells around the yolk The filopodia appear to bind to fibronectin, a laminar protein that is a component of the chick vitelline envelope. If the contact between the marginal cells and the fibronectin is experimentally broken (by adding a soluble polypeptide similar to fibronectin), the filopodia retract, and epidermal migration ceases the ectoderm has surrounded the yolk, the endoderm has replaced the hypoblast, and the mesoderm has positioned itself between these two regions. Gastrulation – epiboly of the ectoderm

28. Axis Formation in the Chick Embryo

29. Axes are specified early in the cleavage stage. Formation of these axes are later formed during gastrulation . Axis formation in chicks

30. DV axis is established when the dividing cells of blastoderm form a barrier between the basic albumin (pH 9.5) above the blastodisc and acidic subgeminal space below it (6.5 ). H2O and Na+ ions are transported from the albumin to the subgeminal space and causes a membrane potential difference of 25 mV. The role of pH in forming the Dorsal-Ventral Axis (DV) The difference in membrane potentials distinguishes two sides of the epiblast : The side facing the negative and basic albumin becomes the dorsal side. The side facing the positive and acidic subgeminal space fluid becomes the ventral side.

31. The bilateral symmetry of the chick blastoderm is determined by gravity. The ovum spins at rate of 10-20 revolutions per hour for about 20 hours through the hen’s reproductive tract. The shifting of yolk makes the lighter components to aggregate beneath one side of the blastoderm . Lighter components tips up the end of the blastoderm and this end becomes the posterior portion of the embryo-the part where the primitive streak formation begins. The role of gravity in forming the Anterior-Posterior Axis (AP)

32. There is still no known interactions that explain how the posterior margin forms and why it is the site of gastrulation . The ability to form primitive streak can be seen throughout the marginal zone and if the blastoderm is separated into parts, each part will form their own primitive streak . However, once a posterior marginal zone (PMZ) has formed, it controls the marginal regions and prevent the other regions to form their own primitive streaks . Also PMZ cells initiate gastrulation and is regarded as the equivalent of the amphibian Nieuwkoop center. Primitive Streak Formation

33. The PMZ region like the Nieuwkoop center is thought to be the place where the localization of β - catenin in the nucleus and TGF- β family signal coincide . Only the PMZ regions secrete VG1 and if the PMZ tissues are grafted to the anterior marginal zone, that region will able to form primitive streak . Koller’s sickle Anterior portion Forms the Hensen’s node from the epiblast and middle layer cells. Posterior portion Contributes to the posterior portion of the primitive streak. Transplantation of Koller’s sickle can cause formation of new axes and middle layer cells in the Koller’s sickle express Goosecoid . Primitive Streak Formation

34. Regarded as the avian equivalent of the amphibian dorsal blastopore lip since it is the site of gastrulation its cells become the chordamesoderm it act like the amphibian organizer which can organize a second embryonic axis when its cells are transplated into other locations The Hensen’s Node

35. The Hensen’s Node Cells of the Hensen’s node secrete chordin , noggin and nodal proteins which antagonize the BMPs and dorsalize the ectoderm and mesoderm. The antagonism of BMPs does not appear to be sufficient for neural induction. In chick embryos, fibroblast growth factors (FGFs) generate neuronal phenotype in epiblast cells rather than BMPs or ectopic expression of chordin . FGFs from the Hensen’s node and primitive streaks and beads induce trunk and hindbrain neuronal expression in the epiblast cells. The Hensen’s Node

36. This formation is regulated by paracrine factor, Nodal and transcription factor, Pitx2. However, there is a different mechanism of regulation in chick embryos . As primitive streak reaches maximum length, the: transcription of sonic hedgehog genes ceases on the right side of the embryo due to the expression of activin activin activated expression of FGF8 and FGF8 prevents the transcription of caronte gene. The absence of caronte genes ables the bone morphogentic proteins (BMPs) to block the expression of nodal and lefty 2 which also activates the snail gene ( cSNR ) that is a characteristic of the right side of avian embryonic organs. The Left-Right Axis Formation (LR)

37. On the left side of the body: Lefty-1 blocks the FGF8 expression Hedgehog activates caronte Caronte genes prevents BMPs to repress nodal and lefty-2 and to inhibit the blocking of lefty-1 expression on ventral midline structures On the left side of the body: Nodal and Lefty-2 activate pitx2 and repress snail ( cSNR ). Lefty-1 in the ventral midline prevents the caronte signals from passing to the right side of the embryo. The Left-Right Axis Formation (LR)

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