The Early Development of Birds & Mammals

1. The Early Development of Birds & Mammals Gilbert - Chapter 11

2. Goals • Become familiar with the cleavage and gastrulation patterns in birds and mammals • Examine the adaptations specific to the amniote egg • Understand the specialized structures that develop in mammals to allow for embryonic implantation

3. Early Avian Development: Chick • Why Chick embryos as a model system? • Easily obtained • Control developmental stage needed by incubating when ready • Relatively large embryo • Good for surgical manipulation • Many similarities to human development • Inexpensive

4. Early Avian Development: Chick • Fertilization occurs internally prior to secretion of shell • Egg is telolecithal • VERY yolky! • Cleavage is discoidal

5. Cleavage in Chick embryo • 1st cleavages occur in a small area of cytoplasm called the blastodisc • These do not penetrate through whole cell, so cells are continuous with each other • As cleavage continues, a small disc of cells about 4-6 layers deep is created in the center of the blastodisc

8. In the center of that disc • Cells in the deeper layers die off, forming a single celled layer of blastoderm called the AREA PELLUCIDA • This will form the actual embryo - these cells are also called the EPIBLAST • On the margins of that disc • Deeper cells do not die off, creating a thicker layer called the AREA OPACA • Underneath this disc is a fluid-filled cavity called the subgerminal space

9. Occurs while inside the hen!

10. Gastrulation in Chick Embryo • First step: form a 2 layered blastoderm • EPIBLAST (embryo proper) & HYPOBLAST (extraembryonic membranes) • 2 mechanisms • 1) Cells delaminate from the epiblast and migrate as “islands” into the subgerminal cavity • Primary hypoblast • 2) Cells near the posterior end migrate as a sheet toward the anterior end (inside the subgerminal cavity) • Secondary hypoblast

13. Avian Gastrulation • Second step: • Formation of germ layers • Avian embryos form a structure called the primitive streak • This is where the formation of germ layers begins • Thickening of the epiblast near the posterior end

15. As the primitive streak extends, a depression forms at the midline, called the primitive groove • It is through the primitive groove that cells will migrate into the blastocoel • Analogous to the amphibian blastopore • At the anterior end, a thickening called Hensen’s nodeis a funnel shaped depression where cells begin to migrate by ingression into the blastocoel • Analogous to the organizer (dorsal blastopore lip)

16. The first cells that migrate through Hensen’s node are endoderm, then chordamesoderm

18. Creation of the primitive streak establishes the anterior-posterior axis of the embryo • Begins at the tail end, moves toward the head end

19. Cells continue to ingress through the primitive streak, laying down the mesoderm layer • At this point, the primitive streak regresses

20. Regression of the Primitive Streak • During regression, the dorsal axis of the embryo is laid down • Notochord, and neural precursors become established • Head ectoderm begins to develop • Hensen’s node regresses to the posterior end, becomes anus

22. In this case, the result is actually a flat embryo with 3 germ layers, rather than a spherical embryo • The avian embryo is still atop the yolky mass • Endoderm separates the embryo from the yolk

27. The Chick Organizer • Hensen’s node! • Transplant to an ectopic location, form a secondary axis

30. Avian Gastrulation: Summary • Complex! • Initiated by Hensen’s node • Sets up the body axes • Establishes the germ layers

31. Mammalian Development • Difficult to study • Internal fertilization and development • Fertilization occurs IN the oviduct! • Few embryos produced • Lengthy development periods • Mice: mother must be sacrificed to harvest embryos • Some early development can be viewed in in vitro culture systems • Fertlization, cleavage, some of gastrulation (mice)

32. Mammalian Cleavage • Very tiny egg! • Meiosis not completed until fertilization • Different from other types of cleavage • Holoblastic, rotational • Slower • Asynchronous • 1st cleavage: normal, meridional cleavage plane • 2nd cleavage: • One blastomere meridional • One blastomere equitorial • Embryonic gene expression occurs right away

35. Cleavage and Compaction • Another major difference = Compaction • After the 3rd cleavage (8 cells), blastomeres express large amounts of adhesion molecules, like cadherins • Cells become a tight, compact ball of cells • Inner cells sealed off from outside environment • At the 16 cell stage, embryo becomes a morula • Small group of cells inside surrounded by layer of external cells

37. Morula • Cells on the outside are called trophoblast and become extraembryonic membranes and embryonic portion of the placenta • Cells on the inside are called Inner Cell Mass (ICM) and produce the embryo proper This is the first differentiation event in mammalian cells

38. The Blastocyst • As the morula develops, a fluid filled cavity forms (blastocoel) • In this case the ICM is positioned on one side of the trophoblast, creating not a blastula, but a blastocyst

40. The trophoblast will develop into specialized tissues that will allow for the embryo to implant into the placenta • The inner cell mass • Develops into two layers: Epiblast and Hypoblast • Gastrulation will occur here • Similar to chick embryo • ICM is the source of pleuripotent stem cells!

45. Early Mammalian Development: Summary • Very different cleavage pattern • Form a blastocyst • ICM, trophoblast • Gastrulation in humans is very similar to chick • The node is the organizer • Mouse - very unusual gastrulation pattern!