Animal Development

1. Animal Development

2. LE 47-1 1 mm

4. Morphogenesis • process by which an animal takes shape • determined by genome and differences between embryonic cells

5. The Acrosomal Reaction • Triggered when sperm reaches egg • releases hydrolytic enzymes that digest material surrounding the egg

6. Contact and fusion of sperm and egg membranes LE 47-3 Entry of sperm nucleus Acrosomal reaction Sperm plasma membrane Sperm nucleus Cortical reaction Contact Acrosomal process Basal body (centriole) Sperm head Fertilization envelope Fused plasma membranes Cortical granule Actin Perivitelline space Hydrolytic enzymes Acrosome Jelly coat Cortical granule membrane Vitelline layer Sperm-binding receptors Egg plasma membrane EGG CYTOPLASM

7. The Cortical Reaction • Initiated by the fusion of egg and sperm • induces a rise in Ca2+ • stimulates cortical granules to release their contents outside the egg • cause formation of a fertilization envelope • block to polyspermy

8. 500 µm LE 47-4 30 sec 10 sec after fertilization 20 sec 1 sec before fertilization Spreading wave of calcium ions Point of sperm entry

9. Activation of the Egg • sharp rise in Ca2+ • increases the rates of cellular respiration and protein synthesis by the egg cell • = activation of egg cell

10. 1 Binding of sperm to egg Acrosomal reaction: plasma membrane depolarization (fast block to polyspermy) 2 3 4 Urchin Egg Cell 6 Seconds 8 10 Increased intracellular calcium level 20 Cortical reaction begins (slow block to polyspermy) 30 40 50 Formation of fertilization envelope complete 1 2 Increased intracellular pH 3 4 Increased protein synthesis 5 Minutes 10 20 Fusion of egg and sperm nuclei complete 30 Onset of DNA synthesis 40 60 First cell division 90

11. Fertilization in Mammals • the cortical reaction modifies the zona pellucida as a slow block to polyspermy

12. LE 47-6 Follicle cell Sperm basal body Cortical ganules Zona pellucida Sperm nucleus Egg plasma membrane Acrosomal vesicle EGG CYTOPLASM

13. Cleavage • a period of rapid cell division without growth • partitions the cytoplasm of one large cell into many smaller cells • blastomeres

14. LE 47-7 Morula Fertilized egg Blastula Four-cell stage

15. Polarity • eggs and zygotes of many animals (except mammals) • Animal pole • Vegetal pole • Most of yolk

16. Animal hemisphere Animal pole Point of sperm entry LE 47-8 Vegetal hemisphere Vegetal pole Point of sperm entry Future dorsal side of tadpole Anterior Gray crescent Right First cleavage Ventral Dorsal Left Posterior Body axes Establishing the axes

17. Cleavage planes • follow a pattern relative to the zygote’s animal and vegetal poles

18. Zygote 0.25 mm LE 47-9 2-cell stage forming 4-cell stage forming Eight-cell stage (viewed from the animal pole) 8-cell stage 0.25 mm Animal pole Blasto- coel Blastula (cross section) Vegetal pole Blastula (at least 128 cells)

19. Meroblastic cleavage • incomplete division of the egg • occurs in species with yolk-rich eggs • Ie. reptiles and birds

20. Disk of cytoplasm Fertilized egg Zygote LE 47-10 Four-cell stage Blastoderm Cutaway view of the blastoderm Blastocoel BLASTODERM YOLK MASS Epiblast Hypoblast

21. Holoblastic cleavage • complete division of the egg • occurs in species whose eggs have little or moderate amounts of yolk • Ie. sea urchins and frogs

22. Gastrulation • rearranges the cells of a blastula into a three-layered embryo • a gastrula • has a primitive gut

23. The three layers produced by gastrulation are called embryonic germ layers • The ectoderm forms the outer layer • The endoderm lines the digestive tract • The mesoderm partly fills the space between the endoderm and ectoderm Video: Sea Urchin Embryonic Development

24. Key Future ectoderm Future mesoderm Future endoderm Animal pole Blastocoel Urchin Mesenchyme cells Vegetal plate Vegetal pole Blastocoel Filopodia pulling archenteron tip Archenteron Mesenchyme cells Blastopore 50 µm Blastocoel Ectoderm Archenteron Blastopore Mouth Mesenchume (mesoderm forms future skeleton) Digestive tube (endoderm) Anus (from blastopore)

25. CROSS SECTION SURFACE VIEW Animal pole Blastocoel Frog Dorsal tip of blastopore Dorsal lip of blastopore Vegetal pole Blastula Blastocoel shrinking Archenteron Ectoderm Mesoderm Blastocoel remnant Endoderm Key Future ectoderm Future mesoderm Yolk plug Yolk plug Gastrula Future endoderm

26. Organogenesis • regions of the germ layers develop into rudimentary organs • the notochord forms from mesoderm • the neural plate forms from ectoderm • curves inward, forming the neural tube Video: Frog Embryo Development

27. Neural folds LE 47-14a LM 1 mm Neural fold Neural plate Notochord Ectoderm Mesoderm Endoderm Archenteron Neural plate formation

28. Neural plate Neural fold LE 47-14b Neural crest Outer layer of ectoderm Neural crest Neural tube Formation of the neural tube

29. Somites • blocks • Formed from mesoderm lateral to the notochord • Coelom • Lateral to the somites • mesoderm splits to form the coelom

30. Somites Eye Tail bud LE 47-14c SEM 1 mm Neural tube Notochord Neural crest Coelom Somite Archenteron (digestive cavity) Somites

31. LE 47-15 Eye Neural tube Notochord Forebrain Somite Heart Coelom Archenteron Endoderm Lateral fold Mesoderm Blood vessels Ectoderm Somites Yolk stalk YOLK Yolk sac Form extraembryonic membranes Neural tube Early organogenesis Late organogenesis

33. Adaptations of Amniotes • develop in a fluid-filled sac in a shell or the uterus • Birds • Reptiles • mammals • Germ layers give rise to membranes surrounding embryos

34. Amnion Allantois LE 47-17 Embryo Amniotic cavity with amniotic fluid Albumen Shell Yolk (nutrients) Yolk sac Chorion

35. Mammalian Development • eggs of placental mammals • Small • store few nutrients • Exhibit holoblastic cleavage • Show no obvious polarity • Gastrulation and organogenesis similar to birds and reptiles • Early cleavage is relatively slow

36. Cleavage forms the blastocyst forms • trophoblast, the outer epithelium of the blastocyst, initiates implantation in the uterus, and the blastocyst forms a flat disk of cells • As implantation is completed, gastrulation begins • The extraembryonic membranes begin to form • By the end of gastrulation, the embryonic germ layers have formed

37. Endometrium (uterine lining) Inner cell mass LE 47-18a Trophoblast Blastocoel Blastocyst reaches uterus. Expanding region of trophoblast Maternal blood vessel Epiblast Hypoblast Trophoblast Blastocyst implants.

38. Expanding region of trophoblast Amniotic cavity Amnion Epiblast LE 47-18b Hypoblast Chorion (from trophoblast Yolk sac (from hypoblast) Extraembryonic membranes start to form and gastrulation begins. Extraembryonic mesoderm cells (from epiblast) Allantois Amnion Chorion Ectoderm Mesoderm Endoderm Yolk sac Extraembryonic mesoderm Gastrulation has produced a three-layered embryo with four extraembryonic membranes.

39. Fate Maps • general territorial diagrams of embryonic development

40. Epidermis LE 47-23a Central nervous system Epidermis Notochord Mesoderm Endoderm Neural tube stage (transverse section) Blastula Fate map of a frog embryo

41. Development • Differentiation • Signal molecules • Influence gene

42. Anterior LE 47-26a AER Limb bud ZPA Posterior Apical ectodermal ridge 50 µm Organizer regions