1. Animal Development

2. Outline I.Early Stages of Embryonic Development A. Intro B. Fertilization C. Cleavage D. Gastrulation II.Morphogenesis

3. Intro Make a tiny dot at the top of your paper… –In your first week of development, that’s how big you were! Less than a month later, brains take form & heart is beating

4. Intro Preformation –Embryo contains all descendants –Russian nesting dolls –Is there any merit to this idea? DNA? Epigenesis –Aristotle (350 B.C.) –Animal emerges gradually from a formless egg –Favored idea as microscopy improved

5. Intro After fertilization, cell division partitions the cytoplasm in such a way that nuclei of different embryonic cells are exposed to different cytoplasmic environments –Expression of different genes in different cells

6. Fertilization Goal = combine haploid sets of chromosomes from two individuals into a single diploid cell Also, must activate the egg to trigger development of the embryo Lab study = sea urchins (deuterostomes)

7. Acrosomal Reaction Acrosome = vesicle at tip of sperm cell Releases hydrolytic enzymes that enable penetration of the egg’s jelly coat “Lock-and-key” fit ensure fertilization occurs solely from within a species

8. Acrosomal Reaction Sperm nucleus into egg’s cytoplasm Fusion of the membranes opens sodium ion channels Depolarization = fast block to polyspermy –Prevents an abnormal number of chromosomes in the zygote

9. Acrosomal Reaction

10. Cortical Reaction Series of changes in the outer cortex of the egg cytoplasm Release of Ca 2+ into cytosol High Ca 2+ concentration changes cortical granules that result in the egg cell separating from the vitelline layer Fertilization envelope resists the entry of additional sperm (slow block)

11. Cortical Reaction

12. Egg Activation High Ca 2+ concentrations also produce metabolic changes –Neither sperm, nor DNA are responsible for activation!!! –Parthenogenesis a possibility Sperm cell nucleus begins to swell & within 20 minutes has merged w/ the egg nucleus First cell division occurs within 90 min.

13. In Mammals Internal vs. external in urchins Female secretions alter the surface of sperm cells & increase the sperm’s motility Zona pellucida = extracellular matrix of the female egg

14. In Mammals

15. Microvilli take in the entire sperm cell Basal body of sperm’s flagellum divides into two centrosomes (w/ centrioles for mitotic spindles)

16. Cleavage Rapid cell division after fertilization Cells skip G 1 & G 2 phases (S & M only) Sea urchin: cleavage begins 45-90 min. after fertilization

17. Cleavage 1.Vegetal pole –Yolk (stored nutrients) is concentrated 2.Animal pole –More rapid cell division –Typically the anterior end of the embryo

18. Cleavage Plasma membrane & cortex rotate towards point of sperm entry exposing a light-gray region (gray crescent) –Opposite of sperm entry –Marks the dorsal side of an amphibian egg

19. Cleavage zygote  morula  blastula  gastrula Frog cleavage Zygote (1 cell)8-celled embryo Morula (16-64 cells)

20. Cleavage Frog blastula

21. Gastrulation Rearrangement of blastula cells into a three-layered embryo 1.Ectoderm 2.Endoderm 3.Mesoderm Cells can now interact w/ each other in new ways

22. Gastrulation

23. Gastrulation Invagination = vegetal plate buckles inward Involution = cells rolling over the edge of a lip into the interior Blastocoel = fluid-filled cavity (hollow blastula) Blastopore = open end of the archenteron (anus in a deuterostome) Archenteron = deep, narrow cavity that is the continuation of invagination (digestive tract)

24. Gastrulation

25. Gastrulation

26. Organogenesis Three germ layers develop into beginnings of organs First organs in chordates are the notochord & neural tube

27. Organogenesis 1.Notochord –From dorsal mesoderm above archenteron –Core of the backbone in which vertebrae are attached 2.Neural tube –Dorsal ectoderm just above notochord –CNS

28. Organogenesis

29. In Mammals Fertilization in oviduct (Fallopian tubes) Journey to uterus implantation takes up to 1 week At this point, embryo has over 100 cells & is known as a blastocyst Blastocyst has two parts: 1.Inner cell mass = developing embryo 2.Trophoblast = fetal portion of the placenta

30. In Mammals

31. Morphogenesis Movement of cell shape & position –Reorganization of cytoskeleton (microtubules & microfilaments)

32. Morphogenesis Convergent extension = cell crawling in which a sheet of cells becomes narrower to grow longer