1. Vertebrate Development
Biology II:
Form and Function

2. The six stages of vertebrate development
Fertilization
Cleavage
Gastrulation
Neurulation
Neural crest formation
Organogenesis

3. Stage of vertebrate development (I)

4. Stage of vertebrate development (II)

5. Fertilization (I) Entry of sperm cell induces activation
prevents other sperm from entering
Intitiates second meiotic division of egg nucleus
Induces polarity

6. Fertilization (II)

7. Fertilization in sea urchins

8. Sperm penetration

9. Polarity in early embryos

10. Cleavage
Division of first cell to many within ball of same volume (morula) is followed by hollowing of that ball to a blastula. Form of cleavage and blastulation depends on orientation of yolk and nucleus
In primitive chordates, division is even, towards a symmetrical blastula composed of cells of equal size
In amphibians, holoblastic cleavage leads to assymetrical blastula
In reptiles and birds, meroblastic cleavage occurs, resulting in a cap of cells on top of the yolk
In mammals, holoblastic cleavage occurs, creating a trophoblast containing a blastocoel, with inner disc of cells equivalent to a blastodisc

11. Yolk distribution in amniotic eggs affects blastula development

12. Holoblastic cleavage
Cells with little yolk, and central nucleus, develop evenly

13. Uneven cleavage
In frog cells, there is more yolk, and nucleus of fertilized egg is to one side:
Yolk slows division, so areas of low yolk content divide quicker, and create smaller cells (see here, front)
Areas of high yolk content divide more slowly, and give rise to larger cells

14. Meroblastic cleavage
Occurring in reptiles, birds and mammals, an uneven division of cells causes a cap of cells on top of the yolk

15. Blastula of mammals and birds
Cap of cells develops into a blastodisc
Blastocoel develops in mammals, surrounded by trophoblast

16. Gastrulation
Invagination of outer layer of cells to inside of the blastula is known as gastrulation, resulting in the formation of the gastrula
Type of gastrulation is a function of type of blastula…
End result is three types of germ layer tissue - endoderm, mesoderm and ectoderm

17. Gastrulation in the lancelet

18. Gastrulation in the frog

19. Gastrulation in birds

20. Gastrulation in mammals

21. Neurulation and neural crest formation
Formation of neural fold (primitive streak) above notocord, begins a channel that eventually seals on the dorsal surface, forming neural groove
Mesoderm derived tissue close to notocord develop into somites, giving rise to muscles, connective tissue and vertebrae
Layer of cells on dorsal surface of groove form neural crest, responsible for formation of several important organs
Associated patches of ectoderm tissue derive into placodes, which evetually result in important neurally related organs

22. Neural tube formation (I)

23. Neural tube formation (II)

24. Induction
Proximity of a cell to certain other cells controls its development - Spemann and Mangold
Cells whose fate can be predicted are termed ‘determined’
Cells whose fate cannot be reversed are termed ‘committed’
Since all cells initially can become any tissue Development is a process of progressive restriction of gene expression

25. Spemann and Mangold’s dorsal lip transplant experiment

26. Induction of the vertebrate eye

27. Organogenesis Ontogeny recapitulates phylogeny
(and a quick word about extraembryonic membranes)

28. Derivation of major tissue types

29. Embryonic development of vertebrates (I)

30. Embryonic development of vertebrates (II)

31. Extraembryonic membranes - Chick embryo

32. Extraembryonic membranes - mammalian embryo (I)

33. Extraembryonic membranes - mammalian embryo (II)

34. The placenta

35. Human development

36. Developing human at 4 weeks

37. Developing human at 7 weeks

38. Developing human at 3 months

39. Developing human at 4 months

40. Ultrasound at 5 months

41. Delivery position of foetus