Vertebrate Development

Vertebrate Development

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. •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
Cleavage

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