1. Animal Embryonic Development
From Fertilization to Organogenesis

2. Early Stages of Development
Fertilization
Cleavage
Gastrulation
Neurulation

3. Figure 20.1

4. Fertilization unequal gamete contributions egg contributes nutrients
proteins, mRNAs
mitochondria
essential developmental genes (imprinted)
sperm contributes
centriole
tubulin

5. Fertilization rearrangements of egg cytoplasm
egg contents are distributed heterogeneously
frog model system
animal hemisphere
contains nucleus
heavily pigmented cortical cytoplasm
lightly pigmented inner cytoplasm
vegetal hemisphere
contains nutrients
unpigmented

6. formation of the gray crescent Figure 20.2
Rotation of cortex relative to inner cytoplasm creates bilateral symmetry.

7. Fertilization rearrangements of egg cytoplasm
imposes bilateral symmetry on egg
site of sperm entry
head (anterior) end
ventral region
gray crescent
tail end
dorsal region
(hence) left-right axis

8. molecular events during rearrangement Figure 20.3
-catenin
GSK-3

molecular events during rearrangement Figure 20.3
Sperm centriole rearranges vegetal pole microtubules.
Microtubules guide movement of cortical cytoplasm, proteins, organelles.
Asymmetric distribution of developmental signals: b-catenin (0range), GSK-3 (blue)

9. Cleavage - blastulation
rapid cell divisions
divisions oriented in specific directions
little gene expression
little cell growth
packaging of cytoplasmic heterogeneity
final product is a hollow ball of cells = blastula
cells = blastomeres
hollow cavity = blastocoel

10. Cleavage - blastulation
yolky eggs alter pattern of divisions
animal hemisphere divides normally
vegetal (yolky) hemisphere
divides less often
produces larger cells

11. yolk affects the cleavage pattern Figure 20.4

12. Cleavage - blastulation
amount of yolk affects cleavage pattern
if yolk is divided into cells
complete cleavage
if yolk is not divided
incomplete cleavage
embryo is a blastodisc atop the intact yolk

13. formation of blastodisc Figure 20.4

14. Mammalian Cleavage in oviduct slow cell divisions
asynchronous cell divisions

15. mammalian cleavage is rotational Figure 20.5

16. Mammalian Cleavage in oviduct slow cell divisions
asynchronous cell divisions
accompanied by gene expression
produces a blastocyst
inner cell mass - primordial embryo
trophoblast - primordial placenta component

17. formation of mammalian blastocyst Figure 20.5
32-cell stage = trophoblast + inner cell mass
32-cell stage leaves zona and implants

18. frog blastula fate map Figure 20.6
cleavage produces cells with different contents
blastulation prevents cell contact
undifferentiated cells of blastula have distinct fates

19. Fate Maps undifferentiated cells of blastula have distinct fates
determination fixes fates of blastomeres
early determination yields mosaic development
a lost blastomere causes a lost body part
later determination yields regulative development
a lost blastomere is compensated during development

20. humans exhibit regulative development Figure 20.7

21. Gastrulation-organizing the body plan
undifferentiated cells produce germ layers
ectoderm - prospective epidermis, nervous system
endoderm - prospective gut tissues
mesoderm - prospective organs, etc.
germ layers migrate to new positions
contact between layers allows inductive interactions to direct differentiation

22. sea urchin involution Figure 20.8
vegetal cells form 1˚ mesenchyme
sea urchin involution Figure 20.8
involution of a tube of cells
vegetal pole flattens
Vegetal pole cells detach - 1˚ mesenchyme
Archenteron tip cells detach - 2˚ mesenchyme
primitive
gut
(archenteron) is
formed
prospective ectoderm, endoderm
& mesoderm are formed

23. Gastrulation-organizing the body plan
blastopore becomes mouth or anus
mouth in protostomes
anus in deuterostomes

24. Gastrulation-organizing the body plan
frog model system
gastrulation begins at gray crescent
“bottle cells” bulge into blastocoel & pull neighbors along
initial involution forms the dorsal lip of the blastopore (d.l.b.)
epiboly
surface cell layers migrate to blastopore
migrating cells form endoderm, mesoderm

25. frog gastrulation Figure 20.9

26. Figure 20.12

27. gray crescent is necessary for normal development Figure 20.10

28. Gastrulation-organizing the body plan
frog model system
ß-catenin activates genes to produce proteins that cause bottle cells to initiate involution
cells of the gastrula are determined during migration over the d.l.b.
dlb is necessary for normal development
dlb is sufficient for normal development

29. role of dlb in development in frog Figure 20.11

30. Gastrulation-organizing the body plan
reptile/bird model
two-layered blastodisc + large yolk mass
upper layer
epiblast
becomes embryo
lower layer
hypoblast
becomes extra-embryonic membranes

31. chick gastrulation Figure 20.13
Primitive streak
Hensen’s node

32. early mammalian gastrulation Figure 20.14
resembles reptile/bird system
inner cell mass produces several layers
hypoblast lines trophoblast - chorion
epiblast upper layer forms amnion
primitive groove and Hensen’s node function as in chick system

33. Neurulation organogenesis formation of organs and organ systems
caused by inductive interactions among germ layers

34. frog neurulation Figure 20.15
some cells determined to be mesoderm
some determined to be chordomesoderm
dorsal, nearest midline
forms notochord
notochord induces ectoderm to form begin nervous system development

35. Neurulation vertebrate body segmentation alongside neural tube
segments of mesoderm = somites
somites direct development of vertebrae, ribs, trunk muscles, limbs, outgrowth of nerves, blood vessels, etc
repeated segments are modified along the anterior/posterior axis

36. somites contribute to vertebrae, ribs & muscles neural crest cells give rise to peripheral nerves Figure 20.16
segments of mesoderm = somites
somites directs development of vertebrae, ribs, trunk muscles, limbs, outgrowth of nerves, blood vessels, etc
.repeated segments are modifed along the anterior/posterior axis
development is guided by Hox
(homeobox) genes

37. HOX genes control anterior-posterior differentiation
families of ~10 HOX genes are on different chromosomes
HOX genes are expressed “in order”
HOX genes guide differentiation from anterior to posterior

38. mouse HOX gene clusters Figure 20.17

39. vertebrate extraembryonic membranes
reptiles, birds and mammals produce membranes that
surround the embryo
originate in the embryo
are not part of the embryo
provide nutrition, gas exchange and waste removal

40. chick extraembryonic membranes Figure 20
chick extraembryonic membranes Figure shell lining embryo compartment waste storage
Yolk sac - hypoblast + endoderm
Chorion - mesoderm + ectoderm
Amnion - mesoderm + ectoderm
Allantoic membrane - endoderm
pantry

41. placenta: chorion + uterine tissues Figure 20.19