1. Cleavage, blastula, gastrula, neurula
Lecture 2

2. Cleavage Mitosis Duplication of cells 1  2  4  8  16  etc.
zygote  morula  blastula  gastrula  neurula  embryo  fetus
Yolk
Contains nutrients for the zygote

3. Cleavage Name Number of cells Zygote 1-8 Morula 16-64 Blastula
128-15,000
Gastrula
>15,000
Neurula
Neural tube formation
Embryo
Period of organogenesis (2-8 weeks)*
Fetus
Period of growth (2-9 months)*
* In humans

4. Distribution of yolk
Oligolecithal (ex. Amphioxus, star fish, sea urchins and mammals)
Eggs with little yolk
Cleavage produces cells of roughly the same size
Mesolecithal (ex. Amphibians)
Moderate amount of yolk
Yolk impedes cleavage formation
Produces cells of unequal size
Holoblastic cleavage
Telolecithal (ex. Reptiles or Birds)
Large amount of yolk
Cell division occurs only at one area
Meroblastic cleavage

5. Amphioxus

6. Cleavage First division begins at the animal pole
Second division perpendicular to the first
And so on… forming blastomeres.

7. Cleavage in different yolk distributions
Holoblastic
meroblastic

8. Blastula formation in Amphioxus
Formation of a fluid filled cavity within the developing embryo, called blastocoele, through a Na+ pump

9. Blastula formation in Amphibians

10. Blastula formation in Birds

11. Blastula formation in Birds
Holoblastic
meroblastic

12. Blastula formation in Mammals
Blastula or blastocyst
Similar to initial cleavage in amphioxus or sea urchins, then follows cleavage similar to birds
At the morula stage  blastula stage  becomes
Process called compaction
Morula Stage
Blastula Stage
Develops into
Inner cell mass
Embryoblast
Embryo proper
Outer cell mass
Trophoblast
Placenta

13. Blastula formation in Mammals

14. Blastula and implantation

15. Blastula formation

16. Formation of Germ layers
Gastrulation = formation of 3 primary germ layers and the primitive gut or archenteron

17. Formation of Germ layers
Mechanisms of Development
Cytoplasmic specification (pre-determined)
Conditional specification (specific development through interactions with the surrounding cells/environment or its position in the developing embryo)
Primary organizer

18. Ectoderm Outer epithelium of body and derivatives Neural tube
Hair, nails, epithelial glands, lining of mouth, enamel of teeth, lens of eyes, inner ear, nasal and olfactory epithelium
Neural tube
Brain, spinal cord, motor nerves
Neural crest
Sensory ganglia and nerves, adrenal medulla, sympathetic ganglia, skull, gill arches, dentine of teeth

19. Mesoderm Notochord  vertebrae
Lining of thoracic and abdominal cavities
Circulatory system
Blood, bone marrow, endothelium, lymphatics
Somites
skeleton and muscle, dermis, connective tissue
Urogenital system
Smooth muscle and connective tissue of digestive tract

20. Endoderm Epithelium of respiratory tract Pharynx Epithelium of gut
Liver, pancreas
Inner lining of urinary bladder
Gut tube

21. Gastrulation Further differentiation into 3 germ cell layers
Formation of the blastopore or primitive streak
Dorsal lip of the blastopore or Spemann organizer
Dominant organizing region of the embryo
Homologous to the Hensen’s node in birds and mammals
Directs differentiation of cells into specific germ layers or organs

22. Gastrulation in Amphioxus

23. Gastrulation in Amphibians

24. Gastrulation in Birds

25. Gastrulation in Birds
1 = Epiblast (forms the ectoderm), 2 = Blastocoel, 3 = Hypoblast (forms the endoderm), 4 = Subgerminal cavity, 5 = Yolk

28. Gastrulation in mammals
Similar to birds
Inner cell mass  embryoblast, forms 2 layers
Epiblast  forms a cavity  amniotic cavity
Epiblast + cytotrophoblast = amnioblast
Hypoblast  forms primitive yolk sac
Outer cell mass  trophoblast, forms 2 layers
Syncitiotrophoblast  secretes beta HCG (human chorionic gonadotropin)
Cytotrophoblast
Forms the placenta

31. Lacunar stage of trophoblast

32. Cytotrophoblast + amnion mesoderm = extraembryonic somatic mesoderm  somatopleure (ectoderm + mesoderm)
Yolk sac endoderm + amnion mesoderm = extraembryonic splanchnic mesoderm  splanchnopleure (endoderm + mesoderm)

33. Cytotrophobast + syncitiotrophoblast = primary villa (precursor of chorionic villi)

34. Neurula Neurulation: Formation of the neural tube
Period of development starting with the first traces of formation of the neural plate and ending with the closure of the neural tube
Initiates formation of the central nervous system
Formation of notochord from the endoderm or mesoderm
Acts as initial organizer of the nervous system (stimulates formation of the vertebrae and spinal cord) and creates the basis of the body axis (head and tail)
Eventually disappears and forms the nucleus pulposus in mammals

35. Neurulation Ectoderm thickens to form the neural plate
Edges of neural plate become raised forming a neural fold, with a depression in the middle, called the neural groove
Anteriorly, neural plate is broadened and flattened
Posteriorly, neural plate becomes narrow and cylindrical

36. Neurula formation in Birds

37. Neurulation Formation of the neural tube
Neural folds grow and meet each other
Closure begins in the middle and proceeds cephalad and caudad
Formation of the anterior and posterior neuropores, which will eventually close
Mesoderm thickens

38. In amphibians

39. In humans

41. Nucleus pulposus