Development Gamete formation  Fertilization  Cleavage  Gastrulation  Organogenesis  Growth

1. Development Gamete formation  Fertilization  Cleavage  Gastrulation  Organogenesis  Growth

2. Determination- cell becomes committed to that pathway of differentiation Cytoplasmic localization Induction

3. Fertilization Restoration of diploidy Activation of egg Parthenogenesis: sperm not required Some species of salamanders and fishes: sperm required for egg activation but not genetic material

4. Egg enlarges due to yolk reserves Also lots of protein synthesis machinery Morphogenesis determinants (transcription factors) Maturing egg becomes bloated with RNA; germinal vesicle Most of this takes place during meiosis I

5. Sea urchin fertilization

6. Sperm and egg nuclei fuse immediately in sea urchins In mammals, fusion occurs after first cell division Sperm receptors are present on vitelline envelope; species- specific Important in marine environments! But seem to be universal

7. Prevention of polyspermy Upon sperm binding: membrane potential changes in egg (fast block) cortical reaction- cortical granules are released into space between plasma membrane and vitelline envelope removes sperm and forms fertilization envelope

10. How does mammalian fertilization differ? Internal Capacitation- female secretions enhance sperm motility Egg surface is protected by secreted follicle cells Extracellular matrix is called zona pellucida; sperm receptors are located there Timing of sperm and egg nuclei is different

12. After fertilization: Lots of DNA and protein synthesis (mRNA was made earlier and stored) Cytoplasm is reorganized; determinants are positioned to direct trasncription during development

13. Cleavage patterns are consistent within species But there are several different patterns Affected by: quantity and distribution of yolk Genetic control of symmetry of cleavage

15. Isolecithal: yolk is distributed evenly throughout egg; cleavage furrow goes straight through (holoblastic) Echinoderms, tunicates, many mammals (including humans as well as marsupials) actual cleavage patterns may vary note mouse and sea star Mesolecithal: moderate amount of yolk in middle cleavage is holoblastic but faster at animal pole than vegetal pole Many more cells seen at animal pole amphibians

16. Telolecithal eggs- yolk concentrated at vegetal pole; diving cells lie on top of yolk mass birds, reptiles, some amphibians, cephalo- pods, monotreme mammals meroblastic (partial) cleavage Meroblastic cleavage also seen in insect eggs, but yolk is in the middle (centrolecithal) Therefore cleavage pattern is affected

17. Effect of yolk on development If mother does not supply nourishment, yolk does Limited yolk (e.g., aquatic invertebrates) zygotes quickly develop into free-swimming larvae Indirect development: larva is distinct part of sequence between embryo and adult will undergo some form of metamorphosis Direct development – enough yolk is present to nourish throughout formation of juvenile

18. Direct development in mammals placental development nourishment obtained form mother

19. Patterns of cleavage radial bilateral (unique to tunicates) spiral rotational (most mammals) forms trophoblast placenta formed on outside, inner cell amss (embryo) formed on inside

20. Meroblastic cleavage Telolecithal- discoidal blastoderm is originally formed as a single layer of cells Centrolecithal- superficial cleavage around rim of egg cytoplasmic cleavage occurs after several rounds of mitosis Some nuclei migrate to form pole cells (later germ cells)

21. (Drosophila)

22. Protostomes vs deuterostomes

23. Modifications of these groups: mammals have radial cleavage reptiles, birds and some fishes: discoidal vertebrates form coelom in a basically schizocoelous (splitting) pattern Cleavage eventually results in formation of blastula (blastocyst)

24. Gastrulation General mechanisms: changes in cell motility changes in cell shape cellular organization formation of cell layers Patterns of gastrulation are affected by presence of yolk

26. Cnidarians and ctenophorans are diploblastic (endoderm and ectoderm only) Coelom formation schizocoel (splitting) seen in protostomes vertebrates form this way too, probably because of yolk otherwise, deuterotomes are enterocoelous

28. (chick)

29. Aqueous environment required for development How, in terrestrial animals? fluid-filled sac amnion (birds, reptiles, mammals) fertilization is internal gastrulation is similar in all

30. Mechanisms of development Delayed-nucleation experiment (Spemann, early 1900s) All cells contained same nuclear information Cytoplasmic determinants are essential for normal development

32. How do cells differentiate? Cytoplasmic specification (not so much in mammals) Cell-cell induction adhesion molecules (extracellular matrix) paracrine signaling Importance of each varies among animals

33. Deuterostomes protostomes

34. Cell induction 1920s- Spemann and Mangold; newts grafted dorsal lip from one embryo onto another Dorsal lip is critical formation of other parts of the embryo

36. Differentiated cells can induce adjacent, undifferentiated cells Thus sequential patterns of development occur: induction cell movement cells adhesions cell proliferation

37. Gene expression during development Initially from stored mRNAs transcribed from maternal DNA Usually takes process to blastula stage or a little beyond Then switches to zygote DNA; homeotic genes Model animal: Drosophila; genes are conserved throughout animal kingdom

38. What is a Hox gene? Contains a “homeobox” that enables it to bind to DNA as a transcription factor (3 -helices) Remainder of the protein helps determine which gene it regulates Some of these gene products help regulate body part development (homeotic genes) So far, homeotic genes have been found only in animals

39. Model animals: Drosophila, nematodes; frogs

40. Genes are arranged linearly Found on third chromosome in fruit fly Mice and humans have four clusters, each on different chromosome Clusters are expressed in different parts of the body Gene expression could be blocked by antibodies to homeobox proteins

41. Organogenesis Pattern formation Positional information Techniques for study: tissue culture hormones growth factors genetic engineering

48. Heart is first functioning organ formed from mesoderm, guided by endoderm Beating heart seen by day 2 in the 21-day incubation of a chick embryo- before there is blood or blood vessels! (ventricle, then atria) How else will food be delivered throughout the growing embryo?