Bio 127 - Section III Late Development

1. Bio 127 - Section IIILate Development Germ Line Development Gilbert 9e – Chapter 16

2. Section 4 Encompasses : • Development of the Tetrapod Limb • Sex Determination • The Saga of the Germ Line • Post-Embryonic Development

3. Student Learning Objectives 1. You should understand that sexual reproduction requiring the fusion of gametes from male and female gonads occurs in specific organisms. 2. You should understand that the primordial germ cells that give rise to gametes arise outside of the gonads and must migrate to them. 3. You should understand that in most organisms the primordial germ cells are specified conditionally, while in some they are specified autonomously by cytoplasmic determinants in the egg. 4. You should understand that migration of the germ cells from their site of origin to the gonads is an essential part of reproductive success .

4. In all plants and some animals, somatic cells can readily form new organisms • Cnidarians, flatworms, tunicates • In many animals, there is an early division between somatic and germ cells • Insects, roundworms, vertebrates

5. Two step process; • Primordial germ cells (PGCs) are determined in a specific location in the embryo • PGCs migrate to the gonad and become the progenitor population for eggs and sperm

6. Two Methods of Germ Cell Determination • Autonomous Specification • Egg cytoplasmic determinants • Called ‘Germ Plasm’ • Nematodes, flies, frogs • Conditional Specification • Signals from surrounding cells • Majority of sexually reproducing organisms • Including mammals

7. The nematode Caenorhabditis elegans Remember cleavage and gastrulation: Asymmetrical divisions produce a stem cell (P-lineage), “founder” cell. Stem cell divisions are meridional Founder cell divisions are equatorial

8. Gastrulation in C. elegans

9. P-granules hold cytoplasmic determinants in C. elegans PIE-1: Blocks all transcription, thus all differentiation Germ plasm also has blocks to translation, stem cell factors, controls for asymmetric divisions and meiosis inducing agents. P4 Blue is DNA marker, Green is P-granule marker

10. Blue stain marks transcriptional activity P4

11. Synctitial cleavage in Drosophila is followed by cellularization

12. Pole plasm forms during cellularization • mitochondria • fibrils • polar granules • no transcription • no translation • germline stabilization anterior posterior

13. Localization of germ cell-less (gcl) gene products Human males with mutant homolog are often sterile

14. Germ plasm at the vegetal pole of frog embryos Marker for frog homolog of fly/worm translation blocker, Nanos

15. The frog cells that take up these granules will become PGCs and migrate to the gonads as the kidney forms • Again, no transcription or translation • Therefore, no differentiation

16. Conditional Specification of mammalian PGCs • Posterior of epiblast at the junction of the primitive streak and extraembryonic ectoderm • Cells are no different from other epiblast • Specified in gastrulation before 3 layers form • Wnts from endoderm make them competent • BMPs from extraembryo ectoderm finish it

17. Picture the blastocyst full of yolk.....

18. Poor old Henson discovered this node as well but didn’t get the naming rights

19. Conditional Specification of mammalian PGCs • Same deal as the others: • Repress differentiation by repressing gene expression • Specified outside embryo forming cells • Once expression is shut down they can go back into embryo and not respond to signals

20. Germ Cell Migration • Drosophila • Zebrafish • Frogs • Mice • Birds and Reptiles

21. Germ Cell Migration: Drosophila As the endoderm invaginates, the ectoderm and mesoderm extend and converge to wrap around the dorsal side to form the “germ band”

22. Germ Cell Migration: Drosophila • mitochondria • fibrils • polar granules • no transcription • no translation • germline stabilization anterior posterior

23. Germ Cell Migration: Drosophila Germ cells passively ride endoderm

24. Germ Cell Migration: Drosophila Endoderm expresses repellent molecules Germ band is retracting PGCs and gonad progenitors in 2 migration streams

25. Germ Cell Migration: Drosophila Combination of chemoattraction and repulsion drive them to gonad E-cadherin MET forms epithelium around PGCs

26. Germ Cell Migration: Drosophila • Both mesoderm and PGCs divide through the larval stage, differentiate at metamorphosis • At larval-pupal transition anterior PGCs in gonad become germ line stem cells • In ovaries, the cells attach top stromal cap In testes, the cells attach to hub cells

27. Remember: Zebrafish development occurs very rapidly 24-hours from 1 cell to vertebrate embryo!

28. Germ Cell Migration: Zebrafish specification: germ plasm determination: PGCs by 32-cells four clusters join into two migration of bilateral clusters into developing gonad follows signal Sdf-1 using receptor CXCR4

29. Remember: Germ plasm at vegetal pole in frogs Marker for frog homolog of fly/worm translation blocker, Nanos

30. Germ Cell Migration: Frogs During cleavage the germ plasm rises up until it ends up in the endoderm at top and back near lip

31. Germ Cell Migration: Frogs The endoderm below mesdoderm are PGCs

32. Germ Cell Migration: Frogs Migration anterior to gonads at endoderm-mesoderm boundary ~30 PGCs reach gonads by fibronectin and Sdf-1

33. Remember Sdf-1 • Soluble signal whose receptor is CXCR4 • Common signal for vertebrate germ cells • Also used by humans to call HSC to bone marrow, guide lymphocytes, MSC?

34. Germ Cell Migration: Mice PGCs formed in extraembryonic epiblast 10-100 cells @ Day 6.5 in mice

35. Germ Cell Migration: Mice Once formed, they migrate directly into the hindgut endoderm and migrate anteriorly through Day 9 dividing the entire time They leave the gut by the dorsal mesentary and enter the genital ridges by Day 12 as 2500-5000 PGCs.

36. Germ Cell Migration: Mice • The travelling stem cell niche • Support cells travel with PGCs to maintain the undifferentiated stem cell phenotype • They secrete stem cell factor (SCF) • The cells follow fibronectin trail • Sdf-1 also required

37. Germ Cell Migration: Birds and Reptiles Germ line cells determined in the area pellucida, migrate to hypoblast Migrate to gonads via blood stream when extraembryonic vessels form

38. Germ Cell Migration: Birds and Reptiles Sdf-1 from intermediate mesoderm draws them out of vessels and through the mesodermal tissues to the gonad

39. Bio 127 - Section IIILate Development Post-Embryonic Development Gilbert 9e – Chapter 15

40. Section 4 Encompasses : • Development of the Tetrapod Limb • Sex Determination • The Saga of the Germ Line • Post-Embryonic Development

41. Student Learning Objectives 1. You should understand that development never stops during the life of the organism and that three major processes occur in the post-embryonic animal: metamorphosis, regeneration and aging. 2. You should understand the Direct Development involves young organisms with the same body plan as the adult; whereas Indirect Development involves major changes to form the adult body plan. 3. Indirect Development, or metamorphosis, is hormonal reactivation of 4. You should understand that regeneration is the reactivation of developmental process to restore missing tissues. 5. You should understand that aging and physiological senescence are an interplay of genetic and environmental influences.

42. Metamorphosis • Development of a larval stage and an adult stage specialized for different functions • Larvae often specialized for growth, dispersal, etc. • Adults usually specialized for reproduction • Example Cecropia moths: • Larvae are wingless eating machines • Adults have one day to mate – don’t even have mouth parts! • Two major types of larvae • Primary: little to no similarity to adult (sea urchins) • Secondary: add and subtract parts from similar form • (insects, amphibians)

43. Metamorphosis: Sea Urchins Pluteus Larvae

44. Metamorphosis: Sea Urchins Primary Larvae: No trace of adult morphology

45. Metamorphosis: Amphibians • Hormone(s): T3 and T4 • Four Major Morphological Processes • Growth of new structures • Cell death in existing structures • Remodeling of existing structures • Biochemical respecification • Shift in the genes expressed and the physiological functions they control

46. Metamorphosis: Amphibians Tadpole eyes are on the sides of the head, frog eyes are on the front and top Binocular Vision New neurons differentiate and form new ipsilateral tracts

47. Metamorphosis: Amphibians • Cell death in existing structures • Apoptosis • T3 induces apoptosis in tail and gills • Apoptosis occurs in gut epithelium due to ECM loss • Phagocytosis • Macrophages finish off the cells of the tail • Also destroy larval RBCs to make fresh ones with the adult hemoglobin protein

48. Metamorphosis: Amphibians Remodeling: - Eyes - Skull - Skeleton - Gut - Sensory

49. Metamorphosis: Amphibians Biochemical Respecification NH3 = ammonia, amino group NH4+ = ammonium ion T3 causes a shift in transcription factor expression that upregulates these genes.

50. Metamorphosis: Amphibians 2NH3 + CO2 + H2O (urea)