Early Development and Axis Formation in Amphibians

Transcription

1 Biology 4361 Early Development and Axis Formation in Amphibians October 25, 2006

2 Overview Cortical rotation Cleavage Gastrulation Determination the Organizer mesoderm induction Setting up the axes: dorsal/ventral anterior/posterior left/right

3 Cortical Rotation

4 Cleavage Patterns

5 Yolk classification Holoblastic (complete cleavage) Species echinoderms, amphioxis Cleavage Radial Isolecithal annelids, molluscs, flatworms Spiral tunicates Bilateral mammals, nematode Rotational Mesolecithal Telolecithal amphibians Meroblastic (incomplete cleavage) cephalopod molluscs fish, reptiles, birds Displaced radial Bilateral Discoidal Centrolethical most insects Superficial

6 Unequal Radial Holoblastic Cleavage Cell cycles regulated by mitosis promoting factor (MPF) no G phases Mid blastula transition promoters demethylated transcription factors, e.g. VegT formed in vegetal cytoplasm

7 Xenopus Fate Map Fate mostly dependent on whether the cells are located in superficial or deep layers in the blastula. ectoderm and endoderm precursors: superficial layer on surface mesodermal precursors: mostly in deep layers in Xenopus, notochord lies beneath the surface in the marginal areas Gastrulation moves cell layers into proper conjunction

8 Gastrulation Vegetal Rotation Involuting marginal zone motive force

9 Xenopus Gastrulation

10 Xenopus Gastrulation IMZ = involuting marginal zone Cell movements: invagination involution epiboly (animal cap) intercalation and convergent extension migration

11 Xenopus Gastrulation NIMZ

12 Determination Spemann s Ligation Experiment demonstrated that early nuclei were capable of generating an entire organism however, further experimentation complicated story

13 Asymmetrical Egg Ligation if one blastomere received no gray crescent material resulted in belly piece Belly piece blood, mesenchyme, gut cells no dorsal structures (notochord, somites) Gray crescent area (future dorsal lip of the blastopore) was critical to development

14 Dorsal Lip Transplant Doral lip = Organizer organizes secondary D V axis induced ventral cells to change fates

15 The Organizer Spemann and Mangold s experiments established the organizing properties of the dorsal blastopore lip. self differentiates establishes Dorsal Ventral axis specifies multiple tissues dorsal mesoderm, which includes: head mesoderm (prechordal plate) chordamesoderm (notochord) dorsalizes surrounding mesoderm into paraxial mesoderm induces the neural tube initiates the movements of gastrulation How is the dorsal lip specified? back to the fate map for re orientation and overview:

16 Xenopus Fate Map The general fate map is imposed by transcription factors and paracrine factors in the vegetal region. Anterior posterior, dorsal ventral, and left right axes specified by events triggered at fertilization and realized during gastrulation. Of all tissues in the early Xenopus gastrula, only the dorsal lip of the blastopore has its fate determined. The dorsal lip will determine the fates of notochord and head endomesoderm and determine the primary embryonic axis.

17 Mesoderm & Organizer Induction Animal cap factors released by vegetal cells Neither animal fragments nor vegetal fragments alone produce mesoderm but placed in conjunction = mesoderm induction Vegetal cells induce mesoderm VegT transcription factor Vg1 paracrine factor VegT Vg1 Mesodermal fate depends upon origin of vegetal inducing cells ventral vegetal cells induce mesenchyme, blood intermediate muscle, kidney dorsal (organizer) notochord, somites

18 Dorsal Signal $ catenin $ catenin anchor for cadherins nuclear transcription factor (Wnt pathway) in sea urchins, specifies micromeres, endomesoderm in Xenopus $ catenin is initially synthesized throughout the embryo from maternal mrna accumulates in dorsal cells concentrates in the Nieuwkoop center and organizer How? It starts in the egg organizer Initially, $ catenin is evenly distributed throughout the egg VegT Vg1 dorsal signal $ catenin Nieuwkoop center But eventually appears just in dorsal cells Dorsalization is accomplished by: 1. protecting $ catenin in dorsal area only 2. degrading $ catenin everywhere else

19 Disheveled/$ Catenin/Cortical Rotation $ catenin induces cells to dorsal fates $ catenin is initially distributed throughout oocyte Dsh Disheveled GBP GSK3 binding protein GSK3 glucose synthase kinase

20 Disheveled/GSK3/$ Catenin At the blastula stage, $ catenin is located exclusively in the future dorsal region GSK3 degrades $ catenin but Disheveled and GBP block GSK3 activity resulting in $ catenin survival only in the marginal area opposite the point of sperm entry

21 Organizer Induction $ catenin acts with Tcf3 (a transcription factor) to stimulate expression of several dorsalizing genes Twin and Siamois TFs activate Xlim, goosecoid dorsal determinants Goosecoid protein TF responsible for organizer properties goosecoid also plays a part in specifying dorsal mesoderm however, additional vegetal factors are needed: vegetal TGF $ signals

22 Mesodermal Signals Xnr Xenopus nodal related genes Xnrs: TGF $ genes

23 Determination of Ectoderm early cells are uncommitted exhibit regulative development later cells are determined exhibit autonomous development

24 Establishment of Axes D V axis set up at fertilization A P axis established by gastrulation movements across the dorsal lip of the blastopore L R axis (still to come)

25 Dorsal Lip Inductions dorsal mesoderm (paraxial mesoderm somites) chordamesoderm notochord head mesoderm (prechordal plate) pharyngeal endoderm

26 Left Right Axis wild type Xnr1 / Left Right axis established by Xenopus nodal related (Xnr1) Nodal expression: common to all vertebrates expressed on the left side! Xnr1 expression is limited to the left side in a process involving cortical rotation and Vg1 heart loops to the left gut coiling counter clockwise Block Xnr1 expression = random gut coiling, heart looping