Lesson 7: Frog Gastrulation and Mesoderm Induction

When are growth phases not active

during reductive cleavage

What establishes asymmetry

maternal determinants

Point of sperm entry

anywhere on the animal hemisphere, specifies DV axis, initiates cortical rotation, and specifies where gastrulation will occur

Cortical rotation

a mechanism for symmetry breaking and specifying where gastrulation will occur

What kickstarts cortical rotation

membrane bound kinesin drags along microtubules

First step of cortical rotation

reorganization of the cytoskeleton

Second step of cortical rotation

formation of gray crescent by rotation ~30 in Rana pipiens

What is the site of gastrulation

the gray crescent

Do all amphibians exhibit a gray crescent

No but they cortical rotation

KRP

Kinesin related protein

GBP

Gsk3 binding protein

KHC

Kinesin heavy chain

KLC

Kinesin light chain

What happens when b-catenin is stabilized everywhere in the sea urchin

Loose positional information and turns into funky thing

Sea urchins

micromeres, autonomous specification and inductive capacity

Frogs

dorsal ventral polarity

Dorsal

note presence of b-catenin in nuclei

Ventral

note absence of b-catenin in nuclei

What does Dsh and GBP do

prevents b-catenin degradation

Third step of cortical rotation

establishment of the first cleavage plane

What occurs after cortical rotation

holoblastic complete cleavage that is radially displaced towards the animal pole

What is the vegetal pole like after cortical rotation

higher density than animal pole, making metaphase plate more difficult

Maternal cdc25a mRNA

present before fertilization

Cdc25A protein

accumulates during early cleavage and is known as an M-phases inducer

Role of blastocoel

permits cell movement

Role of bottle cells

“squeeze in” at the start of gastrulation

What is the name of the type morphogenetic movement for bottle cells during gastrulation

invagination

Myosin is the motor for what

actin contractile ring

What zone does gastrulation occur in

marginal zone, the opposite site of sperm entry

IMZ

involuting marginal zone

Embryonic induction does what

specifies cell fates

Why do all animals gastrulate

to form the three germ layers

What do bottle cells later contribute to

the endodermal lining of the archenteron

What is so special about the DBL

it's all about induction

DBL

dorsal blastophore lip

The Hans Spemann and Hilde Mangold experiment

established embryonic induction

Embryonic Induction

cell and tissue fate can be determined by signals received from other cells

Nieukoop Center

(real organizer) VegT, Vg1, b-cat, twin, SMADs, siamois

Functions of the organizer

initiates gastrulation, specifies dorsal mesoderm, dorsalizes ectoderm, dorsalizes into paraxial mesoderm, specific pharynx endoderm

How does the organizer initiates gastrulation

forms germ layers and AP axis, DV-axis goes from “presumptive” to established

Why does the organizer dorsalizes the ectoderm

to acquire neutral fate

What is Smad2/4 activated by

TGF-b family of paracrine factors (Xnr, Vg1)

Organizer genes

chordin, noggin, goosecoid, cerberus

Siamois and twin proteins

transcription factors with homeodomains

Is b-catenin sufficient enough to specify the organizer

no

What maternal determinants are necessary to specify the organizer

VegT and Vg1

VegT

a transcription factor

VG1

a ligand that belongs to the Tgf b family

What does the Tgf-b pathway activates SMAD transcription factors

VegT promotes what

mesoderm inducing signals

Dorsal b-catenin activity helps to what

restrict organizer specification to the dorsal side

Mesoderm inducing signals

VegT, Vg1, Xnr, Xbra, SMADs

Praxial mesoderm

Mesoderm that forms somites, which develop into muscles, vertebrae, and dermis.

Ventral mesoderm

Mesoderm that forms structures such as blood cells, blood vessels, and the heart.

Ectoderm derivatives

nervous system, skin, sensory neurons in head

Endoderm derivatives

lymphatic system, digestive system, respiratory system

Mesoderm derivatives

bones, muscles

Involution

cells are spreading out but also diving in

Epiboly

cells are just spreading over the yolk

Organizer pathway

TGF b-ligang → S/T receptor kinase → SMAD transcription factor → TGF b-specific gene suppression

Xnr

ligand of TGFb pathway

Xwnt

ligand of wnt pathway

Nieukoop established that

the endoderm has inductive properties and induces cells within the marginal zone to form mesoderm

Posterior mesodermal cells

will form the notochord

Anterior mesodermal cells

will form head muscle and bone

The BMP activates what

SMADs

The BMP pathway is an epidermal inducer, meaning

ectoderm becomes epidermis

The BMP pathway is a powerful ventralizer, meaning

mesoderm becomes ventral mesoderm

If the BMP pathway is active, the

neural pathway is shut down

Anti-BMP

chordin, noggin, follistatin

MZ

marginal zone

CM

chordamesoderm

Would you get two frogs if you cut the blastocoel stage embryo in half

No. At the blastula (blastocoel) stage, cutting a frog embryo in half produces incomplete embryos.