Gastrulation begins 180 ° from site of sperm entry

1. Blastopore Formation

sperm entry (That looks familiar!) Fig. 7.6

How do cells choose fates before gastrulation?

The dorsal-ventral axis in the frog

A frog’s mother sets up the initial axis of polarity Animal pole Vegetal pole

An outline of frog gastrulation

Future Ventral Side (belly) Sperm always enters at the animal hemisphere Future Dorsal side (back)

Sperm entry is the critical asymmetric cue in setting up the dorsal-ventral axis of the frog Xenopus See Figure 7.1

Sperm entry triggers a rotation of the outer cortex relative to the inner cytoplasm , fixing the site of gastrulation initiation Animal cytoplasm Animal cortex Gray crescent cytoplasm = junction of vegetal cortex with animal Sperm entry point Vegetal cortex Yolky cytoplasm 30



cortex rotation gastrulation initiates here.

Rotation occurs during the first division, and requires the sperm centriole

Experimental alteration of cortical rotation changes the blastopore location Sperm entry point 1. Rotate egg so that sperm entry point is now on top.

Sperm entry point 2. Prevent normal cortical rotation gray crescent 3. Gravity rearranges cytoplasm so that the junction of vegetal pole cortex with animal pole cytoplasm is next to sperm entry point instead of opposite it.

Sperm entry point 4. Gastrulation now occurs next to sperm entry point

Information from neighbors:

Mother cell Cell division

Cell division places daughter cells in different environments, which can lead to different cell fate choices

Cell type A Cell type B

Induction:

information from neighbors influences cell fate

inducer responder

Competence:

inductive signal ability to respond to a certain responder inducer Cell not competent to respond

Succesive inductions:

can generate many cell types from just a few interactions inducer responder Cell not competent to respond

Types of signals Inducer Responder

Signals can act globally throughout the body Release into the Fallopian tube

Signals can also act in a graded fashion

Induction:

An initial difference can be amplified into many cell types

How is cell fate determined -- the dorsal-ventral axis in newts See p. 255

Spemann & Mangold: the organizer can influence neighboring cells to form a secondary body axis: Figure 7.17

Induction of the organizer can influence neighboring cells to form a secondary body axis:

If one transplants a second inducer of the organizer the embryo forms two body axes Figure 7.19

Induction of mesoderm in the frog embryo

Initial asymmetry of Xenopus egg leads to some cell fate differences Animal pole Vegetal pole Mesoderm arises at endoderm/ ectoderm junction

Experiment: Neither vegetal or animal pole alone can make mesoderm.

Figure 7.18

Vegetal pole cells can induce animal pole cells to make mesoderm

Fig. 7.18

Conclusion: a signal in the vegetal part is needed to induce mesoderm in the animal cap.

Induction of dorsal mesoderm by dorsalmost vegetal cells

The 3-step model of mesoderm induction #3 #2 #1

Figure 7.18

The 3-step model of mesoderm induction #3 #2 #1

Model for mesoderm induction and organizer formation

Figure 7.23

Neurulation separates the ectoderm into neural and epidermal cell fates

Neurulation is driven by cell shape and adhesion changes similar to those that drive gastrulation

Alberts Figure 20.26

Dorsal-Most Cells Neural tube formation Figure 9.4

Differential adhesion helps the neural tube and skin to separate from each other ectoderm E-cadherin N-cadherin Hatta and Takeichi (1986) Nature

See Fig 9.6

Experiment: each other Differential adhesion helps the neural tube and surface ectoderm separate from ectoderm

Inject N-cadherin mRNA so that surface ectoderm expresses N-cadherin Fig 9.6

Neural crest cells migrate from the CNS to form the peripheral nervous system

Defects in neural tube closure are among the most common birth defects Figure 9.5

Numbers indicate regions of neural tube closure.

It is estimated that 50% of human neural tube defects would be prevented if all women of childbearing age take supplemental folic acid (vitamin B 12 ) 400

m

g/day

Expression of a folate receptor in mice neural tubes prior to fusion Figure 9.7