Question

1. Describe cleavage and gastrulation in frogs. Then describe the steps in specifying the germ layers and the dorsal-ventral and anterior-posterior axis. Include cortical rotation at fertilization, endoderm specification including what is special about the dorsal vegetal cells (Nieuwkoop center), mesoderm induction including how dorsal mesoderm is induced, and the function of the organizer (dorsal lip of the blastopore). Describe the transplantation experiment that established the dorsal lip as the “organizer”. Also describe the experiments that were used to determine if secreted proteins from the organizer (such as bighead) could induce neural differentiation?

Answer 1

Ans) The point of sperm entry is on ventral side and 180^o opposite to the point of sperm entry is Dorsal side where lies the grey crescent.It gets exposed by cortical rotation.Gastrulation begins in the gray crescent.First,cleavage furrow bisects the gray crescent. Gray crescent is found in the marginal zone (below the equator).

Animal pole (Ectoderm cells) cells divide rapidly as compared to the cells at the vegetal pole (Endodermal cells).

Invagination of endoderm cells occurs to form the blastopore (Dorsal blastopore lip).Then,involution of marginal zone cells occurs and in the last step, ectodermal cells undergo epiboly and convergent extension which ultimately close the blastopore.

Transplantation experiment - When the dorsal blastopore lip (From donor) was transplanted to the ventral region of epidermis (on Host or recipient) of early gastrula stage gastrula,it was observed that the donor tissue invaginates to form secondary archenteron and also second embryonic axis.Thus, spemann termed the Dorsal blastopore lip as an organizer (Becaue, it organized the tissues into secondary embryo which also showed anterior-posterior and dorsal-ventral axis).

Dorsal vegetal cells induce Dorsal mesoderm to form somites,notochord etc. Signal is released by Niewkoop center.Beta catenin forms Niewkoop center. When the mesoderm was separated from the animal and vegetal cap, it was observed that neither of them have produced mesoderm. But when, animal and vegetal cap cells were combined,it was observed that the signal released by the vegetal cap cells travels to the animal cap cells and induce them to form mesoderm.This is how the mesoderm is induced. Beta catenin work with Vg1 and VegT proteins to activate Nodal related (Xnr) genes.Xnr gradient specify mesoderm. Xnr concentration is highest in the dorsal and lowest in the ventral side.

Anterior-posterior axis is determined by retinoic acid gradient (found at higher concentration in posterior and lower concentration in anterior end).

Neural differentiation- To form neural tissues is the default fate but some parts secrete BMPs (Bone morphogenetic proteins) which induce ectoderm to form epidermal tissue. (BMPs repress neurogenin gene which is responsible for neural tisse formation). Organizer secrete BMP inhibitors like Chordin,Noggin and Follistatin.These will inhibit BMP and thus neural tissues will be formed. For epidermis tissue formation, Both Wnt and BMP pathway should be "ON", For trunk and spinal cord formation, BMP pathway should be blocked and Wnt pathway should be "ON",For the formation of Head and Brain,both the BMPand Wnt pathways should be turned "OFF".