What are fate maps; how are they produced; Give the fate map of blastula in frog?

What are fate maps; how are they produced; Give the fate map of blastula in frog?

Certain areas are found in the blastula which can be clearly marked out. These are referred to as the presumptive organ forming areas. This name is given because of the presumption that these areas give rise to particular organs or tissues in the adult frog.

The arrangement of the presumptive organ forming areas has been confirmed by marking experiments. In these experiments certain dyes are used in the early stages of division to ultimately find out which part of the future embryo consists of the known dye.

By this method the presumptive organ forming areas of the blastula have been mapped and these maps are called the fate maps. A fate map may be defined as a topographical surface of the blastula with respect to the ultimate fate of that area in the adult frog.

The significance of blastulation lies in the fact that presumptive organ forming areas are arranged in a specific plane in order to facilitate their rearrangement during the next stage of development.

The ectoderm region consists of two presumptive regions viz., the pre­sumptive epidermis and the presumptive neural plate region. While the presumptive epidermis is present on the anterior, ventral and lateral parts of the animal hemisphere, the presumptive neural plate occupies the dor­sal and lateral part of the vegetal hemisphere. The other organ forming areas are

1. Head develops from the anterior parts of neural plate and epidermal region.

2. In the epidermal region of the head are found the presumptive areas of ears.

3. The presumptive eye region occupies the mid dorsal part of the neural plate.

The mesoderm forms a ring between ectoderm and endoderm. This is divided into presumptive notochordal and somite region. The endoderm occupies the posterior half of vegetal hemisphere of the blastula.

Though the presumptive organ forming regions are marked off in blastula it is not complete and final. Hence the egg of frog is said to be of the intermediate type. The blastomeres at this stage are capable of totipotency (capable of developing into all the structures). However there is one specific region which is not totipotent from the very beginning of the de­velopment. This region is the grey crescent area. This grey crescent area gives rise to the notocord and mesoderm. In addition to this it can induce the neighbouring cells to develop into a neural tube.