Fate map -meaning and construction

Fate map is a chart showing the end result of the different  areas of an embryo. Towards the end of cleavage the organ forming areas arc well marked. In a blastula, cell areas are markable which will give rise to thegerm layers and later, the different parts of organs ot the future embyo. Such prospective celt areas are called presumptive areas or organ forming areas. A chart showing the fate of organ forming areas in blastula is called a fate map.

Construction of a Fate Map

Various techniques have been devised forthe construction of fate map. Of these, tracing the course with natural colours and artificial markings are most important.

1. Natural markings : The cytoplasm of certain eggs such as those of

ascidians has natural pigments. Thus, in the fertilized egg of Sryeta four coloured centres have been recognised - an upper hemisphere of tight proloplasrn, an yellow crescent postero-ventrally, a grey crescent anterodorsally and a vegetal area of dark grey yolky substance. The tate of these areas can be followed very easily. lt has been revealed that the upperclear cytoplasm contains the material for epidermal ectoderm. The grey crescent area differentiates into the prospective rteurectoderm and notochord. The yellow crescent becomes the prospective mesoderm and the dark grey yolky area forms the prospective endoderm.

I. Artificial Markings: There are three methods to mark or label the only blastomeres by which their fate can be traced.

(I) Vital staining : This method was devised by Vogt (1928). Vital stains in the dyes which are retained by living cells for considerable period and in no way interfere with normal life activities. The persistence of the stains in the cells makes it possible to determine the ultimate location of the  marked areas in the late embryo. Methylene blue, neutral red, Bismark brown, Janus green and Nile blue sulphate are some of the common Vital stains. The vital staining technique involves soaking a piece of agar with a vitaldye and placing it on a surface ot the embryo in the required location. immtdiately, the stain ditfuses into the blastomeres and the cells are stained. The descendents of the marked cells also become stained. In the dllerentiated embryo, the parts stained indicate the fates of the original stained blastomeres. For example, it the stained cells are seen in the gut of  the ditterentiated embryo, the tate of the marked blastomeres is said to bé t he development of gut. Several such stain marks can be made on the surtace of the same embryo using different colours. In this way, the various organ forming areas can be marked.

ii)Carbon Particle marking : This technique was introduced by Spratt to demonstrate the process involved in primitive streak formation in chick. This consists of applying tiny particles of carbon over the surtaceof blastomeres. They stick to the cell surface and enable the investigator to follow the movements of the cells and to determine the fate of these blastomeres.

iii)Radioactive isotope labelling : The radioactive isotopes such as

C14 and P32 are used to label the early blastomeres. By carefully following the course ot these radioactive isotopes the fate of blastomeres can be determined.

The following presumptive areas are marked in a chordate blastula

1. There is a broad ectodermal area in the animal hemisphere which form the epidermal layer of the skin. This is known as epidermal ectoderm.

2. A relatively smaller ectodermal area lies below the epidermal ectoderm. This area is known as neurectoderm since it contributes to the formation of neural tube and nervous system.

3. A crescemic area below the neurectoderm is designated as Notochordal area which gives rise to the notochord of the embryo. -

4. On either side of the notochordal area are two lateral areas. These zonstitute the prospective mesoderm.

5. Most of the yolky vegetal hemisphere blastomeres collectively form the prospective endoderm.

6. At the caudal margin oi the notochorclal area is a small strip oi blastomere called prechordal plate region. This region mainly gives rise to some of the head mesoderm.