long note on fate map...........
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Early development occurs in a highly organized and orchestrated manner and has long attracted the interest of developmental biologists and embryologists. Cell lineage, or the study of the developmental differentiation of a blastomere, involves tracing a particular cell (blastomere) forward from its position in one of the three germ layers. Labeling individual cells within their germ layers allows for a pictorial interpretation of gastrulation. This chart or graphical representation detailing the fate of each part of an early embryo is referred to as a fate map. In essence, each fate map portrays the developmental history of each cell.
Fate maps were developed as a way of tracing a particular region as it develops from an early embryo into a differentiated body plan. The first fate maps date back to the 1880s and in 1905 the first comprehensive collection of Ascidian (sea squirt) fate maps was published by Edwin Conklin. It is now common to find fate maps in introductory embryology texts. For example, Scott Gilbert’s Developmental Biology (2006) shows fate maps for several different model organisms, including the zebrafish, frog, mouse, and chick embryos. Methods used for fate mapping include, but are not limited to, histological staining, genetic, and genetic inducible fate mapping. The ultimate goal in creating a fate map is to construct a lineage diagram that not only gives spatial information about cell fates, but can also allow the observer to trace the parental lineage of each mitotic division. This type of information can be particularly hard to achieve, but when acquired it can be used to trace the development of complex organ systems such as the central nervous system (a process that involves extensive cell migration).
In the fertilized eggs of many organisms, the progenitor cells are totipotent, meaning that they are capable of expressing every gene in their genome and that each individual cell has the potential to create an identical organism. The commitment of a cell to a specialized developmental pathway is called determination. By removing cells that are already determined and implanting them into a host embryo, one can deduce what the original cells were specified to become. The first visible cell positioning in the embryo of most organisms is during gastrulation, when the embryo rearranges itself into three distinct germ layers: endoderm, ectoderm, and mesoderm. As each cell migrates to its position in the embryo, chemical signals are released, inducing the cell to a particular fate. The developmental fates of the ectoderm, for instance, can be epidermis, central nervous system, sensory organs, and neural crest. Mesoderm cells can become part of the skeleton, muscles, blood vessels, heart, and gonads. The lining of the digestive and respiratory tracts, liver, and pancreas can all derive from the endoderm.
