Question

Discuss how axis formation occurs in amphibian development by answering each of the following question:

What are the properties of the primary organizer?

Answer 1

During amphibian development, the entry of the sperm in the egg cell, anywhere at the dark coloured animal pole (cortical cytoplasm) is the key for initiating embryonic development. This is followed by rotation towards the site of entry by 30 degrees resulting in formation of a lighter wedge like region calle dthe gray crescent. Thus, the gray crescent marks the dorsal side or the future back side of the embryo while the site of entry of the sperm mark sth eventral side or future belly.

Thus, the rotation of the cortical cytoplasm plays a critical role in defining the axis for further development. The molecules specifying dorsal, or back, fate— are initially located in the cortical cytoplasm of the yolky bottom part of the egg. They get shifted upwards, towards the animal pole of the zygote. They ae henceforth placed in contact with other molecular factors—different from those in the cytoplasm near the vegetal pole—triggering events that lead to dorsal fate specification.

The gray crescent in the blastula stage consists of cells found on the dorsal side of the embryo which later in the gastrula stage start migrating into the interior of the embryo, causing the tissue to fold inward. The site where the cells migrate into the interior of the embryo is called the blastopore, and the gray crescent cells make up its dorsal lip. These cells migrate inward instruct the cells above them, a type of tissue called ectoderm, to develop into neural—nervous system—tissue.

The cells of the dorsal blastopore lip and their descendants are called the Spemann-Mangold organizer ( primary organizer). It is a cluster of cells in the developing embryo of an amphibian that induces development of the central nervous system.The key induction wherein the progeny of dorsal lip cells induce the dorsal axis and the neural tube is called primary embryonic induction. The cells of the primary organizer (1) induce the ventral tissues to change their fates to form a neural tube and dorsal mesodermal tissue (such as somites), and (2) organize tissues into embryo with clear anterior-posterior and dorsal-ventral axes.

Therefore, the organizer has key properties are:

• specifies dorsal—back, rather than belly—fate and to make nearby ectoderm turn into neural tissue,
• the organizer also guides head-tail axis development and other processes.
• acts largely by releasing secreted proteins that diffuse into the surrounding tissues and affect their behavior and cell- fate specification. For instance, some of the proteins released by the organizer bind to and neutralize other secreted proteins, which instruct cells to develop as skin
• it blocks BMP-4 ( bone morphogenetic protein-4- secreted by developinfg blastula) from binding to the surrounding ectoderm by secreting the proteins chordin and noggin. Chordin and noggin bind to BMP-4 in the organizer-affected area to prevent BMP-4 from binding to ectoderm receptors. Therefore, instead of becoming skin cells, the ectodermal cells in the area of the organizer take the default path of becoming central nervous system tissues.