Blastula Development and Organizer Substances in Embryos

The Blastula

At the end of segmentation a clearly formed blastocoel is present. The blastodisc is the equivalent of the blastoderm of amphibians and consists of several layers of cells.

Organizer Substances and the Theory of Induction

Theory of Induction explains the differentiation of cells. When a structure is induced, it may in turn induce other structures. Successive waves of induction could explain the organized embryonic development of living organisms.

It was found that organizer properties can be maintained even after the organizer cells are dead. Induction occurs by the transfer of chemical signals from the organizer to the responding cells. The inducing substance instructs the affected cells to differentiate in a specific way.

How can various cells originate from a single egg?

The composition of a cytoplasmic region stimulates blastula cells to release regulatory chemicals. These cells acquire characteristic properties that allow them to become organizers of development. By disseminating chemicals, they instruct adjacent cells to develop in a specific way. As those adjacent cells develop, they will produce substances that program the differentiation of other cells.

Identification of Inducing Substances

Induction can occur through an agar bridge. In processes involving large molecules, many inducing substances are proteins.

Genetic Information and Totipotency

Genes and genetic information guide organismal development. At certain stages of development cells must differentiate, even though the zygote divides to produce cells with the same genetic material. During the early stages of development all cells are totipotent; each has the ability to develop into a complete embryo. The nucleus retains full potential and can differentiate into different cell types.

Cellular Differentiation During Embryonic Development

Differentiation is influenced by inhibitory substances from neighboring cells. Differentiated cells may produce inhibitory factors that prevent adjacent cells from differentiating in the same way. As embryonic development progresses, organs and other structures develop by interactions between groups of cells.

• The course of embryonic development depends on interactions between different cell groups.
• Normal embryonic development depends on appropriate exchange of materials between tissues.
• Differential gene activity explains morphological and physiological differences between cell types.