Cytoplasmic Determinants

The embryo is bisected horizontally, separating animal pole cells from vegetal pole cells.

Vegetal pole

The embryo is bisected vertically, leaving each half with both animal and vegetal cells.

8-Cell stage

Embryo Cell Stage

Cells remain Abnormal larva Normal, but small, larvae embryonic

Conclusion: The animal and vegetal pole halves differ in their developmental potential.

Cells remain Abnormal larva Normal, but small, larvae embryonic

Conclusion: The animal and vegetal pole halves differ in their developmental potential.

19.7 Asymmetry in the Early Embryo The upper (animal) and lower (vegetal) halves of the sea urchin egg differ in the cytoplasmic determinants they contain. Cells from both halves are necessary to produce a normal larva.

Polarity results from cytoplasmic segregation

As we learned from the cloning experiments described above, cell nuclei do not undergo irreversible changes during early development, so we must look for explanations of some embryological events in the cytoplasmic differences between cells. The development of polarity—the difference between one end of an organism and the other—is one such phenomenon. Polarity is obvious throughout development. Our heads are distinct from our feet, and the distal ends of our arms (wrists and fingers) differ from the proximal ends (shoulders). An animal's polarity may develop early, even in the egg itself, in which yolk and other factors may be distributed asymmetrically.

An experiment with sea urchins demonstrates the effects of cytoplasmic segregation on development (Figure 19.7). Very early development in this species occurs by equal mi-totic divisions of the fertilized egg; there is no increase in size at this stage. If an 8-cell embryo is cut vertically, both halves develop normally. On the other hand, if the embryo is cut horizontally, the top half does not develop at all and the bottom half develops into a small, abnormal embryo.

Clearly, then, there must be at least one factor essential for development that is segregated in the bottom half of the egg, such that the bottom cells of the embryo have it, but the top ones do not. This and many other experiments have established that certain materials, called cytoplasmic determinants, are distributed unequally in the egg cytoplasm, and that these materials play a role in directing the embryonic development of many organisms (Figure 19.8).

Animal pole

Animal pole

Cytoplasmic Determinants

Uneven distribution of a 1 cytoplasmic component.

...is retained as the cell divides...

.and sets up polarity in the early embryo.

Uneven distribution of a 1 cytoplasmic component.

...is retained as the cell divides...

.and sets up polarity in the early embryo.

19.8 The Principle of Cytoplasmic Segregation The distribution of a cytoplasmic substance may determine cell fate.

Tissues direct the development of their neighbors by secreting inducers

Experimental work on developing embryos has clearly established that in many cases, the fates of particular tissues are determined by interactions with other specific tissues in the embryo. In developing animal embryos there are many such instances of induction, in which one tissue causes an adjacent tissue to develop in a particular manner. These effects are mediated by intercellular biochemical communication—that is, by chemical signals and signal transduction mechanisms. We will describe two examples of such induction: one in the developing vertebrate eye, and the other in a developing reproductive structure in the nematode C. elegans.

The development of the lens of the vertebrate eye is a classic example of induction. In a frog embryo, the developing forebrain bulges out at both sides to form the optic vesicles, which expand until they come into contact with the cells at the surface of the head (Figure 19.9). The surface tissue in the region of contact with the optic vesicles thickens, forming a lens placode. The lens placode bends inward, folds over on itself, and ultimately detaches from the surface tissue to produce a structure that will develop into the lens.

If the growing optic vesicle is cut away before it contacts the surface cells, no lens forms. Placing an impermeable barrier between the optic vesicle and the surface cells also prevents the lens from forming. These observations suggest that the surface tissue begins to develop into a lens when it receives a signal—an embryonic inducer—from the optic vesicle.

The interaction of tissues in eye development is a two-way street: There is a "dialogue" between the developing optic vesicle and the surface tissue. The optic vesicle induces lens development, and the developing lens determines the size of the optic cup that forms from the optic vesicle. If head surface tissue from a frog species with small eyes is grafted over the optic vesicle of one with large eyes, both lens and optic cup will have an intermediate size.

The developing lens also induces the surface tissue over it to develop into a cornea, a specialized layer that allows light to pass through and enter the eye. Thus a chain of inductive interactions participates in the development of the parts required to make an eye. Embryonic inducers trigger a sequence of gene expression in the responding cells. Tissues do not induce themselves; rather, different tissues interact and induce one another. We will look at embryonic induction in more detail in Chapter 20.

Single cells can induce changes in their neighbors

The tiny nematode Caenorhabditis elegans is used as a model organism in many biological studies, but it is especially useful for studying development. It normally lives in the soil, where it feeds on bacteria, but can also grow in the laboratory if supplied with its food source. The process of development from fertilized egg to larva takes only about 8 hours, and the worm reaches the adult stage in just 3.5 days. The process is easily observed using a low-magnification dissecting microscope because the body covering is transparent (Figure 19.10a). For all these reasons, C. elegans is a favorite experimental organism. The development of C. elegans does not vary, so it has been possible to identify the source of each of the 959 somatic cells of the adult form.

The adult nematode is hermaphroditic, containing both male and female reproductive organs. It lays eggs through a pore called the vulva on the ventral (belly) surface. During development, a single cell, called the anchor cell, induces the vulva to form. If the anchor cell is destroyed by laser surgery, no vulva forms. The eggs develop inside the parent, and a "bag of worms," which eventually consume the parent, results.

The anchor cell controls the fates of six cells on the animal's ventral surface through two molecular switches. Each of these cells has three possible fates: It may become a pri-

Blastocyst Complementation

Surface tissue

Frog head (dorsal view)

Lens placode tissue

Developing lens

Lens

Surface tissue

Frog head (dorsal view)

Lens placode tissue

Developing lens

Lens

19.9 Embryonic Inducers in the Vertebrate Eye

The eye of a frog develops as different tissues take their turns inducing one another.

(a) Caenorhabditis elegans

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Responses

  • afwerki
    How the distribution of cytoplasmic determinants occurs?
    7 years ago
  • pentti
    Which of the following play a role as cytoplasmic determinants in directing embryonic development?
    7 years ago
  • hannes
    What role do cytoplasmic determinants play in regulating organismal growth?
    6 years ago
  • swen
    How are maternal cystoplasmic determinants distributed in early embryonic development?
    4 years ago

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