Determinate v. Indeterminate Cleavage & Embryonic induction

The basic body plan of most organisms is determined very early in development, often before any noticeable tissue differentiation has begun. Symmetrical or asymmetrical distribution of cytoplasmic determinants (mRNAs and proteins produced by the mother's genes and deposited in the egg) will generally determine what portion of an embryo maintains totipotency and for how long.

If the first cleavage furrow divides these cytoplasmic determinants evenly, then both cells formed will retain totipotency. If initial cell divisions result in asymmetric distribution of cytoplasmic determinants, then totipotency will be lost and the developmental fate of individual cells will have been at least superficially determined. This would be considered determinate cleavage and is the type of cleavage seen in many invertebrates. Deuterostomes, including humans typically exhibit indeterminate cleavage in at least the early stages of their development.

As mentioned above, early mammalian blastomeres (as the smaller cells resulting from cleavage of the zygote are called) receive equal amounts if cytoplasmic determinants and retain totipotency up to the 8-cell blastula stage.

In the figures below the early development of an amphibian is shown. Unlike mammals, but like most other animals, amphibian eggs have definite polarity (based on yolk distribution - yolk being concentrated in the vegetal hemisphere). Fertilization establishes further polarity in the zygote. The gray crescent, which forms opposite the point of sperm entry, will be split in half by the first cleavage division (which runs from animal to vegetal pole). The gray crescent is also the region of the zygote that will develop into the dorsal side of the developing embryo. So, immediately upon fertilization and prior to the first cleavage division an amphibian zygote has all three axes established.

Formation of the gray crescent. Notice that the outer layer of cytoplasm rotates upon fertilization, exposing the gray crescent.

The establishment of the body axes and the first cleavage plane in amphibians.
Fig. 47-7 from Campbell, Biology, 6th ed.

Since the first cleavage division normally divides the embryo into two equal blastomeres, experimental separation of the blastomeres at this point leads to normal development of both blastomeres into complete embryos. Note though, that if an artificial and uneven first cleavage is created then only the blastomere that receives the entire gray crescent develops normally. This further demonstrates the role of cytoplasmic contents in developmental determination.

Experimental demonstration of the importance of cytoplasmic determinants in amphibians.
Fig. 47-21 from Campbell, Biology, 6th ed.

Embryonic induction

Definition: The induction of differentiation in one tissue as a result of proximity to another tissue arising, for example, during gastrulation. One of the best known examples is the induction of the neural tube in the ectoderm by the underlying chorda-mesoderm. Although the information to form the tube is present in the competent determined ectoderm, it must be elicited by the inducing tissue. In some cases it is known that cell-cell contact between epithelium and mesenchyme is necessary. (from the Dictionary of Cell and Molecular Biology - Online!)

Induction of neurulation and axis development by the notochord / chordamesoderm

Figure 12.20 from K. Kalthoff. 2001. Analysis of Biological Development. McGraw Hill, Boston.

Experiments like the one shown in Figure 12.20 (above) demonstrated that the chordamesoderm (the tissue that initially forms the dorsal lip of the blastopore and then moves into the embryo later in gastrulation) is important in inducing the proper development of the overlying ectodermal tissue into the neural tube.

• For more information about cytoplasmic determinants and early embryology see Embryonic Development: Getting Started.

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