recap

43.2 recap

Cleavage divides up the cytoplasm of the zygote such that different blastomeres contain different combinations of informational molecules. The amount of nutrients stored in the egg influences the pattern of cell cleavage that produces the blastula. Blastulation in mammals is different compared to other vertebrates. The cells that will give rise to germ cell lineages are set aside very early in development.

learning outcomes

You should be able to:

  • Compare the three main types of cleavage.

  • Explain why cleavage in mammals is different from other vertebrates.

  • Compare and contrast regulated and autonomous specification.

  • Compare and contrast identical and non-identical twins.

  • Compare the timing of determination in reproductive germ cells with that in other types of cells in the embryo.

Question 1

What characteristic of the eggs of different species relates to whether cleavage is complete or incomplete?

Complete cleavage occurs in eggs that have little yolk. In these eggs the early cleavage furrows can divide the egg completely. In eggs with a lot of yolk, the cleavage furrows cannot penetrate completely and the blastula forms as a disc of cells that sits on top of the yolk mass.

Question 2

Why can genes expressed during cleavage play roles in development in mammals but not in sea urchins and frogs?

Cleavage in mammals is very slow, allowing enough time for gene expression to occur. In sea urchins and frogs, cleavage occurs very rapidly, precluding gene expression between cell divisions.

Question 3

Why can identical twins arise from a single egg in mammals but not in most invertebrates?

Mammals have regulative development, meaning that each cell of the blastula is not irreversibly determined, so if the blastula is divided in half, identical twins can develop. Since development is mosaic in most invertebrates, if the blastula is divided, each part will be lacking certain elements and will not be able to complete development.

Question 4

Why is there a longer time between determination and differentiation in germ cells versus skin cells?

Germ cells are determined at the time of early cell divisions of the embryo that take place during formation of the blastula. These presumptive germ cells will not differentiate until the gonads develop and they migrate into those gonads. The origin of skin cells is ectoderm that is determined during a later developmental stage — the gastrula — and these ectodermal cells differentiate rapidly to form the skin of the embryo.

Of the next stage of development—gastrulation—the developmental biologist Lewis Wolpert once said, “It is not birth, marriage, or death, but gastrulation which is the most important time in your life.” During gastrulation, cell movements create new cell-to-cell contacts, which in turn set up signaling cascades that initiate the differentiation of cells and tissues and set the stage for the emergence of the body plan.