CONTENTS

Cleavage

How are the building blocks of the embryo produced?

There are two major consequences of cleavage: production of a multicellular organism from a single-celled zygote and acquisition by individual cells or groups of cells of differences that will facilitate their later development into different cell types.

The increase in cell number occurs without intervening growth between successive cell divisions. Thus, blastomere volume is decreased during cleavage to produce the small, individual cellular units that participate in the subsequent morphogenic events that mold the embryo. Cleavage also increases the number of nuclei, which amplifies the number of templates that will facilitate the later production of specialized proteins needed for formation of a functional organism.

Partitioning of the cytoplasm during cleavage can result in segregation of specialized cytoplasmic components that will have profound effects on development. Remember the localized transcripts that we discussed earlier? Cleavage segregates them and localized proteins into different cells. You can imagine, then, how important the pattern of cleavage is for establishment of an orderly array of functional entities in the embryo. If it is important to the embryo, it is important for you to understand and appreciate differences in cleavage patterns among different groups of organisms. Study the cleavage patterns carefully in your text.

Fertilization is a mitogenic stimulus that triggers the zygote to re-enter the cell cycle. The cell cycle during cleavage differs from that in somatic cells by having no G1- or G2-phase and a very short S-phase. The shortening of the S-phase is accomplished by the simultaneous activation of multiple units of DNA replication.

(See Browder et al., Fig. 5.1; Gilbert, Fig. 5.39; Kalthoff, Fig. 5.27)

Recent evidence implicates a group of three genes in regulating the S-phase of the early embryonic cell cycles in Drosophila (Elfring et al., 1997). Further investigation of these genes and the proteins they encode should lead to an understanding of the molecular basis for the altered cleavage cell cyle.

At the end of cleavage, the somatic cell cycle is established. In frogs, this occurs at the mid-blastula stage and is called the mid-blastula transition (MBT). The important consequences of this transition during development will be discussed in detail later.

(See Browder et al., Fig. 5.2; Kalthoff, Fig. 5.28; Wolpert et al., Fig. 3.33)

Danilchik and Funk (1996) have recently reported evidence for the presence of microtubule-containing structures in the cleavage furrows of Xenopus embryos. Microtubule depolymerizing agents (cold shock and nocodazole) cause furrow regression. They have observed thick, short bundles of radially-arranged microtubules emanating from the contractile ring. This structure was found at the leading edge of all cleavage furrows until the mid-blastula stage. They hypothesize that the role of this structure is to recruit membrane vesicles to the furrow's leading edge for new membrane assembly. Another role may be in subcortical ingression.

Use your textbook to study the various forms of cleavage patterns, and the factors that control them.

Learning Objectives

What is distinct about cleavage-stage cell cycles?
What is the MBT?
What parameters influence patterns of cleavage?
How is cytokinesis regulated?
What are the major cleavage patterns, and what organisms utilize them?
What is compaction?
What do breeding experiments with snails tell us about the control of cleavage patterns?

Digging Deeper:

Recent Literature

Elfring, L.K.Axton, J.M., Fenger, D.D., Page, A.W., Carminati, J.L. and Orr-Weaver, T.L. 1997. Drosophila PLUTONIUM protein is a specialized cell cycle regulator required at the onset of embryogenesis. Mol. Biol. Cell 8: 583-593.

Ohsugi, M., Larue, L., Schwarz, H. and Kemler, R. 1997. Cell-junctional and cytoskeletal organization in mouse blastocysts lacking E-cadherin. Develop. Biol. 185: 261-271

References

Browder, L.W., Erickson, C.A. and Jeffery, W.R. 1991. Developmental Biology. Third edition. Saunders College Pub. Philadelphia.

Danilchik, M. and C. Funk. 1996. Microtubules in early cleavage furrows. 6th International Xenopus Conference, Estes Park, Colorado.

Gilbert, S.F. 1997. Developmental Biology. Fifth Edition. Sinauer. Sunderland, MA.

Kalthoff, K. 1996. Analysis of Biological Development. McGraw-Hill. New York.

Wolpert, L., Beddington, R., Brockes, J., Jessell, T., Lawrence, P. and Meyerowitz, E. 1998. Principles of Development. Current Biology. London.

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Leon Browder & Laurie Iten (Ed.) Dynamic Development
Last revised Wednesday, June 17, 1998