Cell division by constriction in zygotes (fertilized egg cells ) at the beginning of embryogenesis (embryonic development) in multicellular animals is referred to as cleavage (also cleavage division ) . The embryo does not enlarge in the process . Since no new formation of material is necessary, these divisions are the fastest of all, a core division can take place every eight minutes. After a large number of furrows, a ball densely filled with cells forms, the morula . The cells created by the furrows are called blastomeres . Another difference to the "normal" cell divisions is that all divisions are synchronous. In addition, the core-plasma relationship changes constantly, while otherwise it remains constant during cell division. The cell cycle is shortened, it only consists of S and M phases, which also explains the enormous speed of cell division.
The English physiologist Martin Barry (1802–1855) first described the furrows on fertilized mammalian eggs in 1839.
There are three types of grooves: holoblastic, meroblastic and spiral groove.
( Greek holos 'whole') - complete furrowing
Isolecithal - yolk uniformly distributed (Greek iso 'uniform')
- radial: echinoderms , lancet fish
- bilateral: tunicates
- rotational: theria , roundworms
Anisolecithal - yolk unevenly distributed
- offset radial: amphibians
(Greek meros , part ') - partial furrowing
Telolecithal - yolk concentrated at one pole (Greek telo 'end')
- discoidal: cephalopods , fish , reptiles , birds and monotremes
- Centrolecithal - yolk concentrated in the center
- helical: The cells are helically offset from one another.
With the spiral furrowing, which is characteristic of annelids and molluscs (except cephalopods), the furrowing spindles are arranged at an oblique angle to the animal-vegetative axis, so that the blastomeres are helically offset from one another.
Developmental biology: furrowing
- W. Westheide, R. Rieger: Special Zoology. Part 1: Protozoa and invertebrates. 2nd Edition. Spektrum Akademischer Verlag, Munich 2007, ISBN 978-3-8274-1575-2 .
- ↑ Reinhard Hildebrand: Barry, Martin. In: Werner E. Gerabek , Bernhard D. Haage, Gundolf Keil , Wolfgang Wegner (eds.): Enzyklopädie Medizingeschichte. de Gruyter, Berlin / New York 2005, ISBN 3-11-015714-4 , p. 147.
- ^ SF Gilbert: Developmental Biology. 8th edition. Sinauer Associates, 2006, Fig.8.3
- ^ WC Warren et al. a .: Genome analysis of the platypus reveals unique signatures of evolution . In: Nature . Volume 453, 2008, pp. 175-183.
- ^ R. Wehner, W. Gehring: Zoologie. 24th edition. 2007, ISBN 978-3-13-367424-9 , pp. 220, 716, 724, 736, 771.