Trophoblast and hypoblast in the monotreme, marsupial and eutherian mammal: evolution and origins
Version of Record online: 24 JAN 2006
Copyright © 2006 Wiley Periodicals, Inc.
Volume 28, Issue 2, pages 128–145, February 2006
How to Cite
Selwood, L. and Johnson, M. H. (2006), Trophoblast and hypoblast in the monotreme, marsupial and eutherian mammal: evolution and origins. Bioessays, 28: 128–145. doi: 10.1002/bies.20360
- Issue online: 24 JAN 2006
- Version of Record online: 24 JAN 2006
- MHJ acknowledges support from the Wellcome Trust
- LS from the Australian Research Council
The pregastrula stage mammalian conceptus consists of both embryonic and non-embryonic components. The latter forms the bulk of the tissues, provides nutrition for the developing embryo and also contributes developmental signals that influence events within the embryo itself. Understanding the origins and relationships between the embryonic and extraembryonic cell lineages is thus central to understanding development in mammals. Despite the apparent gross differences in early developmental strategy and form, the conceptuses of eutherian, marsupial and monotreme mammals show some remarkable similarities in the lineage allocation to trophoblast and hypoblast and in the emergent properties of the two cell types. We suggest that the gross differences can be explained by two relatively small evolutionary timing changes affecting cell adhesion patterns and the polarisation of developmentally significant information. These changes result in the conversion of a unilaminar blastocyst to a morula form composed of blastomeres with increased regulatory capacity. BioEssays 28: 128–145, 2006. © 2006 Wiley periodicals, Inc.