Clonal analysis of early mammalian development

Abstract

Various extrinsic markers have been used to label single cells in the early mouse embryo. However, they are appropriate only for short-term experiments because of their susceptibility to dilution. Studies on cell lineage and commitments have therefore depended mainly on exploiting genes as markers by combining cells from embryos that differ in genotype at particular loci. Tissue recombination and transplantation experiments using such indelible intrinsic markers have enabled the fate of different cell populations in the blastocyst to be determined with reasonable precision. The trophectoderm and inner cell mass (i.c.m.) give rise to distinct complementary groups of tissues in the later conceptus, as do the primitive endodermal and primitive ectodermal components of the more mature i.c.m. When cloned by blastocyst injection, single i.c.m. cells colonize only those parts of host conceptuses that are derived from their tissue of origin. Thus, while clonal descendants of early i.c.m. cells can contribute to all tissues other than those of trophectodermal origin, primitive endodermal and primitive ectodermal clones are restricted, respectively, to the extraembryonic endoderm versus all i.c.m. derivatives except the extraembryonic endoderm. Interestingly, individual primitive ectoderm cells can include both germ cells and somatic cells among their mitotic descendants. By using the genetically determined presence versus absence of cytoplasmic malic enzyme activity as a cell marker, the deployment of clones has been made visible in situ in whole-mount preparations of extraembryonic membranes. Very little mixing of donor and host cells was seen in either the endoderm of the visceral yolk sac or the mesodermal and ectodermal layers of the amnion. In contrast, mosaicism in the parietal endoderm was so fine grained that, in all except 1 of 15 fields from several specimens that were analysed, the arrangement of donor and host cells did not differ significantly from that expected on the basis of their random association.

Germline and Pluripotent Stem Cells

“…Further on in development, the mammalian embryo gives rise to a blastocyst, a structure with an outer group of trophectoderm cells destined to form the placenta, and an inner group of cells that will give rise to the entire fetus and, eventually, a new organism (Gardner 1985). These inner cells will therefore differentiate into all the known 200 or so specialized somatic cells found in adults and they are, therefore, referred to as "pluripotent."…”

The hypoblast (visceral endoderm): an evo-devo perspective

“…These include the observations of Enders and colleagues in a number of different mammalian species (Enders et al, 1990;Enders et al, 1986;King and Enders, 1970), explant studies in rat Svajger and Levak-Svajger, 1974), transplantation experiments by Gardner and colleagues in the mouse (Gardner, 1982;Gardner, 1984;Gardner, 1985;Gardner and Papaioannou, 1975;Gardner and Rossant, 1979), and eventually the clonal analyses of VE cells (Lawson et al, 1986;Lawson and Pedersen, 1987) and other important observations (Kadokawa et al, 1987;Lamers et al, 1987; for reviews, see Rivera-Perez et al, 2003;Rossant and Tam, 2009). …”

Biallelic expression of imprinted genes in the mouse germ line: implications for erasure, establishment, and mechanisms of genomic imprinting.

“…Szab6 and J.R. Mann, unpubl.). This is the germ layer from which PGCs are derived (McLaren 1983;Gardner 1985;Lawson and Hage 1994).…”