OBJECTIVES: We currently understand very
little about the cell fate decisions in early placental development. Whether
placental villi contain trophoblast stem cells has not yet been established.
When stained with Hoescht and analysed by flow cytometry, many stem cells form
a ‘side-population’ as a result of their ability to rapidly efflux the stain.
We used this trait to examine whether trophoblast stem cells reside in first
trimester villi and to isolate these cells to use as a model of human
trophoblast differentiation. METHODS:
Villi
from first trimester placentae were dissected from the membranes, digested
overnight in trypsin, and harvested by repeated washing with PBS. Cells were
stained with Hoescht-33342 and Propidium Iodide. Live Hoescht side-populations
were isolated using a Fluorescence Activated Cell Sorter. Expression
of cytokeratin, vimentin, HLA-G and FGFR4 were determined by
immunohistochemistry. RESULTS:
94.7%
(±2.3%SE, n=3) of live cells obtained from villi
were cytokeratin positive. 99.5% of live
cells obtained from villi were HLA-G negative (n=4), indicating that these
cells were primarily cytotrophoblasts. 1.04%
(±0.3SE, range 0.1-3.9%, n=13) of these
cells formed a Hoescht side-population. There was no effect of gestational age
on the proportion of cells that formed a side-population (p=0.38, n=13). 98.4%
(±0.2% SE, n=3) of side-population cells
were cytokeratin positive and vimentin negative, confirming they were
trophoblasts. Hoescht side-population cells express FGFR4. CONCLUSION:
We
have isolated a trophoblast population from first trimester villi that form a
Hoescht side-population characteristic of stem cells. Side-population cells expressed FGFR4, which
is characteristic of adult stem cells, but this receptor is ubiquitously
expressed in placenta, highlighting that trophoblast stem cells may have a
phenotype that lies between embryonic and adult stem cell populations.Future characterisation trophoblast
side-population cells will determine whether they express key stem cell markers
and can differentiate into syncytiotrophoblast and extravillous trophoblasts.