Early heart development involves the transformation of endocardial
cells in the atrioventricular canal and outflow tract
regions into mesenchymal cells, a process called endocardial
mesenchymal transformation (EMT). This process is initiated
by factors, termed the particulate matrix, that are secreted
by the myocardium. The particulate matrix causes a
subset of endocardial cells to hypertrophy, lose their cell-cell
contacts, form migratory processes, transform into mesenchymal
cells, and migrate into the underlying endocardial
cushions. The particulate matrix can be extracted using
EDTA and the EDTA extract can initiate the EMT process. Earlier
reports from our laboratory have shown that the particulate
matrix can be detected with the hLAMP-1 antibody in
immunostaining and Western blot analysis. In addition, similar
proteins have been isolated from the growth media of
stage 15–16 chick embryo myocardial cultures (MyoCM).
Since other investigators have identified a possible role for
bone morphogenetic protein (BMP)-2 during the EMT pro-cess in the heart, we asked whether BMP-2 is a part of the
chick hLAMP-1-positive particulate matrix. To answer this
question, we double stained stage 15–16 chick embryo sections
with hLAMP-1 and BMP-2 antibodies. We found that
BMP-2 signals do not colocalize with hLAMP-1-stained particles.
In addition, using immunoprecipitation-Western blot
analysis, we demonstrated no association of BMP-2 with the
hLAMP-1-bound fraction of the EDTA extract or MyoCM. Our
results indicate that BMP-2 is not a component of the hLAMP-
1-positive particulate matrix in the chick.