Chick Amniogenisis and Actin

Chick amniogenesis has been described, but less is known of the
forces that may drive the development and closure of the amnion.
Actin cable formation is believed to be one of the mechanisms
involved to explain the fusion of tissue, such as dorsal closure in
the Drosophila embryo. This study examined the possible involvement
of an actin cable during closure of the amniotic sac. Chick
amnion development was studied from Stage 10 through to Stage
18 when amnion closure is completed. This study was carried out
under the provisions of the Scientific Procedures on Animal (1986)
Act (United Kingdom) from which embryonic forms are exempt
until the halfway point of gestation, for chick embryos this is
10.5 days. Staining of the developing amnion with FITC-phalloidin
indicated the presence of an actin cable during amniogenesis and
was evident at the closure of the amnion at Stage 17/18. The actin
cable at various stages of amnion development was similar to that
described at the leading edge in embryonic epithelial wound healing
and embryonic tissue fusion. Transmission electron microscopy
(TEM) at Stage 14 of the developing amnion, treated with a fixative
to preserve actin, revealed linearly arranged microfilaments
in the elongated cells at the leading edge adjacent to either side
of a ‘nodule’ of cells representing the point of midline fusion. These
microfilaments were approximately 6 nm in diameter. Filopodia
and lamellopodia have been described in epidermal embryonic
wound healing and other examples of embryonic epithelial fusion.
In this study lamellopodia were absent and filopodia were almost
absent at the leading edge cells. TEM showed apoptotic cells and
a mesh of cytoplasmic actin filaments dispersed throughout the
accumulated cells of the nodule. This study supports the hypothesis
that an actin cable is involved in amniogenesis, although it reveals
differences in the ultrastructural morphology from that reported
in other embryonic tissue movements.