This “live image” shows
that cells of a mouse
embryo are connected by a
stable bridge which direct
the growth of microtubules,
a cell’s transport railways.
Within every embryo are the raw
ingredients of life. A new lab at Penn
points the camera lens to watch how
these ingredients come together in
early mammalian development.
By Lauren Ingeno
A single cell forms when sperm and egg meet. It multiplies, and those cells rearrange and transform, giving rise to some 37 trillion cells in the human body. It’s a familiar story. Yet, the mysteries of the earliest stages of mammalian reproduction—how these cells resolve their fate, shape, and position—remain largely unsolved.
In the lab of Nicolas Plachta, PhD, who joined the Department of Cell and Developmental Biology at the Perelman School of Medicine this fall from A*STAR in Singapore, cell biologists are watching these processes play out in real time. With the help of laser scanning microscopy, Plachta is creating vivid time-lapse images of live mouse embryos—and, along the way, discovering new details about how life begins.
Investigating how cells adopt their specific positions within a tissue or organ is key to understanding how a body forms. In Plachta’s lab, researchers combine live imaging with computer segmentation methods to reveal how cells segregate to form the pluripotent inner mass in living mouse embryo. This inner mass acts as precursor for most cells in our body and is one of the first structures to form during development.
Q: What is live cell imaging, and why did you decide to use this approach to study developmental biology?
In the first seven days of mammalian development, the embryo divides into two, four, eight, and then into a cluster of cells called the blastocyst. During that division, the embryo is just floating around the uterus, and if you take it out, it does the exact same thing in a dish that it would do in the body. And then you can use microscopy to image it. Watching these stages in a model organism is accessible, it’s easy to manipulate, and it’s simple. All the factors the embryo needs for those early divisions are already inside the cells. That means you can do a lot of basic cell biology, in a real group of mammalian cells, in real time.
When I arrived at Caltech for my post-doctoral research, no one was imaging embryonic development in mice, even though the mouse is one of the most important mammalian models for research in biology. People thought that a mouse embryo would be hard to image because it would be too sensitive and easy to damage with the microscope. So, sort of by default, to stay away from the crowds, I started to play around with mouse embryos and microscopes, and I found it’s not so difficult.
The transformation from morula (a collection of around 30 cells) to blastocyst is a defining event of mouse and human development. Preceding the blastocyst stage, an F-actin ring forms in the early morula at the apical pole of the cells of the embryo. Unlike stereotypical actin rings that constrict, Plachta’s in vivo imaging reveals that these rings expand all the way to the cell-cell junctions, where they couple and trigger a zippering process that seals the embryo to enable blastocyst formation.
Q: What questions in particular is your lab trying to answer?
We are not a hypothesis-driven lab. We have two or three microscopes filming embryos overnight, and the next morning, we have 10 to 20 movies of different embryos to watch and analyze and see what happened in each case. And every night, we’re imaging completely different types of components inside the cell or different types of cells inside the embryo. We have different projects: Some are more focused on dynamics in the cell nucleus, which later controls the behavior of the cells; or on the cytoskeleton. Others focus on mechanical interactions—how the cells veer and pull against each other in the embryo. Most of what we find are these random discoveries about how the embryo does what it does.
Featured on the cover of the March 2016 issue of Cell is a four-cell mouse embryo imaged by Plachta and colleagues: Individual cells in the early mouse embryo may look similar, but these images show that cells in which Sox2 engages in more specific DNA interactions contribute more progeny to the pluripotent inner cell mass.
Q: What can studying live mouse embryos teach about human reproduction?
What we do is purely basic research—it’s just to know more about how this thing works, how you build an embryo. However, there is biomedical relevance. We may be able to study what happens when these mechanisms fail. With in vitro fertilization (IVF), for instance, many seemingly healthy embryos that are transferred to the uterus fail to implant because of abnormalities that doctors do not fully understand, nor can currently detect. With imaging, we can find out which structures and processes are essential for the embryo to form normally during the pre-implantation stage. Down the line, if we could use some non-invasive ways to visualize these structures in human embryos, we could find new ways to screen which ones would be the most medically appropriate for implantation and would have the best chance of developing into a baby. Right now, we are trying to figure out as much as we can about how an early mammalian embryo is put together at the earliest stage of life.