After more than 30 years studying embryo development, Claudio Stern has finally realized his dream: to see exactly how cells behave during one of the earliest periods of embryo formation. Technology allowing the viewing of individual cells within an intact organism has only recently become available, and Stern, a developmental biologist at University College London, was eager to try it out.

Gastrulation is a period in early embryonic development during which massive cell migration leads one layer of cells to develop into three. These will go on to form different organs. Chick embryos — Stern's chosen animal model — begin this process as a flat disc of cells. A crease called the primitive streak forms across the disc and acts as a passageway, through which cells flow inside the embryo, forming two new layers.

Stern first began working as an embryologist in the 1970s, when he was a graduate student in Brian Goodwin's lab at Sussex University, UK. Goodwin offered him an array of projects: salamander limb regeneration, slime-mould migration, moth wing patterning, regeneration in the unicellular plant Acetabularia and “the chick.”

“I asked him, 'What about the chick?' and he said, 'Anything you like',” recalls Stern, who chose to study gastrulation. Goodwin gave him a Second World War 16-millimetre Vinten cine camera to use for time-lapse filming.

This had been done before. German anatomist Ludwig Gräper was among the first to study cell movements during gastrulation in living chick embryos. In 1926, he made three-dimensional time-lapse movies of embryos. Although Gräper couldn't see individual cells, he could see their global movements, especially the shuffling of the cells as they made their way to the embryo's centre to create the primitive streak. He dubbed these 'Polonaise movements', after a popular dance in which dancers move along opposite sides of the room before turning inwards at the end, whereupon they meet and form pairs, then move down the centre of the room.

“It has taken more than 80 years to see details of the cells during these movements,” says Stern. His group now uses the recently developed multi-photon microscope, which can penetrate deep into tissues and obtain high-resolution three-dimensional images. Because the chick embryo is fairly transluscent and flat, imaging is easier than in organisms such as the mouse, whose cup-shaped embryo and maternal tissues obscure the view.

To see the movements of individual cells, postdoctoral fellow Octavian Voiculescu used single lines to connect groups of three cells into triangles on each video frame. He then compared the movement of triangles in different regions of the embryo. From this, he discovered that embryonic epithelial cells push in between their neighbours — similarly to the Polonaise dancers who pair up as they weave into the middle of the room — before the primitive streak starts to form (see page 1049 and http://tinyurl.com/275aur).

“At low power, the movements look organized. At high power, it looks like rush hour, with people fighting for a door,” says Stern. “It is only by looking at the relationships between cells — the triangles — that the order becomes apparent.”

The team also showed that a major signalling pathway, known as the Wnt planar-cell-polarity pathway, guides these movements. The group now hopes to analyse cell movements from the side, perpendicular to this current view, to try to reveal how these movements are possible, given that the cells are held rigidly together by tight junctions.