How to Turn an Embryo Inside Out

Abstract

We all start life as a small ball of cells. These cells proliferate and organize into tissues, which in turn grow and fold into the complex form that defines a human being. The process that leads from a formless ball to a functional organism—known as morphogenesis—is an active area of research in biology. For physicists, it presents an exciting challenge because a growing organism can be thought of as a type of “active” matter that is capable of reshaping itself [1, 2]. Physics models are therefore increasingly being used to investigate morphogenetic events in plants [3] and animals [4]. Now, researchers at the University of Cambridge in the UK have compared images of a growing algal embryo with a physics-based morphogenesis model, providing one of the few experimental tests of such models [5]. Stephanie Hohn and her colleagues imaged embryos of the algal species Volvox globator as they underwent invagination—a fundamental event in morphogenesis in which a sheet of cells folds inside, triggering the formation of an internal cavity. The researchers modeled the organism as a spherical elastic shell that actively bends and contracts at certain segments, and they found the necessary balance between these motions for inversion to occur. Volvox are freshwater algae generally found in ponds [6]. An adult Volvox consists of a single layer of a few thousand cells embedded within a gelatinous extracellular matrix that encloses a spherical cavity. Each cell has two flagella that face outwards and beat in the water, allowing the spheroid to swim towards light and nutrients. Embryo Volvox develop within a parent spheroid (Fig. 1). And, unlike adults, their flagella are oriented inwards. To form an adult, the embryonic sheet of cells must therefore turn itself inside out. This stunning process of inversion varies from species to species [7]. In V. globator, one half of the spherical sheet of cells shrinks in radius and invaginates, initiating FIG. 1: Photograph of an adult Volvox. The small green spheres are embryos. (S. Hohn et al.[5])