Spatiotemporal Pattern Formation in Early Development: A Review of Primitive Streak Formation and Somitogenesis

Abstract

The basic body plan of a number of vertebrates results from two processes that occur early in the development of the blastoderm: large scale rearrangements of tissue via a process called gastrulation, and axial subdivision of tissue in a process called somitogenesis. The first step of gastrulation in avians is formation of the primitive streak, which marks the first clear manifestation of the anterior-posterior axis. Cell movements that occur through the streak ultimately convert the single layeredblastoderm into a trilaminar blastoderm comprising prospective endodermal, mesodermal and ectodermal tissue. During streak formation a group of cells moves anteriorly as a coherent column from the posterior end of the blastoderm, and as it proceeds other cells stream over the lateral edges of the furrow left behind. The anterior end of the streak is a specialized structure called Hensen’s node, which serves as an organizing center for later axis formation and determination of the left-right asymmetry of the body. Soon after the primitive streak forms, Hensen’s node regresses towards the tail, leaving the notochord and a pair of segmental plates parallel to the primitive streak in its wake. The posterior end of the segmental plate moves down the cranio-caudal axis with the node, as more cells are added to it by cell division within the plate and by cells entering from the primitive streak. A pair of somites forms from the anterior ends of the two plates at regular intervals. Despite the fact that much is known about the basic biological processes, the mechanisms that underlie the formation of the primitive streak and somitogenesis are still unknown, and elucidating them is one of the major unsolved problems in developmental biology. Mathematical modelling has been a useful tool in this process, as it provides a framework in which to study the outcome of proposed interactions and can make experimentally testable predictions. In this paper we outline the biological background of these processes and reviewexisting models of them.