Our internal body organs and tissues, such as muscles, heart, kidneys and backbone, are made from a special cell layer called the mesoderm. This forms in the 2-week-old embryo from a unique structure called the Primitive Streak.

At this time the embryo is made of two layers of cells. To make the mesoderm, cells crawl from the top layer through the primitive streak to form a new, third, layer – which is a bit like the filling in a sandwich. Film Cedit: Kees Weijer, University of Dundee Images © Kate Storey

The inward movement of cells through the primitive streak is called gastrulation. "It is not birth, marriage, or death, but gastrulation which is truly the important event in your life" - quote from famous Developmental Biologist Lewis Wolpert, spoken while convincing a clinician of the importance of studying early development. (From his book 'The triumph of the embryo' Oxford University Press, 1993; now Dover 2008 www.doverpublications.com) A key gene that controls gastrulation is called Snail. This gene ultimately directs the cellular machinery that controls cell movement. It causes cells packed tightly together in an epithelial sheet, to loose contact with each other and crawl through the primitive streak and move away as individual mesodermal cells. Secreted molecules, such as Fibroblast growth factors, also direct the movement of these cells.

Find out more: Overview of human embryonic development leading up to primitive streak formation Key labs that work on the Primitive Streak: Claudio Stern, University College London Guojun Sheng, Riken Institute, Japan Kees Weijer, University of Dundee Selected research papers: A research paper investigating the mechanisms that control primitive streak formation Review of the mechanisms underlying formation of the primitive streak Review of recent research into the role of the primitive streak in embryonic development Regulation of primitive streak position and twinning Recapitulating early embryonic structures using human ES cells