During both gastrulation and neurulation certain tissue regions that start out as epithelial – portions of the blastula wall and the neural plate – undergo changes in physical state whereby their cells detach from one another and become more loosely associated. Tissues consisting of loosely packed cells are referred to as “mesenchymal” tissues, or mesenchymes. Such tissues are susceptible to a range of physical processes not seen in the epithelioid and epithelial tissues discussed in Chapters 4 and 5. In this chapter we will focus on the physics of these mesenchymal tissues.
We encountered this kind of tissue when we considered the first phase of gastrulation in the sea urchin, the formation of the primary mesenchyme, in which a population of cells separate from the vegetal plate, and from one another (except for residual attachments by processes called filopodia), and ingress into the blastocoel (Fig. 5.7). The secondary mesenchyme forms later, at the tip of the archenteron, and these newly differentiated cells help the tube-like archenteron to elongate by sending their own filopodia to sites on the inner surface of the blastocoel wall. In other forms of gastrulation, such as that occurring in birds and mammals, there is no distinction between primary and secondary mesenchyme: cells originating in the epiblast, the upper layer of the pregastrula, ingress through a pit (the “blastopore”) and the “primitive streak” that forms behind it as the mound of tissue (“Hensen's node”) surrounding the blastopore moves anteriorly along the embryo's surface (Fig. 6.1).