Left: A L. variegatus mesenchyme blastula, viewed from the side, showing ingressing primary, or skeletogenic, mesenchyme cells (PMCs). Right: A L. variegatus mesenchyme blastula processed for scanning electron microscopy and fractured through the vegetal plate to show the process of ingression (photograph courtesy of Dr. John Morrill, Univ. of South Florida).
Primary mesenchyme cells undergo ingression at the onset of gastrulation. Although the mechanisms of ingression are not fully understood, several events are known to accompany ingression. The best characterized changes in PMCs involve changes in their adhesive properties. Go to the next page to learn about classic experiments that were done to elucidate these changes...