|
|
|
 |
|
|
|
Gastrulation: Shaping the Embryo
|
|
| |
by José Vázquez
New York University
New York, New York
|
|
| |
The History of Gastrulation
The process by which cells are translocated to new positions in the embryo, producing the three primary germ layers, has been an object of intense research for more than 100 years. Developmental biologists are no longer considered embryologists, and the focus has shifted to cellular and molecular biology. The history of gastrulation begins in the late 1800s when Haeckel first used the term. After the process was named, it took almost a century to turn gastrulation into a coherent picture (Brauckman and Gilbert, 2004). Nowadays, gastrulation implies massive cell movements that reorganize cells within the embryo.
The hallmark of gastrulation in some organisms is the formation of the blastopore. The blastopore is the site at which endoderm and/or mesoderm formation begins. The physical forces involved in molding the gastrula are currently being revisited, and much research remains to be done. More recently, gastrulation is being seen as a composite of processes highly regulated by gene expression. In addition, the evolutionary significance of gastrulation is evident when similarities are observed across animal phyla.
The cellular events of gastrulation have been studied since the development of molecular biology and biochemical techniques that allow the manipulation of molecules regulating morphogenesis. In addition, the study of gastrulation requires the study of biomechanics in order to understand the forces that -- at the cellular, molecular, and tissue levels -- lead to morphogenesis. The three-dimensional aspects of invagination challenge our notions of geometry and architectural design. What is clear right now is the careful orchestration of morphogenetic movements that incorporate genetic and cellular signals, and the decoding of these signals into a force-generating event that results in cell and tissue modeling.
Chemotaxis and Unanswered Questions
The coordination of cell movements during gastrulation involves chemotaxis. An impressive number of chemoattractants has been identified, including cAMP and many growth factors. The focus on cell-substrate interactions has been gaining momentum and is an area of intensive focus in prominent research labs around the world. The evolution of gastrulation points toward the development of a complex body plan. Prior to the use of DNA sequence comparisons to create phylogenies, embryonic development was a reliable tool. Recent techniques involve the analysis of superfamilies of genes (such as cadherin genes) and the resulting proteins involved in the sorting behavior observed during gastrulation. Developmental biologists have been surprised by the consistency of gene expression of the same family in unrelated organisms such as Drosophila and humans. Future studies may elucidate a more thorough understanding of molecular and developmental mechanisms across multiple species.
Defects may take place during gastrulation and result in deformities. One of the most clinically relevant defects occurs during the early patterning of the left/right (LR) axis. The internal organs in vertebrates exhibit an asymmetric placement (e.g. heart, liver, spleen, and gut). Errors of LR patterning during embryogenesis are relevant to several human birth defects (Adams and Levin, 2004). The developmental event that distinguishes left from right suggests asymmetric gene expression, and a large number of proteins have been implicated in this process. Obviously, this is just one of the puzzles that developmental biologists face in their quest to understand cell migration and cell determination.
Students in AP Biology need to become aware that there is a myriad of questions that remain unanswered in developmental biology. Gastrulation is a versatile process that combines embryology, cellular events, molecular biology, and evolution to allow the establishment of the massive complexity that an embryo represents.
Further Reading
Adams, D.S, and M. Levin. 2004. Early Patterning of the Left/Right Axis. In Stern, C., ed. Gastrulation: From Cells to Embryo. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press.
Brauckmann, S., and S.F. Gilbert. 2004. Sucking in the Gut: A Brief History of Early Studies on Gastrulation. In Stern, C., ed. Gastrulation: From Cells to Embryo. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press.
A series of movies depicting various events of gastrulation in several species such as chick, C. elegans, Drosphila, Xenopus, and zebrafish:
 Gastrulation: From Cells to Embryo
José Vázquez joined the College Board's Content Development Group in January 2005. He completed his undergraduate work in biology at the University of Puerto Rico and his graduate work in cell and developmental biology at the University of Pennsylvania. José taught AP Biology for three years in Puerto Rico and another three years at Northwestern University's Center for Talent Development in Evanston, Illinois. In addition to being an AP Reader since 1998, he is the classroom technology reviews editor for American Biology Teacher.
|
|
|
|
|
|
