DEVELOPMENTAL BIOLOGY CLASS NOTES
Morphogenesis: Gastrulation and Neurulation
Reading : Chp 10: 291-302, 337-342; Chp 12: 373-380
Be able to:
Define the terms blastula, blastocoel, blastopore, archenteron, gastrula.
Explain the difference between epithelial and mesenchymal cells, and why transitions between the two
cell types are a fundamental part of development.
Explain what gastrulation, in general, accomplishes for an embryo.
Predict what happens in the absence of gastrulation (with regard to cell type specification and
Compare gastrulation movements in different species and hypothesize why the differences (and
Compare the movements of gastrulation to the movements (and the purpose of) neurulation.
Gastrulation reorganizes the blastula into a three-layered embryo with a recognizable body plan. In contrast
to cleavage, the primary mechanisms of gastrulation involve cell movements; cell divisions occur but are
less important to the morphogenetic process. In this class we will discuss aspects of gastrulation in some
favorite model organisms; don’t worry about details, but rather focus on what gastrulation accomplishes for
the embryo. Central to this topic are the characteristics of
, and the
, which we will encounter examples of throughout the course. After an
embryo gastrulates, the next set of morphogenetic movements are called neurulation.
This is the process of
forming a hollow tube of neural cells, and segregating this tube of cells from the overlying epithelial cells.
Obviously, both of these processes are crucial for setting up the normal structure of an embryo, and the two
processes also share some features, discussed below.
Epithelia and mesenchyme
There are two basic states of cell organization and behavior termed epithelial and mesenchymal. Both are
found during development in all three germ layers.
Many developmental events involve conversion of one state to the other.
Regular in shape, arranged in layer or sheet, often only one cell thick, called an epithelium. Major
feature is polarity.
Bounded on one side (basal surface) by a basal lamina, thin mat of ECM secreted by the cells. Further
from cells may be a thicker basement membrane.
Opposite side is the apical surface; differs for different epithelia. Often secretory; small protrusions
called microvilli increase surface area. Different transport proteins in apical and basal plasma membranes
allow transfer of specific molecules across an epithelium.
Three general types of junctions hold epithelial cells together, allowing them to communicate: 1) Tight
junctions prevent leakage between cells. 2) Gap junctions allow passage of small molecules (< 1 kD). 3)
Other structural junctions include transmembrane linker proteins, e.g. cadherins.