recap

43.4 recap

Gastrulation sets up tissue interactions that initiate organogenesis. Neurulation is initiated by organizer mesoderm that forms the notochord. Neurulation results in the formation of the central nervous system, and associated neural crest cells form the peripheral nerves. Suites of Hox genes are expressed sequentially in vertebrates, but in overlapping patterns providing a variety of signaling molecule patterns that govern differentiation on the anterior–posterior axis.

learning outcomes

You should be able to:

  • Describe how nervous system development begins with neurulation.

  • Describe the formation of somites and their role in development.

  • Explain how genetic and environmental factors can influence neural tube formation.

  • Explain how a limited number of Hox genes can organize complex anterior−posterior patterning of the vertebrate embryo.

Question 1

How does the ectoderm overlying the notochord respond to chordamesoderm signals to form a neural tube as well as neural crest cells?

In response to signaling factors such as Chordin and Noggin, the ectoderm over the notochord thickens on either side of the midline to form ridges aligned on an anterior-posterior axis. These ridges grow until they meet on the midline, forming the neural tube. The most lateral cells of these ridges break free to form neural crest cells, while still more lateral ectoderm comes together and merges to cover the neural tube.

Question 2

What might be a consequence of a failure in somite formation in the embryo?

A somite is a segmental and bilateral block of mesodermal cells that gives rise to vertebrae, ribs, trunk muscles, and limbs.

Question 3

How do Hox genes generate a diversity of signals sufficient to control differentiation of tissues and organs on the anterior−posterior axis?

Vertebrates have groups of Hox genes arranged in linear order on multiple chromosomes. These genes are expressed along the anterior-posterior axis in the same sequence as they occur on the chromosomes. As a result, each anterior-posterior region of the embryo receives different combinations of Hox gene products in different amounts, producing a large combinatorial mix of signaling molecules.

You have seen how the basic structure of the developing embryo arises, through the establishment of the anterior–posterior and dorsal–ventral axes, the formation of the neural tube, and the emergence of a segmented body plan. In the next section we will examine the developmental events that result in the formation of structures that support the developing embryo: the extraembryonic membranes and the placenta.