Establishing Basic Tissue Types In Vertebrates

In contrast to the fruit fly, where the mechanisms employed by the mother to polarize the egg and initiate patterning of the primary body axes are being rapidly uncovered, the early steps in patterning of the vertebrate body axes are less well defined. It is not clear that great generalities can be made about vertebrate embryonic development since distinct maternal mechanisms or external conditions appear to guide early axis patterning in different vertebrate species. In frogs, which are discussed below in more detail, maternal information does play a role in establishing the position of the primary body axes of the embryo. In some fish embryos, however, gravity seems to be the primary early polarizing agent. Furthermore, in other vertebrates, including mammals, there is no evidence for axis patterning of any kind at the blastoderm stage because it is possible to dissociate cells from two different mouse embryos at this stage, mix them together and have these harshly treated balls of cells go on to develop into normal mice. The limitations in our current state of knowledge of maternally controlled vertebrate embryonic development derive primarily from the impracticality of conducting large-scale genetic screens for maternally acting mutations in vertebrate systems. In contrast, such genetic screens have been possible in fruit flies due to their short generation time, and have served as cornerstones for analyzing maternal mechanisms in flies. As shown below, significantly more is known about how various vertebrate embryos interpret maternally provided positional information than about how the egg is initially polarized by the mother or other external factors.

The vertebrate egg that is best understood with respect to early axis formation is the frog oocyte (i.e., unfertilized egg). The frog oocyte is a visibly polarized structure consisting of two differently pigmented hemispheres. The darkly pigmented half of the egg is called the animal hemisphere and the nonpigmented half is referred to as the vegetal hemisphere. Although the difference in pigmentation does not appear to play any role itself in defining the tissues deriving from these two halves, it does reflect a fundamental asymmetry in the egg that will ultimately define the anterior-posterior (A/P) axis of the embryo. Despite what the names seem to imply, the animal and vegetal portions of the embryo do not give rise to a frog versus a cucumber, but rather to different tissue layers within the frog embryo (Fig. 5.1). Most of the animal hemisphere becomes ectoderm (i.e., the outer layer) giving rise to skin and nervous system, whereas the vegetal hemisphere primarily becomes endoderm (i.e., inner layer), which generates the gut. The mesoderm (i.e., middle layer) derives from the equatorial or "marginal" zone of the animal hemisphere and gives rise to a structure called the notochord, which is a rod-like structure that serves as a rigid support (like a backbone), as well as heart, muscle, and blood. Thus, the three layers of the embryo formed during gastrulation correspond to basic tissue types. These primary tissue layers derive from a series

Stratification of the frog embryo into the three basic tissue layers depends on the activity of a transcription factor called VegT, which functions both to define endodermal cell fates in the vegetal hemisphere and to induce mesodermal fates in adjacent animal hemisphere cells (Fig. 5.1). VegT is expressed in the vegetal cells, where it serves a dual function to promote endoderm development and to suppress mesoderm formation. In addition to controlling the expression of genes relevant to the development of vegetal endodermal cells, VegT activates expression of secreted signals, which are related in structure to the fly morphogen Dpp. These secreted signals activated by VegT function to promote mesoderm development. As a result of its combined activities within vegetal cells, VegT specifies endoderm by activating expression of genes required for endoderm development while preventing these cells from responding to the mesoderm-inducing signals also produced in these cells.

A critical consequence of VegT activation of mesoderm-inducing signals is induction of mesodermal cell fates in adjacent animal hemisphere cells. These animal hemisphere cells are close enough to the VegT-expressing vegetal hemisphere cells to receive the mesoderm-induc-ing signals activated by VegT. The future marginal cells do not express VegT, which also functions to prevent a response to these mesoderm-inducing factors. In many respects, the dual activity of VegT in defining endoderm locally and inducing mesoderm in adjacent cells parallels that of engrailed during fly wing development, in specifying posterior compartment cell fates and inducing formation of central organizer in adjacent anterior compartment cells (see Chapter 4).

of adjacent domains lying along the animal-vegetal axis of the earlier blastoderm-stage embryo.

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