Question

Paracrine signaling mechanisms and transcription factors have been seen to play key roles in several different developmental processes throughout both the invertebrate and vertebrate animal kingdom.

a. Pick ONE signaling mechanism. Using two different developmental stages/processes, within ONE organism, and describe HOW it is used in each.

Pick ONE signaling mechanism. Using two different model organisms studied this semester, Compare and Contrast how that ONE mechanism is used at the SAME developmental stage/process in each.

Answer 1

Table 1.

Key signaling pathways that orchestrate development—receptors, ligands, transcription factors, and outputs are shown for each

The response to signaling-pathway activation is usually complex and involves the regulation of many processes, such as control of cell fate, apoptosis, cell proliferation, cytoskeletal reorganization, cell polarity, adhesion, and cell migration. Importantly, each pathway does not specifically regulate a single biological process but can elicit diverse effects, depending on the state of the cell at the time the pathway is activated. Furthermore, because few pathways exist, there are no unique signals for induction of each cell type. Instead, the response of a given cell to a signal depends on its amplitude, duration, interactions between pathways, and integration of transcription factor effectors at promoters and enhancers of target genes. It may also be predetermined by the set of transcription factors expressed in the cell at the time the signal is received.

Here, we use specific examples, mostly taken from Drosophila, to illustrate general principles and mechanisms by which signaling pathways operate in development to specify cell fates. Thus, this is not a comprehensive review of the structures and roles of all the pathways that have been implicated in developmental processes. A number of excellent reviews elsewhere describe in detail the roles of individual pathways in development.

Figure 1.

Patterning of the early Drosophila embryo. (A) Anterior–posterior patterning and segmentation of the embryo is initiated by maternally deposited gene products that regulate the expression of gap genes. Gap genes in turn control the expression ...

Hierarchies of transcription factor expression that progressively dictate distinct cell fates are common at later developmental stages in a variety of tissues. For example, in response to Dorsal signaling, Twist is activated to define the mesoderm and in turn activates MEF2 and Tinman in different cells to induce the skeletal muscle and cardiac muscle fates, respectively (Fig. 2A) (Sandmann et al. 2007). Another example of progressive specification owing to hierarchical expression of transcription factors and activity of signaling pathways is the specification of Drosophila blood cell types (Jung et al. 2005). In the Drosophila hemocyte (blood cell) lineage the blood cell precursors are specified in the embryo by expression of the transcription factors Serpent (SRP) and Odd paired (ODD) and progressively express Hemese (HE) and activate the RTK PDGF/VEGF receptor (PVR), as well as the cytokine receptor Dome, to finally reach the prohemocyte stage. Then, cell-type-specific transcription factors are activated in response to signaling by the Notch, PVR or Notch and JAK/STAT pathways, which specify the different populations of mature hemocytes, namely, the plasmatocytes, the crystal cells, and lamellocytes, respectively, that are destined to perform specialized functions (Fig. 2B)

Figure 2.