Trachealess Plant Example

Figure 9-10. Branching patterns are important and widespread in life. (A) Branching of the major nerves in humans; (B) branching of the venous drainage system in humans. There are corresponding branching structures in the arteries and lymph ducts. From Andreas Vesal-ius' classical drawings of 1543, that helped introduced the modern era in anatomic studies (Vesalius 1543).

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Figure 9-10. Continued

That the result has a fractal dimension (a numerical way to characterize the richness of the branching pattern) would be an incidental finding rather than something built into the process: there seems to be nothing about fractal geometry that is physiologically necessary per se.

Along an insect body, tracheal sacs form and invaginate to begin a branched tracheal system that carries oxygen to the internal tissues. The openings are known as spiracles. Once located along the embryo, each sac generates a structure of six primary, about 25 secondary, and hundreds of tertiary branches. There is considerable variation among species, but in the "archetypal" condition (i.e., in species like Drosophila where this has been specifically studied) the entire tracheal structure is formed by movement and morphological changes in an initial population of cells rather than by the cell growth by which mammalian bronchial trees develop. It is in this sense an autonomous developmental unit. It is adaptable, however, in that eventually the smaller branches grow toward signals produced by oxygen-deprived cells and connect with branches from other trees.

In Drosophila, spiracles initiate in 20 sites of expression of the Trachealess TF gene along the midgestation fly embryo, involving about 80 cells each (what locates the spots is not known) (Davidson 2001; Metzger and Krasnow 1999). Trachealess and Tango protein (bHLH class TFs) expression presumably regulates downstream genes necessary for tracheal sac development. A cascade of expression of the same genes occurs in each sac, which develops autonomously, by growing inward, forming secondary and tertiary branches, each of which appears to be a module involving similar genes.

The secreted Branchless (an Fgf homolog) is activated in five additional locations surrounding the initial Trachealess expression zone; expression appears to be delimited by determinants of the overall AP and DV axes relative to the sac (Metzger and Krasnow 1999; Samakovlis et al. 1996; Sutherland et al. 1996). Breathless Fgf Receptor homologs on nearby cells receive the Branchless signal, triggering a cyto-dynamic cascade that guides the migration of cells to their budding locations in each of the five primary branch sites. Meanwhile, a second secreted factor represses Fgf genes to create an inhibition zone surrounding the cells forming the primary branches. Branchless turns off in the cells as they move along the branch but then switches on again in the distal cells as the next round of branching takes place. Cells moving toward the Branchless signal express branch-related genes not found earlier, as well as a second inhibitor that confines the location of the secondary branching. Terminal branching again involves Branchless signaling but also oxygen-sensitive signaling; together, they generate the more variable subsequent branching that is needed to respond to local tissue oxygen requirements. Tens of downstream SF and TF genes that affect the different steps in this process have been identified by mutation analysis (that is, mutations lead to aberrant development).

The process of tracheal ramification is, however, not simply a repeated invocation of the same branching signal (Metzger and Krasnow 1999). At different stages along the tree different genes are expressed, and mutational analysis suggests that different branches within a tree are controlled by specific genes (although a common set of Fgf pathway genes are also expressed). In the latter sense, as with so many similar systems, subsequent branching may be more like a single process analogous to reaction-diffusion processes, a type of mechanism that can generate roughly fractal branching. At least, although there may be some differences along the developing tracheal tree, hundreds of branch-specific gene signals are not required. In having formed from a single primordial set of cells rather than by mitotic growth, the tracheolar system can be viewed as the unfolding of a program somehow latent or prepatterned in those cells.

Homologous genes are expressed in mammalian tracheal and bronchial branching, which goes through 6 to 20 or so generations depending on the size of the animal. The major initial stages (trachea to two primary bronchi, etc.) are essentially the same among individuals and species. Fgf10 and its receptor are involved in triggering branching in the end of a growing tracheal bud, with inhibitory signal (perhaps of Shh) expressed in the center, so that the outer sides grow but the center does not (Figure 9-11). Experiments have indicated that mesenchyme is important in providing patterning cues, but the epithelial layer is also needed. As with flies,

Sprouty Family

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Sac formation tracbealess tango branchless breathless stumps pointed anterior open sprouty hypersprouty blistered

TCF cropped

Figure 9-11. Animal budding and branching patterns. (A) Mammalian lung; (B) a fractallike pattern generated by computer resembles natural budding and branching of this sort; (C) similar branching in the tracheole system of insects; (D) the steps in forming the tra-cheolar branching as in C are shown along with gene expression specific to each. Modified from (Metzger and Krasnow 1999), reprinted with permission.

the lung budding process is basically autonomous (can be achieved in isolated cultures of bud primordium), and the same signaling is reused in subsequent branches. Receptors are expressed more diversely than the cells secreting Fgf, which is expressed in a temporally dynamic pattern in the mesenchyme. Buds grow toward the nearby source region at any given time. As elsewhere in the embryo, Bmps are antagonists of Fgf, and in the lung they appear to inhibit growth and limit branch formation. Mesenchymal differentiation produces supporting structures that are more complex than in insects and involves further gene cascades.

A simple computer-generated fractal pattern is shown in the center of Figure 911. This has been said by many authors (but not those of the paper from which the figure was derived) to represent the kind of branching divergence found in traits like lungs where, for example, such a process generates the maximum surface area packed into a given volume. The figure shows only the first few branching iterations of what would be a space completely packed with ever more nested versions of exactly the same branching (what the term "fractal" refers to). The idea is a good metaphor, perhaps representing the "objective" of a branched lung to pack as much surface into a given volume as possible, but the lung is not exactly fractal nor symmetric. The left and right human lungs have two and three lobes respectively and are more different when seen in cross section (though not easily seen in the format of Figure 9-11), and this is a standard pattern not a chance difference.

Nested Complexity by Budding and Branching

Branching and budding can go together, and feathers provide an interesting and well-studied example (Yu et al. 2002). Feathers have three levels of branching: rachis or main axis to barb, barb to barbule, and barbule to cilia or hooklike structures that hold feathers together. As noted earlier, feathers form by a periodic patterning process that sets up initial buds separated into rows and columns by inhibition zones. Each of the buds then branches internally, then branches again (Figure 9-12), in a nested hierarchy that, unlike tree or lung branching, leads to different structures at each level. The hierarchy of branching provides multiple opportunities for diversity in final structure, which is manifest in the world of (and within single) birds. Not only is each feather differentiated by structure, and by color, but the array of feather types is different on different regions of the bird, and there are different types of feathers patterned within the regions as well, such as down, contour, and flight feathers. Each has its own modified morphology (and the whole complex can be repeated over space).

Key facts in branching morphogenesis are signals that specify (1) the location of the original bud, (2) the location and number of subsequent branches, and (3) the size, shape, and final histological differentiation of the structures at the ends of the branches (Hogan 1999; Metzger and Krasnow 1999). Feathers involve EMI and are also closely related in this respect to nonbranching structures, including hair and teeth and scales before that). It is not a surprise that despite such differences we find developmental friends including Bmp2, Bmp4, Shh, and Noggin (a Bmp inhibitor) involved in feather production (Chuong, Patel et al. 2000; Jiang et al. 1999;

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Figure 9-12. The nested, hierarchical process of feather development. Redrawn with permission from Nature (Yu, Wu et al. 2002) copyright 2002 Macmillan Publishers Ltd.

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Figure 9-12. The nested, hierarchical process of feather development. Redrawn with permission from Nature (Yu, Wu et al. 2002) copyright 2002 Macmillan Publishers Ltd.

Jung, Francis-West et al. 1998; Metzger and Krasnow 1999; Patel et al. 1999; Yu, Wu et al. 2002).

Shared gene use is no surprise, but some aspects of the presumed gene action are interesting. Although Bmp4 inhibits tooth initiation and may be involved in inter-feather inhibition zones, it appears at the initiation sites within feathers. Bmp2 is a Bmp4 antagonist in teeth but has a generating effect in barbule generation. Shh is here not a growth stimulator but instead appears to be involved in apoptosis that removes tissue to make spaces between barbs. We noted above that Shh may have inhibitory effects in bronchial branching. There may be signaling gradient effects in feathers as well as switchlike effects. Overall in these and other systems, a nested, repeated use of the same genes occurs in different and sometimes at least partly opposite roles.

"Exploratory" Branching

Branching development may be "random" or fractal or stereotypical, but it can also be "exploratory" (e.g., Gerhart and Kirschner 1997). NC cells migrate through meso-derm to peripheral locations where, if they get there, they are stimulated to proliferate, aggregate, or differentiate by SFs emitted by overlying ectoderm of the appropriate type. Angiogenic factors secreted by tissues (including tumors) induce differentiation of angioblasts to form new blood vessels. Vascular branching at the local level seems largely random. Endothelial (vessel lining) cells present Fgf and other surface receptors, and chemotactic growth occurs as the cells proliferate to follow concentrations of growth factor ligands produced by oxygen-deprived cells. Tracheogenesis in insects may "seek" local oxygen-deprived cells, but in vertebrates oxygen is provided by the circulation rather than directly by the lungs; this means that vertebrates must supply lung branches with ample vessels.

In plant branching, which has some similar exploratory characteristics, the meristem acts as an organizer, as mentioned above. Central meristem cells express the homeobox TF Wus that maintains meristem status, but induces expression of a secreted protein, Clv3 in adjacent cells. These cells are maintained in undifferenti-ated state by the TF Stm (Shoot meristemless), that represses a cascade of expression of differentiation genes (including TFs called As1 and Knat1) (e.g., Laux 2003; Weigel and Jurgens 2002). As the meristem grows upward these cells are displaced peripherally, away from this repressive signal, lose their Clv3 expression, and become capable of differentiating into primordial cells for a new branch, flower or leaf (as part of the repetitive induction of competent states, the central cells in a flower regain Clv3 expression). There are new genes here, but also some homologs. Zwille/Pinhead in plants are homologous to Piwi and its relatives in animals, and both help maintain stem cell state (Benfey 1999). This brief description is mainly from the Drosophila of the plant world, the mustard relative Arabidopsis, but similar and/or homologous mechanisms are found in other plants, differing, of course, as the plants themselves differ. Even where the mechanisms are not homologous, however, there is nothing new here in that the logic of the processes is similar to those in animals.

Leaves are also internally branched. They provide rich venation for the cells through diverse patterns among species from reticular patterns in broadleaf plants to parallel veins in many grasses. As with lung or other branching, there can be a hierarchy of branching order and branch (vein) size. Reaction-diffusion-like mech anisms seem plausible, perhaps involving transport of signaling factors like auxin, or other interaction gene products (Dengler and Kang 2001). That a quantitative kind of process is involved in leaf shape is suggested by experiments showing that changes just in the timing of expression of a single gene, Knox1, can modify a leaf from a simple to a complex shape (Bharathan et al. 2002).

These various patterns are exploratory in the sense that growth takes place without a predetermined autonomous plan, and without a prepatterning process, and can occur in response to local factors. Indeed, there can be great variation in a trait among individuals with the same genotype (e.g., inbred identical plants or animals), even if the environmental conditions are essentially the same. The genetic program is the mechanism for the branching, not the branch.

Flower Segmentation

Plant apical meristem tissue corresponds in some senses to the apical tissue in animal organ buds, but the analogous process does not involve homologous genes. Plant differentiation decisions are controlled more by external than internal conditions, including temperature, light, and humidity. Time since last branching also has determining power. Plants grow and bud in response to signaling molecules (generally small nonprotein molecules). Diffusible substances, including auxins and cytokinins, act as hormones to trigger these differentiation processes (see Chapter 10). Basic branching patterns in plants are shown in Figure 9-13.As with other examples in life, the apparent complexity of variation is probably brought about by relatively simple modifications of a basic process (Sussex and Kerk 2001). One of these appears simply to be in the differential timing of the development of axillary (side) branches off the main ("dominant") apical meristem; this appears to be controlled by hormones as will be seen in Chapter 10. Gene mapping studies (Chapter 5) have been done and some QTLs (candidate chromosome regions) have been found, that is, genes that quantitatively affect branch pattern and proliferation. One gene, a TF called Teosinte-branched 1 (Tf1), related to tissue proliferation, appears to affect branching architecture in maize by suppressing lateral branching (Sussex and Kerk 2001).

In some cases, particularly of flower differentiation, a cascade of transcription factor expression follows (Ng and Yanofsky 2000; Weigel and Meyerowitz 1994). The stereotype of this cascade is the "ABC" system (Figure 9-14), in which three different sets of genes in classes denoted A, B, and C are expressed in different positions around the meristem (looking at it end-on). The genes in the model species in which the system has been studied are mainly plant members of the Mads TF family, and the system is very similar in spirit to the combinatorial use of members of the Hox family in axial patterning in animals.

Essentially, a meristem is induced to produce flowers by meristem identity genes including Leafy, Unusual Floral Organs (UFO), and Apetala1, 2 (Ng and Yanofsky 2000). The meristem is arranged in roughly concentric whorls in which different A, B, and C genes are expressed. Cells expressing the A genes Apetala1 and Apetala2 alone lead to differentiation of sepals (green leaflike parts) around the outside of the inflorescence. Expression of the C gene Agamous leads to carpel (female part) formation in the inside. A + B gene expression generates flowers, whereas B + C generates stamens (male parts). Mutually antagonistic interactions among these and

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  • ren
    Which one of the following plants is vessel less or trachealess?
    3 years ago

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