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Figure 8-5. Basic stages of Xenopus development, from fertilization to cleavage (cell divisions that produce the blastula, a spherical layer of cells that surround a fluid-filled cavity), gastrulation (invagination of cells from the blastula to form the gastrula, a two-layered sphere consisting of ectoderm and endoderm surrounding an archenteron that communicates with the exterior via the blastopore that will form the gut), neurulation (embryonic stage at which the neural tube develops), organogenesis and metamorphosis to the adult frog. Shaded spots indicate site of development of germ cells and gonads at each stage.

Among the defining characteristics of vertebrates is a cell lineage known as the neural crest (NC), which derives from the early dorsal neural ectoderm. The NC cells migrate from positions along the dorsal midline, intercalating through meso-dermal tissue to reach various locations where, in contact with overlying ectoderm, inductive tissue interactions trigger the initiation of various organs, like hair, feathers, mammary gland, teeth, and others (NC cells migrating from the head region are sometimes specially referred to as Cranial Neural Crest, or CNC). These inductive epithelial-mesenchymal interactions (EMI) are location and context specific and are responsible for so many important characteristics of vertebrates that some researchers give the NC a special place as a fourth primary germ layer (Hall 1999; Hall 2000), making them "quadroblastic." Among other structures, the segmental patterning of parts of the head have been correlated with EMI and NC migration (but the role of EMI is not entirely clear; see Chapter 9).

At least one reason EMI is important in vertebrate development is that many organs can develop only after the embryo has reached a certain stage of complexity. At that stage, however, it may be difficult to initiate a dispersed structure because the tissue bed is too large for effective signaling. For example, teeth cannot develop until there is a pharynx or jaws to develop in, taste buds cannot develop until there is a tongue, and so on. The use of NC cells and EMI allows tissues to be preprogrammed for certain developmental fates, but then not be activated until an appropriate organizational level. Neural crest cells can then come into contact with the inductive tissue located where the structure will develop. Other organ systems form in vertebrates by inductive interactions that involve mesoderm. Other forms of differentiation occur in cells programmed in some home location but that then circulate freely, as in the blood and lymphatic systems. Consistent with the notion that early program is important in later development, invertebrates establish basic regional differentiation very early in their development.

In addition to the widespread presence of epithelia, vertebrate organs typically contain both primary "functional" tissue, called the parenchyma, often in the form of repeated units, like pancreatic islets, nephrons in the kidney, osteons in bone, hairs, taste buds, or ovarian follicles. The functional tissue in each unit secretes hormones or enzymes, filters blood, transmits neural signals, and so on. There is usually also supporting tissue, called the stroma, such as Schwann or glial cells to protect neurons, or a cellular matrix to hold and structure the organ. Invertebrates are also organized in a similar patterned way, for example, with discrete eye subunits including sensory elements of various kinds that will be described later (not to mention the joints for which arthropods were named).

Many if not most organ systems have periodic, modular, or segmented, organization, with multiple regularly spaced and perhaps regionally differentiated subunits. Some systems do not segment linearly but ramify—literally—by physical branching, as in the nervous, vascular, and respiratory systems. Some organ systems, like limbs in tetrapods, are both segmented and branching.

It is easy to think of development as having to do with morphology, but much if not most of what goes on in life is "virtual," that is, is physiological and has to do with chemical interactions rather than physical structure. Some of these take place within the cell, but some take place in the internal circulating fluid, like lipid transport and respiratory gas exchange—in some ways comparable to those of the external environment, to the extent that some believe that the environment should not be considered as separate from the organism itself (Turner 2000). Metabolic differentiation at the gene level involves many of the same characteristics found in genes responsible for physical systems: the use of multiple subunits often involving related gene products, which interact with each other. The processes of differentiation of physiological systems are often hierarchical in time or cell lineages and demonstrate homeostatic interaction with the environment, in response to signaling such as of hunger, hormones, pheromones, exogenous infections, and the like.

Plants

Plant life histories differ from those of most animals in several basic ways; in part, their system is different because the rigid cell walls prevent the kind of migration so important in animal development. Many plants have life cycles that include diploid and haploid stages, perhaps in alternating generations. Although not reproduction in the sense we have already described, because everything stays connected, some plants nonetheless effectively propagate via rhizomes, part of their root systems, that push up through the soil to start a new above-ground plant. Plants that reproduce via single fertilized cells do not sequester a separate germ line, and gametes are independently produced in many places (different flowers) on the same plant. Plants also continue to undergo differentiating development and morphogenesis throughout their lives, as stem and root meristems retain the ability to develop new structures at their ends because they maintain a core of pluripotent stem cells, resulting in plants being able to respond to their environments in ways that are less stereotypical or fixed than most animals.

Plant embryos also generally form three basic tissues, known as dermal, ground, and vascular tissues, but this is not established by gastrulation. Plants have an active vascular system for the upward and downward transport of nutrients, but it forms differently from these tissues (Figure 8-6). Like animals, plants use a limited repertoire of basic processes, including growing, elongating along the stem and root axes to form branches, and differentiating some stem tips into flowers, leaves, or other specialized structures. Thus plants have a vertical axis (separately behaved in stems and roots), as well as corresponding axes along stem and root branches.

The First Few Cell Divisions

Not surprisingly, early plant development varies among the many plant species that have been studied (Weigel and Jurgens 2002). Under normal conditions, the fertilized zygote undergoes a few rounds of development to form initial root-tip and leaf structures, thus providing the means of survival through its early days. The new plant quickly develops a few primary cell types or layers, which lead to the apical root and apical shoot meristems. Development occurs radially around the meristems. Several primary tissue layers form distinct parts of the plant: dermal tissue forms the outer protective layer, vascular tissue forms the hollow fluid transport tubes, and ground tissue grows in between these layers.

The Basic Plant Body Plan

The two meristems grow to become roots and stems, respectively. Cells behind the growing meristem divide to allow elongation of the shoot or root. At intervals determined by growing conditions and the genotype of the plant, buds of meristem tissue

heart stage

Figure 8-6. Schematic of plant development. Figure depicts the stages of a dicot; monocot development is basically the same.

heart stage

Figure 8-6. Schematic of plant development. Figure depicts the stages of a dicot; monocot development is basically the same.

separate laterally, to initiate branches. In the shoot, each such branching normally develops into a leaf and a small meristem, which itself can generate a new branch or shoot (depending on species, conditions, and the like). Signaling from the primary shoot meristem may control the timing of development of other shoot meristems, maintaining "apical dominance" of the primary shoot. An interesting feature of plant development is that it achieves a kind of spiral symmetry as new shoot meristems are formed at relatively species-specific angles of rotation around the developing apical meristem. Furthermore, plants have hierarchical branching symmetry, with each new shoot or root having radial and PD axes of their own. At points determined by internal and external conditions, a shoot meristem will differentiate into a specialized structure, a flower. This can be male, female, or both. Plants vary in whether they have only one, both, or "bisexual" flower types. Root meristems also send off lateral branches at intervals.

The notion of "stem" cells in animals and plants is similar, and there are at least some genetic similarities shared between stem cells in the two kingdoms (Benfey 1999; Laux 2003; Weigel and Jurgens 2002). This suggests that in the common ancestral period, multicellularity may have been a widespread sometime trait among otherwise single-celled organisms, perhaps due to the effects of gene(s) (today with homologs Zwille in plants, Piwi in animals, of those currently identified) related to basic cell division or adhesion. In any case, the properties of stem cells are similar in the two kingdoms, but plant stem cells are essentially totipotent in adults, able under appropriate signaling to generate all the plants stem, or root tissues, unlike animal cells which are "stem" for a tissue but not all tissues (Laux 2003; Weigel and Jurgens 2002). It is this that makes each stem and root rather independent, unlike the case in animals where each tissue can further differentiate, but generally only down its respective fate-map pathways. A comparable notion in animals would be the cells that give rise to each hair or limb. But in a more substantial way, each branch of stem or root is like a separate organism (see below). Plant cells have less rigid developmental fates in that differentiated cells can more easily dedifferenti-ate to become stem cells.

Plants are simpler in their developmental patterns than animals because plants do not develop as many organs or as much interstructure communication as animals may. Plants do not have to deal with mobility and hence have simpler communication systems. Plants can respond to predation, but not in ways that require the degree of movement flexibility and hence muscular or nervous system complexity of animals. This does not mean that plants are all alike or are simple or that they have little sense of their environments. Plants do differentiate many different cell types and tissue structures, they have to maintain rigidity often at great body sizes, and they have to maintain mutational integrity over very long lifespans. They must be able to find nutrients, light, and water (and mates) and must be able to protect against infection or predation while remaining immobile (or, for floating plants, without real control over their mobility). Plants have distribution systems (although they are not closed as are the circulatory systems of vertebrates) and internal as well as interindividual communication. Although the systems differ in detail, there are many fundamental similarities in the molecular mechanisms by which they are achieved. Plants sense many aspects of their environments and integrate that information to "make decisions" relevant to their kind of life. To achieve this, plants also have evolved a variety of diverse ways of branching (Sussex and Kerk 2001), and developmental fate maps, even if they are not implemented in as unitary way as animals' are (Jurgens 1994).

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