Opportunities in Biology (1989) / Chapter Skim
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5. Development
5. Development
Pages 140-174
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From page 140... ...
As they learned more about the elaborate processes of gastrulation, neurulation, and pattern formation, however, biologists yearned for mechanistic explanations. Emerging theories emphasized the importance of the structure of the egg, the lineage of cell divisions, the accurate timing of these divisions, the importance of cell-cell interactions, and the role of inductions.
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From page 141... ...
Differential cell adhesion can now be described in terms of specific molecules, receptors, and known elements of the extracellular matrix. Communication between cells, hitherto mysterious, can be explained in terms of such components as soluble growth factors, second messengers, cell receptors, and cell junctions.
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From page 142... ...
Eliminating the precursors from a fertilized egg renders He resulting animal sterile. It is lilcely that specific molecules in the egg, the so-called determinants, are responsible in such cases for the development of the germ cells.
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From page 143... ...
In each sex, the differentiation of germ cells into mature gametes is a process of key importance since it represents the basis for understanding the initiation of development; such understanding is likewise of great practical importance. The Egg Contains Both Nutritive Materials and Positional Information Needed for the Early Stages of Development The process of oogenesis, or egg production, results in cells that contain sufficient stored material to support at least the first stages of development.
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From page 144... ...
Since the changes in chromatin structure are probably associated with changes in transcriptional activity, one can speculate that gene expression in the sperm genome is specifically controlled. Unique methylation changes occur during spermatogenesis in many species, and these changes might cause specific information storage in the sperm DNA.
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From page 145... ...
The process of fertilization represents a case in which two cell types that have spent some time in a quiescent storage state must rapidly resume a high level of metabolic activity and fuse with each other. This change in state has been termed activation.
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From page 146... ...
i. Perhaps the most interesting aspect of this story is that this same process of inositol lipid cleavage occurs in a wide variety of other cell types that respond to the binding of a variety of external agents, including hormones, growth factors, neurotransmitters, and chemotactic molecules at their plasma membrane.
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From page 147... ...
In such embryonic systems, intracellular factors that regulate the cell cycle have been described for the first time. Improved methods for protein separation, immunological characterization, and cloning have recently increased our understanding of the extracellular growth factors that may cause quiescent cells to begin to divide.
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From page 148... ...
Yeast Provide a Good Model System for Studying Molecular Mechanisms of Cell Division Yeast are unicellular fungi, simple in some respects, but with many of the same features that characterize mammalian and all other eukaryotic cells. Like all euk~uyotes, yeast cells have a nucleus, chromosomes, mitotic activity, and a cell cycle divisible into G1, S
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From page 149... ...
Although embryonic cells are endowed with a generous supply of all known structural proteins needed for cell division, they must still synthesize proteins in order to pass through the cell cycle. This has suggested to some that protein synthesis serves a regulatory function, a view that was reinforced when the amounts of specific proteins were found to oscillate during the embryonic cell cycle in clam and sea urchin eggs.
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From page 150... ...
It seems likely that some of these factors will ultimately prove to be known growth factors or proteins encoded by protooncogenes. Although the well-studied growth factors have been soluble proteins, embryological studies have provided evidence that the extracellular matrix can be an important inducer of differentiation events and can control cell proliferation.
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From page 151... ...
Are the inducers influencing simple properties such as the decision to proliferate, or are they specifying detailed positional information? At this point, we can say only that there appear to be several classes of signaling molecules: trophic factors, which prevent cell death; growth factors, which induce proliferation; transforming growth factors, which change expression; and differentiation factors, which may determine cell type.
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From page 152... ...
Is this pattern of cell division merely a reflection of the underlying developmental events? Or are the division patterns in some way linked to the expression of a specific fate?
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From page 153... ...
An exciting finding has been the identification of enhancers, short sequences that can act from a distance of thousands of nucleotide pairs to increase transcription of nearby genes; since they act in either orientation, they apparently accomplish their task by affecting chromatin structure. Gene rearrangements, such as those that initiate transcription of immunoglobulin genes, can act by bringing the coding sequence into the vicinity of a tissue-specific enhancer.
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From page 154... ...
When membranes form, isolating embryonic nuclei into individual cells, those cells are already partially instructed about their role in morphogenesis. As embryonic development proceeds past the blastoderm stage, the pattern of cells with detectable amounts of product from any given segmentation gene changes.
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From page 155... ...
Each of the major known homeotic loci is expressed in a distinct, relatively nonoverlapping pattern along the anterior-posterior axis of young embryos. The expression of each homeotic gene is controlled by the segmentation genes and by interactions with other homeotic genes.
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From page 156... ...
For example, the mouse has at least 16 genes containing the homeobox. Transcripts of several of these genes have been detected in embryos, but there is not yet enough information to determine whether they play a role in the control of developmental processes in vertebrates.
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From page 157... ...
This area is virtually unexplored, in part because the experimental material~efined cell types of the appropriate developmental potential is limiting.
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From page 158... ...
Antisense RNA is naturally used for regulating some prokaryotic genes. More work will be required to make this technique reliable and useful for eukaryotic cells.
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From page 159... ...
Th~us the ~specific translat~na I inhibition of :~ this: ~ pa~tticular :: ::: messeng:er ~ RNA :~acts: as:: a: :rapidly: :reversible block to DNA~:~nthesis~:~ before:~ ~:: fertilizatio:n.: :;: :~ _ :: CELL MOVEMENT AND CELL ADHESION We Can Now Begin to Unravel the Mechanisms That Control Cell Movement and Cell Adhesion As we have seen, animals begin development as a single cell (the fertilized egg) , which cleaves into a single-layered ball of cells (the blastula)
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From page 160... ...
What factors guide neuronal growth cones along their highly specific pathways? And what factors control the events of cell-cell recognition as neuronal growth cones recognize specific target cells?
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From page 161... ...
A Variety of Cell-Adhesion Molecules Have Been Discovered, and Several Have Been Purified and Characterized Cell-adhesion molecules, or CAMs, were first definitively identified by means of immunologically based adhesion assays in which specific antibodies capable of blocking cell adhesion were used to purify cell-surface molecules. Over the past few years, a variety of CAMs have been discovered; several have been purified and characterized chemically.
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From page 162... ...
Whether these specific proteins and carbohydrates actually function as neuronal recognition molecules, and whether in so doing they either directly mediate or modulate specific cell adhesion, awaits future studies. In addition to the more transient forms of cell adhesion involved in cell movement and cell recognition, a variety of more specialized and long-term cellsurface contacts, called cell junctions, provide more stable adhesion and communication.
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From page 163... ...
It will be important to elucidate the structure of the laminin site that mediates the binding of neuronal growth cones and to understand the role that laminin plays in promoting neurite outgrowth in the developing embryo. The glycoproteins in the ECM exert their effects on cell movement and cell adhesion through the interaction of individual matrix components with cell-surface receptors.
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From page 164... ...
With the discovery of a number of major cell and substrate adhesion molecules and their receptors, and with the recent progress in uncovering additional adhesion and recognition systems, we can expect great advances over the next decade in our understanding of what mechanisms control these basic events of morphogenesis and how these events help control the development of tissues and organs throughout the body. POSITIONAL INFORMATION Species Differ in the Patterns in Which the Cell Types Are Arranged, Not in Their Cell Types What distinguishes one group of organisms from another, and indeed one part of an individual organism's body from another, is the way in which cell Apes are arranged with respect to one another.
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From page 165... ...
In fish embryos, early cleavages seem to yield reproducible patterns of cell lineage, but these bear no relation to the final pattern of the body; during early gastrulation, prior to the establishment of the main body axis, cells from different lineages migrate individually and mix randomly. Even in nematodes and leeches, in which cell lineage under undisturbed conditions is invariant, examples are accumulating that suggest that cell lineage and the determination of cell fate are not obligatorily coupled.
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From page 166... ...
With sensitive molecular biological techniques it has been possible to examine embryonic tissues, where it has been found that embryos in the earliest stages of development contain both the mRNAs for growth factors and the molecules themselves. In the frog the mRNA for a relative of a known growth factor involved in wound healing has been found localized in the egg in a region where the earliest inductive signals are generated.
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From page 167... ...
After removal of part of the appendage, wound healing brings normally nonadjacent cells into contact, producing a discontinuity in the normally smooth gradation of positional values, which stimulates cell division and intercalation to reduce the discontinuity. The studies on amphibians raise the intriguing possibility that mammals may one day be stimulated to regenerate their limbs if a way can be- found to reactivate the developmental programs used for forming limbs in the embryos.
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From page 168... ...
Gap junctions are small channels in cell membranes that connect neighboring cells in venous tissues, including the liver. These junctions allow the free diffusion between cells of small molecules; theoretically, the concentrations of such molecules within the liver cell could serve as a measure of the total size of the organ.
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From page 169... ...
By this means the immune system builds up a pool of long-lived T lymphocytes, which are useful in fighting off the types of infections that its host will encounter during life. A single encounter with an antigen, and burst of cell division, changes the life expectancy of the human T cell, without any further cell division, from less than a week to more than 10 years.
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From page 170... ...
In plant embryology, there is no phenomenon comparable to the sudden contact of a group of cells with a new cellular environment, as occurs in animal gastrulation. Nonetheless, plants exhibit diverse cell types and complex morphogenesis.
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From page 171... ...
Thus, a portrayal of this subject is a convenient format for the illustration of the unique features of plant development. Plant Growth and Morphogenesis Are Dominated by the Plant Cell Wall Plant cells cannot move appreciably relative to each other; to cover distance they must grow across it.
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From page 172... ...
The compounds attain and gibberellic acid play major roles in this process, and their structures, synthetic pathways, and modes of degradation are relatively well known. The investigation of the precise ways in which they affect growth, however, is one of the central problems of plant development.
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From page 173... ...
Control of Direction of Growth Depends on the Direction of Cellulose Synthesis Control of the shape of plant cells can be achieved by the highly localized control of growth rate, as is the case for some unicellular structures such as root hairs and pollen tubes. The cells in multicellular plant organs, such as roots, stems, and leaves, however, grow throughout their length and hence change shape by a different method.
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From page 174... ...
Meristems are zones of continuous cell division located at the tips of stems and roots in plants. For example, the shoot tip returns repeatedly to the same configuration while continuously producing leaves and additional stem.
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