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Differential Gene Expression in Development. 19 Differential Gene Expression in Development. 19.1 What Are the Processes of Development? 19.2 How Is Cell Fate Determined? 19.3 What Is the Role of Gene Expression in Development? 19.4 How Does Gene Expression Determine Pattern Formation?
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19 Differential Gene Expression in Development • 19.1 What Are the Processes of Development? • 19.2 How Is Cell Fate Determined? • 19.3 What Is the Role of Gene Expression in Development? • 19.4 How Does Gene Expression Determine Pattern Formation? • 19.5 Is Cell Differentiation Reversible?
19 Differential Gene Expression in Development Stem cells are actively dividing, unspecialized cells that have the potential to produce different cell types. In stem cell therapy, stem cells are injected into damaged tissues, where they will differentiate and form new, healthy tissues. Opening Question: What are other uses of stem cells derived from fat?
19.1 What Are the Processes of Development? • Development: the process in which a multicellular organism undergoes a series of progressive changes that characterizes its life cycle. • In its earliest stages, a plant or animal is called an embryo. • The embryo can be protected in a seed, an egg shell, or a uterus.
19.1 What Are the Processes of Development? • Four processes of development: • Determination sets the fate of the cell • Differentiation—the process by which different types of cells arise • Morphogenesis—organization and spatial distribution of differentiated cells • Growth—increase in body size by cell division and cell expansion
19.1 What Are the Processes of Development? • Determination and differentiation occur largely because of differential gene expression. • Cells in the early embryo arise from repeated mitoses and soon begin to differ in terms of which genes are expressed.
19.1 What Are the Processes of Development? • Morphogenesis involves differential gene expression and the interplay of signals between cells. • It occurs in several ways: • Cell division • Cell expansion in plants (position and shape are constrained by cell walls)
19.1 What Are the Processes of Development? • Cell movements are important in animals • Apoptosis (programmed cell death); essential in organ development • Growth occurs by increasing the number of cells or enlargement of existing cells.
19.1 What Are the Processes of Development? • Cell fate: which type of tissue the cell will ultimately become. • Cell fate is usually determined quite early in development. • The timing can be determined by transplanting cells from one embryo to a different region in a different embryo.
19.1 What Are the Processes of Development? • Cell fate determination is influenced by gene expression and the extracellular environment. • Determination is a commitment. • Determination is followed by differentiation—the changes in biochemistry, structure, and function that result in different cell types.
19.1 What Are the Processes of Development? • During animal development, cell fate becomes progressively more restricted. • Cell potency: potential to differentiate into other cell types. • Totipotent—can differentiate into any cell type (early embryo) • Pluripotent—can develop into most cell types, but cannot form new embryos
19.1 What Are the Processes of Development? • Multipotent—can differentiate into several related cell types • Unipotent—can produce only one cell type: their own (mature organism) • Many of these processes can be manipulated in the laboratory.
19.2 How Is Cell Fate Determined? • How does one egg cell produce so many different cell types? • Two processes for cell determination: • Cytoplasmic segregation (unequal cytokinesis) • Induction (cell-to-cell communication)
19.2 How Is Cell Fate Determined? • Cytoplasmic segregation: • Factors within a zygote or egg are not distributed evenly and end up in different daughter cells after division. • Polarity—developing a “top” and a “bottom.” Can develop very early; yolk and other factors are distributed asymmetrically.
19.2 How Is Cell Fate Determined? • In animal development, the animal pole is the top, the vegetal pole is the bottom. • Sea urchin embryos
19.2 How Is Cell Fate Determined? • If the sea urchin eight-cell embryo is cut vertically, it develops into two small larvae. • If it is cut horizontally, the bottom develops into a larva, the top remains embryonic. • This indicates that the top and bottom halves have already developed distinct fates.
19.2 How Is Cell Fate Determined? • Model of cytoplasmic segregation: • Cytoplasmic determinants are distributed unequally in the egg cytoplasm. • Includes specific proteins, regulatory RNAs, and mRNAs that play a role in development of many organisms.
19.2 How Is Cell Fate Determined? • The cytoskeleton contributes to asymmetric distribution of cytoplasmic determinants: • Microtubules and microfilaments have polarity. • Cytoskeletal elements can bind motor proteins that transport the cytoplasmic determinants.
19.2 How Is Cell Fate Determined? • In sea urchin eggs, a protein binds to the growing (+) end of a microfilament and to an mRNA encoding a cytoplasmic determinant. • As the microfilament grows toward one end of the cell, it carries the mRNA along with it. • The asymmetrical distribution of the mRNA leads to a similar distribution of the protein it encodes.
19.2 How Is Cell Fate Determined? • Induction: cells in a developing embryo influence one another’s developmental fate via chemical signals and signal transduction mechanisms.
19.2 How Is Cell Fate Determined? • Development of the lens in the vertebrate eye: • The forebrain bulges out to form optic vesicles, which come in contact with cells at the surface of the head. These surface cells ultimately become the lens. • The optic vesicle must contact the surface cells, or the lens will not develop.
Figure 19.4 Embryonic Inducers in Vertebrate Eye Development
19.2 How Is Cell Fate Determined? • The surface cells receive a signal, or inducer, from the optic vesicles. • Inducers trigger sequences of gene expression in the responding cells. • How genes are switched on and off to govern development is studied using model organisms.
19.2 How Is Cell Fate Determined? • Vulval development in Caenorhabditis elegans: • Adults are hermaphroditic; eggs are laid through a ventral pore called the vulva.
Figure 19.5 Induction during Vulval Development in Caenorhabditis elegans (Part 1)
19.2 How Is Cell Fate Determined? • During development, one cell, the anchor cell, induces the vulva to form from six cells on the ventral surface. • Two signals are involved: the primary (1) and secondary (2) inducers. • The concentration gradient of the primary inducer (LIN-3) is key. It is produced by the anchor cell and diffuses out to form the gradient.
Figure 19.5 Induction during Vulval Development in Caenorhabditis elegans (Part 2)
19.2 How Is Cell Fate Determined? • The inducers control activation or inactivation of genes through signal transduction cascades. • This differential gene expression leads to cell differentiation.
19.3 What Is the Role of Gene Expression in Development? • All cells in an organism have the same genes, but each cell expresses only certain ones. • The mechanisms that control gene expression during cell fate determination and differentiation work at the level of transcription.
19.3 What Is the Role of Gene Expression in Development? • Cell fate determination can occur by induction. • When an inducer molecule binds to a receptor on the cell surface, a signal transduction pathway leads to activation of transcription factors.
19.3 What Is the Role of Gene Expression in Development? • Development is often controlled by these kinds of molecular switches, which allow a cell to proceed down one of two alternative paths. • In nematodes, LIN-3 is a growth factor; it binds to receptors on vulva precursor cells, starting a signal transduction pathway that includes Ras protein and MAP kinases.
19.3 What Is the Role of Gene Expression in Development? • The gene for b-globin (part of hemoglobin) is expressed in red blood cells. • This gene exists in other cells, but is not expressed. This can be shown using nucleic acid hybridization. • A probe for the b-globin gene will find its complement in DNA from brain cells but not in mRNA from brain cells.
19.3 What Is the Role of Gene Expression in Development? • Differentiation in muscle cells: • Muscle precursor cells come from an embryonic layer called the mesoderm. • When these cells commit to becoming muscle cells, they stop dividing. • In most embryonic cells, cell division and cell differentiation are mutually exclusive.
19.3 What Is the Role of Gene Expression in Development? • Cell signaling activates the gene for a transcription factor called MyoD. • MyoD activates the gene for p21, an inhibitor of cyclin-dependent kinases that normally stimulate the cell cycle. • The cell cycle stops so that differentiation can begin.
Figure 19.7 Transcription and Differentiation in the Formation of Muscle Cells
19.4 How Does Gene Expression Determine Pattern Formation? • Pattern formation: The process that results in the spatial organization of tissues and organisms. • Linked to morphogenesis, creation of body form • Morphogenesis involves cell division and differentiation, as well as apoptosis (programmed cell death).
19.4 How Does Gene Expression Determine Pattern Formation? • In human embryos, connective tissue links the fingers and toes. Later, the cells between the digits die.
19.4 How Does Gene Expression Determine Pattern Formation? • Model organisms are used to study apoptosis. • Mutants with altered cell death phenotypes are used to identify the genes and proteins involved.
19.4 How Does Gene Expression Determine Pattern Formation? • C. elegans produces exactly 1,090 somatic cells as it develops, but 131 of those cells die. • The sequential activation of two proteins controls this cell death. • A third gene codes for an inhibitor of apoptosis in cells not programmed to die.
19.4 How Does Gene Expression Determine Pattern Formation? • A similar system controls apoptosis in human development. • The proteins are similar to those of C. elegans. • The conservation of this pathway in evolution indicates its importance: Mutations are harmful, and evolution selects against them.
19.4 How Does Gene Expression Determine Pattern Formation? • Flowers are composed of four organ types (sepals, petals, stamens, carpels) arranged around a central axis in whorls. • In Arabidopsis thaliana, flowers develop from a meristem (undifferentiated, rapidly growing cells)at the growing point on the stem.
Figure 19.9 Organ Identity Genes in Arabidopsis Flowers (Part 1)
19.4 How Does Gene Expression Determine Pattern Formation? • The identity of each whorl is determined by organ identity genes: • Class A genes, expressed in sepals and petals • Class B genes, expressed in petals and stamens • Class C genes, expressed in stamens and carpels
