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Sequential Regulation of Gene Expression During Cellular Differentiation
Determination commits a cell to its final fate Determination precedes differentiation Cell differentiation is marked by the production of tissue-specific proteins
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Myoblasts produce muscle-specific proteins and form skeletal muscle cells
MyoD is one of several “master regulatory genes” that produce proteins that commit the cell to becoming skeletal muscle The MyoD protein is a transcription factor that binds to enhancers of various target genes
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Master regulatory gene myoD Other muscle-specific genes
Fig Nucleus Master regulatory gene myoD Other muscle-specific genes DNA Embryonic precursor cell OFF OFF Figure Determination and differentiation of muscle cells
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MyoD protein (transcription Myoblast factor) (determined) Nucleus
Fig Nucleus Master regulatory gene myoD Other muscle-specific genes DNA Embryonic precursor cell OFF OFF mRNA OFF MyoD protein (transcription factor) Myoblast (determined) Figure Determination and differentiation of muscle cells
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(fully differentiated cell)
Fig Nucleus Master regulatory gene myoD Other muscle-specific genes DNA Embryonic precursor cell OFF OFF mRNA OFF MyoD protein (transcription factor) Myoblast (determined) Figure Determination and differentiation of muscle cells mRNA mRNA mRNA mRNA Myosin, other muscle proteins, and cell cycle– blocking proteins MyoD Another transcription factor Part of a muscle fiber (fully differentiated cell)
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Pattern Formation: Setting Up the Body Plan
Pattern formation is the development of a spatial organization of tissues and organs In animals, pattern formation begins with the establishment of the major axes Positional information, the molecular cues that control pattern formation, tells a cell its location relative to the body axes and to neighboring cells
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Pattern formation has been extensively studied in the fruit fly Drosophila melanogaster
Combining anatomical, genetic, and biochemical approaches, researchers have discovered developmental principles common to many other species, including humans
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The Life Cycle of Drosophila
In Drosophila, cytoplasmic determinants in the unfertilized egg determine the axes before fertilization After fertilization, the embryo develops into a segmented larva with three larval stages
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Figure 18.17 Key developmental events in the life cycle of Drosophila
Head Thorax Abdomen 0.5 mm Dorsal Right BODY AXES Anterior Posterior Left Ventral (a) Adult Follicle cell 1 Egg cell developing within ovarian follicle Nucleus Egg cell Nurse cell 2 Unfertilized egg Egg shell Depleted nurse cells Fertilization Laying of egg 3 Fertilized egg Figure Key developmental events in the life cycle of Drosophila Embryonic development 4 Segmented embryo 0.1 mm Body segments Hatching 5 Larval stage (b) Development from egg to larva
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Head Thorax Abdomen 0.5 mm Dorsal Right BODY AXES Anterior Posterior
Fig a Head Thorax Abdomen 0.5 mm Dorsal Right BODY AXES Anterior Posterior Figure Key developmental events in the life cycle of Drosophila Left Ventral (a) Adult
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(b) Development from egg to larva
Fig b Follicle cell 1 Egg cell developing within ovarian follicle Nucleus Egg cell Nurse cell 2 Unfertilized egg Egg shell Depleted nurse cells Fertilization Laying of egg 3 Fertilized egg Embryonic development Figure Key developmental events in the life cycle of Drosophila 4 Segmented embryo 0.1 mm Body segments Hatching 5 Larval stage (b) Development from egg to larva
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Genetic Analysis of Early Development: Scientific Inquiry
Edward B. Lewis, Christiane Nüsslein-Volhard, and Eric Wieschaus won a Nobel 1995 Prize for decoding pattern formation in Drosophila Lewis demonstrated that genes direct the developmental process
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Eye Leg Antenna Wild type Mutant Fig. 18-18
Figure Abnormal pattern formation in Drosophila Wild type Mutant
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Eye Antenna Wild type Fig. 18-18a
Figure Abnormal pattern formation in Drosophila Antenna Wild type
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Fig b Figure Abnormal pattern formation in Drosophila Leg Mutant
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Nüsslein-Volhard and Wieschaus studied segment formation
They created mutants, conducted breeding experiments, and looked for corresponding genes Breeding experiments were complicated by embryonic lethals, embryos with lethal mutations They found 120 genes essential for normal segmentation
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Animation: Development of Head-Tail Axis in Fruit Flies
Axis Establishment Maternal effect genes encode for cytoplasmic determinants that initially establish the axes of the body of Drosophila These maternal effect genes are also called egg-polarity genes because they control orientation of the egg and consequently the fly Animation: Development of Head-Tail Axis in Fruit Flies
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Bicoid: A Morphogen Determining Head
Structures One maternal effect gene, the bicoid gene, affects the front half of the body An embryo whose mother has a mutant bicoid gene lacks the front half of its body and has duplicate posterior structures at both ends
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Fig EXPERIMENT Tail Head T1 T2 A8 T3 A7 A1 A2 A3 A4 A5 A6 Wild-type larva Tail Tail A8 A8 A7 A6 A7 Mutant larva (bicoid) RESULTS Figure Is Bicoid a morphogen that determines the anterior end of a fruit fly? Fertilization, translation of bicoid mRNA 100 µm Anterior end Bicoid mRNA in mature unfertilized egg Bicoid protein in early embryo CONCLUSION Nurse cells Egg bicoid mRNA Developing egg Bicoid mRNA in mature unfertilized egg Bicoid protein in early embryo
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EXPERIMENT Tail Head Wild-type larva Tail Tail Mutant larva (bicoid)
Fig a EXPERIMENT Tail Head A8 T1 T2 T3 A7 A1 A6 A2 A3 A4 A5 Wild-type larva Tail Tail Figure Is Bicoid a morphogen that determines the anterior end of a fruit fly? A8 A8 A7 A7 A6 Mutant larva (bicoid)
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RESULTS Anterior end Bicoid mRNA in mature unfertilized egg
Fig b RESULTS Fertilization, translation of bicoid mRNA 100 µm Anterior end Figure Is Bicoid a morphogen that determines the anterior end of a fruit fly? Bicoid mRNA in mature unfertilized egg Bicoid protein in early embryo
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CONCLUSION Nurse cells Egg bicoid mRNA Developing egg
Fig c CONCLUSION Nurse cells Egg bicoid mRNA Developing egg Figure Is Bicoid a morphogen that determines the anterior end of a fruit fly? Bicoid mRNA in mature unfertilized egg Bicoid protein in early embryo
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This phenotype suggests that the product of the mother’s bicoid gene is concentrated at the future anterior end This hypothesis is an example of the gradient hypothesis, in which gradients of substances called morphogens establish an embryo’s axes and other features
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The bicoid research is important for three reasons:
– It identified a specific protein required for some early steps in pattern formation – It increased understanding of the mother’s role in embryo development – It demonstrated a key developmental principle that a gradient of molecules can determine polarity and position in the embryo
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