How does a fertilized egg cell become an animal? Figure 16.2 (a) Fertilized eggs of a frog 1 mm (b) Newly hatched tadpole 2 mm
Differential gene expression results from genes being regulated differently in each cell type Cell differentiation - the process by which cells become specialized in structure and function Morphogenesis - The physical processes that give an organism its shape Materials in the egg can set up gene regulation that is carried out as cells divide
Cytoplasmic Determinants maternal substances in the egg that influence early development As the zygote divides by mitosis, cells contain different cytoplasmic determinants, which lead to different gene expression Figure 16.3 (a) Cytoplasmic determinants in the egg Unfertilized egg Sperm Fertilization Nucleus Two-celled embryo Mitotic cell division Zygote (fertilized egg) Molecules of two different cytoplasmic determinants
Induction signal molecules from embryonic cells cause transcriptional changes in nearby target cells Thus, interactions between cells induce differentiation of specialized cell types Figure 16.3 (b) Induction by nearby cells Early embryo (32 cells) Signal transduction pathway Signaling molecule (inducer) Signal receptor NUCLEUS
Figure 16.7 0.5 mm Head Thorax Abdomen Dorsal Ventral Posterior Anterior Right Left (a) Adult BODY AXES Larval stage (b) Development from egg to larva Segmented embryo Body segments Fertilized egg Unfertilized egg Egg developing within ovarian follicle Hatching 0.1 mm Embryonic development Egg shell Depleted nurse cells Fertilization Laying of egg Egg Nurse cell Follicle cell Nucleus 1 2 3 4 5 In Drosophila, cytoplasmic determinants in the unfertilized egg determine the axes before fertilization
Regulation and Development: What are hox genes? Genes that control organ and tissue development in various parts of the embryo. Determines organism’s basic body plan.
Regulation and Development How important are hox genes? If they are mixed up, an animal’s body plan will be mixed up; In the wrong order.
Regulation and Development Figure 16.8 Wild type Eye Antenna Mutant Leg
Apoptosis “programmed cell death” While most cells are differentiating in a developing organism, some are genetically programmed to die also occurs in cells that are infected, damaged, or at the end of their functional lives Figure 16.5 2 m
Apoptosis is essential to development and maintenance in all animals Figure 16.6 1 mm Interdigital tissueCells undergoing apoptosis Space between digits
Can a differentiated cell generate the entire organism again? organismal cloning- organism develops from a single cell without meiosis or fertilization The cloned individuals are genetically identical to the “parent” that donated the single cell
Figure 16.11 Frog embryo Less differ- entiated cell Results Enucleated egg cell Donor nucleus trans- planted Experiment Frog egg cell Frog tadpole Donor nucleus trans- planted Fully differ- entiated (intestinal) cell Egg with donor nucleus activated to begin development Most develop into tadpoles. Most stop developing before tadpole stage. UV The older the donor nucleus, the lower the percentage of normally developing tadpoles nuclear potential is restricted as development and differentiation proceeds
Figure 16.12 Grown in culture Results Cell cycle arrested, causing cells to dedifferentiate Implanted in uterus of a third sheep Cultured mammary cells Embryonic development Technique Mammary cell donor Egg cell donor Egg cell from ovary Nucleus removed Nucleus from mammary cell Surrogate mother Cells fused Early embryo Lamb (“Dolly”) genetically identical to mammary cell donor 1 2 3 4 5 6 Dolly cloned by nuclear transplantation of a differentiated cell In 1997 Dolly died prematurely in 2003; possibly incomplete reprogramming of the original transplanted nucleus
CC (for Carbon Copy) was the first cat cloned Cloned animals do not always look or behave exactly the same as their “parent” Figure 16.13
Faulty Gene Regulation in Cloned Animals only a small percentage of cloned embryos have developed normally to birth; many with defects Why?
Answer Epigenetic changes must be reversed in the nucleus from a donor animal in order for genes to be expressed or repressed appropriately for early stages of development
Stem Cells of Animals stem cell is a relatively unspecialized cell that can: reproduce itself indefinitely and differentiate into specialized cells of one or more types
Figure 16.15 Cultured stem cells Embryonic stem cells Liver cells Nerve cellsBlood cells Adult stem cells Different culture conditions Different types of differentiated cells Cells that can generate all embryonic cell types Cells that generate a limited number of cell types