Development The process of regulated growth and differentiation Differentiation The formation of different types of cells, tissues and organs from undifferentiated cell Differentiated cells have characteristic structural and functional properties Done through specific regulation of gene expression different cells express different genes Two common types of development Mosaic Different regions of the one-cell zygote are already specified as to what that region will develop into (ex. Styela, C. elegans, protostomes) Is this caused by a loss of genetic material (Rous, 1905)? No, the DNA content of the cells stays constant Transplanted cytoplasm will alter development must be cytoplasmic determinants (bicoid) Regulative Embryonic cells are totipotent, their differentiated state is caused by their environment. Change the environment and the cell can change it's type. (frogs, vertebrates, deuterostomes) F. C. Steward (1950s) grew whole carrots from single root cells Briggs and King (1950s) transplanted nuclei from Rana blastula cells into enucleated eggs and got normal development - wouldn't work with gastrula stage cells I. Wilmut (1996) transplanted nuclei from sheep epithelial cells into enucleated eggs and got normal development to adult As the embryo develops cells lose the capability to do some things (they adopt specific fates) and this determination is part of the differentiation process is this caused by a loss of genetic material (Rous, 1905)? No, the DNA content of the cells stays constant must be stable transcription patterns that can't be altered most development is a combination of mosaic determination and regulative processes (some totipotent cells, some stem cells and some terminally differentiated cells) Model organisms used in the study of developmental genetics Xenopus large easily manipulated eggs Drosophila what else - good genetics C. elegans small size (959 cells in male) 3 1/2 day life cycle small genome - 8x107 mouse mammalian, not an elephant Arabidopsis plant, small genome, short life cycle Zebra fish transparent embryos How is pattern created? localized determinants in the egg muscle determinants in Styela Bicoid gradient in Drosophila Different promoters respond to particular concentrations of transcription factors The genes produced from these promoters form their own gradients of transcription factors or signaling molecules More complex promoters now sum the effects of various gradients to produce a pattern Example: regulation of the homeotic genes in Drosophila development Homeotic mutations change one body part to look like a different body part antennae to leg, haltere to wing macromutations clues to how development works clues to how evolution might work the result of changes in expression of transcription factors (homeobox genes) A homoetic selector protein must turn on all of the genes needed to make a particular body part or organ First ones found were the homeobox genes The homeobox is a very strongly conserved DNA binding domain - a transcription factor Can also be signal transduction proteins (wingless) In Drosophila the initial pattern is layed down by the diffusion of transcription factors During early devlopment the nuclei divide but the egg does not - nuclei are in a syncetium Bicoid mRNA is planted at one end of the egg, the bicoid protein forms a gradient which determines the head to tail axis Bicoid is a transcription factor that binds to sequences in the promoters of other transcription factors These genes are turned on if the concentration of bicoid is appropriate and then form their own gradients The embryo is divided up into stripes, each of which has a unique combination of transcription factors Eventually, the homeotic genes such as antennapedia are turned on only in particular stripes antennapedia is normally on only in the three thoracic segments - legs a mutant where it is expressed in the head develops legs on the head Changes in the gradients will change the number and charcteristics of the various segments provides a mechanism for rapid evolution of new body plans Vertebrates don't have segments so does this have any implications for vertebrate evolution? The homeobox genes were found in vertebrates Homeobox is strongly conserved The genes are organized the same as in Drosphila Expression pattern is similar Vertebrates do have segments in the nervous system Homeobox genes are also expressed in developing limb bud and other tissues, always in gradients or defined patterns Inducers - signals from one cell to another diffusible - organic compounds or peptides that bind to receptors and alter gene expression (either directly or through a signal transduction pathway) retinoic acid - a morphogen (an inducer that effects shape) that can cause the development of extra fingers in the limb steroid hormones (estrogen regulation of ovalbumin) cell surface - boss and sevenless in Drosophila eye development determination alterations in chromatin (Xenopus 5S genes, globin genes) DNA methylation (an epigenetic change) [ Biol 207 ] [ Bell ] [ CSU Chico ] [ Library ] This document is maintained by: Jeff Bell Last Update: Thursday, December 10, 1998