Principles of Experimental Embryology Biology 4361 – Developmental Biology September 19, 2006Major Research Questions How do forces outside the embryo affect its development? (Environmental Developmental Biology) How do forces within the embryo cause the differentiation of cells? (Developmental Dynamics) How do cells organize themselves into tissues and organs? (Morphogenesis and Cell Adhesion)The Embryonic Environment What is the “embryonic environment”? Internal (e.g. intrauterine) developmental stimuli:  chemicals (e.g. maternal hormones, caffeine, nicotine)  competitors (e.g. littermates) Environmental regulation pathway:  external stimulation triggers signaling event in embryo  signal stimulates an embryonic pathway, usually hormonal, that changes the developmental pathway. Organisms are connected to their environment External development stimuli:  light  temperature  humidity  predators  competitors  intraspecific signalsEnvironmental Regulation of Development Light, temperature & moisture = environmental signals that produce changes in embryonic hormones. Hormones induce changes in color, reproduction, hibernation, behavior Environmentallyproduced changes in hormones during development can affect the adult phenotype; e.g. Bicyclus anynana seasonal polyphenism  common in butterflies Nemoria bistriaria wet season dry season wet season dry seasonSeasonal polyphenism Nemoria arizonaria larvae early spring – oak catkin (flower) summer – oak twig Environmental signal – tannins in oak leaves (probably)  developmental response – change in cuticle morphologyAnthropogenic effects pepper moths Industrial melanism (anthro – man)Environmental signals – UV radiation UV = mutagenic, damaging to cell and tissue development  lower wavelengths have more energy; cause more damage  developmental stages are the most sensitive! melanin Mycosporinelike amino acid = sunscreen HO HO N OH COOH NH COOH OCH 3 R Exposure can induce protective mechanismsBonellia viridis (marine echiuroid worm) ♂ ♀ Environmental Signals  Intraspecific  larvae settles on sand/rock surface = ♀  larvae settles on proboscis = ♂ 10 cm 2 mm Therefore, signal (most likely chemical) to become ♂ is from ♀Temperaturedependent sex determination  sex determined by the egg temperature during weeks 2 & 3 ≤ 30° C = female ≥ 34° C = maleDevelopmental Dynamics of Cell Specification undifferentiated differentiated specification determination 1. Specification – capable of differentiating autonomously when placed in a neutral environment.  reversible 2. Determination – capable of differentiating autonomously even when placed into another region of the embryo.  essentially irreversible Stages of cell commitment:Cell Type Specification I. Autonomous specification II. Syncytial Specification III. Conditional SpecificationAutonomous Specification (mosaic development)Autonomous Specification  Blastomere fates are generally invariant.  Specification by differential distribution of cytoplasmic components during cleavage.  Each embryo of the species has the same cell lineages.  Specification precedes largescale cell migration.  “Mosaic” development: cell cannot change fate if a blastomere is lost.Autonomous Specification Tunicate (sea squirt)  each dissociated blastomere pair forms original structures  blastomeres are committed at a very early stage in mosaic development each blastomere will contain positional information in the form of specific proteins and genesSyncytial Specification syncytium – nuclear division without cell division; results in cytoplasm with many nucleinuclei & cytoplasm form syncytial blastoderm Drosophila Cleavage DrosophilaSyncytial Specification through Morphogen Gradients Drosophila egg bicoid – anterior determinant nanos – posterior determinant Maternal messages: Each morphogen establishes a gradient throughout the embryo (like a diffusion gradient) Cells establish identity depending on their position in multiple gradientsBicoid protein = headBicoid Manipulation = morphogen gradientConditional SpecificationConditional Specification Cell fate depends on interactions with neighboring cells Embryonic cells can change fates to compensate for missing parts = regulation Conditional specification produces Regulative DevelopmentMorphogen Gradients Cells respond to protein concentration by turning different colors.Morphogen Gradients in Conditional Specification Cell commitment and differentiation are programmed by various morphogen gradients. Transplants of flag “cells” shows that they retain their identity (nationality), but grow according to the cells around them.An example of Gilbert’s flag analogy illustrating conditional specification in an actual developmental situation. (this is also mentioned in the text) Drosophila legs and antennae are structurallyrelated, but obviously different morphologically. Let’s assume that “leg” and “antenna” are like the national identities (French and American) in the flag analogy. If one would transplant cells that would ordinarily make up the proximal (close to the body) area of the fly leg to an area that would ordinarily produce the tip of the fly antenna, the resulting structure would be an antenna with a claw at the end. antenna cell fields (imaginal discs in the fly – see your lab book) that will develop into legs and antenna this set of cells will develop into the proximal (close to the body) portion of the leg this set of cells will develop into the distal (away from the body) portion of the antenna claws see next slideleg field antenna field proximal area distal area transplant claws at the tip (really, that’s what they look like!! Look at the picture in the previous slide if you don’t believe me! The transplanted leg cells have kept their “leg” identity, but have modified their development from their original location (proximal to the body), to that of their new location (the distalmost point). In this example of conditional specification, a morphogen gradient that started at the body (source) would specify proximal structures. As the morphogen concentration decreased more distal structures would be formed. Therefore, while the leg cells kept their leg identity, they were “conditioned” by the very low morphogen concentration at the tip (sink) to form the most distal leg structures –