Biology 4361 Developmental Biology Principles of Experimental Embryology September 19, 2006
Major 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 Organisms are connected to their environment What is the embryonic environment? External development stimuli: light temperature humidity predators competitors intraspecific signals 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.
Environmental 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. seasonal polyphenism common in butterflies wet season dry season dry season wet season Nemoria bistriaria Bicyclus anynana
Seasonal polyphenism Nemoria arizonaria larvae early spring oak catkin (flower) summer oak twig Environmental signal tannins in oak leaves (probably) developmental response change in cuticle morphology
Anthropogenic effects (anthro man) Industrial melanism pepper moths
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! Exposure can induce protective mechanisms melanin R OH COOH N OCH 3 HO HO NH COOH Mycosporine like amino acid = sunscreen
Environmental Signals Intraspecific Bonellia viridis (marine echiuroid worm) larvae settles on sand/rock surface = larvae settles on proboscis = 2 mm Therefore, signal (most likely chemical) to become is from 10 cm
Temperature dependent sex determination sex determined by the egg temperature during weeks 2 & 3 30 C = female 34 C = male
Developmental Dynamics of Cell Specification Stages of cell commitment: 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 undifferentiated differentiated specification determination
Cell Type Specification I. Autonomous specification II. Syncytial Specification III. Conditional Specification
Autonomous 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) blastomeres are committed at a very early stage in mosaic development each dissociated blastomere pair forms original structures each blastomere will contain positional information in the form of specific proteins and genes
Syncytial Specification syncytium nuclear division without cell division; results in cytoplasm with many nuclei
Drosophila Cleavage Drosophila nuclei & cytoplasm form syncytial blastoderm
Syncytial Specification through Drosophila egg Morphogen Gradients Maternal messages: bicoid anterior determinant nanos posterior determinant Each morphogen establishes a gradient throughout the embryo (like a diffusion gradient) Cells establish identity depending on their position in multiple gradients
Bicoid protein = head
Bicoid Manipulation = morphogen gradient
Conditional Specification
Conditional Specification Cell fate depends on interactions with neighboring cells Embryonic cells can change fates to compensate for missing parts = regulation Conditional specification produces Regulative Development
Morphogen 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. 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 see next slide antenna claws
leg field antenna field proximal area distal area transplant transplant cells from the proximal portion of the leg field to the distal portion of the antenna field structure of transplanted antenna 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 claws.
Stem Cells and Commitment Pluripotent makes many types of cells. Totipotent ability to make all cell types.
Stem Cell Derived Blood Cells
Morphogenesis and Cell Adhesion How are tissues formed from populations of cells? How are organs constructed from tissues? How do organs form in particular locations and how do migrating cell reach their destinations? How do organ and their cells grow, and how is their growth coordinated throughout development? How do organs achieve polarity?
Cell Interactions Cells interact with each other either through paracrine signaling at some distance, or through direct contact. Cell membrane protein components bind cells together; e.g. cadherins catenins
Cadherin mediated Cell Adhesion Different cells have different cadherins. Different cadherins have different affinities for each other. Thus, cell types can segregate themselves based on membrane components. actin microfilament system = anchoring and movement NOTE Ca 2+ dependent binding: Ca 2+ can control both strength and reversibility of binding