Biology 4361 – Developmental Biology Principles of Experimental Embryology June 16, 2008
Overview What forces affect embryonic development?
The embryonic environment: external and internal How do forces within the embryo cause the differentiation of cells? Differentiation ­ definition Specification, commitment, and determination ­ concepts Types of specification: autonomous, syncytial, and conditional Morphogens and morphogen gradients Stem cells and commitment How do cells organize themselves into tissues and organs? The Embryonic Environment What is the “embryonic environment”? External influences: ­ light ­ temperature ­ humidity ­ predators ­ competitors ­ intraspecific signals
Internal influences (e.g. intrauterine): ­ chemicals (e.g. maternal hormones, caffeine, nicotine) ­ competitors (e.g. litter­mates) Environmental regulation pathway: ­ external stimulation triggers signaling event in embryo ­ signal stimulates an embryonic pathway (e.g. endocrine), that changes the developmental pathway. Differentiation Differentiation – development of cellular specialization How does the intraembryonic environment direct cellular differentiation? Differentiation is a process ­ preceded by the commitment of cell to certain fate(s) Commitment is also a staged process:
­ specification ­ determination Commitment Stages Stages of cell commitment: 1. Specification. Capable of differentiating autonomously when placed in a neutral environment; not when placed in non­neutral environment (functional definition) ­ reversible 2. Determination. Capable of differentiating autonomously even when placed into another embryonic region. (functional definition) ­ essentially irreversible undifferentiated differentiated specification determination Specification Types I. Autonomous specification II. Syncytial Specification III. Conditional Specification
Autonomous Specification ­ Cells are specified by differential distribution of cytoplasmic components during cleavage of the egg and early embryo. ­ proteins ­ RNA
Autonomous Specification Tunicate (sea squirt) blastomere separation
Autonomous Specification ­ 2 Tunicate (sea squirt) Blastomeres are committed at a very early stage in mosaic development dissociated blastomeres If split, each dissociated blastomere pair forms original structures
Each blastomere contains positional information in the form of specific proteins and genes Autonomous (Mosaic) Development differentiated cell mitosis early embryo later embryo
Syncytial Specification Syncytium – nuclear division without cell division; results in cytoplasm with many nuclei
Drosophila Cleavage nuclei & cytoplasm form syncytial blastoderm FELICE FARBER
Syncytial Specification through Morphogen Gradients Drosophila egg U Irion & D St Johnson
Maternal messages: bicoid – anterior determinant nanos – posterior determinant Syncytial Specification through Morphogen Gradients Maternal messages: bicoid – anterior nanos – posterior Bicoid & Nanos proteins = morphogens Each morphogen establishes a gradient throughout the embryo (like a diffusion gradient)
1:0 10:1 1:1 1:5 1) each region has a distinct Bicoid:Nanos ratio 2) Bicoid:Nanos determines anterior­posterior identity Syncytial Specification through Morphogen Gradients Maternal messages: bicoid – anterior nanos – posterior 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
Conditional Specification differentiated cell mitosis later embryo early embryo ?
Conditional Specification Legs and antennae: structurally­related; different morphologies. Experiment: Transplant embryonic cells that would produce proximal leg (close to the body) to an area that would ordinarily produce antenna tip. antenna proximal leg ? distal antenna claws I. Duncan claw
Morphogen Gradients ­ Conditional Specification Cell commitment and differentiation are programmed by various morphogen gradients.
e.g. cells respond to protein concentration by turning different colors Morphogen Gradients ­ 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.
Conditional Specification: Leg­Antenna Transplant I. Duncan Transplanted leg cells keep “leg” identity ­ but modify development from their original location (proximal to the body), to that of their new location (the distal­most point). A morphogen gradient started at the body (source) specifies proximal structures. As the morphogen concentration decreases more distal structures form. Therefore, while transplanted leg cells kept their identity, they were “conditioned” by the low morphogen concentration at the tip (sink) to form the most distal leg structures – claws.
Stem Cells and Commitment * *
Totipotent* – ability to make all cell types; embryo and trophoblast (fetal portion of the placenta). Pluripotent* – uncommitted; makes many types of cells. Multipotent* – committed; makes several different types of cells *all stem cells regenerate copies of themselves Stem Cell Derived Blood Cells
Overview How do forces outside the embryo affect its development? How do forces within the embryo cause the differentiation of cells? How do cells organize themselves into tissues and organs? Morphogenesis and Cell Adhesion
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 cells 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. Cadherin Calcium­dependent adhesion ­ multiple forms
Cadherin­Mediated 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