Developmental Bio - Intro Lecture Flashcards
definition of embryology, general research methods
descriptive study of series of events that occur when various types of embryos form
relies more on dissection, light, and electron microscopy (“classical” techniques)
what is epigenesis? who was the first person to hypothesis this?
formation of structures de novo from preexisting ones - first thought of by Aristotle in “On the Generation of Animals”
what was preformationism, and who were the ovists and spermists?
preformationism was a widely popular belief between the mid-late 16th and early 19th centuries that posits that embryos develop as pre-made, miniature organisms of the full adult (e.g., that embryos grew from “homunculi”, mini humans; these mini organisms contained mini sized organs, tissue, etc.).
ovists were those that hypothesized that the miniature human is located in the egg and grows there during development, whereas spermists believed that spermatozoa held the miniature human, and the egg was simply required to activate growth.
what are the types of changes or growth one would see over time if preformationism existed?
the “animalcule” would simply scale up over time until birth; there would be no development of new tissue or organs, as they would already exist within the miniature organism.
what was major evidence against preformationism?
more sophisticated microscopy revealed clearly that little people were not inside sperm and egg cells
embryo features were found to change a great deal over the course of development, e.g. body plan development, bone growth, formation of new structures, and not simply a “scaling” process of a tiny human.
what is haeckel’s biogenetic “law”, and what does it mean?
“ontogeny recapitulates phylogeny”
- embryonic development goes through stages that “repeat” stages of animal evolution.
ontogeny = embryonic development
phylogeny = evolutionary development (evo-devo) of organisms in time or the set of ancestry to descendent relationships of all living creatures
what was an example of “ontogeny recapitulates phylogeny”?
“gill arches” of mammalian embryos - actually pharyngeal pouches (superficially look like gills). are not involved in respiratory functions - develop into portions of mid ear, tonsils, thymous, thyroid.
fish embryo pharyngeal pouches -> gill slits
do human embryos pass through the stages of “lower” animals?
no; however, embryos have certain common rudimentary structures. mutations have allowed for a variety of uses of these common structures.
what is the current consensus about the relationship between ontogeny and phylogeny?
ontogeny does not recapitulate phylogeny (not simplistically); changes in ontology could be argued to create phylogeny, though.
what is one fact about early embryos that emphasizes the complexity of the relationship between ontogeny and phylogeny?
early embryos are very diverse between species (e.g. blastula and gastrula stages)
what is the “phylotypic” stage?
the most conserved stage of embryogenesis between related species (such as phylum)
e.g. stage at which embryos develop maximal similarities between each other (maximal gene expression conservation or gene sequence divergence).
for vertebrates: stage w/ tail bud, pharyngeal pouches; high levels of homeotic gene (genes regulating dev. of anatomical structures) expression, continued body plan execution + organogenesis
what are the two models (or hypotheses) of the phylotypic stage?
1) funnel model - the most conserved stage occurs early in development, with increasing divergence throughout the rest of development
2) hourglass model - the most conserved stage occurs in the middle of development; high divergence in early stages (decreases to middle stage) and final stages
evidence/arguments for the funnel model of the phylotypic stage
- period characterized by large amount of interactions as body plan is being developed - lots of similar signaling and early information movement between related species
- experiemnts using gene knockout found that the amount of essential developmental genes decreased over time (stronger constraints early that would “funnel” embryogenesis into similar states)
evidence/arguments for the hourglass model
- early morphological work showed greatest divergence at early and late development
- Hox genes (highly conserved gene sequences involved in anterior-posterior body axis formation) are activated in middle of dev at phylotypic stage
- genes expressed were found in studies to be oldest at the middle of development compared to early/late stages
- gene expression profiles for various vertebrates have found the most conserved expression to occur mid-dev, at the pharyngular stage
what are reasons that some argue that the “phylotypic” stage of development is more resistant to change than the earlier or later stages (and that some embryonic stages are “recapitulated”)?
phylo stage is most resistant to change bc of the expression of vital genes (e.g. Hox genes) in body plan development and execution and organogenesis. the phylotypic stage is the stage at which critical rudimentary structures are formed that allow for greater complexity as development continues.
many genes expressed in this stage are also important in later processes post-embryogenesis. thus, this stage is highly conserved as it is key for the development of the rest of the organism.
lifecycle practice - step after fertilization
cleavage - series of divisions gives rise to blastomeres. @ end of cleavage, a blastula (hollow ball of blastomeres surrounding fluid called blastocoel) is formed.
lifecycle practice - step after cleavage
gastrulation - cells (either individually or as sheets) move w/ respect to one another (often, a “blastopore” lip is formed and cells migrate inside of it). embryo @ this stage is called a gastrula, resembles ball folded in on itself.
what is the process of neurulation? what does it entail?
neurulation is the formation of the rudimentary nervous system
entails:
1) migration of ectodermal cells give rise to raised neural plate
2) margins of plate fuse to form neural tube - eventually forms brain + spinal cord
3) archenteron (rud. gut) formed by endodermis
4) mesoderm gives rise to regions that will become backbone, dosral musculature, kidneys, heart, etc.
