1 history of embryology

8/6/2019 1 History of Embryology

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DEFINITIONS:Embryology - study of development Phylogenetic development- gradualhistorical transformation of the speciesOntogenetic development- transformationof a rudiment derived from the parent

organism into a new adult individual

SCOPE OF EMBRYOLOGY:

Molecular Embryology, Biotechnology,Ecological Developmental Biology, ClinicalGenetics, Evolutionary Developmental

Biology, Bioethics



Types of ontogenetic development

1.Embryogenesis - development of theegg

2.Blastogenesis- development of new

individuals by asexual reproduction

Phases of ontogenetic development:Gametogenesis, Fertilization,

Cleavage and Blastula formation,Gastrulation, Organogenesis, Growth

and histological Differentiation



Development DOES NOT happen by magic.

Information and mechanisms at thecellular and/or molecular levels are

needed to accomplish development.



A SCIENCE WITH LOTS OF QUESTIONSTO BE A NSWERED«

Where are the plans or instructions fordevelopment?How are they interpreted and used?Where are the raw materials for development?How did those raw materials get to the embryo?How are they used?How do different cells and tissues know what tobecome?How do cells migrate? How do they know where togo?How is neural circuitry established?How do axons find their target? ETC.. ETC., ETC«



1. A ristotle (340 BC)Do all parts of a developing organism come into existencetogether and simply grow larger?Is development a stepwise process characterized byprogressive organization and an increase in complexity?The embryo was formed from menstrual blood interactingwith a male vital factor present in the semen. This creative

force forms the maternal substance into embryonic bodyparts.He observed that new structures arose progressively inembryos (e.g. blood, blood vessels, heart, blood vesselsaround organs). These supported the concept of epigenesis , i.e., the organism develops in a stepwise

fashion from an unorganized state.This term would not be used until the mid to late 17thcentury.

HISTORIC A L BA CKGROUND



2.Bonnet &Swammerdam :(17th century)Preformation

Theory : embryonicparts are alreadypresent in thesperm or egg

(animalculists orovists) whichsimply grow in sizein development.

Homunculus



3.Caspar Friedrich Wolff (1759)

Similar to epigenetic theorybut postulated that adevelopment force inherent in the matter of the embryo

directs the laying down of body parts in sequence.He laid the foundation forthe germ layer theory by showing that thematerial out of which the embryo isconstructed is, in an early stage of development, arranged in the form of leaf-like layers.



4. Karl Ernst von Baer (1828) -most coherent embryologicaldataBaer·s law: More general

features that are commonto all members of groups of animals are developed in the embryo earlierthan the more special features which distinguish

the members of the group.Ex: all vertebrates (brain & spinal cord,notochord, segmented muscles, aortic arches);vs. various classes (hair, feathers, limbs)

Descriptive and Comparative Embryology



5.Muller (1864), Haeckel (1868)

Biogenetic law : features that are inherited from the common ancestor of the group havean ancient origin, & develop earliest duringontogeny

Example: ́ Big 4µ characteristics of allembryonic chordates despite differences inadult appearanceDivergent features are adaptations of theembryo to its surroundings (e.g. placenta)However, an embryo does not ́ pass throughµthe adult stages observed in lower animals.



ONTOGENY REC A PITUL ATES PHYLOGENY?



Ontogeny does not recapitulate phylogeny;

ontogeny is a shortenedand modified

recapitulation of phylogeny



This bird would note that: (1) adult humans lackbeaks, just as embryonic birds do (2) their adult

extremities are very much alike (having not differentiated into wings and legs) just as is the

case of embryonic birds (3) mammals showonly the most primitive form of feather

production. Therefore, birds are far moreadvanced than humans, and the adult human

resembles the embryonic bird.

But what if a bird was

THEevolutionary biologist«



6. A ugust Weismann (1883)Germ plasm theory - every germ cell duringearly development receives a complete set of units of heredity (´idsµ or Mendel·s ´genesµ)Development involves orderly unpacking of anembryo as dictated by ids; interactionsbetween parts make epigenetic development possibleEach egg nucleus contain discrete localizeddeterminants which result to unequaldistribution of nuclear components duringcleavageCells cannot change their fate if a blastomereis lost ( mosaic model of development )



7. Wilhelm Roux (1905)Heat-killed one of the 2 blastomeres of a frog·s eggSurviving cell developed half of a complete embryoResults either support both preformation and mosaicdevelopment, or reflect use of crude techniqueswhich possibly caused defects in the other half

Experimental Embryology



8.H. Driesch (1891),Endres (1895),Spemann (1901),Schmidt (1903)If cleavage cells of a sea urchin were

completelyseparated, eachcould develop intoa whole embryo

(regulative model of development ).Massive cell rearrangements and migrationsprecede or accompany specifications of development, allowing cells to acquire different

functions



9.T.H. Morgan (1919), Watson & Crick (1953) ² units of heredity composed of sequenceof DN A base triplets are transformed into anarray of proteins, which acting partly on theirown or through other chemical components,system that is an adult organism.

A nalytical (modern) Embryology



Development is epigenetic.A lthough it is regulated by the nucleus,

it takes place primarily in the

cytoplasm.It involves interactions between parts.The parts arise within gradient

patterns or fields.Differentiation is in essence thedevelopment of the macromolecular

pattern within the cell.

Summary Points:



REFERENCES

Dudek, R. High-Yield EmbryologyGilbert, S. Developmental BiologyLarsen, W. Human EmbryologySadler, T. Langman·s Medical Embryology

http://www.ncbi.nlm.nih.gov/books/NBK9983/http://www.uoguelph.ca/zoology/devobio/dbindex.ht m#http://people.ucalgary.ca/~browder/virtualembryo/db

_tutorial.htmlhttp://www.luc.edu/faculty/wwasser/dev/devm.htmhttp://www.biology.arizona.edu/developmental_bio/developmental_bio.html

1 history of embryology

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