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Developmental Biology

By Levi Clancy for Student Reader on

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Model organisms

Invertebrate model organisms
  • Nematode

  • Drosophila

  • Sea urchin

Vertebrate model organisms
  • Xenopus

  • Chicken

  • Zebrafish (Danio rerio)

  • Mouse

Different model organisms are used to study development, chosen for their: length of embryonic development: length of life cycle; size of organism; ease of lab growth; size of genome; number of chromosomes; and experimental techniques available for that organism.

Embryos that develop internally are difficult to see and manipulate, as they embed into the mother's uterus. Large embryos are easier to directly manipulate (ie, tissue transplantation). Large adult organisms are space- and money-consuming versus smaller organisms, which can be easier to analyze and in large numbers.

External development allow operations to be easily performed on the embryo and their effect on development to be observed.

DrosophilaSmall organisms.
Large litters.
Fast development
External development
Genetic screens.
Other excellent tools.
MouseMammal.Internal development.
Small litter.
Relative large.
Many excellent tools.
XenopusLarge (1.2mm) eggs.
Large embryos.
External development.
Tetraploid.Few genetic tools.Operations on the developing embryo are relatively easy, to study the role of different tissues and their interactions with each other. Genetic tools are being developed for diploid frog species.
Large embryos.
External development.
Many progeny.
Excellent genetic tools.Danio rerio is the zebrafish. This small tropical fish has 25 pairs of chromosomes, with its entire genome being 1.7x109 base pairs. Its life cycle from fertilization to fertile adult is three to four months. From fertilization to hatching takes only two days and feeding starts at five days. The genome is being sequenced.
Mammal-like development.
Large embryos.
External development.
Sea urchinExternal developmet.
Easy egg and sperm collection.
Large batches of synchronous embryos.
An echinoderm with small (100µm) eggs. Embryo readily takes up radioactive precursors into DNA, RNA and protein. The sea urchin has been used extensively for studies of fertilization and the activation of macromolecular synthesis.


DevelopmentThe process by which a single cell (the zygote, a fertilized egg) cleaves and grows to a multicellular organism with: differentiated cells, tissues and organs; and an organized body plan. Development involves cell proliferation (growth) and progressive cell specification (differentiation).
EmbryogenesisEmbryogenesis gives rise to the embryo by generating increasing complexity via differential gene expression (the action of different genes in different cells).
BlastomereAny of the embryonic cells arising from the fertilized ovum.
GameteThe female gamete is the oocyte and the male gamete is the sperm. Gametes arise from germ cells.
OocyteThe female's large nonmotile gamete, aka egg, plural ova, singular ovum.
SpermatocyteThe male's small motile flagellated gamete, aka sperm.
OogenesisThe gametogenesis of oocytes.
SpermatogenesisThe gametogenesis of sperm.
Germ CellGerm cells are the type of stem cell that give rise to gametes: male spermatogonia (singular spermatogonium); and female oogonia (singular oogonium). Germ cells are distinct from the rest of the body and arise in the earliest embryonic cell divisions.
Stem CellDivides throughout life of organism, making more cells like itself and providing precursor cells for a pathway of differentiation. Examples: blood-forming erythroblasts; stem cells in gut and skin; stem cells in nervous system; spermatogonia.
SpermatogoniaSpermatogonia are stem cells that persist throughout adult life in all animals.
DeterminationDetermined cells have acquired sets of regulatory factors that drive development along a specific pathway.
Genomic EquivalenceAll cells of an embryo have identical genetic information -- they are genomically equivalent. Blastomere isolation, nuclear transplantation and in situ hybridization all proved this. (link)
TotipotencyThe ability for a cell to divide and give rise to all the differentiated cells of an organism.
Midblastula TransitionIn some embryos (including Xenopus) there is a midblastula transition whereby zygotic genes activate, the cell life cycle grows longer, different cells start dividing at different times (becomes asynchronous) and the cells grow motile. It occurs when the DNA:cytoplasm ratio grows high enough, which normally happens after successive divisions. This can be artificially induced by directly injecting DNA or adding extra sperm, or delayed by using a haploid embryo. (link)
MorphogenA substance whereby different concentrations will specify different cell fates. In Drosophila, removing removing bcd mRNA will result in a lack of the transcription factor Bicoid and thus lead to no anterior formation. (link)
Zygotic Lethal Gene
Maternal Effect GeneA gene whose mRNA is transcribed by the mother during oogenenesis within the egg or translocated to the egg.
Zygotic GeneA maternal effect gene acts before any zygotic genes, which are encoded by the embryonic genome itself.
Zygotic LethalA zygote gene causing a fatal phenotype when homozygously mutant.
Gap Gene
Pair Rule Gene
Dominant Negative Mutation
Transcriptional SynergyIf binding of a single transcription factor activates one transcription unit, then binding of two or three transcription factor can activate thousands of transcription units.
SyncitiumA syncitium contains many nuclei but no cell boundaries
Temporal ColinearityWhen genes are activated in a time sequence that follows their physical order in a cluster.
EpitheliumAn epithelium is an organized sheet of cells that has the characteristic of being polarized. The basal side is bordered by a basement membrane. The apical side faces a lumen (like the cavity of the gut) or the outside surface of the organism.
MesenchymeCells that are not organized into an epithelium are called mesenchymal. A mesenchyme is a loose arrangement of cells (that may be individually mobile) surrounded on all sides by a matrix of extracellular material.
Extracellular Matrix
Homophilic InteractionAssociation of identical molecules interacting with each other.
Animal PoleContains the germinal vesicle (nucleus).
Vegetal PoleContains the yolk.
Gray CrescentSome amphibians contain pigment granules in the animal pole, leading to the appearance of the gray crescent.
CortexThe cortex is the region proximal to the cell surface, directly underneath the cell membrane
Cortical CytoplasmThe cortical cytoplasm is the cytoplasm of the cortex region. In some amphibian eggs, the animal cortex contains pigment granules that darken it.
Cortical Rotation
Veg-TA transcription factor secreted by Xenopus vegetal cells required for mesoderm determination and also differentiation of vegetal cells into endoderm.
Vg-1A secreted factor from Xenopus vegetal cells required for mesoderm determination.
EctodermDerives from animal cap cells. Vertebrates: gut, liver lungs. Insects: gut.
MesodermDerives from the marginal zone, part of the animal cap. Vertebrates: muscle, heart, blood, skeleton, kidney. Insects: muscle, heart, blood.
EndodermDerives from vegetal cells. Vertebrates: nervous, system, skin. Insects: nervous, system, cuticle.
Differential ScreenDifferential screening (aka subtractive hybridization) identifies genes expressed at specific times and places.
Nieuwkoop CenterNieuwkoop center establishes a gradient of Xnr, which stands for Xenopus nodal related molecules (TGF-β-like cell signaling proteins)
Homeotic TransformationIn homeotic transformation, a normal body part is replaced by a body part which is regularly found in other regions. For example, Antennapedia mutants of Drosophia have antennae replaced by legs. Also, Ultrabithorax (Ubx) mutants of Drosophila have halteres (T3) replaced by wings (T2), imparting four total wings.
InvaginationOccurs during gastrulation to form mesoderm in urchins. Like a hole poking into a soft balloon. Epithelium is maintained.
InvolutionA sort of passive invagination, such as in cells near a site of invagination. Most organisms in which invagination occurs in fact use a combination of invagination and involution.
Convergent Extension
Tetragenic EffectCausing morphological deformities in the fetus.
HomeodomainA highly conserved protein domain that binds to DNA.
HomeoboxThe DNA sequence that encodes a homeodomain is a homeobox.
BMP and WntVentral-promoting growth factors in amphibians.
Chordin, Noggin, Follistatin and FrzbDorsalizing factors in amphibians; antagonize the ventralizing factors BMP and Wnt.