Developmental Biology

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.

Organism	Pros	Cons	Tools	Notes
Drosophila	Small organisms. Large litters. Fast development External development		Genetic screens. Other excellent tools.
Mouse	Mammal.	Internal development. Small litter. Relative large.	Many excellent tools.
Xenopus	Large (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.
Zebrafish	Vertebrate. 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.
Chicken	Vertebrate. Mammal-like development. Large embryos. External development.
Sea urchin	External 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.

Glossary

Development	The 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).
Embryogenesis	Embryogenesis gives rise to the embryo by generating increasing complexity via differential gene expression (the action of different genes in different cells).
Somitogenesis
Blastomere	Any of the embryonic cells arising from the fertilized ovum.
Gamete	The female gamete is the oocyte and the male gamete is the sperm. Gametes arise from germ cells.
Oocyte	The female's large nonmotile gamete, aka egg, plural ova, singular ovum.
Spermatocyte	The male's small motile flagellated gamete, aka sperm.
Oogenesis	The gametogenesis of oocytes.
Spermatogenesis	The gametogenesis of sperm.
Germ Cell	Germ 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 Cell	Divides 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.
Spermatogonia	Spermatogonia are stem cells that persist throughout adult life in all animals.
Determination	Determined cells have acquired sets of regulatory factors that drive development along a specific pathway.
Genomic Equivalence	All cells of an embryo have identical genetic information -- they are genomically equivalent. Blastomere isolation, nuclear transplantation and in situ hybridization all proved this. (link)
Totipotency	The ability for a cell to divide and give rise to all the differentiated cells of an organism.
Midblastula Transition	In 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)
Morphogen	A 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 Gene	A gene whose mRNA is transcribed by the mother during oogenenesis within the egg or translocated to the egg.
Zygotic Gene	A maternal effect gene acts before any zygotic genes, which are encoded by the embryonic genome itself.
Zygotic Lethal	A zygote gene causing a fatal phenotype when homozygously mutant.
Gap Gene
Pair Rule Gene
Dominant Negative Mutation
Transcriptional Synergy	If binding of a single transcription factor activates one transcription unit, then binding of two or three transcription factor can activate thousands of transcription units.
Enhancer
Determination
Syncitium	A syncitium contains many nuclei but no cell boundaries
Temporal Colinearity	When genes are activated in a time sequence that follows their physical order in a cluster.
Epithelium	An 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.
Mesenchyme	Cells 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.
Cytoskeleton
Microtubule
Microfilament
Extracellular Matrix
Cadherin
Integrin
Homophilic Interaction	Association of identical molecules interacting with each other.
Animal Pole	Contains the germinal vesicle (nucleus).
Vegetal Pole	Contains the yolk.
Gray Crescent	Some amphibians contain pigment granules in the animal pole, leading to the appearance of the gray crescent.
Cortex	The cortex is the region proximal to the cell surface, directly underneath the cell membrane
Cortical Cytoplasm	The 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-T	A transcription factor secreted by Xenopus vegetal cells required for mesoderm determination and also differentiation of vegetal cells into endoderm.
Xnr
Vg-1	A secreted factor from Xenopus vegetal cells required for mesoderm determination.
Ectoderm	Derives from animal cap cells. Vertebrates: gut, liver lungs. Insects: gut.
Mesoderm	Derives from the marginal zone, part of the animal cap. Vertebrates: muscle, heart, blood, skeleton, kidney. Insects: muscle, heart, blood.
Endoderm	Derives from vegetal cells. Vertebrates: nervous, system, skin. Insects: nervous, system, cuticle.
Differential Screen	Differential screening (aka subtractive hybridization) identifies genes expressed at specific times and places.
Nieuwkoop Center	Nieuwkoop center establishes a gradient of Xnr, which stands for Xenopus nodal related molecules (TGF-β-like cell signaling proteins)
Homeotic Transformation	In 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.
Invagination	Occurs during gastrulation to form mesoderm in urchins. Like a hole poking into a soft balloon. Epithelium is maintained.
Involution	A 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.
Ingression
Convergent Extension
Segmentation
Tetragenic Effect	Causing morphological deformities in the fetus.
Homeodomain	A highly conserved protein domain that binds to DNA.
Homeobox	The DNA sequence that encodes a homeodomain is a homeobox.
BMP and Wnt	Ventral-promoting growth factors in amphibians.
Chordin, Noggin, Follistatin and Frzb	Dorsalizing factors in amphibians; antagonize the ventralizing factors BMP and Wnt.