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. |