Xenopus

The South African clawed frog Xenopus laevis is a flat, smooth frog with lidless eyes and webbed feet (in Latin, xenopus means "peculiar foot," and laevis means "smooth"). The lateral-line system, which consists of sensory hair cells covering the body that are used to detect movements in the water column, persists in adults. The lidless eyes, webbed feet, and maintenance of the lateral-line system in the adult stage are all adaptations to these frogs' lifelong environment. Xenopus can live in virtually any amount or quality of water, a necessary adaptation when ponds begin to dry up and become stagnant in the summer. They may also aestivate in the mud of dried up ponds, shutting down most of their life processes until more favorable (i.e., wet) conditions arise. These frogs can survive for several months without food or water in aestivation. Xenopus are not entirely out of their element on land, either. They have been known to migrate overland in times of drought and to move from overcrowded ponds to colonize new areas at the onset of torrential rains.

Xenopus has become a model organism for studies of vertebrate development, and can be found in labs around the world. In fact, escapees have established themselves in a number of wild areas in the Americas. Several characteristics make Xenopus amenable to laboratory experimentation. First, they are aquatic and can complete their entire life cycle in water. Thus, unlike most amphibians, they can be kept in water tanks instead of terraria , which are harder to maintain. Second, they are hardy animals. They eat virtually any type of food and are not prone to disease. Third, they are extremely fertile and have a relatively short life cycle. Females can lay up to 1,000 eggs at one spawning, and these eggs can develop into reproductive adults within one year.

The life cycle of Xenopus, as in most other frogs, is made up of three main stages: fertilized egg, tadpole, and adult frog. Mating occurs in fresh water with the male clinging to the back of the larger female. Fertilization occurs externally—the female lays her eggs in the water while the male releases sperm over them. The tadpoles hatch within three days and start to filter-feed from detritus in the water. After about two months, the tadpoles undergo metamorphosis , a marked structural transformation in which the legs sprout and the tail is lost. Internally, the skeleton of the head changes shape and teeth appear; and the circulatory, digestive, immune, and other systems are differentiated. Adult Xenopus use the claws of their front fore-limbs to tear up food which they then manipulate into their tongueless mouths.

Xenopus first gained widespread use in labs when researchers discovered that they could be used in human pregnancy tests. The injection of a small amount of urine from a pregnant female under the skin of female Xenopus causes them to lay eggs. In fact, an injection of reproductive hormones from many vertebrate species causes the same reaction. This proved to be an extremely useful trait, as biologists could now induce egg laying to study the embryology and development of Xenopus eggs at any time of year.

Another reason that Xenopus are useful in labs is that the large size of their embryos and cells makes visualization of the various developmental stages significantly easier than in most other vertebrates . Various staining and labeling techniques are used to follow the cellular and molecular changes occurring in the developing embryo. For example, specific tissues of the embryo may be stained, and then several of these stained embryos may be collected at later life stages. To determine the distribution of this tissue in the embryo, histological sections, or thin slices of embryos from each life stage, are made and observed for staining patterns. In this way, scientists can follow the three-dimensional migration and differentiation of a particular tissue type over time.

Scientists use similar techniques to follow spatial and temporal gene transcription patterns. For example, to determine when the embryo begins making its own mRNA (and thus its own proteins), rather than relying on maternal mRNA, scientists inject radioactively labeled mRNA precursors into developing embryos. As before, sections of the embryos are then made and analyzed for mature radioactive mRNAs. Scientists can also follow the function of a particular gene over time by injecting developing embryos with radioactive probes that bind specifically to mRNA of that gene. This is a very important technique as it lends insight into which genes control which developmental processes.

Probably the most striking aspect of Xenopus development is metamorphosis. The tadpole and adult stages have fundamentally different body plans and life styles. This leads to the question: How are the cells and tissues of the larval stage rearranged and redistributed to form the adult stage, at the same time allowing the developing organism to maintain necessary life functions?

Metamorphosis is initiated by hormones released from the pituitary and thyroid glands. One hormone in particular, thyroxine, affects several tissues and organs of the larvae. In the brain, thyroxine causes most cells to undergo mitosis , and the brain becomes larger. However, in the hindbrain, giant Mauthner's cells, which extend down the spinal chord and allow the rapid darting movements tadpoles use to avoid predation, degenerate when exposed to thyroxine. Similarly, thyroxine causes the deposition of collagen, a fibrous skin protein, in most areas of the body, but causes the breakdown of collagen in the regressing tail. Thyroxine also causes the initiation of bone development in the limbs.

Thyroxine sets off a cascade of developments in the maturing frog. The resorption of the tadpole's tail begins with the resorption of the notochord from the tip of the tail to the base. Next the vertebral rudiments and connective tissue that surrounded the notochord degenerate. Finally the tail muscles are resorbed and disappear rapidly. This may be because the organism does not want to lose the tadpole form of locomotion (its tail) before the adult form of locomotion (webbed feet) is fully developed. Once the webbed feet are developed, the tail becomes a hindrance and is quickly lost.

The appearance of limbs occurs in the following manner. The hind legs start to bud off just before the front limbs, and shortly thereafter are innervated by new nerve cells. The pelvic girdle (the hip) ossifies first, then the bones of the legs ossify one by one outward from the body. The fore-limbs, however, ossify before the shoulder girdle is completed. This is possibly an adaptation which allows filter feeding from the mouth through the gills to continue during development of the forelimbs.

Other than metamorphosis, another unique feature of amphibians such as Xenopus is their ability to regenerate their limbs, tail, and many internal organs. If a limb is severed, dead cells at the site are quickly removed through the blood stream. Then nearby mesodermal cells, cells with specialized functions in connective or other tissues, multiply and "dedifferentiate" so that they can act as precursors for the new types of cells necessary to form a complete limb. In general, the ability to regenerate body parts declines over time, especially after metamorphosis is completed.

Todd A. Schlenke

Bibliography

Bernardini, Giovanni, M. Prati, E. Bonetti, and G. Scari. Atlas of Xenopus Development. New York: Springer-Verlag, 1999.

Deuchar, Elizabeth M. Xenopus: The South African Clawed Frog. New York: John Wiley and Sons, 1975.

Duellman, William E., and Linda Trueb. Biology of Amphibians. Baltimore: Johns Hopkins University Press, 1986.

Nieuwkoop, Pieter D., and Jacob Faber, eds. Normal Table of Xenopus Development (Daudin): A Systematical and Chronological Survey of the Development from the Fertilized Egg until the End of Metamorphosis, 2nd ed. Amsterdam: North-Holland Publishing Company, 1975.

Tinsley, R. C., and H. R. Kobel. The Biology of Xenopus. Oxford, U.K.: Oxford University Press, 1996.