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From Sperm and Egg to Embryo

Genetic Regulation of Development

Organizing the Multicellular Embryo

Generating Cell Diversity

Dynamic Development at a Glance

Development of the Nematode Caenorhabditis elegans

In this week's tutorial, the development of C. elegans will be examined.  As you read the following text, you will come across a number of Web links.  These links will provide information that outlines various aspects of worm development for which you will be responsible.  There are also four movies to examine.

The nematode worm Caenorhabditis elegans has taken the world of developmental biology by storm over the past few years. The advantages for the study of development are legendary: transparent embryo and adult, ease of doing genetic analyses and a complete lineage map of every single cell in the body of the adult. It will also be the first multicellular organism to have its genome completely sequenced.

C. elegans is a free-living organism, which normally lives in the soil, where it feeds mainly on bacteria. In the laboratory, it is grown on agar plates or in liquid culture, with E. coli as the food source. These small (~1 mm long and 80 micrometers in diameter) worms have a life cycle lasting 3.5 days. If a single adult hermaphrodite is placed on a plate, up to 500,000 worms can be produced in just over a week (Hope, 1994). C. elegans has two sexes: a male and a self-fertilizing hermaphrodite.

We shall begin our study of C. elegans development by becoming acquainted with the worm itself. Notice that all organ systems are easily seen through the body wall. The worm is shown with the anterior end at the top. The body is comprised of a tube (the gut, comprised of a muscular pharynx and an intestine) within a tube (the cuticle, hypodermis, muscle and nerve cells). The space between them is the pseudocoelom, which contains the gonad. The image shown here is a hermaphrodite. The gonad of the hermaphrodite is bi-lobed; each lobe connects to a single mid-ventral vulva via an oviduct, spermatheca and a common uterus. The gonad of the male is single-lobed. It is located in a mid-ventral location. Sperm travel from the gonad, seminal vesicle and vas deferens, exiting through the cloaca.

We shall next study still micrographs and movies of early development. An overview of worm development has been prepared by Dr. Susan Strome.  There are four movies to help in the visualization of the development of C. elegans, they can be accessed from the hard drive if you are in the Mac Lab.

• Movie #1:  P0.mov

The earliest events after fertilization are summarized in the movie by Steven Herd in the Strome lab of the 1-cell embryo (note that anterior is to the right.) In this sequence, the maternal pronucleus migrates from the anterior to the posterior, through the pseudocleavage furrow. It meets the paternal pronucleus in the posterior, and they migrate anteriorly before fusing and entering mitosis. There are two sets of images here. The images on the right show the segregation of the P-granules to the posterior cortex. The P-granules become localized to the germ cell lineage. Now, examine the series of micrographs showing localization of P-granules.

• Movie #2:  P1.mov

Next, look at movies of the 2-cell embryo (note that anterior at the top).  As cleavage continues to the four cell stage, the P-granules continue to be segregated in the P-lineage (germ cell lineage) at the posterior of the embryo.

• Movie #3:  P2.mov

The 4-cell embryo (anterior on the right, dorsal at the bottom).  There are two sets of images presented; on the right, we see the segregation of the P-granules to the germ cell line as the embryo divides to the 8-cell stage.  On the left, we see the microscope images displaying the process of the third cleavage.

• Movie #4:  annotated_embryo1.MOV

Our next stop will be to a movie of early development prepared at The University of Wisconsin.  Note the asynchronous cleavage of the blastomeres and that the AB blastomere is larger than the P1 blastomere.  You should be able to identify the different blastmeres according to their spatial arrangement.

For many years, C. elegans investigators were convinced that development was directed entirely by the parceling of morphogenic determinants during cleavage. However, we now know that embryonic induction plays a significant role in determination of cell fate. You should now examine the lineage map of founder cells. Return to the overview of worm development and trace the formation of the MS cell. Now do the same thing using the movie of early development (return to movie #4).

For more C.elegans movies, see the webpage maintained by Bob Goldstein's lab.

Reference

Hope, I. 1994. Caenorhabditis elegans, the nematode worm. In J. Bard (ed.), Embryos. Color Atlas of Development, Mosby-Year Book Europe, London, pp. 55-75.

Dynamic Development at a Glance
Main Page Dynamic Development