I had the chance to visit Memorial Sloan Kettering Cancer Center to work with Professor Bao on some experiments with Caenorhabditis elegans (C. elegans).
These C. elegans are very commonly used in research, as they reproduce very quickly in huge numbers.
C. elegans are transparent nematodes that are about 1 mm in length, and live in temperate soil environments.
One very interesting fact about these worms is that they have a male sex and a hermaphrodite! No female sex! The hermaphrodites are simply self-fertile worms that have the ability to make the limited number of sperm used solely for internal self-fertilization. There are way more hermaphrodites in a population than there are males. Males only make up around 0.2% of the worm population.
Another interesting fact about the sex's of these worms is that scientists often forcefully change the sex of the worms through heat-shock. By putting embryo's at 30 degrees celsius for a couple of hours, we can forcefully produce male larvae. Another example of this is turtle eggs. If turtle eggs are buried in colder sand, the babies will be male and vice versa.
In terms of reproduction, C. elegans reproduce incredibly quickly. In 3.5 days, 300 progeny can be made!
Internal structure
C. elegans are very simple creatures. The main structures of the worm is the pharynx and the gut/intestine.
Using fluorescent microscopy, Professor Bao and I were able to pinpoint and showcase the pharynx of the worm.
The pharynx contains roughly 91 cells. The pharynx is a neuromuscular tube of which the function is to pump and crush bacteria, and inject them into the intestine.
Not only that, but the pharynx also resembles the "heart" of the worm.
Moreover, the gut/intestine of the C. elegans receives the bacteria from the pharynx. From here, the bacteria is pushed through down due to the rhythmic movements of the intestine. These rhythmic movements are similar to ours in our small intestine. In order to move food around, our intestines do something called peristalsis, which are these movements of the gut that can transport food easily down.
Next, we'll cover exactly how embryogenesis works in C. elegans.
Embryogenesis
The stages of embryogenesis in C. elegans can be subdivided into 7 parts.
Fertilization. Hermaphrodites produce their own sperm and eggs. The haploid oocyte and haploid sperm created by the worm combine to create a diploid, single cell embryo in the spermatheca. For a brief period, the fertilized embryo becomes covered by a membrane that may prevent polyspermy which is followed by the formation of a harder eggshell consisting of three layers secreted by the egg. The newly fertilized embryo exits prophase arrest and leaves the spermatheca to continue its development in the uterus.
Proliferation. After fertilization, the single cell embryo begins a series of highly stereotyped cell divisions. During this phase, all embryonic cells look similar in their cytoplasmic structures, and many cells also begin to make short distance migrations away from the their sister cells, through a process that has been termed “global cell sorting”. Early proliferation events span from 0 to 150 min post-fertilization and take place within the uterus. Proliferation events continue in later stages, including gastrulation and morphogenesis and these occur after the embryo is laid. Egg-laying occurs through a simple motor program involving specialized smooth muscle cells, whose contraction opens the vulva and compresses the uterus so that eggs can be deposited into the environment.
Gastrulation. Gastrulation is the process in which an embryo transforms from a one-dimensional layer of epithelial cells and reorganizes into a multilayered and multidimensional structure. After the egg has been laid, the embryo has divided a bit. Now having around 30 cells, gastrulation encompasses the process by which single cells begin to become internalized and migrate into the center of the embryonic mass to eventually create separate ectodermal, endodermal and mesodermal compartments.
Morphogenesis. This phase overlaps with the end of gastrulation, when most cells have ended proliferation and are joining tissue subgroups. Cells become structurally specialized to adopt shapes and cell contents that reflect their eventual cell fates within specialized tissue compartments. Here, we can see the shape of the developing embryo's.
The embryo at the top right has entered the first stage of morphogenesis, which is the "bean stage". The other embryo's showcase later developments which are covered in the next section.
Elongation. Once the embryo’s developing tissues begin to form a longer worm-like shape, they become folded within the eggshell as shown above. First through the two-fold and finally the three-fold stage, the embryo decreases in circumference and increases in length until it is ready for hatching.
Quickening. The first signs of muscle movement are seen around 430 minutes post-fertilization, and by the three-fold stage, the worm can move in a coordinated fashion within the egg. A series of squeezing events, and later, active wriggling motions, precedes hatching. Here are some videos of the worms moving in their shells.
These squeezing activities surely aid in further elongation of the animal, though they occur quite late. Quickening occurs coincidentally with the last stages of morphogenesis, but will be considered separately.
Hatching. When the embryo contains about 600 cells and many immature tissues, the young larva breaks through the eggshell to emerge into the world. At this point the larva will begin post-embryonic development where it will grow ten-fold in length and ten-fold in breadth before achieving adulthood.
Sources:
“Introduction to the Germ Line*.” WormBook Header Image, http://www.wormbook.org/chapters/www_introgermline/introgermline.html#:~:text=C.,proliferates%20and%20undergoes%20meiotic%20development.
Embryo Introduction, https://www.wormatlas.org/embryo/introduction/EIntroframeset.html.
JD;, Kormish JD;Gaudet J;McGhee. “Development of the C. Elegans Digestive Tract.” Current Opinion in Genetics & Development, U.S. National Library of Medicine, https://pubmed.ncbi.nlm.nih.gov/20570129/.
“Spermatogenesis*.” WormBook Header Image, http://www.wormbook.org/chapters/www_spermatogenesis/spermatogenesis.html#:~:text=Spermatogenesis%20creates%20functional%20sperm%20from,during%20the%20L4%20larval%20stage.
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