Embryogenesis

The process by which an embryo forms and develops, marking a time of rapid cell proliferation, specification, and differentiation. Embryogenesis in C. elegans takes about fourteen hours at 22 degrees C, starting with fertilization of the oocyte with self-sperm from the hermaphrodite or sperm from a male. During the first two hours, the zygote forms and early cleavages establish the embryonic axes. Somatic and germ-line founder cell fates are also determined. During the next five hours, most cell proliferation completes, the embryo undergoes gastrulation, and cell differentiation and organogenesis begins. Cell differentiation, organogenesis and morphogenesis are completed during the final stage of embryogenesis. The nervous system becomes active and muscles are stimulated during this last stage, resulting in the embryo twitching within its egg shell and eventually hatching as an L1 larva.

Asymmetric cell division

In C. elegans the three principal axes of the body (Anterio/Posterior, Dorsal/Ventral, and Left/Right) are established by a series of asymmetric cell division. It's through these types of division that cell determinants are unequally segregated into daughter cells resulting in the specification of cell fates. The first cell division in C. elegans, in the one-cell embryo, results in the establishment of the A/P axis.

Cuticle biogenesis

The C. elegans cuticle is a protective exoskeleton of specialized extracellular matrix (ECM) consisting primarily of collagen, lipids, and glycoproteins and is required for viability. (Chisholm and Hardin 2005; Page and Johnstone 2007). The cuticle determines the shape of the body and, through connection from the epidermis to muscle, provides anchoring points for muscle contraction. The cuticle also serves as a model for ECM formation and function with molecules and pathways involved in cuticle biogenesis conserved in vertebrates (Page and Johnstone 2007). The outer epithelial layer, the epidermis, of the embryo undergoes a series of cell fusions to make large multinucleate, or syncytial, epidermal cells, which secrete the materials needed to make up the cuticle. This protective layer is produced five times during C. elegans development, with each molt ending with an entirely new cuticle.

Programmed cell death

Programmed cell death (PCD), or apoptosis, is an integral component of C. elegans development. During development, 131 cells are fated to die by apoptosis. PCD is easily observed with Nomarski optics in the C. elegans embryo; the nucleus of the apoptotic cell becomes refractile, resembling a flat button, making C. elegans an easy model for following apoptotic cell death. PCD in C. elegans can be divided into three main phases (with participating genes): specification of which cells should live or die (ces-1, ces-2, in general); activation of the cell killing machinery (egl-1, ced-9, ced-4 and ced-3); and an execution phase where the dying cell is dismantled and removed through phagocytosis (ced-1,-2,-5,-6,-7,-10 and-12), which occurs in concert with apoptotic DNA degradation (cps-6, nuc-1). Molecular and biochemical studies in C. elegans revealed programmed cell death mechanisms conserved in humans. The regulatory pathway that controls cell death is composed of conserved cell death activators and inhibitors: EGL-1 and BH3-domain-only proteins, CED-9 and Bcl-2, CED-4 and Apaf-1, and CED-3 and caspases, in nematodes and mammals, respectively. Further the degradation of chromosomal DNA involves a mitochondrial proapoptotic endonuclease: endonuclease G (EndoG), and AIF in mammals and their orthologs CPS-6 and WAH-1 in worms.