Axis specification and Gastrulation in C elegans Introduction C elegans is an excellent system for studying how individual cells become different in the eatly embryo because of its simplicity reproducible cell lineage and genetics Having said that the situation is still quite complex We will restrict our discussion to cells that end up in the interior of the embryos and which form two of the primary germ layers MS which forms mesoderm including pharynx and muscle and E which forms the endoderm These cells are daughters of a cell called EMS which in turn is descended from the posterior cells at the twocell stage P1 Our goal is to identify molecular pathways that establish the fates of MS and E Anterior progeny make pharynx via induction from MS and epidermis Posterior makes mesoderm muscle pharynx makes gut The establishment of different blastomeres involves several steps Step 1 The sperm entry point determines the posterior end of the zygote sperm adapted from Goldstein and Hird 1996 Development 122 1467 1474 The sperm enters the oocyte at the future posterior end of the zygote The sperm astral microtubules are thought to organize the cytoskeleton of the zygote resulting in an initial polarization of the actin rich cortex This leads to segregation of P granules germ plasm components to the posterior and leads to the next step Step 2 Anterior and posterior differences arise in the cortex of the zygote involving the PAR proteins PAR 3 6 PAR 2 MEX 5 AB P1 Adapted from Nance 2005 Bioessays 27 126 135 Different PAR for partitioning defective proteins PAR 3 and PAR 6 in the anterior and PAR 2 in the posterior localize to the anterior and posterior reinforcing differences in these two ends of the zygote PAR proteins promote accumulation of mRNAs and proteins in each end of the zygote such as MEX 5 in the anterior Step 3 SKN 1 and PIE 1 proteins accumulate in P1 MEX 5 and other proteins accumulate in AB The MEX proteins keep SKN 1 and PIE 1 levels low in the anterior cell of the two cell embryo AB by causing them to be degraded i e after they are translated via the ubiquitin pathway AB P1 MEX 5 PIE 1 MEX 5 SKN 1 SKN 1 PIE 1 Step 4 PIE 1 represses the activity of SKN 1 in P2 resulting in a single cell in which SKN 1 acts the EMS cell Step 5 SKN 1 activates transcription of med genes MED proteins are transcription factors that allow differentiation of mesoderm and endoderm in the daughters of EMS MS and E respectively MED proteins MS mesoderm E endoderm Step 6 A Wnt signal from P2 results in the posterior daughter of EMS becoming endoderm Without this signal E becomes a second mesodermal precursor instead The Wnt signal called MOM 2 is produced by P2 EMS has a Frizzled MOM 5 that allows it to detect this signal Without the activity of these two genes no endoderm forms and instead extra mesoderm forms Thus they are named mom for more mesoderm ABp ABa MOM 5 Frizzled PIE 1 P2 MOM 2 Wnt EMS MS mesoderm E P3 endoderm some progeny make germline Step 7 Cells are internalized during gastrulation via multiple ingression movements The endodermal precursor cells Ea and Ep ingress to internalize the endoderm One mechanism aiding ingression involves contraction of the outer or apical surfaces of Ea and Ep causing them to undergo apical constriction This constriction presumably involves actin and myosin At the same time neighboring cells probably migrate to cover over Ea and Ep Putzke Rothman 2003 Curr Biol 13 R223 R225 myosin Adapted from Lee Goldstein 2003 Development 130 307 320 Step 8 After these ingression movements the epidermis seals the outside of the embryo in a movement known as ventral enclosure Ventral enclosure involves the migration of epidermal cells which make contact at the ventral midline This contact requires cadherins