Presentation on theme: "Nematodes early development of Caenorhabditis elegans 9e; Page 243-251."— Presentation transcript:
Nematodes early development of Caenorhabditis elegans 9e; Page 243-251
Caenorhabditis elegans many attributes for studying developmental biology Compact genome (approximately same number of genes as humans) but 3% of the size Completely sequenced genome (first multicellular organism) Short life cycle (3 days); good for genetics Can freeze and thaw them with no apparent effect on mortality. So, can keep genetic stocks easily. Completely known cell lineage
3 germ layers, but no body cavity Unsegmented, cylindrical body shape
Complete cell lineage. This allows highly precise investigation into cellular interactions and cell fate specification. Keep marking cells later and later in development with tracers until you only get a single cell type Only 959 cells in adults and 558 in larva. The number of cells is invariant!
Germline is set aside at the very first cell division Eggs fertilized internally, but laid at the 28C stage
Worms have rotational holoblastic cleavage Site of sperm entry determines the anterior- posterior axis. Sperm usually enters the opposite side from egg pronucleus (maternal polarity) rotational means that each daughter cell divides at a 90 degree angle relative to its sister Par-3 Mex-5
The mother segregates cell fate determinants in response to a spatial cue (sperm entry) to establish a specific axis of polarity Par-3 Par-2 The Par proteins (meaning partition deficient) are important proteins for establishing and maintaining polarity Mex-5 +CYK protein
Par- 2 domain expands, while Par-3 contracts Female nucleus begins to migrate toward the male pronucleus Pseudocleavage
Egg now polarized along the A-P axis Pronuclei meet in the middle Par-2 and Par-3 are segregated to cortical cytoplasm of future daughter blastomeres
Cytokinesis completes the segregation of determinants
Similar partitioning of determinants and asymmetrical division creates founder cells. Founder cells have a particular cleavage rate and fate. C. elegans uses both autonomous and conditional cell fate specification: If blastomeres at first cleavage are separated, AB the fate of AB descendents are less than their potential. When separated from the P- lineage ABa blastomeres fail to make pharyngeal muscle
P-granules determine germ cell fate P granules are segregated into the lineage that will become the germ line. At each cell division the P granules are segregated into only one of the daughter cells. Microfilament but not microtubule inhibitors prevent segregation of P granules
Gastrulation occurs at the 24 cell stage E cells move in ventrally first (2), then the P4 (1), MS cells migrate in from the anterior and CD cells move in from the posterior. very unusual! Compare urchins to worms
Recall, that dorsal ventral polarity determined by early cell interactions
L-R axis is also determined by cell interactions.