1. Nematodes
early development of Caenorhabditis elegans
9e; Page

2. 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)
Completely known cell lineage
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.

3. Life cycle

4. 3 germ layers, but no body cavity
Unsegmented, cylindrical body shape

5. 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!

6. Eggs fertilized internally, but laid at the 28C stage
Germline is set aside at the very first cell division

7. A simplified cell lineage, grouped by fate.

8. Worms have rotational holoblastic cleavage
rotational means that each daughter cell divides at a 90 degree angle relative to its sister
Site of sperm entry determines the anterior- posterior axis. Sperm usually enters the opposite side from egg pronucleus (maternal polarity)
Par-3
Mex-5

9. The mother segregates cell fate determinants in response to a spatial cue (sperm entry) to establish a specific axis of polarity
The Par proteins (meaning partition deficient) are important proteins for establishing and maintaining polarity
Par-3
+CYK protein
Par-2
Mex-5

10. Par- 2 domain expands, while Par-3 contracts
Pseudocleavage
Female nucleus begins to migrate toward the male pronucleus

11. 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

12. Cytokinesis completes the segregation of determinants

13. Founder cells have a particular cleavage rate and fate.
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

14. 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

15. Gastrulation occurs at the 24 cell stage
Compare urchins to worms
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!

16. Recall, that dorsal ventral polarity determined by early cell interactions

17. L-R axis is also determined by cell interactions.

18. Flies are next!