ch 9

• The entire lineage of C. elegans has been traced through its transparent cuticle
• Germ cell development in C. elegans

  1. Germ cells undergo mitosis near the distal end & meiosis further from the distal tip

  2. Early meiosis forms sperm (stored in the spermatheca)

  3. Later meioses form eggs that become fertilized as they roll through the spermatheca
• Early cleavage in C. elegans

  Each asymmetrical division produces one founder cell (denoted AB, MS, E, C, and D), which produces differentiated descendants, and one stem cell (the P1-P4 lineage)
• The adult body of C. elegans contains exactly 959 somatic cells
• C. elegans can reproduce either by self-fertilization or by cross-fertilization with the infrequently occurring males
• C. elegans

  • Easy to maintain in the lab (petri dishes)
  • Short generation time: 3 days & short life-span (2-3 weeks)
  • Fully sequenced genome
  • Nematodes have a small number of chromosomes, a small number of cells, and an invariant cell lineage
  • Each cell gives rise to the same number & type of cells in every embryo, allowing you to determine which cells have the same precursor cells
• The predominant adult form of C. elegans is hermaphroditic, each worm producing both eggs & sperm
• Meroblastic cleavage is partial cleavage; occurs when there is an indentation, but not complete division
• Holoblastic cleavage is complete cleavage
• The germ line segregates into the posterior portion of the most posterior (P) cell
• The two daughters of the E cell in C. elegans migrate from the ventral side into the center of the embryo
• Gastrulation in C. elegans starts just after the generation of the P4 cell in the 24-cell embryo
• Apical constriction in C. elegans involves the internalization of cells from the ventral side
• Gastrulation in C. elegans

  1. Starts just after the generation of the P4 cell in the 24-cell embryo

  2. Apical constriction- internalization of cells from the ventral side

  3. The two daughters of the E cell migrate from the ventral side into the center of the embryo
• Within 6 hours of gastrulation stage, cells move to develop organs & embryo stretches out (556 somatic cells & 2 germline cells)
• 1. granules


  • Ribonucleoprotein complexes that function in specifying the germ cells
• Fig. 9.20 Cell lineage map for all gastrulating cells


  • Final positions of cell lineages in the larval worm
• Further modeling occurs (apoptosis; molts) yielding final worm of 959 somatic cells with numerous sperm and eggs
• Anterior-Posterior Axis Formation


  1. A/P axis formation is determined by the location of sperm entry

  2. When sperm enters the oocyte, the centriole associated with the sperm pronucleus initiates cytoplasmic movements pushing the male pronucleus to the nearest end of the oocyte which becomes the posterior pole
• Segregation of the P-granules into the germ line lineage of the C. elegans embryo

  1. At each successive division, the P-granules enter into the P-lineage blastomere, the one that will form the germ cells

  2. P-granules are ribonucleoprotein complexes that function in specifying the germ cells
• Fig. 9.20 Gastrulation in C. elegans


  • Time lapse imaging showing progressive internalization of the endodermal precursor cells in the C.elegans gastrula
  • Cross-section of the invagination of these endodermal precursor cells with their more apical neighbors indicated
  • Only the apical surface of the invaginating cells shows activated myosin
  • Model of apical constriction through the contraction of actin-myosin contractile networks linked to apical adherens junctions all localized to the apical surface. F-actin & nonmuscle myosin II
• PAR proteins


  • Involved in establishing cell polarity (mutations lead to failures in asymmetric segregation of cytoplasmic determinants)