Worm Breeder's Gazette 14(1): 68 (October 1, 1995)
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
Dept. of Genetics, Washington University School of Medicine, St. Louis, MO 63110, mccarter@genetics.wustl.edu
We are interested in the regulation of the meiotic cell cycle in
oocytes. Specifically, how does each C. elegans oocyte maintain its
cell cycle in diakinesis of prophase I, and how is maturation (cell
cycle progression) triggered? To address these questions, we have
followed oocyte development, maturation, and ovulation by time-lapse
Nomarksi microscopy of anesthetized animals 1. Some of the landmark
events, shown in Figure 1, have been briefly described previously 2,3.
Timing and standard deviations were compiled from observations of 45 N2
oocytes. Time zero is defined as the completion of ovulation into the
spermatheca, which coincides closely with fertilization.
Oocyte Development
Oocytes develop while in diakinesis of meiotic prophase I. As they
move proximally, cell and nuclear volumes increase, and chromosomes
become more condensed. In hermaphrodites, the nucleolus disappears
about 70 minutes before ovulation. Subsequently, the nucleus migrates
to the distal surface of the cell. The distal surface of the oocyte can
also bend to meet the nucleus, suggesting a physical connection under
tension. Both the time when nuclear migration occurs and its rate are
variable.4
Meiotic Maturation
Progression from prophase to metaphase of meiosis I is termed meiotic
maturation.5 The first indication of maturation is nuclear envelope
breakdown (NEBD) which begins at -5.7 minutes. At -3.0 minutes, the
oocyte begins to change shape from a cube to a sphere, a process we have
termed oocyte cortical rearrangement. While vigorous sheath
contractions occur at the same time, analyses of sheath ablated animals
and mutants defective in sheath activity appear to indicate that oocyte
cortical rearrangement does not depend on somatic activity. The events
of maturation are highly reproducible with low standard deviations.
Ovulation
Ovulation, the exit of the oocyte from the gonad arm into the
spermatheca, requires contraction of the myoepithelial sheath and
dilation of the distal spermatheca. The rate of sheath contractions
increases preceding NEBD and peaks at ovulation. The sheath appears to
tonically contract as it pulls the dilating distal spermatheca over the
most proximal oocyte. As the distal spermatheca closes, cytoplasmic
streaming is visible in the oocyte, possibly indicating fertilization.
Entry into the uterus occurs 4.2 minutes after ovulation. The meiosis I
and II divisions occur in the uterus.
Meiotic Prophase Arrest
In unmated females, oocytes arrest in diakinesis, failing to undergo
meiotic maturation and ovulation for hours or days. Numerous oocytes
accumulate, each with an enlarged, distally positioned nucleus lacking a
nucleolus. This phenotype defines the arrest point, and separates the
events belonging to oocyte development from those of meiotic maturation.
Additionally, sheath contractile activity in females is lower than the
background level observed between ovulations in hermaphrodites. Oocytes
in females do stochastically exit arrest and ovulate at a very low rate
(approximately 1/15 the rate for hermaphrodites), perhaps reflecting an
inability of C. elegans oocytes to maintain arrest indefinitely.
In hermaphrodites, oocyte development, maturation, and ovulation
occur in an assembly-line fashion with an ovulation rate of about once
per 45 minutes while sperm are present. The time an oocyte spends in
diakinesis may reflect a developmental requirement for the execution of
certain events and thus might not be a true 'arrest' of the cell cycle.
Prospects
While the events of oocyte development, maturation, and ovulation
occur in a reproducible order, the dependency relationships for these
events are not yet defined. Mutant analysis may aid in inferring
causality. For instance, we are currently investigating mutants which
appear to interfere with the somatic events of ovulation even though
meiotic maturation occurs normally within the oocyte.
1 Kirby, C., M. Kusch, K. Kemphues (1990). Dev. Bio., 142:203-215. 2
Ward, S. and J.S. Carrel (1979). Dev. Bio. 73:304-321. 3 Doniach, T
(1988). WBG 10(2):64-65. 4 Figure 1 indicates the completion of slow
migration (39% of oocytes) or fast migration (36% of oocytes). In 25%
of oocytes, the nucleus did not migrate, and in 18%, the nucleus drifted
from distal after migrating. Nuclear position is most variable in the
first oocyte produced by the arm. 5 Meiotic Maturation is the term used
in other systems for oocyte cell cycle progression (i.e. MPF =
maturation promoting factor), Masui, Y. and H.J. Clark (1979). Int.
Rev. Cytol. 57:185.