A summary of oogenesis in Xenopus laevis... |
| Primordial germ cells: Approximately 20 primordial germ cells (PGCs) migrate to the sexually indifferent gonad during stage 48-52 (7-21 days) of larval development. These PGCs are the descendants of cells that inherited the "germ plasm" found at the vegetal pole of stage VI oocytes. In the gonad, PGCs proliferate to a population of approximately 1000 cells by day 40 of oogenesis, and eventually reach a population size of about 10,000 cells. The gonad becomes sexually dimorphic at the forelimb stage of development. During their migration and proliferative phase, PGCs can be identified by their round morphology, approximately 15-20 um in diameter, and their lobed nucleus with 1-2 nucleoli. |
Completion of the four mitotic divisions of secondary oogonia gives rise to a cluster of sixteen post-mitotic oocytes in the pre-diplotene stages of meiotic prophase. Initially, these pre-diplotene oocytes remain clustered in a "nest" and undergo synchronous development. These early oocytes exhibit a distinct polarity resulting form their final mitotic division. They have a distinctive pear shape, with the large, round nucleus at the broader, distal end of the cell. Cytoplasmic organelles, including mitochondria (JPEG: 68 KB), are concentrated in the narrow, proximal, end of the cell. As oocytes complete their final S-phase and enter the classic, recombination stages of meiotic prophase (leptotene->zygotene->pachytene), the oocyte chromosomes condense into the classic "bouquet" organization (JPEG: 39 KB), where synaptonemal complexes are associated with the nuclear envelope on the side of the nucleus facing the cap of cytoplasmic organelles, and ribosomal DNA sequences form a highly condensed mass on the opposite side of the nucleus By late pachytene stage, individual oocytes are becoming surrounded by follicle cells, disrupting the organization of oocyte nests. Following completion of recombination during the pachytene stage of prophase, oocytes enter a prolonged diplotene phase of growth and differentiation. To signify the distinct changes in nuclear, cytoplasmic, and cytoskeletal organization during the transition to diplotene, we have adopted the term "stage 0" to refer to small, highly polarized, pre-diplotene oocytes (Gard et al., 1995) Dumont (1972) has classified the long (4-8 month) diplotene phase of oogenesis in Xenopus into six stages, according to the external appearance and cytoplasmic organization of the oocytes. These stages have been commonly accepted as a normal table for oocyte differentiation. However, it should be remembered that oogenesis is not a discrete series of stages, but a continuum of oocyte growth and differentiation. There is some overlap in the size range of the different stages of oogenesis, and the range of diameters associated with a given stage can vary between individual frogs. |
|
 Stage
II (300-400 um): The
onset of vitellogenesis, or the production of yolk, marks the
beginning of stage II of oogenesis. From this point on, Xenopus oocytes
are opaque, making traditional microscopy more difficult. Cortical
granules and pre-melanosomes begin to appear in the oocyte cortex.
During late stage I-early stage II (200-300 um diameter), components
of the mitochondrial mass, including mitochondria, germ plasm,
and maternal RNAs disperse to the future vegetal pole of the
oocyte. However, no outward sign of A-V polarity is evident at
this stage of oogenesis |
 Stage
III (400-500 um): Stage
III oocytes are characterized by the onset of pigmentation, which
gives them a grey color. Pigment is equally distributed in the
cortex, and stage III oocytes retain an unpolarized appearance.
Lampbrush chromosomes reach their peak near the end of stage
III. Mitochondrial proliferation halts until stage 30 of embryonic
development, with each oocyte containing approximately 105 mitochondria |
 Stage
IV (0.5-1.0 mm): Stage
IV of oogenesis is marked by the onset of visible polarization
of the oocyte along the animal-vegetal (A-V) axis. During this
stage, pigment becomes unequally distributed between the cortex
of the animal and vegetal hemispheres, resulting in the darkly
pigmented animal hemisphere and lightly pigmented vegetal hemispheres
characteristic of many amphibian oocytes. A-V polarity is also
apparent in the distribution of large yolk platelets, which are
concentrated in the vegetal cytoplasm, and the position of the
GV, which moves into the animal cytoplasm. Lampbrush chromosomes
and nucleoli are concentrated in the center of the GV |
 Stage
V (1.0-1.2 mm): During
stage V, the A-V polarity of the oocyte continues to develop,
as the GV moves further into the vegetal cytoplasm and a thin
band of basophilic, yolk-free cytoplasm forms near the basal
(vegetal) surface of the GV. A sharp equatorial boundary exists
between the pigmented animal cortex and the vegetal cortex, although
dispersal of pigment in the animal cortex results in a lighter
appearance relative to stage IV oocytes. Stage V oocytes will
undergo meiotic maturation in response to progesterone. |
 Stage
VI (1.2-1.3 mm): Fully-grown
stage VI Xenopus oocytes often exhibit a light "equatorial
band" separating the pigmented animal and unpigmented vegetal
cortex (not easily seen in this image). The vegetal surface of
the GV appears convoluted or sacculated, and is bounded by a
cap of basophilic yolk-free cytoplasm. |
 Unfertilized
egg (1.2-1.3 mm): Exposure
of stage VI oocytes to progesterone (in vitro or in
vivo) releases them from their prophase arrest, and induces
resumption of their meiotic cell cycle, a process called "maturation." Maturation
is accompanied by substantial re-organization of the oocyte cytoplasm
and cytoskeleton, culminating in the assembly of the meiotic
spindles near the animal pole. The unfertilized egg arrests its
cell cycle in second meiotic metaphase (M2), with an axially-aligned
M2 spindle located near the center of a white "maturation
spot," which is formed by breakdown of the germinal vesicle
at the onset of maturation. |
 Fertilized
eggs and the early embryo: Upon
fertilization by sperm (or artificial activation) eggs complete
the second meiotic divsion and enter the mitotic cell cycle.
A large MT aster (the sperm aster) is rapidly assembled from
the sperm centrosome. MTs of the sperm aster play an important
role in pronuclear migration. In addition, upon reaching the
subcortical cytoplasm of the vegetal hemisphere, MTs of the sperm
aster from a dense array of "cortical" MTs that play
a critical role in establishing the dorsal-ventral axis of the
embryo. |
References: Beetschen, J.-C., and Gautier, J. (1989). Oogenesis. In "Developmental Biology of the Axolotl"(J.B. Armstrong and G.M. Malacinski, Eds.), pp. 25-35. Oxford Univ. Press, New York. Bement, W.M.,
Gallicano, G.I., and Capco, David G. (1992). Role of the cytoskeleton
during
early development. Microscopy Research and Technique 22, 23-48. Gard, D.L. (1995)
Axis formation during amphibian oogenesis: reevaluating the role of the cytoskeleton.
Current Topics in Developmental Biology 30, 213-250. Houlistan, E.,
and Elinson, R.P. (1992). Microtubules and cytoplasmic reorganization
in the frog
egg. Curr. Top. Dev. Biol. 26, 53-70. Nieuwkoop, P.D.,
and Sutasurya, L.A. (1979). "Primordial germ cells in the chordates." Cambridge Univ.
Press. Cambridge. Smith, L.D., Michael,
P., and Williams, M.A. (1983). Does a predetermined germ line exist
in amphibians? In: Current problems in Germ Cell Differentiation
(A. McLaren and
C.C. Wylie, eds.). Cambridge Univ. Press. Cambridge. pp 19-39. Wylie, C.C., Brown,
D., Godsave, S.F., Quarmby, J., and Heasman, J. (1985). The cytoskeleton
of Xenopus oocytes and its role in development. J. Embryol. Exp.
Morph. 89 (Supplement), 1-15. |
Oogonia
(15-20 um) and stage 0 oocytes (< 35 um):
Stage
I (35-300 um):
