How the Oocyte Nucleolus Is Turned into a Karyosphere: The Role of Heterochromatin and Structural Proteins

Abstract

Oocyte meiotic maturation includes large-scale chromatin remodeling as well as cytoskeleton and nuclear envelope rearrangements. This review addresses the dynamics of key cytoskeletal proteins (tubulin, actin, vimentin, and cytokeratins) and nuclear envelope proteins (lamin A/C, lamin B, and the nucleoporin Nup160) in parallel with chromatin reorganization in maturing mouse oocytes. A major feature of this reorganization is the concentration of heterochromatin into a spherical perinucleolar rim called surrounded nucleolus or karyosphere. In early germinal vesicle (GV) oocytes with non-surrounded nucleolus (without karyosphere), lamins and Nup160 are at the nuclear envelope while cytoplasmic cytoskeletal proteins are outside the nucleus. At the beginning of karyosphere formation, lamins and Nup160 follow the heterochromatin relocation assembling a new spherical structure in the GV. In late GV oocytes with surrounded nucleolus (fully formed karyosphere), the nuclear envelope gradually loses its integrity and cytoplasmic cytoskeletal proteins enter the nucleus. At germinal vesicle breakdown, lamin B occupies the karyosphere interior while all the other proteins stay at the karyosphere border or connect to chromatin. In metaphase oocytes, lamin A/C surrounds the spindle, Nup160 localizes to its poles, actin and lamin B are attached to the spindle fibers, and cytoplasmic intermediate filaments associate with both the spindle fibers and the metaphase chromosomes.

Keywords: cytoskeleton; germinal vesicle; karyosphere; meiosis; meiotic spindle; nuclear envelope; nuclear lamina; oocytes.

Figure 1

Early GV mouse oocytes before the formation of karyosphere: (A) reaction for lamin B, alpha-tubulin, and chromatin; (B) reaction for lamin A/C, Nup160, and chromatin, epifluorescence. These cells were labeled as a part of the research described in [24]. Chromatin is stained with Hoechst 33258 and the proteins are visualized by indirect immunofluorescence. Bar = 20 μm.

Figure 2

Mouse oocyte in the GV stage before karyosphere formation—reaction for alpha-tubulin, fibrillar actin, and chromatin, epifluorescence. The cell was labeled as a part of the research described in [29]. Actin is visualized using labeled phalloidin. Bar = 20 μm.

Figure 3

Mouse oocyte in the GV stage during karyosphere formation—reaction for lamin A/C and chromatin, epifluorescence. The cell was labeled as a part of the research described in [24]. Bar = 20 μm.

Figure 4

Mouse oocyte in the GV stage during karyosphere formation—reaction for lamin B and chromatin, epifluorescence. The cell was labeled as a part of the research described in [24]. Bar = 20 μm.

Figure 5

Mouse oocyte in the GV stage during karyosphere formation—reaction for lamin A/C, Nup160, and chromatin, epifluorescence. The cell was labeled as a part of the research described in [24]. Bar = 20 μm.

Figure 6

Mouse oocyte in the GV stage with well-contoured karyosphere—reaction for alpha-tubulin, fibrillar actin, and chromatin, epifluorescence. The cell was labeled as a part of the research described in [29]. Bar = 20 μm.

Figure 7

Mouse oocyte at the GV stage with fully formed karyosphere and condensed chromosomes—reaction for alpha-tubulin, fibrillar actin, and chromatin, epifluorescence. The cell was labeled as a part of the research described in [29]. Bar = 20 μm.

Figure 8

Mouse oocyte in the GV stage with fully formed karyosphere and condensed chromosomes—reaction for chromatin, vimentin, and fibrillar actin, epifluorescence. The cell was labeled as a part of the research described in [29]. Bar = 20 μm.

Figure 9

Mouse oocyte in the late GV stage with visible signs of nuclear envelope disintegration—reaction for lamin A/C, Nup160, and chromatin, epifluorescence. The cell was labeled as a part of the research described in [24]. Bar = 20 μm.

Figure 10

Mouse oocyte in the GVBD stage—reaction for lamin B and chromatin, confocal microscopy. The chromosomes are arranged in a prometaphase belt. The cell was labeled as a part of the research described in [24]. Bar = 10 μm.

Figure 11

Prometaphase I mouse oocyte showing chromosomes and alpha-tubulin in the process of the construction of meiotic spindle, epifluorescence. The cell was labeled as a part of the research described in [29]. Bar = 20 μm.

Figure 12

Metaphase I mouse oocyte stained for lamin B, alpha-tubulin, and chromatin, epifluorescence. The cell was labeled as a part of the research described in [24]. Bar = 20 μm.

Figure 13

Mouse oocyte in metaphase I stained for lamin A/C, Nup160, and chromatin, epifluorescence. The cell was labeled as a part of the research described in [24]. Bar = 20 μm.

Figure 14

Mouse oocyte in metaphase I, reaction for cytokeratins, fibrillar actin, and chromatin, confocal microscopy. The cell was labeled as a part of the research described in [29]. Bar = 20 μm.

Figure 15

Mouse oocyte in metaphase I, reaction for fibrillar actin, cytokeratins, and chromatin, epifluorescence. The cell was labeled as a part of the research described in [29]. Bar = 20 μm.

Figure 16

Summary of the dynamics of the main cytoskeletal proteins during meiotic chromatin rearrangement in mouse oocytes. GV, germinal vesicle; GVBD, germinal vesicle breakdown; ZP, zona pellucida; CC, cumulus cells; k, karyosphere.

Figure 17

Summary of the dynamics of the selected nuclear envelope proteins during meiotic chromatin rearrangement in mouse oocytes. GV, germinal vesicle; GVBD, germinal vesicle breakdown; ZP, zona pellucida; CC, cumulus cells; k, karyosphere.