Cohen Lab

Pictures

		Electron micrograph of a chromosome pair during mouse meiosis. Two homologous chromosomes are aligned and interacting by virtue of their proteinaceous Synaptonemal complex (the paired gray lines that run from top left to bottom right). The centromere region is highlighted by the anti-CREST serum that is labeled with 18 nm gold grains (the large black dots on the left hand end of the chromosomes), while a meiotic nodule is shown by anti-MLH1 staining, labeled with 12 nm gold grains (towards the opposite end of the chromosome pair). Image by Nadine Kolas, former graduate student in the lab.
		Human oocyte from a trisomy 21 individual. Immunofluorescence image of the synaptonemal complex (labeled in green), together with MLH1 (labeled in red), showing 22 pairs of synapsed chromosomes and one triplet of chromosomes 21 (bottom and inset). The centromere is labeled in blue with anti-CREST serum. Image by Michele Lenzi, former graduate student in the lab.
		Pairing of chromosomes during pachytene of prophase I in mouse spermatocytes. All chromosomes (19 autosome pairs, plus the XY) are shown at their complete degree of pairing (or synapsis). The autosomes are fully synapsed along their entire length, while the X and Y pair only at their pseudoautosomal (PAR) region. The red signal indicates SYCP3, a protein the localizes along each homologous chromosome, while the green signal indicates SYCP1, the major component of the central “zipper” region that tethers the homologs together during synapsis. Both SYCP3 and SYCP1, together with SYCP2, are the major components of the synaptonemal complex.
		Synaptic failure and meiotic disruption in spermatocytes from Msh4-/- male mice. The red signal shows SYCP3 staining of a spermatocyte nucleus, together with blue signal for the centromere. In the absence of MSH4, spermatocytes fail to undergo complete synapsis, with the majority of pairing events being between non-homologous partners (extreme examples are indicated with an asterisk).
		Metaphase mouse oocyte. This image was captured 10 hours after the oocyte was released from the ovary. The spindle is stained for actin (in green) and the DNA is shown in blue. Image by Nadine Kolas, former graduate student in the lab.
		Mouse spermatocyte at diplotene of meiosis I. The central zipper region of the synaptonemal complex has broken down at this stage, so the two SYCP3-positive (green) chromosomes begin to repel each other, but remain attached at their sites of chiasmata (crossovers). The faint blue signal all over the nucleus is the DNA, which is present throughout the area, and not just at the cores where the SYCP3 signal is found.
		Progression of spermatogenesis in wildtype males. In Mlh1, Msh4 and Msh5 deficient mice, spermatogenesis is arrested earlier than this stage, so that no mature spermatozoa are found (spermatozoa are found in the center of the tubular lumen).
		Oogenesis in the mouse. At day 3 postpartum, all of the oocytes in the mouse ovary have progressed through the first stages of meiosis I and enter dictyate arrest. Meiosis is not re-initiated until after puberty. In this image, the red staining is GCNA1, which localizes to early meiotic oocytes, becomes more punctate as they approach dictyate arrest, and is lost when the oocytes enter this arrest period. In the bottom left hand corner, one such arrested oocyte is observed. Note that the stromal cells (granulosa cells) have reorganized around this oocyte to form a primordial follicle structure.
		Role for Mlh1 in Meiotic Recombination. Mlh1 has been shown to participate in mismatch repair and meiosis in organisms from yeast through to humans. The gene for Mlh1 is highly conserved, the yeast and mouse genes sharing approximately 35% sequence identity, while the human and mouse genes share >77% identity. In mouse spermatocytes, Mlh1 (green on red = yellow) is localized exclusively to meiotic chromosomes (red) at pachytene, when chromosomes are fully synapsed.