The leptotene stage, alternatively referred to as the leptonema, is the first of five substages of meiotic prophase I. The name leptonema is derived from the Greek for “thin threads.” After its chromosomes are duplicated during interphase, a cell destined to become a gamete enters the leptotene stage. During the leptotene stage, those duplicated chromosomes—each of which is composed of two sister chromatids—condense from diffuse chromatin into long, thin strands that become increasingly apparent inside the nucleoplasm. The zygotene stage of meiosis is the next stage of prophase I.
During this stage, the chromosomes adhere to the nuclear envelope’s inner membrane by their ends (telomeres). Telomeres typically collect at a nuclear envelope sector during the transition to the zygotene stage, generating a meiotic bouquet. Additionally, lateral (axial) synaptonemal complex members are generated. It is the initial stage of Meiosis 1’s Prophase 1.
It is possible to discern two distinct types of DNA repair responses immediately following DNA damaging therapy (gamma irradiation) during male mouse meiosis. Exogenous DNA damage induces a huge accumulation of gammaH2AX in the nucleus of leptotene to early pachytene cells, which is related with DNA repair mediated by the homologous recombination proteins DMC1 and RAD51. From mid-pachytene through diplotene, non-homologous end joining is the primary DNA repair process.
Meiotic Arrest at the Diplotene Stage of Prophase I
Leptotene, zygotene, pachytene, diplotene and diakinesis are the five stages of meiotic prophase I. Chromatin is organised into long and thin strands during the leptotene stage, and synapsis of homologous chromosomes occurs during the zygotene stage, helped by the construction of key parts of the synaptonemal complex. Pachytene is the stage during which homologous recombination takes place, including chromosomal crossover. The oocyte then advances to the diplotene stage, during which it frequently enters a protracted resting phase (Snustad and Simmons, 2015).
Diplotene is frequently coupled with substantial chromatin architectural rearrangements in the oocyte. Different species employ a variety of techniques to ensure that the oocyte matures properly at this stage. The chromosomes of Xenopus laevis and Danio rerio significantly enlarge in size and produce symmetric loops, generating what are referred to as lampbrush chromosomes. Each loop in these chromosomes shows a high level of transcriptional activity and is repeatable in size and pattern. Because Xenopus laevis and Danio rerio oocytes are far larger than those of mammals, it is critical that RNA production be sufficient to sustain oocyte growth. At the conclusion of the diplotene I arrest, these lampbrush chromosome structures vanish, development is slowed to a crawl, and the oocyte enters metaphase I. (Appels et al., 2012).
Mammalian oocytes are substantially smaller than their vertebrate counterparts and lack identifiable lampbrush chromosomes, owing to the absence of the requirement to collect such huge stocks of store proteins to sustain embryonic development following fertilisation. Prior to birth, primary oocytes enter meiotic diplotene I arrest, surrounded by supportive granulosa cells and creating quiescent ovarian structures called primordial follicles (Pan et al., 2012). Following puberty, activation of the primordial follicles is characterised by a change in the architecture of the supporting granulosa cells from flat to cuboid, a significant alteration in the oocyte transcriptome, and resumption of oocyte growth and maturation (Ernst et al., 2017).
The arrest of the Drosophila oocyte at the diplotene stage of prophase I results in the condensation of the oocyte chromatin into a structure called the karyosome and the development of transcriptional quiescence. This state of quiescence has little detrimental effect on oocyte maturation, as the majority of growth is given by the supportive nurse cells (Bastock and St Johnston, 2008; Lake and Hawley, 2012). Contrary to expectations and despite the nurse cells’ role, prophase I-arrested oocytes transiently resume gene expression roughly 36 hours after karyosome formation and shortly before they advance into metaphase I. (Navarro-Costa et al., 2016). This temporary transcriptional reactivation is coupled with a worldwide change in the chromatin architecture of the oocyte, implying a function in meiotic development.
Development of the Synaptonemal Complex
The schematics in the given figure depict the SC’s development. The chromosomes are unpaired and tiny pieces of the chromosomal core appear in the nucleus during the leptotene stage of meiotic prophase, as illustrated in the given figure below . Immunofluorescent microscopy was used to visualise the early unpaired core segments in the picture, utilising antibodies against one of the core proteins and a secondary antibody coupled with a green fluorochrome.
During the zygotene stage of meiosis, the short segments are connected together to form longer stretches of cores, and at the same time, the cores of homologous chromosomes begin to associate with one another, resulting in the formation of the first SC segments during the zygotene stage. The creation of transverse filaments between the cores is associated with the formation of synapsis between the cores. When the transverse filament proteins are reacted with their respective antibodies conjugated to a red fluorescent secondary antibody, the transverse filament proteins become visible. Due to the overlap of the red and green fluorochromes in the zygotene nucleus, yellow segments appear in the areas where the cores have begun to synapse in the nucleus. At this point, the antibody against transverse filaments is present along the whole length of the synaptic cleft (SC) membrane.
When the transverse filaments are eliminated, the chromosomal cores begin to separate. The separated cores of the diplotene stage are luminous green, while the final points of contact with transverse filaments are yellow. At a few spots, some filament material remains adhered to the split cores. Where two cores exhibit a steep convergence, it is assumed that a reciprocal recombination event occurred, resulting in the formation of a chiasma.
Prophase I
This is a lengthy and complicated phase that is markedly different from mitotic prophase. The prophase is separated into five distinct stages. The chromosomes divide longitudinally during the leptotene stage, forming pairs of sister chromatids. Genetic material has already self-replicated. At the zygotene stage, homologous chromosomes are paired (synapsis), which is required for intrachromosomal recombination to occur. When pairing is complete during the pachytene phase, chromosomes begin to spiralize and appear thick and short. The diplotene stage reveals the bivalent’s four chromatids as a tetrad. At the undivided centromere, the two sister chromatids stay linked. When a pair of chromatids separates, non-sister chromatids frequently cross over, a process called chiasma. At this period, diakinesis, the coiling and subsequent shortening of chromosomes, is at its peak. The spindle is formed at the conclusion of prophase, and the nucleolus and nuclear membrane are dissolved.
Conclusion
Leptotene is the first prophase of meiosis. Meiosis one is the decreased division of cells in which the offspring cells’ chromosomes are half.
Chromosomes uncoil and form thread-like structures during the leptotene stage (leptos = fine threads). Chromomeres, the structure resembling beads, can be seen.
Due to the unique arrangement of chromosomes in the nucleus, the leptotene stage is also referred to as the bouquet stage. Chromosomes converge toward the centrosome on one side of the nucleus.
Centrioles undergo duplication and then migrate to the opposing poles of the nucleus, where they undergo additional duplication.
The leptotene stage is immediately followed by the zygotene stage.