**1. Introduction**

In 1958, Australian electron microscopists H. Hoffman and G. W. Grigg, when analyzing ultrathin sections of lymph nodes of adult mice found clustering of mitochondria around the concavities in the nuclear membrane, some lying in very close juxtaposition to the membrane [1]. They even suggested the presence of mitochondria inside of the nucleus but given that the quality of electron microscopic images was not perfect, this suggestion stayed hypothetical. However, in 1960, H. Mori described mitochondria in nuclei of cells from four types of ascites cancer, as well as of tongue cancer, pancreatic cancer, and in regenerating hepatocytes of newts [2]. Later, this phenomenon was reproduced by D. Brandes et al., who published in 1965 in *Science* a brief article entitled "Nuclear Mitochondria?" In their study, similar to that of Mori, cancer (leukemic) cells were used [3]. Since then, mitochondria in nuclei have been found in white blood cells [4,5], lymph nodes of patients with Hodgkin's disease [6], leukemic myoblasts [7], in cardiomyocytes of a patient with rheumatic heart disease [8], and certain other cardiac pathologies [9–12]. Given that the presence of intranuclear mitochondria has been exclusively proven in abnormal cells, these facts were attributed to the manifestation of the pathology.

Two main issues elicit discussion: how do mitochondria ge<sup>t</sup> into the nucleus and what advantages or disadvantages arise as a result of such organelle interaction? Several explanations of such observations have been suggested. Most frequently, the appearance of mitochondria inside nucleus was assigned to the improper execution of mitosis. Using immunofluorescence techniques, it has been shown that a brief opening of the nuclear membrane can occur in the interphase nucleus. Nuclear membrane remodeling was found during viral infections [13–15], laminopathy [16–18], muscular dystrophy, cardiomyopathy, lipodystrophy [19,20], Hutchinson–Gilford progeria syndrome [21], and cancerogenesis [18,22–25]. However, it may be premature to consider this phenomenon as specific for pathological processes only. For example, mitochondria in nuclei were observed by Zhao et al. at the final stage of erythropoiesis in mice [26]. Immunofluorescence assays as well as focused-beam and scanning electron microscopy methods have shown that erythroblast nuclei can be in the opened and fragmented state for 3–5 min. The opening is followed by relatively stable periods of closure lasting about an hour with caspase-3 to be essential for this cyclic process. Loss of caspase-3 blocks not only the opening but also erythroid di fferentiation, leading to hematologic disorders.

There is no doubt that in terms of energy, nucleus function is quite costly in using, for many processes, cytosolic ATP, which is mostly generated by mitochondria. Limiting di ffusion distance for intracellular ATP transport to the site of its use may be an issue to facilitate ATP transport directly to the site of priority use. On the other hand, mitochondria and the nucleus possess genomes of di fferent nature and properties, and numerous data have reported on their interaction and cross-talk. A common opinion is that the transfer of mitochondrial DNA to the nucleus has contributed to the evolution of eukaryotic genomes [27–29]. Mitochondrial DNA transfer to the nucleus is an established fact, possibly playing both normal [30] and pathological [31,32] roles. Vice versa, anterograde signaling (from nucleus to mitochondria) includes numerous regulatory factors coordinating the function of subcellular organelles and integrating cellular and environmental signals, such as nuclear respiratory factor 1 (NRF1) [33], nuclear factor erythroid 2-like 2 [34] (NFE2L2 or NRF2), peroxisome proliferator-activated receptors (PPARs), and estrogen-related receptors (ERRs) [35], as well as many others that regulate mitochondrial-specific activities.

It is reasonable to consider that increasing nuclear membrane surface would facilitate the exchange rate between nucleoplasm and cytosol. Indeed, numerous deep and branching invaginations of the nuclear envelope [36,37], especially in cancer cells [38], were found.

Unlike nuclear envelope invaginations possibly serving as mechanism for importing cytosolic components to the nucleoplasm, envelope herniations may serve the opposite role through exporting nuclear content to cytosol [25,39]. Recently found mitochondria-derived vesicles [40,41] may play a role as a vehicle providing transport of genetic material to the nucleus.

In this study, we made an attempt to analyze the appearance of mitochondria in the nucleus, comparing the heart cells of two species of animals, radically di fferent in life expectancy: rats and naked mole-rats (*Heterocephalus glaber*), as well as the line of rats named by the breeder as OXYS, characterized by accelerated aging. To analyze the structure of the mitochondrial network and its relationship with the nucleus, two microscopic techniques were used: confocal and electron microscopy. Confocal microscopy by itself is not able to resolve mitochondria in nuclear membrane invaginations of those residing in the nucleoplasm. A combination of confocal and electron microscopy may become not only the instrument to address this question but also it would help to address the functionality of nuclear mitochondria.

Indeed, our analysis using confocal microscopy revealed a specially organized functional mitochondrial network in the vicinity of the nuclei in normal cardiac myocytes, whereas electron microscopic images convincingly demonstrated the absence of nuclear membrane over relatively large areas of the nucleus. Regardless of the disruption of the nuclear membrane integrity, the content of the nucleus preserved its specific morphology. Here, we present the ultrastructure of open nuclei containing mitochondria in normal cardiomyocytes of Wistar and OXYS rats, as well as naked mole-rat (*Heterocephalus glaber*). The latter species is of particular interest because it is very long-lived and is

resistant to many pathologies inherent in other species. Therefore, it can be an example of a mammal that has succeeded in maintaining a long and healthy life.
