Sci. Aging Knowl. Environ., 26 February 2003
Vol. 2003, Issue 8, p. pe4
[DOI: 10.1126/sageke.2003.8.pe4]


Germline Genomes--A Biological Fountain of Youth?

Christi A. Walter, Ronald B. Walter, and John R. McCarrey

Christi A. Walter is at The University of Texas Health Science Center at San Antonio, San Antonio, TX 78229-3900, USA and the South Texas Veteran's Health Care System, San Antonio, TX 78229, USA. Ronald B. Walter is at Southwest Texas State University, San Marcos, TX 78666, USA. John R. McCarrey is at The University of Texas at San Antonio, San Antonio, TX 78249, USA. E-mail: walter{at} (C.A.W.);2003/8/pe4

Key Words: aging • germ cells • DNA repair • mutagenesis • oogenesis • spermatogenesis • apoptosis


Gametes have perhaps the most dramatic and important biological function: the creation of a new organism. Because their DNA will direct the development of the next generation, from a genetic perspective gametes are profoundly different from most somatic cells. Indeed, despite their highly differentiated state, gametes may be considered penultimate stem cells. Recognizing the importance of germ cells, many biology textbooks state that in multicellular organisms, somatic cells exist primarily to ensure the survival and propagation of germ cells. In other words, germ cells are "privileged" and are supported by a vast array of "underprivileged" somatic cells. How is the biological importance of reproduction manifest in gametes and other cells in the male and female germ lines? Do protective mechanisms fail as individuals attain older ages?

Are Germ Cells Privileged?

The success of a species is dependent on successful reproduction. Successful reproduction, in turn, is dependent in large part on gamete genetic integrity. Accordingly, it would seem that the integrity of germline DNA must be stringently guarded. However, genetic adaptation, which is also necessary for the success of a species, requires mutagenesis. Thus, in germ cells there must be a delicate balance between maintaining adequate genetic integrity at the individual level to ensure the propagation of the species while allowing sufficient mutagenesis to support genetic adaptation.

Development of Germ Cells in Mammals

Primordial germ cells (PGCs), progenitors of the germ cell lineage, derive from the totipotent cells of the epiblast (that is, cells derived from the inner cell mass of the blastocyst that give rise to all cells that will generate the embryo proper) in the early mammalian embryo. The PGCs emerge from the epiblast and take up residence in extraembryonic locations. After gastrulation, the PGCs proliferate mitotically (Fig. 1) and migrate to and colonize the genital ridges. Proliferation continues as the germ cells populate the developing gonads. During the embryonic proliferative phase, a large number of germ cells are lost through apoptosis (1). Germ cells in the developing ovary follow an intrinsically programmed pathway to enter meiosis and progress to the diplotene stage of the first meiotic prophase (Fig. 1). Because female germ cells follow a linear path of differentiation and do not replenish themselves at any stage of development (2), there is no true stem cell in the oogenic lineage. A majority of fetal oocytes are lost through apoptosis. Those that remain constitute the entire germ cell population available to the female for the remainder of her life-span. The surviving oocytes remain frozen in the first meiotic prophase until they are either lost at a later stage of life because of apoptosis, or, one by one, are induced to complete meiosis coincident with maturation of the follicle and ovulation (Fig. 2A).

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Fig. 1. Stages of mammalian gametogenesis. Fetal/embryonic stages: PGCs migrate from extraembyronic sites to the genital ridges. The early undifferentiated gonad undergoes differentiation into an ovary or testis, and the GCs (germ cells) differentiate into oogonia or prospermatogonia, respectively, and proliferate to populate the gonads. Many germ cells are lost through apoptosis during the embryonic phase. Mitotic proliferation ceases in the ovary, and the oogonia enter meiosis (1° oocytes) but stop in the diplotene phase of prophase I. GCs in the embryonic testis proliferate by mitosis and then enter a mitotic freeze. Postnatal stages: Primary oocytes begin to undergo additional differentiation and follicle growth after birth, which is accompanied by apoptosis. The 1° oocytes complete meiosis I and become 2° oocytes. Most of the cytoplasm is retained by the oocytes, with only a small portion going to the first polar body, which also carries a haploid set of replicated chromosomes. As individual follicles mature and become ready for ovulation, they progress through meiosis II but freeze at metaphase until they become fertilized. Only after fertilization is meiosis II completed, generating thereby a large ovum and three small polar bodies. All postnatal phases of GC differentiation in the ovary are meiotic, and no replenishment of oocytes occurs. Prospermatogonia are released from the mitotic freeze postnatally and once again proliferate mitotically. The GCs differentiate to stem spermatogonia, and (although not shown in the figure) cytoplasmic bridges between daughter cells, which will persist until late in spermiogenesis, connect the differentiating spermatogonia. Stem spermatogonia divide to give rise to daughter cells, some of which replenish the stem cell populations, whereas others enter the differentiative pathway of spermatogenesis as type-B spermatogonia. There is a major wave of apoptosis at this point, so that a majority of the differentiating spermatogonia are lost, whereas a minority proceed to enter meiosis as primary spermatocytes. Primary spermatocytes also are susceptible to apoptosis, especially as triggered by meiotic checkpoint mechanisms that ensure proper pairing of homologous chromosomes. The surviving cells proceed through the meiosis I reductional division to produce 2° spermatocytes. Secondary spermatocytes undergo the second meiotic division to yield haploid, postmeiotic round spermatids. These cells then undergo spermiogenesis, which involves significant additional differentiation with no further cell division, leading to the production of mature spermatozoa. Once released from the seminiferous epithelium, bridges no longer connect mature spermatozoa. All cell types are artistic renderings.


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Fig. 2. Murine male and female gametes. (A) Fertilized ova from C57BL/6 mice. (B) A crude preparation of epididymal spermatozoa from C57BL/6 mice. Photographs were taken at x40 magnification. PB, polar body; ZP, zona pellucida, a protective noncellular structure around the oocyte from which the embryo must hatch before it can implant in the uterine wall.

Germ cells in the fetal testis are inhibited from entering meiosis and remain mitotic until midway through fetal stages, when prospermatogonia enter a mitotic freeze and do not proliferate further until after birth, when the prepuberal stage (that is, the postnatal period just prior to puberty) is reached. Stem spermatogonia are present at the base of the seminiferous epithelium in the testis. These stem spermatogonia are classical stem cells: They divide to produce some cells that maintain the stem cell population and others that initiate the process of spermatogenesis, which culminates in the production of mature spermatozoa (Fig. 2B). There is a major postnatal wave of apoptosis during spermatogenesis that affects premeiotic spermatogonia (3). Thus, there are large depletions of male and female germ cells during embryogenesis, soon after birth, and presumably during each subsequent wave of spermatogenesis as spermatogonia differentiate to form spermatocytes. It has been speculated that the apoptotic-mediated depletion that occurs during oogenesis and spermatogenesis is involved in regulating the numbers of germ cells (1). However, this process of atresia may also be a mechanism to prevent less-than-pristine germ cells from contributing to the production of future generations (1). Notably, there are relatively few mitotic divisions in either germ cell population before birth, and both germ cell lineages enter an inactive state with regard to proliferation, thereby reducing the risk of mutagenesis during replication.

Protective Mechanisms in Testes and Ovaries

Additional potentially protective mechanisms are found within the testes and ovaries themselves. Meiotic and postmeiotic spermatogenic cells reside on the protected side of a blood-testis barrier and may thus avoid exposure to noxious substances circulating in the blood. The extraperitoneal location of the testis facilitates a lower core temperature in the testis than in the main body cavity. This could function, at least in part, to decrease spontaneous DNA damage mediated by heat. Arrest in prophase of meiosis I may be a protective mechanism for oocytes. These cells are stalled at a stage when chromosomes have been replicated and paired, which may enhance the cell's ability to repair some types of DNA damage through homologous recombination. The highly condensed state of oocyte DNA may also lessen exposure to damaging agents.

DNA Repair in Germ Cells

Genetic integrity is intimately linked with DNA repair capabilities; thus, one might speculate that repair should be more robust in germ cells than in somatic cells. In general, a variety of DNA repair genes are highly expressed in the adult testis, specifically in male germ cells, as compared to adult somatic tissues. Less is known about the quantitative expression of DNA repair genes in oocytes, because it is difficult to retrieve sufficient numbers to assay at a variety of oogenic stages.

Although it is clear that male and female germ cells do possess DNA repair activities, very little information is available regarding (i) quantitative aspects of specific DNA repair pathways during oogenesis or spermatogenesis and (ii) how the nature and functions of these activities compare with those of the corresponding activities in somatic tissues (Fig. 3). However, among the tissues tested, short-patch base excision repair (see Shcherbakova Review) has been shown to be most active in spermatogenic cell types (4-6). In comparison, nucleotide excision repair appears to have limited activity in pachytene spermatocytes [primary spermatocytes in which meiotic recombination is occurring (Fig. 1)] and round spermatids [a postmeiotic cell type (Fig. 1)] as compared to somatic tissues (7). Double-strand break repair and single-strand break repair have been demonstrated in spermatogenic cell types (8). Xenopus oocytes have been a particularly helpful tool in the dissection of the base excision repair pathway (9). The majority of DNA repair studies using oogenic cell types have been indirect, and many experiments have been performed with Xenopus oocytes, but repair of base-pair mismatches and excision repair of ultraviolet (UV)-mediated damage have been studied directly to a small extent. The results suggest that mismatch repair and nucleotide excision repair function in oocytes, although the amount of activity may vary in oogenic cell types. Disruption of spermatogenesis and/or oogenesis has been noted in various DNA repair knockout mouse models, suggesting that the corresponding proteins are essential for normal gametogenesis (Fig. 4).

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Fig. 3. Assessment of DNA repair mechanisms in mammalian germ cells. *GC = germ cells; YSSBs = single-strand breaks, y'DSBs = double-strand breaks.


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Fig. 4. Mouse models deficient in DNA repair that display disrupted meiosis.

The frequency of mutagenesis and chromosomal aberrations can provide direct evidence about the extent to which germline DNA is maintained. The specific locus test (SLT) has been among the most extensively used assays for examining spontaneous and induced mutant frequencies in germ cells. For example, it has been used efficaciously to determine the effects of suspected genotoxins on germline mutagenesis in mice. For the SLT, progeny are examined for readily assessed phenotypic deviation from the expected homozygous recessive phenotype of multiple loci. Thus, these experiments are limited to data derived from offspring and are not obtained directly from the germ cells. Regardless, literally tens of thousands of progeny have been examined and spontaneous and induced mutant frequencies determined for male and female germ lines. The compiled data reveal spontaneous mutant frequencies of 1.4 x 10-6 and 6.6 x 10-6 for female and male germ cells, respectively (10). These data demonstrate that the spontaneous mutant frequency is lower in female germ cells than it is in male germ cells. Unfortunately, the SLT cannot be used to determine the mutant frequencies in somatic tissues. Consequently, it is not possible to compare the mutant frequencies in germ cells with those in somatic cells using this approach.

Since the advent of transgenic mouse technology, new models for assessing mutagenesis have been developed. The power of the transgenic systems lies in the ubiquitous presence of a transgene that can act as a reporter for in vivo mutagenesis and can be recovered from virtually any cell or tissue type at any stage of the life-span. Using transgenic models, in vivo mutant frequencies can be determined directly for replicating and nonreplicating cell types. In these models, the transgene is a phage or plasmid DNA that contains a gene that can be used to score mutations (for example, lacI or lacZ from the bacterial lac operon). Using the lacI model, DNA is isolated from the cell or tissue of choice, and the transgene is recovered by packaging with appropriate phage extracts and then used to infect Escherichia coli. The infected E. coli are plated on medium containing a chromogenic substrate that produces a blue-appearing plaque for cells carrying a phage with a mutant lacI gene. Thus, mutations that occur in vivo in the mouse are determined in ex vivo assays after facile recovery of the transgene. Germ cell and somatic cell mutant frequencies can be examined using samples from the same animals.

As with any model, there are limitations to transgenic systems. To date, the transgenes that have been used extensively to assess mutant frequencies are prokaryotic genes (lacI and lacZ) that have high GC contents relative to mammals. Thus, this approach yields a high prevalence of GC->AT transition mutations in the recovered transgenes, presumably resulting from the failure to repair deaminated 5-methylcytosine in the mouse. Although mammalian genes do not generally have such a high GC content, many de novo mutations associated with human disease involve GC->AT transition mutations. Therefore, because they contain a high proportion of CpG sequences, the transgenes are good mutation reporters to use in gaining a better understanding of this type of transition mutation.

Determination of spontaneous mutant frequencies using a lacI transgene has revealed that male germ cells have a significantly lower spontaneous mutant frequency than do somatic cells and tissues (11-13), but when the lacZ transgene is used in a similar manner, it does not reveal a substantial difference between male germline and somatic mutagenesis (14). The reason for this discrepancy is not known, although the low mutant frequency in the male germ line detected with the lacI transgene is certainly consistent with the high levels of expression of the DNA repair genes and of repair enzyme activities that have been reported for these cells, as well as with the spontaneous mutant frequency revealed by the SLT.

Another specific limitation of the transgenic approach for examining mutant frequencies in female germ cells is the difficulty associated with testing oocytes. Because substantial numbers of cells are required to obtain an accurate determination of mutant frequency, the only feasible sources of sufficient numbers of oocytes are fetuses. Experiments that require commercially supplied transgenic mouse fetuses are very expensive. Consequently, the mutant frequency for female germ cells has not yet been examined extensively by means of transgenic approaches.

The available data lead one to believe that germ cells are indeed privileged and that their DNA is stringently guarded, so that a pristine set of haploid genomes is passed to the next generation. This is particularly important because it effectively resets the mutation load to baseline for each new generation. If no such mechanism were in place and mutations were allowed to accumulate in germ cells at each generation and then be passed to the offspring, the mutant frequency in a population would be expected to steadily increase with each succeeding generation. No such progressive increase in mutant frequency has been observed in successive generations. Presumably, the observed germ cell mutant frequency is a product of adaptation. As such, the germ cell would have achieved a balance between producing viable offspring and allowing for adaptation at the population level.

Do Germ Cells Age?

Female Germ Cells

There is a well-recognized maternal age affect associated with chromosomal aberrations. Indeed, the risk of aneuploidy increases significantly with maternal age. Amniocentesis in conjunction with karyotype analysis is now routinely offered to women of advanced maternal age to determine whether the fetus displays a chromosomal aberration.

What is the reason for this increase in chromosomal aberrations with age? It seems reasonable to speculate that because oocytes remain frozen in meiotic prophase I for months ( in rodents) or years (in humans), the oocytes themselves undergo an aging process. Aging in the oocyte may in turn increase the risk of chromosomal nondisjunction when meiosis resumes (that is, just before ovulation). According to this logic, the longer an oocyte sits dormant in prophase I, the greater the likelihood that a chromosomal aberration will occur.

However, there are other mechanisms that might lead to this increase in the frequency of chromosomal aberrations observed in older females. For example, there are processes that cause a pregnancy to be aborted when chromosomal aberrations are present. It is possible that these processes also suffer the ravages of aging and no longer function properly in older females. In addition, oocytes may be released from a follicle (ovulated) essentially in the order in which they entered meiosis (15). In this model, aberrant oocytes might exhibit delayed entry into meiosis and thus take longer to become ready for ovulation than do "nondefective" oocytes. Consequently, the defective oocytes are more likely to be ovulated in older women rather than in young women. So we must ask: Why did these particular oocytes enter meiosis later than the others? Do they already house defects that slowed their entry into meiosis, and do these defects also enhance the probability of chromosomal nondisjunction? Finally, because biological processes are not 100% efficient, it is also possible that some "defective" oocytes escape the massive atresia that occurs during embryogenesis and again soon after birth.

There is a paucity of information about specific DNA repair pathways that function during oocyte maturation or as females age. The limited numbers of experiments that have been reported suggest that oocyte DNA repair activity does not change detectably with age (16). Thus, it is difficult to arrive at firm conclusions with respect to the ability of specific oogenic cell types to repair a variety of DNA lesions as the host female ages.

Male Germ Cells

Many human autosomal dominant disorders (for example, achondroplasia, Crouzon syndrome, and Apert syndrome) are associated with a paternal age effect. The majority of mutations associated with paternal age are de novo germline mutations; that is, the father does not carry the mutation himself. Instead, it arose in his germ cells. Fluorescence in situ hybridization studies have shown that the number of chromosomal aberrations in spermatozoa also increases with paternal age (17). Differentiating spermatogenic cells are continually produced from stem spermatogonia once the male reaches reproductive age, and the time for progressing through spermatogenesis has not been reported to change substantially with age. Thus, it seems unlikely that aging occurs in meiotic or postmeiotic spermatogenic cell types. Regardless, an increased mutant frequency and increased chromosomal aberrations can be detected in these cell types. There are several possible explanations for these observations: (i) The stem spermatogonia themselves could age, and the aging process could be associated with increased mutagenesis. (ii) The apoptotic machinery may respond less efficiently with age and fail to remove "defective" cells during spermatogenesis that would be removed in young animals. (iii) DNA repair and other protective mechanisms (for example, antioxidant systems) may undergo a functional decline with age in any or all of the spermatogenic cell types, leading to an increase in DNA damage and mutagenesis. (iv) Finally, various combinations of the above possibilities cannot be excluded.

Because of the relatively small fraction of stem spermatogonia in the adult testis, aging effects in spermatogonia have not been assessed and are not likely to be examined rigorously in the near future. The spermatogenic lineage is able to repopulate after cytotoxic insults that severely deplete extant differentiating spermatogenic cells (18). It is not clear whether the repopulation is from the primary pool of stem spermatogonia, or whether the original stem cell population is replaced by a second stem cell population that was previously quiescent. The quality of the DNA in the repopulating stem cell pool has not been rigorously examined. Similarly, it is not known whether the original stem cell pool becomes depleted or senescent with old age or is replaced by a second stem cell population. It must therefore be concluded that we do not know whether spermatogonia age by mechanisms other than by increased cell divisions with time.

Base excision repair was examined in nuclear extracts prepared from male germ cells (Fig. 1) isolated from mice of various ages and was found to decline with increased age (5). However, it is not known whether this decline contributes directly to the increased mutation frequency in meiotic and postmeiotic germ cells (that is, pachytene spermatocytes, round spermatids, and eipididymal spermatozoa). It is also not known whether stem spematogonia exhibit a decrease in base excision repair with age, or whether a secondary stem cell pool with lower levels of DNA repair gets used during old age. The activities of other DNA repair pathways relative to age have not been examined quantitatively in spermatogenic cells.

The role of apoptosis in maintaining genome integrity in male germ cells at any age is largely unknown. A peak of apoptosis occurs as stem spermatogonia proliferate, and those that do not undergo apoptosis progress to type-B spermatogonia, the cell type that becomes committed to meiosis, during spermatogenesis (3). This peak of apoptosis corresponds in time to a decline in spontaneous mutant frequency in spermatogenic cells (11), leading to the speculation that apoptosis does provide a mechanism for removing stem cells that have intolerable levels of genomic instability. One might expect that as numbers of aberrant cells increase with age, the level of apoptosis would also increase. Indeed, spermatogenic cell apoptosis has been shown to increase with age in rats (19).

Is Germ Cell DNA a Biological Fountain of Youth?

Yes, largely--at least as compared to somatic cells. But do germ cells age? The answer to this question is much less certain. Experiments that will distinguish definitively between germ cell aging and other age-related effects that affect germ cells have not been reported and are not currently being done, in part because of technical constraints (for example, the availability of stem spermatogonia in adult animals). It is interesting to note that one study found an association between parental age and decreased life-span of female offspring (20). If the conclusion stands the test of additional experiments, it would seem that organismal age is relative to the age of the germ cells that produced the organism. Regardless of the source, there are clearly parental age effects that are associated with decreased germline genetic integrity.

February 26, 2003

Suggested ReadingBack to Top

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Citation: C. A. Walter, R. B. Walter, J. R. McCarrey, Germline Genomes--A Biological Fountain of Youth? Science's SAGE KE (26 February 2003),;2003/8/pe4

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