Meiosis occurs only in germ cells and gives rise to male and female gametes
Gametes derive from a specialized lineage of embryonic cells—the germline—known as germ cells. They are the only cells that can divide by mitosis and meiosis and differentiate into sperm or ova. Germ cells are therefore the critical link between generations. The process by which cells decide between becoming somatic cells of the body or germ cells occurs in the early embryo and involves factors and processes that prevent the somatic fate and induce germline differentiation. Studies in experimental model systems are beginning to unravel the complex process of germline determination, which involves germline-specific transcription factors (see pp. 81–88) and small noncoding RNAs (see pp. 99–100) and DNA methylation (see pp. 95–96) to control expression of specific genes. The process by which germ cells develop into either sperm or ova is referred to as gametogenesis and involves meiosis.
Except for the gametes, all other nucleated cells in the human body—somatic cells—have a diploid number (2N) of chromosomes. Human diploid cells have 22 autosome pairs consisting of two homologous chromosomes, one contributed by the father and one by the mother. Diploid cells also contain a single pair of sex chromosomes comprising either XX or XY. Each somatic cell in human females has 44 autosomes (i.e., 22 pairs) plus two X chromosomes, and each somatic cell in males has 44 autosomes plus one X and one Y chromosome. The karyotype is the total number of chromosomes and the sex chromosome combination, and thus in normal females is designated 46,XX, and in normal males, 46,XY (Fig. 53-1). Gametes have a haploid number (N) of chromosomes and contain either an X or Y sex chromosome.
FIGURE 53-1 Normal human karyotype. The normal human has 22 pairs of autosomal chromosomes (autosomes) as well as a pair of sex chromosomes. Females have two X chromosomes, whereas males have one X and one Y chromosome. N53-7
Contributed by Emile Boulpaep, Walter Boron
The representation in Figure 53-1 is obtained by taking photomicrographs of chromosomes and then rearranging them as shown. The chromosomes are numbered according to size, the largest chromosomes having the smallest number. Pairs of homologous chromosomes are identified on the basis of size, patterns of banding, and the placement of centromeres.
In the case of humans, one generally uses leukocytes (white blood cells) that have been treated with a hypotonic solution to cause swelling and thus help disperse the chromosomes, and with colchicine to arrest mitosis in metaphase. A dye is then applied to visualize the chromosomes better.
Mitosis is the only kind of cell division that occurs in somatic cells. Mitosis results in the formation of two identical daughter cells (Fig. 53-2A), each having the same number of chromosomes (i.e., 46 in humans) and the same DNA content as the original cell. After interphase, during which nuclear DNA in the form of chromatin replicates, mitosis proceeds in a continuum of five phases:
1. Prophase. The replicated chromatin condenses into 46 chromosomes that comprise two identical sister chromatids bound together at the centromere.
2. Metaphase. The nuclear envelope breaks down, the chromosomes align along the midplane of the cell known as the metaphase plate, and microtubules enter the nuclear space and attach to the centromere of each chromosome.
3. Anaphase. The centromeres dissolve and the microtubules pull apart the sister chromatids toward opposite poles of the cell.
4. Telophase. A new nuclear membrane envelopes each cluster of chromatids, which decondense back into chromatin.
5. Cytokinesis. The cell divides into two genetically identical daughter cells, each containing one of the nuclei.
FIGURE 53-2 Mitosis and meiosis. A, In mitosis, the two daughter cells are genetically identical to the mother cell. B, In male meiosis, the four daughter cells are haploid. Cell division I produces both recombination (i.e., crossing over of genetic material between homologous chromosomes) and reduction to the haploid number of chromosomes. Cell division II separates the chromatids of each chromosome, just as in mitosis. C, Female meiosis is similar to male meiosis. A major difference is that instead of producing four mature gametes, female meiosis produces only one mature gamete and two polar bodies (or, if the first polar body divides, three polar bodies). N53-8
Meiosis in Males versus Females
Contributed by Emile Boulpaep, Walter Boron
Figure 53-2B in the text shows meiosis in the male, whereas Figure 53-2C shows meiosis in the female.
In both males and females, the primordial germ cell (PGC) enters the gonad and undergoes many rounds of mitotic divisions. At some point, both a spermatogonium (in the case of males) and an oogonium (in the case of females) enter the first of two meiotic divisions (top cell in Fig. 53-2B, C).
In the case of males (see Fig. 53-2B), one primary spermatocyte (diploid 4N DNA)—a cell that has just entered prophase I—ultimately gives rise to two secondary spermatocytes (haploid 2N DNA) at the completion of the first meiotic division, and four spermatids (haploid 1N DNA) at the completion of the second meiotic division. Thus, one primary spermatocyte yields four mature gametes.
In the case of females (see Fig. 53-2C), one primary oocyte (diploid 4N DNA)—a cell that is arrested in prophase I until shortly before ovulation—ultimately gives rise to one secondary oocyte (haploid 2N DNA) and one diminutive first polar body (haploid 2N DNA) at the completion of the first meiotic division. The polar body is equivalent to one of the two cells at telophase I in Figure 53-2B. In the second meiotic division, which the cell completes at the time of fertilization, the secondary oocyte gives rise to one mature oocyte (haploid 1N DNA) and a diminutive second polar body (haploid 1N DNA). Thus, unlike the situation in males, one primary oocyte yields one mature gamete—equivalent to one of the four cells at the bottom of Figure 53-2B.
Note that the first polar body sometimes divides during meiosis II, thereby yielding a total of three polar bodies and one oocyte. This is the same amount of DNA produced in spermatogenesis (i.e., four spermatids from one spermatogonium).
Daughter cells produced by mitosis are genetically identical because there is no exchange of genetic material between homologous chromosomes and the sister chromatids of each chromosome split, one going to each daughter cell during anaphase of the single mitotic division.
Meiosis occurs only in germ cells—spermatogonia in males and oogonia in females—still with a complement of 2N DNA (N = 23). Germ cells initially multiply by mitosis and then enter meiosis when they begin to differentiate into sperm (see Fig. 53-2B) or ova (see Fig. 53-2C). Gametogenesis reduces the number of chromosomes by half, so that each gamete contains one chromosome from each of the original 23 pairs. This reduction in genetic material from the diploid (2N) to the haploid (N) number involves two divisions referred to as meiosis I and meiosis II. Because of this halving of the diploid number of chromosomes, meiosis is often referred to as a reduction division.
Meiosis is a continuum composed of two phases: the homologous chromosomes separate during meiosis I, and the chromatids separate during meiosis II. Prior to the start of meiosis I, the chromosomes duplicate so that the cells have 23 pairs of duplicated chromosomes (i.e., each chromosome has two chromatids)—or 4N DNA. During prophase of the first meiotic division, homologous pairs of chromosomes—22 pairs of autosomal chromosomes (autosomes) plus a pair of sex chromosomes—exchange genetic material through a process known as recombination or crossing over at attachment points known as chiasmata. This results in a random, but balanced, exchange of chromatid segments between the homologous maternal and paternal chromatids to produce recombinant homologous chromosomes comprising a mix of maternal and paternal DNA. At the completion of meiosis I, the daughter cells have a haploid number (23) of duplicated, crossed-over chromosomes—or 2N DNA.
During meiosis II, no additional duplication of DNA takes place. The chromatids simply separate so that each daughter receives a haploid number of unduplicated chromosomes—1N DNA. Gametes produced by this process are genetically different from each other and from either parent. The genetic diversity that arises from recombination during meiosis and the combining of gametes from different parental lineages causes significant phenotypic variation within the population, providing an efficient mechanism for adaptation and natural selection.
A major difference between male and female gametogenesis is that one spermatogonium yields four spermatids (see Fig. 53-2B), whereas one oogonium yields one mature oocyte and two or three polar bodies (see Fig. 53-2C). We discuss the details regarding timing and process for spermatogenesis on page 1100, and for oogenesis on page 1120.
Fertilization of an oocyte by an X- or Y-bearing sperm establishes the zygote's genotypic sex
Fusion of two haploid gametes, a mature spermatozoan from the father and a mature oocyte from the mother—referred to as fertilization—produces a new diploid cell with 2N DNA, a zygote, that will become a new individual.
The sex chromosomes that the parents contribute to the offspring determine the genotypic sex of that individual. The genotypic sex determines the gonadal sex, which in turn determines the phenotypic sex that becomes fully established at puberty. Thus, sex-determining mechanisms established at fertilization direct all later ontogenetic processes (processes that lead to the development of an organism) involved in male-female differentiation.
Fusion of a sperm and an egg—two haploid germ cells—results in a zygote, which is a diploid cell containing 46 chromosomes (see Fig. 53-1): 22 pairs of somatic chromosomes (autosomes) and a single pair of sex chromosomes. In females, these sex chromosomes are both X chromosomes, whereas males have one X and one Y chromosome.
When the karyotypes of normal females and males are compared, two differences are apparent. First, among the 23 pairs of chromosomes in females, 8 pairs—including the 2 X chromosomes—are of similar size, whereas males have only such pairs. Second, instead of a second X chromosome, males have a Y chromosome that is small and acrocentric (i.e., the centromere is located at one end of the chromosome): this chromosome is the only such chromosome that is not present in females.
In the offspring, 23 of the chromosomes—including 1 of the sex chromosomes—are from the mother, and 23—including the other sex chromosome—come from the father. Thus, the potential offspring has a unique complement of chromosomes differing from those of both the mother and the father. The ovum provided by the mother (XX) always provides an X chromosome. Because the male is the heterogenetic (XY) sex, half the spermatozoa are X bearing whereas the other half are Y bearing. Thus, the type of sperm that fertilizes the ovum determines the sex of the zygote. X-bearing sperm produce XX zygotes that develop into females with a 46,XX karyotype, whereas Y-bearing sperm produce XY zygotes that develop into males with a 46,XY karyotype. The genotypic sex of an individual is determined at the time of fertilization. The Y chromosome appears to be the fundamental determinant of sexual development. When a Y chromosome is present, the individual develops as a male; when the Y chromosome is absent, the individual develops as a female. In embryos with abnormal sex chromosome complexes, the number of X chromosomes is apparently of little significance.
Genotypic sex determines differentiation of the indifferent gonad into either an ovary or a testis
The indifferent gonad is composed of an outer cortex and an inner medulla. In embryos with an XY sex chromosome complement (i.e., 46,XY), the medulla differentiates into a testis and the cortex regresses. On the other hand, in embryos with an XX sex chromosome complement (i.e., 46,XX), the cortex develops into an ovary and the medulla regresses. Thus, the Y chromosome exerts a powerful testis-determining effect on the indifferent gonad. In the absence of a Y chromosome, the indifferent gonad develops into an ovary.
Interestingly, two X chromosomes are necessary for normal ovarian development. In individuals with the karyotype 45,XO—Turner syndrome—the ovaries fail to develop fully and appear as streaks on the pelvic sidewall (Box 53-1). Even the absence of only some genetic material from one X chromosome in XX individuals (e.g., due to chromosome breakage or deletion) may cause abnormal gonadal differentiation. However, a complete Y chromosome is necessary for development of the testes. Indeed, individuals with the karyotype 47,XXY—Klinefelter syndrome—are not phenotypic females (based on the presence of two X chromosomes) but males. Taken together, the data on 45,XO and 47,XXY individuals tell us that the absence of a Y chromosome causes female phenotypic development.
The best-known example of gonadal dysgenesis is a syndrome referred to as Turner syndrome, a disorder of the female sex characterized by short stature, primary amenorrhea, sexual infantilism, and certain other congenital abnormalities. Cells in these individuals have a 45,XO karyotype (i.e., they lack one of the X chromosomes). The gonads of individuals with Turner syndrome appear as firm, flat, glistening streaks (referred to as streak gonads) lying below the fallopian tubes with no evidence of either germinal or secretory elements. Instead, they are largely composed of connective tissue arranged in whorls suggestive of ovarian stroma. Individuals with Turner syndrome have normal female differentiation of both the internal and external genitalia, although these genitalia are usually small and immature.
Turner syndrome can also be caused by partial deletion of the X chromosome, particularly if the entire short arm of the X chromosome is missing, or by formation of an X-chromosome ring that develops as a result of a deletion and subsequent joining of the two free ends of the chromosome.
In at least half of affected individuals, Turner syndrome is caused by the total absence of one X chromosome. In others, the lesion is structural (i.e., partial deletion or ring chromosome). In at least a third of cases, the genetic lesion appears as part of a mosaicism; that is, some of the cells carry the aberrant or absent chromosome, whereas the rest are normal.
We have just seen that the absence of a Y chromosome and the presence of two complete X chromosomes lead to normal ovarian development. Why? The X chromosome is far larger than the Y chromosome (see Fig. 53-1) and contains nearly 10% of the human genome compared to <100 genes on the Y chromosome. Thus, the large number of X-linked diseases—affecting such processes as blood clotting and color vision—is hardly surprising. Compared with males, females have a double dose of X-chromosome genes. To avoid an overdosage of X-derived gene products, each somatic 46,XX cell at the blastocyst stage separately and randomly inactivates either the maternal or paternal X chromosome in a process called lyonization. Once inactivated, that X chromosome remains inactivated for the life of the cell and all of its descendants. The inactivated X chromosome is visible at interphase as a small dark dot of condensed chromatin in the nucleus known as the Barr body. Presence of a Barr body can be used to determine the genotypic sex of a cell. If one X chromosome is normally inactivated in females, then why are two X chromosomes necessary for normal ovarian development, as evidenced by the deficiencies in 45,XO individuals? The answer is that many genes on the inactivated X chromosome somehow are not silenced and are necessary for normal ovarian development.
The testis-determining gene is located on the Y chromosome
With rare exceptions (see below), a Y chromosome (Fig. 53-3A) is necessary for normal testicular development. Thus, it stands to reason that the gene that determines organogenesis of the testis is normally located on the Y chromosome. This so-called testis-determining factor (TDF) has been mapped to the short arm of the Y chromosome and indeed turns out to be a single gene called SRY (for sex-determining region Y). The SRY gene encodes a transcription factor that belongs to the high-mobility group (HMG) superfamily of transcription factors. The family to which SRY belongs is evolutionarily ancient. One portion of the SRY protein, the 80–amino-acid HMG box, which actually binds to the DNA—is highly conserved among members of the family.
FIGURE 53-3 Location of the testis-determining region of the Y chromosome and an example of translocation. A, The Y chromosome is much smaller than the X chromosome. Giemsa staining of the chromosome results in alternating light and dark bands, some of which are shown here. The short or “p” arm of the Y chromosome is located above the centromere, whereas the long or “q” arm is located below. The numbers to the left of the chromosome indicate the position of bands. The TDF is the SRY gene. B, Crossing-over events between normal X and Y chromosomes of the father can generate an X chromatid that contains a substantial portion of the TDF region and a Y chromatid that lacks its TDF. The figure shows both an equal and an unequal recombination event. If a sperm cell bearing an X chromosome with a translocated TDF fertilizes an ovum, the result is a male with a 46,XX karyotype, because one of the X chromosomes contains the TDF. Conversely, if the sperm cell carries a Y chromosome lacking its TDF, the result can be a 46,XY individual that appears to be female.
Rarely, the TDF may also be found translocated on other chromosomes. One example is a 46,XX male (see Fig. 53-3B), an individual whose sex chromosome complement is XX but whose phenotype is male. During normal male meiosis, human X and Y chromosomes pair and recombine at the distal end of their short arms. It appears that most 46,XX males arise as a result of an aberrant exchange of paternal genetic material between X and Y chromosomes during spermatogenesis. In such cases, the TDF is transferred from a Y chromatid to an X chromatid. If the sperm cell that fertilizes the ovum contains such an X chromosome with a TDF, the resultant individual will be a 46,XX male.
Endocrine and paracrine messengers modulate phenotypic differentiation
Just as an individual's genes determine whether the indifferent gonad develops into an ovary or a testis, so does the sex of the gonad dictate the gonad's endocrine and paracrine functions. Normally, chemical messengers—both endocrine and paracrine—produced by the gonad determine the primary and secondary sexual phenotypes of the individual. However, if the gonads fail to produce the proper messengers, if other organs (e.g., the adrenal glands) produce abnormal levels of sex steroids, or if the mother is exposed to chemical agents (e.g., synthetic progestins, testosterone) during pregnancy, sexual development of the fetus may deviate from that programmed by the genotype. Therefore, genetic determination of sexual differentiation is not irrevocable; numerous internal and external influences during development may modify or completely reverse the phenotype of the individual, whatever the genotypic sex (Box 53-2).
Discordance Between Genotype and Gonadal Phenotype
Discordance between genotypic and phenotypic sex are referred to as disorders of sex development (DSD), a term that describes congenital conditions in which development of chromosomal, gonadal, or anatomical sex is atypical. The term DSD avoids gender labeling in the diagnosis as well as names with negative social connotation (e.g., hermaphrodite, pseudohermaphrodite, intersex) that some patients and parents may perceive to be harmful.
Most DSD conditions are caused by aberrant expression of SRY. For example, a number of patients have no recognizable Y chromosome but do have testes. Some of these individuals are 46,XX and possess both male and female sex organs. Others have mixed gonadal dysgenesis—a testis in addition to a streak ovary—and a 45,XO karyotype. Some affected individuals have only one type of gonadal tissue but morphological characteristics of both sexes. All these patterns can result from mosaicisms (e.g., 46,XY/46,XX) or from translocation of the SRY gene (see Fig. 53-3B)—which normally resides on the Y chromosome—to either an X chromosome or an autosome. A “normal” testis in the absence of a Y chromosome has never been reported.
Another group of individuals with a karyotype of 46,XY has pure gonadal dysgenesis—streak gonads, but no somatic features of XO. In the past investigators assumed that these individuals possessed an abnormal Y chromosome. Perhaps the SRY gene is absent or its expression is somehow blocked.