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1282 Chapter 21: Sexual Reproduction: Meiosis, Germ Cells, and FertilizationPRIMORDIAL GERM CELLS AND SEXDETERMINATION IN MAMMALSSexual reproductive strategies vary enormously among different organisms. Inthe rest of this chapter, we focus mainly on the strategies used by mammals.In all vertebrate embryos, certain cells are singled out early in developmentas progenitors of the gametes. These diploid primordial germ cells (PGCs)migrate to the developing gonads, which will form the ovaries in females and thetestes in males. After a period of mitotic proliferation in the developing gonads,the PGCs undergo meiosis and differentiate into mature haploid gametes—either eggs or sperm. Later, the fusion of egg and sperm after mating initiatesembryogenesis. The subsequent production of new PGCs in this new embryobegins the cycle again (see Figure 21–3A).In this section, we consider how mammalian PGCs arise, how the sex of amammal is determined, and how sex determination dictates whether the PGCsdevelop into sperm or eggs.Signals from Neighbors Specify PGCs in Mammalian EmbryosIn many animals, including many vertebrates, the unfertilized egg contains specificmolecules localized in a particular region of the cytoplasm that determinewhich cells will become the germ cells. When the egg is fertilized and dividesrepeatedly to produce the cells of the early embryo, the cells that ultimatelyinherit these germ cell determinants become PGCs (Figure 21–16). Although themolecular nature and functions of the determinants are largely unknown, proteinsof the Vasa family are a required component in all of these animals. Vasaproteins are structurally similar to ATP-dependent RNA helicases, but their precisefunction in germ cell determination remains a mystery.By contrast, in other animals, including mammals, the egg cytoplasm doesnot contain localized germ cell determinants. Instead, signals from neighboringcells dictate which cells become PGCs. In mammals, all cells produced by thefirst few divisions of the fertilized egg are totipotent—that is, they have thepotential to give rise to any of the cell types of the animal, including the germcells and cells of extraembryonic tissues such as the placenta. Only later is asmall group of cells induced by signals from neighbors to become PGCs. In mice,for instance, about 6 days after fertilization, signals (including bone morphogenicprotein 4, BMP4) secreted by cells in tissue lying outside the embryoproper induce about 10 cells in the adjacent underlying embryo to become PGCprecursors. These cells divide and mature to become PGCs, turning off theexpression of a number of somatic cell genes and turning on the expression ofgenes involved in maintaining the special character of the germ cells.Although different mechanisms specify PGCs in different animals, some ofthe mechanisms that control their proliferation and development have beenFigure 21–16 Segregation of germ celldeterminants in the nematodeC. elegans. The micrographs in the upperrow show the pattern of cell divisions,with the cell nuclei stained blue; below,the same cells are stained with anantibody that labels (in green) smallgranules (called P granules) that functionas germ cell determinants. The P granulesare composed of RNA and proteinmolecules and are distributed randomlythroughout the cytoplasm of theunfertilized egg (not shown). As shown inthe far left-hand panels, after fertilization,the granules accumulate at one pole ofthe zygote. The granules are thensegregated into one of the two daughtercells at each cell division. The single cellcontaining the P granules in the embryoshown in the far right-hand panels is theprecursor of the germ line. (Courtesy ofSusan Strome.)
PRIMORDIAL GERM CELLS AND SEX DETERMINATION IN MAMMALS 1283(A)100 mmnotochord(B)hindgutneuraltubesomitefuturegonadprimordialgerm cellFigure 21–17 Migration of mammalianPGCs. (A) Fluorescence micrographshowing migrating PGCs in a crosssection of an early mouse embryo. ThePGCs are stained with a monoclonalantibody (in green) that specifically labelsthese cells at this stage ofembryogenesis. The remaining cells inthe embryo are stained with a lectin (inred) that binds to sialic acid, which isfound on the surface of all cells.(B) Drawing corresponding to themicrograph shown in (A). (A, courtesy ofRobert Anderson and Chris Wylie.)conserved in evolution from worms to humans. For example, PGC developmentin all animals that have been studied depends on the suppression of somatic cellfates by gene repression, as well as on the inhibition of translation of specificmRNAs by Nanos RNA-binding proteins.PGCs Migrate into the Developing GonadsAfter mammalian PGCs develop, they proliferate and migrate to their final destinationin the developing gonads (Figure 21–17). As they migrate through theembryo, various extracellular signal proteins produced by adjacent somatic cellssignal them to survive, proliferate, and migrate. Among the secreted signal proteinsthat help attract PGCs into the developing gonad are chemokines, whichbind to G-protein-coupled receptors (GPCRs) and guide the migration of variouscell types, including PGCs and white blood cells (discussed in Chapter 23).After the PGCs enter the developing gonad, which at this stage is called thegenital ridge, they go through several more mitotic cell divisions, in the course ofwhich they become specified to follow a pathway that will lead them to developas either eggs or sperm.When the PGCs first migrate into the embryonic gonad, however, they arenot irreversibly committed to becoming gametes. When removed from theembryo and cultured in the presence of appropriate extracellular signal proteins,they convert into cells that can be maintained in culture indefinitely as acell line that can produce any of the cell types of the body of the animal,although not the extraembryonic cells that go on to form structures such as theplacenta; for this reason, they are said to be pluripotent, rather than totipotent.In these respects, these so-called embryonic germ (EG) cells resemble embryonicstem (ES) cells (discussed in Chapter 23). EG and ES cells are promising sourcesof various human cell types—both for drug testing and for the treatment of diseasessuch as heart attacks, strokes, and various neurodegenerative diseases, inwhich specific cell types die.What determines whether the PGCs that migrate into the developing mammaliangonad develop into eggs or sperm? Surprisingly, it is not their own sexchromosome constitution but rather whether the genital ridge has begun todevelop into an ovary or a testis, respectively. The sex chromosomes in thesomatic cells of the genital ridge determine which type of gonad the ridgebecomes. Although many genes influence the outcome, a single gene on the Ychromosome has an especially important role.The Sry Gene Directs the Developing Mammalian Gonad toBecome a TestisAristotle believed that the temperature of the male during sexual intercoursedetermined the offspring’s sex: the higher the temperature, the greater thechance of producing a male. Had he been referring to lizards or alligators ratherthan to humans, he would have been closer to the truth, as in many egg-laying
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