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You are watching: Whether an organism will be genetically male or female is determined by __________.

Alberts B, Johnson A, Lewis J, et al. Molecule Biology the the Cell. 4th edition. Brand-new York: Garland Science; 2002.

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Sexual reproductive strategies have the right to vary enormously in between different organisms. In the remainder of this chapter, we emphasis mainly top top the strategies provided by mammals.

In all vertebrate embryos, certain cells room singled out at an early stage in advancement as progenitors that the gametes. This primordial germ cells move to the emerging gonads, i beg your pardon will type the ovaries in females and the testes in males. ~ a duration of mitotic proliferation, the primordial germ cells undergo meiosis and also differentiate into mature gametes—either eggs or sperm. Later, the combination of egg and sperm after adjustment initiates embryogenesis. The subsequent manufacturing in this embryo of new primordial germ cells starts the bike again.

In this section, we think about how mammalian primordial germ cell arise, how the sex that a mammal is determined, and also how sex decision dictates whether the primordial germ cells construct into sperm or eggs.

Primordial Germ cell Migrate right into the developing Gonad

In most animals, including plenty of vertebrates, the unfertilized egg is asymmetrical, with various regions of cytoplasm containing various sets the mRNA and also protein molecule (discussed in chapter 21). As soon as the egg is fertilized and divides continuously to produce the cells of the at an early stage embryo, the cells that inherit certain molecules localized in a particular an ar of the egg cytoplasm end up being primordial germ cells. In mammals, by contrast, the egg is more symmetrical, and also the cells produced by the first couple of divisions that the fertilized egg are all totipotent—that is, lock can provide rise to any type of of the cell species in the body, including germ cells. A small group of cells in the at an early stage mammalian embryo is induced to end up being primordial germ cells by signals produced by bordering cells. In mice, because that example, 1 mainly after fertilization, around 50 cell in organization lying external the embryo ideal are induced by their neighbors to come to be primordial germ cells. In the next few days, these cells proliferate and are swept earlier into the embryo proper in addition to the invaginating hindgut. Lock then actively migrate with the gut to your final destination in the occurring gonads (Figure 20-16). Together the primordial germ cells migrate through the embryo, they are signaled to survive, proliferate, and migrate by miscellaneous extracellular proteins created by surrounding somatic cells.


Figure 20-16

Migration of mammalian primordial germ cells. (A) illustration showing the last stages the migration with the hindgut into the two genital ridges, each of i m sorry will develop into a gonad—either one ovary or a testis. (B) Micrograph showing moving (more...)

After the primordial germ cells get in the occurring mouse gonad, which in ~ this stage is referred to as the genital ridge, they continue to proliferate because that 2 or 3 much more days. At this point, they commit to a developmental pathway that will certainly lead them to become either egg or sperm, depending no on their own sex chromosome constitution but on whether the genital ridge has started to build into one ovary or a testis, respectively. The sex chromosomes in the somatic cells of the genital ridge recognize which type of gonad the ridge becomes. A single gene on the Y chromosome has specifically important function in this decision.

The Sry Gene ~ above the Y Chromosome deserve to Redirect a mrs Embryo to come to be a Male

Aristotle thought that the temperature of the male during sexual intercourse identified the sex that offspring: the higher the temperature, the greater the opportunity of creating a male. Us now know that the sex that a mammal is identified by that is sex chromosomes, quite than by the setting (although for part animals, such together crocodiles and also many fish, opposing is true). Mrs mammals have two X chromosomes in every one of their somatic cells, whereas males have one X and also one Y. The Y chromosome is the determining factor. People with a Y chromosome construct as males no issue how plenty of X chromosomes castle have, whereas individuals without a Y chromosome build as females, even if they have actually only one X chromosome. The sperm the fertilizes the egg identify the sex that the resulting zygote: eggs have actually a solitary X chromosome, conversely, the sperm can have one of two people an X or a Y.

The Y chromosome influences the sex the the separation, personal, instance by inducing the somatic cell of the genital ridge to construct into a testis rather of an ovary. The crucial gene ~ above the Y chromosome that has actually this testis determining duty is called Sry, because that “sex-determining region of Y.” Remarkably, as soon as this gene is introduced into the genome of one XX computer mouse zygote, the transgenic embryo created develops together a male, even though it lacks every one of the other genes top top the Y chromosome (Figure 20-17). Together mice, however, cannot produce sperm, in part, at least, because the visibility of 2 X chromosomes suppresses sperm development.


Figure 20-17

The Sry gene, injected into the cell nucleus of one XX mrs zygote, resulted in the transgenic embryo created to build into a male. The external genitalia of the transgenic mouse are equivalent from those of a regular XY masculine mouse. (From P. Koopman et (more...)

Sry is expressed only in a subset that the somatic cell of the emerging gonad, and also it reasons these cells to distinguish into Sertoli cells, which room the main type of sustaining cells found in the testis. The Sertoli cells straight sexual advancement along a male pathway by affecting various other cells in the genital ridge in in ~ least 4 ways:


They stimulate the newly getting here primordial germ cell to build along a pathway the produces sperm.


They secrete anti-Müllerian hormone, which suppresses the development of the female reproductive street by causing the Müllerian duct come regress (this duct otherwise provides rise to the oviduct, uterus, and also upper part of the vagina).


They stimulate particular somatic cells the lie surrounding to the developing gonad to migrate right into the gonad and also form an important connective tissue frameworks that are forced for typical sperm production.


They aid to induce various other somatic cell in the arising gonad to come to be Leydig cells, i beg your pardon secrete the male sex hormone testosterone; this hormone is responsible for inducing all male second sexual characteristics. These encompass the structures of the male reproductive tract, such together the prostate and seminal vesicles, which build from another duct, referred to as the Wolffian duct system. This duct device degenerates in the occurring female since it calls for testosterone come survive and develop. The testosterone additionally masculinizes the beforehand developing mind and in order to plays a major part in identify male sexual identity and also orientation, and also thereby behavior: mrs rats that are treated v testosterone roughly birth, because that example, later display screen malelike sexual behavior.

Sry encodes a gene-regulatory protein (Sry) the activates the transcription of other gene-regulatory proteins forced for Sertoli cabinet development, consisting of the Sry-related protein Sox9. In the absence of one of two people Sry or Sox9, the genital ridge creates into an ovary. The supporting cells end up being follicle cells rather of Sertoli cells. Various other somatic cells become theca cells rather of Leydig cell and, start at puberty secrete the female sex hormone estrogen instead of testosterone. The primordial germ cells construct into eggs rather of sperm (Figure 20-18, and see number 20-28), and the animal develops as a female.


Figure 20-18

Influence that Sry top top gonad development. The germ line cells are shaded in red, and also the somatic cells are shaded in green and blue. The change from light to darker color indicates the the cell has matured or differentiated. The Sry gene action in a subpopulation (more...)

If the genital ridges are removed before they have started to build into testes or ovaries, a mammal develops into a female, nevertheless of the sex chromosomes the carries. It seems that female advancement is the “default” pathway of sexual breakthrough in mammals.

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A small variety of cells in the gastrulating mammalian embryo room signaled through their next-door neighbors to come to be primordial germ cells. These cells migrate right into the genital ridges, which construct into the gonads. Here, the primordial germ cells start to develop into one of two people eggs, if the gonad is becoming an ovary, or sperm, if the gonad is becoming a testis. A developing gonad will develop into one ovary uneven its somatic cell contain a Y chromosome, in which instance it develops into a testis. The Sry gene top top the Y chromosome is responsible because that this testis-determining function: the is expressed in a subset the somatic cell in the occurring gonad, and it induces these cells to identify into Sertoli cells. The Sertoli cell in turn create the signal molecules that promote the advance of masculine characteristics, suppress the advance of woman characteristics, and induce the primordial germ cells to commit come sperm development.