Indifferent stage
Male development
Female development



Early in pregnancy, (within 10-15% of the pregnancy’s expected length) a genital ridge is formed in the sides of the embryonic tissue, ventral to the mesonephros or temporary kidneys. Initially, the genital ridge is made of coelomic epithelium with mesenchyme tissue. Subsequently, this area is invaded by germinal cells originating in the yolk sac (Figs. 8-1, 8-2). 

Germinal cells, gonocytes

  • Future gamete precursors
  • Originate in the yolk sac
  • Migrates to gonadal region
  • Accumulates due to:
    • Attraction by chemotactic substance
    • Failure to survive elsewhere

Figure 8-1. Generalities about gonocytes



Figure 8-2. Origin of germinal cells prior to migrating to the area of the future indifferent gonads

These clusters of cells are the indifferent gonads (Fig. 8-3).

Indifferent stage

  • Gonads begin
  • Pair of ducts capable of becoming male organs
  • Pair of ducts capable of becoming female organs
  • Underdeveloped potential external genitalia
  • Nervous system to be primed

Figure 8-3. Conditions existing at the indifferent stage of development


Normally the indifferent gonads will evolve into testis if the genotype of the conceptus is XY and ovaries if the conceptus is XX. The name “indifferent gonads” or bipotential tissue is given because at this developmental stage, regardless of the genetic make up of the conceptus, the gonads have the potential to be either ovaries or testis.  Up until recently, the understanding was that unless other substances were present, these indifferent gonads were destined to become female organs. Recent discoveries, however, reveal that the indifferent gonads have to be induced to become either male or female organs. Following the establishment of the indifferent gonads, the embryo develops two pairs of ducts. One pair of mesonephric ducts, also known as the Wolfian ducts, which have the potential to further develop into the epididymis, ductus deferens, and ureter of the male reproductive tract. Their original role is to drain fluids from the mesonephros to the urogenital sinus. The other pair is the paramesonephric ducts, also known as Müllerian ducts, which have the potential to develop into the oviducts, uterus, cervix and a portion of the vagina (Fig. 8-3). 


Figure 8-4. Schematic representation of the structures visible at the indifferent stage

At this stage, the embryo has a urogenital sinus, also known as the cloaca, with the potential to develop into female or male external genitalia, as well as an incipient central nervous system which could be primed to function in the pattern of a male or female animal. At this developmental point the embryo is considered to be in an indifferent stage (Fig. 8-4).


Male development

The sex determining region Y gene (SRY), located in the Y chromosome appears to be responsible for the initial organization of the indifferent gonad into the male structures.  The SRY gene is activated, by still unknown mechanisms, during a very precise window of time in the early developmental period.  As a result of its activation, a protein named testis-determining factor is produced and acts upon some of the mesonephric cells in the indifferent gonadal region.  These cells then develop into pre-Sertoli cells (Fig. 8-5).



Figure 8-5. Pathway leading to the establishment of male sex organs and development of male like behaviour


The pre-Sertoli cells synthesize Anti-Müllerian Hormone (AMH), also known as Müllerian Inhibiting Substance.  AMH belongs to a superfamily of polypeptide hormones called Transforming Growth Factor beta (TGFbeta).

AMH triggers the migration of the epithelial cells to the central part of the mesonephros. The mesonephric cells become pre-Sertoli cells which surround the germinal cells or gonocytes. AMH also triggers the regression of the Müllerian or paramesonephric ducts through induced cellular apoptosis (Figs. 8-5, 8-6, 8-7).



Figure 8-6. Schematic representation of the structures visible during early development in the male and the female


Gonocytes will be eventually converted to non-motile spermatogonia, which will divide by mitosiss until stopped by the Sertoli cells.  The final outcome is the formation of the primitive sex cords which will then remain quiescent until puberty without any further formation of spermatogonia. Simultaneously, some mesenchymal cells will differentiate into interstitial or Leydig cells.  These cells will start to produce testosterone (T), which in turn will be converted to dihydrotestosterone (DHT) in the urogenital sinus.  DHT is responsible for the virilisation of the external genitalia and for the formation of accessory secretory glands in males (Fig. 8-7).



Figure 8-7. In late development the new metanephros develops, the accessory sex glands and the bladder


Figure 8-7a. Changes taking place during sexual differentiation of the male


Female development

If the indifferent tissue is not exposed to AMH, the Müllerian or paramesonephric duct is not reabsorbed as a consequence of induced apoptosis. In the female another gene, the DAX1, (dosage-sensitive sex reversal, adrenal hypoplasia critical region, on chromosome X, gene 1) is activated at this time.  This gene is located in the X chromosome. The activation of the DAX1 gene results in the development of the indifferent gonad into ovaries, the growth of the paramesonephric or Müllerian duct into oviduct and uterine structures (Figs. 8-6, 8-8). 

 At the same time, the Wolffian or mesonephric ducts start atrophying.  The metanephros starts developing into the future kidneys and starts functioning  by 30% of the pregnancy, while the mesonephros becomes the ovary (Fig. 8-10). The changes from the indifferent stage towards the establishment of female organs starts approximately two weeks after the change towards males (Figs. 8-8, 8-9).



Figure 8-8. Sequence of events leading to the development of females


This time lapse permits a clear division between males and females and appears to allow for the potential manifestation of the male characteristics.  In the developing female there is a rapid proliferation of germinal cells in the incipient ovary.  The number of germinal cells then decreases towards parturition with a further decrease at the time of puberty (Fig. 8-9).

Female development

  • Later than male’s
  • Germinal cells
    • 6 to 7 X 106 at six months
    • 1 to 4 X 105 at puberty
    • 400 potential ova

Figure 8-9. Generalities about female development


Figure 8-10. Pictorial description of the organs in female late development



Pseudohermaphroditism refers to the condition whereby an animal has chromosomes and gonads of one sex and the external genitalia of the opposite sex, or with components of both sexes (Figs. 8-11, 8-12). 



Result of a developmental abnormality where the genetic make up of the gonads are of one gender while the external and / or internal genitalia is of the other gender, or an incompletely differentiated organ

Figure 8-11. Pseudohermaphroditism


Figure 8-12. Pseudohermaphroditism in a pig. Two descended testicles and a clear vulva are present


This abnormality may be observed in genetic females which have been exposed to elevated levels of androgens during gestation (Fig. 8-13).

Pseudohermaphroditism in a genetically female animal

  • Female exposed to androgens
    • Adrenal abnormality
    • Maternal androgens

Figure 8-13. Possible causes of a female developing pseudohermaphroditism


The source of androgens can be an abnormality of the fetal adrenal or maternal secretions. In the case of males, pseudohermaphroditism may be triggered by a lack of sufficient testosterone production by the fetal testes. It can also be induced by a resistance or a lack of response of the urogenital sinus to androgens.  This resistance can be the consequence of a lack of androgen receptors in this tissue.  The phenomenon can also be triggered by the failure in the conversion of testosterone to DHT by the enzyme 5α- reductase (Fig. 8-14).


Pseudohermaphroditism in a genetically male animal

  • Male testicular abnormality
    • No MIS = female internal genitalia
    • No T = female genitalia
    • Androgen resistance
      • No DHT
      • No receptors

Figure 8-14. Possible causes of a male developing pseudohermaphroditism


A common occurrence in cattle is the production of freemartins.  A freemartin occurs as a consequence of a translocation of testosterone produced by the bull calf into the circulation of the heifer calf at the site of the cotyledons.  This takes place because the chorion of both conceptuses anastomoses is forming a common blood supply (Fig. 8-15).



  • Sexual modification of a female twin, by the in utero transfer of blood from a male fetus
  • Vascular anastomoses between the placentas permits the transfer of blood forming and germ cells among them
  • 92% of female twin calves born with a bull calf are freemartin
  • A single born female may be freemartin if the twin calf dies early in gestation

Figure 8-15. Characteristics of a freemartin


A common occurrence in cattle is the production of freemartins.  A freemartin occurs as a consequence of a translocation of testosterone produced by the bull calf into the circulation of the heifer calf at the site of the cotyledons.  This takes place because the chorion of both conceptuses anastomoses is forming a common blood supply (Fig. 8-15).

The testosterone acts upon the reproductive tract of the female inducing the formation of male-like internal organs, at the same time that the AMH causes partial regression of the paramesonephric ducts of the female conceptus.  Usually, the result is an ovotestis and a fragmented uterus lacking some of its parts. The final outcome is a phenotypic female that is sterile in 92% of the cases.  Furthermore, the freemartin displays bullish-like behaviour because the exposure of the CNS to testosterone during fetal life has primed her to function in a male-like fashion.  A sound management practice is to treat any female born as a twin to a bull, as a male and to  not keep it for breeding.   







Notes on Sexual Differentiation