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Common Urological Problems

Genital tracts

  • - Paediatric Urology - Embryology
  • Jun 17, 2010
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Tags: | adrenal hyperplasia | androgenic stimulation | cryptorchidism | embryonic gonad |

The internal and external genitalia of both sexes are genetically ‘programmed’ to differentiate passively down a ‘default’  female pathway unless actively switched down a male pathway by the genetic information carried by the testis-determining gene (SRY).  Until the sixth week of gestation,  the genitalia of both sexes share identical embryonic precursors.  Differentiation   of   the   gonads   and genital tracts is initiated by the migration of primordial germ cells from the yolk sac,  across the coelomic cavity to condensations of primitive mesenchyme in the lumbar region of the embryo.

By a process of reciprocal interaction, the germ cells and surrounding mesenchyme form the primitive sex cords within the embryonic gonad.

At around this time (6 weeks),  the paired paramesonephric ducts appear as cords of coelomic epithelium lying lateral to the mesonephric ducts.  From this stage onwards, the pathways of male and female differentiation diverge.

Female

Internal genitalia
Although the primitive sex cords degenerate, secondary sex cords derived from genital ridge mesoderm enfold the primordial germ cells to form primitive follicles.  Differentiation of the genitalia down a female pathway cannot be entirely explained by the absence of the SRY gene. Normal development of the ovary does appear to at least be partly dependent on the presence of the two normal X chromosomes, since patients with Turner syndrome who have a single X chromosome (X karyotype)  typically have poorly formed, dysgenetic ovaries.

In the female, the initial phase of gametogenesis (transition from primordial germ cell to primary oocyte)  occurs within the fetal ovaries. 

The  undifferentiated genital   tract   is   genetically   programmed   to   proceed   down   the pathway  of  female  differentiation unless  switched  down  the  male pathway by the SRY geneFigure 1.10   The undifferentiated genital   tract   is   genetically   programmed   to   proceed   down   the pathway of female differentiation unless switched down the male pathway by the SRY gene.

During fetal life these primary oocytes embark on the first phase of meiotic division before entering a long phase of arrested division,  which resumes again only at puberty. In the absence of testosterone, the mesonephric ducts regress (leaving only vestigial remnants -  the epoöphoron,  paroöphoron and Gartner’s cysts). The paramesonephric ducts persist in the form of the fallopian tubes. Distally, the fused portions of the paramesonephric ducts give rise to the uterus and upper two-thirds of the vagina.

At the junction of the paired paramesonephric ducts with the urogenital sinus a condensation of tissue, the sinuvaginal bulb, develops. Between the 10th and 20th weeks of gestation displacement of the sinuvaginal bulb in the direction of the fetal perineum separates the developing vagina from the urethra.  During this process,  canalisation of the vagina occurs. The upper two-thirds of the vagina is derived from the paramesonephric ducts,  whereas the distal third has its origins in the urogenital sinus and the introitus and external genitalia are derived from ectoderm.

Development   of   the   lower   female genital tract between 10 and 20 weeksFigure   1.11   Development   of   the   lower   female genital tract between 10 and 20 weeks.

External genitalia
In the absence of androgenic stimulation,  the external genitalia of the embryo and fetus differentiate passively down a female pathway. The genital tubercle gives rise to the clitoris,  the urogenital sinus contributes the vestibule of the vagina, while the urogenital folds persist as the labia minora, and the labioscrotal folds persist as the labia majora.

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