In the previous section we observed late embryonic male genital development and now in fetal development we will observe early fetal female development. Then we will explore fetal development of the external genitalia and gonadal descent.
Week 10 Female
Fetus (week 10) Planes A (most lateral), B (lateral), C (medial) and D (midline) from lateral towards the midline.
- Image Source: UNSW Embryology, no reproduction without permission.
- week 10 to 12 - female in absence of Sry expression, within the gonad cortical cords extend from the gonad surface epithelium. Primordial germ cells which have entered the gonad form oogonia primordia, surrounding mesenchyme forms the follicular primordia.
- The selected images below show the general anatomy of the pelvic region of the an early female fetus (10 week, 40mm).
- This first image shows the relative positions of the kidney (plane A most lateral) and developing ovary (plane A and B away from the midline) and internal genitalia (plane C and D in the midline).
- The selected images below show sections through the pelvic region showing anatomical relationships between the developing female gonad (ovary), internal genital tract and external genitalia.
- Note the relative immaturity of the external genitalia, which at this time would appear identical to the male.
|Plane A (midline)
|| Plane B (medial)
|Plane C (lateral)
||Plane D (most lateral)
Uterus and Vagina
Mouse paramesonephric duct (Müllerian duct)
|This mouse image shows the relationship between the mesonephric and paramesonephric ducts opening into the urogenital sinus.
- The paramesonephric duct began as an infold of surface epithelium lying along the surface of the genital ridge.
- Estrogens, both maternal and fetal, stimulate its development and that eventually of the external female fetal genital structures.
- In contrast, the mesonephric duct regresses, remnants of this duct may remain lying within the broad ligament.
||Female Uterus and Vagina (between week 9 and 20)
- The initially paired ducts fuse in the midline forming the single body of the uterus.
- The ducts remain separate laterally where they form the uterine tubes (Fallopian tubes, uterine horns).
- The ducts peripheral attachment site to the urogenital sinus wall (yellow) is is described as the Müllerian tubercle.
- The fused ducts also generate the vagina, under the influence of BMP4.
- Estrogen will also later alter the vaginal epithelium.
The uterus and broad ligament will eventulaly divide the pelvic cavity into two separate pouches.
- posteriorly - uterorectal pouch (pouch of Douglas)
- anteriorly - uterovesical pouch
||This graph shows the growth during the fetal period of the uterus between week 19 and 38.
- During this time the uterine circumference increases from 20 mm to just under 60mm and the width increases from less than 10mm to just over 20 mm.
- Uterine horn fimbrial development begins after week 20 and continues after birth.
- Uterine growth continues postnatally, increasing outer muscle thickness and cyclic changes in the lining with puberty.
This next section will look at the development of the external genitalia using a series of animations and online resources.
Female External Genitalia
||Linked animation showing the development of external female genitalia from the indifferent external structure (week 9 to 12 approximately).
- original cloacal membrane becomes separated into the urogenital membrane and anal membrane
- urogenital folds beneath the genital tubercle remain separate (unfused)
- forming the inner labia minora
- second outer skin folds form the larger labia majora
- either side of the developing vestibule of the vagina
- genital tubercle (top of the animation) forms the glans of the clitoris
Male External Genitalia
||Linked animation showing the development of external male genitalia from the indifferent external structure (week 9 to 12 approximately).
- original cloacal membrane becomes separated into the urogenital membrane and anal membrane (identical to female).
- urogenital folds beneath the genital tubercle begin to fuse in the midline
- skin folds either side for the scrotum
- which also has a midline fusion, the raphe
- scrotal sac is initially empty and is an attachment site for the gubernaculum
- descent of the testes begins generally during week 26 and may take several days.
External Genitalia Comparison
||Linked animation shows the descent of the gonads and their blood supply.
- both male and female gonads undergo this relative descent within the peritoneal cavity
- this descent carries the gonads away from the kidneys which are ascending
- during the fetal period, male testes will continue to descend out of the peritoneal cavity
Internal Gonad Descent
||The linked animation shows the descent of the testes (between week 7 to 38, birth).
Descent of the testes into the scrotal sac begins generally during week 26 and may take several days.
- testis (white) lies in the subserous fascia (spotted)
- a cavity processus vaginalis evaginates into the scrotum
- gubernaculum (green) attached to the testis shortens drawing it into the scotal sac
- as it descends it passes through the inguinal canal extends
- from the deep ring (transversalis fascia)
- to the superficial ring (external oblique muscle)
Incomplete or failed descent can occur unilaterally or bilaterally, is more common in premature births, and can be completed postnatally. (see also cryptorchidism).
|Start of testis descent
||End of testis descent
Testes Descent Timeline
Data from a study of male human fetal (between 10 and 35 weeks) gonad position.
- 10 to 23 weeks - (9.45%) had migrated from the abdomen and were situated in the inguinal canal
- 24 to 26 weeks - (57.9%) had migrated from the abdomen
- 27 to 29 weeks - (16.7%) had not descended to the scrotum
A second study looked at the position of the testes
- 33 weeks fetal testes had descended to the scrotum
- between 33 to 40 weeks (term) both testes have normally descended to the scrotum
Failure of descent (cryptorchidism) either unilateral or bilateral testicular descent, occurring in up to 30% premature and 3-4% term males.
Cryptorchidism in common eutherian mammals.- Species comparison of descent timeline
Historic Genital Images
Broad ligament of adult showing Epoöphoron
Urogenital Sinus of Female Human Embryo of 8.5 to 9 weeks old
Transverse section of Human Embryo 8.5 to 9 Weeks Old
Longitudinal Section of Ovary of Cat Embryo of 9.4 cm long
Section of the Ovary of a Newly Born Child
Human Embryo (3.5 cm long) Testis Section of a Genital Cord
Tail end of Human Embryo 25 to 29 Days Old
Tail end of human embryo eight and a half to nine weeks old
Primitive Kidney and Bladder
Stages in the development of the external sexual organs in the male and female
- ↑ <pubmed>12297555</pubmed>
- ↑ <pubmed>10374123</pubmed>
- ↑ <pubmed>9649288</pubmed>
- ↑ <pubmed>10510117</pubmed>
- ↑ <pubmed>17379650</pubmed>
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Cite this page: Hill, M.A. (2021, April 16) Embryology BGDB Sexual Differentiation - Fetal. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/BGDB_Sexual_Differentiation_-_Fetal
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- © Dr Mark Hill 2021, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G