BGDB Sexual Differentiation - Fetal

From Embryology
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Practical 12: Sex Determination | Early Embryo | Late Embryo | Fetal | Postnatal | Abnormalities | 2011 Audio


Introduction

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

  • 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.

Uterus and Vagina

Data from the developing mouse showing 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.

(Image modified from: Drews U, Sulak O, Schenck PA. Androgens and the development of the vagina. Biol Reprod. 2002 Oct;67(4):1353-9. PMID: 12297555)


This looped animation shows the anterior view of development of the female uterus and vagina between Week 9 and 20.

The paramesonephric ducts (red) fuse in the midline to form the genital canal.

The urogenital sinus (yellow), in contact with the paramesonephric duct, thickens to form the sinusal tubercle which extends as a solid vaginal plate, then becomes hollow as the sinovaginal bulb, finally forming the vagina. (More? Genital System - Female Uterus)

The uterus and broad ligament will eventulaly divide the pelvic cavity into two separate pouches.

posteriorly the uterorectal pouch (pouch of Douglas) anteriorly the 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.

(Data: Soriano D, Lipitz S, Seidman DS, Maymon R, Mashiach S, Achiron R. Development of the fetal uterus between 19 and 38 weeks of gestation: in-utero ultrasonographic measurements. Hum Reprod. 1999 Jan;14(1):215-8.)

External Genitalia

This next section will look at the development of the external genitalia using a series of animations and online resources.

Female External Genitalia

This looped animation shows the development of external female genitalia from the indifferent external structure, covering the approximate period of week 9 to 12.

Note the original cloacal membrane becomes separated into the urogenital membrane and anal membrane. The urogenital folds beneath the genital tubercle remain separate (unfused), forming the inner labia minora and second outer skin folds form the larger labia majora either side of the developing vestibule of the vagina. Note at the top of the animation, the changing relative size of the genital tubercle as it forms the glans of the clitoris.

Male External Genitalia

This looped animation shows the development of external male genitalia from the indifferent external structure, covering the approximate period of week 9 to 12.

Note the original cloacal membrane becomes separated into the urogenital membrane and anal membrane (identical to female). The urogenital folds beneath the genital tubercle begin to fuse in the midline. The skin folds either side for the scrotum, which too has a midline fusion, the raphe. The 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

Gonad Descent

This looped animation shows the descent of the gonads and their blood supply.

Internal Gonad Descent

Testes Descent

This looped animation shows the descent of the testes between Week 7 and 38 (birth).

The testis (white) lies in the subserous fascia (spotted) a cavity processus vaginalis evaginates into the scrotum, and the gubernaculum (green) attached to the testis shortens drawing it into the scotal sac. As it descends it passes through the inguinal canal which extends from the deep ring (transversalis fascia) to the superficial ring (external oblique muscle). Descent of the testes into the scrotal sac begins generally during week 26 and may take several days. The animation shows the path of a single testis. Incomplete or failed descent can occur unilaterally or bilaterally, is more common in premature births, and can be completed postnatally. (see also cryptorchidism).

Abdominal muscles : external oblique (purple), internal oblique (blue), transverse abdominus (pink)

Fascia: transversalis (red), subserous (spotted), peritoneum (orange)

Testes Descent | Testes Descent Large


Testes Descent Timeline

Data from a recent 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 showed that from 33 weeks fetal testes had descended to the scrotum and that 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.

References: Sampaio FJ, Favorito LA. Analysis of testicular migration during the fetal period in humans. J Urol. 1998 Feb;159(2):540-2. | Malas MA, Sulak O, Ozturk A. The growth of the testes during the fetal period. BJU Int. 1999 Oct;84(6):689-92.


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Practical 12: Sex Determination | Early Embryo | Late Embryo | Fetal | Postnatal | Abnormalities | 2011 Audio


References



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BGDB: Lecture - Gastrointestinal System | Practical - Gastrointestinal System | Lecture - Face and Ear | Practical - Face and Ear | Lecture - Endocrine | Lecture - Sexual Differentiation | Practical - Sexual Differentiation | Tutorial


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Cite this page: Hill, M.A. (2021, April 18) 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