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
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- 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.
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Plane A (most lateral)
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Plane B (lateral)
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Plane C (medial)
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Plane D (midline)
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Uterus and Vagina
Mouse paramesonephric duct (Müllerian duct)[1]
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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.
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This graph shows the growth during the fetal period of the uterus between week 19 and 38.[2]
- 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.
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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
<mediaplayer width='270' height='380' image="http://php.med.unsw.edu.au/embryology/images/3/37/Male_external_001_icon.jpg">File:Female_external_001.mp4</mediaplayer>
External Genital Female Development Movie
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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
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Male External Genitalia
<mediaplayer width='270' height='380' image="http://php.med.unsw.edu.au/embryology/images/3/37/Male_external_001_icon.jpg">File:Male_external_001.mp4</mediaplayer>
External Genital Male Development Movie
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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.
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External Genitalia Comparison
Gonad Descent
Internal Gonad Descent
Testes Descent
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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).
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Start of testis descent
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End of testis descent
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Additional Information
Additional Information - Content shown under this heading is not part of the material covered in this class. It is provided for those students who would like to know about some concepts or current research in topics related to the current class page.
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Testes Descent Timeline
Data from a study of male human fetal (between 10 and 35 weeks) gonad position.[3]
- 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[4]
- 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.[5]- Species comparison of descent timeline
Historic Genital Images
Keith, A. (1902) Human Embryology and Morphology. London: Edward Arnold.
Chapter 9 - The Uro-genital System
Fig. 79. Scheme of the Wolffian Body of the right side.
Fig. 80. Position of the Wolffian and Genital Ridges on the dorsal wall of the abdomen.
Fig. 81. Remnants of the Wolffian Body in the Female.
Fig. 82. Remnant of the "Wolffian Body in the Male.
Fig. 83. The Origin of the Renal Bud (diagrammatic).
Fig. 84. The Termination of the Ureter in the Bladder and Sub-division of the Renal Bud
Fig. 85. A transverse section to show Wolffian and Müllerian Ducts arise, and their position in the Wolffian Ridge.
Fig. 86. Diagram of the Genital Ducts at -the commencement of the 3rd month of foetal life. Lateral view.
Fig. 87. Diagram of the Müllerian Ducts at the commencement of the 3rd month. Ventral view.
Fig. 88. Evolution of the Human Form of Uterua.
Fig. 89. Showing the manner in which the Mulleriau Ducts fuse to form the Uterus and Vagina.
Fig. 90. A section of the Prostate showing the Hemnants of the lower ends of the Mttllerian Ducts in the male.
Fig. 91. A section of a Prostate showing an unusually developed Uterus Masculinus. (After Primrose.)
Fig. 92. Section showing the Uro-genital Sinus. A. 4th month female human foetus. B. 5th month female human foetus.
Fig. 93. Section showing the Uro-genital Sinus in the male foetus.
Fig. 94. A section to show the condition of the Vagina and Uterus at the 7th month of foetal life.
Fig. 95. The Division of the Cloaca into Rectal and Uro-genital Parts.
Fig. 96. Imperforate Anus due to a persistence of the Anal Plate.
Fig. 97. Rectal part of the Anal Plate has persisted and the Cloacal Septum has failed to fuse with the Perineal Septum.
Fig. 98. The Uro-genital Cleft or Depression and the Genital Tubercle and Folds towards the end of the 2nd month.
Fig. 99. A section of the male bladder and urethra at birth.
Fig. 100. A A section to show the condition of parts in Ectopia Vesicae.
Fig. 101. A diagram to show the position at which the Prostatic Tubules arise.
Fig. 102. The Position of the Testis in a foetus of 2£ months .
Fig. 103. Showing the Position of the Testis at the 6th month, and the Formation of the Gubernaculum Testis.
Fig. 104. Structures in the wall of the abdomen are carried out so as to form the Inguinal Canal and Coverings of the Testis.
Fig. 105. A diagram of the Processus Vaginalis.
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
Retroperitoneal structures
References
- ↑ <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. (2024, June 15) Embryology BGDB Sexual Differentiation - Fetal. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/BGDB_Sexual_Differentiation_-_Fetal
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