BGDB Sexual Differentiation - Early Embryo

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



Week 4 to 5

<html5media height="580" width="550">File:Stage13 GIT3d.mp4</html5media>

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Week 4-5 embryo (stage 13) Movie

Begin by observing the internal structure of the embryo at the end of week 4 and the beginning of week 5.

Stage13 bf1.jpg Stage13 sem1.jpg

  • The gastrointestinal tract now has 3 defined regions
  • The intermediate mesoderm has differentiated to first form a primitive kidney (pronephros) and now forms an intermediate kidney (mesonephros)
  • Associated with the intermediate kidney are 2 ducts (mesonephric ducts')
  • The mesonephric ducts run rostro-caudally and end opening into the cloacal cavity.

Colour code:

  • Foregut - oropharyngeal membrane, oesophagus, pharynx (dark salmon - Foregut, oropharyngeal membrane, oesophagus, pharynx)
  • Trachea and Lung Buds (dark blue - Trachea and Lung Buds)
  • Mesonephros (yellow - mesonephros)
  • Mesonephric Ducts (light blue - mesonephric ducts)
  • Hindgut and Cloaca (brown - hindgut and cloaca)


Mesonephros and Mesonephric Duct

Planes of sections

Section 29-46: (E1-G4) Section passes dorsally to the mesonephros. Return to G4 and then follow the caudal route of the mesonephric duct into the sacral region.

Stage 13 image 078.jpg Stage 13 image 079.jpg Stage 13 image 080.jpg Stage 13 image 081.jpg Stage 13 image 082.jpg Stage 13 image 083.jpg Stage 13 image 084.jpg
29 (E1L) 30 (E2L) 31 (E3L) 32 (E4L) 33 (E5L) 34 (E6L) 35 (E7L)
Stage 13 image 085.jpg Stage 13 image 086.jpg Stage 13 image 087.jpg Stage 13 image 088.jpg Stage 13 image 089.jpg Stage 13 image 090.jpg Stage 13 image 091.jpg
36 (F1L) 37 (F2L) 38 (F3L) 39 (F4L) 40 (F5L) 41 (F6L) 42 (F7L)
Stage 13 image 092.jpg Stage 13 image 093.jpg Stage 13 image 094.jpg Stage 13 image 095.jpg
43 (G1L) 44 (G2L) 45 (G3L) 46 (G4L)

Kidney and Mesonephric Duct

<html5media height="520" width="360">File:Urogenital_sinus_001.mp4</html5media>

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Urogenital Sinus Movie

Now go back and observe the development of the intermediate mesoderm.
  • This looped animation shows the 3 stages of kidney development (pink) in relation to the hindgut region between Week 4 and 5.
  • The earliest stage of kidney development begins up near the pharyngeal arches as the pronephros (cervical nephrotomes) which quickly degenerate, beside these the mesonephric duct begins to form (purple).
  • The next stage is the extensive mesonephros (red) which extends down towards the developing hindgut region (yellow). Associated with the mesonephros is the mesonephric duct (purple).
  • The mesonephric duct gives off a lateral branch forming the ureteric bud.
  • The ureteric bud interacts with the surrounding mesenchyme to begin the final stage in renal development, formation of the metanephros (pink) which will form the adult kidney.


Next observe the changes occuring in the hindgut region (yellow) ending in the embryo at the cloaca.

  • The posterior portion (nearest the mesonephros) remains as the gastrointestinal tract hindgut forming the rectum.
  • The anterior portion (extends into the connecting stalk as the allantois)
    • becomes separated from the hindgut and forms the urogenital sinus, primordia of the urinary bladder.
    • The two mesonephric ducts initially open into the urogenital sinus.

Primitive Urogenital Sinus

<html5media height="440" width="420">File:Urogenital_septum_001.mp4</html5media>

Click Here to play on mobile device Urogenital Septum Movie

Stages in septation of the urogenital sinus between Week 4 and 6.
  • The initial cloaca is divided by the urorectal septum.
  • The posterior portion forms the rectum and the anterior portion forms the primitive urogenital sinus.
  • The superior part of the primitive urogenital sinus forms the urinary bladder, this and the rectum are the same for both sexes.
  • The inferior part of the primitive urogenital sinus forms different structures in males and females. The constricting part forms the prostatic urethra (male) or the membranous urethra (female), beneath this region the expanded space is the definitive urogenital sinus forms the penile urethra (male) or vagina vestibule (female). Note the animation is simplified, as the cloacal membrane appears to break down before the septum can fuse with it.

Several different defects in both sexes can occur if this septation process is not correctly aligned (rectourethral fistula, rectovaginal fistula).

Stage12 sem9 cloacal membrane.jpg

Cloacal Membrane (intact)

(week 4 Carnegie stage 12)

Gonad Development

Male Testis

<html5media height="650" width="350">File:Testis 001.mp4</html5media>

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Testis Development Movie

Development of the male gonad showing medullary sex cords

  • Paramesonephric duct (red left, Müllerian duct) degenerates under the influence of Mullerian duct inhibitory factor (MDIF) secreted by sertoli cells (differentiated by SRY expression).
  • Mesonephric duct (purple, Wolffian duct) differentiates under the influence of testosterone secreted by Leydig cells. Within the testes these mesonephric tubules grow towards the medullary sex cords and will form the rete teste. The mesonephric duct extending out of the gonad forms the ductus deferens.
  • Medullary sex cords (orange) form testis cords.
    • these later differentiate into solid seminiferous tubules that during puberty become hollow and actively produce spermatozoa.
  • The surrounding connective tissue (pink) differentiates to form stromal cells.
  • The tunica albuginea (white) covers the testis and bands extend inward to form connective tissue septa.


Female Ovary

<mediaplayer width='350' height='650' image="http://php.med.unsw.edu.au/embryology/images/f/fa/Gonad-icon.jpg">File:Ovary_001.mp4</mediaplayer>

Ovary Development Movie

Development of the female gonad showing medullary sex cords

  • Mesonephric duct (purple, Wolffian duct) degenerates, small remnants may remain as epoophoron and paroophoron (in the mesentry of the ovary) and Gartner's cycts (near vagina).
  • Paramesonephric duct (red left, Müllerian duct) grows forming the oviduct (uterine horn) and the end opens into the peritoneal cavity and terminates in fimbria (finger-like extensions). Away from the ovary, the two paramesonephric ducts fuse in the midline to form the uterus.
  • Cortical sex cords (orange) form after the primary sex cords degenerate and mesothelium forms secondary cords.
  • The surrounding connective tissue (pink) differentiates to form follicle cells.

Gonadal Development Comparison

Now directly compare the development of the male and female gonad (click image to start, these may not be synchronised).

Ovary Testis
<mediaplayer width='350' height='650' image="http://php.med.unsw.edu.au/embryology/images/f/fa/Gonad-icon.jpg">File:Ovary_001.mp4</mediaplayer> <mediaplayer width='350' height='650' image="http://php.med.unsw.edu.au/embryology/images/f/fa/Gonad-icon.jpg">File:Testis 001.mp4</mediaplayer>
Ovary Development Movie Testis Development Movie


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



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.

Male pathway

  • initially Sry, Sox9 and Fgf9 are up-regulated.
  • then FGF9, PDGF and DHH drive structural and functional changes.

Female pathway

  • during sex determination period RSPO1 (R-spondin-1) a secreted protein interacts with WNT4.

References


<pubmed>17237341</pubmed>| Physiol. Rev.

Nussey S, Whitehead S. Endocrinology: An Integrated Approach. Oxford: BIOS Scientific Publishers; 2001. Chapter 6 The gonad

History




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Cite this page: Hill, M.A. (2024, March 28) Embryology BGDB Sexual Differentiation - Early Embryo. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/BGDB_Sexual_Differentiation_-_Early_Embryo

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© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G