BGDB Sexual Differentiation - Early Embryo

Week 4 and 5

(Week 5, stage 13/14)

The gastrointestinal tract (GIT) develops primarily from the endoderm, with inputs from the mesoderm (splanchnic mesoderm) and the ectoderm (neural crest cells form its primary nerve plexus). After the endoderm is folded into a blind-ended tube in week 4, it begins to develop into the primary lining of the GIT, while the mesoderm provides its muscular wall and connective tissues components. The GIT is typically partitioned into 3 parts, based on their separate blood supplies: foregut, midgut and hindgut, with the allantois coming off the hindgut. A bud coming off the tract forms the liver; eventually buds appear that form gallbladder and pancreas. The rostral end of the tube ends at the buccopharyngeal membrane, where it interacts with ectoderm directly (no mesoderm in between). This area forms the future mouth. A similar endodermal-ectodermal interaction occurs at the tail – the cloacal membrane. The section of the GIT here is called the cloaca.

Kidney

The kidneys develop from intermediate mesoderm, which lies between the lateral plate mesoderm and the somites. The kidney develops in multiple stages, which occur in a rostrocaudal sequence. The earliest structure to form is the pronephros, in week 4, featuring a pronephric duct with associated nephrogenic mesenchyme. This degenerates early on, leaving only the duct system running down to the cloaca – this becomes known as the mesonephric duct, in the embryo. The next stage is the formation of the mesonephros, also in week 4. Its differentiation is induced by the pronephros. However, the mesonephros is also a transient structure. It provides a template for the adult metanephros, beginning on day 35-37.

Kidney and Mesonephric Duct

This looped animation shows the 3 stages of kidney development in relation to the development of 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.

The final stage in kidney development is formation of the metanephros (pink) which will form the adult kidney.

Now 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. The anterior portion, which extends into the connecting stalk as the allantois, becomes separated from the hindgut and forms the primordia of the urinary bladder, but remains connected at the common urogenital sinus.

Primitive Urogenital Sinus

This looped animation shows the next stage 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).

Gonad Development

=Male Testis

This looped animation shows the development of the male gonad showing medullary sex cords.

The paramesonephric duct (red, left) degenerates under the influence of Mullerian duct inhibitory factor (MDIF) secreted by sertoli cells (differentiated by SRY expression).

The mesonephric duct (purple) 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.

The medullary sex cords (orange) form testis cords that later differentiate into solid seminiferous tubules which become hollow and actively produce spermatazoa during puberty.

The tunica albuginea (white) covers the testis and bands extend inward to form connective tissue septa.

Female Ovary

This looped animation shows the development of the female gonad showing cortical sex cords.

The mesonephric duct (purple) degenerates, small remnants may remain as epoophoron and paroophoron (in the mesentry of the ovary) and Gartner's cycts (near vagina).

The paramesonephric duct (red, left) 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.

The 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 (these may not be synchronised).

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Cite this page: Hill, M.A. (2024, April 20) 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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