BGDB Sexual Differentiation - Late Embryo

Week 8

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(Carnegie stage 22, male)

Begin by observing the internal structure of the embryo at the end of week 8.

Colour code:

Adrenal Glands (brown - fetal adrenal cortex and neural crest medulla)

Kidneys (orange - metanephros)

Gonads (green - developing testes)

Urinary Bladder (red - urogenital sinus)

Urethra (yellow - urethra)

Then this page shows small excerpts from whole cross-sections of the late embryonic Carneigie stage 22 human.
Read the description with the serial section excerpt and then use the link below each image (click section number) to see the full cross-section image.
They are organised in sequence as if you were travelling downward through the embryo (that is why the kidney comes first).

Embryo (week 8, Stage 22) Renal

E6: R,L adrenal glands under diaphragm. E7: Large adrenal glands. Inferior vena cava. Thoracic aorta.
	Fl: Adrenal glands. R. Kidney. Autonomic ganglia (partly the adrenal medulla precursors).	F2: Kidneys (note retroperitoneal location). Cortex. Medulla. L. Adrenal gland. Superior mesenteric artery. Inferior vena cava.	F3: R testis (note its location relative to the R adrenal). L adrenal. R renal hilus. large channels are branches of ureteric tree.

F4: R kidney and R ureter. Inferior vena cava. L. kidney, L renal hilus and L ureter. R testis with R mesonephric duct (precursor of vas deferens). L testis. Umbilical arteries passing into umbilical cord allantois between them.	F5: Kidneys. Ureters. Note umbilical arteries and allantois. Also note how R testis and mesonephric structures are attached to parietal peritoneum by a mesogonad.	F6: Kidneys. Ureters. Note umbilical arteries and allantois. Also note how R testis and mesonephric structures are attached to parietal peritoneum by a mesogonad.	F7: In F7, (dorsal to R testis and liver) note with the distinct lumen of the mesonephric duct, almost solid column of paramesonephric cells and remnants of mesonephric tubules. "mesogonad". Ureters. Bladder with submucosa and detrusor muscle. Umbilical arteries. Division of aorta.

G1: Ureters, Bladder. Umbilical arteries. Testis with remains of mesonephros (dorsal), mesonephric duct and paramesonephric cells. Sigmoid colon and mesocolon.	G2: Ureters being displaced ventrally, crossing common iliac arteries. Sigmoid colon. Bladder. Mesonephric ducts (lateral) and paramesonephric ducts (smaller, medial) located dorsal to bladder.	G3: Ureters (cut twice): descending dorsal to bladder and ascending ventrally to enter the bladder at trigone, through the submucosa). Fusion of paramesonephric ducts. Paired mesonephric ducts. Umbilical arteries looping off common iliac arteries. Pubic symphysis. Colon.	G4: Most caudal part of loop of ureters. Urethra emerging from bladder. Mesonephric ducts. Rectocolic junction.

G5: Urethra (in region of future prostate gland - note crescentic shape). Rectum. Rectovesical pouch. Between G4 and G5, each mesonephric duct (vas deferens) has joined the prostatic urethra (caudal to the ureters), thereby increasing the caliber of the latter.	G6: Penile urethra, emerging inferiorly to the glans penis. Scrotal swellings (appear before testis descends).	G7: Penile urethra, emerging inferiorly to the glans penis. Scrotal swellings (appear before testis descends).	Note F7 MS term: "inebriated Puffin" (dorsal to R testis and liver) lumen of the mesonephric duct (eye), almost solid column of paramesonephric cells (beak) and remnants of mesonephric tubules (body).

Urinary System Development

The adult kidneys – the metanephroi – form from day 35, from a portion of the intermediate mesoderm called the metanephric blastema (or metanephric mesenchyme). They are induced to form by the ureteric buds, outgrowths from the end of the mesonephric ducts, which come into contact with the metanephric blastema. Upon contact, they begin to lengthen and bifurcate rapidly in the metanephric blastema – these branches differentiate into the collecting ducts. Both the ureteric buds and the metanephric blastema begin to differentiate; interestingly each induces differentiation in the other structure. The ureteric bud is induced by the metanephric blastema to form the collecting tubules, renal pelvis and ureters. The metanephric blastema is induced to form the nephrons.

Development of the kidney is starts in week 5 and is completed by week 15. However, in week 6 the kidneys begin to ascend to their correct anatomical position. This movement is completed by week 9. During the ascent, the kidneys also become vascularised via the dorsal aorta. As this ascent occurs, the mesonephric ducts and the ureters enter the wall of the eventual bladder.

Simultaneously, between weeks 4 and 6, the cloaca is partitioned into the urogenital sinus anteriorly, and the rectum posteriorly. This is achieved by downward growth of the urorectal septum, a portion of endoderm from the hindgut. The urogenital sinus has an area of enlargement – the bladder – and is superiorly continuous with the allantois.

Further development of the urinary system varies depending on the sex of the embryo. In males, the pelvic urethra forms the membranous urethra, the prostatic urethra and penile urethra; in females it forms the membranous urethra and the vestibule of the vagina. The sex of the above model is male.

Genital System Development

Till the end of the 6th week the male and female genital systems are indistinguishable. Sex differentiation is based upon the presence of specific sex chromosomes. The female has two X chromosomes, while the male has an X and a Y chromosome. The specific gene involved in determining the male sex is the SRY transcription factor, which activates specific genes for male sex development. If SRY is damaged or absent, female development occurs.

In the 5th week, the primordial germ cells migrate to the region of the future gonads. Cells from the coelomic epithelium and the mesonephros proliferate, forming genital ridges medial to the mesonephros. In the 6th week, these cells surround the germ cells, together forming the primitive sex cords. They contain distinct cortical and medullary regions. Also in the 6th week, the paramesonephric or müllerian ducts form, lateral to the mesonephric ducts.

In male genital development, the medullary cells of the sex cords differentiate into Sertoli cells, while the cortical cells degenerate. These primitive cells also secrete anti müllerian hormone (AMH), which causes regression of the paramesonephric ducts between the 8th and 10th weeks. In weeks 9-10, the gonadal cells also begin to produce testosterone, which maintains the mesonephric ducts. These eventually go on to form the vas deferens, ductuli efferentes and other structures in the male.

In the female, the opposite occurs. The sex cords degenerate and the genital ridge forms secondary cortical sex cords. These induce the primordial germ cells to form the ovarian follicles. Due to the lack of AMH and testosterone, the mesonephric ducts degenerate, and the paramesonephric ducts go on to form the fallopian tubes, the uterus and superior part of the vagina. In both sexes, the external genitalia appear similar, until the 12th week.

Trigone

This looped animation shows the posterior of the developing bladder between Week 4 and 6.

The mesonephric duct (purple) has lateral branches forming the uteric bud (kidney) both these fuse into the wall of the bladder (yellow). The mesonephric duct then moves inferiorly to the level of the pelvic urethra.

References

Glossary Links

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Cite this page: Hill, M.A. (2024, April 27) Embryology BGDB Sexual Differentiation - Late Embryo. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/BGDB_Sexual_Differentiation_-_Late_Embryo

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