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(Carnegie stage 22, male) Quicktime
|Begin by observing the internal structure of the embryo at the end of week 8.
- 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)
- This page shows selected excerpts from whole cross-sections of the human embryo (week 8, stage 22) late embryonic period.
- 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.
||F1: 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
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- starts in week 5 and is completed by week 15.
- week 6 - the kidneys begin to change their relative position, described as "ascent of the kidneys", to their correct anatomical position.
- week 9 - the rising movement is completed.
- 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 developing bladder.
Development of the Urinary Bladder
||Division of the Cloaca
- weeks 4 to 6 - the cloaca is separated into:
- anteriorly - urogenital sinus
- posteriorly - rectum
- Separation is achieved by downward growth of the urorectal septum
- a portion of hundgut endoderm and mesoderm
- urogenital sinus enlargement forms the primitive bladder
- surrounding mesoderm will contribute the detrusor muscle
- this is superiorly continuous with the allantois
Development of the Urethra
- Further development of the urinary system varies depending on the sex of the embryo.
- Males - the pelvic urethra forms the membranous urethra, the prostatic urethra and penile urethra. (The sex of the above animation and sections is male)
- Females - the pelvic urethra forms the membranous urethra and the vestibule of the vagina.
Genital System Development
Male Human Embryo (week 8, stage 22)
Detail from cross-section showing mesonephros, mesonephric duct, paramesonephric duct and developing testis.
|Until the end of the 6th week the male and female genital systems are indistinguishable. Sex differentiation is based upon the presence of the specific sex chromosomes.
- week 5 - 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.
- week 6 - 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.
- week 8 - males Sertoli cells secrete anti müllerian hormone (AMH), which causes regression of the paramesonephric ducts between the 8th and 10th weeks.
- week 9 to 10 - gonadal cells begin to produce testosterone, which maintains the mesonephric ducts.
- mesonephric ducts go on to form the internal genital tract:
- rete testis
- ductuli efferentes
- vas deferens
- during the same time course as above, 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
- paramesonephric ducts go on to form the internal genital tract:
- uterine (fallopian) tubes
- uterus body
In both sexes, the external genitalia appear similar until the 12th week.
||This 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.
- Glossary: A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | Numbers | Symbols | Term Link
Cite this page: Hill, M.A. (2020, April 6) Embryology ANAT2341 Lab 8 - Late Embryo. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/ANAT2341_Lab_8_-_Late_Embryo
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- © Dr Mark Hill 2020, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G