Week 4 to 5
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Week 4-5 embryo (stage 13)
| Begin by observing the internal structure of the embryo at the end of week 4 and the beginning of week 5.
- 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.
- 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
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.
Kidney and Mesonephric Duct
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| 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
|| 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).
|| 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 spermatozoa during puberty.
- The tunica albuginea (white) covers the testis and bands extend inward to form connective tissue septa.
|| Development of the female gonad showing medullary 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 (click image to start, these may not be synchronised).
The following figures are from a recent review of Sex determination and gonadal development in mammals.
- Signaling in genital development
- Sex reversal in humans caused by abnormal X-Y exchange
- Mesonephric tubules in the 11.5 dpc mouse urogenital ridge
- Development and differentiation of the genital duct system
- The migratory pathway of primordial germ cells
- Structure of the early fetal testis
- Differentiation of pre-Sertoli cells into Sertoli cells
- Model for cell-autonomous and prostaglandin-mediated upregulation of Sox9 in pre-Sertoli cells
- Visualization of testicular cell types
- Ovary and follicle development and differentiation
- Postulated molecular pathway leading to the formation of the bipotential genital ridge
- Structure of mouse and human SRY protein
- Postulated interaction of molecular players involved in early testicular development
Nussey S, Whitehead S. Endocrinology: An Integrated Approach. Oxford: BIOS Scientific Publishers; 2001. Chapter 6 The gonad
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Cite this page: Hill, M.A. (2019, January 18) Embryology 2011 Lab 8 - Early Embryo. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/2011_Lab_8_-_Early_Embryo
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- © Dr Mark Hill 2019, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G