Embriology - Reproductive System 2023-01.pdf

Development of the reproductive system Gabriela Paola Garcia Ordoñez, MD, MSc Modified by: Dra. Flores Urogenital system can be divided into two entirely different components: Urinary system Genital System Intimately interwoven ✓ Both develop from a common mesodermal ridge (intermediate mesoderm) al...

Development of the reproductive system Gabriela Paola Garcia Ordoñez, MD, MSc Modified by: Dra. Flores Urogenital system can be divided into two entirely different components: Urinary system Genital System Intimately interwoven ✓ Both develop from a common mesodermal ridge (intermediate mesoderm) along the posterior wall of the abdominal cavity, ✓ Initially, the excretory ducts of both systems enter a common cavity, the cloaca Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 47.16 Adult derivatives of the three embryonic germ layers in vertebrates Genital system The key to sexual The genetic sex of an dimorphism is the Y embryo is determined at the chromosome time of fertilization. – It contains the SRY gene (sex It depends on wether determining region on Y gene) the spermatocyte The SRY protein is the carries an X or Y testis-determining factor chromosome. – Under its influence, male development occurs – In its absence, female development occurs Gonads The gonads do not acquire male or female morphological characteristics until the seventh week of development They appear initially as a pair of longitudinal ridges Genital or gonadal ridges Germ cells do not appear in the genital ridges until 6th week of development Gonads Primordial germ cells: Originate in the epiblast → migrate through the primitive streak By the 3rd week, these germ cells reside among endoderm cells in the wall of the yolk sac close to the allantois During 4th week, these cells migrate along the dorsal mesentery of the hindgut These cells arrive at the primitive gonads at beginning of 5th week and invade the genital ridges in the 6th week ✓ If the primordial germ cells fail to reach the ridges, the gonads do not develop ✓ It can be said that the primordial germ cells have an inductive influence on development of the gonad into ovary or testes Gonads Shortly before and during arrival of primordial germ cells→ the epithelium of the genital ridge proliferates → epithelial cells penetrate the underlying mesenchyme Form a number of irregularly shaped cords, the primitive sex cords Gonads The primitive sex cords are connected to surface epithelium – It is imposible to differentiate between male and female gonad – The gonad is known as indifferent gonad Testis The PRIMORDIAL GERM CELLS carry an XY chromosome SRY gene encodes the testis-determining factor → the primitive sex cords continue to proliferate and penetrate deep into the medulla to form the testis or medullary cords Testis Toward the hilum of the gland, the cords break up into a network of tiny cell strands that later give rise to the tubules of the rete testis Tunica albuginea – Dense layer of fibrous connective tissue – Separates testis cords from the surface epithelium Testis During 4th month, testis cords become horshoe-shaped Their extremities are continuous with those of the rete testis Testis cords are now composed of primitive germ cells and sustentacular cells of Sertoli – Derived from the surface epithelium of the gland Testis Interstitial cells of Leydig – These are derived from the original mesenchyme of the gonadal ridge – These lie between the testis cords – Development begins shortly after onset of differentiation of the cords – By 8th weekof gestation, Leydig cells begin production of testosterone Testis Testis cords remain solid until puberty – By puberty they will acquire a lumen→ leads to the formation of the seminiferous tubules – Once the seminiferous tubules are canalized, they join the rete testis tubules→ these enter the ductuli efferentes – They link the rete testis and the Wolffian (mesonephric) duct and it becomes the ductus deferens XX sex chromosome No Y chromosome Primitive sex cords dissociate into irregular cell clusters These clusters are made up of primitive germ cells and they occupy the medullary part of the ovary These clusters will disappear and be replaced by a vascular stroma that forms the ovarian medulla Ovary The surface epithelium, during the 7th week, gives rise to a second generation of cords → cortical cords – These penetrate the underlying mesenchyme but remain close to the surface In the 3rd month, these cords split into isolated cell clusters Cells in these clusters proliferate and begin to surround each oogonium → these are called follicular cells Follicular cells + oogonia = primordial follicle Genital ducts INDIFFERENT STAGE – Both male and female embryos have 2 pairs of genital ducts Mesonephric (Wolffian) ducts Paramesonephric (Müllerian) ducts – The paramesonephric duct arises as a longitudinal invagination of the epithelium on the anterolateral surface of the urogenital ridge Genital ducts INDIFFERENT STAGE – Cranially, the paramesonephric duct opens into the abdominal cavity with a funnel-like surface – Caudally, it first runs lateral to the mesonephric duct, then crosses it ventrally to grow caudomedially INDIFFERENT STAGE – In the midline, the mesonephric tubes comes in close contact with the paramesonephric duct from the opposite side The caudal tip of the combined ducts projects into the posterior wall of the urogenital sinus → causing a small swelling called: sinus tubercule – The mesonephric ducts open into the urogenital sinus on either side of the sinus tubercule Genital ducts END 8 WEEK Genital Ducts in the Male They are stimulated to develop by testosterone They are derived from parts of the mesonephric kidney system Some of the original excretory tubules, the epigenital tubules, establish contact with cords of the rete testis and form the efferent ductules of the testis Genital Ducts in the Male Paragenital tubules do not join the cords of the rete testis – Their vestiges are collectively known as the paradidymis Except for the most cranial portion (the appendix epididymis), the mesonephric ducts persist and form the main genital ducts Below the entrance of the efferent ductules, the mesonephric ducts elongate and become convoluted → forms the Epididymis Genital Ducts in the Male Genital Ducts in the Male From the tail of the epididymis to the outbudding of the seminal vesicle, the mesonephric ducts obtain a thick muscular coat and form the Ductus Deferens The region of the ducts beyond the seminal vesicle is the ejaculatory duct In the presence of estrogen and absence of Müllerian inhibiting susbstance, the paramesonephric ducts develop into the main genital ducts of the female Genital Ducts in the Female Initially 3 parts can be recognized in each duct: 1. A cranial vertical portion that opens up into the abdominal cavity 2. A horizontal part that crosses the mesonephric duct 3. A caudal vertical part that fuses with its partner from the opposite side When the second part of the paramesonephric ducts moves mediocaudally, the urogenital ridges gradually come to lie in a transverse plane Genital Ducts in the Female Genital Ducts in the Female After the ducts fuse in the midline, a broad transverse pelvic fold → the broad ligament of the uterus The uterine tube lies in its upper border, and the ovary lies on its posterior surface The uterus and broad ligaments divide the pelvic cavity into the uterorectal pouch and the uterovesical pouch In the absence of testosterone, the mesonephric ducts in the female degenerate. Vagina 2 solid evaginations grow out from the pelvic part of the urogenital sinus – Sinovaginal bulbs – This is shortly after the solid tip of the paramesonephric ducts contact the urogenital sinus The sinovaginal bulbs proliferate and form a solid vaginal plate By the 5th month, the vaginal outgrowth is entirely canalized VAGINA The vagina has dual origin – Upper portion is derived from the uterine canal – The lower portion is derived from urogenital sinus Vagina The hymen separates the lumen of the vagina from the urogenital sinus External genitalia Indifferent stage (3rd week of development) Cloacal fold Cloacal membrane: Cranial: genital tubercle Caudal: urethral folda and anal folds Genital swelling: Male: scrotal swellings Female: labia majora External genitalia male By end of 3rd month, urethral folds close → forms the penile urethra – This distal portion of the urethra is formed during the 4th month → ectodermal cells from the tip of the glans penetrate inward ad form short epithelial cord – This cord obtains lumen → external urethral meatus Genital swellings → scrotum Estrogens stimulate development of the external genitalia in females – Genital tubercle elongates →clitoris – Urethral folds do not fuse → labia minora – Genital swelling → labia majora External genitalia female External genitalia female Differentiation of the external genitalia of embryos Descent of the Testes Develop retroperitoneally in the abdominal region and must move caudally and pass through the abdominal wall to reach the scrotum Passage through the abdominal wall is via the inguinal canal, which is about 4 cm long and lies just superiorly to the medial half of the inguinal ligament. Descent of the Testes Normally, the testes reach the inguinal region by approximately 12 weeks’ gestation, migrate through the inguinal canal by 28 weeks, and reach the scrotum by 33 weeks Descent of the Ovaries The ovaries finally settle just below the rim of the true pelvis The cranial genital ligament forms the suspensory ligament of the ovary, whereas the caudal genital ligament forms the ligament of the ovary proper and the round ligament of the uterus. The latter extends into the labia majora. Bibliography Langman's medical embryology. Sadler, T W (Thomas W); Langman, Jan. Medical embryology. 14th ed. / T.W. Sadler. c2019. Chapter 16 https://www.youtube.com/watch?v=MureNA-RSZM The development of the reproductive system. Animation is derived from Keith L. Moore, T.V.N. Persaud, Mark G. Torchia, "Before We Are Born: Essentials of Embryology and Birth Defects", 8th edition. Elsevier, 2012