Reproductive Embryology Flashcards
Typical development of the reproductive system is dependent on dimorphic gene expression from the sex chromosomes to control gonad formation and differentiation into ovaries or testes. Subsequently four different hormone signaling pathways are required to generate the differences in the male and female fetal reproductive structures at birth, which are primarily controlled by the testes. These include:
- Testosterone to maintain and further develop male duct structures
- Anti-Müllerian hormone (AMH), aka Müllerian inhibiting substance (MIS) to regress female duct structures
- Dihydrotestosterone (DHT) to promote male genital and urethral development in males.
- Testosterone and possibly Estrogens to act on the brain
- In the absence of these hormonal pathways, development toward a female phenotype will occur (e.g., maintenance of female reproductive ducts formation of female genital structures, feminization of the brain). However, typical healthy female reproductive development (including full fertility) requires two X chromosomes. Furthermore, healthy development of female structures requires exposure to estrogens synthesized from the placenta.
Turner’s Syndrome
- sex chromosome
- 45 XO
- 1 X chromosome
- Short stature, broad chest, webbed neck, low hairline, normal external genitalia and uterus
- Infertility, lack of puberty, skeletal abnormalities, lymphedema, kidney problems, heart defects, deafness, occasional verbal or developmental delays
Klinefelter Syndrome
- sex chromosome
- 47 XXY
- 2 X, 1 Y
- Underdeveloped male external genitalia and testes, incomplete puberty, gynecomastia, variable cryptorchidism, hypospadias, or micropenis
- Infertility, decreased muscle mass and bone density, depression, anxiety and other behavioral disorders, diabetes and other metabolic disorders
Gonadal Dysgenesis
- sex chromosome
- X/XY or XX/XY
- variable chromosome makeup in different cells (chimerism)
- Variable development of gonads, including pure gonadal dysgenesis, ovotestis (gonad with both ovarian and testicular tissue), or testis on one side and ovary on the other. Genitalia can be underdeveloped male, ambiguous, or female, depending on androgen exposure. Variable development of vagina and reproductive ducts
- Variable fertility, cancer risk from underdeveloped gonads, variable puberty
Gonadal Dysgenesis - 46XX
- 46XX
- variable
- Variable development of gonad (streak gonad or underdeveloped ovary). Rarely an ovotestis
- Infertility, cancer risk from underdeveloped gonads, absent menstruation, lack of puberty
De La Chapelle Syndrome
- 46 XX
- SRY gene transposed to X chromosome
- Variably developed male external genitalia (often underdeveloped, sometimes with hypospadias), variable cryptorchidism
- Infertility, underdevelopment of male post-pubertal characteristics without supplemental hormones, short stature
Meyer-Rokitansky-Kuster-Hauser Syndrome (MRKH) Type I and Type II aka Mullerian agenesis)
- 46 XX
- unknown cause
- Type I: Agenesis or incomplete Müllerian duct structures (e.g., fallopian tube, uterus), variable vaginal atresia
- Type II: Same as Type I with kidney malformation
- Ectopic ovary, absent menstruation, Type II has major implications for salt and water balance, heart defects, and bone defects.
Congenital Adrenal Hyperplasia (CAH)
- 46 XX
- High exposure to androgens during development (due to overproduction of androgens from Adrenal Glands
- Variably enlarged clitoris and fused labia, greater amount of body hair
- Fertility problems, irregular menstruation, severe acne, irregular heartbeat, salt and water balance issues, short stature
Gonadal Dysgenesis - 46 XY
- 46 XY
- Variable-SRY mutations common
- Female external genitalia, streak gonads or underdeveloped testes, normal uterus and fallopian tubes. Rarely an ovotestis.
- Infertility, lack of puberty, cancer risk from underdeveloped gonads.
Androgen Insensitivity Syndrome
- 46 XY
- Variable response to androgens, ranging from complete to partial insensitivity
- Cryptorchidism, variable development of female genitalia (partial or mild AIS can result in masculinized female or ambiguous genitalia. Complete AIS results in normal external genitalia. Variable development of vagina. Uterus and fallopian tubes are absent.
- Cancer risk from undescended testes, variable fertility, variable vaginal development can cause sexual difficulties.
5-α Reductase Deficiency
- 46 XY
- Inability to synthesize DHT
- Underdeveloped male, ambiguous, or female external genitalia, hypospadias, cryptorchidism
- Maturation of male genitalia upon puberty, underdevelopment of male secondary characteristics (e.g., facial and body hair).
Dysgenesis
- failure of gonadal differentiation results in dysgenesis
- Multiple DSDs can have failed differentiation and development of the Gonad = Gonadal Dysgenesis
- Usually manifests as an undifferentiated streak of tissue (Streak Gonad)
- Extremely rarely can have mixed ovarian/testicular tissue (Ovotestis) or different gonads between L/R.
- Most common in 46 XX individuals.
- Impact on other structures variable-can often have normal female duct/genital development.
Urinary and reproductive structures are closely associated in development and derive from many of the same tissues. The primary germ layers involved in urogenital development are: [], [], and some contributions from [] and [].
Urinary and reproductive structures are closely associated in development and derive from many of the same tissues. The primary germ layers involved in urogenital development are: Intermediate Mesoderm, Undifferentiated Mesoderm (mesenchyme), and some contributions from Endoderm and Ectoderm.
The definitive urogenital system develops predominantly in the caudal part of the embryo, and therefore fairly late in the embryonic period (remember, things develop temporally beginning in the rostral embryo and progressing caudally).
Week 3: The [] forms an elongated swelling of mesoderm that extends inferiorly down the posterior abdominal wall called the []. As the name implies, the [] contributes to both the urinary system and the reproductive system.
The intermediate mesoderm (located between the somite and the lateral plate mesoderm) forms an elongated swelling of mesoderm that extends inferiorly down the posterior abdominal wall called the urogenital ridge. As the name implies, the urogenital ridge contributes to both the urinary system and the reproductive system.
Week 3-5: The first and superior-most structures to develop are the tubules and ducts of the [], which form and then regress (3-4 weeks). Next the abdominal [] and ducts form and becomes the functional kidney until around week 9. The ducts that drain the [] kidney, the mesonephric duct (aka []), persist in males as the male reproductive tract.
The first and superior-most structures to develop are the tubules and ducts of the pronephros, which form and then regress (3-4 weeks). Next the abdominal mesonephros and ducts form and becomes the functional kidney until around week 9. The ducts that drain the mesonephric kidney, the mesonephric duct (aka Wolffian duct), persist in males as the male reproductive tract.
Week 5-7: The [] (definitive kidney) begins to develop near the hindgut, and the indifferent gonad begins to develop on the medial side of the urogenital ridge near the mesonephros.
The metanephros (definitive kidney) begins to develop near the hindgut, and the indifferent gonad begins to develop on the medial side of the urogenital ridge near the mesonephros.
Indifferent Stage
At the superior end of the intermediate mesoderm (at the level of the liver), the adrenal cortex receives migrating neural crest cells (medulla) to form the bipartite adrenal gland. The metanephric kidney migrates superiorly, changing its blood supply as it goes, until it stops under the adrenal gland. Kidney formation frees the mesonephric tubules and ducts to participate in male reproductive tubule development. During this period, the paramesonephric ducts (aka Müllerian duct) develop in preparation for female reproductive structure formation. At Embryonic Week 7, before sex-specific gene expression, both male and female duct structures are present
Primordial germ cells (PGCs) migrate into the body from the yolk sac, through the mesoderm via the embryonic mesenteries and into the gonad in the 4th to 6th week. Upon reaching the urogenital ridge, the germ cells invest the cortex and the medulla of the indifferent gonad. The differentiation of the PGCs into precursors to ova or sperm is determined by the []: in females the [] and [] induce the production of egg precursors, whereas in males the [] and [] cells inhibit this process which allows precursors of sperm to develop. If the cortex develops into an ovary, they will become eggs. If the medulla develops into a testis they will give rise to sperm.
Primordial germ cells (PGCs) migrate into the body from the yolk sac, through the mesoderm via the embryonic mesenteries and into the gonad in the 4th to 6th week (Fig. 3). Upon reaching the urogenital ridge, the germ cells invest the cortex and the medulla of the indifferent gonad. The differentiation of the PGCs into precursors to ova or sperm is determined by the surrounding extracellular environment: in females the mesonephros and ovary induce the production of egg precursors, whereas in males the Leydig and Sertoli cells inhibit this process which allows precursors of sperm to develop. If the cortex develops into an ovary, they will become eggs. If the medulla develops into a testis they will give rise to sperm.
Sex is typically determined in humans by an XX (female) or XY (male) chromosomal complement. Expression of genes from these chromosomes differentiates the indifferent gonads into either ovaries (female gonad) or testes (male gonad) The X chromosome expresses a gene called [] that facilitates testes formation when there is only one copy but suppresses testis development and facilitates ovary development when there are two or more. Thus, abnormal gonads develop in XO females and XXY males due to atypical [] expression.
Sex is typically determined in humans by an XX (female) or XY (male) chromosomal complement. Expression of genes from these chromosomes differentiates the indifferent gonads into either ovaries (female gonad) or testes (male gonad) The X chromosome expresses a gene called Dax1 that facilitates testes formation when there is only one copy but suppresses testis development and facilitates ovary development when there are two or more. Thus, abnormal gonads develop in XO females and XXY males due to atypical Dax1 expression.
Sex is typically determined in humans by an XX (female) or XY (male) chromosomal complement. Expression of genes from these chromosomes differentiates the indifferent gonads into either ovaries (female gonad) or testes (male gonad). In males, the Y chromosome also expresses a gene called [] that inhibits ovary differentiation and induces the development of Sertoli and Leydig cells in the testes, which are required for typical male sexual development. Differentiation of the gonads are crucial to further development of the reproductive system because hormones from ovaries/testes cue subsequent developmental events. Inability to respond to these cues leads to specific types of DSD.
Sex is typically determined in humans by an XX (female) or XY (male) chromosomal complement. Expression of genes from these chromosomes differentiates the indifferent gonads into either ovaries (female gonad) or testes (male gonad). In males, the Y chromosome also expresses a gene called SRY that inhibits ovary differentiation and induces the development of Sertoli and Leydig cells in the testes, which are required for typical male sexual development. Differentiation of the gonads are crucial to further development of the reproductive system because hormones from ovaries/testes cue subsequent developmental events. Inability to respond to these cues leads to specific types of DSD.
- As the gonads are differentiating and acquiring their germ cells, the two intermediate mesodermal reproductive duct systems, the mesonephric and paramesonephric ducts, form. Subsequent development (or atrophy) of these duct systems is dependent on hormone production controlled by the gonads.
- In females the development of the ovary results in lower concentrations of androgens (e.g., testosterone) relative to a developing male. The mesonephric duct system requires androgens to stay maintained, and so they regress in females (duct remnants exist that can become cystic and cause problems). The paramesonephric ducts continue to develop, in part due to influence of estrogens produced by the placenta. During this process, the left and right paramesonephric ducts fuse at their inferior ends, becoming the uterus, cervix, and fallopian tubes, as well as the uterine ligaments.
- The degree that the paramesonephric ducts contribute to the vagina has long been controversial, with some researchers arguing for an origin from the urogenital sinus (endoderm), origin from the paramesonephric ducts (intermediate mesoderm) or contributions from both (2-4). Recent evidence shows that the fused caudal end of the paramesonephric ducts (uterovaginal primordium) contacts the urogenital sinus, which induces the development of paired sinovaginal bulb (4). The bulbs fuse into a solid midline vaginal plate, which pushes the uterovaginal primordium cranially. Then, the deep cells of this plate degenerate, creating the vaginal lumen. Therefore, current evidence supports the hypothesis that the adult vagina is primarily derived from the urogenital sinus, transitioning into paramesonephric derivatives at the uppermost vagina and cervix (4). Note the paramesonephric duct helps to induce formation of the sinovaginal bulbs, which is why it is common to observe vaginal atresia in cases of MRKH syndrome
- Fusion of the paramesonephric duct and other mechanisms of uterine development can fail, resulting in various congenital uterine anomalies.
- Clinical consequences can include menstrual complications, increased likelihood of endometriosis, and associated problems. Implantation is typically not affected, but complications with pregnancy can occur, including spontaneous abortion, miscarriage, preterm delivery, restriction of fetal growth, and even uterine rupture
- The degree that the paramesonephric ducts contribute to the vagina has long been controversial, with some researchers arguing for an origin from the urogenital sinus (endoderm), origin from the paramesonephric ducts (intermediate mesoderm) or contributions from both (2-4). Recent evidence shows that the fused caudal end of the paramesonephric ducts (uterovaginal primordium) contacts the urogenital sinus, which induces the development of paired sinovaginal bulb (4). The bulbs fuse into a solid midline vaginal plate, which pushes the uterovaginal primordium cranially. Then, the deep cells of this plate degenerate, creating the vaginal lumen.
- Therefore, current evidence supports the hypothesis that the adult vagina is primarily derived from the urogenital sinus, transitioning into paramesonephric derivatives at the uppermost vagina and cervix. Note the paramesonephric duct helps to induce formation of the sinovaginal bulbs, which is why it is common to observe vaginal atresia in cases of MRKH syndrome
- In males, development of the testes and differentiation of Sertoli and Leydig cells enables development of male duct structures. Sertoli cells produce a hormone called AMH/MIS (anti-Müllerian hormone/ Müllerian inhibiting substance), which causes the degeneration of the paramesonephric ducts. Small remnants of the paramesonephric ducts exist in the prostate and testes, which can become cystic and cause problems.
- The Leydig cells produce testosterone that maintains the mesonephric ducts, which eventually form ductus deferens, epididymis, ejaculatory duct, and seminal vesicles.
Cryptorchidism and Indirect Inguinal Hernias
- Failure of testes to descend leads to cryptorchidism (hidden testes). Cryptorchidism is the most common congenital defect in young males (2-4%) and is a risk factor for cancer & infertility (Fig. 7). Cryptorchidism also frequently occurs in common XY DSDs (e.g., AIS).
- Descent of the testes through the inguinal canal also creates a zone of weakness susceptible to herniation, specifically indirect inguinal hernias (herniation of peritoneal contents into the inguinal canal within the internal spermatic fascia). Indirect hernias are associated with a patent processus vaginalis, which is an outpocketing of the peritoneal bursa that helps form the space for the testes in the scrotum. Normally, the processus vaginalis closes after the testes moves through, remaining as a small serous bursa (tunica vaginalis) that surrounds the testis.
Prior to week 7, the external genitalia develop as an undifferentiated structure with outgrowth controlled much like a limb bud. This structure consists of the[], [] and []. Each of these structures differentiate into specific homologous male and female structures.
Prior to week 7, the external genitalia develop as an undifferentiated structure with outgrowth controlled much like a limb bud. This structure consists of the genital tubercle, urethral fold and surrounding genital swelling composed of left and right labioscrotal folds. Each of these structures differentiate into specific homologous male and female structures.
- In females, the genital tubercle becomes the glans of the clitoris (-DHT), and parts of the corpus spongiosum erectile tissue.
- The two urethral folds become the labia minora and deeper corpus spongiosum erectile tissue surrounding the vagina and urethra (bulb of the vestibule). The crura of the corpora cavernosa erectile tissue are incorporated into the deep clitoris and run along the pubic rami.
- The labioscrotal folds become the labia majora.
- Development of female external genitalia takes longer than in males and is mediated by estrogen.
- In males, testosterone from the testes is converted by the enzyme 5-α reductase into dihydrotestosterone (DHT) in the skin of the external genitalia during week 7. DHT is a more potent androgen than testosterone, and it causes the indifferent genital tubercle to enlarge into the glans of the penis and end of the corpus spongiosum.
- The urethral folds also fuse around the urethral groove to form a tubular urethra; the folds then increase in size to become the shaft of the penis, which incorporates the internal erectile tissues (corpora cavernosa, most of corpus spongiosum).
- The new tubular urethra which connects to an epithelial cord in the developing glans that degenerates to create the external urethral orifice.
- The labioscrotal folds also to fuse into the scrotum.
- The prostate, gubernaculum and urethra are also dependent on DHT for typical development.
