12. Reproductive Embryology

Question 1

RECAP: During early embryonic development, what three germ layers arise?

Answer 1

1. Ectoderm
2. Mesoderm
3. Endoderm

Question 2

RECAP: what happens in the 4th week and what does it create?

Answer 2

1. Embryo folds
2. Creates a cavity inside the embryo with a gut tube suspended in it

Question 3

Three tracts share a common caudal opening. What are the three tracts and what is the name of the opening?

Answer 3

1. Reproductive tract
2. Urinary tract
3. GI tract
4. The cloaca (the hindgut ends in this) and is closed to the ouside by cloacal membrane

Question 4

What is the sequence (indifferent and different) of things forming from the primordial germ cells?

Answer 4

1. Indifferent means the same in both males and females

Question 5

What is the urogenital ridge?

Answer 5

1. Region of intermediate mesoderm giving rise to both the embryonic kidney and the gonad
2. The indifferent gonad is derived from the urogenital ridge intermediate mesoderm plus primordial germ cells (extragonadal)

Question 6

What are Primordial Germ Cells?

Answer 6

1. Common origins of spermatozoa and oocytes and thus represent the ancestors of the germline
2. Arise in the yolk sac and migrate into the retroperitoneum along the dorsal mesentery

Question 7

How is the gender of the foetus determined?

Answer 7

1. The male gamete is either carrying:
2. Y chromosome: XY when it combines with egg = male conceptus = primordial germ cells carry Y chromosome = Expresion of SRY genes drives development of male = gonad (testis), testicular hormone production and internal genitalia (male duct system)
3. X chromosome: XX when it combines with egg = female conceptus = primordial germ cells DO NOT carry Y chromosome = absence of Y chromosome leads to development of female = gonad (ovary), internal genitalia (duct system - tubes and uterus)

Question 8

What is the indifferent stage regarding the ducts?

Answer 8

1. Mesonephric ducts develop in both male and female embryos
2. Paramesonephric ducts develop in both male and female embryos
3. Both ducts end at the urogenital sinus part of the cloaca

Question 9

What are the fates of the ducts?

Answer 9

1. Depend on whether there is a functional testis
2. Testis produce Mullerian Inhibiting Hormone - suppress mullerian (paramesonephric) duct development. Testis producing androgens - supporting wolffian (mesonephric) duct
3. No testis producing Mullerian Inhibiting Hormone - no suppression of Mullerian (paramesonephric) duct developing. No testis producing androgens - wolffian (mesonephric) duct degenerates

Question 10

What happens if theres exogenous androgen development?

Answer 10

1. supports wolffian duct
2. but no testis so no MIH
3. therefore mullerian ducts develop

Question 11

What is androgen insensitivity hormone?

Answer 11

1. Receptors for testosterone do not work
2. Wolffian ducts don’t survive
3. But MIH present so Mullerian ducts degenerate
4. i.e. nothing generates

Question 12

How is the male reproductive ducts formed?

Answer 12

1. The mesonephric (wolffian) duct first acts as the duct for the embryonic kidney
2. Drains into the urogenital sinus (this will become the bladder)
3. Surplus to requirement once true kidney develops
4. Mesonephric duct is maintained by testis derived androgens
5. Converted into the vas deferens and epididymis
6. Migrates with the testis as it descends
7. The mesonephric duct forms the vas deferens

Question 13

How are the female reproductive ducts formed? How is the uterus etc formed?

Answer 13

1. Paramesonephric duct
2. appear as invaginations of the epithelium of the urogenital ridge
3. Caudally: make contact with the cloaca (urogenital sinus)
4. Cranially: open into the abdominal cavity
5. Paramesonephric duct forms the uterus/cervix/upper vagina/fallopian tubes

Question 14

How is the uterus, cervix, fornix, vagina etc formed?

Answer 14

Question 15

How is the indifferent stage of external genitalia formation reached?

Answer 15

1.

Question 16

What are the basic components of the indifferent external genitalia?

Answer 16

1. Genital Tubercle (GT)
2. Genital Folds
3. Genital Swellings

Question 17

How does the male external genitalia form?

Answer 17

1. The Genital Tubercle elongates and genital folds fuse to form the spongy urethra (GT develops into glans penis)
2. Influence of testis-derived androgen hormones - dihydrotestosterone

Question 18

How does the female external genitalia form?

Answer 18

1. No fusion occurs in the female
2. development of labia majora and labia minora
3. Genital tubercle develops into clitoris
4. Urethra opens into the vestibule

Question 19

Describe the descent of the testis?

Answer 19

1. Testis descend
2. through the processus vaginalis and gubernaculum leading the testes into the scrotal swelling
3. The processus vaginalis will then normally condense to form the tunica vaginalis

Note: gubernaculum = retroperitoneum

Question 20

Picture describing descent of testis

Answer 20

Question 21

Describe the descent of the ovary

Answer 21

1. Gubernaculum attaches ovary inferiorly to labio-scrotal folds
2. Ovary descends to the pelvis
3. Uterus has developed - prevents further descent
4. Round ligament of the uterus in inguinal canal

Gubernaculum becomes the ovarian ligament (connecting ovary to uterus), round ligament of the uterus (connecting uterus to labia) - round ligament occupies space in the inguinal canal

Question 22

On examination, a 3 year old boy is found to have a urethral opening approximately 1.5 cm from the glans penis which extends posteriorly along the ventral surface of the shaft of the penis. What is the most likely cause of this structural anomaly?

Answer 22

1. failure of the urogenital folds to fuse
2. the penile urethra is formed as a consequence of fusion of the urogenital folds that surround the orifice of the embryonic urogential sinus. fusion of these folds and the labioscrotal folds (that form the scrotum in the male) is under the influence of testis derived androgen. the Wolffian duct develops into the vas deferens and the ureteric bud induces development of the definitive kidney and the ureter.

Question 23

From which tissue do the gonads develop?

Answer 23

1. intermediate mesoderm in the abdomen
2. the gonads develop from an indifferent gonad derived from the intermediate mesoderm in the abdomen. As development proceeds, the gonad undergoes descent through the abdomen. Blood supply and venous and lymphatic drainage of the gonads reflects this abdominal origin.

Question 24

The gubernaculum connects the developing testis to _________?

Answer 24

1. the developing scrotum and in so doing creates the “pull” to guide the testis caudal through the abdomen to enter the scrotum by the end of gestation.

Question 25

From which structure does the uterus develop?

Answer 25

1. paramesonephric duct
2. the paramesonephric develops in the absence of testis derived MIH to develop into the uterus, uterine tubes and the upper vagina. the mesonephros and metanephros are the embryonic and definitive kidneys respectively. the vitelline duct allows continuity between the midgut and the proximal umbilical cord permitting rotation of the midgut loop,

Question 26

From which structure does the vas deferens develop?

Answer 26

1. mesonephric duct
2. the mesonephric duct is supported by testicular androgens to develop into the vas deferens. the mesonephros and metanephros are the embryonic and definitive kidneys respectively. the vitelline duct allows continuity between the midgut and the proximal umbilical cord permitting rotation of the midgut loop,

Question 27

A boy, aged 15 years old, is brought to see his GP by his mother. They are concerned that he does not seem to be “as well developed” as his cousin who is the same age as he is. Investigations revealed high levels of GnRH, LH and FSH. What is the defect most likely to be?

Answer 27

1. a gonadal defect
2. this scenario depicts one of hypergonadatrophic hypogonadism, where despite production of GnRH and LH and FSH the gonad is not responding by producing testosterone. as a consequence, there is no negative feedback and therefore GnRH, FSH and LH become elevated.

Question 28

What is Kallman syndrome?

Answer 28

1. hypogonadotropic hypogonadism
2. This failure in GnRH activity can either be due to the absence of the GnRH releasing neurones inside the hypothalamus [22] or the inability of the hypothalamus to release GnRH in the correct pulsatile manner to ensure LH and FSH release from the pituitary

Question 29

What is Klinefelter syndrome?

Answer 29

1. Klinefelter syndrome (KS) also known as 47,XXY or XXY, is the set of symptoms that result from two or more X chromosomes in males. The primary features are sterility and small testicles.
2. onset of puberty is not normally delayed, although infertility is associated with Klinfelter’s syndrome.