Lecture 3: Development of the reproductive tracts

Question 1

What is the first stage in the development of a fetus?

Answer 1

Fertilisation

Question 2

What occurs after fertilisation?

Answer 2

• 30 hours after fertilisation, the fertilised egg (oocyte), splits into two cells of equal size (blastomeres)
• after 3 more divisions (16 cells), the group of cells is referred to as the morula
• in the first week, the cells of the morula reorganise themselves to form a cavity called the blastocyst cavity
• from now on the morula is known as the blastocyst

Question 3

What cell types make the blastocyst?

Answer 3

Outer cell mass: trophoblast, contacts with the endometrium of uterus to facilitate implantation and formation of the placenta
Inner cell mass: embryoblast, resposible for the formation of the embryo itself

Question 4

At what week do we see the process of gastrulation?

Answer 4

Week 3
3 germ cell layers formed
Gastrulation is a process of cellular rearrangement which involves migration, invagination, and differentiation of the epiblast

Question 5

What controls gastrulation?

Answer 5

The primitive streak
Cells differentiate at the primitive streak and migrate to form the middle mesodermal layer, as well as displace most of the hypoblast cells to form an endodermal layer
(ectoderm forms from the epiblast)

Question 6

When is the sex of the embryo determined?

Answer 6

At fertilisation by the kind of sperm (X or Y) that fertilises the oocyte
-male and female characteristics do not develop until 7th week

Question 7

What is the indifferent stage of sexual development?

Answer 7

Early genital systems in the 2 sexes are similar so therefore the initial period of genital development is an indifferent stage of sexual development

Question 8

What does the cloaca become?

Answer 8

Found in the hindgut and becomes the urogenital sinus: the common opening for the reproductive and urinary systems

Question 9

What does the cloaca give rise to?

Answer 9

Urogenital sinus which gives rise to:

• undifferentiated gonad
• early kidney structures

Question 10

What are primordial cells?

Answer 10

Primordial germ cells which migrate from the yolk sac into the retroperitoneum, along the dorsal mesentery
-this leads to cloaca
They carry chromosomal information to determine whether we will be looking at a male/female

Question 11

How does the gonad develop?

Answer 11

Gonads (testes/ovaries): organs that produce sex cells
Derived from 3 sources
-mesothelium (mesodermla epithelium lining posterior abdominal wall)
-underlying mesenchyme (enbryonic connective tissue)
-primordial germ cells (earliest undifferentiated sex cells)
Primordial germ cells migrate along retroperitoneum to the gonad (which is indifferent)
-the karyotype of the fertilised oocyte determines the karyotype of the primordial germ cells (XX or XY), so determine whether the gonad is going to differentiate to become male/female

Question 12

How do the male internal genitalia develop?

Answer 12

Male gamete > primordial germ cells carrying a Y chromosome
Expression of SRY gene (sex determining region of the Y chromosome) which derives the male reproductive system (testis, production of testicular hormones, internal genitalia- male duct system)

Question 13

What ducts in the internal genetalia form male/female ducts?

Answer 13

Wolffian duct- male: go onto develop testis, epididymis, vas deferens
Mullerian duct- female

Question 14

How do the female internal genetalia develop?

Answer 14

Primordial germ cells do not carry a Y chromosome

-absence of Y chromosome leads to development of female (ovary, internal genitalia-duct system)

Question 15

What are other names for the wolffian and mullerian ducts?

Answer 15

Wolffian: mesonephric duct
Mullerian: paramesonephric duct

Question 16

What is MIH?

Answer 16

Mullerian-inhibiting substance produced by seminiferous tubules to prevent the female pathway from developing in males

Question 17

What is the function of testosterone?

Answer 17

Absence of testosterone= female side develops

Presence of testosterone= male side develops

Question 18

How do the ducts develop?

Answer 18

Male
-testis produce MIH, suppressing mullerian duct development
-testis produce androgens, support development of wolffian duct
Female
-no testis producing MIH, so no suppression of mullerian duct development
-no testis producing androgens, so wolffian duct degenerates

Question 19

How do both ducts remain patent?

Answer 19

• exogenous androgen
• supports wolfarian duct
• but no testes, so no MIH
• so mullerian ducts develop

Question 20

How do both ducts degenerate?

Answer 20

• androgen-resistant male
• receptors for testosterone don’t work
• wolffian ducts don’t survive
• but MIH is present so mullerian ducts degenerate

Question 21

How does the wolffian duct develop?

Answer 21

First acts as duct for the embryonic kidney, drains into the urogenital sinus, this will become the urinary bladder

• this is surplus to requirement once true kidney develops
• mesonephric duct is maintained by testis derived androgens
• converted into the vas deferens and epididymis
• migrates with testis as it descends

Question 22

How does the mullerian duct develop?

Answer 22

• appear as invaginations of the epithelium of the urogenital ridge
• derived from urogenital sinus

Question 23

What does the mesonephric duct do in male/female embryos?

Answer 23

Function as a duct of the embryonic kidney

-it will cease to be of use to the urinary system and will degenerate unless testis-derived testosterone is present

Question 24

What are the components of the external genitalia in indifferent stage of development?

Answer 24

At around 7 weeks

• genital tubercle (GT)
• genital folds
• genital swellings

Question 25

How does the external genitalia develop in a male?

Answer 25

-GT elongates: develops into the glans penis
-genital folds fuse to form the spongy urethra
All influenced by testis derived androgen hormones (dihydrotestosterone)

Question 26

How does the external genitalia develop in a female?

Answer 26

• no fusion occurs
• development of labia majora and minora
• GT develops into the clitoris
• urethra opens into vestibule

Question 27

How do the gonads descend?

Answer 27

• the gubernaculum (caudal genital ligament) attaches to the gonad inferiorly to the labio-scrotal folds
• as the abdominopelvic cavity increases in volume the gonad begins its descent inferiorly

Question 28

How do the ovaries descend?

Answer 28

• gubernaculum attaches ovary inferiorly to lbio-scrotal folds
• ovary descends to pelvis
• uterus and ductal system has already developed preventing any further decent
• round ligament of the uterus, found in th eiguinal canal, maintains position of the utertus to prevent decent of ovaries