3. Development of the reproductive tracts

Question 1

describe the stages that occur to form the blastocyst

Answer 1

•fertilisation
•~30 hours after fertilisation, the fertilised oocyte (egg) splits into two cells of equal size; called blastomeres.
•After three more divisions, the group of cells is referred to as the morula.
•Within the first week, the cells of the morula re-organise to form a cavity, known as the blastocyst cavity (blastocoel). From this point, the morula is
known as the blastocyst.

Question 2

What are the two cell types of the blastocyst?

Answer 2

–Outer cell mass (trophoblast) – contacts with the endometrium of the uterus to facilitate implantation and the formation of the placenta.
–Inner cell mass (embryoblast) – responsible for the formation of the embryo itself

Question 3

What is gastrulation ?

Answer 3

process of cellular rearrangement which involves migration, invagination and differentiation of the epiblast. It is largely controlled and orchestrated by the primitive streak.two cell layers become three germ cell layers, and the bodily axes observed in the mature adult are created.

Question 4

What germ layer does the reproductive tract mainly arise from?

Answer 4

Intermediate mesoderm

Question 5

What determines the sex of an embryo?

Answer 5

•The chromosomal sex of an embryo is determined at fertilization by the kind of sperm (X or Y) that fertilizes the oocyte

Question 6

When do Male and female morphologic characteristics begin to develop?

Answer 6

• seventh week.

Question 7

What is the initial period of genital development called and why?

Answer 7

indifferent stage of sexual development

- the early genital systems in the two sexes are similar

Question 8

When and how does the primitive gut tube develop and what is it divided into?

Answer 8

• The Primitive gut tube develops during week 3-4 by incorporating the yolk sac during craniocaudal and lateral folding of the embryo.
• The tube is divided into 3 distinct sections; foregut, midgut and hindgut.

Question 9

What is the dilated end of the hindgut called?

Answer 9

Cloaca

Question 10

What is the cloaca?

Answer 10

Common chamber and outlet into which the intestinal, urinary, and genital tracts open. (The terminal end of the hindgut).

Question 11

What does the cloaca divide into?

Answer 11

Urogenital sinus (anteriorly) and anal canal (posteriorly)

Question 12

What divides the cloaca?

Answer 12

urorectal septum,

Question 13

What is the urogenital ridge?

Answer 13

Area of intermediate mesoderm in the posterior abdominal wall that gives rise to the embryonic kidney and the gonad.

Question 14

What part of the urogenital ridge gives rise to embryonic kindey?

Answer 14

Mesonesphros

Question 15

WHat are primordial germ cells and where do they arise from?

Answer 15

• Special population
• “seed” for the next generation
• Allocated shortly after initiation of the current generation
• Arise in the yolk sac and migrate into the retroperitoneum, along the dorsal mesentery

Question 16

What are the gonads and where are they derived from?

Answer 16

The gonads (testes or ovaries) are the organs that produce sex cells (sperms or oocytes). The gonads are derived from three sources;
–Mesothelium (mesodermal epithelium) lining the posterior abdominal wall
–Underlying mesenchyme (embryonic connective tissue)
–Primordial germ cells (earliest undifferentiated sex cells)

Question 17

Describe the role of genital ridges in formation of gonads

Answer 17

The gonads begin as genital ridges – a pair of longitudinal ridges derived from intermediate mesoderm and overlying epithelium. They initially do not contain any germ cells.

In the fourth week, germ cells begin to migrate from the endoderm lining of the yolk sac to the genital ridges, via the dorsal mesentary of the hindgut. They reach the genital ridges in the sixth week.

Simultaneously, the epithelium of the genital ridges proliferates and penetrates the intermediate mesoderm to form the primitive sex cords. The combination of germ cells and primitive sex cords forms the indifferent gonad – from which development into the testes or ovaries can occur.

Question 18

Describe the migration of the primordial germ cells

Answer 18

The primordial germ cells migrate along the retroperitoneum to the gonad, which at this point is indifferent (neither ‘male’ nor ‘female’). The karyotype of the fertilised oocyte determines the karyotype of the primordial germ cells (XX or XY), and will therefore determine whether the gonad differentiates to become male or female.

Question 19

Describe the path in males from indifferent gonad to repro system

Answer 19

Y influence –> indifferent gonad –> testis
medullary cords develop
no cortical cords
thick tunica albuginea

The Y chromosome contains the SRY gene, which stimulates the development of the primitive sex cords to form testis (medullary) cords.

Question 20

Describe the path in females from indifferent gonad to repro system

Answer 20

absence of Y --> indifferent gonad --> 
ovary
medullary cords degenerate 
cortical cords develop
no tunica albuginea

As there is no Y chromosome, there is no SRY gene to influence development. Without it, the primitive sex cords degenerate and do not form the testis cords.

Instead, the epithelium of the gonad continues to proliferate, producing cortical cords.

Question 21

Describe what leads to development of the internal genitalia in males

Answer 21

•Male gamete carrying Y chromosome
•XY male conceptus
•primordial germ cells carry Y chromosome
•Expression of SRY genes drives development of male
–gonad (testis)
–Production of testicular hormones
–internal genitalia (male duct system)

Question 22

Describe what leads to development of the internal genitalia in females

Answer 22

•Male gamete carrying X chromosome
•XX 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 –i.e. tubes & uterus)

Question 23

At the indifferent gonad stage, which ducts are present in males and females and where do they end?

Answer 23

•Mesonephric/wolffian ducts develop in both male and
female embryos
•Paramesonephric/ mullerian ducts develop in both male and female embryos
•Both the mesonephric and paramesonephric ducts end at the urogenital sinus part of the cloaca

Question 24

what does the fate of the ducts depend on?

Answer 24

Depends on whether there is a functional testis

Question 25

Describe the fate of the ducts in male

Answer 25

–Testis producing Mullerian inhibiting hormone
•Suppressing Mullerian(paramesonephric) duct development
–Testis producing androgens
•Supporting mesonephric (Wolffian) duct

Question 26

Describe the fate of the ducts in female

Answer 26

–No testis producing Mullerian inhibiting hormone
•No suppression of Mullerian(paramesonephric) duct development
–No testis producing androgens
•mesonephric (Wolffian) duct degenerates

Question 27

What is the effect of exogenous androgen in embryo (without presence of testes)?

Answer 27

–Exogenous androgen
–Supports Wolffian duct
–But no testis, therefore no MIH
–Therefore Mullerian ducts develop

Question 28

What is androgen insensitivity syndrome?

Answer 28

Male individual that has all female secondary characters
–testes develop
–the testes produce testosterone
–Receptors for testosterone don’t work
–Wolffian ducts don’t survive
–But MIH present so Mullerian ducts degenerate
-oestrogen from the adrenal glands direct feminization due to lack of effect of testosterone to counter it

Question 29

What does the mesonephric duct first act as in the fetus?

Answer 29

• The mesonephric (Wolffian) duct first acts as the duct for the embryonic kidney
• Drains into the urogenital sinus
• Urogenital sinus will become the urinary bladder
• Surplus to requirement once true kidney develops

Question 30

What does the mesonephric duct become in later stages>?

Answer 30

• Mesonephric duct is maintained by testis derived androgens
• Converted into the vas deferens and epididymis
• Migrates with the testis as it descends

Question 31

Describe the Paramesonephric duct development

Answer 31

• Paramesoneprhic ducts (aka Mullerian duct) appear as invaginations of the epithelium of the urogenital ridge
• Caudally: make contact with the cloaca (urogenital sinus)
• Cranially: open into the abdominal cavity

Question 32

What does the mesonephric duct become in males??

Answer 32

Forms the epididymis and the vas deferens. Also produces ureteric buds which form the ureters

Question 33

What does the pararmesonephric duct become in females?

Answer 33

Fallopian tubes, uterus, cervix and 1/3 of vagina

Question 34

What are the basic components of the external genitalia at the indiffferent stage?

Answer 34

–genital tubercle (GT)
–genital folds
–genital swellings

Question 35

Describe the development of the external genitalia in males

Answer 35

•In the male the Genital Tubercle (GT) elongates & genital folds fuse to form the spongy urethra
•GT develops into glans penis
•Influence of testis-derived androgen hormones
–dihydrotestosterone

Question 36

Describe the development of the external genitalia in males

Answer 36

•No fusion occurs in the female
–development of labia majora and labia minora from genital swelling
–Genital tubercle develops into clitoris
–urethra opens into the vestibule

Question 37

What does the genital tubercle form in the male and female?

Answer 37

In male it elongates to form the phallus and then eventually the glans penis.
In the female it develops into the clitoris.

Question 38

What does the genital folds form in the male and female?

Answer 38

In males they fuse to form the spongy urethra. In females they do not fuse and form the labia minora