Lecture 4 Testes and Their Development

Question 1

What is the bipotential precursor of the gonds called?

Answer 1

Genital ridge primordia or gonadal ridge

Question 2

What is the SRY?

Answer 2

Sex-determining region of Y chromosome contains SRY gene which produces SRY protein, a testis-determining DNA-binding protein acting as a transcription factor, that initiates differentiation of testis

Question 3

What is encouraged and suppressed during the development of internal reproductive organs in the female?

Answer 3

Gonadal cortex becomes ovary in the absence of SRY protein
Wolffian duct is degenerated in the absence of testosterone
Mullerina duct becomes Fallopian tube, uterus, and upper vagina (all the structures downwards of ovary to upper vagina) in the absence of anti-Mullerian hormone

Question 4

What is encouraged and suppressed during the development of internal reproductive organs in the male?

Answer 4

Gonadal cortex becomes testis in the presence of SRY protein
Wolffian duct becomes seminal vesicle, vas deferens, and epididymis due to testosterone
Mullerian duct degenerates due to AMH from testis

Question 5

What do the Sertoli cells secrete?

Answer 5

Anti-Mullerian hormone

Question 6

What do the Leydig cells secrete?

Answer 6

Testosterone

Question 7

How are the PGCs specificated?

Answer 7

By BMP4 (stimulatory signal) secretion from the extraembryonic ectoderm, causing a gradient of activity

Question 8

What cells do PGCs arise from?

Answer 8

Proximal epiblast cells (adjacent to developing early placental tissue)

Question 9

What are primordial germ cells (PGCs)?

Answer 9

Precursors of spermatozoa and oocytes

Question 10

What determines the site of PGC speciation?

Answer 10

A set of inhibitory and stimulatory signals creating a concentration gradient

Question 11

What are the signals affecting the concentration gradient that results in PGC formation and where are they released from?

Answer 11

Stimulatory: BMP4 from posterior extra-embryonic ectoderm
Inhibitory: CER1, LEFTY1, DKK1 released from the primitive ectoderm-derived anterior visceral endoderm

Question 12

What is induced in PGCs and what do they do?

Answer 12

PRDM1 (Blimp1): repression of somatic mesoderm in all cells except PGCs
PRDM14: reacquisition of pluripotency by PGCs

Question 13

Following speciation of PGCs, they proliferate and migrate. Outline the order of migration.

Answer 13

First in yolk sac near allantois, moves to hind gut, then into gut mesentery, followed by developing kidney, finally into genital ridge primordia (bipotential gonad)

Question 14

What directs PGCs migration?

Answer 14

Chemokines secreted by genital ridges attract PGC which has complimentary receptors

Question 15

How are sex chords formed?

Answer 15

Germinal epithelium that overlie the genital ridge migrate into genital ridge as primitive sex chords. These cells have SRY expression cause proliferation and migration to surround PGCs and form testis chords. These cells eventually become Sertoli cells, and PGCs become prospermatogonia

Question 16

How is sex chord formation different in females?

Answer 16

Due to the lack of SRY expression, these cells do not penetrate deeply into ridge and are ill-defined, only form small clusters around PGCs. Initiates primordial ovarian follicle formation, where PGCs are oocytes and small clusters are granulosa cells.

Question 17

How are Leydig cells and seminiferous tubules formed?

Answer 17

Mesonephric cells migrate into genital ridges. Either cluster between sex chords and become Leydig cells, or condense around Sertoli cells like basement membrane and become seminiferous tubules

Question 18

What is the role of SRY in testis development?

Answer 18

Drives differentiation of genital ridge to testis, by acting as a transcription factor thus activating Sox9

Question 19

What is the role of Sox9 in testis development? (3)

Answer 19

Suppress development of ovaries by inhibiting Wnt4/ß-catenin both directly and indirectly via Fgf9/Fgfr2
Stimulates Mullerian duct regression
Induces Sertoli cell proliferation and differentiation which will secrete AMH

Question 20

What are the positive feedback mechanisms to ensure Sox9 production? (2)

Answer 20

Sox9 protein binds to and activates Sox9 gene, so is a positive feedback on itself.
PGD2 produced by pre-Sertoli cell causes positive feedback on nearby pre-Sertoli cells to produce Sox9

Question 21

What occurs in Sox9 KOs? (4)

Answer 21

Absence of AMH
Expression of Wnt
No Leydig cells, or no patterned aggregation
Meiotic oocytes

Question 22

What role do Sertoli cells have in testis formation?

Answer 22

Drive foetal Leydig cell differentiation by secreting Dhh and Pdgf.

Sertoli cell retinoid metabolising protein (CYP26b1) causes irreversible masculinisation of PGCs.

Question 23

How does CYP26B1 cause irreversible masculinisation of PGCs?

Answer 23

Inhibits the action of retinoic acid. Retinoic acid stimulates Stra8 which is required for initiation of meiosis.
(no retinoic acid activity, no Stra8 activity, so no initiation of meiosis - hence Sox9 null mice have meiotic oocytes, deficiency).