REPRO: Sexual Determination and Differentiation

Question 1

What is sexual determination?

Answer 1

A genetically controlled process dependant on the ‘switch’ on the Y chromosome. The prescence of a Y chromosome determines the differentiation into the male and absence of a Y chromosome determines the differentiation into the female.

It is the chromosomal determination of being male or female.

Question 2

What is sexual differentiation?

Answer 2

It is the process by which internal and external genitalia develop as male or female.

Sexual determination and differentiation are both contiguous and consist of several stages.

Question 3

What are the different levels of sexual differentiation, from embryo to adult?

Answer 3

• Genotypic sex (if the embryo has XX or XY)
• Gonadal sex (either ovaries or testes develop)
• Phenotypic sex (outward appearance)
• Legal sex (what’s written on the passport)
• Gender identity (how you feel)

Question 4

Describe how the SRY gene is involved in gonadal development.

Answer 4

The SRY gene is a transcription factor that transcribes itself (positive feedback), along with a transcriptional cascade of events that eventually leads to the development of a testis. In the absence of this gene, ovaries develop. SRY is the triggering event for sexual differentiation.

The testis develops cells that make 2 important hormones:

• SERTOLI cells produce anti-Mullerian hormone (AMH).
• LEYDIG cells make testosterone.

Products of the testis influence further gonadal and phenotypic sexual development.

Question 5

Describe gonadal development after fertilisation.

Answer 5

After fertilisation, a pair of gonads develops which are bipotential. Their precursor is derived from common somatic mesenchymal tissue precursors called the genital ridge primordia (3.5 to 4.5 weeks) on the posterior wall of the lower thoracic lumbar region.

Wolfian duct - can develop into internal male genitali (e.g. - vas deferens, prostate, seminal vesicles.)

Mullerian duct - can develop into female internal genitalia (e.g. - uterine tube, upper parts of vagina.)

Question 6

What are the three waves of cells that invade the genital ridge?

Answer 6

1. Primordial Germ Cells - become sperm (male) or oocytes (female)
2. Primitive Sex Cords - become Sertoli cells (male) or Granulosa cells (female)
3. Mesonephric Cells - become blood vessels and Leydig cells (male) or Theca cells (female)

Question 7

Describe primordial germ cell migration.

Answer 7

An initially small cluster of cells in the epithelium of the yolk sac expands by mitosis at around 3 weeks. They then migrate to the connective tissue of the hindgut, to the region of the developing kidney and on to the genital ridge - completed by 6 weeks.

Question 8

Describe the formation of the sex cords (Sertoli/Granulosa).

Answer 8

Cells from the germinal epithelium that overlies the genital ridge mesenchyme migrate inwards as columns called the primitive sex cords.

In MALES:

1. There is SRY expression
2. The sex chords penetrate the medullary mesenchyme and surround the PGCs to form the testis chords.
3. They eventually become Sertoli cells, which express anti-Mullerian hormone (AMH).

In FEMALES:

1. There is no SRY expression.
2. The sex chords are ill-defined and do not penetrate deeply, but instead condense in the cortex as small clusters around PGCs.
3. They eventually become Granulosa cells.

Question 9

Describe the role of mesonephric cells in gonadal development.

Answer 9

These originate in the mesonephric primordium, which is just lateral to the genital ridges.

In MALES, they act under the influence of pre-Sertoli cells to form:

• vascular tissue
• Leydig cells (synthesise testosterone, don’t express SRY)
• basement membrane
• contributing to the formation of seminiferous tubules and rete-testis

In FEMALES, they form:

• vascular tissue
• Theca cells (synthesise androstenedione, which is a substrate for estradiol production by the granulosa)

Question 10

Gonadal Sex Summary (male and female)

Answer 10

Males - Invading cells:

• Primordial germ cells –> Spermatozoa
• Primitive sex cords –> Sertoli cells (SRY, AMH)
• Mesonephric cells –> Leydig cells (Androgens)

Females - Invading cells:

• Primordial germ cells –> Oocytes
• Primitive sex cords –> Granulosa cells
• Mesonephric cells –> Theca cells

Question 11

What are the two main structures involved in developing internal reproductive organs?

Answer 11

A control mechanism is involved in the differentiation of the internal genitalia. All foetuses have BOTH - the Mullerian and Wolffian ducts.

MULLERIAN ducts:

• most important in female
• inhibited in males by AMH

WOLFFIAN ducts:

• most important in the male, stimulated by testosterone
• lack of stimulation by testosterone means regression in female

The Mullerian ducts develop unless it is repressed by AMH. The Wolffian regresses unless it is stimulated by testosterone. In the male there is AMH and testosterone so the Wolffian develops. In the female there is neither AMH nor testosterone so the Mullerian develops.

Question 12

Explain what happens in the differentiation of the External Genitalia.

Answer 12

The differentiation of the external genitalia mediated by an enzyme called 5-alpha reductase which is present in the genital skin. In males this enzyme causes testosterone to be converted to dihydrotestosterone (DHT) which causes differentiation into the penis and scrotum. In females there is still 5-alpha reductase but no testosterone substrate, so no DHT and the external genitalia develops into clitoris and labia.

Question 13

Describe the role of 5-α-reductase in external differentiation.

Answer 13

Testosterone is converted to the more potent androgen DHT (dihydrotestosterone) by 5-α-reductase in the genital skin. DHT binds to the testosterone receptor but is more potent than testosterone.

DHT causes differentiation of the male external genitalia:

• clitoral area enlarges into the penis
• labia fuse and become ruggated to form the scrotum
• the prostate forms.

It is present in both males and females, but because there is no substrate (testosterone) in females, its effects don’t play out.

See summary images

Question 14

List some disorders of sexual differentiation.

Answer 14

• Gonadal dysgenesis: sexual differentiation is incomplete; also used as a general description of the abnormal development of the gonads
• Sex reversal: phenotype not matching the genotype.
• Intersex: have some components of both tracts, or have ambiguous genitalia.

Patients prefer to be known as someone with a ‘disorder of sexual differentiation’ or DSD.

Terms such as ‘pseudohermaphrodite’ and ‘testicular feminisation’ are now obsolete.

Question 15

What is it called when testosterone is being made in an XY individual, but it has no effect? What does that result in?

Answer 15

Androgen Insensitivity Syndrome (AIS). The testes form and make AMH so the Mullerian ducts regress. However, there is no differentiation of the Wolffian ducts, and thus no external male genitalia. There will be feminine or ambiguous external genitalia.

Question 16

What’s the difference between complete and partial AIS?

Answer 16

Complete AIS:

• incidence 1:20,000
• appear completely female at birth, and are assigned the female gender despite being XY.
• Have undescended testes.

Diagnosis:

• usually present with primary amenorrhoea, lack of body hair is a clue.
• would be diagnosed by ultrasound scan and karyotype with male levels of androgens.
• would never respond to androgen so they appear and feel female.

Partial AIS:

• present with varying degrees of penile and scrotal development, from ambiguous genitalia to large clitoris.
• surgery was universal, but now fortunately considered optional (at least best, delayed).
• decisions made on potential, very difficult for parents

Question 17

What is it called when testosterone is made in an XY individual, but not DHT? What does that result in?

Answer 17

5-α-reductase Deficiency. The testes form and make AMH so the Mullerian ducts regress. The Wolffian ducts then develop, but there is no external male genitalia.

Question 18

Describe 5-α-reductase deficiency.

Answer 18

The incidence varies enormously as it is autosomal recessive and can depend on inter-related marriage.

• Testes form, AMH acts, testoesterone acts.
• Internal structures form.
• External structures do not develop.

The individual may appear mainly female, or may have ambiguous genitalia. The degree of the enzyme block varies, and so, therefore, does the presentation. At puberty, you need to assess the potential as the high testosterone levels which will occur at adrenarche and puberty may induce virilisation (development of male physical characteristics in a female/precociously boy).

Question 19

Describe Turner Syndrome.

Answer 19

Turner syndrome 1:3000 incidence.

It is when an individual has the chromosome XO. They have a failure of ovarian function. They have what are called ‘streak’ ovaries, which are a form of ovarian dysgenesis. It illustrates that we need 2 X’s for proper ovarian development.

Although one X is repressed and one X isn’t, the repressed X still has pseudo-autosomal regions that are probably required for normal ovarian development.

The uterus and tubes are present but other, small defects in growth and development may be present. There may be a need for steroid hormone support of the bones and uterus. They may be fertile.. many have mosaicism (some of the cells have 2Xs and some don’t).

Question 20

Describe Steroidogenesis. (See diagram)

Answer 20

Steroid hormones are derivatives of cholesterol that are synthesized by a variety of tissues, most prominently the adrenal gland and gonads.

• Cholesterol has 27 carbons.
• Progestogens (progesterone, pregnenalone etc.) have 21 carbons.
• Androgens (testosterones, DHT etc.) have 19 carbons.
• Oestrogens (oestrone, oestradiol, oestriol) have 18 carbons.
• Mineralocorticoids (aldosterone, cortisol etc.) have 21 carbons.

SOME enzymes that convert substances (there are many other enzymes involved):

• Cholesterol side-chain cleavage enzyme(P450scc): converts cholesterol to progestagens.
• 21-hydroxylase: Progesterone to Mineralocorticoids.
• 17,20-lyase: Progestogens to Androgens.
• Aromatase: Androgens to Oestrogens.

Question 21

Briefly describe the Hypothalamic Pituitary Adrenal Axis.

Answer 21

Corticotropin releasing hormone stimulates the anterioir pituitary to secrete ACTH.

Adrenocorticotropic hormone:

• Stimulates the rapid uptake of cholesterol into the adrenal cortex.
• Upregulates cholesterol side-chain cleavage enzyme (P450scc).
• Increases glucocorticoid secretion.

Hypothalamus –> CRH –> Pituitary –> ACTH –> Adrenal glands –> Cortisol

Cortisol controls its own production via negative feedback (to hypothalamus and pituitary).

Question 22

What happens when an XX female is exposed to high levels of androgens in utero? What does that result in?

Answer 22

Congenital Adrenal Hyperplasia (CAH) is the most common cause.

No SRY is expressed, so we don’t get any testes or AMH. The Mullerian ducts remain. The individual ends up with masculinised external genitalia (5-alpha reductase converts testosterone to DHT), but the androgen levels are not usually high enough to rescue the Wolffian ducts.

Question 23

How would an XX female be exposed to high levels of androgens in utero?

Answer 23

There could a failure of the enzyme 21-hydroxylase which converts steroid hormone precursors, to eventually end up as cortisol and aldosterone. This means that there will be a build up of precursors, which will be ushered along to make more androgens. Also, in the event of the lack of negative feedback from cortisol/aldosterone, more cholesterol will be pushed into steroid genesis for more cortisol/aldosterone, which worsens the situation.

Question 24

Describe the effects of Congenital Adrenal Hyperplasia (CAH).

Answer 24

CAH - 1:15,000

The adrenal gland grows (due to the hyperplasia).

The completeness of the enzyme (21-hydroxylase) block varies. If the enzyme is absent, then children may be wrongly gender assigned at birth, or may have ambiguous genitalia. They may have external male looking genitalia but uterine tubes, uterus inside (little bit of wolffian ducts).

They may need treatment with glucocorticoids to correct the feedback. Also, in CAH, we need to be aware of the possibility of ‘salt-washing’ due to the lack of aldosterone; this can be lethal.

Question 25

Glossary

Answer 25

Mullerian duct – embryonic ducts developing into female internal internal genitalia

Wolffian duct - embryonic ducts developing into male internal internal genitalia

Sex determining region Y (SRY) – important transcription factor on Y chromosome

Primordial germ cell – cells that develop into sperm or oocytes

Primitive sex cords – cells that develop into gonadal cells associated with germ cells

Mesonephric cells - cells that develop into gonadal cells that produce androgens

5-alpha reductase – enzyme involved in development of male external genitalia

Gonadal dysgenesis – sexual differentiation is incomplete, usually abnormal development of gonads

Sex reversal – phenotype does not match genotype

Intersex – some components of both male and female or ambiguous genitalia