S4: Introduction to Reproduction and Sexual Differentiation

Question 1

What is sexual determination?

Answer 1

It is the genetically controlled process dependent on the ‘switch’ on the Y chromosome. It is the chromosomal determination of male or female.

Question 2

What is sexual differentiation?

Answer 2

The process by which internal and external genitalia develop as male or female (expression of the male or female phenotype). This starts to occur at about 7 weeks, before then the foetus is sexually indifferent.

Question 3

Describe the process of sexual determination and differentiation

Answer 3

The two processes are contiguous.

• First stage is the embryo having a genotypic sex which is the karyotype (whether it has two X chromosomes or an X and a Y. Usually XX is female and XY is male.
• Gonadal sex is the ovaries or testes being made. These release hormones and are involved in determining the phenotype (external genitalia).
• Phenotypic sex is the physical characteristics on the individual (external and internal genitalia) and what form it takes.
• Legal sex is the one on your passport of driving license. It use to be that we were allocated a gender at birth and that was based on external genitalia. In 2007, a law was passed that allowed us to change our legal sex if you feel that the sex you are assigned is not the one you identify as.
• Gender identity is the gender that the patient feels that they are.

Question 4

What determines gonadal sex?

Answer 4

• SRY gene creates the testis.
• In the absence of Y chromosome ovaries develop.
• Y chromosome is the smallest chromosome and has few genes (not enough to create a male).

Question 5

Describe Sex Determining Region Y (SRY)

Answer 5

Sex determining region Y (SRY) switches on briefly during embryo development (>week 7) to make the gonad into a testis. In its absence or if it is damaged, an ovary is formed.

• It is located on short arm of a chromosome.
• It is a gene for a transcription factor that transcribes itself once activated, and also causes a cascade of various transcription factors to be expressed and proteins to made which determines differentiation down the male pathway.

Question 6

What are the two different cells in testes and what hormones do they release?

Answer 6

1. Sertoli cells that produce anti-Mullerian hormone (AMH). These cells also express SRY for testes development.
2. Leydig cells make testosterone.

Question 7

What is gonadal development?

Answer 7

• After fertilisation, a pair of gonads develop which are bipotential.
• Their precursor is derived from common somatic mesenchymal tissue precursors called the genital ridge primordia (3½ - 4½ weeks) on posterior wall of lower thoracic lumbar region.
• Genetic ridges either become a pair of testes or ovaries.

Question 8

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

Answer 8

1. Primordial Germ Cell become sperm (male) or oocytes (female). At this stage, the they are still diploid and can divide by mitosis.
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 9

Describe primordial germ cell migration into the genital ridge

Answer 9

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 hind gut, to the region of the developing kidney and on to the genital ridge completed by 6 weeks.

Question 10

Describe primitive sex cord migration into the genital ridge

Answer 10

Cells from the germinal epithelium that overlies the genital ridge mesenchyme migrate inwards as columns called the primitive sex cords. These next wave of cells are already on epithelial surface and they migrate inwards.
~MALE~:
- Primitive sex cords (pre-sertoli cells) form long tubes with germ cells inside them. These are the precursor to structure of testes. There is also SRY expression in sertoli.
- They penetrate the medullary mesenchyme and surround the primordial germ cells (PGCs) to form testis cord.
- Eventually become sertoli cells which express Anti-mullerian hormone (AHM).
~FEMALE~:
- Cells move in and cluster around germ cells as a surface layer and eventually become granulosa cells that become part of the ovary. There is no SRY expression.
- Sex cords are ill defined and do not penetrate deeply but instead condense in the cortex as small clusters around PGCs. Eventually become granulosa cells.

Question 11

Describe mesonephric cells migration into the genital ridge

Answer 11

These originate in the mesonephric primordium which are just lateral to the genital ridges.
~MALES~:
- They act under the influence of pre-sertoli cells (which express SRY) and form vascular tissue, leydig cells and basement membrane.
- Leydig cells synthesis testosterone and do not express SRY. The basement membrane contributes to the formation of seminiferous tubules and rete-testis.
~FEMALES~:
- Without the influence of SRY they form vascular tissue and theca cells.
- Theca cells synthesise androstenedione (a androgen) which is a substrate for estradiol production by the granulosa cells. This occurs later in life but at this point, the female developing gonad does not secrete anything.

Question 12

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

Answer 12

Mullerian and Wolffian ducts.

Question 13

Describe Mullerian ducts for the development of internal reproductive organs

Answer 13

• Most important in females.
• Develops into uterus, uterine tube and upper 1/3 of vagina.
• Inhibited and repressed in the male by AMH (anti-Mullerian hormone).

Question 14

Describe Wolffian ducts for the development of internal reproductive organs

Answer 14

• Forms vas deferens, epididymis, prostate, seminal vesicles etc.
• Most important in the male stimulated by testosterone.
• These will automatically undergo apoptosis unless stimulated so lack of stimulation by testosterone means regression in female.

Question 15

How is DHT synthesised (role of 5-a-reductase)?

Answer 15

Testosterone is converted in the genital skin to the more potent androgen DHT (dihydrotestosterone) by 5-a-reductase. The 5-a-reductase enzyme is found in the genital skin of males and females. However, the testosterone substrate is only found in male genital skin so DHT is only made in males.

Question 16

What is the role of DHT?

Answer 16

DHT also binds to the testosterone receptor, but is more potent than testosterone (10x more powerful). DHT also involved in descent of testes. DHT causes differentiation of the male external genitalia.

• Clitoral area enlarges into penis.
• Labia fuse and become reggated to form scrotum.
• Prostate forms.

Question 17

Describe external differentiation in male and female

Answer 17

Male: The presence of DHT however causes the genital tubercle to become the phallus of the penis, the urethral fold folds over on itself to form a hollow tube which will become the shaft of the penis.
Female: There is little testosterone thus little DHT. The genital tubercle becomes the clitoris and the genital swelling becomes the labia majora and minora. The urethral fold becomes the opening to the vagina.

Question 18

Sex differentiation summary in males

Answer 18

1. Initially you have an undifferentiated gonad
2. If you have XY sex chromosomes you will have SRY
3. SRY says make testis (will form at genital ridge)
4. Sertoli cells will express AMH causing regression of mullerian ducts (no female internal architecture)
5. Leydig cells will be made under influence of sertoli, they will release testosterone causing growth of wolffian duct
6. Testosterone produced by the primitive testes will be reduced to DHT in the genital skin causing fusion of labial scrotal folds, growth of phallus and prostate

Question 19

Sex differentiation summary in females

Answer 19

1. If you have XX chromosomes you’ll have no SRY
2. Therefore you will not develop testis, rather you will develop ovaries with granulosa and theca cells
3. Mullerian ducts will grow into uterine tubes, uterus, cervix and upper 1/3rd of vagina.
4. Lack of testosterone causes wolffian duct to regress
5. Lack of DHT leads to formation of lower 2/3rds of vagina, labia and clitoris

Question 20

List steps from genotypic sex to phenotypic sex

Answer 20

Genotypic sex -> Gonadal sex -> Gonads determine internal genitalia -> Gonads determine external genitalia

Question 21

What are the disorders of sexual differentiation?

Answer 21

• Gonadal dysgenesis: Sexual differentiation is incomplete. Usually missing SRY in male, or partial or complete deletion of second X in female. Also used as a general description of abnormal development of the gonads.
• Sex reversal: Phenotype does not match the genotype. For example, person may be male genotypically but externally look like a female.
• Intersex: Have some components of both male and female tracts or ambiguous genitalia. Sex of infant is too difficult to determine e.g. small micropenis, large clit.

Question 22

Clinically acceptable terms for disorders of sex

Answer 22

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 and are oldfashioned.

Question 23

List gonadal dysgenesis disorders

Answer 23

• Androgen insensitivity syndrome.
• 5-a-reductase deficiency.
• 45 XO.
• Congenital adrenal hyperplasia.

Question 24

Describe androgen insensitivity syndrome (AIS)

Answer 24

It is when testosterone is made but it has no effect. This might happen due to testosterone receptors not working or a problem with the receptor transducing signal.

• Person is XY so genetically male.
• SRY present so will make testis.
• Testis will produce AMH so mullerian ducts regress = no female internal reproductive organs.
• Testis will produce testosterone but it will not be able to function therefore wolffian duct cannot grow and will also regress.
• Therefore there is no internal genitalia at all.
• DHT is made but doesn’t work as it also binds to the testosterone receptor, therefore external genitalia that will develop will be female.
• Might have more female secondary sexual characteristics as they are usually due to androgens and in puberty, oestrogen.

Question 25

What are most secondary sexual characteristics due to?

Answer 25

Most secondary sexual characteristics are due to androgens (a little oestrogen during puberty).

Question 26

How is AIS diagnosed?

Answer 26

• Usually present with primary amenorrhoea (absence of period).
• Lack of body hair is a clue.
• Ultrasound scan and karyotype with male levels of androgens. Never responded to androgen so appear and often feel female.
• Usually diagnosed by lack of periods at 17 + age.

Question 27

What is complete or partial AIS?

Answer 27

Complete AIS:
- Incidence 1:20,000.
- Appear completely female at birth and assigned female gender despite being XY.
- Have undescended testes.
Partial AIS:
- Incidence is unknown as it is probably a spectrum.
- Present with varying degrees of penile and scrotal development from ambiguous genitalia to large clitoris.
- Surgery was universal but now fortunately considered optional or at least best delayed. Decisions made on potential. Very difficult for parents.

Question 28

Describe 5-a-reductase deficiency

Answer 28

Testosterone is made but there is no DHT due to 5-a-reductase deficiency. Incidence varies enormously as autosomal recessive and can depend on inter-related marriage. May appear mainly female or may have ambiguous genitalia
The degree of the enzyme block varies and so therefore does the presentation.
- Person is XY.
- They form testes which produce AMH and testosterone.
- AMH causes regression of mullerian ducts.
- Testosterone causes growth of the woffian ducts so there is internal male genitalia.
- Lack of DHT causes female external genitalia to develop.

Question 29

What happens at puberty with 5-a-reductase deficiency?

Answer 29

Need to assess potential as high testosterone level which will occur at adrenarche and puberty may induce virilisation and develop male external genetalia.

Question 30

Describe 45 XO

Answer 30

This is Turner’s syndrome with incidence 1:3000.

• Individual will develop ovaries as no SRY.
• Mullerian ducts will grow as no AMH.
• Wolffian ducts will regress as no/little testosterone.
• External genitalia will be female.
• XO have failure of ovarian function.
• ‘Streak’ ovaries = ovarian dysgenesis. This illustrates that we need 2 X’s for ovarian development. Streak (Small ovaries) that don’t produce as much oestrogen and progesterone as normal.
• Uterus and tubes are present but small, other defects in growth and development. We usually give hormone support of bones and uterus.
• They are usually infertile, but mosaicism will increase liklihood of fertility. Spectrum of presentation can be seen.

Question 31

Describe steroidogenesis

Answer 31

Steroidogenesis occurs in the adrenal glands.

• Cholesterol - three 6 sided rings, a 5 sided ring and a chain of carbon sticking out (structure). A 27 carbon structure.
• If chain is removed down to 21 C -> these are the progesterones (so cholesterol with parts cleaved off). The only difference between the progesterones are the OH groups and double bonded O2 moved around.
• Another two carbons removed from progesterone, 19 C. They are all androgens. This includes testosterone.
• Another C off androgen down to 18 C, they are all oestrogens. Oestrogen is sometimes abbreviated to E2 because they are essentially they same molecule but with different number of OH groups (oestriol is E1, oestrodiol has two OH groups to E2, oestrotial to E3).
• Cortisol and aldosterone are also 21 C-> steroids.

Question 32

Describe pathway block in adrenal cortex in congenital adrenal hyperplasia (CAH)

Answer 32

• The enzyme 21-hydroxylase enables cortisol and aldosterone to be made. If this enzyme doesn’t work, we can’t produce cortisol or aldosterone.
• Normally cortisol production negatively feedbacks and inhibits release of CRH and ACTH from the hypothalamus and anterior pituitary.
• However in CAH, there is no cortisol production. So a lack of –ve feedback causes high levels of CRH and ACTH to be released. ACTH acts on the adrenal cortex to increase cholesterol uptake and increase side chain cleavage by P450 enzymes. It increases glucocorticoid secretion.
• However as 21-hydroxylase isn’t working, there is a build up of androgens as cortisol can’t be produced.

Question 33

Describe congenital adrenal hyperplasia (CAH)

Answer 33

• Female with XX.
• No SRY, so develop ovaries not testis.
• Therefore no sertoli cells so no AMH, mullerian ducts will develop so will have female internal genitalia.
• No leydig cells to produce testosterone, but testosterone produced due to CAH.
• Testosterone leads to growth of wolffian duct therefore will have male internal genitalia as well.
• Testosterone will be converted to DHT at genital skin, leading to male external genitalia.
• The extent of block (in adrenal cortex) varies. If the enzyme absent at birth then children may be wrongly gender assigned at birth or may have ambigious genitalia. Also in CAH there can be salt wasting due to lack of aldosterone which can be lethal.
• Treatment with glucocorticoids is required to correct feedback.