Normal Sexual Differentiation Flashcards
1
Q
Define sexual determination and sexual differentiation.
A
- Sexual determination = Genetically controlled process dependent on the ‘switch’ on the Y chromosome. Chromosomal determination of male or female.
- Sexual differentiation = The process by which internal and external genitalia develop as male or female.
- The two processes are contiguous and consist of several stages.
2
Q
What gene creates the testis and how?
A
- SRY gene creates the testis.
- In the absence of Y chromosome, ovaries develop. The Y chromosome is the smallest chromosome; doesn’t contain many genes (obviously doesn’t contain all the genes to make a boy). One of the genes it does contain is SRY (very important in terms of sex differentiation). SRY is located on the short arm of the Y chromosome.
- Sex determining region Y (SRY) switches on briefly during embryo development (>week 7) to make the gonad into a testis. In its absence, an ovary is formed.
- Testis develops cells that make 2 important hormones:
1) Sertoli cells produce anti-Mullerian hormone (AMH).
2) Leydig cells make testosterone. - Products of testis influence further gonadal and phenotypic sexual development. Absence of hormones form the ovary = differentiation towards the female line (causes female structures to develop).
3
Q
How does SRY and SOX9 act?
A
- If there is a Y chromosome, it is assumed there is SRY
- SRY is a DNA binding protein that acts as a transcription factor.
- The critical site of DNA binding is upstream of SOX9 which is also a transcription factor.
- SOX9 can bind its own upstream activation site so its levels remain high even if SRY declines. This is a positive feedback mechanism.
- SRY turns off many ovarian genes and turns on many testicular genes. These are direct targets of SRY (there are thousands of genes that are direct targets of SRY).
SRY binds DNA upstream of itself (of its own gene). It is a transcription factor that causes its own transcription (positive feedback). Once switched on, it is really on. - It also transcribes SOX9, which is a transcription factor for itself; SRY is a very positive switch on the Y chromosome. SOX9 is a little bit downstream; turns on lots of testicular genes and turns off many ovarian genes. Around 900 targets identified are shared between SRY and SOX9
- Once SRY is switched on, it switches on SOX9; SRY determines executive policy and SOX9 implements it. Strong positive feedback mechanism.
- SOX9 then acts to stimulate the expression of further downstream genes.
- It is the Sertoli cell that expresses SRY. All of this early action occurs in one of the cells in the testis that is destined to become a Sertoli cell.
4
Q
The main role of SRY is switching on SOX9. How does SOX9 achieve its effects?
A
- Main role of SRY is switching on SOX9
- SOX9 has 4 effects
- One of the primitive cells in the developing testis in the embryo is the Sertoli cell. It is the Sertoli cell that expresses SRY. All of this early action occurs in one of the cells in the testis that is destined to become a Sertoli cell.
1) Stimulates production of prostaglandin D2 (PGD2) by the pre-sertoli cells. PGD2 acts as a paracrine hormone to stimulate further SOX9 production in these cells. This second positive feedback mechanism serves to re-enforce all precursor cells along the sertoli pathway.
2) SOX9 stimulates production of fibroblast growth factor 9 (FGF9). FGF9 acts as a chemotactic factor for cell migration into the developing testis. It also further stimulates the production of SOX9 a third positive feedback mechanism.
3) SOX9 stimulates the production of anti-Mullerian hormone (AMH). AMH causes the regression of the mullerian ducts which develop into the uterus and fallopian tubes in the female.
4) Inhibition of female transcription factors WNT4 and FOXL2 (involved in the female cascade to become a female).
5
Q
Describe the transcriptional pathway induced by the presence/absence of the Y chromosome and the three types of cell produced.
A
- If there is a Y chromosome, SRY will activate SOX9 which then leads to the production of AMH. AMH has important effects. Both of these have effects on the germ cells that arrive in the developing testes (become sperm). The cell where this happens becomes a Sertoli cell. Leydig cells and sperm are produced.
- In the female, there is no SRY so WNT4 and FOXL2 are the main transcription factors (both inhibited by SOX9). The cell where this happens becomes the granulosa cell and there are theca cells and oocytes in the ovary.
- Three types of cells in either the testis or ovaries. These cells go down different pathways depending on whether they have SRY or FOXL2/Wnt4
6
Q
How do the gonads develop?
A
- After fertilisation, a pair of gonads develop which are bipotential (can develop into either male or female).
- 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.
- Genital ridges will become a pair of testis/ovaries; can become either. There are two series of ducts; Mullerian ducts become the uterus, uterine tubes and the upper third of the vagina (internal genitalia of female) + the Wolffian ducts become the vas deferens, seminal vesicles etc. (the internal genitalia connecting the testes to the urethra of the male).
- Not only are the genital ridges capable of becoming male or female but of these two sets of ducts, one will grow and the other will not (depending on what the effects of the hormones from the early gonads are).
7
Q
There are three main types of cells in the testes or ovary – they are analogues of each other (come from the same cell lineage). What are the three waves of cells that invade the genital ridge?
A
- The first stage after the SRY switch is to make the gonads (testes or ovaries). These will influence the rest of the development. There are three lots of cells that invade these genital ridges. There are three main types of cells inside the testes and three types of cells inside the ovary; they come from the same cell lineage (analogues of each other).
- 3 waves of cells invade the genital ridge
1) Primordial Germ Cells invade up from the yolk sac = become Sperm or Oocytes (can become either at this stage).
2) Primitive Sex Cords – become Sertoli cells (male) or Granulosa cells (female). The Sertoli cells are the main cells nursing the developing sperm in the male and the granulosa cells do the same job in the female. The primitive sex cords are precursors to them both (whether they become Sertoli or granulosa depends entirely on the Y chromosome). SRY is expressed in cells that become Sertoli cells. SRY and that cascade causes Sertoli cell production (without SRY, they become granulosa cells)
3) Mesonephric Cells – become blood vessels and Leydig cells (male) or Theca cells (female). In both males and females, these produce androgens in later life (but not at the moment).
8
Q
How do the first wave of cells (primordial germ cells) invade the genital ridge?
A
- The first wave come from the hindgut (from the yolk sac). 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. They slowly migrate up and start to inhabit these two genital ridges. By 6 weeks, these cells are inhabiting them and waiting to see whether they receive signals from a granulosa cell or a Sertoli cell to develop into either sperm or eggs (oocytes).
9
Q
How do the second wave of cells (primitive sex cords) invade the genital ridge?
A
- Cells from the germinal epithelium that overlies the genital ridge mesenchyme migrate inwards as columns called the primitive sex cords.
- The primitive sex cords are a layer of cells on the surface of the genital ridges. They migrate inwards. These are the cells that become either Sertoli cells or granulosa cells.
- When there is SRY expression, they penetrate inwards and surround the germ cells to make chords. In the testes, there are seminiferous tubules, which are long tubes with the Sertoli cells and the sperm inside. Even at this point, they start to make a very basic testicular-like structure. They express SRY, they become Sertoli cells, they get mixed up with the germ cells in a way that looks a little bit like a primitive testis and they make one hormone (AMH). Penetrate medullary mesenchyme & surround primordial germ cells to form testis cords – precursor of seminiferous tubules. Eventually become Sertoli Cells which express AMH. AMH has a negative effect on the Mullerian ducts.
- If there is no SRY, WNT4 and FOXL2 is produced; pathway stimulated. The same cells still migrate in, but now they don’t become Sertoli cells or make AMH. They surround the germ cells more like a clustering around them in a ball (reminiscent of the follicle in the ovary so even then it looks like a very primitive ovary already). These eventually become the granulosa cells. Sex cords ill defined and do not penetrate deeply but instead condense in the cortex as small clusters around primordial germ cells – precursor of ovarian follicle. Eventually become Granulosa cells.
10
Q
How do the third wave of cells (mesonephric cells) invade the genital ridge?
A
- These originate in the mesonephric primordium, which are just lateral to the genital ridges; these cells move in as well and act under the influence of already present cells.
- In males, they act under the influence of pre-sertoli cells, or developing Sertoli cells, (which themselves express SRY) to form:
1) Vascular tissue
2) Leydig cells (synthesize testosterone, do not express SRY themselves as this is the role of the Sertoli cells; they are influenced by the Sertoli cells).
3) Basement membrane – contributing to formation of seminiferous tubules and rete-testis - In females, without the influence of SRY (under the influence of granulosa cells surrounding the germ cells), they form:
1) Vascular tissue
2) Theca cells. - Most importantly, they become Leydig cells in men and theca cells in women.
11
Q
Summarise gonadal sex formation with the three types of invading cells.
A
- The first stage after the SRY switch is to make the gonads (testes or ovaries). These will influence the rest of the development.
- There are three lots of cells that invade these genital ridges. There are three main types of cells inside the testes and three types of cells inside the ovary; they come from the same cell lineage (analogues of each other)
1) Primordial germ cells = spermatozoa/oocytes
2) Primitive sex cords = sertoli (SRY, AMH)/granulosa cells
3) Mesonephric = leydig (androgens)/theca cells - The mesonephric cells are influenced by the Sertoli cells to become Leydig in the male, which produce testosterone. They are influenced by the granulosa cells in females to become theca cells.
- Theca cells are analogues of Leydig cells in the female (come from the same cell origin)
- Granulosa cells and Sertoli cells arrive from the same cellular origin etc.
- Theca cells in later life do produce some androgen that gets aromatised to oestrogen, but not in the embryo. In the embryo, the Leydig cells produce androgens and the theca cells do not produce anything. In the male, AMH and testosterone are produced while in the female, nothing is produced in terms of these hormones that affect development.
12
Q
How do the male internal reproductive organs develop?
A
- Wolffian ducts are most important in the male and they are stimulated by testosterone
- Mullerian ducts regress in men due to the presence of AMH
- The Mullerian duct is inhibited by anti-Mullerian hormone (will grow unless inhibited by AMH). The Wolffian duct is rescued by testosterone (or else it dies).
- In the presence of testosterone, the Wolffian duct is rescued and can develop into the seminal vesicles, the vas deferens and the epididymis.
- A single Y chromosome switch decides whether we get a testis or ovary. Then, AMH and testosterone, or a lack of them, determine the internal genitalia.
13
Q
How do the female internal reproductive organs develop?
A
- In the female, there is no testosterone or AMH, so the Mullerian ducts differentiate into the female internal reproductive organs (the uterus, uterine tubes and the upper third of the vagina).
- Lack of stimulation by testosterone causes Wolffian ducts to regress in females. In the absence of testosterone, the Wolffian duct is going to atrophy.
- The Mullerian duct will grow regardless, unless it is inhibited by AMH. In the female, there is no AMH or testosterone, so the Mullerian duct will grow and the Wolffian duct will die (as there is no testosterone).
- A single Y chromosome switch decides whether we get a testis or ovary. Then, AMH and testosterone, or a lack of them, determine the internal genitalia.
14
Q
How does the male external genitalia develop?
A
- In the early embryo, the genital skin is the same in everyone. There is an enzyme in the skin that converts testosterone to DHT. Testosterone is converted in the genital skin to the more potent androgen DHT (dihydrotestosterone) by 5-a-reductase.
- DHT only has an extra hydrogen, but it is about 10 times more potent than testosterone.
- In the male, there is testosterone as a substrate and the 5-a-reductase converts testosterone into DHT. It is the DHT that causes differentiation of the male external genitalia.
- DHT binds to the testosterone receptor, but is more potent than testosterone (~10x).
- DHT causes differentiation of the male external genitalia:
1) Clitoral area enlarges into penis
2) Labia fuse and become ruggated to form scrotum
3) Prostate forms - If there is testosterone present, the 5-a-reductase converts it to DHT so external differentiation can occur down the male pathway. The genital tubercle becomes the phallus, which eventually becomes the glans penis or the head of the penis. The urethral fold folds over to form a hollow tube that will become the shaft of the penis. The line on the underside of the penis down the scrotum is caused by the fusion of the urethral folds and the scrotal swellings. The scrotal swellings come from the urethral fold and the genital swellings become the scrotum. Part of the urethral fold becomes the shaft of the penis.
15
Q
How does the female external genitalia develop?
A
- In the early embryo, the genital skin is the same in everyone. There is an enzyme in the skin that converts testosterone to DHT.
- The female has the enzyme but not the testosterone substrate. There is differentiation into the female external genitalia in the absence of DHT.
- In the female, there is no testosterone. Even though 5-a-reductase present, there is no DHT. In this situation, the genital tubercle becomes the clitoris, the urogenital groove becomes the opening of the vagina, the urethral fold and genital swelling become the minor and major labia, respectively.
