Development Of The Urogenital System Flashcards
What are the names of the three sets of kidney structures during development?
Pronephros -cervical region
Mesonephros - abdominal region
Metaneohros - pelvic region
Describe the pronephros.
Develops first and regresses around week 4.
It’s rudimentary and non-functional.
It has 7-10 solid cell groups
Describe the mesonephros.
This is first seen in week 4 after the pronephros regresses
The excretory tubules form first and drain into the mesonephric duct .
It contributes supporting cells to the genital ridge
Describe the metanephros.
This is the definitive kidney.
It appears in week 5 and is functional by week 11
The excretory units develop from the metanephric mesoderm.
It’s made of two parts - ureteric bud
- metanephric cap
When do the nephrons stop developing?
They continue developing until birth, but urine production starts early in gestation.
How does the urorectal septum divide the cloaca, and when does this happen?
It’s fuses with the cloaca membrane to form the anterior urogenital sinus and the posterior rectal/anal canal
This happens between weeks 4 and 7
What is the bladder eventually formed from?
The urogenital sinus and caudal parts of the mesonephric duct (trigone)
What is the bladder lined with at this stage?
Endoderm
What is the allantois?
Outpouching of the hindgut
It’s a sac like structure involved in nutrition and excretion.
What is the urachus?
It’s a duct between the bladder and the yolk sac, formed form the allantois. It eventually becomes the medial umbilical ligament
What is the cloaca?
A posterior orifice that serves as the only opening for the early stages of the intestinal, reproductive and urinary tracts.
What is the ureteric bud?
A protrusion of the mesonephric duct. It allows drainage from the developing kidney.
What is the difference between genetic sex and physiological sex?
Genetic sex is determined at fertilisation. Gonads only develop make or female characteristics around week 7.
What three stages are there in the development of the reproductive tract?
1) Genital duct development (the indifferent stage)
2) Gonadal development
3) External genitalia development.
What happens in the genital duct development stage?
Two pairs of genital ducts develop in weeks 5-6.
- the paramesonephric duct (female)
- the mesonephric duct (male)
The paramesonephric ducts are lateral to the mesonephric ducts
What is the function of the mesonephric (male) ducts, and how do they develop?
Drain urine from the mesonephric kidney.
Play an essential role in development of the male reproductive system.
Under the influence of testosterone it forms the vas deferens and ejaculatory duct once the mesonephros regresses
In females the lack of testosterone allows the duct to disappear, leaving only a few non-functional remains.
What is the function of the paramesonephric ducts and how do they develop?
They develop laterally to the gonads and mesonephric ducts
They form funnel shaped cranial ends that open into the peritoneal cavity
They migrate caudally, parallel to the mesonephric ducts until they reach the future pelvic region
They approach each other in the midline, with the cranial part forming uterine tubes and the caudal part forming the uterus and superior vagina (uterovaginal primordium)
What happens to the paramesonephric ducts in males?
They degenerate due to the action of anti-mullerian hormone, a protein made by the serotoli cells of the testes.
How do they gonads usually appear in week 5 in both of the sexes?
As longitudinal ridges. Also known as the urogenital or gonadal ridges.
What forms the urogenital or gonadal ridges?
Projection of the mesoderm structure into the coelomic cavity
Where do primordial germ cells originate from and how doe they get to the gonadal ridge?
They originate in the yolk sac and move to the gonadal ridge via the dorsal else try to form the primordial gonad.
What happens if the primordial germ cells don’t reach the gonadal ridge by week 6?
The ridges can’t develop any further
What happens to the primordial germ cells when they reach the gonadal ridge?
They form in a cord like structure to become the primitive sex cords
What is the testis determining factor and what does it do?
If the embryo is genetically male, the Y chromosome encodes the testis determining factor SRY (sex determining region of Y chromosome)
This acts on somatic cells and allows proliferation of the sex cords
Describe gonad differentiation in males.
The cords become horseshoe shaped, and then break up into tubules.
- the cords contain both somatic and primitive germ cells
Leidig cells begin to produce testosterone
Serotoli cells begin to produce AMH (anti-mullerian hormone)
Dense connective tissue forms
- tunica alberguinea
- this separates the cords from the surface epithelium
Testis cords are solid until puberty, when what occurs?
They acquire a lumen to form seminiferous tubules
They join with the rete testes, which joins with the efferent ductlules
The rete testis and the mesonephric duct link to form the ductus deferens
What is the active signal needed for the maintained of the paramesonephric ducts and thus female development?
Wnt4 - the ovary determining gene
What hormone is needed for the development of the female gonads?
No hormones. Just the XX chromosome means SRY can’t be produced.
What effects do the primordial germ cells have on the gonadal ridge in females?
It stimulates somatic cells to grow from the coelomic epithelium
Allows surface epithelium to continue to proliferate
They also divide by mitosis to form a pool of oogonia
How do oogonia become oocytes?
They enter meiosis arrest at 4 months - now called oocytes
They then become associated with follicular cells and are called primordial follicles
When does the development of the external genitalia begin?
Week 3
How does the development of th external genitalia begin?
A pair of cloacal folds develop around the cloacal membrane, joining at the cranial end to form a genital tubercle
In what ways are the cloacal folds then subdivided?
They form urethral folds in front (labia minora in females)
Anal folds behind
Genital swellings then appear in either side of the urethral folds
- form scrotal swellings in men
- form labia majora in females
What forms the urethra?
The middle pelvic part of the urogenital sinus
What causes the gentian tubercle to elongate into phallus in men?
Androgens from the fetal testis
What happens when the genital tubercle elongates into a phallus in men?
The phallus pulls the urethral folds forward
These then form the lateral walls of the urethral groove and close over the urethral plate to form the penile urethra
The terminal part of the male urethra forms surface ectoderm
What is the prostate gland derived from?
Outgrowths from the prostatic urethra
What are the bulbourethral glands derived from?
Outgrowths of the penile urethra
What is the lower part of the vagina formed from?
Two outgrowths of the urogenital sinus (sinovaginal bulbs) fuse to forms vaginal plate.
This then hollows to form a cavity
Name some of the things that can go wrong in female development.
Double uterus - failure of fusion
Vaginal atresia - failed canalisation
Absense of vagina and uterus
- failure of sinovaginal bulbs means no vagina
- because the formation of these bulbs are normally induced by the uterus, it is normally absent as well.
Name some of the possible developmental problems in males.
Hypospadias (penile, perineal)
Epispadias
Name some of the possible intersex disorders
Klinefelter’s syndrome - XXY or XXXY
Testicular feminising syndrome - female external phenotype
Turners syndrome - X0
Dihyrdotestosterone- lack of gene for 5-alpha reductase
Describe klinefelter’s syndrome
When a man has the chromosomes XXY or XXXY, causing the leidig cells to not produce enough steroids, causing low sperm production
(1 in 500 males)
Causes infertility, gynaecomastia and impaired sexual maturation
Describe testicular feminising syndrome.
When someone is genetically male, but has female external phenotype, with internal testes.
The testes produce testosterone, but a mutation on the X chromosome causes a deficiency in androgen receptors so the testosterone can’t act in target tissue
The uterus and upper vagina is absent because they can still produce AMH
Describe turners syndrome.
This is a woman with the chromosomes X0
It causes primordial germ cells to degenerate shortly after arriving at the gonadal ridge
This causes failure of gonadal development - infantile genitalia and mesonephric duct regression
What happens when men don’t get dihydrotestosterone in utero?
This is caused by a lack of the gene that encodes for 5-alpha reductase
This enzyme normally converts testosterone to dihydrotestosterone
This means their penis doesn’t develop properly, they have an enlarger clitoris, they have a blind vaginal pouch and internal testis.
They look female
Suddenly grow a penis and normal secondary sex characteristics at puberty
What part of the gastrula does the urogenital tract originate from?
The intermediate mesoderm
