Development of the urogenital system Flashcards
What mesoderm region does the urogenital system arise from?
Intermediate mesoderm
In kidney development, it is somewhat unique in that they form in 3 distinct steps/structures. They develop in what direction?
Name the 3 structures
Cranio-caudal
Pronephros
Mesonephros
Metanephros
The pronephros, develops in the future “cervical” region of the embryo. It is not a functional kidney - basically formed when the intermediate mesoderm starts to thicken and form solid cell clumps. How many of these clumps do you get?
After they have formed, they regress almost immediately. At the end of what week does this occur?
- 7-10
- End of week 4
As the pronephros starts to dissappear, the mesonephros starts to form. This is inferior to where the pronephros has informed, and appears a rod-like structure at the abdominal region. Whilst the mesonephros does the capability to act like a proper kidney, but does NOT.
Some cells from this mesonephron will migrate and contribute to what structure?
Like the prenephros, the mesonephros will also quickly???????
Genital ridge.
Regress.
What structure is the “definitive kidney”?
The Metaneprhos
The metenephros also forms (as the mesenphros starts to regress) from the intermediate mesoderm, but again a bit lower down. What week does it appear? What week does it become functional?
Appears at week 5
Functional by week 11
The Metenephros has two main parts - What are they called (again remember they are formed from the intermediate mesodern)
Ureteric bud
metanephric cap (metenephric mesenchyme also a name for this) - the mesdodern forms mesenchyme around the ureteric bud - TOGETHER THESE STRUCTURES WILL FORM THE DEFINITIVE KIDNEY.
The urteric bud and meteneprhic cap are both budded structrues that are from the MESONEPRHIC DUCT.
Outline what the cloaca does
- Required for waste removal from the devloping kidney, as well as for the intestinal and reproductive tracts at the early stages.
As mentioned elsewhere, the metenephric mesoderm (metenephric cap) and uteritic duct are both the key intermediate mesoderm derived structures that go on to form the definitive kidney.
What broad category of structures do each of these structures go onto form?
Urine production starts very early on in gestation. How many nephrons would you expect to find in the kidney at birth?
Metenephric mesoderm - Excretory units
Uteritic duct - collecting system
1 million nephrons at birth
What is the function of the uteric bud?
Allows urine drainage from the developing kidney
In order to consider how the bladder and the urethra form - we need to start thinking about the cloaca. The cloaca is hind gut in origin, and has an endodermal lining.
The cloaca must be divided, so that it can form exits for both the uro/genital and digestive systems. What weeks does this happen? What structures form?
Weeks 4-7.
Urogenital sinus anteriorly - which will form the bladder (as well as imput from caudal parts of mesonephric duct)
Rectal/anal canal posteriorly
Let’s look at how division of the cloaca leads to formation of the primitive bladder and anal/rectal canal.
Cloacal membrane will be typically found at the end of the embryo. In this membrane, a septum begins to form at the hindgut, which separates out where the rectum and primitve bladder will be. This septum travels in what direction? What is the name for this septum?
What region separates the two
Anteriorly
Uro-rectal septum
Perineal body
Whilst division of the cloaca forms the primitive bladder, further work must be done to make the full bladder. The bladder comes from the Cranial part Urogenital sinus. The only part that does not is what, and where does it come from?
What type of lining is the bladder?
- Trigones
- Mesenephric duct
Endoderm
At what week do gonads get male or female characteristics?
Week 7
The first part of genital duct development is known as the “indifferent stage”. The intermediate mesoderm will form the gonads. Along side the gonads, we get two pairs of genital ducts. These will become fully formed at weeks 5-6. One set of the ducts are associated with the female, whilst the other are associated with the male. Which is which?
Paramesonephric ducts (Mullerian) ducts - more laterall
Mesonephric (Wolffian) ducts
The mesonephric duct has a few roles. Firstly, it drains urine from the mesonephric kidney. They also play a really important role in the development of the male reproductive system this is HIGHLY hormone driven.
In males, this fate of this structure is determined by Testosterone. What structures are formed?
Conversley - what happens to the mesenphric duct in the female after the kidney has developed?
- Ductus deferens
- Ejaculatory duct
- Epidiymys
In females, it disappears - does leave a few non-functional remenants though.
How do the paramesonephric ducts (lateral to both gonads and mesonephric duct) contribute to genital duct development in females? What happens to them in males?
- The ends of the paramesonephric ducts start to widen - this forms the funnel shaped end of the uterine tubes - open into the peritoneal cavity
- They then undergo a lot of migration caudally, parallel to the mesonephric ducts until they reach the future pelvic region
-
Once that happens, they come together and FUSE at the caudal end - thus forming the uterus and the top part of the vagina
- important to note that the more cranial parts do not fuse and form the uterine ducts
MALES
- degenrate - due to the action of anti-mullerian hormone (ALH)
- made by the sertoli cells in the testis.
We have looked at the gentical duct development. Let us look at gondal development. This starts primarily at week ????
Gonads initially appear as a little bumps in intermediate mesoderm. (!Note that germ cells are not yet present in these structures). These bumps are known as what?
5
Pair of Urogenital ridges/gonadal ridges
An interesting point on gonadal development - the cells that will go onto form egg cells and gametes come from OUTSIDE the gonads. The primordial germ cells come from the epiblast, where they then migrate through the primitive streak and then reside amongst endoderm cells in the yolk sac (basically outside of the main embryo) at around Week?????
Why? Nobody knows. At around Week ????, the primordial germ cells migrate along the ????, arriving at the ?????? at around week ????, and invade the ??????? in the 6th week.
he primordial germ cells come from the epiblast, where they then migrate through the primitive streak and then reside amongst endoderm cells in the yolk sac (basically outside of the main embryo) at around Week 3
At around Week 4, the primordial germ cells migrate along the dorsal mesentery of the hindgut, arriving at the primitive gonads at around week 5, and invade the urogenital/gonadal ridges in the 6th week.
Shortly before and during arrival of primordial germ cells to the primitive gonads, the epithelium of the ????? proliferates, and epithelial cells pentrate the underylying mesenchyme.
When the primordial germ cells reach the urogenital ridges, they form a number of irregularly shaped cords - called ?
Shortly before and during arrival of primordial germ cells to the primitive gonads, the epithelium of the urogenital ridge proliferates, and epithelial cells pentrate the underylying mesenchyme.
When the primordial germ cells reach the urogenital ridges, they form a number of irregularly shaped cords - called Primitive sex cords, on top of the proliferating epithelium.
If SRY is present then males are go for launch. The cords (remember now reffered to as Medullary/Testis cords) will undergo the further proliferation, making them look horseshoe shaped. The primitive germ cells move in and mix with somatic cells.
- This movement breaks up with cords into tubules
- The somatic cells start to differentiate into ????? and ???? cells
- First cells produce testosterone - and lots of it
- second cells produce Anti-mullerian hormone which gets rid of the paramesonphric duct
- You then get a dense connective tissue around the gonad , forming the tunica albuginea - separating the cords from the surface epithelium
Leydig
Sertoli
Testis cord remain solid until puberty, where the form a lumen - making them turn in the ????????. Once they are canalised, they join the retis testis, efferent ductules and then the mesonephric duct (which forms what?)
Testis cord remain solid until puberty, where the form a lumen - making them turn in the seminiferous tubules. Once they are canalised, they join the retis testis, efferent ductules and then the mesonephric duct - forming the epididymys and ductus efferents
So what are the stages that lead to fermale gonad development?
Need to be XX and not have SRY.
The primitive sex cords will the dissocitae in clusters - not that they are not tubular structures like those we saw in men - that are full of primitive germ cells. The clusters are located in the MEDULLARY (i.e centerish parts) part of the ovary.
The surface epithelium contibues to proliferate a lot - this is important as at around week 7 it gives rise to second generation of cords- which penetrate the underlying mesenchyme - called ??? cords. The primitivee germ cells that have made it to the gonadal ridge dividing by mitosis - to produce the pool of ????? -which then enter miotic arrest (at around 4 months) . Obviously these oocytes need follicular cells to form the primordial follices - this is where the new cords come in - these cells split into isolated clusters. These clusters continue to proliferate andsurround each oogonium with a layer of epithelium called ?????? cells - thus forming primordial follices.
NOTE - That follicular cells are just primitive theca and granulosa cells
Cortical cords
Oogonia
Follicular cells
Brief summary of formation of male and female gonads
primitive germ cells migrate to the genital/gonadal ridge and form the primitive sex cords in both male and female.
- Males
- primitive sex cords develop into medullary/testis cords
- Secondary coritcal cords fail to develop
- Females
- primitive sex cords regress - no medullary/testis cords
- Secondary cortical cords develop from the epithelium
Development of the external genitalia starts at around week ???? after fertilisation. It is makred by pair of cloacal folds developing around the cloacal membrane. These cloacal folds will fuse together to form a structure called the???, cranial to the cloacal membrane. This structure will go onto form ???? in males and the ???? in females.
Caudally, the cloacal folds are subdivided into the ????? folds anteriorly and the ???? folds posteriorly.
On each side of the urethral folds (form labia minora in females,whilst cover the male urethra in the penis) another pair of cellular aggregations called the genital swellings form. In the male,these will form the ????, and in the female they will form the ????
Genital tubercle
glans Penis
Clittoris
Urethral folds and anal folds
Scrotum and labia majora
Obviously when their is either a male or female pathway - all of the decisions will be governed by HORMONES.
As mentioned before, paramesonephric ducts fuse together to form the top part of the vagina and the uterus. The lower part of the vagina has different origins- comes from the ????. This produces two outgrowths called sinovaginal bulbs. These bulbs proliferate and form a solid vaginal plate- this will eventually become canalised.
Urogenital sinus.
Variations and things that can go wrong in the development of the female reproductive tract.
Double uterus - failure of fusion of the paramesonephric ducts
Vaginal atresia - failure of canalisation of the vagina
Absence of vagina and uterus - failure of formation of sinovaginal bulbs. Can also be due to failure of formation of the paramesonpheric ducts - whcih is called Mayar Rokitansky syndrome - the upper vagina and uterus fail to develop - but lower part of vagina still forms.
