Development if the urogenital system Flashcards
Where does the urogenital system originate from?
intermediate mesoderm
What are the structures formed prior to the kidney?
o Pronephros – cervical region
o Mesonephros – abdominal region
o Metanephros – pelvic region
formed in a cranial to caudal and chronological sequence
Describe the pronephros
Rudimentary and non-functional
Mesoderm solidifies into cell clusters
7-10 solid cell groups form in the cervical region
These cell clusters are called the PRONEPHROS
Cells organise themselves to try and emulate what a kidney should look
like
Regresses by week 4 – once they form, disappear after clustering
(so never functional as a kidney)
This is like the “fore-kidney”
Tubules don’t do much, disappear early
o probably because don’t have time to become functional before disappear
What is the mesonephric duct?
– very important structure
o Associated with the mesonephros
o Appears around week 5
o Does not disappear when the rest of the mesenephros does duct system remains
o Tubules from mesonephros drain into the mesonephric duct at this stage
o NB: Although it is functional as a kidney and forms tubule shapes that have all the necessary
equipment to become functional, it does not go on to form the functional kidney
o Rudimentary in humans but has functions in other animals
Describe the metanephros
appears in week 5 after degeneration of mesonephros
Fully functional by week 11 (produces urine from week 11/12)
Forms excretory units of mature kidney
Excretory units develop from metanephric mesoderm
formed from 2 distinct parts:
- Ureteric bud
- Metanephric cap
Describe the uteric bud
Outgrowth in the bottom of the mesonephric duct
Derived from intermediate mesoderm
Source of all duct systems of the kidney
Becomes associated with the cells in the vicinity – metenephric mesenchyme
Forms a circular area surrounding the ureteric bud metanephric
Describe the metanephric cap
Mesenchyme
Becomes associated with cells in the vicinity of the ureteric bud
Forms a circular area surrounding the ureteric bud
Cloaca:
o Structure at the bottom of the developing gut
o Posterior orifice that serves as the only opening for the
intestinal, reproductive and urinary tracts at early stages
o Acts like a sewer at the bottom end of the embryo
o Collects waste – urine and digestive waste
o Lined with endoderm
Ureteric bud:
o Protrusion of mesonephric duct
o Allows urine drainage from the developing kidney
Allantois
o Sac like structure involved in nutrition and excretion
o Invagination of hindgut
o Greek for “sausage shape”
o Not functionally developed at this stage in humans
o Vessels in the allantois become important in the vessels of the umbilical cord
o Taken over by the placenta later in development => route to the umbilicus
Urachus:
o Duct between the bladder and yolk sac
o Forms from the allantois at around 5-7 weeks
Give an overview of the formation of the bladder and urethra
The cloaca is hindgut to begin with (Endodermal lining)
Urorectal septum divides cloaca into two halves by fusion with cloaca
membrane to form:
o Anterior urogenital sinus forms the bladder
o Posterior rectal/anal canal exit for digestive waste
o Happens between weeks 4-7
NB: septum is mesodermal in origin
Bladder forms from the urogenital sinus and caudal parts of the
mesonephric duct
Describe the division of the cloaca
o Septum pushes through until it reaches the membrane
o Critical to separate digestive/urinary waste
Everything above it gives rise to the bladder
Ureter needs to grow from bladder into the kidney to connect things together => provide a natural exit
for excretory waste
Describe the formation of the bladder
Forms from cranial part of the urogenital sinus
o Apart from the trigone region
Right at the back of the bladder
Derived from mesonephric ducts
All of the mesonephric duct that grows into the bladder becomes absorbed
Ureter starts to grow out from where the mesonephric duct used to be
Bladder is lined with endoderm
Endoderm lining the bladder is derived from the cloaca, but the structures
surrounding it are mesodermal in origin
When does the genital system develop?
Genetic sex is determined at fertilisation
Gonads do not acquire male or female morphological characteristics until week 7
Development then follows a sequence of events
What are the three sections of reproductive tract development?
Three main sections:
- Genital duct development
- Gonadal development
- External genitalia development
Describe the genital duct development during the indifferent stage
2 pairs of genital ducts develop in weeks 5-6
Form laterally to the mesonephric ducts (Wolffian) = MALE
o NB: females initially have mesonephric ducts, but they need testosterone to be maintained
o This only happens in males; without testosterone, the
mesonephric ducts degenerate
The paramesonephric ducts (Mullerian) = FEMALE
o Develop lateral to mesonephric ducts
What are the mesonephric (wolffian ducts)?
Drain urine from the mesonephric kidney
Switch from being a kidney structure to a reproductive structure
What is the function of the mesonephric (wolffian ducts) in the male?
Play an essential role in the development of the male reproductive system
Under the influence of testosterone, ducts differentiate to form
o ductus deferens
o ejaculatory duct
o These develop when mesonephros vanishes
What is the function of the mesonephric (wolffian ducts) in the female?
o mesonephric ducts almost completely disappear, leave a few non-functional remnants
o Females don’t need ductus deferens/ejaculatory duct therefore does not need to be
maintained (redundant)
How do the paramesonephric (mullerian) ducts develop in the female?
Develop lateral to the gonads and mesonephric ducts
Form funnel shaped cranial ends which open into the peritoneal cavity
o Form infundibulum
Initially 2 duct structures
Migrate caudally, parallel to the mesonephric ducts until they reach the future pelvic region
5
o Duct systems migrate into the middle
o Approach each other in the midline
Cranial portion forms the uterine tubes
Caudal portions fuse to form the uterovaginal primordium
o Uterus and superior vagina
How do the paramesonephric (mullerian) ducts develop in the male?
Paramesonephric ducts degenerate due to the action of
anti-mullerian hormone (AMH).
o Causes regression of the mullerian
(paramesonephric) ducts
This is a protein made by the Sertoli cells of the testis
Describe the development of the gonads
Gonads initially appear as a pair of longitudinal ridges – week 5
o Indifferent at this stage
o Urogenital or gonadal ridges => give rise to future gonad
Mesoderm structure – projecting into the coelomic cavity
o Space in the embryo => mesoderm is growing a lot at this point and growing into the space
o This ridge forms the urogenital ridge => this is where the future gonad comes from
How do gametes fill the gonads?
Gonad comes from mesoderm but needs to be filled by the gametes
o Future eggs and sperm (building blocks of germ cells) need to be in the area where the gonads
are forming as they cannot enter the gonads after they have formed
o Germ cells are derived from primordial germ cells
Primordial germ cells – originate in yolk sac
o Migrate into the body from the yolk sac (an external area)
o Subset of epiblast cells
Move into the genital ridge via dorsal mesentery
o Forms the primitive gonads
- The germ line is separate from somatic cells.
- Primordial germ cells form in a “cord like” structure, the primitive sex
cords