development of urogenital system Flashcards
(23 cards)
urogenital system
The urinary and genital systems are closely associated, particularly in the penis where the final duct, the urethra is shared by sperm and urine
The urogenital tract originates from INTERMEDIATE MESODERM
urinary system- kidney structures
In humans, there are 3 sets of kidney structures during development
PRONEPHROS – cervical region
MESONEPHROS – abdominal region
METANEPHROS – pelvic region
They are formed in a cranial to caudal and chronological sequence
pronephros
Rudimentary and non functional
7-10 solid cell groups in the cervical region
Regresses by week 4
mesonephrons
Derived from intermediate mesoderm from the upper thoracic and upper lumbar segments
Week 4 after regression of the pronephros the first excretory tubules of the mesonephros appear
Contributes supporting cells to the genital ridge
metanephrons- definitie kidney
This appears in week 5 and is functional by week 11
Excretory units develop from the metanephric mesoderm
Formed from 2 parts
Ureteric bud and metanephric cap
cloaca
Posterior orifice that serves as the only opening for the intestinal, reproductive and urinary tracts at early stages
ureteric bud
Protrusion of mesonephric duct
Allows urine drainage from the developing kidney
bladder and urethra
The cloaca is hindgut (Endodermal lining)
Urorectal septum divides cloaca by fusion with cloaca membrane to form the
ANTERIOR UROGENITAL SINUS
POSTERIOR RECTAL/ANAL CANAL
Happens between weeks 4-7
bladder
From cranial part of the urogenital sinus
Apart from the trigone
Mesonephric ducts
Bladder is lined with endoderm
genital duct development
The indifferent stage
2 pairs of genital ducts develop in weeks 5-6
- The paramesonephric ducts– uterine tube, uterus and vagina
- Form laterally to the mesonephric ducts – ductus deferens, epididymis
mesonephric ducts
Drain urine from the mesonephric kidney
Under the influence of testosterone, duct forms the ductus deferens and ejaculatory duct when mesonephros vanishes
In an XX embryo – no SRY gene -almost completely disappears, leaves a few nonfunctional remnants
paramesonephric ducts
Develop lateral to the gonads and mesonephric ducts
Form funnel shaped cranial ends which open into the peritoneal cavity
Migrate caudally, parallel to the mesonephric ducts until they reach the future pelvic region
Approach each other in the midline
- Cranial portion forms the uterine tubes
- Caudal portion fuse to form the uterovaginal primordium
- Uterus and superior vagina
paramesopnephric - XY embryo
In a XY embryo – the paramesonephric ducts degenerate due to the action of anti-mullerian hormone. This is a protein made by the Sertoli cells of the testis
gonadal development
Gonads initially appear as a pair of longitudinal ridges – week 5 - indifferent
- Urogenital or gonadal ridges
Mesoderm structure – projecting into the coelomic cavity
Primordial germ cells – originate in yolk sac
Move to the genital ridge via dorsal mesentery
- Forms the primitive gonads
gonadal development- germ line/cells
Germ line is separate from somatic cells.
If PGCs don’t arrive into the gonadal ridge by week 6, the ridges develop no further
Gonad is “indifferent” until week 7
Primordial germ cells form in a “cord like” structure – primitive sex cords
gonads- testis
If the embryo is genetically XY, the Y chromosome encodes testis determining factor SRY
- Sex determining Region of Y chromosome
Acts on somatic cells
Proliferation of the sex cords
gonad differentiation- testis
Cords become horseshoe shaped
- Primitive germ cells and somatic cells
Cords break up into tubules
Leydig cells begin to produce testosterone
Sertoli cells produce anti-Mullerian hormone
Dense connective tissue forms
-Tunica albuginea
-Separates the cords from the surface epithelium
gonads in males
Testis cords are solid till puberty
- Acquire a lumen forming the seminiferous tubules
- Join with the rete testis
- Join with efferent ductules
- Rete testis and mesonephric duct link to form the ductus deferens
Wnt 4
Wnt 4 - “ovary determining gene”
Wnt 4 knockout mice have no paramesonephric ducts, similar human mutation
gonads- ovary
XX chromosome (or really the absence of SRY)
PGCs are necessary -failure to reach the genital ridge the gonad regresses
Week 6, somatic cells grow from the coelomic epithelium
Surface epithelium continues to proliferate
PGCs divide by mitosis → pool of oogonia
(enter meiotic arrest at beginning of 4th month of gestation, now “oocytes”
Oocytes become associated with follicular cells
primordial follicles)
external genitalia
Indifferent stage
Mesenchymal cells migrate to the cloacal membrane and form a pair of cloacal folds
They fuse to form the genital tubercle at the cranial portion
Caudally the cloacal folds are subdivided:
- urethral folds in front - form labia minora OR ventral aspect of the penis
- anal folds posteriorly
Genital swellings then appear on either side of the urethral folds
development of penis and scrotum
Driven by androgens (mainly DHT)
Rapid elongation of the genital tubercle which becomes the phallus
Urethral folds get pulled together and close which forms the urethra during the 4th month.
The genital swellings start to move caudally and become the scrotum
development of clitoris, labia and vestibule
Driven by oestrogens
Genital tubercle elongates only slightly to form the clitoris
Urethral folds and genital swellings do not fuse
- Urethral folds form the labia minora
- Genital swellings form the labia majora
The urogenital groove stays open, this forms a structure called the vestibule, which is where the vagina and urethra open into.
