Development of the Urogenital System Flashcards
What is the major urogenital structure formed by the intermediate mesoderm? What will this germ layer generally give rise to?
Forms the urogenital ridge in the dorsal body wall
Germ layer gives rise to kidneys and gonads
What are the three stages of kidney development?
- Pronephros - 4th week
- Mesonephros - 4th to 9-10th week
- Metanephros - 5th - maturity
What does the pronephros do and what is its fate? What region of the body is it in?
It induces the underlying mesoderm in the cervical region to segment and form the pronephric ducts, but it never forms a functional nephotome, and completely disappears when the mesonephros starts forming
What is the kidney functioning in the mesonephros stage? How does it degrade?
One pair of excretory tubules will be formed per somite, with an afferent glomerular arteriole arising directly from the dorsal aorta. Forms functional renal corpuscles
Degrades segmentally from cranial to caudal.
Where does the mesonephric duct extend? Where does it empty?
From the upper thoracic region through the middle lumbar region, empties into the UG sinus and cloaca. The lower section of this duct will become the genital ducts in the male.
What is the ureteric bud and where is it capped?
An outpocketing of the distal mesonephric duct which will form the metanephric kidney (intermediate mesoderm)
It is capped everywhere but medially, which will form the hilus
How many major calyces are formed, and how are the minor calyces + collecting tubules formed?
3 major calyces are formed from subdivisions of the renal pelvis (dilation of ureteric bud).
Minor calyces - formed by consolidation of 3rd and 4th divisions
Collecting tubules - divisions 5-12 of the branching of the ureteric tree
How are nephrons induced in the developing metanephric mesoderm?
Collecting tubule induces metanephric tissue caps to form cell clusters called renal vesicles. Capillaries approach the renal vesicles and form the glomerulus, while the renal vesicles form all parts of Bowman’s capsule up until the distal convoluted tubule, which links with the collecting tubule and drains into collecting duct.
Where do the kidneys ascend from and why?
Kidneys form within pelvic region, ascend because of diminished body curvature of embryo and increased growth and elongation of lumbar and sacral regions because of development.
They stop when they contact the suprarenal glands
How does blood supply to the kidney change overtime?
Initially they recieve blood from the common iliac, but they receive new branches from the aorta as they ascend
What is the role of WT1 in nephron formation? What are two things that are important for tubule formation?
WT1 is secreted by metanephric mesoderm which stimulates them to release two factors, which induce the ENDODERM (from UG sinus) of the collecting tubule to release FGF2.
FGF2 stimulates proliferation of the mesoderm and continued release of WT1. Also remember PAX2 and WNTs for tubule formation
What forms the trigone of the urinary bladder?
Initially, the ureteric bud empties into the mesonephric duct which empties into the UG sinus. Whenever the bladder is formed, the caudal part of the mesonephric duct is absorbed into the bladder, causing the ureteric bud to empty directly into the wall of the bladder
How is the mesothelial lining of the ureter changed to endothelium?
Whenever it is replaced by the epithelium which covers the allantois and cloaca
What is the urogenital sinus formed from, and what separates it from the anorectal canal?
It is formed from allantois, and is in communication with the anorectal canal via the cloaca until the urorectal septum partitions it.
What does the terminal part of the allantois become?
A thick fibrous cord called the urachus or median umbilical ligament, which remains connected to the apex of the urinary bladder
What does the mid portion of the allantois / UG sinus become?
Urinary bladder
What does the narrowed part of the UG sinus become in males and females?
The pelvic part of the UG sinus.
Male: Prostatic and membranous urethra
Female: Membranous urethra only
What does the terminal part of the UG sinus become in males?
penile urethra
What germ layer does the prostate form from?
Endoderm, it is a proliferation of the epithelium of the urethra just below the neck of the bladder. It grows into the surrounding mesoderm
What germ layers give rise to the adrenal cortex layers?
All mesoderm from dorsal mesentery + developing gonad.
First wave: acidophilic mass forming fetal cortex which will become zona reticularis
Second wave: definitive cortex which will give rise to zona fasciculata and glomerulosa
What gives rise to the adrenal medulla?
Neural crest cells (chromaffin cells are basically postganglionic neurons)
What is SRY?
the sex-determining region of the Y chromosome, on its short arm. Signals male development, away from default of female development.
When does sexual differentiation begin?
Around the 7th week - prior to this is the indifferent stage of sexual development
Where do the primordial germ cells come from?
They migrate from the epiblast of the embryo via the primitive streak, and make their way to the endoderm of the yolk sac in close proximity to the allantois. They then migrate through the dorsal mesentery and reach the primitive gonads by week 5
Where is the gonadal ridge and what happens when primordial germ cells arrive?
Condensation of mesoderm on inferior/medial side of the developing mesonephros. When primordial germ cells arrive, they become the epithelium of the ridge and penetrate the underlying mesoderm to form the primitive sex cords
What are indifferent gonads?
Primitive sex cords that formed on the inferior/medial side of the mesonephros
What does SRY gene encode for?
Testis determining factor, which signals the primordial gonadal tissue to form the testis
What two cords arise in the male in the developing testis?
- Medullary cords / testis cords - arise from proliferation of primitive sex cords
- Rete testis - smaller tubules dorsal to the medullary cords which will connect with the efferent ductules
What are the origins of Sertoli cells, primordial germ cells, and Leydig cells?
Sertoli cells, primordial germ cells - endoderm
Leydig cells - mesoderm
What does not happen in males until puberty?
Testis cords remain solid until onset of puberty, when they canalize and become seminiferous tubules.
How do the tunica albuginea of males and females differ?
Over testes: thick
Over ovaries: very thin
What suspends the ovary off the posterior pelvic wall?
The mesovarium
What do the primitive sex cords become in females? What is the other relevant set of cords and what is its function?
Primitive sex cords proliferate into primordial germ cells which will form the 2 million primordial follicles of the ovaries.
Epithelium (endoderm) will give rise to cortical cords which will encircle each primary oocyte and act as the follicular cells.
What is the function of the follicular cells?
Surrounds oocytes and releases oocyte maturation inhibitor until puberty begins
What is the paramesonephric duct also called? Where does it form and how?
Mullerian duct - forms on anterior-lateral surface of UG ridge via an invagination of epithelium
What happens to the paramesonephric duct cranially and caudally?
Cranially - opens into abdominal cavity via a funnel-like opening
Caudually - crosses mesonephric duct and merges with its partner before opening into the UG sinus
What does the opening of the paramesonephric duct into the UG sinus become?
Becomes the sinus tubercle in both sexes.
What does the sinus tubercle become in males and females?
Males: Seminal colliculus (posterior wall of prostatic urethra)
Females: Sinovaginal bulbs - forms distal part of vagina and the hymen
What is the mesonephric duct called and what happens to it in males and females?
Wolffian duct - stimulated to stay by SRY
Males: Seminal vesicles, ejaculatory ducts, ductus deferns, epididymis, and efferent ductules of testis
Females: Degrades due to lack of SRY
Why does the Mullerian duct degrade in males?
Antimullerian inhibiting substance is secreted by Sertoli cells in the testis causes its regression
What is the fate of the Mullerian duct in females and why does it develop?
Lack of SRY stimulates WNT4, which promates formation of ovary. Mullerian duct will differentiate into uterine tubes, fimbriae, uterus, cervix, and upper vagina (distal part is from sinovaginal bulbs)
What part of the Wolffian duct + mesonephric tubules are retained by SRY signalling?
Most of the duct except the most cranial portion as well as the excretory mesonephric tubules near the developing testes.
What forms the efferent ductules of the testes?
The epigenital tubules of the mesonephric kidney which are on the cranial pole of the testis. The paragenital tubules will degenerate
What does the most cranial portion of the Wolffian duct become in males?
The appendix epididymis
What two structures does the Mullerian duct become in males?
- Appendix of testes
- Prostatic utricle
both blind-ended sacs
What does the Wolffian duct become in males?
Epididymis, vas deferens, seminal vesicles, and ejaculatory duct in developing prostate glands
What is the primary transcription factor that makes it so the Wolffian duct degrades in females and what are its remnants?
WNT4, inhibits synthesis and release of AMI, and SRY absence prevents FGF9 synthesis and release.
It persists as epoophoron and paroophoron in the mesovarium
What are the three parts of the paramesonephric duct?
- Cranial vertical-oriented part opening into abdominal cavity
- Horizontal part that crosses the regressing mesonephric duct
- Caudal part that fuses with its partner and is anchored to UG membrane
What do the first two parts of the paramesonephric duct form?
Fimbrae and uterine tubes
What happens to the urogenital ridges in women? What does this do to the relative orientation of the paramesonephric ducts and mesonephric ducts?
They rotate medial-caudally so that the right and left distal portion of the second part of the paramesonephric duct become approximated.
Paramesonephric ducts become more medially approximated, with the regressing mesonephric ducts lateral to them
What is the bridge across the floor of the pelvis containing the ducts of the female called after the UG ridges converge / rotate?
Broad ligament of the uterus
What are the components of the broad ligament? What does its mesoderm do?
Mesoderm gives rise to its mesenteries: Mesometrium, mesovarium, and mesosalpinx, as well as muscular layers + connective tissues of tubular structures.
Makes the uterus, ovaries, and uterine tubes intrapreitoneal
What two compartments does the formation of the broad ligament create?
- Uterorectal pouch - posterior to ligament
2. Uterovesical pouch - anterior to ligament
What is the uterine septum?
The septum between the right and left 3rd part of the paramesonephric tubes which is canalized to form the uterus
What forms the two parts of the vagina?
Upper 1/3 - paramesonephric dcts
Lower 2/3 - elongating and recanalizing sinovaginal bulbs, from the sinus tubercle opening into the UG sinus
What forms the hymen?
Double-layed membrane, inner mesodermal outer epithelial. It is derived from the urogenital membrane, and is normally slightly opening by recanalization prior to birth
What are the cloacal folds and what do the two halves become?
The two folds of the cloacal membrane in the indifferent stage of external genitalia development.
Cranial half: urethral folds
Caudal half: anal folds
What are the genital tubercle and the genital swellings?
Genital tubercle - midline swelling which will become penis or clitoris
Genital swellings - lateral to cloacal folds - becomes scrotal swellings in male or labia majora in female
What are the urethral groove and urethral plate?
Elongating phallus pulls urethral folds forward, which will become the lateral walls of the urethral groove. The groove does not extend into the phallus. These folds fold over the groove and urethral plate to form the penile urethra. The endoderm epithelial lining is the urethral plate.
What is the penile raphe?
Seam joining two urethral folds on the ventral surface
How does the penile urethra extend into the glands of the penis?
Infiltration of ectodermal cells forming an epithelial cord, which will recanalize to form the external urethral meatus and navicular fossa
What is the scrotal raphe?
The structure formed by the increased size and fusion of the scrotal swellings and they move caudually and towards midline, before the testes descend
What do the urethral folds become in the female?
Labia minora
The genital swellings become the labia majora
What forms the vestibule of female external genitalia?
Urogenital groove which remains open between the labia minora
How does the descent of the testes and vas deferens differ from the ovaries?
They remain retroperitoneal structures in the posterior abdominal wall, within the urogenital mesentery
What is the gubernaculum and what is its function?
A mesenchymal condensation on the caudal pole of the gonad which passes through the inguinal canal between the transversus abdominis and the internal abdominal oblique of the anterior wall
Acts as a guided path for forming the process vaginalis
What is the extraabdominal portion of the gubernaculum?
Part that passes through the inguinal canal out to the scrotal swellings?
How will the testis follow the process vaginalis? What does it become in the adult?
It will follow the posterior wall of the process vaginalis, and in the adult the anterior surface will have a visceral and parietal layer of peritoneum known as the tunica vaginalis
What causes renal agenesis, and what is its primary fetal symptom if it is bilateral?
Caused by ureteric bud not forming due to problem with metanephric duct. Causes oligohydramnios in bilateral cases, and is fatal
What is an ectopic kidney and what is one difference from normal kidneys?
Abnormally positioned kidney normally in pelvis or caudal region of abdomen. Kidneys maintain their function but often have accessory renal arteris
What stops the ascension of a horseshoe kidney?
Inferior mesenteric artery
What is one possible complication of an accessory renal artery?
Arteries can impinge on ureter and restrict urine flow
What are several forms of multiple kidney and what is the root cause?
Results from a bifurcation of ureteric bud, which each metanephric cap can form a different ductal system.
- Divided kidney
- Two kidneys sharing same ureter
- Two independent kidneys with separate ureters
What is a urachal fistula? What is the symptom?
Structure forming as a result of failure of intraembryonic portion of allantois to regress and form urachus, leading to urine draining in umbilicus
What is a urachal cyst vs sinus:?
Cyst - portion of allantois fails to regress into median umbilical ligament
Sinus - Allantois remains open to umbilicus but not in communication with bladder
What is an ectopic ureter?
Rare opening of a ureter into places like wall of vagina, urethra, or vestibule (bypassing bladder)
What is uterus bicornis?
Two uteruses opening into a single vagina, actually the case in most mammals. Very common
What is uterus didelphys?
Double uterus with double vagina
What is the benign uterus malformation condition?
Uterus arcuatus, where internal region of fundus of uterus is indented
What are some atresias of the vagina / uterus?
Vaginal atresia - there is no vaginal opening to the outside
Cervical atresia - there is no cervical opening to the vagina (can be unilateral or bilateral since two tubes come together to form the uterus)
What is hypospadias?
Incomplete fusion of urethral folds on ventral surface
What is epispadias and what often accompanies it?
Where urethral meatus is located on dorsum of penis and is often associated with exstrophy, congenital malformation of anterior wall of bladder + abdominal wall in front of it
What is the most common anomaly resulting in obstruction of the female reproductive tract?
imperforate hyman / atresia. However, the vaginal orifice may have many abnormal types of openings
What causes rectourethral or rectovaginal fistulas?
Incomplete segregation of the cloaca by the urogenital septum. It is a hindgut abnormality
