Embryology Flashcards
When is the organogenetic period?
Weeks 4-8
When does maturation of organ systems occur?
Week 9 onwards
When does organogenesis of the reproductive system occur?
Extends beyond week 8 and even beyond the post-natal period
What system is the reproductive system linked closely to?
Urinary system (it forms a template for the reproductive system)
How many sets of kidneys develop?
3 sets
What is the first set of kidneys to develop?
Pronephros is the cervical region. They are vestigal
What is the second set of kidneys to develop?
Mesonephros in the thoracolumbar region. They are briefly functional
What is the last set of kidneys to develop?
Metanephros in the pelvic region. They are functional
What does the cloaca divide to form?
Rectum posteriorly and the urogenital sinus anteriorly
What does the mesonephric duct drain into?
The urogenital sinus
What does the urogenital sinus form?
The urinary bladder and the caudal end forms the urethra
What happens to the mesopnephros at about 6 weeks?
It forms a long ovoid structure in association with the thoracic and upper lumbar vertebra
What happens to the medial part of the mesonephric ridge?
It becomes thickened to form a genital/gonadal ridge
How are the indifferent gonads formed?
During week 6 germ cells migrate from the yolk sac, via the dorsal mesentery and embed into the primitive sex cords formed from the mesenchyme of the genital ridge
When are no gonads formed?
When there is failure of migration of germ cells to the genital ridge
How is the para-mesonephric duct formed?
Epithelium of the mesonephros invaginates to form a tube
Where does the paramesonephric duct develop?
Lateral to the developing gonads and mesonephric duct
What is the structure of the paramesonephric duct?
- Funnel shaped cranial end opens into peritoneal cavity
- Caudally, cross ventral to mesonephric ducts
- This projects into dorsal wall of urogenital sinus
What is the male reproductive system formed from?
- Gonad forms testis
- Mesonephric duct develops into epididymis, vas deferens, seminal vesicle
- Urogenital sinus forms bladder, urethra and prostate
What is the female reproductive system formed from?
- Gonad forms the ovary
- Paramesonephric duct forms fallopian tube, uterus, cervix part of vagina
- Urogenital sinus forms bladder and lower part of vagina
What happens to the paramesonephric duct in the male?
Degenerates
What happens to the mesonephric duct in the female?
Degenerates
How does the indifferent gonad know whether to develop into a testes or an ovary?
- SRY gene is expressed in certain cells (such as the primordial supporting cells within the sex cord) and it encodes for a protein called TDF
- TDF makes the indifferent gonad develop into a testis
How does TDF produce male genitalia?
- TDF transforms the primordial supporting cells into sertoli cells
- Sertoli cells secret MIS (MUllerian inhibiting substance) which transforms mesenchymal cells between cords to form Leydig cells
- Leydig cells produce testosterone leading to production of male external genitalia
- MIS also leads to paramesonephric duct degeneration
Describe the development of the testis.
- Week 5: primary sex cords, medulla and cortex
- Week 6: primordial germ cells incorporated into primary sex cords
- Week 7: TDF stimulates primary sex cords > extend into medulla> seminiferous cords> tubules
- Mesothelium lost when thick capsule forms
- Mesenchyme between cords five rise to Leydig cells
- Week 8: Leydig cells produce testosterone and other hormones which stimulate masculine differentiation
What are the seminal vesicles embryologically ?
Outgrowths from the caudal end of the mesonephric duct
What is the prostate embryologically?
Outgrowth from urethra (urogenital sinus)
What is the male external genitalia formed under the influence of?
Testosterone
How does the indifferent gonad develop into an ovary?
In a female foetus there is no SRY region and no TDF
-By default the indifferent gonad develops into an ovary
Describe the development of the ovary.
- Primary sex cords degenerate leaving the primordial germ cells
- Week 10: secondary sex cords develop from surface epithelium
- Primordial germ cells become incorporated into them
- Week 16: Primordial germ cells then bud off with single surrounding layer of cortical cells to form primordial follicles
- Active mitosis of these early primordial follicles to form additional follicles
- All follicles formed by 7th month
- Oocytes enter meiosis and arrested I prophase
What postnatal ovarian development occurs?
- ~2 million primordial follicles at birth
- Many become atretic before puberty
- After puberty, hormonal influences stimulate 12-15 follicles per 28 day cycle to develop
- Only one follicles reaches maturity and ovulation normally
- Continues until the menopause
Where do the tubular components of the female reproductive system come from?
- Absence of testosterone causes degeneration of mesonephros and mesonephric duct
- Paramesonephric ducts develop to give rise to female genital tract
What is the origin of the female genital tract?
- Cranial (open funnel) end of the paramesonephric duct becomes associated with ovary
- Caudal end of paramesonephric duct (uterovaginal primordium) becomes uterus and superior part of vagina
- Inferior part of vagina from urogenital sinus
- Mesonephric duct degenerates
Where do the gonads initially form?
In the lumbar region
Describe the descent of the testis.
- Week7: At T10 (diaphragm undescended)
- Week 12: level of the deep inguinal ring. Remain here until 7th month
- Enters scrotum just prior to birth (39 weeks)
Describe the descent of the ovaries.
- Week 7: At T10
- Week 12: Broad ligament
- Round ligament drawn through inguinal canal to attach to labrum majorum
What changes occur due to the descent of the gonads?
- Formation of the larger inguinal canal in males
- In the female the presence of the round ligament and its passage through the inguinal canal
What developmental defects can occur?
- Undescended testis
- Remnants (Gartner’s cysts)
- Anomalies of the paramesonephric duct in the female
What is cryptorchidism?
One or both testis fail to descend
What are the types of crytochidisim?
- Undescended
- Ectopic
- Retractile testes
- Absent testes
What are the possible complications of undescended testis?
- Infertility
- Malignant transformation (germ cell tumours)
- Testicular torsion
What anomalies can occur with the uterus?
- Complete duplication
- Bicornuate uterus
- Arcuate uterus
- Septate uterus
