Embryology of the Reproductive System Flashcards
How is genetic sex determined?
- genetic sex is determined at fertilisation by the sperm
- the SRY (sex-determining region of the Y chromosome) gene initiates production of Testis-determining factor (TDF)
- if there is no Y chromosome, there is no TDF and female development commences
What is the reproductive system derived from?
intermediate mesoderm
- the intermediate mesoderm gives rise to the urogenital ridge
- the urogenital ridge gives rise to the urinary system and reproductive system
How do the gonads first appear?
What type of cell appears in the gonads?
- gonads appear as a pair of longitudinal ridges (genital / gonadal ridges)
- gonadal ridges are formed by proliferation of the epithelium and condensation of the underlying mesenchyme
- the germ cells** do not appear in the genital ridges until **week 6
Where do primordial germ cells originate?
When do they enter the genital ridges?
- PGCs originate in the epiblast and migrate through the primitive streak
- they arrive at the primitive gonads at the beginning of week 5 and invade the genital ridges in week 6
- if PGCs fail to reach the genital ridges, the gonads do not develop
What is meant by the indifferent gonad?
How and when does this develop?
- shortly before and during arrival of the PGCs, the epithelium of the genital ridge proliferates and penetrates the underlying mesenchyme
- the epithelial cells form irregularly shaped cords - the primititve sex cords
- in male and female embryos, the cords are connected to surface epithelium
- at this stage (weeks 5-6) it is impossible to determine between the male and female gonad so it is the “indifferent gonad”
How can the indifferent gonad be split into 2 different regions?
- the indifferent gonad can be split developmentally into 2 distinct regions:
- outer cortex
- inner medulla
- the fate of medullary and cortical regions is dependent on TDF production
What is formed from the primitive sex cords if an embryo is genetically male?
What is carried by the PGCs?
- the PGCs carry an XY sex chromosome complex
- under the influence of the SRY gene on the Y-chromosome, which encodes TDF, the primitive sex cords continue to proliferate and penetrate deep into the medulla
- they form the testis** or **medullary cords
Following formation of the medullary cords, what else develops in pursuit?
Rete testis:
- towards the hilum of the gland, the cords break up into a network of tiny cell strands
- these will later give rise to the tubules of the rete testis
Tunica albuginea:
- this is a dense layer of fibrous connective tissue that separates the medullary cords from the surface epithelium
What happens to the testis cords in the 4th month?
What cell types are they composed of at this stage?
- the testis cords become horseshoe-shaped
- their extremities are now continuous with those of the rete testis
- the cords are composed of primitive germ cells and Sertoli cells
- Sertoli cells are derived from the surface epithelium of the gland
What cell type lies between the testis cords?
When do they begin development and what is their function?
interstitial cells of Leydig
- they are derived from the mesenchyme of the gonadal ridge
- they begin development shortly after onset of differentiation of the testis cords
- by week 8, they begin production of testosterone
- the testis is able to influence sexual differentiation of the genital ducts and external genitalia
What happens to the testis cords at puberty?
What does this structure enter and join with?
- testis cords remain solid until puberty, when they canalise to form the seminiferous tubules
- the seminiferous tubules join the rete testis tubules, which in turn enter the ductuli efferentes
- these are the remaining parts of the excretory tubules of the mesonephric system
- the ductuli efferentes link the rete testis and the Wolffian (mesonephric duct), which becomes the ductus deferens
What happens to the primitive sex cords in female embryos?
- female embryos have an XX chromosome complex and no Y chromosome
- the primitive sex cords dissociate into irregular cell clusters, which contain groups of primitive germ cells
- the cell clusters occupy the medullary part of the ovary
- later on they disappear and are replaced by a vascular stroma that forms the ovarian medulla
What happens to medullary cords in a female embryo?
What type of cord develops?
- the medullary cords degenerate
- the surface epithelium continues to proliferate (unlike in males) and gives rise to cortical cords in week 7
- cortical cords penetrate the underlying mesenchyme but remain close to the surface
What happens to the cortical cords in month 3?
What is formed at the end of this process?
- the cortical cords split into isolated cell clusters
- cells in the clusters continue to proliferate and begin to surround each oogonium with a layer of epithelial cells - follicular cells
- the oogonia and the follicular cells together constitute a primordial follicle
What is the difference in the structures that develop and regress depending on whether the spermatocyte carries an X or Y chromosome?
In presence of Y chromosome (44 + XY):
- medullary cords develop
- there are no cortical cords
- there is a thick tunica albuginea
In absence of Y chromosome (44 + XX):
- medullary cords degenerate
- cortical cords develop
- there is no tunica albuginea
During which week do the gonads become distinguishable?
the male and female gonads become distinguishable at 7 weeks
At the stage of the indifferent gonad, what pairs of genital ducts are present?
- both male and female embryos initially have 2 pairs of genital ducts:
- mesonephric (Wolffian) ducts
- paramesonephric (Müllerian) ducts
- the paramesopnephric duct arises as a longitudinal invagination of epithelium on the anterolateral surface of the urogenital ridge
- cranially, the duct opens into the abdominal cavity with a funnel-like structure
- caudally, it comes into contact with the paramesonephric duct from the opposite side in the midline
- the 2 ducts project into the posterior wall of the urogenital sinus to form the sinus tubercle (small swelling)
- the mesonephric ducts open into the urogenital sinus on either side of the sinus tubercle
What is the fate of the genital ducts in the male?
What hormones / substances drive these changes?
Sertoli cells:
- secrete anti-Müllerian hormone (AMH) which suppresses development of the paramesonephric ducts
- a small portion of the paramesonephric ducts remain as the appendix testis
Leydig cells:
- secrete testosterone which stimulates development of the mesonephric ducts, which will go on to form:
- efferent ductules
- epididymis
- vas deferens
- seminal vesicles
- dihydrotestosterone stimulates development of the external genitalia, including growth of the penis, scrotum and prostate
What happens to the genital ducts in the female?
What structures do they go on to form?
- in the presence of oestrogens (maternal and placental sources) and absence of AMH and testosterone…
- the paramesonephric ducts are stimulated and go on to form:
- uterine tube
- uterus
- upper portion of the vagina
- the external genitalia are stimulated:
- labia
- clitoris
- lower portion of vagina
At the indifferent stage, what does the external genitalia consist of?
- in week 3, mesenchyme cells migrate around the cloacal membrane to form a pair of elevated cloacal folds
- cranial to the cloacal membrane, the folds unite to form the genital tubercle
- caudally, the folds are subdivided into urethral folds and anal folds
- the genital swellings appear on either side of the urethral folds
- these will go on to form the scrotal swellings and labia majora