Reproduction Week 1 Flashcards
Where does the urogenital tract develop from?
Intermediate mesoderm
Name and describe the three kidney structures involved in the development of the urinary system:
1) pronephros = 7-10 solid cell groups in cervical region, regresses by week 4 of development
2) mesonephros = develops in upper thoracic and upper lumbar regions, produces supporting cells for genital ridge, appears in week 4 after regression of pronephros
3) metanephros = the DEFINITE kidney that appears in week 5 and is functional by week 11.
- formed by two parts: metanephric cap and ureteric bud
What is the cloaca?
Posterior orifice that acts as an opening for the intestinal/reproductive/urinary systems in early development.
Is made from hindgut with endodermal lining
What is the ureteric bud?
A protrusion from the mesonephric duct that allows drainage of the developing kidney
How is the bladder formed?
- cloaca is divided by the urorectal septum and fuses with the cloacal membrane (weeks 4-7)
- anterior urogenital sinus and porterior rectal canal formed
- bladder forms from cranial part of urogenital sinus
- bladder begins to get lined with endoderm
- the trigone of the bladder forms from caudal part of the mesonephric duct
What is the allantois?
- sac-like outpouching of the hindgut involved in nutrition and excretion
What is the urachus?
- called median umbilical ligament later in life
- is fibrous remnant of channel between the bladder and umbilicus that used to drain urine in foetal during 1t trimester
- forms from the allantois during weeks 5-7
What are the three main steps in the formation of the genital system?
- Genital duct formation
- Gonad formation
- External genitalia formation
How are the genital ducts formed?
- 2 pairs of genital ducts form in weeks 5-6
- paramesonephric ducts (müllerian ducts) are more lateral and will remain in females
- mesonephric ducts (wolffian ducts) are more medial and will remain in males
What do the mesonephric ducts do in the foetus and what happens to them?
- they drain urine from the mesonephric kidney (2nd kidney)
- in males, testosterone causes these ducts to form the ductus deferens, ejaculatory duct and part of the epididymis
- in females, there is no testosterone made by the ovaries so the mesonephric ducts regress and form non-functional remnants
Describe the paramesonephric ducts and what happens to them:
- only remain in females as in males the sertoli cells make AMH = the development of these ducts is switched off and they degenerate
- have funnel shaped ends that open into the peritoneal cavity
- migrate caudally to pelvic region and approach in midline and fuse
- upper unfused parts form uterine tubes
- lower fused part forms uterovaginal primordium (will become uterus and vagina)
What cellular movements occur in the formation of the indifferent gonad?
- at week 5, gonads are indifferent and initially appear as pair of longitudinal ridges made from mesoderm called UROGENITAL RIDGES
- primordial germ cells from the yolk sac move to the genital ridge and form primitive gonads
- primordial germ cells must arrive by week 6 or else the ridges will not develop any further
- the primordial germ cells then form cord-like structures called primitive sex cords
How does the testes develop from the indifferent gonad?
- the Y chromosome of male has SRY (sex-determining region of the Y chromosome) and this is the testes determining factor
- causes sex cords to become horse-shoe shaped and split into somatic and germ cells
- sex cords break up forming seminiferous tubules
- tunica albuginea forms around each testes and separates cords from the epithelium giving structure
- tubules in testes are solid until puberty when sperm production begins
What do leydig cells produce?
Testosterone
What do sertoli cells produce?
AMH (anti-mullerian hormone)
What is the pathway of seminiferous tubules to form the ductus deferens?
Seminiferous tubules Retes testes Efferent ductules Epididymis Ductus deferens
How does the ovary develop from the indifferent gonad?
- females have no SRY and so no testosterone production
- other active signals are involved in female development (WNT4 = ovary determining gene)
- primordial germ cells undergo mitosis forming oogonia pool
- oogonia start meiosis but are then in meiotic arrest at month 4 of gestation and are called oocytes
- oocytes associate with follicular cells forming primordial follicles
How does the external genitalia of the foetus develop?
- cloaca and its membrane thickens forming a pair of cloacal folds
- cloacal folds join cranially forming genital tubercle but are divided caudally
- from the cloacal folds, urethral folds form anteriorly (called labia minora in F) and anal folds form posteriorly
- on either side of the urethral folds there are genital swellings
- genital swellings become scrotal swellings in males and labia majora in females
When does the external genitalia start to develop?
3 weeks after fertilisation
Where does the urethra form from?
The middle part of the urogenital sinus
How does the penis form?
Foetal testes produces androgens which cause the genital tubercle to elongate into phallus
What is the end part of the male urethra formed from (the external urethral meatus)?
Surface ectoderm
How do the bulbourethral glands form?
Bulbourethral glands: as outgrowths from penile urethra
How does the prostate form?
Prostate: as an outgrowth from prostatic urethra
How does the vagina form?
Upper part of vagina forms from fused mullerian ducts, lower part from two sinovaginal bulbs which are outgrowths from the urogenital sinus and fuse forming the vaginal plate (becomes a cavity)
What physical deformations can occur in female development?
- bicornuate uterus (double uterus due to failure of fusion)
- vaginal atresia (vaginal canal doesn’t form properly = block of solid tissue)
- absent uterus and vagina = failure of sinovaginal bulb formation and no uterus)
(the uterus is needed for inducing sinovaginal bulb formation)
What is Kleinfelter’s syndrome?
Males with XXY
- infertility
- gynaecomastia
- impaired sexual maturation
- leydig cells don’t make enough testosterone so low sperm count
What is androgen insufficiency syndrome/testicular feminising syndrome?
- male with internal testes and female phenotype
- testes make testosterone but a mutation on the X chromosome causes androgen receptor deficiency
- the testosterone cannot act on receptors so female looking genitalia
What is 5-alpha reductase deficiency?
- lack of 5-alpha reductase enzyme which converts testosterone -> DHT
- DHT needed for masculinisation of external male genitalia
- males have enlarged clitoris and internal testes
- at puberty they get hormone surge and grow penis
What is Turner’s syndrome?
XO
- failure of gonad development as the primordial germ cells degenerate once they reach the urogenital ridge
- there is mesonephric duct regression
- no proper gonads but a ‘streak gonad’
- features include webbed fingers and webbed low-set neck
How are steroid hormones synthesised and what type of receptor do they bind to?
From enzymatic modification of cholesterol
Nuclear receptor
How are oestrogens formed?
- two main oestrogens are oestrone and oestradiol
- oestrone forms from ovaries direct OR from conversion of androstenedione
- oestradiol forms from granulosa cells OR is converted from oestrone
Cholesterol -> pregnenolone -> progesterone ->
androstenedione —-> testosterone
I I (aromatase)
I I
v ————————-> v
oestrone oestradiol
How are androgens produced?
- adrenal cortex makes ~50% and ovaries make some
- in females, testosterone gets converted to oestrogen, so it is essential we make other androgens
- androgens are DHEA and androstenedione
See week 5 page 6 lecture revision
How are progestogens (progestins) produced?
- from cholesterol -> pregnenolone -> progesterone
What are the functions of progestogens?
- endometrial development
- smooth muscle control
- maintaining placenta in pregnancy
- mammary gland development
What is the function of FSH?
- initiates recruitment of follicles and follicle growth and development
What is the function of LH?
- supports theca cells and the LH surge triggers ovulation
What is the function of inhibin?
Is secreted by egg after ovulation and this inhibits FHS secretion providing negative feedback so that no more follicles are triggered to develop at that time
What hormone changes occur if an egg is implanted?
The CL does not degenerate and it is supported by hCG that the developing embryo produces until the placenta has developed and can take over
How do ovulation testing kits work?
measure LH levels, as when these are at a peak you are about to ovulate