Embryology Flashcards
Define indifferent stage
First stage of gonadal development
- impossible to distinguish between male and female gonad
What happens during gonadal indifferent stage
Gonads begin as genital ridges
- pair of longitudinal ridges derived from intermediate mesoderm and overlying epithelium
In 4th week germ cells migrate from endoderm lining of yolk sac to genital ridges via dorsal mesentery of hindgut
Epithelium of genital ridges proliferates and penetrates intermediate mesoderm to form primitive sex cords
Testes development
Y chromosone contains SRY gene which stimulates the development of primitive sex cords to form testis cords
Tunica albuginae forms around cords - fibrous connective tissue layer
A portion of the testis cord breaks off to form future rete testis
The remaining cord contains two cell types
- germ cells
- sertoli cells - derived from surface of epithelium of gland
Leydig cells are located between testis cords
- begin testosterone production at 8th week
Ovary development
Primitive sex cords degenerate
Epithelium of gonad continues to proliferate producing cortical cords
In 3rd month cords break into clusters surrounding each oogonium (germ cell) with a layer of epithelial follicular cells - forming a primordial follicle
Indifferent stage of internal genitalia
In first weeks all embryos have 2 pairs of ducts ending at the cloaca
- mesonephric (Wolffian) ducts
- paramesonephric (Mullerian) ducts
Development of male internal genitalia
In presence of testosterone mesonephric ducts develop to form primary male genital ducts
- give rise to efferent ductules, epididymis, vas deferens and seminal vesicles
Paramesonephric ducts degenerate in presence of anti-Mullerian hormone
- produced by sertoli cells
Development of female internal genitalia
In absence of testosterone mesonephric ducts degenerate
Absence of anti-Mullerian hormone allows for development of paramesonephric ducts
- cranial -> fallopian tubes
- horizontal -> fallopian tubes
- caudal -> fuses to from uterus, cervix and upper 133 of vagina
Indifferent stage of external genitalia
Mesenchymal cells from primitive streak migrate to clocal membrane to form pair of cloacal folds - 3rd week
Cranially folds fuse to form genital tubercle
Caudally divide into urethral folds and anal folds
Genital swellings develop either side of urethral folds
Development of male external genitalia
Driven by presence of dihydrotesosterone (DHT)
Rapid elongation of genital tubercle -> phallus
Urethral folds pulled to form urethral groove - extends along caudal aspect of phallus
- folds close by 4th month forming penile urethra
Genital swellings become scrotal swellings
- move caudally -> scrotum
Development of female external genitalia
Driven by oestrogens
Genital tubercle only elongates slightly to form clitoris
Urethral folds and genital swellings do not fuse but form labia minora and labia majora
Urogenital groove remains open -> vestibule
Descent of testes
Testes become more caudal as foetus grows
Pass through inguinal canal at 28th week
Reach scrotum by 33rd week
Retain original blood supply - testicular arteries branch from lumbar aorta
Scrotal ligament is remnant of gubernaculum
Descent of ovaries
Ovaries migrates caudally from position on posterior abdominal wall - stop within true pelvis
Gubernaculum -> ligament and round ligament of uterus
Spermatogenesis
Occurs in seminiferous tubules - blood/testis barrier formed by sertoli cells
Spermatogonia - initial pool of diploid cells divide my mitosis to give:
- Type A spermatogonia - replenish
- Type B spermatogonia - divide by meiosis
- meiosis I -> two haploid cells - secondary spermatocytes
- meiosis II -> four haploid cells - spermatids
Oogenesis
Primordial germ cells move to colonise cortex of primordial gonad
Begin meiosis I before birth -> primary oocytes
- arrested in prophase
Once puberty begins further maturation
- pre-antal - theca folliculi
- antral - secondary follicles
- preovulatory - first polar body and secondary oocyte
Complete meiosis II following fertilisation
