Sex Determination and Development Flashcards
What is the difference of sexual differentiation and sex determination
Sexual differentiation - Process by which females and males become structurally and functionally dissimilar.
Sex determination - initiation of the male differentiation pathway
What is the genetic difference between males and females
Females are homogametic (46, XX) Males are (46, XY) heterogametic
What is the genetic determinant of sex?
The presence or absence of the Y chromosome. If it is present then male gonads will form but if absent the female gonads will form.
Where are many of the genes for testies development located?
On autosomes or the X chromosome
How can males have a genetic profile of XX?
Because section of the Y chromosome has translocated onto an autosome or X chromosome
What gene causes sex determination
SRY gene as it encodes a transcription factor that is responsible for turning on genes for testes differentiation and turning off ovarian genes. Once gonadal differentiation is initiated, gonadal hormones drive the rest of differentiation.
Describe the formation of the indifferent gonads
They develop from two cell types (somatic mesenchyme or primordial germ cells) PGCs originate from the epiblast they then migrate to the genital ridge (driven by chemotaxis) while undergoing mitosis. PGCs are then surrounded by primitive medullary sex cords. In males and females the gonads are identical until week 7.
What is the genital ridge formed from?
Proliferation of surface epithelium and condensation of mesenchyme forming sex cords.
Describe how the indifferent gonad differentiates into the male gonads
Under influence of SRY gene, the primative sec cords proliferate and penetrate the medulla forming the testis cords. They become looped and contact the ingrowing mesonephric tubule called the rete testies. Mesodermal cells differentiate into sertoli cells. Mesenchyme tissue in interstital spaces develop into leydig cells.
Describe how the indifferent gonad differentiates into the female gonads
In the absence of the Y chromosome degeneration of the medullary cords occurs. Cortical cords from proliferating surface epithelium form distinct cell clusters. Epithelial cells proliferate around each oogonium forming primordial follicles.
Describe the formation of the ginital ducts
Mesonephric (wolffian) ducts give rise to male genital ductal system.
- Paramesonephric (Mullerian) ducts give rise to female
Summarise the development of the internal genitalia in the male
SRY results in activation of SOX9 gene which inhibits the WNT4. SOX9 goes on to activate SF1 and other genes resulting in the formation of the testes (medullary cords then develop)
Summarise the development of the internal genitalia in the female
WNT4 causes activation of DAX1 which inhibits SOX9, and activation of other genes both of which result in the formation of the ovaries (cortical cords develop).
How does the vagina form?
- Lower vagina forms from paramesonephric tubercle. Two evaginations grow out from the pelvic side and proliferate forming the vaginal plate
Describe the development of the external genitalia in the females
In the absence of androgens then the Genital tubercle will form the clitoris, the genital swellings will form the labia majora, urethral fold forms labia minora and the urethral groove forms urethral orifice.
Describe the development of the external genitalia in the males
Occurs in the presence of androgens. Genital tubercle forms the glans, the genital swellings becomes the scrotum
What is Turner syndrome?
45 XO - where female lacks an X chromosome. Therefore they have a deficiency in ovarian steroids and a lack of secondary sex characteristics and infertility. They have a short stature, webbed neck and skeletal deformities.
What is Klinefelter syndrome?
47 XXY - Males who have an extra X chromosome. Therefore there is incomplete virilization and breast enlargement after puberty. They have small testes with decreased spermatogonia
What is psudo-hermaphroditism?
Individual with gonads of the appropriate to their genotype but their external genitalia is of the opposite sex. Eg, androgen-insensitivity syndrome, congenital adrenal hyperplasia
Describe features of Androgen-Insensitivity syndrome
- Testes are initially normal but the tissue lacks androgen receptors so cannot respond to testosterone. Therefore the mesonephric ducts degenerate but since the testes secrete AMH the female ducts also degenerate. Female external genitalia develop but have undescended testes.
Describe features of congenital adrenal hyperplasia
Genotype is XX however foetal adrenals are over-active so secrete large amounts of steroid hormones, some of which have androgenic action. This causes development of mesonephric ducts and formation of male external genitalia but no AMH is secreted so female ducts persist.
