St A - 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.
