15 Sex Determination

Sex Determination and Gonadal Development

chromosomal sex determination  development of gonads  environmental sex determination 

Chromosomal Sex Determination in Mammals

Primary sex determination  determination of the gonads  strictly chromosomal X and Y  Y carries gene that encodes testisdetermining factor 

2ndary sex determination determination of bodily phenotype related to the organism's sex  does not affect the nature of the gonad  hormones from 

Primary sex determinatio n

Secondary sex determination ->

Mechanisms of mammalian primary sex determination

Male sex determinants 

SRY  Sexdeterminin g Region of the Y chromoso me

Association of DNA with the SRY protein can cause the DNA to bend 7080 degrees. The black structure represents the HMG box of the SRY protein. The red coil is the double helix of DNA specifically bound by SRY.

indirect mechanism  induces cells to secrete a chemotactic factor (g.r)  -> migration of mesonephric cells into XY gonad  ->gonadal

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SOX9  autosomal gene involved in sex determination of males  encodes a transcription factor that contains HMG (high-mobility group)

SOX9 (continued)  activates Amh gene in Sertoli cells (antiMullerian)

activated by Sry in mammals but not in other vertebrates

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1 functional allele ->

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SF1  steroidogenic factor  for the formation of bipotential gonad  remains at high levels in the XY genital ridge, not in XX

masculinizes the Leydig and Sertoli cells  works with SOX9 to activate Amh in the Sertoli cells  activates genes encoding enzymes that make

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Female sex determinants 

DAX1  potential ovarydetermining gene on the X chromosome  expressed in the genital ridge after Sry

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DAX1 (continued)  antagonist to function of Sry  downregulates SF1 expression

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WNT4  on the autosome  expressed in the genital ridge at it bipotential state  maintained in XX gonads in the absence

Secondary sex determination

Temporal phases  within embryo during organogenesis  during adolescence

AMH  from the Sertoli cells  causes degeneration of the Müllerian duct 

binds with mesenchyme around M duct  -> mesenchyme secrete paracrine factor  -> apoptosis of 

Testosterone  from fetal Leydig cells  causes Wolfian duct to differentiate into  epididymis  vas deferens

causes urogenital swellings to develop into:  scrotum  penis  if converted into 5 α-

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Insulin-like hormone 3  from the Leydig cells  required for the descent of the gonads into the scrotum

Estrogen  in females, causes differentiation of m.d.>   

uterus oviducts cervix

in males, regulates function of vas efferens in absorbing water from rete testis (> females)

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-> concentrates sperm

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Environmental sex determination

Temperature-dependent

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aromatase  converts testosterone to estrogen  < aromatase = male  > aromatase = female  activity is dependent

Location-dependent