2009 BGD-B Lecture Sexual Differentiation

From Embryology


Historic Image of female urogenital sinus development

This lecture is still available online with links to powerpoint slides as PDF for printing online lecture.

This current page is only in the early developmental stage and not yet complete. The genital system is anatomically and developmentally associated with the urinary system and is often described as the urogenital system.

Lecture Slides

The links below to PDF versions of lecture slides.


  • Understand the role of the Y chromosome in sex determination.
  • Understand the differences in male/female duct develpoment (mesonephric/paramesonephric).
  • Compare the development of the cloaca in the male and female.
  • Understand the developmental abnormalities in male and female development.

Lecture Overview

  • Gonad
  • Internal Genitalia
  • Reproductive Tract
  • External Genitalia
  • Development
  • Function
  • Abnormalities

Background Reading

UNSW Embryology

NIH Bookshelf

Textbook References

  • Human Embryology (3rd ed.) Larson Chapter 10 pp266-313
  • The Developing Human (6th ed.) Moore & Persaud Chapter 13 pp303-346
  • Before We Are Born (5th ed.) Moore & Persaud Chapter 14 pp289-326
  • Essentials of Human Embryology, Larson Chapter 10 pp173-205
  • Human Embryology, Fitzgerald and Fitzgerald Chapter 21-22 pp134-152

Genital System Development

3 stages

  1. Differentiation of gonad (sex determination)
  2. Differentiation of internal genital organs
  3. Differentiation of external genital organs

2nd and 3rd stages dependent on endocrine gonad

  • Long Maturation Time-course
    • Begins in embryo
    • Continues through fetal
    • Finishes in puberty

Sex Chromosomes

X Chromosome

  • 1400+ genes
  • 150 million base pairs
  • 95% determined

Y Chromosome

  • 200+ genes
  • 50 million base pairs
  • 50% determined


  • gastrulating mammalian embryo
  • cells signaled by neighbours
  • form primordial germ cells (pgc)
  • migrate into genital ridges which develop into gonads

Primordial germ cells develop into

  • eggs, if gonad is becoming an ovary
  • sperm, if gonad is becoming a testis

Gonad will develop into an ovary unless its somatic cells contain a Y chromosome

Human Molecular Genetics - Development of the germ line

Sex Determination

  • Humans (week 5-6)
    • Germ cells migrate into gonadal ridge
    • Gonads (male/female) identical at this stage (Indifferent)

Gonad development dependent on sex chromosome

  • Y present testes
  • No Y ovary

DNA with SRY Protein SRY protein binds DNA

Testes determining factor (TDF)

  • Transcription factor
  • Bends DNA 70–80 degrees

Internal Genital Tract

  • Female - paramesonephric - uterus
  • Male - mesonephric - ductus deferens

All embryos form paired

  • Mesonephric duct - see kidney development
  • Paramesonephric duct - Humans 7th week Invagination of coelomic epithelium Cord grows and terminates on urogenital sinus
  • Male Gonad (testes) secretes
    • Mullerian duct inhibitory factor (MDIF) which causes regression of paramesonephric duct
    • Testosterone which retains mesonephric duct

Caspar Friedrich Wolff (1733 – 1794) in 1759 was the first to describe the "Wolffian duct" also called the mesonephric duct.

Johannes Peter Muller (1801 – 1858) in 1830 was the first to describe the "Mullerian duct" also called the paramesonephric duct.

External Genital Organs

  • All embryos initially same (indifferent)
  • Testosterone - differentiates male
    • Male Hormone dependent anatomy

Endocrinology - External Genital Development

Genital Abnormalities

  • Chromosomal
  • Hermaphroditism
  • Gonadal Dysfunction
  • Tract Abnormalities
  • External Genitalia
  • Gonadal Descent


  • Turner’s Syndrome (Monosomy XO)
    • 99% non-viable embryos
    • Fail to sexually mature at puberty
  • Klinefelter’s Syndrome (47, XXY)
    • Begin normal male, become infertile
    • Tall, mental dullness, behaviour problems
  • Males (46, XX)
    • Develop as male, infertile adults
    • Part of SRY gene located on one X (see Sex Reversal in Humans)


  • True Hermaphroditism (46,XX)
    • Gonads both ovary and teste tissues
    • Ovotestes or ovary and testes
  • Male Pseudohermaphrodites (46,XY)
    • Gonads of one sex, external genitalia of opposite
    • Various causes
  • Female Pseudohermaphrodites (46,XX)
    • Gonads are ovaries, external genitalia ambiguous
    • Hyperplastic adrenals secrete androgens

Gonadal Dysfunction

Gonads fail to develop properly

  • Gonadal Dysgenesis
    • Swyer’s syndrome
    • 46,XX

Mixed Gonadal Dysgenesis (45,X/46,XY)

  • Primary Hypogonadism
    • Affected females 46,XX

Primary Hypogonadism

    • Defective anterior pituitary production of gonadotropin
    • Lack of gonadotropin-releasing hormone

Tract Abnormalities

Many different forms


  • Associated with other anomalies
  • Paramesonephric duct (0.1-0.5% of women)
  • several classifications
  • Unicornuate, bicornuate uterus


Ductus Deferens

  • Uni- or bilateral absence
  • Failure of mesonephric duct to differentiate

External Genitalia

  • Multi-factoral
  • Chromosomal, single gene, environmental
  • Developmental arrest gives ambiguous


  • Common male (1 in 300)
  • failure of urogenital folds to fuse
  • results in a proximally displaced urethral meatus

Gonadal Descent


  • One or both testes fail to descend into scrotum
  • 1:30 live male births
  • May be associated with other abnormalities

Undescended Ovaries

  • reasonably rare, also be associated with other uterine malformations (unicornuate uterus)

Virilization of a Genetic Female with Ovaries

  • Fetal androgens
    • congenital adrenal hyperplasia (CAH)
    • adrenal adenoma or hyperplasia
  • Maternal androgens
    • ovarian or adrenal tumors
  • Latrogenic
    • exogenous androgens or progestagens with androgenic activity

Androgen Insensitivity Syndrome

  • XY karyotype and presence of testes
  • Externally - develop female secondary sex characteristics
  • Internally - women lack Müllerian duct derivatives, have undescended testes


  • Hormone Axis LH & FSH
  • Difference in timing
  • increased gonadotrophin secretion and stimulate gonadal maturation
  • rise in testosterone and estradiol secretions in males and females respectively
  • positive feedback of estradiol in females also occurs during puberty
  • secondary sexual characteristics

Tanner Stages

  • External Genitalia
  • Mammary

Endocrinology - Tanner Stages

Mammary Glands

  • sex hormone secretion of ovarian hormones estrogen and progesterone
  • lead to reciprocal signaling between the epithelium and stroma
  • branching and elongation through the stroma forming extensive network of ducts
  • full development approx 20 years
  • mainly fat and connective tissue deposition
  • growth also influenced by other hormones
  • prolactin, corticoids, growth hormone

Integumentary Development - Mammary Glands

Sex and Brain Differentiation

  • Brains of males and females differ
    • regions specialized for reproduction
    • Also in other regions (controlling cognition, etc) where sex differences are not necessarily expected
  • Differentially susceptible to neurological and psychiatric disease
  • 2 sources of sexually dimorphic information
    • complement of sex chromosome genes
    • mix of gonadal hormones
  • sex differences in brain attributed to differential action of gonadal hormones
  • evidence for sex chromosome effects on both neural and non-neural systems
  • XX and XY cells differentiate even before they are influenced by gonadal hormones
    • even if exposed to similar levels of gonadal steroids


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Cite this page: Hill, M.A. (2021, December 9) Embryology 2009 BGD-B Lecture Sexual Differentiation. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/2009_BGD-B_Lecture_Sexual_Differentiation

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© Dr Mark Hill 2021, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G