Sex Determination & Development Flashcards

Chromosomal and Genetic Determination of Sex

• Genotypic Sex Determination at Fertilization:

  • Chromosomal sex (genotype) is established during fertilization upon recombination of paternal and maternal gametes.
  • Mammalian sex determination:
  • Males possess an heteromorphic pair: $XY$.
  • Females possess a homomorphic pair: $XX$.
  • Avian sex determination:
  • Males possess a homomorphic pair: $ZZ$.
  • Females possess an heteromorphic pair: $ZW$.

• Environmental (Temperature-Dependent) Sex Determination:

  • Some reptiles (certain lizards and tortoises) lack heteromorphic sex chromosomes.
  • Sex is determined by environmental temperature during critical embryonic windows.
  • Alligator example:
  • Critical sensitivity period: $7\text{--}21\text{ days}$ post-oviposition.
  • Incubation temperature below $86^\circ\text{F}$ ($30^\circ\text{C}$) yields $100\%$ female offspring.
  • Intermediate temperatures yield a mix of male and female offspring.
  • Incubation temperature above $93^\circ\text{F}$ ($34^\circ\text{C}$) yields $100\%$ male offspring.

• Dosage Compensation and X-Inactivation:

  • In female mammals ($XX$), one of the two $X$ chromosomes randomly undergoes transcriptional inactivation early in development.
  • The condensed, inactive $X$ chromosome is cytologically visible as a Barr body along the nuclear membrane of female somatic cells.

• Tortoiseshell Cat Model:
  • Serves as a classical phenotypic example of $X$ chromosome inactivation mosaicism.
  • Each somatic cell expresses alleles from only one $X$ chromosome, resulting in a mosaic coat pattern of black and orange patches.

Molecular Markers and Embryo Sexing Technologies

• H-Y Antigen (Histocompatibility Y Antigen):

  • A male-specific cell-surface protein present on $XY$ cells and absent on $XX$ cells.
  • Immunological assays against H-Y antigen can determine embryonic sex with approximately $80\%$ accuracy.

• SRY Gene (Sex-Determining Region Y):

  • Located on the short arm of the $Y$ chromosome; encodes Testis-Determining Factor ($TDF$).
  • Testis differentiation is directly initiated by the $SRY$ gene rather than the H-Y antigen.
  • Transgenic/Mutational evidence:
  • Mouse models with $46,XX$ carrying a translocated $SRY$ fragment develop a male phenotype.
  • $46,XY$ individuals with non-functional $SRY$ gene mutations differentiate phenotypically as females.

• Diagnostic Sexing Methods:

  • Reverse Transcription-Polymerase Chain Reaction (RT-PCR):
  • Uses targeted primers to amplify $Y$-specific sequences ($SRY$) along with universal control sequences ($ZFY\text{--}ZFX$) across humans, horses, dogs, mice, cattle, sheep, and goats.

• Fluorescent In Situ Hybridization (FISH):
  • Utilizes fluorescently labeled DNA probes specific to the $Sry$ gene to hybridize against metaphase chromosome spreads or interphase nuclei (e.g., porcine lymphocytes or embryos).
  • Presence of a green fluorescent punctate signal confirms $Y$-chromosome identity.

Gestational Timeline and Comparative Embryology

• Stages of Embryonic and Fetal Reproductive Development:

  • First Trimester:
  1. Migration of primordial germ cells from the yolk sac endoderm to the genital ridge.
  2. Primary sex cords form within the gonadal ridge; paramesonephric (Müllerian) ducts develop parallel to mesonephric (Wolffian) ducts.
  3. Gross histological structural divergence makes phenotypic sex evident.
  • Second Trimester:
  1. Divergent differentiation of duct systems: male Wolffian development vs. female Müllerian development.
  2. Formation of the broad ligament of the uterus.
  • Third Trimester:
  1. Testicular descent into the scrotum.

• Species-Specific Timing of Testicular Descent:

  • Bull & Ram: Descends during mid-to-late gestation ($2\text{nd}$ to $3\text{rd}$ trimester).
  • Boar & Human: Descends during late gestation ($3\text{rd}$ trimester, near term).
  • Colt (Equine): Descends late in gestation or shortly after birth.

• Comparative Developmental Timing in Livestock:

• Cattle (Gestation: $275\text{--}290\text{ days}$):
  • Implantation: Days $20\text{--}40$
  • Testes Formation: Days $35\text{--}45$
  • Masculinization of Genitalia: Days $45\text{--}70$
  • Defeminization of Sexual Receptivity: Prenatal, Days $80\text{--}110$
  • Defeminization of LH Secretion: Prenatal/Postnatal, Days $80\text{--}60$
• Sheep (Gestation: $145\text{--}155\text{ days}$):
  • Implantation: Days $15\text{--}25$
  • Testes Formation: Days $30\text{--}35$
  • Masculinization of Genitalia: Days $35\text{--}45$
  • Defeminization of Sexual Receptivity: Prenatal, Days $50\text{--}80$
  • Defeminization of LH Secretion: Prenatal, Days $50\text{--}80$
• Swine (Gestation: $112\text{--}115\text{ days}$):
  • Implantation: Days $13\text{--}24$
  • Testes Formation: Days $26\text{--}30$
  • Masculinization of Genitalia: Days $30\text{--}45$
  • Defeminization of Sexual Receptivity: Postnatal, Days $90\text{--}180$
  • Defeminization of LH Secretion: Prenatal, Days $40\text{--}70$

Morphological Differentiation of the Urogenital System

• The Three Embryonic Renal Systems:

  • Pronephros: Primitive, non-functional kidney that completely regresses.
  • Mesonephros: Intermediate functional kidney during early embryonic development.
  • Drained by the Mesonephric Ducts (Wolffian Ducts).
  • Mesonephric tubules penetrate the primitive gonad and connect with sex cords via the rete testis to form the efferent ducts.
  • In males, mesonephric ducts form the epididymis and vas deferens (ductus deferens).
  • Metanephros: Permanent functional adult kidney.
  • Functional by the end of the first trimester ($1/3$ of gestation).
  • Originates as a ureteric bud arising from the caudal portion of the mesonephric duct.

• Ductal Systems and Morphogenesis:

  • Paramesonephric Ducts (Müllerian Ducts):
  • Form adjacent to the mesonephric ducts on the coelomic surface.
  • In females, fusion of the caudal paramesonephric ducts forms the uterus (uterine body and horns), cervix, and cranial vagina.
  • Urogenital Sinus Differentiation:
  • Cranial region forms the urinary bladder.
  • Caudal region forms the vestibule and caudal vagina in females, and the urethra/penile structures in males.

• Endocrine Cascades of Morphological Sex Differentiation:

• Male Pathway ($XY$):
  • $SRY$ gene expression $\rightarrow$ production of Testis-Determining Factor ($TDF$) $\rightarrow$ primitive gonad differentiates into a testis.
  • Sertoli cells secrete Anti-Müllerian Hormone ($AMH$):
    • Directs degeneration and regression of paramesonephric (Müllerian) ducts.
    • Induces differentiation of interstitial Leydig cells.
  • Leydig cells secrete Testosterone and $5\alpha\text{-dihydrotestosterone (DHT)}$:
    • Testosterone: Drives stabilization and differentiation of Wolffian ducts into the male duct system (epididymis, vas deferens, seminal vesicles).
    • $DHT$ (via $5\alpha\text{-reductase}$ conversion): Drives development of male external genitalia (penis, scrotum) and accessory sex glands (prostate, bulbourethral/Cowper's glands).
• Female Pathway ($XX$):
  • Absence of $SRY$ / $TDF$ $\rightarrow$ primitive gonad defaults to an ovary.
  • Absence of $AMH$ $\rightarrow$ paramesonephric ducts persist and differentiate into oviducts, uterus, cervix, and cranial vagina.
  • Absence of testosterone $\rightarrow$ mesonephric (Wolffian) ducts regress.
  • Female external genitalia and complete female tract form by default.

• Experimental Surgical/Physiological Evidence:
  • Fetal castration (male or female) prior to ductal differentiation results in female phenotypic development.
  • Presence of a functional testis (or testicular autotransplantation into a female fetus) forces male duct system development and Müllerian regression.

Pathophysiology and Clinical Disorders of Sex Development

• Definitions and Terminology:

  • Virilization: Biological development of masculine secondary sexual characteristics, typically mediated by elevated androgen exposure.
  • Hermaphroditism Classification:
  • True Hermaphrodite: Possesses both ovarian and testicular tissue (ovotestes, or separate testis and ovary). Karyotype mosaicism ($XX/XY$) is present in some cases, though over $60\%$ are $46,XX$.
  • Male Pseudohermaphrodite ($XY$): Possesses testes (frequently intra-abdominal) with female or ambiguous external genitalia.
  • Female Pseudohermaphrodite ($XX$): Possesses ovaries with masculinized external genitalia.

• Androgen Insensitivity Syndrome (AIS / Testicular Feminization):

  • Genotype: $46,XY$.
  • Pathophysiology: X-linked recessive mutation in the androgen receptor gene.
  • Endocrine Status: Testes produce normal male levels of testosterone and $AMH$, but target tissues lack functional $5\alpha\text{-dihydrotestosterone}$ receptors.
  • Phenotype: Appears phenotypically female at birth; paramesonephric ducts regress normally (due to $AMH$), but Wolffian ducts fail to develop and external male genitalia fail to masculinize.

• Congenital Adrenal Hyperplasia (CAH):

  • Genotype: $46,XX$.
  • Pathophysiology: Genetic defect in the $21\text{-hydroxylase}$ enzyme required for converting progesterone to cortisol in the adrenal cortex.
  • Mechanism: Lack of cortisol blunts negative feedback, elevating ACTH secretion. Excess progesterone precursors are shunted into adrenal androgen synthesis pathways.
  • Phenotype: Overproduction of adrenal testosterones causes severe virilization and masculinization of external genitalia in $XX$ females (female pseudohermaphroditism).

• $5\alpha\text{-Reductase}$ Deficiency:

  • Genotype: $46,XY$.
  • Enzymatic Block: Inability to convert Testosterone to $5\alpha\text{-dihydrotestosterone}$.
  • Phenotype: Internal male duct structures develop normally (testosterone-dependent), but external genitalia appear female or ambiguous at birth.

• Freemartinism in Cattle:

  • Etiology: Occurs in heterosexual twin pregnancies when chorionic vascular anastomosis occurs between twin placentas by Day $39$ (range: Days $23\text{--}42$).
  • Mechanism:
  • Male twin testes differentiate earlier (Days $35\text{--}45$) than female twin ovaries.
  • Testicular hormones ($AMH$ and testosterone) pass through shared placental circulation into the female fetus.
  • $AMH$ arrests paramesonephric duct development in the female, preventing normal formation of uterus, cervix, and anterior vagina.
  • $\text{XY}$ cellular chimerism is established in the female twin.
  • Diagnostic Marker: Y-chromosome specific DNA sequences detected in female peripheral blood via PCR.

• Chromosomal Aneuploidies and Dysgenesis Syndromes:
  • Klinefelter's Syndrome ($XXY$):
  • Nondisjunction event yields an extra $X$ chromosome.
  • Phenotype: Male with testicular hypoplasia/hyperplasia; sterile.
  • Turner's Syndrome ($XO$ / Monosomy X):
  • Absence of second sex chromosome ($45,XO$).
  • Incidence: $1\text{ in } 2500$ live female births; approximately $98\%$ of $XO$ fetuses undergo spontaneous abortion.
  • Phenotype: Female with non-functional streak/inactive ovaries; sterile.
  • Swyer Syndrome (Pure Gonadal Dysgenesis - PGD):
  • Karyotype: $46,XY$.
  • Phenotype: External female phenotype with normal vagina and uterus, but gonads exist only as non-functional connective tissue streaks; fails to undergo normal puberty due to absence of gonadal steroids.
  • Leydig Cell Hypoplasia:
  • Inadequate differentiation of fetal testicular Leydig cells.
  • Phenotype spectrum ranging from complete female external genitalia to males with micropenis, despite normal androgen receptor function.

Sexual Differentiation of the Brain and Hypothalamic Axis

• Endocrine Mechanism of Neuro-Sexual Differentiation:
  • Sexual differentiation of the brain is mediated by fetal gonadal steroid exposure during critical perinatal windows.

• Male Brain Differentiation (Defeminization & Masculinization):

  • Fetal testes secrete Testosterone ($T$).
  • Testosterone does NOT bind to $\alpha\text{-fetoprotein}$ ($\alpha\text{FP}$) and freely crosses the blood-brain barrier.
  • Neuronal aromatase inside hypothalamic neurons converts testosterone into estradiol ($E_2$).
  • Intracellular estradiol defeminizes the hypothalamus by eliminating the hypothalamic Surge Center.

• Female Brain Differentiation (Feminization):

  • Fetal ovaries secrete Estradiol ($E_2$).

  • Fetal liver produces $\alpha\text{-fetoprotein}$ ($\alpha\text{FP}$), a glycoprotein that binds circulating estradiol with high affinity.

  • The large $\alpha\text{FP}\text{--}E_2$ complex cannot cross the blood-brain barrier.

  • Unbound estradiol is prevented from entering the brain; lack of intracranial estrogen exposure allows the hypothalamic Surge Center to develop intact.

  • Hypothalamic Center Functionality:

• Female: Possesses both a Tonic Center and a Surge Center.

  • Tonic center regulates basal pulsatile GnRH release.
  • Surge center drives massive, preovulatory GnRH/LH surges in response to positive estrogen feedback.

• Male: Possesses ONLY a Tonic Center.

  • Surge center is defeminized (inactivated) during prenatal development.
  • GnRH release occurs exclusively in low-amplitude, tonic pulses.

• Structural Neuroanatomical Dimorphism (SDN-POA):
  • The Sexually Dimorphic Nucleus of the Preoptic Area (SDN-POA) within the medial preoptic area exhibits marked morphological divergence between sexes:
  • Male SDN-POA volume is $2.2\text{ times}$ larger than female SDN-POA volume.
  • Male SDN-POA contains $2.1\text{ times}$ more neurons than female SDN-POA.

• Experimental Neuro-Hormonal Manipulations (Rat Models):
  • Male castrated at birth: Deprived of neonatal testosterone $\rightarrow$ develops a female-like (smaller) SDN-POA and retains hypothalamic surge capability.
  • Female administered testosterone at birth: Exposed to neonatal androgen/estradiol $\rightarrow$ undergoes defeminization, developing a male-sized SDN-POA and losing the surge center capability.