Sexual Differentiation and Endocrine Control of Development

Comprehensive practice flashcards covering the hormonal, genetic, and morphological pathways of sex differentiation, classical experiments, and human disorders of sex development.

What are the three distinct biological categories used to classify sex?

Genetic sex, hormonal sex, and phenotypic sex.

In Alfred Jost's classical embryonic experiment depicted below, what occurred when a testis was grafted against one ovary of a rabbit fetus?

The grafted testis caused local stimulation of the Wolffian duct and inhibition of the Müllerian duct on that side, but the effect did not extend to the contralateral side.

What anatomical outcome was observed in later studies when local crystalline implants of testosterone were placed in a female fetus without a testis?

The testosterone implants stimulated the Wolffian duct, but the Müllerian duct remained intact because anti-Müllerian hormone was absent.

What was the original proposition of the Jost hypothesis regarding embryonic sexual differentiation?

Secretion of androgens and anti-Müllerian hormone ($\text{AMH}$ or $\text{MIH}$) by fetal testes during critical stages of development accounts for the full range of sexually dimorphic urogenital traits observed at birth.

What type of molecule is anti-Müllerian hormone (AMH), and to which family does it belong?

It is a large molecular weight glycoprotein homodimer that belongs to the $\text{TGF-}\beta$ superfamily.

Where is the testis-determining SRY gene located on the human Y chromosome?

On the short arm of the Y chromosome, just below the pseudoautosomal region ($\text{PAR}$).

According to Yamashiro et al. (2018), what is the developmental timeline for primordial germ cell (PGC) migration, genital ridge colonization, and sex differentiation in canines?

Canine PGCs migrate at $20\text{--}22\,\text{dpf}$, colonize the genital ridges at $23\text{--}25\,\text{dpf}$, mature around $27\text{--}30\,\text{dpf}$, and initiate morphological sexual differentiation at $35\text{--}40\,\text{dpf}$.

What is the length of an individual seminiferous tubule, and what is their total combined length in a human testis?

Individual tubules measure approximately $30\text{--}80\,\text{cm}$, and their total combined length reaches approximately $600\text{--}900\,\text{m}$.

During testis differentiation, what cell type differentiates directly in response to SRY gene expression in the genital ridge?

Sertoli cells.

What molecule produced by Sertoli cells serves as the preferred energy source for spermatocytes and spermatids?

Lactate.

How do Sertoli cells concentrate testosterone locally within the seminiferous tubules?

By secreting androgen-binding protein ($\text{ABP}$).

What are the major structural regions and components of the mature sperm cell shown in the diagram?

The head (containing the acrosome, plasma membrane, and nucleus), the midpiece or connecting piece (containing the centriole, mitochondria, and axial filament), the tail, and the end piece with terminal disc.

Why do germ cells die and fail to form ovarian follicles in individuals with Turner's syndrome ($45,\text{XO}$)?

Female germ cell survival and follicle formation require an $X$-linked gene dosage mechanism provided by two active $X$ chromosomes ($\text{XX}$).

Why do $47,\text{XXY}$ germ cells fail to survive in the testicular environment in Klinefelter's syndrome?

The presence of two $X$ chromosomes is hypothesized to render the germ cells sensitive to the cytotoxic effects of anti-Müllerian hormone ($\text{AMH}$).

Which adult male and female reproductive structures arise from the embryonic Wolffian and Müllerian ducts?

The Wolffian duct differentiates into the epididymis, vas deferens, and seminal vesicle; the Müllerian duct differentiates into the oviduct, uterus, and upper vagina.

According to Hughes (2001), what is the timing of Müllerian duct regression and Wolffian duct differentiation during human gestation?

Müllerian duct regression occurs between weeks $8\text{--}10$, and Wolffian duct differentiation occurs between weeks $8\text{--}12$.

Which enzyme converts testosterone into $5\alpha$-dihydrotestosterone ($5\alpha\text{-DHT}$), and what anatomical process is directly dependent on $5\alpha\text{-DHT}$?

$5\alpha$-reductase; it is required for external virilization and the development of male external genitalia.

Which enzyme converts testosterone into oestradiol-$17\beta$, and what is its role in male sexual differentiation?

Aromatase; it mediates sexual differentiation of the brain.

What is the testosterone receptor (NR3C4), and how does it execute its cellular function?

It is a nuclear hormone receptor superfamily member that binds both testosterone and $5\alpha\text{-DHT}$, translocates to the nucleus, and directly binds to DNA to regulate gene transcription.

What can happen to a genetic female fetus ($46,\text{XX}$) if the mother is accidentally exposed to synthetic androgens such as danazol during pregnancy?

The fetus can become externally virilized and may develop a masculinized brain.

Why do castrated males (eunuchs) exhibit unusually tall stature and elongated limbs?

The absence of sex steroids prevents androgen-mediated epiphyseal plate closure, allowing long bone growth to continue unchecked.

What was the primary conclusion of the historical study on Korean eunuchs by Kyung-Jin Min et al. (2012)?

Korean eunuchs had significantly longer lifespans than intact men of similar social standing, supporting the hypothesis that male sex hormones decrease male longevity.

What are the primary karyotypic and anatomical characteristics of Klinefelter's syndrome?

A $47,\text{XXY}$ karyotype, small intra-abdominal testes, low testosterone, azoospermia, scant body hair, limited breast development (gynecomastia), and tall stature.

What is the genetic cause and anatomical presentation of Swyer syndrome (XY gonadal dysgenesis)?

Caused by SRY gene mutations in approximately $15\%$ of cases; presents with a $46,\text{XY}$ karyotype, female appearance, streak gonads, normal uterus and vagina, lack of sex steroids, tall stature, and primary amenorrhea.

Why do individuals with complete Androgen Insensitivity Syndrome (AIS) develop well-formed breasts despite a $46,\text{XY}$ karyotype?

Because androgen receptors are nonfunctional, testicular testosterone is converted to oestrogen via aromatase, which stimulates breast development without opposing androgenic action.

Why do individuals with Androgen Insensitivity Syndrome (AIS) lack a uterus and have a short, blind-ending vagina?

Their functional testes produce anti-Müllerian hormone ($\text{AMH}$), causing normal regression of the embryonic Müllerian ducts.

What is the genetic origin of 46,XX testicular difference of sex development (de la Chapelle syndrome)?

It results from an abnormal crossover during paternal meiosis that translocates the SRY gene onto the X chromosome passed from the father to the child.

What are the hallmark internal and external anatomical characteristics of Turner's syndrome ($45,\text{XO}$)?

Streak gonads devoid of germ cells, absent Wolffian ducts, intact Müllerian ducts (uterus and oviducts present), short stature, thickened neck, and absent sex steroid production.

What biochemical defect underlies 'guevedoces' in the Dominican Republic, and what happens to affected individuals at puberty?

A mutation causing $5\alpha$-reductase-2 deficiency; at puberty, the surge in testosterone virilizes the body, triggering phallic growth, testicular descent, and male muscle development.

According to Reik and Lewis (2005), how does X-chromosome inactivation (XCI) differ between marsupials and eutherian mammals?

Marsupials exhibit imprinted $X$-chromosome inactivation maintained by histone modifications, whereas eutherians evolved random $X$-chromosome inactivation stabilized by DNA methylation.