Sex Differences That Arise During Development

The hypothalamus, pituitary gland, and adrenal glands of both males and females produce the same hormones in similar amounts—it is the hormones secreted by the ovaries and testes that determine biological sex differences. Once a fetus develops either testes or ovaries they produce hormones that influence the development of other structures involved in reproduction.

Sex differences in reproductive organs are the result of the developmental pathways followed by males and females. These pathways lead to different structures for making sperm and egg cells, called gonads; different structures for carrying the sperm and egg cells, called ducts; and different external reproductive structures, called genitalia.

Gonad Differentiation During the first seven weeks of development, a male and female fetus are indistinguishable unless one looks at their chromosomes. The embryonic gonads are composed of two masses that can become either the paired ovaries of a female or the paired testes of a male. It is only when the cells of the embryonic gonads become specialized during development, or differentiate, that development in females and males begins to diverge.

For undifferentiated gonads to develop into testes, a gene on the Y chromosome, the SRY gene, must be present. The SRY (Sex-determining Region of the Y chromosome) gene encodes a protein that causes expression of the genes required for testicular development.

While less is known about ovarian development, it is becoming clear that many genes must be expressed for ovarian development to occur. These genes, some of which are called the Gpbox genes, seem to act in a similar manner to the SRY gene by turning on other genes involved in the production of ovaries— these genes would be expressed only in the absence of the SRY gene.

Males, with their one X and one Y chromosome, express the SRY gene and not the Gpbox genes. Since females have two X chromosomes and no Y chromosome,

Figure 12.3 Genes involved gonadal differentiation. When the Y-linked SRY gene is expressed, testes develop; when the SRY gene is absent, Gpbox genes are expressed and ovaries develop.

Undifferentiated gonads

Gpbox genes on X

chromosome turn on ovarian development (only in absence of SRY gene of Y chromosome).

Undifferentiated gonads

Gpbox genes on X

chromosome turn on ovarian development (only in absence of SRY gene of Y chromosome).

SRY gene on Y

chromosome turns on testicular development.

SRY gene on Y

chromosome turns on testicular development.

Embryonic ovaries

Embryonic ovaries

Embryonic testes

they express the Gpbox genes and not the SRY gene (Figure 12.3). The expression of these genes leads to the differentiation of the male and female gonads.

Once differentiated, the gonads produce their sex-specific hormones. The cells of the embryonic testes make testosterone, which directs the development of the internal duct systems and formation of external genitalia. Once the embryonic external genitalia have been masculinized, the testes will not secrete testosterone until puberty. The cells of the ovaries do not produce estrogen during development. Estrogen production occurs only after puberty.

Differentiation of the Duct System The ducts of the reproductive system are the structures that carry the sperm and egg cells, or gametes. In males these sperm-carrying ducts include the epididymis, vas deferens, and urethra, in females, egg cells travel through the oviducts, uterus and cervix, and vagina (Figure 12.4).

Prior to differentiation, the embryonic duct system consists of two separate sets of tubes that lie side-by-side. For each sex, one duct system stays in place, or persists, and the other degenerates or regresses. These embryonic duct systems are called the Mullerian duct system and Wolffian duct system.

For male duct development, the Mullerian duct system regresses and the Wolffian duct system persists. Wolffian duct development occurs when testosterone is present. The testes also produce a hormone called anti-Mullerian hormone that causes regression of the Mullerian ducts.

Female duct development is stimulated by the presence of gene products from many different genes. When these genes are expressed in the developing ovary, the female pathway is followed, thus preventing testosterone synthesis and secretion. Since the Wolffian duct system requires testosterone to stimulate its development, this structure regresses in females.

External Genitalia Differentiation In male and female embryos, one embryonic structure can be molded into either the penis and scrotum of the male external genitalia or the clitoris and vulva of the female external genitalia.

Undifferentiated duct system

Mullerian ducts

In female embryos, there is no testosterone or anti-Mullerian hormone, so the Mullerian duct system persists.

Undifferentiated duct system

Wolffian ducts

In male embryos, testes secrete testosterone and anti-Mullerian hormone that allow the Wolffian duct system to persist.

Female duct system

Oviduct

Oviduct

Vagina

Figure 12.4 Adult reproductive ducts. During embryological development, the two separate duct structures exist side-by-side in the abdomen of the embryo. In an XY male, development of testes and secretion of testosterone and anti-Mullerian hormone allows the Wolffian duct system to persist. In female embryos, there is no testosterone or anti-Mullerian hormone and the Mullerian duct system persists.

Vagina

Male duct system

Bladder

Seminal vesicle

Prostate gland

Penis

Male duct system

Bladder

Seminal vesicle

Prostate gland

Penis

-Urethra

-Vas deferens

Epididymus

-Urethra

-Vas deferens

Epididymus or

Figure 12.4 Adult reproductive ducts. During embryological development, the two separate duct structures exist side-by-side in the abdomen of the embryo. In an XY male, development of testes and secretion of testosterone and anti-Mullerian hormone allows the Wolffian duct system to persist. In female embryos, there is no testosterone or anti-Mullerian hormone and the Mullerian duct system persists.

(Figure 12.5). Thus, it is said to be a bipotential structure; this structure consists of a genital tubercle, a urogenital sinus, and two labioscrotal swellings.

In the presence of dihydroxytestosterone (DHT), an androgen formed from testosterone, the genital tubercle forms the penis, the urogenital sinus fuses to form the urethra, and the labioscrotal swellings fuse to form the scrotum.

Figure 12.5 Differentiation of the external genitalia. Male and female external genitalia are fashioned from the same bipotential structure consisting of a genital tubercle, a urogenital sinus, and two labioscrotal swellings. In the presence of DHT, male genitalia develops, but when there is no DHT, female genitalia results.

Undifferentiated genitalia

Genital tubercle

Labioscrotal swelling Urogenital sinus

Undifferentiated genitalia

Female genitalia

Clitoris

Urethra

Labia minora

Labia majora

Anus

Female genitalia

Clitoris

Urethra

Labia minora

Labia majora

Vaginal opening

Male genitalia

Male genitalia

Urethra

Penis

Urethra

Penis

Testes

Anus

In females, whose bodies produce very little DHT, the genital tubercle becomes the clitoris, the urogenital sinus becomes the labia minora and further differentiates into the vagina and urethra, and the labioscrotal swellings fold to form the labia majora. Therefore, the penis and clitoris arise from the same starting tissue, as do the scrotum and vulva.

While many sex differences arise during development, it is when hormones begin to be expressed at puberty that real sex differences arise. In addition, only a fraction of the hormones that are expressed affect athleticism—many hormones act on structures and processes that have no impact on athleticism.

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