Caitlain's Corner

With all of the differences between males and females, it is sometimes hard to imagine that we all started out identically. When fertilization occurs, the egg, which contains an “X” chromosome from the mother, and the sperm, which may contain an “X” or a “Y” chromosome, from the father, unites. If the sperm contained the “X” chromosome, the baby will be born genetically female. Conversely, if the baby receives the “Y” chromosome, the baby will be born genetically male.

While genetically sex is determined at fertilization, biologically, we all begin as undifferentiated embryos. Somewhere around the 6th week of development, a gene on the Y chromosome triggers the release of a series of hormones that trigger the masculinization of the body, resulting in the development of a male. In other words, the “default” pathway is female, in the absence of anything to cause the fetus to become a male. This gene is referred to as the “SRY” gene, for “sex-determining gene of the Y chromosome.” The presence of the SRY gene results in a biologically male child, whereas the absence of it results in a female child.* This process is known as sexual differentiation.

The result, then, is that all of the male’s reproductive organs and appurtenances develop from pre-existing tissue in an otherwise female fetus. The development of these organs from similar tissues results in what is know as “homologous” structures – structures that have similar developmental histories even though they might eventually perform disparate functions. For example, the testes and the ovaries are homologous, though they have disparate (though similar) functions.
In the developing fetus, genital ridges appear at around 4 weeks, and within a week or so later, they begin to develop into one set of gonads. The presence of the SRY gene causes a specific process (the production of androgens such as testosterone) to begin that will result in the development of testes, whereas the lack of the gene results in the development of ovaries. Around the aforementioned 6th week, the gonads and external genitalia begin to differentiate and follow different developmental paths from this point forward.

Also during that 4th week, the embryo’s anogenital region develops a slit known as the “cloaca.” This structure is enclosed in a membrane, and is flanked by two urethral folds, resulting in a region known as the “genital swelling.” Approximately two weeks later, at the front end of the cloaca these folds have fused with each other near the rear and in what will become the anus, urinary bladder and other tissue groups in the lower pelvic region. The fused region becomes the perineum – you can easily see the fused area in the form of a ridge that runs along the perineum (this ridge is known as the “raphe”). At the front of the region, development begins in what will become the external genital structures of the child. The genital swelling becomes the labia majora or the scrotum, and the internal urethral folds become the labia minora or fuse into the structure of the scrotum to become the scrotal raphe (ridge) and the shaft of the penis (enclosing the urethra).

Here are some examples of the homologous structures in females and males:

* This article does not cover intersexed or other atypical differentiation issues.

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