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Human sexual anatomy and development

Sexuality is about appreciating the human body and sexual anatomy. We should understand the function of the male and female genitalia as well as the changes that occur throughout our lives. It is also useful to know what kind of stimulation may be pleasurable or is involved in achieving orgasm.

The step of evolving two sexes was inevitable because of the huge advantage that sexual reproduction provides. Sexual reproduction involves combining gene pools from two parents and allows more rapid development of genetic diversity. Lifeforms can adapt more quickly to changing environments. Those individuals with a more advantageous set of genes reproduce and survive more successfully on average than those who are less advantaged. Many organisms reproduce by using this method, called natural selection.

Many plants and animals reproduce sexually. Animal reproduction requires insemination of a female by a male. The corresponding transfer of genetic material in flowering plants in called pollination. The male genetic material is mobile and plentiful. The female genetic material is immobile. The male part (even in plants) is the active agent that effects the reproductive transfer. After fertilisation, the embryonic cells (formed from egg and sperm cells) divide and grow within the female reproductive anatomy or part (of a plant). This standard definition of male and female is used throughout nature.

The reason we have genitals is because we have two sexes: male and female. Sexual reproduction involves combining genetic material from two parents. Every human embryo develops a pair of gonads. The sex chromosomes determine whether the gonads in the embryo become the testes (for a boy) or the ovaries (for a girl). The gonads become centres for the production of genetic material (eggs or spermatozoa). A girl is born with her genetic material: immature eggs. Once the testes of a male foetus start producing androgens (male hormones such as testosterone) they drive further change, which is concluded at puberty. This process is called sexual differentiation.

There are relatively few anatomical differences between the sexes. Sometimes we have one body part, for example head and heart. Sometimes we have two for backup. Even in our sexual anatomy there is symmetry. A woman has two ovaries and a man has two testes. A man has nipples but he never develops the mammary glands that form the female breast.

Every foetus starts out with the building blocks for both male and female reproductive organs. Regardless of sex, they have Wolffian and Müllerian ducts. Development of our internal reproductive anatomy depends on hormones produced by the testes (not for a female). In most foetuses only one set of reproductive ducts (male or female) develops. A process called atrophy inhibits the development of the superfluous anatomy.

In a boy the Wolffian ducts develop into the male reproductive organs. The male ducts form the vas deferens (that connects the testes to the penis) as well as other tubes required for ejaculation of semen. In a female these ducts waste away. Women cannot ejaculate because their equivalent organ (the clitoris) does not have these tubes. Nor do they have equivalent glands.

In a girl the Müllerian ducts form the female reproductive organs. The female ducts form the vagina, which leads into the womb (uterus) where the foetus develops before birth. In a male embryo these Müllerian ducts disappear. This is clear anatomical evidence that the vagina (anatomy that is not present in the male) is not involved in orgasm, which is a basic human response common to both sexes. Male ejaculation is the only justification for orgasm. Female orgasm is a hangover from how the sexes have evolved.

Unlike most of the internal reproductive anatomy, the phallus is not an either-or option. Every foetus has a rudimentary phallus, called the genital tubercle This is the organ responsible for the human orgasmic response. It is impossible to determine the sex of a foetus until after 14 weeks. The extent of the development of the phallus varies depending on hormones sent out by the testes (or not). The genital tubercle undergoes significantly more development in a boy. The labioscrotal folds of the embryo become the scrotum (for a boy) and the labia (for a girl). Women’s genitalia (clitoris and labia) are anatomically equivalent to men’s genitalia (penis and testicles). We all achieve orgasm through the same mechanism regardless of gender and orientation. The penis and clitoris are easily reached by the hands, which makes self-stimulation (masturbation) is possible.

Girls and boys have equal amounts of androgens until the age of 7 or 8. At puberty levels of androgens rise markedly in boys but less so in girls. Women have around two thirds of men’s levels of androgens, including testosterone (involved in libido and responsiveness). Boy and girls also have equal amounts of oestrogens. At puberty the levels of oestrogens increase abruptly in girls but only slightly in boys. Women’s levels of oestrogens are much higher than men’s. There is a steep increase in male responsiveness at adolescence but a much lower increase in female responsiveness.

… the reported levels of the 17-ketosteroids in the human male differ from the reported levels of the 17-ketosteroids in the human female in a manner which more or less parallels the differences which we have found between the median frequencies of orgasm at various ages in the two sexes. (Alfred Kinsey 1953)