Major Ideas Presented on Human Sexuality

There are three major components to sexual anatomy in humans (and most mammals):

• the gonads: your testes or ovaries

• the internal genitalia: the derivatives of the wolffian ducts [in males the sperm transport system; in females no derivatives]; and the derivatives of the mullerian ducts [in males there are none; in females they are the oviducts, uterus, and upper vagina.

• the external genitalia: the derivatives of the genital tubercle, folds, and swellings [in males the penile head or glans; the penile shaft; and the scrotum; in females they are the clitoris, labia minora, and labia majora - in toto for the female these external genitalia are called the vulva]

• chromosomal sex: whether you are 46,XX [a female] or 46,XY [a male]. This is established when the zygote is formed.

• genetic factors: These include major regulatory genes such as the SRY in the short arm [known as the p arm] of the Y chromosome. The SRY gene is involved in changing the neutral gonads into testes. It also includes genes that make sex hormones. And it includes genes that make receptors to those hormones.

• How hormones work: hormones are chemicals produced by cells in one part of the body that act as switches to turn on or to turn off gene activity in another part of the body. The tissue upon which a hormone acts is called a target tissue. Thus testosterone, a steroid hormone ;produced by Leydig cells in the testes will act on the wolffian ducts to make them form the sperm transport system [i.e., the collecting tubules, epididymis, vas deferens, seminal vesicles, and ejaculatory duct]. Testosterone will also be converted by an enzyme in the genital tubercle, folds, and swellings (the target tissue) into a penis and scrotum.

• What are the major sex hormones in males? From the testes there are two--testosterone which is from the Leydig cells of the interstitial tissue; and mullerian inhibiting substance [MIS] which is from the Sertoli cells of the seminiferous tubules. Both interstitial tissue and seminiferous tubules are found in the testes. They are not present in the ovaries. Testosterone is needed to make the sperm transport system from the wolffian ducts and to make a penis and scrotum form the genital tubercle, folds, and swellings. MIS is needed to destroy (induce apoptosis) the mullerian ducts.

• What are the major sex hormones in females? Ovaries will form if both MIS and testosterone are absent in the embryo From the ovaries (appearing at puberty) come two hormones - estrogen which is produced by the maturing Graafian follicle that bears the maturing egg; and progesterone, which comes primarily from the corpus luteum formed after the follicle ovulates or releases its egg. Estrogen prepares the uterus for implantation; progesterone maintains a pregnancy throughout the nine months. Progesterone also sends a signal to the brain to shut off hormones that stimulate egg production (that's why a woman who is pregnant cannot be fertilized while she is pregnant).

• Are there other sex hormones? Yes, both male and female have a hormone from the hypothalamus of the brain called gonadotropin releasing hormone [GnRH] which tells the anterior pituitary gland to release two hormones. These are follicle stimulating hormone [FSH] and luteinizing hormone [LH]. In females FSH causes oocytes to ripen from day 1 to day 14 of the menstrual cycle. In females LH stimulates the ovary to produce progesterone especially from day 14 to 28 when it ceases altogether. LH converts the emptied follicle that has released the egg to the oviduct into a corpus luteum. LH also is released in large doses on day 14 to make the follicle burst and release the egg. In males GnRH keeps a steady supply of FSH and LH going to the testes. The FSH is primarily involved in spermatogenesis. LH is primarily involved in making leydig cells produce testosterone.

• What about the sequence of events in embryology? The first of the major components that gets altered are the neutral gonads. They become testes if invaded by primordial germ cells [PGCs] that are 46,XY. They become ovaries if invaded by primordial germ cells that are 46,XX. If you cut out the PGCs (or if they are of an abnormal type such as 45,X or any combination lacking Y and less than two full X chromosomes) the ovaries that form will degenerate into scar tissue-these are called streak ovaries. The second of the major components to be altered (about the seventh week of pregnancy) are the wolffian and mullerian ducts which form the internal genitalia. The third of the major components (beginning in the eighth week of pregnancy) are the genital tubercle, folds, and swellings which form the external genitalia. The process begins about the sixth week of pregnancy (neutral gonads present) and ends in the third month of pregnancy when very clear vulva or penis and scrotum can be detected by ultrasound. Thus the time sequence is: gonads --> internal genitalia --> external genitalia.

• What are major abnormalities of sexual differentiation? These are described in detail in a separate item on the web site. But for purposes of clarification on the major biological implications of these syndromes, we have:
  • chromosome abnormalities [true hermaphrodites; non-disjunctional syndromes]
  • sex regulatory mutations leasing to sex reversals
  • male pseudohermaphrodites due to gene mutations affecting hormone production or receptors for those hormones.
  • female pseudohermaphrodites due to gene mutations affecting hormone production.

• Doesn't sexuality include behavior? Yes, there are several factors involved. These include a) sexual orientation or the awareness that one is attracted to one's opposite sex or to one's own sex. The latter are gay or lesbian or technically, homosexual. b) sexual drive or energy called libido which is induced by rising levels of testosterone in males (puberty to adult) and rising levels of estrogen (puberty to adult) in females. c) paraphilias which are often abnormal ways people get themselves sexually aroused or deal with their sexual identity. These are sometimes called perversions. But it also includes people who seek sex change surgery as a solution to their conviction that the sex of their minds is the reverse of the sex of their bodies. So called gender roles are mostly cultural traditions.

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