The human life cycle includes fertilization, cleavage, blastocyst
formation, implantation, differentiation of the inner cell mass into
embryonic tissues, organogenesis, fetal development, birth, infancy,
childhood, adolescence, young adulthood (parenting), middle age, old age,
and death.
The sperm brings in a haploid chromosome number which includes an X
(female determining) or a Y (male determining). At fertilization the
chromosomal sex is determined because eggs only bring in one X. Thus a
zygote that is 46,XX is female and a zygote that is 46,XY is male.
There are genes that control all the developmental events. One
of the first genes in genetic sex is on the Y chromosome. It is
called SRY. The SRY is on the Y chromosome. It is absent in the
X chromosome. If it is present, the neutral gonads become
testes. If it is absence the neutral gonads become
ovaries. The testes and ovaries are your gonads (NOTE:
THEY ARE NOT YOUR GENITALS]. The developmental fate of your
gonads is your gonadal sex.
In the embryo when the embryonic kidney forms, there are two ducts that
form, the wolffian ducts (closer to those kidneys and the future gonad)
and the mullerian ducts (somewhat outside the wolffian ducts). The
neutral gonad pinches off from the embryonic kidney and the two ducts
surround it. If the neutral gonad is a testis, it will release a hormone
called MIS - mullerian inhibiting substance - which destroys the
mullerian ducts. Cell destruction by genetic signal is called
apoptosis. The testis also secretes a hormone called testosterone. This
is used to stimulate the wolffian ducts to form the sperm transport
system. The sperm transport system includes the epididymis, the
vas deferens, the seminal vesicles, and the ejaculatory duct.
In the embryonic XX female, the neutral gonad is an ovary. The ovary does
NOT produce MIS. Thus the mullerian ducts develop into the female
reproductive tract. This includes the oviducts, the uterus, and
the upper vagina (the lower vagina is from a different tissue). The
ovary also does NOT produce testosterone. Thus the wolffian ducts
fail to develop and they are induced to apoptosis (self destruction).
Three structures form towards the end of organogenesis (about days 50-55)
in the thigh region: the genital tubercle, genital folds, and genital
swellings. In the absence of testosterone produced from the gonads,
they develop into a female pathway and produce a
clitoris (from the genital tubercle); labia minora (from
the genital folds); and labia majora (from the genital
swellings). In the developing XY embryo the genital tubercle responds to
testosterone from the testes and produces the penile head (called a
glans); the genital folds form a tube called the penile
shaft; and the genital swellings enlarge and form a sac called the
scrotum. These are the external genital sex.
At about age 13 hormonal changes begin to take place. Hormones from the
pituitary gland are released during sleep and stimulate enlargement of
the gonads. In boys entering adolescence the scrotum darkens and
wrinkles, public hair forms, the voice deepens,. Bone growth is stimulated
and height is increased, and the shoulders widen. All these changes are
produced by huge amounts of testosterone produced. Also the
pituitary releases a hormone to stimulate sperm formation in the testes
and the male develops erotic feelings and begins to ejaculate through
night dreams, masturbation, or later in the teens (or later in some)
through sexual intercourse. In the female the same pituitary hormones
lead to ovarian maturation. The female experiences menarche (her
first menstrual period) and ripens one egg (a graafian follicle) every 28
days. She also develops pubic hair, breast enlargement, and a widening of
the hips in response to ovarian hormones, especially estrogens
and progesterone.
There are two major psychological aspects of sexuality. There are
cultural features such as gender roles (in the US until
the 1970s MDs were males and nurses were females) Today half the students
in medical school are female. Older ideas of chivalry (walking on gutter
side of a sidewalk, getting up for a female in a subway, or holding a door
for a female) are not frequently practiced since the 1970s. The other
aspect is harder to prove but most scientists believe that our brains are
hormone influenced especially during fetal development and men
and women respond differently emotionally (on capacity for crying, bonding
to infants, our attraction and fidelity to a partner). These are also
influenced culturally but few scientists believe sexual orientation, for
example, is purely a matter of upbringing. Unlike anatomical and
physiological processes that can be objectively put to experimental test,
psychological aspects of sexuality tend to be more controversial.
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