Secondary Sexual Characteristics
Primary sexual characteristics are present during development in the uterus
and are the differences in reproductive organs etc between males and
females
Secondary sexual characteristics are the changes that occur during
puberty as children become adolescents
They are controlled by the release of hormones - oestrogen in girls
and testosterone in boys
Human secondary sexual characteristics
Female secondary sexual characteristics:
Male secondary sexual characteristics:
Some changes occur to both boys and girls, including growth of sexual
organs and growth of body hair
Emotional changes also occur due to the increased levels of hormones in
the body
These include more interest in sex and increased mood swings
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The Menstrual Cycle
Starts in early adolescence in girls (around age 12) and is controlled
by hormones
The average menstrual cycle is 28 days long
Ovulation (the release of an egg) occurs about halfway through the cycle
(day 14) and the egg then travels down the oviduct to the uterus
Failure to fertilise the egg causes menstruation (commonly called a period)
to occur - this is caused by the breakdown of the thickened lining of the
uterus
Menstruation lasts around 5 - 7 days and signals the beginning of the next
cycle
After menstruation finishes, the lining of the uterus starts to thicken again in
preparation for possible implantation in the next cycle
Change
s in the lining of the uterus during the menstrual cycle
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Hormones of the Menstrual Cycle
The menstrual cycle is controlled by hormones released from the ovary and
the pituitary gland in the brain
The roles of FSH and LH
Cha
nges in the levels of the pituitary hormones FSH and LH in the blood during
the menstrual cycle
FSH (follicle-stimulating hormone) is released by the pituitary gland and
causes an egg to start maturing in the ovary
It also stimulates the ovaries to start releasing oestrogen
The pituitary gland is stimulated to release luteinising hormone (LH)
when oestrogen levels have reached their peak
LH causes ovulation to occur and also stimulates the ovary to
produce progesterone
The roles of oestrogen and progesterone
Changes in the levels of oestrogen and progesterone in the blood during the
menstrual cycle
Oestrogen levels rise from day 1 to peak just before day 14
This causes the uterine wall to start thickening and the egg to mature
The peak in oestrogen occurs just before the egg is released
Progesterone stays low from day 1 – 14 and starts to rise once ovulation has
occurred
The increasing levels cause the uterine lining to thicken further; a fall in
progesterone levels causes the uterine lining to break down (menstruation /
‘period’)
Interaction between all four of the menstrual cycle hormones
The pituitary gland produces FSH which stimulates the development of
a follicle in the ovary
An egg develops inside the follicle and the follicle produces the
hormone oestrogen
Oestrogen causes growth and repair of the lining of the uterus wall and
inhibits production of FSH
When oestrogen rises to a high enough level it stimulates the release
of LH from the pituitary gland which causes ovulation (usually around day 14
of the cycle)
The follicle becomes the corpus luteum and starts producing progesterone
Progesterone maintains the uterus lining (the thickness of the uterus wall)
If the ovum is not fertilised, the corpus luteum breaks down and progesterone
levels drop
This causes menstruation, where the uterus lining breaks down and is
removed through the vagina - commonly known as having a period
If pregnancy does occur the corpus luteum continues to produce
progesterone, preventing the uterus lining from breaking down
and aborting the pregnancy
It does this until the placenta has developed, at which point it starts secreting
progesterone and continues to do so throughout the pregnancy
Where hormones involved in the menstrual cycle are made and act
