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Chapter 13: Hormones and SexMen-Are-Men-and-Women-Are-Women Assumption
→ The men-are-men-and-women-are-women assumption is the tendency to think about femaleness and
maleness as discrete, mutually exclusive, opposite categories.
- In regards to hormones and sex, this leads to the assumption that females have female sex hormones that
give them female bodies and make them do female things and that males have male sex hormones that
give them male bodies and make them do male things.
Developmental and Activational Effects of Sex Hormones
→ Hormones influence sex in two fundamentally different ways
- (1) by influencing the development from conception to sexual maturity of the anatomical, physiological,
and behavioral characteristics that distinguish one as female or male
- (2) by activating the reproductionrelated behavior of sexually mature adults
Neuroendocrine System
→ The endocrine glands are the only organs whose primary function appears to be the release of hormones.
→ However, other organs (the stomach, liver, and intestine) and body fat also release
hormones into general circulation, making them part of the endocrine system.
Glands
→ There are two types of glands:
- Exocrine glands: (sweat glands) release their chemicals into ducts, which carry
them to their targets, mostly on the surface of the body.
- Endocrine glands: (ductless glands) release their chemicals directly into the
circulatory system. Once released by an endocrine gland, a hormone travels via
the circulatory system until it reaches the targets on which it normally exerts its effect (other endocrine
glands, sites in the nervous system).
Gonads
→ Central to any discussion of hormones and sex are the gonads, the male testes and the female ovaries. The
primary function of the testes and ovaries is the production of sperm cells and ova, respectively.
- After copulation (sexual intercourse), a single sperm cell may fertilize an ovum to form one cell called a
zygote, which contains all of the information necessary for the typical growth of a complete adult organism
in its natural environment.
- With the exception of ova and sperm cells, each cell of the human body has 23 pairs of chromosomes.
- In contrast, the ova and sperm cells contain only half that number, one member of each of the 23
pairs. Thus, when a sperm cell fertilizes an ovum, the resulting zygote ends up with the full
complement of 23 pairs of chromosomes
→ The pair of chromosomes called the sex chromosomes contain the genetic programs that direct sexual
development.
- The cells of females have two large sex chromosomes, which are called X chromosomes.
- In males, one sex chromosome is an X chromosome, and the other is called a Y chromosome.
→ The sex chromosome of every ovum is an X chromosome, whereas half the sperm cells have X chromosomes,
and half have Y chromosomes.
- Your sex with all its social, economic, and personal ramifications is determined by which of your father’s
sperm cells fertilize your mother’s ovum.
- If a sperm cell with an X sex chromosome won, you are a female
- If one with a Y sex chromosome won, you are a male.
→ Once a chromosome has duplicated, the two products remain joined at one point, producing an X shape. This
is true of all chromosomes, including Y chromosomes.
- Because the Y chromosome is much smaller than the X chromosome, early investigators failed to see one
small arm and thus saw a Y.
, - In humans, the smaller Y-chromosome genes appear to control the synthesis of only 66 proteins, where
the larger X-chromosome genes control 615
Hormones
→ Vertebrate hormones fall into one of three classes:
- (1) amino acid derivatives: hormones that are synthesized in a few simple steps from an amino acid
molecule. Example: epinephrine, which is released from the adrenal medulla and synthesized from
tyrosine.
- (2) peptides and proteins: hormones are chains of amino acids; peptide hormones are short chains, and
protein hormones are long chains.
- (3) steroids: hormones that are synthesized from cholesterol, a type of fat molecule
→ The hormones that influence sexual development and the activation of adult sexual behavior (the sex
hormones) are all steroid hormones.
- Most other hormones produce their effects by binding to receptors in cell membranes.
→ Steroid hormones can influence cells in this fashion, however, because they are small and fat-soluble, they can
readily penetrate cell membranes and often affect cells in a second way.
- Once inside a cell, the steroid molecules can bind to receptors in the cytoplasm or nucleus and directly
influence gene expression
- Amino acid derivative hormones and peptide hormones affect gene expression less commonly and
by less direct mechanisms
- Steroid hormones tend to have the most diverse and long-lasting effects on cellular function.
Sex Steroids
→ The gonads also produce and release steroid hormones. The two main classes of gonadal hormones are
- Androgens: testosterone is the most common androgen
- Estrogens: estradiol is the most common estrogen.
- Progestins: progesterone is the most common progestin where in females it prepares the uterus and the
breasts for pregnancy. Whether it serves a function in males is unclear, but in both males and females it
seems to be involved in various forms of neuroplasticity
→ The adult ovaries tend to release more estrogens than androgens and the adult testes release more
androgens than estrogens has led to the common, but misleading, practice of referring to androgens as “the male
sex hormones” and to estrogens as “the female sex hormones.”
→ Because the primary function of the adrenal cortex (the outer layer of the adrenal glands) the regulation of
glucose and salt levels in the blood, it is not generally thought of as a sex gland.
- However it does release small amounts of all of the sex steroids released by the gonads.
The Pituitary
→ The pituitary gland is frequently referred to as the master gland because most of its hormones are tropic
hormones.
- Tropic hormones’ primary function is to influence the release of hormones from other
glands. Example: gonadotropin is a pituitary tropic hormone that travels through the
circulatory system to the gonads, where it stimulates the release of gonadal
hormones.
→ The pituitary gland is really two glands which fuse during the course of embryological
development:
- The posterior pituitary: develops from a small outgrowth of hypothalamic tissue that
eventually comes to dangle from the hypothalamus on the end of the pituitary stalk
- The anterior pituitary: begins as part of the same embryonic tissue that eventually
develops into the roof of the mouth. During the course of development, it pinches off
and migrates upward to assume its position next to the posterior pituitary. The anterior
pituitary releases tropic hormones; and is the real master gland.
,Female Gonadal Hormone Levels Are Cyclic; Male Gonadal Hormone Levels Are Steady
→ Although males and females release exactly the same hormones, these hormones are not present at the same
levels, and they do not necessarily perform the same functions.
→ The major difference between the endocrine function of females and males is that in human females, the levels
of gonadal and gonadotropic hormones go through a cycle that repeats itself every 28 days or so.
- It is the regular hormone fluctuations that control the female menstrual cycle.
- In contrast, human males levels of gonadal and gonadotropic hormones change little from day to day.
→ Because the anterior pituitary is the master gland, many early scientists assumed that there must be difference
between the male and female anterior pituitary
- This hypothesis was discounted by a series of transplant studies conducted by Geoffrey Harris in the 1950s
→ In these studies, a cycling pituitary removed from a mature female rat became a steady-state pituitary when
transplanted at the appropriate site in a male, and a steady-state pituitary removed from a mature male rat began
to cycle once transplanted into a female.
- This showed that the anterior pituitaries are not inherently female (cyclical) or male (steady-state) their
patterns of hormone release are controlled by some other part of the body.
Control of the Pituitary
→ The nervous system was assumed to have control of the anterior pituitary by behavioral research on birds and
other animals that breed only during a specific time of the year.
- It was found that the seasonal variations in the light–dark cycle triggered many of the breeding-related
changes in hormone release.
- If the lighting conditions under which the animals lived were reversed, the breeding seasons were also
reversed.
- This showed that visual input to the nervous system was controlling the release of tropic hormones from
the anterior pituitary.
→ It was then assumed that control of the anterior pituitary was done by the hypothalamus, the structure from
which the pituitary is suspended.
- Hypothalamic stimulation and lesion experiments quickly established that the hypothalamus is the
regulator of the anterior pituitary, but how the hypothalamus carries out this role was unknown as unlike
the posterior pituitary, the anterior pituitary receives no neural input from the hypothalamus, or other
neural structures
Control of the Anterior and Posterior Pituitary by the Hypothalamus
→ There are two different mechanisms by which the hypothalamus controls the pituitary:
- one for the posterior pituitary
- one for the anterior pituitary
→ The two major hormones of the posterior pituitary, vasopressin and oxytocin, are peptide hormones that are
synthesized in the cell bodies of neurons in the paraventricular nuclei and supraoptic nuclei on each side of the
hypothalamus
- They are then transported along the axons of these neurons to their terminals in the posterior pituitary and
are stored there until the arrival of action potentials causes them to be released into the bloodstream.
- Neurons that release hormones into general circulation are called neurosecretory cells
- Oxytocin stimulates contractions of the uterus during labor and the ejection of milk during suckling
- Vasopressin (antidiuretic hormone) facilitates the reabsorption of water by the kidneys; and both seem to
influence stress-coping and social responses
→ The means by which the hypothalamus controls the release of hormones from the neuron-free anterior
pituitary is more complex where Harris (1955) suggested that the release of hormones from the anterior pituitary
was itself regulated by hormones released from the hypothalamus. Two findings supported this:
1. The discovery of a vascular network, the hypothalamo-pituitary portal system, which seemed to carry
hormones from the hypothalamus to the anterior pituitary. A network of hypothalamic capillaries feeds a
, bundle of portal veins that carries blood down the pituitary stalk into another network of capillaries in the
anterior pituitary. (A portal vein is a vein that connects one capillary network with another.)
2. The discovery that cutting the portal veins of the pituitary stalk disrupts the release of anterior pituitary
hormones until the damaged veins regenerate
Discovery of Hypothalamic Releasing Hormones
→ It was hypothesized that the release of each anterior pituitary hormone is controlled by a different
hypothalamic hormone.
- Each hypothalamic hormone that was thought to stimulate the release of an anterior pituitary hormone
was referred to as a releasing hormone.
- In contrast, each hormone thought to inhibit the release of an anterior pituitary hormone was referred to as
a release-inhibiting hormone.
→ Efforts to isolate the hypothalamic releasing and release-inhibiting hormones led to a major breakthrough in
the late 1960s.
- Guillemin and his colleagues isolated thyrotropin-releasing hormonefrom the hypothalamus of sheep, and
Schally and his colleagues isolated the same hormone from the hypothalamus of pigs.
Thyrotropin-releasing hormone triggers the release of thyrotropin from the anterior pituitary, which in turn
stimulates the release of hormones from the thyroid gland.
- Schally’s and Guillemin’s isolation of thyrotropin-releasing hormone confirmed that hypothalamic releasing
hormones control the release of hormones from the anterior pituitary
→ Subsequent isolation of gonadotropin-releasing hormone by Schally and his group found a releasing hormone
that stimulates the release of both of the anterior pituitary’s gonadotropins: follicle-stimulating hormone(FSH) and
luteinizing hormone(LH).
- All hypothalamic-releasing hormones, like all tropic hormones, have proven to be peptides.
Regulation of Hormone Levels
→ Hormone release is regulated by three different kinds of signals:
- Signals from the nervous system
- Signals from circulating hormones
- Signals from circulating nonhormonal chemicals
Regulation by Neural Signals
→ All endocrine glands, with the exception of the anterior pituitary, are directly regulated by signals from the
nervous system.
- Endocrine glands located in the brain (the pituitary and pineal glands) are regulated by cerebral neurons.
- Endocrine glands located outside the CNS are innervated by the autonomic nervous system, usually by
both the sympathetic and parasympathetic branches, which have opposite effects on hormone release
→ Hormone release can be influenced by experience. For example, many species that breed only in the spring
are often prepared for reproduction by the release of sex hormones triggered by the increasing daily duration of
daylight. .
Regulation by Hormonal Signals
→ The hormones themselves also influence hormone release. For example, tropic hormones of the anterior
pituitary influence the release of hormones from their respective target glands.
→ However, the regulation of endocrine function by the anterior pituitary is influenced by circulating hormones
that provide feedback to the structures that influence their release: pituitary gland, hypothalamus, other sites →
The function of most hormonal feedback is the maintenance of stable blood levels of the hormones.
- High gonadal hormone levels usually have effects on the hypothalamus and pituitary that decrease
subsequent gonadal hormone release
- Low levels usually have effects that increase hormone release.
Regulation by Non-hormonal Chemicals
→ Circulating chemicals other than hormones can play a role in regulating hormone levels.
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