Reproductive Physiology

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Reproductive Physiology

• Focus on Reproductive Hormones

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Overview of Endocrine Glands and Hormones

• 3 characteristics that define hormones
• They are synthesized by specific tissues or
  glands
• They are secreted into the bloodstream, which
  carries them to their site (s) of action
• They change the activities of target tissues or
  organs

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Overview of Endocrine Glands and Hormones cont

• those hormones found in metazoans belong to 4
  structural categories
• Amines e.g. catecholamines epinephrine
  norepinephrine thyroid hormones
• Eicosenoids e.g. prostaglandins, leukotrienes
• Steroids e.g. testosterone estrogen - cyclic
  hydrocarbon derivatives synthesized from
  cholesterol
• Peptide protein hormones e.g. insulin this
  group has largest hormones are most complex

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Overview of Endocrine Glands and Hormones cont

• In contrast to NT, which mediate rapid signaling
  over short distances, hormones communicate over
  longer distances on a longer time scale
• Endocrine systems are well suited for regulatory
  functions that are sustained for minutes, hours
  or days
• Functions include maintenance of blood osmolarity
  (ADH) blood sugar (insulin), regulation of
  metabolic rates (growth hormone thyroxine),
  control of sexual activity reproduction (sex
  hormones) modification of behavior

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Overview of Endocrine Glands and Hormones cont

• The rapid activity of nervous system slower,
  more sustained activity of endocrine system work
  together in integration of physiological
  metabolic functions to the point that a given
  molecule may serve as a NT in some circumstances
  a hormone in others
• Hormones are generally secreted at a relatively
  low resting level that is modulated up or down by
  signals acting on the endocrine tissues signals
  are often neurohormones, which are released from
  specialize neurons act directly on the
  endocrine tissue
• Table 9-1 p. 313-314 overview of vertebrate
  endocrine glands, tissues their hormones

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Physiological Effects of Hormones

• Most hormones produce tissue-specific
  physiological effects - a given hormone generally
  induces responses only in selected tissues may
  induce different responses in different tissues
• specificity in hormonal action depends partly on
  the distribution of the components of
  hormone-triggered signaling pathways (especially
  receptors) partly on the preferential
  expression of effector proteins in different
  tissues - categories range from metabloic
  developmental hormones (e.g. catecholamines
  glucocorticoids) to those that regulate water
  electrolyte balance (e.g. ADH) to reproductive
  hormones (estrogens androgens)

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Reproductive Hormones (vertebrates)

• Produced from cholesterol in gonads (ovaries or
  testes) adrenal cortex of either sex
• Cholesterol is 1st converted to progesterone
  which is then transformed into the androgens
  (androstenedione testosterone)
• These are then converted into the estrogens
  (estradiol-17 beta is most potent)
• Bind to intracellular receptors modify the
  expression of specific genes

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Reproductive Hormones (vertebrates) cont

• (also 2 peptide hormones produced in pituitary
  gland function in parturition oxytocin
  lactation prolactin)
• Estrogens androgens are NB in both sexes in
  various aspects of growth, development
  morphological differentiation as well as the
  development regulation of sexual reproductive
  behaviors cycles
• Androgens predominate in males estrogens in
  females table 9-9 p. 344

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Reproductive Hormones (vertebrates) cont

• Production secretion of steroid sex hormones in
  both sexes are promoted by follicle-stimulating
  hormone (FSH) luteinizing hormone (LH) which
  are synthesized in anterior pituitary gland
• Are released from anterior pituitary in response
  to gonadotropin-releasing hormone (GnRH) from the
  hypothalamus
• The steroid sex hormones exert a negative
  feedback on GnRH-secreting neurons of
  hypothalamus on anterior pituitary endocrines
  cells that produce FSH LH

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Steroid Sex Hormones in Males Fig 9-46

• Seminiferous tubules of mammalian testes are
  lined with germ cells Sertoil cells
• Binding of FSH to receptors on Sertoli cells
  stimulates spermatogenesis in germ cells after
  sexual maturity (either continuously or
  seasonally depending on species)
• Sertoli cells support development of sperm are
  responsible for synthesis of androgen-binding
  protein inhibin

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Steroid Sex Hormones in Males

• Between seminiferous tubules are interstitial
  cells (Leydig cells) that produce secrete sex
  hormones, particularly testosterone
• Testosterone inhibin provide inhibitory
  feedback to hypothalamic centers controlling GnRH
  production (thus diminish release of FSH LH)
• Androgens trigger development of primary male sex
  characters (penis, vas deferens, seminal
  vesicles, prostate gland epididymis) in the
  embryo 20 sex characters (lions mane,
  roosters comb, mens facial hair) at puberty
• Androgens also contribute to general growth
  protein synthesis (especially myofibrillar
  proteins in muscle)

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Steroid Sex Hormones in Females

• Estrogen does not play same role androgens do in
  prenatal differentiation of embryonic genital
  tract however, estrogens stimulate later
  development of primary sexual characteristics
  (e.g. uterus, ovaries vagina) 20 sex
  characters (e.g. breasts) for regulation of
  reproductive cycles
• Reproductive cycles arise from within the animal
  under control of neuroendocrine system, but these
  inner cycles are constrained by environmental
  signals such as changes in day length that
  accompany the changing seasons (e.g. climate that
  increases survivability of young)

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Steroid Sex Hormones in Females cont

• Female mammals birds are born with a full
  complement of oocytes, each of which becomes
  embedded in a follicle within the ovary is
  capable of developing into one ovum
• Most follicles oocytes degenerate early, but
  even before puberty some develop just short of
  yolk formation or maturation (e.g. humans 400
  ova are available for release between menarche
  onset of menstruation menopause lower
  vertebrates oogenesis occurs throughout life)

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Steroid Sex Hormones in Females

• Female mammals reproductive cycle is composed
  of follicular phase luteal phase fig 9-47 p.
  346
• Follicular phase
• begins with release of FSH which stimulates
  development of 15-20 ovarian follicles which are
  fluid-filled cavities containing an ovum
  enclosed by a membranous sac of several cell
  layers (including theca interna ovarian
  granulosa)
• LH then stimulates theca interna to synthesize
  secrete androgens

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Steroid Sex Hormones in Females cont

• FSH stimulates production of enzyme that converts
  androgens to estrogens in ovarian granulosa
  leading to substantial increase in estrogen
  levels
• These high levels of estrogens just prior to
  ovulation, activate hypothalamus anterior
  pituitary gland producing surge in FSH LH
  release (example of positive feedback) FSH
  accelerates maturation of developing follicles
  (depending on species, 1 or more ova matures
  under influence of LH, follicle ruptures
  releasing ova ovulation)

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Steroid Sex Hormones in Females cont

• Luteal phase
• Begins with ovulation, estrogen secretion
  declines LH transforms ruptured follicle into a
  temporary endocrine tissue corpus luteum
• Corpus luteum secretes estrogens progesterone
  which exert negative feedback on GnRH released by
  hypothalamus leading to decreased secretion of
  FSH LH inhibin released along with ovum acts
  on anterior pituitary inhibiting FSH (not LH)

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Steroid Sex Hormones in Females cont

• Progesterone stimulates secretion of endometrial
  fluid by endometrial tissue, preparing it for
  implantation of fertilized ovum (in absence of
  fertilization implantation of ovum, corpus
  luteum degenerates secretion of estrogens
  progesterone subsides (in humans some primates,
  this precipitates menses i.e. shedding of
  endometrial lining)
• With reduction in estrogen, progesterone
  inhibin levels, FSH LH secretion by pituitary
  increases again initiating a new cycle

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Steroid Sex Hormones in Females cont

• If ovum is fertilized as it travels down ciliated
  fallopian tube, it becomes implanted in
  endometrium
• the developing placenta begins to produce
  chorionic gonadotropin (CG)
• CG (action similar to LH) induces further growth
  of active corpus luteum so that estrogen
  progesterone secretion continues
• Placenta begins secreting CG within 1 day of
  implantation effectively takes over
  gonadotropic function of anterior pituitary
  during early pregnancy by maintaining corpus
  luteum

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Steroid Sex Hormones in Females cont

• Pituitary FSH LH are not secreted again until
  after parturition corpus luteum continues to
  grow to secrete estrogen progesterone until
  placenta fully takes over production (then corpus
  luteum degenerates) some species differences
  occur as to timing role of CL
• Duration of follicular luteal phases vary among
  mammal groups (in primates they are equal in
  nonprimate mammals, luteal phase is much shorter)
  - cycles/year varies e.g. 1/year in spring to
  multiple cycles/year

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Steroid Sex Hormones in Females cont

• During gestation, progesterone estrogens
  secreted from CL or placenta initiate growth of
  mammary tissues of mother in preparation for
  lactation
• Prolactin placental lactogen (produced in
  placenta) also aid in preparing mammary glands
  for lactation (milk synthesis is inhibited by
  progesterone during pregnancy)
• Negative feedback of estrogens progesterone on
  hypothalamus anterior pituitary prevents
  release of FSH LH during pregnancy, preventing
  ovulation (birth control pills contain small
  amounts of progesterone estradiol or their
  analogs mimic steroids of early stages of
  pregnancy preventing ovulation act on
  endometrium to provide highly effective means of
  avoiding conception)