Reproductive System 2 - Introduction to Reproductive Hormones Flashcards
The Endocrine System
A collection of glands located throughout the body that manufacture and secrete hormones to regulate physiological function
generally act in a paracrine or endocrine manner
What are hormones?
A chemical substance produced by ductless glands within the endocrine system
The major endocrine glands are:
Adrenal glands
Hypothalamus
Ovaries and testes
Pancreas
Thyroid and parathyroid
Pineal gland
Pituitary gland
Thymus gland
What are the classes of hormones?
Three general classes
Hormones derived from amino acids
Dopamine, epinephrine (tyrosine precursor)
Proteins and peptides
Insulin (peptide), growth hormone (protein)
Hormones derived from lipids
Prostaglandins (derived from phospholipids)
Steroids (derived from cholesterol)
Most RH steroids but stimulated by other classes
Cholesterol
Large molecule
4 cycloalkane rings
Large hydrocarbon chain
Essential component of all cellular membranes
Source?
Diet and de-novo
Sex steroids / hormones
Oestrogens
Progestogens
Androgens
Precursor for:
Steroid hormones
Adrenal steroids
Glucocorticoids
Mineralcorticoids
Reproductive hormones
Commonly called sex steroids or steroid hormones
Produced mainly by the gonads (ovaries and testes)
3 classes of reproductive hormones:
Oestrogens
Androgens
Progestogens
Oestrogens
Oestradiol
Oestrone
Oestriol
Androgens
Testosterone
5a-dihydrotesterone
androstenedione
dehydroepiandrosterone
Progestogens
progesterone
Overview of major functions of reproductive hormones
Bind to specific receptors in target tissues
AR, ER, PR
Exert a range of effects on both organs and the body as a whole
Primary role: maturation of reproductive organs, fertility and pregnancy
Secondary role: development of physical characteristics
Testes anatomy: sites of testosterone production
Testosterone is produced in Leydig cells
Ovarian anatomy: sites of steroid hormone production
label ovary
Ovarian anatomy: sites of steroid hormone production
Oestrogens: ovarian granulosa cells
Androgens: ovarian theca cells
Progesterone: corpus luteum
Reproductive hormones: molecular action
Generic mechanism of action
Hormones enter cells
Hormones bind to intracellular receptor
Conformation change and translocation
DNA binding (HRE’s)
Gene expression (or repression)
Regulation of gonadal steroidogenesis
Endocrine function of gonads is regulated by HPG axis
Hypothalamus
Pituitary
Gonad
Hypothalamus
Small structure at base of forebrain
Hypothalamus releases GnRH (Gonadotrophin Releasing Hormone)
Peptide hormone
Released into blood
Reach pituitary
Pituitary
Pea-sized gland located at the base of the skull ventral to the hypothalamus
Comprised of two elements
Posterior and anterior lobes
Anterior pituitary contains specialised cells (gonadotropes)
GnRH stimulates gonadotropes to
Produce gonadotrophins
glycoproteins
FSH (follicular stimulating hormone)
LH (Lutenising hormone)
hCG (human chorionic gonadotrophin)
Gonadotrophins
LH and FSH
Mechanism of action
bind to different receptors expressed on cells within ovary and testes
LH main functions
♀ acts on ovarian follicle inducing ovulation
♂ acts on testes producing testosterone
FSH main functions
♀ promotes ovarian follicle development & oestrogen production
♂ stimulates spermatogenesis in testes
HPG axis
Hypothalamus > Pituitary > Gonads (release pf the sex hormones)
Reproductive hormones are subject to feedback control
Both positive and negative – different stages of the cycle
Negative feedback
decreases level of signal and maintains homeostasis
Direct negative feedback
Steroid hormones (testosterone/oestrogen) decrease secretion of gonadotrophins (FSH, LH)
Indirect negative feedback
Steroid hormones inhibit their own production indirectly by inhibiting hypothalamus GnRH
> decreased pituitary gonadotrophin secretion
> reduced steroid hormone production
GnRH get released by the
hypothalamus
Where is the pituitary?
just below the hypothalamus
Negative regulation of steroidogenesis by inhibin
Inhibin
Protein hormone
In females, released by granulosa cells in response to FSH
In males, produced by Sertoli cells in response to androgens
Negative regulator of FSH
Positive feedback of reproductive hormones
Positive feedback
leads to an increase in hormone level
Occurs within follicular phase of uterine (menstrual) cycle
Oestrogen-induced LH to trigger ovulation
Detailed representation of the HPG axis:
Hypothalamus
(Hypothalmic neurons) secreting GnRH > onto Pituitary Gland (anterior gondotrophs) > LH and FSH gets released and reproductive hormone synthesis > physiological effects
