What is the function of the gonads?
gametogenesis
- spermato/oogenesis
steroidogenesis
- males (androgens, small amounts of oestrogen/progesterone), females (oestrogen/progesterone
Describe the activation of germ cells
- Germ cells (spermatogonia/oogonia) proliferate in embryogenesis to 6/7million
- male = high number throughout life, dormant until puberty
- female = oogonia reach maximum at 24wks but no more produced after this (finite number)
- rapid atresia of 6/7 million cells at first so only 2 million remain at birth
- puberty 400,000 left and by menopause the ovary is depleted of these cells
- only 300/400 cells mature and ovulate, and even fewer are fertilised
Describe the process of spermatogenesis
- embryogenesis: germ cells multiply and differentiate to spermatogonia (diploid)
- puberty: FSH released, cells divide by mitosis to more spermatogonia/primary spermatocytes (diploid)
- primary cells enter 1st step of meiotic division to become secondary spermatocytes (haploid)
- secondary enter 2nd step to become spermatids
- these mature and differentiate to spermatozoa
70 DAYS
in cytoplasm of Sertoli cells
What two options for spermatogonia?
self renewal
- pool of spermatogonia available for other spermatogenic cycles throughout life
differentiation to primary spermatocytes
males retain spermatogenic capability throughout life producing 300-600sperm/gm testis/second
Describe the process of oogenesis
BEFORE BIRTH
- germs cells multiply to form diploid oogonia
- these multiply to form primary oocytes
- these enter 1st step of meiotic division and are halted at first stage (PROPHASE)
- layer of cells (primordial follicles) form around oocytes
- dormant for next 12-50 years, atresia causes come to degenerate and die
PUBERTY
- FSH rescues some oocytes
- at ovulation, primary oocytes complete 1st meiotic division to form secondary oocyte and polar body
- secondary oocyte enters 2nd division that is incomplete until fertilisation
FERTILISATION
- secondary oocyte divides to ovum and second polar body
Why is a polar body formed in oogenesis?
meiosis in females causes unequal distribution of cytoplasm (ovum/secondary oocyte retains cell resources, polar body keeps chromosomes and eventually disintegrates)
What happens to primordial follicles before puberty?
enter atresia where some cells start to degenerate and die
Number of oogonia remaining to develop at birth and puberty?
birth - 2 million
puberty - 0.5 million
Why does spermatogenesis take place in the testes in the scrotum?
- testes develop in abdomen and descend into scrotum just before birth (if no descent –> infertility?)
- spermatogenesis is in seminiferous tubules of testes
- scrotum 2-3 degrees cooler than core body temp which is important for spermatogenesis
Describe the structure of the testes?
coiled seminiferous tubules lined by layers of spermatogonia and layers of Sertoli cells
spermatozoa travel to RETE TESTIS to concentrate and drained by VASA EFFERENTIA into EPIDIDYMIS where they are stored
in epididymis nutrients secreted for cells to mature and attain motility
spermatozoa propelled via VAS DEFERENS (surrounded by smooth muscle) to urethra
Describe the structure of the seminiferous tubule?
- sheath of connective tissue surrounds layer of spermatogonia above layer of Sertoli cells
- Sertoli cells connected at periphery by tight junctions
- spermatogonia special mechanism to get through the barrier, move into Sertoli cells for spermatogenesis
- develop and released into lumen
What does Sertoli cell tight junctions ensure?
blood-testis barrier that keeps out large proteins such as antibodies
protect spermatozoa from immune reactions
What is the function of Sertoli cells in spermatogenesis?
structural and metabolic support for spermatogonia
produce FSH and androgen receptors and responds to both
produce inhibin that feeds back on axis to stop FSH secretion
What is the function of LEYDIG CELLS?
just outside Sertoli cells, outside seminiferous tubules
contain enzymes to make and secrete testosterone that is needed for spermatogenesis
synthesise LH receptors - produce androgens (testosterone in particular)
Describe the structure of ovaries?
ovarian follicles all at different stages of development
some undergoing atresia
follicle at maximum size ready for ovulation = graffian follicle
follicles embedded in ovarian stroma
What steroids are produced by the gonads?
progestogens C21
androgens C19
oestrogens C18
Describe the production of testosterone
mainly in testes
- they contain 17-hydroxysteroid dehydrogenase enzyme to convert androstenedione to testosterone
- converted to dihydrotestosterone
Describe the production of 17beta-oestradiol
in ovaries
- aromatise androstenedione and testosterone to oestrone
- converted to 17b-oestradiol
- controlled so in female one egg only release every month
What 2 events occur in the menstrual cycle?
OVARIAN CYCLE
follicular phase
ovulation
luteal phase
ENDOMETRIAL CYCLE
proliferative
secretory
What are the processes of the endometrial phase?
endometrium (uterus lining) = superficial epithelial layer and deeper stromal layer
middle myometrial layer
PROLIFERATIVE PHASE
- oestrogens made in follicular phase cause effects seen in proliferative phase
- cause proliferation (increase mitosis and increase progesterone and oestrogen receptors
- endometrium thicker, larger glands, longer coiled blood vessels
SECRETORY PHASE
- luteal phase produce progesterone and 17b-oestradiol
- progesterone reduced proliferation by oestrogen by reducing oestrogen receptors
- also increase secretory activity of cells in myometrium to make environment suitable for implantation (glycogen and mucopolysaccharides)
if no fertilisation
- oestrogen/prog fall so no longer stimulate endothelium
- spasm of vessels, necrosis of tissue, blood shed
Describe the menstrual cycle
28days (ovulation midcyle, menstruation first 5 days)
FOLLICULAR PHASE
- FSH raised slightly at start to rescue dormant follicles so they grow, produce 17b-oestradiol
- negative feedback of this hormone on gonadotrophin
- one follicle selected to be dominant, others atresia
- dominant produces lots of oestrogen
- if oestrogen high for 36hrs, switches to positive feedback causing a surge of LH/FSH
- this stimulates ovulation
- empty follicle transformed into corpus luteum that continue to make 17b-oestradiol and progesterone
LUTEAL PHASE - progesterone dominant
- oestrogen and progesterone together causes negative feedback effect to gonadotrophs (LH/FSH low)
- if no fertilisation, oestrogen and progesterone fall
- changes to endometrium (vasospasm in vessels, necrosis of tissue, contraction of muscle = menstruation)
- negative feedback effect reduced as oes/pro fall so FSH rises slightly
- back to start of cycle
What can also occur at LH/FSH surge?
small surge in 17-hydroxyprogesterone
adds to positive feedback effect of oestrogen
What is another effect of progesterone?
increase body temperature
seen after ovulation
Describe the development of follicles in the ovary
- follicle develops to pre-antral stage without gonadotrophins
- FSH required to develop to early antral follicle stage (if FSH not high enough undergo atresia)
- under FSH follicles get larger until maximum size Graffian follicle reached for ovulation
- LH surge causes follicle rupture and release of egg
- follicle transformed into corpus luteum that continue to secrete oestrogen and progesterone into luteal phase of cycle
- surge of LH stimulated egg to complete first meiosis
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Introduction to endocrinology46
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Hypothalamo-adenohypophysial system43
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Hypothalamo-neurohypophysial system50
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Insulin secretion34
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Introduction to DM37
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Thyroid gland and iodothyronines21
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Thyroid disorders20
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Adrenal Gland25
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Adrenal Disorders24
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Gonads25
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Gonads 223
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Endocrine control of calcium metabolism35
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Adrenals and corticosteroids17
