The Gonads 1 (10) Flashcards
How do the gonads (an organ that produces gametes) develop in males and females ?
In males they develop as ovaries and in males they develop as the testes
What are the 2 functions of the gonads
Gametogenesis which is the production of gametes for reproduction. In males this is spermatogenesis and in females this is oogenesis. The other function is Steroidogenesis which is the production of steroid hormones. In males this is androgens (and small amounts of oestrogen and progesterone) and in females it is oestrogens and progesterones (and small amounts of androgens).
what are the female germ cells
oogonia
what are the male germ cells
spermatogonia
Gametogenesis
Gametes are derived from germ cells, which multiply and increase in number before birth.
Spermatogenesis
Spermatogonia levels remain relatively constant through life (6-7million). These spermatogonia don’t become activated until puberty. Spermatogonia are diploid. Divide at puberty to form primary spermatocytes. Some primary spermatocytes return to quiescent stage so a pool of spermatogonia remain available for subsequent spermatogenic cycles throughout life. Males retain spermatogenic capability thoughtout life, producing 300-600 sperm/gm testis/second.
What are the steps of the spermatogenesis cycle
Germ cells in embryogenesis multiply and differentiate to produce spermatogonia which are diploid. Around puberty - when FSH starts to be released - the spermatogonia divide by mitosis to produce either more spermatogonia or to produce primary spermatocytes. Primary spermatocytes are also diploid.
Primary spermatocytes enter the first meiotic division to give secondary spermatoctyes which are haploid. Secondary spermatocytes enter second meiotic division to give spermatids . Spermatids then mature and differentiate into spermatozoa. Whole cycle takes approximately 65-70 days.
what are the steps of oogenesis
Germ cells multiply up to produce diploid oogonia. The oogonia multiply again to produce primary oocytes. Primary oocytes immediately enter their first meiotic division. At prophase, development is halted for 12-50 years so they stay in this stage of development. The oocytes form a layer of cells around them - primordial follicles. This all occurs before birth. Rapid atresia of the oogonia occurs before birth. At puberty, FSH rescues some oocytes and they continue development. The oocytes complete meiotic division by ovulation time, to produce secondary oocyte and polar body. The secondary oocyte enters second meiotic division and does complete that division until fertilisation. If fertilsed, it will produce the ovum and the second polar body.
Ooegenesis
Initial total number of oogonia in primordial follicles is approx 6 million; early entry into meiosis halted in prophase. Primordial follicles, arrested in their develipment, enter atresia. By the time of birth the total number of oogonia is approx 2 million and by puberty is is 0.5 million. Only 300-400 reach ovulation.
where does spermatogenesis take place
in the seminiferous tubules of the testes
where do the testes develop
in the abdomen but descend into the scrotum just before birth
what happens if the testes do not descend
could lead to infertility
what is critical for spermatogenesis
that the scrotum is 2-3 degrees cooler than the core temperature
what lines the seminiferous tubules
layers of spermatogonia and layers of sertoli cells
what happens to the spermatozoa after they are produced
are released into the lumens of the seminiferous tubules, where they migrate to the Rete Testis where they are concentrated and drained by the Vasa efferentia into the epididymis where they are stored
what happens to the spermatozoa in the epididymis
nutrients are secreted for them. they mature here and attain their motility. Then they are propelled via the vas deferens (surrounded by smooth muscle) to the urethra
what is the structure of the seminiferous tubule
a lumen surrounded by a layer of elongated Sertoli cells. Above this is a layer of spermatogonia and above that is a layer of connective tissue
how are the sertoli cells connected and why is it important
at the periphery by tight junctions. this forms a blood testis barrier which keeps large proteins i.e antibodies out - so protects spermatozoa from immune reactions
how do spermatogonia move through the seminiferous tubule
Spermatogonia are engulfed into the sertoli cell, where they develop in the cytoplasm into primary and secondary spermatocytes and are released into the lumen of the tubule as spermatozoa
what do the sertoli cells provide
provide structural support for spermatogonia and provide metabolic support
What lies outside the seminiferous tubules
Clusters of Leydig cells which are the site of testosterone production
what are the two types of testicular cells
sertoli cells nd leydig cells
why is it important that the leydig cells are nearby to the seminiferous tubules
because testosterone is needed for spermatogenesis to take place
Summarise the sertoli cells
They form the seminiferous tubules. synthesise FSH and androgen receptors. In response to FSH produce inhibin. Intimately associated with developing spermatocytes
Summarise the leydig cells
Lie outside the seminiferous tubules. Synthesise LH receptors. In response to LH are the principal source of testicular androgens (mainly testosterone)
what does inhibin do
feeds back on the axid and inhibits FSH secretion
Describe the structure of the ovarian stroma
It consists of primordial follicles undergoing atresia. A graffian follicle (maximum size and is ready for ovulation. Remnants of corpus luteum (after ovulation)
What does a graffian follicle consists of
contains a maturing oocyte that is surrounded by fluid. 2 layers of cells surround the follicle:
inner granulosa cells and outer thecal cells
what steroid hormones do the gonads produce
progestogens (C21), androgens (C19) and oestrogens (C18)
why do the gonads produce different steroid hormones to the adrenals
the enzymes present in the tissue determines the hormones produced
what steroid hormones do the adrenals produce
mineralcorticoids and glucocorticoids and small amounts of androgens
What steroidogenesis reactions only occur in the adrenals
progesterone to deoxycorticosterone and eventually to aldosterone. Also 17-hydroxyprogesterone to 11-deoxycortisol and down to cortisol
what steroidogenesis reactions only occur in the gonads
The testes can convert androsteronedione to testosterone. The ovaries will then aromatise androstenedione and testosterone in to oestrone which can then be converted to 17b-oestradiol
How long does the menstrual cycle last
28 days approx. (20 to 35 days)
when does ovulation occur
day 14
what does the menstrual cycle consist of
The ovarian cycle (ovary) and the endometrial cycle (uterus) which both happen simultaneously
what is the ovarian cycle (3 stages)
Follicular phase (first half), Ovulation (mid-cycle), Luteal Phase (second half)
what is the endometrial cycle (2 phases)
proliferative phase and secretory phase
what is the endometrium
the lining of the uterus and contains a superficial epithelial layer and a deeper stromal layer
what causes the proliferative phase in the endometrial cycle
the oestrogens produced in the follicular phase. There is an increase in mitosis and an increase in the progesterone and oestrogen receptors. the endometrium thickens and moistens. the endometrial glands get enlarge, coil and increase blood supply
How are the follicular phase and proliferative phase connected?
The Follicular phase of the ovarian cycle produces oestrogen (17β- OESTRADIOL. This stimulates proliferative phase of endometrial cycle
How are the luteal phase and secretory phase connected?
The luteal phase of the ovarian cycle produces progesterone and 17β-estradiol. This stimulates the secretory phase of endometrial cycle
what is the effect of progesterone
it reduces the proliferation that oestrogen causes by reducing oestrogen receptors. it also causes the endometrium to become secretory. The glands produce glycogen and mucopolysacharides. The glands become engorged with blood
What are the first 5 steps of the menstrual cycle
- Pituitary gland produces FSH so FSH concentration increases.
- Many follicles develop in response to FSH (follicle stimulating hormone)
- Follicles secrete oestrogen as they develop
- Oestrogen has a negative feedback on FSH.
- FSH is inhibited and so the follicles stop producing oestrogen and they undergo atresia
What are steps 6-10 of the menstrual cycle
- One follicle will be dominant and continues to produce oestrogen.
- The dominant follicle develops further, continuing to produce oestrogen.
- Oestrogen levels continue to rise
- High levels of oestrogen cause a switch in the pituitary. There is no longer a -ve feedback on FSH/LH, but a + one instead
- This causes an LH surge, together with some FSH
What are steps 11- 15 of the menstrual cycle
- LH (luteinising hormone) causes ovulation.
- After ovulation, the empty follicle is transformed into corpus luteum which produces large amounts of progesterone
- Progesterone levels continue to rise
- This causes a -ve feedback on FSH and LH so their levels fall.
- Since FSH/LH levels fall, so do progesterone levels
What are steps 16- 18 of the menstrual cycle
- The corpus luteum breaks down (menstruation - the endometrium becomes necrotic and sheds)
- The falling levels of progesterone means there is less inhibition of gonadotrophins (FSH/LH)
- The pituitary gland produces FSH. The concentration of FSH increases and the cycle restarts
why does body temperature increase after ovulation
because of progesterone
What is the pre antral follicle
The follicle containing the oocyte develops over its embryonic life in the absence of gonadotrophins . It is surrounded by layer of cells
what is the early antral follicle
FSH allows further development. If FSH levels aren’t high enough to continue development they undergo atresia. The follicle is surrounded by granulosa cells and thecal cells. Antral filled space
What is the late antral follicle
Same as early antral follicle. Follicle increases in size, therefore more antral filled space surrounding ovum
What is the graffian follicle
Under the influecne of FSH, the follicles get bigger until they reach their maximum size and it is ready for ovulation. LH surge occurs and causes the follicle to rupture and release an egg.
what happens to the graffian follicle after ovulation
it is transformed into a corpus luteum which continues to secrete oestrogen and progesterone in the luteal phase of the cycle
what does the surge of LH stimulate the egg to do
complete` the first meiosis
where are the LH receptors in the follicle
thecal cells
where are the FSH receptors in the follicle
granulosa cells
what produces androgens in the ovary
the thecal cells
how do the granulosa cells produce oestradiol
androgens stimulate them to convert androgens to oestrogen using aromatase enzymes
what does the corpus luteum do after ovulation
it produces large amounts of 17b-oestradiol and Progesterone because the cells are still stimulated by LH and FSH
