The Gonads 1 Flashcards
What are the gonads in males and females?
Males: testes
Females: ovaries
What are the functions of the gonads?
1) Gametogenesis- production of gametes for reproduction (males= spermatogenesis, females= oogenesis)
2) Steroidogenesis- production of steroid hormones (males= androgens and small amounts of oestrogen and progesterone, females= oestrogens and progesterones and small amounts of androgens)
Describe the activation of germ cells
- There are a large number of germ cells during embryogenesis (6-7 million). Males, these germ cells= spermatogonia, females= oogonia
- Males maintain the high number of spermatogonia throughout their life- they are inactive during childhood but become activated during puberty.
- Oogonia multiply up until they reach their max number at 24 weeks- after this, there is a finite number of eggs (6-7 million)
- The oogonia then start to degenerate and die (atresia)
- Atresia is rapid at first and by birth there are only 2 million oogonia left.
- At puberty there is only 400,000 left and menopause is the depletion of all of these cells.
Describe spermatogenesis
Males start off with their germ cell in embryogenesis. They multiply and differentiate to form spermatogonia.
Males hold the spermatogonia until they reach puberty. When they do reach puberty, FSH starts to be released and the spermatogonia divide by mitosis to form primary spermatocytes (diploid).
Primary spermatocytes undergo phase 1 meiosis to form secondary spermatocytes (haploid- (intermediates)). Secondary spermatocytes finish off phase 2 meiosis to form spermatids (haploid).
Spermatids will mature and differentiate into spermatozoa (haploid)
This whole process requires 70 days
Describe oogenesis
- Germ cells multiply to form oogonia- they are diploid
- Oogonia then multiply again to produce primary oocytes
- The primary oocytes enter the first phase of meiosis as soon as they form. However, as soon as they get to prophase 1, they are halted and they stay in that stage of development.
- At this point, a layer of cells form around them called primordial follices.
- The oocytes are in this state for the next 12-50 years. After 12 years, some of oocytes are rescued from this state, under the influence of FSH at puberty
- However, there some cells that will die and disintegrate (atresia)
- At ovulation time in a cycle, is the time that the first meiotic division will be complete, converting a primary oocyte to a secondary oocyte and polar body
- The second meiotic phase will occur at fertilisation, if it occurs. If it DOES occur an ovum and second polar body is produced. The ovum retains all the cell’s resources and therefore becomes one of the largest cells in the body.
What is a polar body?
In female meiosis, there is an unequal distribution of the cytoplasm- one cell retians all of the cytoplasm and material/ resources. The other daughter cells just contain the chromosomes and these cells will die off. The BTEC cells are the polar bodies
Describe the testes and the pathway spermatozoa take
Spermatogenesis occurs in the seminiferous tubules. The scrotum is 2-3 degrees cooler than core temperature- this is critical for spermatogenesis (which is why is on the periphery of the body)
The seminiferous tubules are lined by layers of spermatogonia and layers of sertoli cells. When spermatozoa are produced, they travel down to collection in the rete testis.
They are concentrated here and drained by the vasa efferentia into the epididymis where they are stored. Nutrients are secreted in the epididymis so that the spermatozoa can mature and obtain motility.
From here, they are propelled through the vas deferens (surrounded by smooth muscle) via the urethra.
Describe the seminiferous tubule
- A sheath of connective tissue surrounds the seminiferous tubule.
- The sertoli cells are connected at the peripheral by tight junctions- this provides the blood-testis barrier which keeps antibodies out. This protects spermatozoa from immune reaction.
- Spermatogonia move into the sertoli cells (they can get through the barrier).
- Spermatogenesis actually takes place in the cytoplasm of the sertoli cells.
- They develop towards the lumen.
- Outside the sertoli cells are the leydig cells.
- The leydig cells contain enzymes so that they can make and secrete testosterone.
What is important about the leydig cells?
They make testosterone which is important for spermatogenesis to take place.
They also synthesise LH receptors- when LH binds to the LH receptor, testicular androgens are produced (namely testosterone)
What do the sertoli cells do?
They provide structural support for the spermatogonia and provide metabolic support.
They also synthesise FSH and androgen receptors. They produce various molecules including INHIBIN in response to FSH
They are important for the development of spermatocytes
Histology of the testis
Describe the ovaries
Gametogenesis starts in the ovaries- there will be follices in there at different stages of developments (depending on when their arrest was lifted).
Some follicles will undergo atresia.
A follicle that has reached its maximum size is known as the Graffian Follicle- this follicle is ready for ovulation.
All follicles are embedded in the ovarian stroma.
There are often remenants of the last follicles that have been used- the last corpus luteum.
Which steroid hormones are synthesised in the gonads?
Progestogens (C21)
Androgens (C19)
Oestrogens (C18)
Remember that they all come from cholesterol and that the enzyme present in the tissue determines which hormones are made.
Adrenals= mineralcorticoids + glucocorticoids (and small amounts of androgens)
NOT THE SAME
Reactions to form hormones
Only the adrenals have 21-hydroxylase enzyme that can convert progesterone to deoxycorticosterone (and eventually to aldosterone)
The same enzyme can convert 17 OH-Progesterone to 11-deoxycortisol and down to cortisol.
It is primarily in the testes that the 17-hydroxysteroid dehydrogenase enzyme exists which converts androsteronedione to testosterone.
The ovaries have the ability to aromatise androstenedione and testosterone in oestrone which can be converted to 17 beta- oestradiol.
Menstrual cycle quick facts
Last approx 28 days (can be from 20-35 days)
First day of menstruation is day 1 of cycle
The important reproductive event during the cycle is ovulation which occurs around day 14
The menstrual cycle consists of 2 cycles working together
1) the ovarian cycle- the follicular phase, ovulation and the luteal phase
2) endometrial cycle- proliferative phase to the secretory phase
What is endometrium?
The lining of the uterus and it contains a superficial epithelial layer and a deeper stromal layer.
The oestrogen produced in the follicular phase causes the effects in the proliferative phase of the endometrial cycle (build up of the uterus wall).
Describe the menstural cycle wrt this picture
Oestrogen produced in the follicular phase causes the proliferation of the endometrium- there is an increase in mitosis and an increase in progesterone receptors and increase in oestrogen receptors.
The endometrim gets thicker and the glands get bigger, blood vessels get longer. Ovulation occurs mid cycle.
In the luteal phase, the hormones produced are progesterone and 17-beta oestradiol.
These hormones induce the secretory phase. PROGESTERONE reverses the effects of OESTROGEN- it reduced the proliferation that oestrogen causes by reducing oestrogen receptors.
Progesterone also increases the secretory activity of the cells in the myometrium (middle lining of the uterus)- glands are wider and prepares the uterus just incase of implantation.
How do the hormones vary in the menstrual cycle?
- FSH levels are raised at the beginning- high enough to rescue dormant follicles so they continue to develop
- These follicles start growing and as they grow they produce 17 beta-oestradiol. This has a negative feedback effect on the gonadotrophin secretion.
- As the follicles grow, one will be selected as the dominant one and the others will undergo atresia. The dominant one will produce a lot of oestrogen
- If oestrogen levels are high enough, there will be a positive feedback effect which stimulates the surge of LH and FSH (gonadrotrophins). This stimulates ovulation
- After ovulation, the empty follicle is transformed into the corpus luteum which continues to form the 17- beta oestradiol and large amounts of progesterone
- Luteal phase- progesterone is dominant. Effect of oestrogen and progesterone is a negative feedback effect on the gonadotrophins, reduced LH and FSH produced.
- If fertilisation does not take place, oestrogen and progesterone levels will fall.
- This results in menstruation (vasopasm of blood vessel and necrosis of tissue)
- Reduction of oestrogen and progesterone, negative feedback is reduced and FSH rises again
Body temperature and progesterone
There will be an increase in body temperature after ovulation.
Follicles inside the ovaries
Follicles are stuck in the early stages of meiosis- this follicle develops over embryonic life to come to this pre-antral stage.
FSH is needed to convert it into an early-antral stage. If there is not enough FSH around, atresia will continue to happen.
The outer layers of the follicle are Thecal cells and the inner ones are granulosa cells.
Due to FSH, the follicles will get bigger until they reach Graffian follicle level when its ready for ovulation.
At this point the LH surge occurs and causes the rupture of the follicle and release of the egg.
After ovulation, the follicle is turned into a corpus luteum.
Hormones receptors in the ovaries
Thecal cells- LH receptors. When Lh binds, they will start steroid hormone synthesis and produce androgens- do not have the aromatase to produce oestrogen.
Granulosa cells- FSH receptors- when FSH binds, they stimulate the aromatase enzymes which can convert androgen to oestradiol.
Summary
oestradiol causes the proliferation of the endometrium. It thickens and stimulates mitosis, increase blood supply.
Progesteron influences secretory stage
