The gonads Flashcards
Functions of the gonads
Males?
Females?
- Gametogenesis - Production of Gametes
Males = Spermatogenesis
Females = Oogenesis - Steroidogenesis- Production of Steroid Hormones
Males = Androgens (and small amounts of oestrogen and progesterone)
Females = Oestrogens and Progesterones (and small amounts of androgens)
Activation of Germ Cells During embryogensis? Males cell name? Females cell name? Number of germ cells in males vs females? Puberty in males vs females? Reproductive life of females? Menopause in females?
Germ cells proliferate, reaching large numbers around 6-7 million
In males, germ cells are spermatogonia
In females, germ cells are called oogonia
During embryogenesis in males, this high number of spermatogonia remains throughout life
In females, the oogonia multiply up and reach maximum levels at 24 weeks - after this, no more oogonia are produced = finite number of eggs. Start off with around 6-7 milion however a process of ATRESIA commences:
Atresia - the cells start degenerating and dying off
Atresia is rapid at first - by birth there are only about 2 million oogonia left
In males during childhood, spermatogonia have a dormant period + don’t become activated until puberty
In females, by puberty there are only about 400,000 oogonia left
In the female reproductive life, only about 300-400 of these cells reach maturation and ovulation - very few are fertilised
By menopause the ovary has been depleted of these cells
Spermatogenesis
Where does it occur?
How long does the process take?
Sertoli cells
- Start off with the germ cells in embryogenesis: multiply and differentiate to produce spermatogonia which are DIPLOID
- Around puberty - when FSH starts to be released, spermatogonia divide by mitosis to produce either more spermatogonia or to produce primary spermatocytes= pool of spermatogonia remains for subsequent spermatogenic cycles throughout life
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
This whole process takes around 70 DAYS
Oogenesis
- Germ cells multiply to produce oogonia - at this point they are all DIPLOID
- The oogonia then multiply again to produce Primary Oocytes
- As soon as the Primary Oocytes are formed they enter the first meiotic division
- As soon as they get to the first stage in meiosis (Prophase) the development is halted so they stay in that stage of development
- At this time, the oocytes form a layer of cells around them (primordial follicles - the oocytes are in a follicle)
This all occurs before birth - you have your primordial follicles with your oocytes in it before birth - The oocytes are dormant for the next 12-50 years (stuck in that phase of meiosis)
- At puberty, some of the oocytes will be rescued under the influence of FSH - the FSH will rescue a group of these cells to continue development and the last lot won’t be rescued until around menopause
- While they’re in this stage there is a process of atresia-some of the cells degenerate and start to die
- At the time of ovulation, they complete the first meiotic division to produce the Secondary Oocyte and a Polar Body
In females in meiosis, there is an unequal distribution of the cytoplasm - one cell retains all the cytoplasm and the cells resources and the other daughter cells (the polar body) contains just the chromosomes and these cells will eventually die off - The secondary oocyte enters the second meiotic division and it won’t complete that division until FERTILISATION
- If fertilised, it will produce the Ovum and the Second Polar Body
Ovum and Second Polar Body - again there is unequal distribution of cytoplasm - the ovum retains all the cells resources (becomes one of the largest cells in the body) - the Second Polar Body will eventually disintegrate
The testes
Where in the testes does spermatogenesis occur? Structure?
Development of testes? Consequences of dvelopmental problem?
Conditions of scrotum? Importance?
Pathway of spermatozoa?
Maturation of spermatozoa?
Propogation of spermatozoa?
(slide 9, lecture 10)
Seminiferous tubules of the testes
Seminiferous tubules are lined by layers of spermatogonia and layers of Sertoli cells
Testes develop in the abdomen but descend into scrotum just before birth , if testes don’t descend could lead to infertility
Scrotum =2-3 degrees cooler than core temperature - which is critical for spermatogenesis
The spermatozoa produced travel down to the collection in the Rete Testis where they are concentrated and drained by the Vasa efferentia into the epididymis where they are stored
In the epididymis, nutrients are secreted for them - they mature in here and attain their motility
Then they are propelled via the vas deferens (surrounded by smooth muscle) via the urethra
Seminiferous Tubule
Structure?
Connections of Sertoli cells? Importance?
Movement of spermatogonia?
What are just outside Sertoli cells? Function? Importance of location?
Other functions of Sertoli cells?
(slide 10, lecture 10)
Sheath of connective tissue and underneath that there is a layer of spermatogonia, under that there is a layer of elongated Sertoli cells
The Sertoli cells are connected at the periphery by tight junctions- forms a blood-testis barrier which keeps out large proteins such as antibodies= protects spermatozoa from immune reactions
Spermatogonia can get through barrier by:
Spermatogonia move into the Sertoli cells→ enclosed in the cytoplasm of the Sertoli cells where spermatogenesis actually takes place
As these cells develop they move towards the lumen and are released into the lumen
Just outside the Sertoli cells are Leydig Cells: Contain enzymes so that they can make + secrete testosterone
Important that the Leydig cells = nearby because testosterone is needed for spermatogenesis to take place
Sertoli cells provide structural + metabolic support for spermatogonia
Cross-section of testes
Tubules= Seminiferous tubules packed together Between = clusters of Leydig cells
Hormones that affect testes
Responses to hormones?
Sertoli cells= FSH+ androgen receptors
In response to FSH, produces Inhibin= feeds back on axis+ inhibits FSH secretion
Leydig cells= LH receptors
In response to LH, produces androgens (particularly testosterone)
Ovaries Function? Contains? Where are all follicles embedded in? Other structures?
(slide 13, lecture 10)
Gametogenesis starts off in the ovaries
Ovaries contain follicles at all different stages of development
Graffian Follicle - reached its maximum size and is ready for ovulation
All follicles embedded in the ovarian stroma
Remnants of the last corpus luteum (follicle)
Steroidogenesis in the gonads
What are all steroids synthesised from?
What determines hormones produced?
Cholesterol
The enzymes present in the tissue
Adrenals= mineralocorticoids + glucocorticoids + (small amounts of) androgens GONADS = progestogens (C21) + androgens (C19) + oestrogens (C18)
Reactions in Steroidogenesis
Difference between gonads and adrenals?
Men vs Women?
Control of reproduction involves?
(slide 15, lecture 10)
The first four reactions in steroidogenesis are COMMON to both the gonads and the adrenals:
Cholesterol→ Pregnelanone→ Progesterone→ 17-OH-Progesterone→ Androstenedione
Only adrenals contain the 21-hydroxylase enzyme that convert progesterone →deoxycorticosterone→ aldosterone (eventually)
Or the same enzyme can convert 17-hydroxyprogesterone→ 11-deoxycortisol→ cortisol (eventually)
Primarily the testes contain the 17-hydroxysteroid dehydrogenase enzyme which converts androsteronedione→ testosterone
The ovaries will then aromatise androstenedione and testosterone into oestrone→ 17b-oestradiol (eventually)
The control of reproduction involves the hypothalamus, the pituitary and the gonads
Very controlled in the female (one egg released every month)
Menstrual Cycle Lasts approx? Begins? Important part of cycle? Events of Menstrual Cycle? Layers of endometrium? Effect of oestrogen? Effect of progesterone?
28 days
First day of menstruation
Ovulation which occurs around day 14 (mid-cycle)
- Ovarian Cycle (in ovaries) consists of:
Follicular Phase (first half)
Ovulation (mid-cycle)
Luteal Phase (second half): hormones produced= progesterone and 17b-oestradiol, induces secretory phase - Endometrial Cycle (in uterus) consists of:
Proliferative: Oestrogens (produced in follicular phase) which cause effects in this phase)
Secretory
Endometrium= lining of the uterus, contains
Superficial epithelial layer
Deeper stromal layer
Oestrogen causes proliferation of the endometrium, leads to:
Increase in mitosis
Increase in the progesterone receptors (acts later in the cycle)
Increase in oestrogen receptors
The endometrium gets thicker, the glands get bigger and the blood vessels get longer
Progesterone reverses the effects of oestrogen- reduces the proliferation that oestrogen causes by reducing oestrogen receptors
Also increases secretory activity cells in the myometrium (middle layer of the uterus lining)- glands become wider and produce various substances- make the environment suitable for implantation to occur
Variation in hormones during menstrual cycle
Effect of progesterone on body?
Graphs of hormones?
(slide 19, lecture 10)
The cycle can be divided into the follicular phase (1st half) and the luteal phase (2nd half) - it lasts about 28 days and ovulation occurs mid-cycle
- Menstruation occurs in the first 5 days of the cycle
- FSH levels= slightly raised at the beginning - enough to rescue the dormant follicles so they continue their development
- These follicles start growing start producing 17b-oestradiol- has negative feedback effect on gonadotrophin secretion
- As follicles grow, one of them = selected as DOMINANT follicle, others undergo atresia
- The dominant follicle will produces large amounts of oestrogen
- If oestrogen levels high enough for long enough (36 hours), instead of producing negative feedback effect, switches to positive feedback effect= stimulates a surge of gonadotrophins (LH+ FSH). May be small surge of 17-hydroxyprogesterone = adds to the positive feedback effect that oestrogen has
- Surge of gonadotrophins stimulates ovulation
- After ovulation, empty follicle is transformed into corpus luteum: continues to produce 17b-oestradiol and produces large amounts of progesterone
- Luteal Phase now: progesterone= dominant hormone
- Effect of oestrogen and progesterone together= negative feedback effect on gonadotrophins decrease gonadotrophin levels
- If fertilisation does NOT take place - oestrogen + progesterone levels begin to decrease= changes in endometrium
- Endometrium - vasospasm in some blood vessels, necrosis of tissue, contraction of the muscle, loss of the tissue with the blood - MENSTRUATION
- Because of the decrease in progesterone and oestrogen, decreased negative feedback= FSH levels start to increase= start of cycle
Progesterone has an effect on body temperature- can measure increase in body temperature after ovulation
Ovarian (follicular) cycle?
- Follicles have oocyte that is stuck in early stage of meiosis
- The follicle develops over embryonic life + develops to pre-antral stag. Up until the pre-antral stage, gonadotrophins are NOT needed
- To develop any further requires FSH (to get to early-antral stage)
- If FSH isn’t at a high enough level to continue the development they will undergo atresia
- When exposed to sufficient FSH, develop into Early-Antral Follicle
- The outer layers of the follicle= Thecal cells and the inner layer = Granulosa cells
- Under influence of FSH, the follicles will get bigger until eventually get to maximum size= Graffian Follicle which = ready for ovulation
- LH surge occurs+ causes rupture of the follicle+ release of the egg
- After ovulation, the follicle transformed into corpus luteum: continues to secrete oestrogen and progesterone in Luteal phase of the cycle
- The surge of LH will stimulate the egg to complete the first meiosis
Hormone secretion during ovarian (follicular) cycle
Cells in follicle?
Actions?
After ovulation?
Thecal Cells - outer - LH Receptors
Granulosa Cells - inner - FSH Receptors
Thecal Cells:
In response to LH stimulation, start steroid hormone synthesis and produce androgens
Thecal cells do NOT have the aromatase enzymes to convert androgens to oestrogen
Granulosa cells:
Have FSH Receptors, stimulation of these cells stimulates aromatase enzyme which converts androgens into oestradiol
After ovulation, the corpus luteum is formed
Thecal cells +Granulosa cells will still stimulated by LH +FSH , continue to produce large amounts of 17b-oestradiol +Progesterone
