Jones - taught Flashcards
(155 cards)
1
Q
What is the HPG axis
A
The hypothalamic–pituitary–gonadal axis
multiple endocrine glands working together as a system
2
Q
What does the HPG axis do?
A
regulate development, reproduction, and ageing in animals
3
Q
What do endocrine glands do?
A
secrete hormones into the bloodstream; the hormones travel to and act on other organs or tissues
4
Q
What are the components of the HPG?
A
- Hypothalamus
- Anterior pituitary
- Testes/ovaries
5
Q
What is the hypothalamus?
A
Component of forebrain; part of the diencephalon
6
Q
What does the hypothalamus do?
A
Regulates many core body functions (homeostatic functions) eg. Metabolism, growth, reproduction, stress.
7
Q
How is the hypothalamus in contact with the anterior pituitary gland?
A
Secretes a peptide hormone: gonadotrophin releasing hormone (GnRH)
Critical component of the reproductive system
8
Q
What are the two parts of the pituitary gland?
A
• Posterior and anterior (distinct lobes); derived from separate types of cells during embryogenesis; different functions.
9
Q
What is the anterior pituitary gland?
A
An endocrine gland
10
Q
What is the anterior pituitary gland made up of?
A
different groups of cells: o Thyrotropes (Thyroid-stimulating hormone; TSH) o Somatotropes (Growth hormones) o Gonadotrophs (Follicle-stimulating hormone; FSH and Luteinising hormone; LH) o Corticotropes (adrenocorticotropic hormone; ACTH) o Lactotropes (prolactin; PRL)
11
Q
How does GnRH connect the three systems?
A
GnRH from the hypothalamus travels in the portal blood to the anterior pituitary where it acts on gonadotrophs
12
Q
How does GnRH positively regulate cells proliferation and hormone production?
A
through a G-protein-coupled receptor (GnRHr)
13
Q
What does GnRHr signalling stimulate?
A
Production of FSH and LH
14
Q
What is GnRH?
A
short polypeptide
15
Q
What do FSH and LH do?
A
act on different types of cells and these cells ten produce different hormones
16
Q
What are the three main types of sex steroid?
A
progestagens, androgens and oestrogens
17
Q
What are sex steroids derived from?
A
a common precursor: cholesterol
18
Q
Interconversion of sex steroids is via?
A
a biosynthetic network
19
Q
Action of these steroids is dictated by tissue-specific receptors
A
T
20
Q
Once inside a cell a steroid …
A
steroid-receptor complexes bind to steroid response elements on DNA and impact on transcription
21
Q
What affect does oestrogen have on gonadotrophs?
A
- Oestrogen binds to the oestrogen receptor (ER) on the gonadotroph
- The hormone-receptor complex translocates to the nucleus and mediates negative transcriptional control of target genes through oestrogen response elements (EREs)
- Suppression of transcription
- This regulates the production of FSH and LH
22
Q
What family are th inhibins and activins from?
A
TGFβ
23
Q
What do inhibins/activins do on gonadotrophs?
A
bind to inhibin and activin receptors on the gonadotroph cell
They act to regulate FSH and LH expression
24
Q
What is LH and FSH output predominantly regulated by in females?
A
secretory products from the ovary
25
In females Negative feedback on LH and FSH is by?
: oestrogens, progestogens and inhibins
26
in females Positive feedback on LH and FSH is by?
activin; oestradiol (an oestrogen)
27
What is the effect of oestradiol in females?
at low concentrations acts to negatively regulate LH expression but at high levels it acts to positively regulate expression
28
What is the effect of progesterone in females?
o High concentration seen in luteal phase of menstrual cycle (after ovulation) enhances the negative feedback of oestradiol
o At certain levels, the positive feedback effect of oestradiol is blocked
29
What level does feedback act on the HPG axis?
both the levels of the pituitary and at the level of the hypothalamus
30
How do the waves affect the hypothalamus?
both the magnitude and frequency of the waves
31
Do you get positive feedback at the male HPG axis?
No as don’t need a cycle as in females
32
What do Leydig cells secrete?
androgens
33
What do androgens do in males?
exerts a negative feedback response which causes decrease in GnRH
34
What level do inhibins act at in both males and females?
the level of pituitary to supress FSH secretion
35
How is GnRH secretion regulated?
release is pulsatile - begin at puberty
Pulse generator resides in hypothalamus
Increasing or decreasing the amplitude or frequency of GnRH pulses
36
What is Kisspeptin?
Master player of control of reproduction
54 amino acid neuropeptide
encoded by KISS1
37
What is the function of Kisspeptin?
It binds to the receptor KISS1R found in GnRH neurons
Kisspeptin is a potent GnRH stimulator
Kiss-1 +ve neurons are direct targets of oestrogens.
Binding of oestrogen results in negative feedback by decreasing GnRH expression
38
What is leptin?
peptide hormone produced by adipocytes
May be involved in activating the HPG axis at puberty via KISS1
Relationship between weight and fertility
39
How is there Interplay of leptin and kisspeptins in controlling puberty onset?
puberty is metabolically gated
Kisspeptins are essential upstream regulators of GnRH neurones
Leptin is necessary for puberty to proceed, but is not the sole requirement
Leptin acts on GnRH neurones indirectly via other neurons)
• Some evidence suggests that leptin acts via the Kiss1 neurons
40
What are the organs of the male reproductive system?
* Testes
* Epididymis
* Vas deferens
* Seminal vesicle
* Prostate gland
* Urethra
41
What is the function of the testes?
Houses seminiferous tubules; site of spermatogenesis.
42
What is the function of the Epididymis?
Sperm storage and maturation.
43
What is the function of the Vas deferens?
Transport of sperm from epididymis to urethra during ejaculation.
44
What is the function of the seminal vesicle?
Produces a mucus secretion which aids the mobility of sperm.
45
What is the function of the Prostate gland?
Produces an alkaline secretion that neutralises acidity of any urine in the urethra and aids the mobility of sperm.
46
What is the function of the Urethra?
Tube that carries urine and sperm out of the body
47
What are the two functions of the testis?
* To produce androgens and other hormones for sexual differentiation
* To produce spermatozoa for sexual reproduction
48
Where in the testis are steroid hormones synthesised?
Leydig cells
49
Where are spermatozoa produced?
in the seminiferous tubules with maturation in the epididymis
50
What are Primordial germ cells?
PGCs are the gamete precursors.
51
How does the PGC population exand?
By mitosis
52
Where do the PGCs migrate to?
the genital ridge primordium or in and become sertoli of granulosa cells
53
How do environmental factors influence PGCs?
can lead to infertility
54
Overview of spermatogenesis
* Begins at puberty; about 100 million produced per day
* Involves mitosis and meiosis
* Undergo two rounds of meiosis and become 4 spermatids
* Final differentiation step is called spermiogenesis and makes spermatazoa
* Generates four mature spermatozoa – identical in size but not genetically
55
What are spermatogonial stem cells (SSCs)
* SSCs self-regenerating pool undergo rounds of mitosis
| * At intervals, groups of morphologically distinct cells emerge
56
What influences the SSCs decision to divide or differentiate?
Growth factors
57
Process of spermatogenesis
Development occurs in centripetal direction – from the outside towards the lumen of the seminiferous tubules
Mature sperm are next to the lumen where they are released
58
What is spermiation?
the cytoplasmic bridges are broken and mature sperm can be released
59
What is the sperm morphology?
Highly differentiated head containing nucleus
Acrosome cap; important for fertilization
Tail required for motility
Mid-piece contains the mitochondria
Central axoneme made up of bundles of fibres; allows tail movement
Sperm head shape differs between species
60
process of Spermiogenesis?
Golgi apparatus forms the acrosome cap
Genetic material is really condensed – nucleus changes shape to fit into the sperm head
One of the centrioles of the spermatid elongates to become the tail
The remaining cytoplasm and organelles (residual body) are removed by Sertoli cells via phagocytosis
61
How is the chromatin remodelled?
* X and Y chromosome transcription stops before meiotic divisions
* Autosomal transcriptional activity ceases later, during spermiogenesis
* Massive repackaging of the DNA: histones are replaced by protamines; tightly compressed chromatin with no gene expression
62
What are sertoli cells?
somatic cells
63
Function of sertoli cells?
nurture sperm cell development
Maintain close contact with each other via ‘tight junctional complexes’
Important for the transfer of nutrients and movement of the germ cells
important role in spermiation and phagocytosis – removes excess cytoplasm and remodels before release
64
What is ectoplasmic specialisations?
connections between germ and Sertoli cells
65
What is the blood testes barrier?
separates testes into basal and adluminal compartments
Adluminal
compartment is an ‘immune-privileged site’ protecting the haploid cells from potential immune rejection
66
What is spermatogenesis?
the production of spermatozoa (mature male gametes) from spermatogonial stem cells
67
How many clones are developing in sperm cells at any given time?
4 clones at different stages
68
How often does a section of seminiferous tubule produces sperm?
every 16 days
69
How is constant production of sperm maintained?
different sections of tubule must be at different stages
Gap junctions between adjacent Sertoli cells provide means for communication
Timing is dictated by cross communication between SSCs and Sertoli cells
70
How do androgens act on leydig cells?
autocinologically
| negative feedback
71
What is testosterone converted to within the Sertoli cells?
dihydrotestosterone and oestrogen
72
What are the three main functions of testosterone in sertoli cells?
1) maintains integrity of blood-testis barrier
2) required for Sertoli-spermatid adhesion
3) essential for spermiation
73
How does testosterone stimulate the ducts?
Testosterone travels to the tubule lumen where it binds to androgen binding proteins (ABP) secreted by Sertoli cells
Testosterone-ABP travels to and stimulates the ducts
74
How does steroid conversion occur in the testis?
* Some testosterone and androstenedione from Leydig cells enter Sertoli cells
* Here they may bind to androgen receptors directly or after conversion to the more potent dihydrotestosterone
* Androgens may also be converted to oestrogens
75
How are sperm stored and matured?
* 90% fluid is absorbed in vasa efferentia; dependent on oestrogen
* Passage through epididymis takes 5-11 days; sperm acquire potential to swim and to fertilise oocyte; dependent on androgens
* Mature sperm are stored in the tail end of epididymis ready for ejaculation via the vas deferens
76
What is oogenesis?
the production of oocytes (mature female gametes) from primordial germ cells
77
What makes up the female reproductive system?
Uterus
Ovaries
Ovarian stroma
Uterine (fallopian) tube
78
What is the function of the uterus?
supports pregnancy/embryo
79
What is the function of the ovaries?
produce oocytes and secrete hormones
80
What is the function of the Ovarian stroma?
connective tissue, smooth muscle, stromal cells, developing follicles, interstitial glands
81
What is the function of the Uterine (fallopian) tube?
connects ovary and uterus; important for transport of oocyte/embryo
82
Two functions of the female genital tract?
* Gamete production and transportation
| * Site of implantation; support foetal development
83
What is the function of the first half of the menstrual cycle?
oestrogenic
| a mature oocyte is produced and made ready for fertilisation
84
What is the function of the second half of the menstrual cycle?
progestagenic
| the uterus is made ready to allow implantation and to support pregnancy
85
How doe ovarian function differ from testicular function?
o Far fewer oocytes are produced (around 400 in a lifetime vs. millions each day) –
o Ovulation occurs episodically rather than continuously being produced
o Ovulation stops at menopause; sperm production declines with age but continues into old age
86
What happens to the ovary at puberty?
Becomes an active endocrine gland
87
Outline of oogenesis
* Mitotic divisions all occur during foetal development
* Girls are born with primary oocytes arrested at prophase I
* Resumption of meiosis and development of the oocyte occurs after puberty
* Asymmetric divisions produce only one mature oocyte and two polar bodies that contain chromosomes but very little cytoplasmic material
* Second asymmetric division is dependent on fertilisation
88
How does a folicle develop?
with oocyte and provides supportive environment
89
What happens to the follicle at ovulation?
oocyte leaves and folicle becomes a copus luteum
90
process of Primordial to preantral follicle
* Large amounts of mRNA and rRNA produced to build organelles and generate protein stores
* Oocyte secretes glycoproteins which condense to form the zona pellucida
* Granulosa cells proliferate to form thick layer around oocyte
* Ovarian stromal cells condense to form thecal layer
* Developing cells are being looked after by the somatic cells and there is no connection to the blood vessels for nutrition
91
Process of Antral follicle development
* Thecal layer expands and further develops to form two layers
* Granulosa cells secrete fluid
* Antrum: fluid-filled cavity. This stage is characterised by increasing follicle size (due to antrum)
* Oocyte surrounded by granulosa layer ‘cumulus oophorus’ is suspended in follicular fluid by a thin stalk which connects to ‘mural’ granulosa cells
* Oocyte continues to synthesize RNA and make proteins
92
What are the two thecal layers?
theca interna (highly vascularised) and theca externa (fiborous)
93
Stages of follicle development
1) primordial follicle surrounded by thin layer of granulosa cells
2) Preantral follicle: larger oocyte with zona pellucida; expanding granulosa cell layer
3) Antral follicle: Oocyte with zona pellucida, surrounded by cumulus cells; vast number of granulosa cells; antrum filled with follicular fluid
`
94
How is there Communication between oocyte and granulosa cells?
* Granulosa cells are connected to oocyte through cytoplasmic processes
* Gap junctions form between adjacent granulosa cells and at the oocyte surface
* Extensive network of communication which allows transfer of amino acids and nucleotides to growing oocyte
95
How is follicle development regulated
* Very early primordial follicle development is stimulated locally via growth factors and cytokines
* Further follicle development is dependent on the pituitary; absence of pituitary input results in atresia
96
Which cells bind LH and what do they produce?
cells in theca interna
| Thecal cells produce the androgens androstenedione and testosterone
97
Which cells bind FSH and what do they produce?
```
granulosa cells
convert androgens (from thecal cells) to oestrogens oestradiol 17β and oestrone upon FSH stimulation
```
98
How do follicles respond to FSH and LH?
the follicles grow and the eggs mature
99
What do antral follicels produce as they grow?
increasing amounts of steroid hormones
100
What do thecal cells synthesis?
androgens from acetate and cholesterol; they also generate low levels of oestrogens
101
What doe granulosa cells synthesise?
convert exogenous androgens (from Thecal cells) to oestrogens
• Androgens stimulate aromatase activity and therefore promote oestrogen synthesis
102
What does the LH surge do in ovulation?
• Antral follicles will die unless an LH surge occurs
o Entry into preovulatory phase of growth
o Nuclear membrane breaks down and meiosis resumes up to metaphase II. Half of the chromosomes but majority of cytoplasm go to one cell (the secondary oocyte); rest is the first polar body (dies)
o Cytoplasmic maturation occurs (synthesis of specific set of proteins, reorganisation of microtubules)
o Within two hours of start of LH surge there is a transient rise (then decline) in output of follicular oestrogens and androgens
o Follicle ruptures and oocyte is carried out in follicular fluid
103
After LH surge what do granulosa cells do?
switch from oestrogen production to progesterone production
| positive feedback loop
104
What does the progesterome increase do?
* it is essential for ovulation to take place
* it depresses growth of less mature follicles
* itpromotes transition to progestagenic phase of ovarian cycle
105
What is the corpus leuteum made up of?
granulosa cells and thecal cells
106
What does the corpus leuteum secrete?
progestagens
progesterone and androgens
inhibin A and oxytocin
107
What is required for luteinisation?
LH
108
What is required for maintainance of the corpus luteum?
prolactin, oestrogen and progesterone
109
How long is the corpus luteum maintained?
12-14 days
110
Overview of fertilisation
Mature ovulated egg initially attaches to the epithelium in the uterine tube
Moves down towards the oviduct
Sperm swim into the oviduct to meet the egg – many get lost or trapped in cilia
Some go to the wrong ovary
111
What occurs in sperm maturation?
maturation in the epididymis
• They acquire ability to swim progressively
o More rigid flagellum/powerful
o cAMP content of tail increases
112
What activates sperm?
Capacitation in femal tract
113
What characteristics does a fully capacitated sperm have?
1) hyperactivated motility
| 2) changes in membrane properties that subsequently allow the acrosome reaction to happen
114
Why do sperm undergo capacitation?
* Sperm more responsive to signals from oocyte
| * Sperm able to undergo the acrosome reaction which is essential for fertilisation
115
what do sperm undergo in capacitation?
* Stripping or modification of surface glycoproteins
| * Changes in surface charge; reduction in membrane stability through loss of cholesterol and formation of lipid rafts
116
How do sperm undergo capacitation?
* Mechanisms not fully understood; active research
* Sperm cytoplasm becomes more alkaline
* Higher pH increases calcium permeability and hence intracellular calcium concentration
* Results in increased adenylate cyclase activity and therefore cAMP production
* Activation of spermatozoal protein kinase A (PKA)
* Downstream phosphorylation (including flagellum proteins); signalling pathways
117
What is hyperactive sperm motality?
* Involves high amplitude, asymmetrical beating of the flagellum
* Move in a more erratic way
* Helps the sperm meet its target
* Aids sperm penetration of the zona pellucida
118
How does a sperm find the egg?
* Sperm swim through uterus and into oviduct; cilia may help
* Oocyte and cumulus cells release chemoattractants to aid sperm
* Oocyte moves down the oviduct by muscular contractions and beating cilia
119
What are the survival times of the oocyte and the sperm?
oocytes 6-24 hours after ovulation
| sperm 24-48 hours in female tract
120
What is the acrosome reaction?
terminal phase of the capacitation process’
121
Process of capacitation
* Occurs after sperm head binds to zona pellucida; zona proteins responsible for inducing reaction
* Sperm acrosome swells; the acrosome membrane fuses with the sperm plasma membrane
* Acrosomal vesicle undergoes exocytosis
* Release of hyaluronidase (enzyme which digests hyaluron) and exposure of acrosin
122
Fertilisation process
1. Sperm makes contact with egg
2. Acrosome reacts with zona pellucida
3. acrosome reacts with periviteline space
4. plasma membranes of sperm and egg fuse
5. sperm nucleus enters egg
6. cortical granules fuse with egg plasma membrane, which renders the vitelline layer impenetrable to sperm
123
What is the zona pellucia composed of?
four glycoproteins: ZP1, ZP2, ZP3 and ZP4
124
What is the function of ZP1
structural protein; cross-links others
125
What is the function of ZP2>
contains sperm-binding domain necessary for oocyte-sperm recognition and penetration of the zona pellucida. Also responsible for the primary block to polyspermy
126
What is the function of ZP3?
complexed with ZP4, is involved in primary sperm-egg binding
127
What is the process of gamete binding?
1) Hyaluronidase from acrosome digests cumulus cells and exposes acrosin, a protease on inner membrane of sperm
2) Primary binding: Sperm membrane binds zona pellucida via ZP3 (complexed to ZP4) and a species-specific complementary binding partner on the sperm
3) Secondary binding: Sperm inner acrosomal membrane binds to zona pellucida via ZP2 on zona and acrosin on sperm
4) Acrosin digests zona pellucida and oocyte plasma membrane
5) Adhesion of sperm equatorial region and oocyte membrane
6) Penetration of sperm head into oocyte
128
What is the process of oocyte activation?
* Following sperm/oocyte fusion
* PLCζ is released into the cytoplasm of the oocyte – where it facilitates the hydrolysis of membrane-bound PIP2 to DAG and IP3
* This triggers Ca2+ release from intracellular Ca2+ stores
* leading to Ca2+ induced Ca2+ release and oocyte activation.
* Increase in cytosolic calcium concentration and calcium oscillations
* Stimulates cortical granule release (includes Ovastacin)
* Protein kinase C (PKC) stimulates phosphorylation of other proteins essential for development of the conceptus
129
Why is block to polyspermy important?
polyspermy is multiple sperm in egg
130
how is polyspermy avoided?
* Ca2+ stimulates fusion of zygotic cortical granules with oocyte plasma membrane and release of contents into perivitelline space
* Enzymes including Ovastacin act on zona pellucida which hardens; inactivation of sperm receptors through ZP2 cleavage: Block to polyspermy
131
How is fertilisation completed?
* Calcium pulses stimulate resumption of meiosis; second polar body is extruded and female pronucleus forms
* Sperm nucleus decondenses; protamines are replaced by histones; male pronucleus forms
* Pronuclei come together; DNA replication occurs; pronuclear membranes break down and replicated chromosomes align on mitotic spindle ready for first cleavage division
132
What is syngamy?
Combination of two genomes
133
What happens in pre implantation development?
Embryo undergoes cleavage division to two large cells
This continues with a decrease in cellular size
After 8 cells these divisions are not necessarily in sync
Blastocysts develops and hatches through the zona and implants into the nucleus
134
What is infertility?
fertility decreases with age
| 85% of people concieve within 12 months
135
What causes male infertility?
Problems with sperm
• Other problems e.g. Hormone imbalance, tumours, tube blockages, chromosome defects, undescended testicles (temperature regulation is wrong), infection, neurological problems (and many more)
136
What is o Oligozoospermia?
Low sperm count
137
What is Asthenozoospermia?
Low sperm mobility
138
What is Teratozoospermia?
Abnormal sperm. The sperm may have an unusual shape, making it harder to move and fertilize an egg
139
What is Azoospermia?
No sperm
140
What causes female infertility?
Ovulation problems
• Uterus or fallopian tube damage: infection, pelvic inflammatory disease, birth defect, previous tubal pregnancy, endometriosis, fibroids
141
What ovulation problems cause female infertility?
o Age (most significant factor: Ooycyte age, endometriosis)
o Hormone imbalance
o polycystic ovarian syndrome (PCOS): 30% of infertile women
o thyroid problems – both an overactive thyroid gland and an underactive thyroid gland can prevent ovulation
o premature ovarian failure – a woman's ovaries stop working before the age of 40
142
Stages of IVF?
1. Ovarian hyperstimutation
Fertility drugs firstly supress the natural cycle then stimulate the ovaries into producing a large number of ooycytes e.g. Daily dose of FSH
Must also supress ovulation
2. Egg retrieval
3. Sperm preparation
Sperm must also be matured by the addition of appropriate factors that can stimulate capacitation
4. co- incubation
5. embryo transfer
In the UK the two healthiest are selected
6. pregnancy
143
In vitro maturation
* Oocytes are collected before final maturation
* They are cultured in vitro in the presence of FSH and other factors prior to fertilisation using IVF or ICSI
* Women can avoid use of drugs: important if vulnerable to ovarian hyperstimulation syndrome e.g. PCOS
144
Intra-cytoplasmic sperm injection
Sperm (or spermatids?) are injected directly into the egg cytoplasm using a fine needle
Bypasses normal fertilisation – sperm don’t have to undergo many of the maturation steps
Allows use of non-motile sperm or spermatids that have not been able to mature
145
GIFT
• Gamete intra-fallopian transfer (GIFT): In this procedure, eggs and sperm are combined in vitro, then immediately inserted into the fallopian tubes through a small incision in the abdomen. Fertilization happens inside the body, and the embryo implants naturally. (often used if there is an ethical/religious objection to out of body fertilization)
146
ZIFT
• Zygote intra-fallopian transfer (ZIFT): In ZIFT, eggs and sperm are combed in vitro (as with GIFT). But in this procedure, the doctor waits until fertilization has occurred before transferring the embryos to the fallopian tubes. (IVF – the zygote is put into the uterus)
147
Cryopreservation
* Eggs, sperm or embryos can be frozen
* Slow freezing used to be used – no ice crystals would form
* Vitrification is now used: freezing quickly in the presence of cryoprotectants
* Allows IVF to be less invasive – may only have to go through the process once for multiple children
* Can also be used when individuals have cancer so they can have children after treatment
* Has many ethical issues surrounding ownership/use of the embryos/gametes
148
Mitochodrial transfer
• Techniques involve ‘spindle transfer’ or ‘pronuclear transfer’
149
Cytoplasmic transfer
Used ICSI and inserted sperm along with younger healthier ooplasm
This was banned as developmental abnormalities were found in a small number of the offspring
Offspring were found to have mitochondria heteroplasmy
150
Spindle transfer
Patients spindle is inserted into a health enucleated egg
Egg is then fertilised by ICSI
151
Pro-nuclear transfer
Transfer occurs after fertilisation – develop to pronuclear stage
152
ART and developmental abnormalities
rates of abnormalities may be higher
153
Therapeutic cloning
* Embryo created using SCNT
* Pluripotent embryonic stem cells (ESCs) are harvested from the inner cell mass of blastocyst
* ESCs can differentiate in vitro into specific lineage
154
Artificial gametes
oocytes or sperm generated by manipulation of their progenitors or of somatic cells.
155
routes for making artificial oocytes and sperm
```
o GSCs (Germline stem cells) e.g. SSCs
o ESCs (Embryonic stem cells)
o iPSCs (induced pluripotent stem cells)
o Somatic cells
o In vitro differentiation with/without autotransplantation
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