Repro 7.1 Coitus and Fertilisation Flashcards
During coitus, where is sperm deposited?
Cervix
Define conception.
Fertilization of the ovum by a sperm and the subsequent establishment of pregnancy
What are the phases of coitus?
Excitement phase
Plateau phase
Orgasmic phase
Resolution phase +/- refractory period
What happens to semen immediately after ejaculation?
Coagulation by clotting factors (fibrinogen and veiculae)
What does coagulation of semen prevent?
Sperm being physically lost from the vagina
Do all sperm enter the uterus?
No. The vast majority are lost by leakage from the vagina
How far do sperm have to travel in the uterus to reach the uterine tubes and how long does this take?
15 - 20cm
A few hours
What causes transport of the sperm through the uterus?
Their own propulsive capacity
Fluid currents caused by the action of ciliated cells in the uterine tract
WHat happens to sperm during their journey through the uterus?
Undergo a further series of maturational changes - capacitation and acrosomal reaction that results in their acquiring a full capacity to fertilise the ovum
When do sperm first begin to mature?
When they leave the testes and journey through the epididymis. It continues during their storage until ejaculation
How long does capacitation take?
6 - 8 hours
What changes occur in sperm during capacitation?
Removal of a glycoprotein coat promotes changes in the sperm cell membrane
Tail movements change from waves to whip like thrashing movements
They become responsive to signals from the oocyte
What happens when capacitated sperm comes in contact with the oocyte?
When it contacts the zone pellucida the membranes fuse (sperm head binds to ZP3 proteins) and this marks the commencement of the acrosomal reaction
What happens during the acrosomal reaction?
The acrosome swells and liberates its contents by exocytosis
Proteolytic (acrosomal) enzymes and further binding facilitate penetration of the zona pellucida by the sperm
How long does it take for sperm to penetrate the zona pellucida?
15 minutes
What induces capacitation and the acrosomal reaction?
An influx of calcium ad rise in cAMP in the spermatozoa
What does the secondary oocyte contain?
Haploid chromosomes and the bulk of cytoplasm
Surrounded by follicular cells (cumulus) embedded in a gelatinous matrix
Where are the remaining chromosomes after the first meiotic division of the oocyte?
Contained within the first polar body
Where is the site of fertilisation?
The ampulla of the oviduct
What is a zygote?
The product of fusion of the sperm and ovum pronuclei
How many sperm are required at the site of fertilisation?
300 - Needed to disperse the zona pellucida. Only 1 will fertilise the oocyte
How is polyspermy prevented?
Cortical reaction
What is cleavage?
Series of metabolic changes and rapid mitotic division. Increased number of cells (16-32) without growth (needs to be small enough to pass through isthmus to uterus). Forms a morula
How do identical twins arise?
After cleavage, cells are totipotent therefore if separation of cells occurs, 2 individual embryos can form with identical genetic information
What is a blastocyst?
Consists of an outer layer (trophoblast) which will become the placenta and the inner layer which will become the embryo itself
When the blastocyst forms there is a loss of totipotency
There is also a fluid filled cavity present
What does implantation involve?
Interaction between the trophoblast and the lining epithelium of the uterus (trophoblast develops receptors to adhere). Further imbedding into the endometrium is dependent upon the invasive property of the trophoblast
What part of the trophoblast has invasive properties to further embedding of the embryo into the endometrium?
Syncytiotrophoblast
By what day should the blastocyst be fully embedded in the endometrium?
10th
How is the early placenta established?
Cellular interactions between the blastocyst and the endometrium and the establishment of the relationship between embryonic and maternal tissue
What hormone allows the morula to travel through the oviduct?
Progesterone - Smooth muscle relaxant
On what days of the uterine cycle does the transition from zygote to blastocyst occur?
14 - 21
For how long is the conceptus nourished in intrauterine fluid?
3 days
How many days after conception does implantation occur?
6
What is the purpose of hCG in pregnancy?
Maintains the corpus luteum which is required for its progesterone secretion to maintain the uterine wall
How does an ectopic pregnancy occur?
Failure of transport of egg -> conceptus embeds in uterine tube, ovary or abdomen
Why is an ectopic embryo not viable?
Not enough blood supply or nutrients
Why is an ectopic pregnancy dangerous to the mother?
Sever risk of maternal haemorrhage - ectopic rupture
How long does it take for spermatogonia to mature to spermatozoa?
74 days
50 in the seminiferous tubules
12 - 26 in the epididymis
What happens to spermatogenesis with age?
diminishes in quality and quantity
What happens to sperm in the epididymis?
Maturation
Concentration as fluid from the seminiferous tubules is absorbed
What physiological changes occur in the female with sexual arousal?
Blood engorgement and erection: clitoris, vaginal mucosa, breast and nipples (may be flushing of the skin)
Glandular activity (ease entry of penis into vagina)
Sexual excitement
+/- orgasm (not necessary for reproduction)
No physiological refractory period
What is the innervation of penile erection?
L2 - S3
PNS - Pelvic nerve
Somatic - Pudendal nerve
What haemodynamic changes must occur to produce an errection?
Central arteries of the corpora cavernosa must vasodilate by reduction of intracellular calcium in the smooth muscle of these arteries
Describe the process of haemodynamic changes in the penis causing tumescence.
Inhibition of sympathetic arterial vasoconstrictor nerves
Activation of PNS - release of post-ganglionic Ach
Ach binds to M3 receptor on endothelial cells
Rise in [Ca]i
Activation of NOS and formation of NO
NO diffuses into vascular smooth muscle and causes relaxation (vasodilation)
(NO also released directly from nerves)
What factors block NO and can therefore cause erectile dysfunction?
Alcohol
Anti-hypertensives
Diabetes
How does viagra work?
Inhibits cGMP breakdown so maintains vasodilation
What is emission?
Movement of ejaculate into prostatic urethra (leakage results in sperm in pre-ejaculate)
What causes emission?
Vas deferens peristalsis (SNS). Contribution of secretions from accessory glands (PNS)
What causes ejaculation?
Spinal (and cerebral) reflex but it can be overcome psychologically. Activation of control receptor.
Describe the mechanism of ejaculation.
Contraction of glands and ducts (smooth muscle)
Bladder internal sphincter contracts
Rhythmic striatal muscle contractions
What results if the bladder internal sphincter does not contract?
Dry orgasm
What striated muscle contracts during ejaculation?
Pelvic floor
Ischiocavernosus
Bulbospongiosus
Hip and anal muscles
What is the normal sperm count in ejaculate?
20-200 x10^6 sperm/ml
What is the normal range of volume of ejaculate?
2-4ml
What % of sperm in ejaculate are swimming vigorously forward?
> 60%
What % of sperm have abnormal morphology and why?
<30%
High because of high rates of production
How quickly does semen liquify after ejaculation?
within 1 hr
What is considered oligospermia?
<20 x10^6 sperm/ml
What % of semen is sperm?
2-5%
What produces the majority of ejaculate?
Seminal vesicle 60%
prostate 25%, bulbourethral gland very little volume
