lecture 11: fertilisation Flashcards
1
Q
What is the male reproductive tract?
A
- sperm travels from the testis → epididymus → vas deferens → penis
- long way that it has got to go
- testis = site of sperm synthesis
- epididymus = site of sperm storage
2
Q
What is the female reproductive tract?
A
- egg has to make its own journey to the site of fertilisation in the ampulla of the fallopian tube
- female reproductive tract is a reservoir for sperm after sex
- sperm can survive for quite some time in the reproductive tract (day or so)
- lurk waiting for egg
- may have sex at night but you probably get pregnant in the tram on the way to work the next day
3
Q
What is the spermatozoa?
A
- sperm are highly differentiated, designed for a sole purpose
- designed for one thing and one thing only: to find and fertilise the egg
- the DNA in the sperm head is much more highly condensed than in the sperm cell
- use protamines instead of histones
- across the top of the sperm head is a patch called the acrosome
- acrosome is full of enzymes that are involved in breaking down the investments of the egg and getting through the glycoprotein coat around the egg (zona pellucida)
- midpiece contains the mitochondria
- the mitochondria are almost helical and wrap around the actual inner machinery of the sperm
- tail is the largest part / flagellum for movement
- ‘absolutely exquisite cell’
- like lots of things in reproduction it has lots of idiosyncracies, things that are unique to that cell
- unique biochemistry e.g. isozymes ( LDH) that are only found in sperm
- questions about role, function, etc
4
Q
What are the different cells called along the path of spermatogenesis?
A
- male germ cell in embryo (2n) → mitosis → spermatogonium (2n) in adult → mitosis → primary spermatocyte (2n) →→ first meiotic division
- these cells are diploid
- stem cells undergo rounds of meiosis
- → secondary spermatocytes (n) → spermatids with cytoplasmic bridge → sperm cells (n)
- cytoplasmic bridges are formed due to certain genes that are only found on the X chromosome that produce proteins required for sperm differentiation
- these cells are haploid
5
Q
What is the site of spermatogenesis?
A
- seminiferous tubules
- lots and lots of tightly coiled tubules that make up the testis
- process of spermatogenesis and spermeogenesis can take about 60 days
6
Q
Where are sperm stored?
A
- within the seminferous tubules going into the epididymis
7
Q
Are all sperm the same size?
A
- no
- vertebrate sperm vary in size
8
Q
What is unique about human sperm?
A
- can use fructose as an energy source so seminal fluid is very high in fructose
- not many cells use fructose as an energy source
- cells lining the female reproductive tract can’t use fructose
- this means the sperm don’t have to compete for energy sources once they reach the female reproductive tract
9
Q
How big is the human oocyte? What does it look like?
A
- ~110 µm in diameter
- post-ovulation is immediately surrounded by the zona pellucida
- glycoprotein coat laid down in the ovary around the egg
- contains specific receptors that the sperm need to bind
- surroundind this a cloud of cells called the cumulus ooferous
- these cells are in direct communication with the egg
- prior to ovulation there are gap junction feeding directly from those cells to the egg
- bi-directional communication - dialogue
- these cells around the time of ovulation start to produce hyaluronic acid
- glycosaminoglycan - but no amino groups attached so technically a polysaccharide
- cells don’t grow
- get dispersed into huge matrix
- actually only a few hundred cells even though it looks like several thousand
- inflated due to hyaluronic acid matrix
- one of the reasons for doing this is to ensure its pick up by the fimbrae/oviduct at the appropriate time
10
Q
are all eggs the same size?
A
- no vertebrate eggs vary in size and shape
- e.g. xenopus, chick, mouse, zebrafish
- lobe-finned fish eggs are 9cm big
- size is reflective of whether the egg will develop in vivo or externally
- eggs that are layed need to be provided with lots of nutrients and protection cf eggs that develop internally and have access to maternal nutrients
11
Q
What is interesting about the ovulated egg?
A
- it has not completed meiosis
- the human and the mouse egg do not complete the second meiotic division until the sperm enter
- embryo → oogonia → primary oocyte → meiosis I arrest → adult → secondary oocyte → meiosis II arrest → fertilised egg
- only one egg is produced by meisosis → rest of the genetic material is shunted off in polar bodies
12
Q
What are different mammalian GV stage oocytes?
A
- mouse: 80µm
- cow: 110 µm
- pig: 125 µm
- cat: 110 µm
- human: 110 µm
- mouse and human eggs contain very little lipid/endogenous nutrients
- whereas eggs from domestic species such as cow, pig and even cats contain a lot of lipid and this reflects differences in maternal physiology
13
Q
What is the ovulated oocyte?
A
- in most mammals, the oocyte does not complete meiosis until after fertilisation
14
Q
What is gametogenesis in mammals?
A
- in males, each spermatogonium gives rise to: four hapoloid sperm
- mitosis ceases in the embryo
- meiosis begins at puberty through to end of life
- in females, each oogonium gives rise to ONE haploid egg and 2 polar bodies
- meiosis begins in the embryo
- meiosis I arrested around birth
- a cohort of primary oocytes arrested at Meiosis I resume meiosis at puberty, and at each cycle but ends at menopause
15
Q
What is really important?
A
- that only one sperm gets through to the egg → otherwise polyspermy → dire consequences
16
Q
What is the egg’s journey?
A
- egg collected by fimbrium
- passes through infundimbulum
- ampulla of oviduct
- fimbriae waft across the surface of the ovary around the time of ovulation
- egg ovulates
- oocyte surrounded by
- zona pellucida
- cumulus (corona radiata)
- oocyte surrounded by
- for a successful pregnancy to occur, mature, functional oocyte(s) must be available for fertilisation (in human < 300 / lifetime)
- ovulaton releases one oocyte per follicle
- these must enter the oviduct via the fimbrium
20
Q
How is sperm transported along the male tract?
A
- cauda epididymis and vas deferens → contractions
- smooth muscle in walls of seminal vesicle and prostate contract → seminal fluid
- sperm transferred to vagina via penile urethra
21
Q
What is sperm transport in the female tract?
A
- semen deposited
- in vagina (e.g. human) (pH 5.7, i.e. acidic)
- in uterus (e.g. pig)
- in vagina, but carried into uterus by muscular (e.g. mouse)
- copulatory plug (in humans, a very fine jelly)
- cervix
- cervical mucus is barrier to sperm (varies with cycle)
- filters out sperm with poor motility or abnormal morphology
- ~99% of human sperm do not get past the cervix
- uterus
- uterine contractions speed sperm transport
- utero-tubal junction
- barrier to sperm transport
- sperm need certain proteins on head to pass
- oviduct (fallopian tube)
- storage of sperm in crypts or mucosal folds
- binding to epithelium preserves sperm fertility and slow release reduces polyspermy
- capacitation and hyperactivation
- ampulla
- sperm meets egg
- fertilisation
- typically only a few thousand sperm make it anywhere close to the site of implantation
22
Q
So what is the overall marathon journey of the sperm to the oocyte?
A
- seminiferous tubules → epididymis → vas deferens → ampullar and ejaculatory duct → prostatic urethra → penile urethra → vagina → uterus → oviduct → fertilisation
- motivated by: secretory pressure (cilia), ejaculation, sperm flaggellum
- 40 cm in total
- relative size makes this about 26 miles
- 40% cases of infertility are due to sperm unable to make this journey
30
Q
How long does the acrosome reaction take to complete?
A
- measured by monitoring entry of dyes into human sperm
- takes a matter of minutes
31
Q
What is the sequence of movements leading up to fertilisation?
A
- binding of sperm to zona pellucida acrosomal reaction
- penetration through zona pellucida
- fusion of plasma membranes
- sperm nucleus enters egg cytoplasm
- only the sperm head and one organelle enters the egg
- don’t contribute a single mitochondrion
33
Q
Which organelles does the sperm contribute to the offspring?
A
- we know that it does not contribute mitochondria
- centriole: as well as providing the DNA, the sperm passes on its centriole
- centriole is involved in cell division
- really important
39
Q
What is seen following sperm binding in regards to calcium release?
A
- when the sperm binds you see a spike - very rapid release of calcium
- this is the release that blocks the polyspermy
- following this you see pulses of caclium every 10 minutes
- these oscillations are very important for programming the fertilised egg into becoming an embryo
41
Q
What are stages of fertilisation?
A
- sperm activated by female reproductive tract
- sperm binds to zona pellicida
- acrosome reaction
- sperm lyses hole in zona
- cortical granule reaction
- sperm and egg membranes fuse (triggers egg activation, egg completes meiosis)
- calcium waves (PLC-ζ)
- cortical granule release
- zona hardening
- sperm decondensation
- delivery of male centrioles
- polar body extrusion
- pronuclei x 2
- syngamy
- cleavage
- fertilisation is a very active process
