lecture 11: fertilisation Flashcards
What is the male reproductive tract?
- 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

What is the female reproductive tract?
- 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

What is the spermatozoa?
- 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

What are the different cells called along the path of spermatogenesis?
- 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

What is the site of spermatogenesis?
- seminiferous tubules
- lots and lots of tightly coiled tubules that make up the testis
- process of spermatogenesis and spermeogenesis can take about 60 days

Where are sperm stored?
- within the seminferous tubules going into the epididymis
Are all sperm the same size?
- no
- vertebrate sperm vary in size

What is unique about human sperm?
- 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
How big is the human oocyte? What does it look like?
- ~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

are all eggs the same size?
- 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

What is interesting about the ovulated egg?
- 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

What are different mammalian GV stage oocytes?
- 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
What is the ovulated oocyte?
- in most mammals, the oocyte does not complete meiosis until after fertilisation

What is gametogenesis in mammals?
- 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
What is really important?
- that only one sperm gets through to the egg → otherwise polyspermy → dire consequences
What is the egg’s journey?
- 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

What maintains the 3D structure of the cumulus oocyte complex?
- hyaluronic acid
- this is something that the sperm has to battle
What is sperm formation, maturation and storage?
- spermatogenesis in testis
- passive transport to rete
- sperm entering the vasa are incapable of movement
- rete-testis
- caput epididymis
- fluid resporption
- secretions
- sperm are immotile and incapable of fertilising egg
- peristaltic movements along epididymis
- maturation – morphology, biochem, physiology
- cauda epididymis
- sperm storage
- immotile but capable of motility (activation)
- epididymis
- androgen dependent
- regionally differentiated
- additin of cartinine (profound antioxidant - sperm are very sensitive to oxidation, possibly there to protect mitochondrial function) , fructose and glycoproteins which coat the sperm
- journey through the epididymis takes 5 to 11 days (species dependent)
- sperm are said to mature in the epididymis
- gain motility
What is semen?
- sperm and seminal plasma
- consists of sperm suspended in a fluid that nourishes them and facilitates fertilisation
- components of seminal fluid produced in seminal vesicles, prostate gland and bulbourethral glands
- all components of semen join in the urethra and ejaculated through the penis by muscle contractions
- sperm are less than 5% of the volume of semen
- 5 - 10ml of semen at ejaculation
- sperm count anywhere from 20 - 200 million / ml → so a single ejaculate can contain as many as a billion sperm
- seminal fluid is thick and contains mucus
- semen is alkaline to protect against the acidity of the vagina
How is sperm transported along the male tract?
- 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

What is sperm transport in the female tract?
- 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

So what is the overall marathon journey of the sperm to the oocyte?
- 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

What is the longevity of gametes in the female tract?
- human
- sperm: 24 - 48 h
- oocytes: 6 - 24 h
- mouse
- sperm: 6 - 12 h
- oocytes: 6 - 15 h
- horse
- sperm: 75 - 120 h
- oocytes: 6 - 8 h
- extremes:
- sperm greater than 10 years in species with sperm storage
- sperm less than 1 minute in some species with external fertilisation
What is capacitation?
- stripping of much of the glycoprotein coat on the sperm that was acquired during passage along the epididymis
- sperm exhibit a change in the surface membrane properties
- this prepares sperm to undergo acrosome reaction




