Describe the key features in the life of spermatozoa from the time that they leave the Sertoli cell to a successful natural fertilisation.

The role of the spermatozoa is to successfully deliver the paternal DNA to the oocyte to allow for sexual reproduction to occur. In order for this to happen, several events must occur and they which will be outlined in the overview fig 1:
1)    Movement into the epididymus
2)    Spermatozoa maturation
3)    Spermatozoa release during intercourse into female vagina
4)    Overcoming the cervix
5)    Capacitation in the uterus
6)    Activation of the sperm
7)    Binding to the oocyte
8)    Fusion with the oocyte
9)    Syngamy (not discussed)
This process is a very difficult task and it serves as a selection process so that only the best sperm will be able to fertilise the egg.
We begin with spermatozoa in the seminiferous tubules. In this state the sperm are immature. They do not have proper movement and cannot actually swim. They are bathed in a fluid secreted by the sertoli cells which washes them through the vasa efferentia and into the epididymis.
Maturation
During their time in the epididymis, the spermatozoa undergo maturation. This process appears to be under the influence of androgens (proven by castration experiments where the maturation process was lost but then restored by testosterone). The testosterone is synthesised by the leydig cells (as usual). During this maturation, the sperm acquires the ability of forward motion.
Additionally in the epididymis there is a large reabsorption of fluid which serves to increase the spermatocrit by 100 fold. Subsequently the further transport   of the fluid is dependent upon the epididymis musculature. Thiiis absorption is dependent upon oestrogen (produced both by leydig cells and also by the developing spermatozoa themselves). (absorption is relevant as it means that ligation of vas deferens after doesn’t cause blow up).
 Release during intercourse
The spermatozoa are stored in the epididymis. During coitus, under sympathetic stimulation, they are propelled by peristaltic muscular contractions of the vas deferens  and the urethra, they are ejaculated into the vagina of the female. During this process, the fluid from the vas deferens only makes up 10% of the total emission. It is preceded by the prostate components which make up 30% and proceeded by the seminal vesicles contribution of 60%.
Overcoming the cervix
The female cervix acts as a hormonally dependent filter and during the progesterone phases of the cycle, the external os remains swollen and closed and also the mucus of the cervix is very thick and prevents the sperm from entering the uterus. However, during the oestrogen part of the female menstrual cycle the cervix secretion is much thinner and allows the entry of sperm.
Note that even in the permissive phase of the cervix, over 99% of the spermatozoa will be unable to cross the cervix. Those that do are nourished by uterine secretions.
Capacitation
The fluid of the uterus is thought to be the trigger for this capacitation. This is a further change in the characteristics of the spermatozoa which include an increase in imotility and also a change in the surface membrane. The mechanism appears to be linked to increase in calcium levels causing a rise in cAMP and causing PKA activation resulting in several phosphorylations.
Movement in the uterus
The spermatozoa can swim up the uterus (they may possibly be aided by uterine cilia forming fluid currents. On reaching the isthmus of the oviduct the spermatozoa linger and bind to the oviduct epithelial cells. They are only realease when they receive a signal from the oocyte – chemoattractants. Chemotaxis causes hyperactive flagellar beating towards the isthmus of the oviduct. Note the oviductal cilia are beating to allow the movement of the oocyte, the sperm must swim against this.
Sperm activation
Once in the presence of the oocyte, the sperm becomes activated. The trigger for this again appears to be a calcium influx (calcium ionophores can experimentally induce it). This activation is caused by progesterone  and also by the sperm binding to the ZP3.
A model for the events that occur as follow:
Sperm approaching the oocyte are activated by prostaglandins secreted by the cumulus oophorus. This allows for the activation of some sperm causing the acrosome reaction to occur. The acrosome is the release of the contents of the acrosome including acrosin and hyaluronidase. This breaks down the ECM and loosens the area around the cumulus to aid the penetration of the cumulus and zona pelucida. Other sperm can be activated by binding to ZP3 of the zona pelucida. On activation there is an associated movement change of the sperm tail becoming more whiplash like.
Binding to the oocyte and fusion
The binding of the sperm to the oocyte seems to be an interaction between integrins on the oocyte with the disintegrins ADAM complex on the sperm. Subsequent to this membrane fusion occurs. Gene knockout studies in mice show that this involves CD9, CD55 and CD59 of the oocyte and Izumo1 of the sperm.

Subsequent to this there is a wave of calcium which triggers the cortical reaction, the exocytosis of cortical granules which release proteases which cleave ZP2 and ZP3 preventing polyspermy. This wave also activates meiosis and permits the subsequent syngamy.
Originally written by Neil Douarin

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