FERTILIZATION: the success story of an adventurous sperm cell

Introduction

To the world at large, the word fertilization is not really something to consult a dictionary about. In fact, even a 7 year old kid can correctly tell you what the term means in his own understanding. Well, I am the sperm cell in this particular story you are about to read and to me, fertilization is one of the major processes that occur between when a lady moans and when her tummy is seen to bulge some weeks later.

_{Image of A Lady with a Bulging Tummy (CC0, No attribution required]:
pexels}

As a matter of fact, fertilization is not just the fusion between the two sex cells, it encompasses everything that happens from the moment the sperm cells are deposited in the vagina to when the sperm-egg cell starts dividing by a process known as cleavage.
There are basically three major processes that occur before the increase in Tommy size is noticed. These are: Fertilization, Implantation, Growth and Development

For the purpose of this post, If you do not mind, I would like to concentrate on fertilization; of course this is the only part in the whole moan-bulge interval that i am still respectfully called a sperm cell and not some awful sounding ‘zygote’ as at when I am bound to an egg.

What Runs Through Your Head When You Hear the Word ‘Fertilization’?

Fertilization in common terms is the process of fusion between the male sex gamete (spermatogonia) and the female sex gamete (oogonia) or we could just say it is when a potent sperm cell successfully travels and unites with an egg cell to bring about a new life which at this stage is known as a zygote.

The process of fertilization begins when the male reproductive fluid(semen) is deposited in the female reproductive tract. This happens far from the point where the egg is comfortably situated and waiting for any of the sperm cells that wins the race to come give her a hug. Thus; transportation of the deposited sperm cells becomes an important process to be considered.

So you can understand the concept behind how we eventually found our way to the egg’s position, I will have to first of all tell you about the normal composition of the fluid in which we lived in: Semen, which is of great importance if you must know how i eventually became successful in my adventure.

• Composition of Semen

Normal semen in one ejaculate contains about 100-150 millions of us per mL of semen,
a sugar moiety known as fructose which is what gives us the energy to thrive in such a herculean adventure,
fibrinogen from seminal fluid to cause clotting of the semen once it is exposed to air,
prostaglandins too from seminal fluid to cause anti-peristaltic movement of cervical wall to make sure we move fast and get to our destination before death time comes and
the prostatic fluid which neutralizes the vaginal pH with the clotting enzymes in it to convert fibrinogen to coagulum to cause stickiness of the semen.

How Did We Ever Get Close to the Egg?

Yes, a few of us finally got to the destination, but how we ever got there was not solely our efforts, we had a little help. How we got transported to the position of the egg was one of the most important processes in the whole fertilization adventure, as anything short of the normal would have actually led to our inability to get to the site of fertilization.

Once semen was deposited in the female reproductive tract, with the vagina being the drop off point, the enzymes present in the prostatic fluid converted fibrinogen into coagulum (a sticky plug) which made the semen sticky and held it back in the tract so it was not drained down and we were not driven out of the female tract.

After about 30 minutes, the semen re-liquefied allowing us to assume our initial motile nature and thus giving us the go ahead to go on and have fun.
I guess this is the point where we all said, let the party begin!

At this point, the vaginal pH had already been neutralized by the prostatic fluid present in the semen. The prosthetic fluid has a basic pH and thus, made the cervical tract a little bit basic and fit for our survival. we moved as fast as we could with the aid of our flagella and with the aid of the anti-peristaltic movement of the cervix as an effect of prostaglandins from semen.

Right about then, the cervix in which we found ourselves was of a lady in the proliferative stage of her menstrual cycle.

One of the major determinants of the speed of locomotion of any sperm cell is the ovulatory phase in which the female is In. In the proliferative phase which occurs from about the 5-14th day, the mucous membrane of the cervix is thinner and more alkaline allowing for a faster movement of the sperm cells while in the secretory phase which occurs from about the 15-28th day, the cervical mucous membrane is thicker and more adhesive thus reducing the speed of any sperm trying to move at that time. It takes about 2 to 7 hours for a sperm cell to comfortably travel from the cervix to ampulla region of the fallopian tube. This is the portion of the tube where fertilization usually occurs probably because it is the widest part

Sadly, only about 1% of us finally got to the ampulla region of the uterus. On reaching here, fusion did not just occur spontaneously, we first had to undergo what is known in science as capacitation.

capacitation is the process of interaction or conditioning between the sperm and the uterine wall. In humans, this period lasts for about 7 hours and is used to remove the glycoprotein coat and the plasma proteins of the membrane overlying the acrosomal region. This leaves the sperm cell with its cell membrane at every other part of the cell except the acrosome region and thus permits easy exit of the sperm cell into the egg cell. Only a capacitated sperm can successfully enter and perfectly fertilize an egg.

Am Here Now, What Then?

From this point on, i will be giving an account of myself alone as i was now too busy to take note of any event going on around my few remaining brothers. After capacitation, i reached the egg cell at the ampullary region of the uterus but did not immediately fuse with it. I actually had to scale through some tough barriers surrounding the egg so i could eventually make my way into the egg’s cell membrane. “what a task! after the whole journey, i still have to scale through fences?” that was exactly what i thought when i noticed the barriers.

First, i had to find a way of passing through a zone of small follicular cells called the corona radiata just to show I was really serious about fertilizing and then I also had to find a way past a layer of specialized extracellular matrix called the zona pellucida. I scaled through the first barrier simply. It must have been due to my capacitation because i realized that my uncapacitated brothers could not pass through this barrier. the barrier i would say, was merely a test of capacitation.

Once i came in contact with the zona pellucida, i automatically released two enzyme; hyaluronidase and acrosin. These enzymes then caused what i call an acrosome reaction. In this reaction, my plasma membrane fused with the membrane over my acrosome, exposing my acrosomal contents. These enzymes helped me penetrate the zona pellucida and come in contact with the egg cell membrane and also caused the dissolution and disappearance of the plasma membrane covering my acrosomal head.

Once my head had successfully entered the zona pellucida and touched the egg cell, permeability of the zona pellucida changed restricting all other sperms from crossing in. “I think she likes me”, I thought. Or maybe I was just being lucky

change in permeability of the zona pellucida prevents a situation known as polyspermy where more than a single sperm fertilizes one egg

At this point, my cell membrane adhered and fused with the egg’s cell membrane and with the opening that was created over my acrosine during the acrosin reaction, i entered majestically into the egg leaving behind my cell membrane but carried my tail along with me.
As soon as i was in, the egg made its cell membrane impenetrable and completed its second meiotic division which was on a halt at metaphase[II].

Now it became clear to me that she did not do all those for me, and not because I was finally in and she liked me, but because I became the activation factor that triggered everything that happened as soon as I entered. we then lay side by side forming a pronuclei of both of us, grew and replicated our DNAs, then we divided mitotically initiating the first step of a process known as cleavage.

cleavage is the process of division by mitosis, of a fused sperm-egg cell (zygote) from a single cell after fertilization to an 8 or 16 cell mass

I wish I did not have to end my story here but I have to because from this point on, I am now known as an embryo. Though some people choose to call me blastocyst in my 16 cell stage, I do not think I will ever love any other name like I loved ‘sperm’

Summary

Fertilization is like a quest for the sperm cells. only the one who finds and collects what he set out for gets to wear the crown and enjoy a new life with his new found egg.

References

_{Sadler, T W (Thomas W); Langman, Jan. Medical embryology.
12th ed. / T.W. Sadler.
Philadelphia : Wolters Kluwer Health/Lippincott Williams & Wilkins, c2012.
Moore, K. L., Persaud, T. V. N., & Torchia, M. G. (2008). The developing human: Clinically oriented embryology. Philadelphia, PA: Saunders/Elsevier.
Moore, K. L., & Dalley, A. F. (1999). Clinically oriented anatomy. Philadelphia: Lippincott Williams & Wilkin
Guyton, A. C., & Hall, J. E. 1. (2006). Textbook of medical physiology (11th ed.). Philadelphia : New Delhi: Elsevier Saunders.}

Image Sources

All images are from pexels and wikicommons licensed under creative commons and eligible for commercial use.

Thanks for reading