Tag: reproduction

Posted in Science & Nature

From Cell To Birth: Fertilisation

Posted on by Jinavie

Once the sperm enters the vagina, the real battle begins. The vagina is highly acidic, an environment in which sperm can only survive 2~3 hours. It is crucial for the sperm to enter the uterus through the cervix, but only 1% of the 200~300 million sperm make it through.

Even within the uterus, they must brace harsh conditions as they travel against gravity. After about 5 hours of intense swimming, the sperm reach the top of the uterus. Here they face a choice: go left or go right. Half the sperm make the wrong choice and head down the eggless fallopian tube and ultimately die. The rest navigate their way through the maze of folds in the fallopian tube, often getting lost or sticking to the wall thinking that it is an egg.

About 200 sperm finally make it to the egg, which sits in the ampulla of the fallopian tube. But as always, there is competition even at this final moment. Only one sperm can win the race, and the fastest one will ultimately produce a new life.

When the first sperm touches the egg, a series of chemical reactions occur, essentially “priming” the sperm. This causes it to start the acrosome reaction, where it releases a hoard of enzymes from its head, digesting away the covering shell (zona pellucida) of the egg. It then becomes supercharged, using all of its energy to drive itself inwards until it reaches the oocyte within. As soon as this happens, the tail breaks off, and one final chemical reaction as the calcium level spikes occurs to release more enzymes that prevent the acrosome reaction in other sperm. It also solidifies the zona, forming an impenetrable shield to prevent other sperm coming in (polyspermy can lead to a failed pregnancy).

The calcium spike that causes the above cortical reaction also triggers the egg to divide, so that it reaches the most mature stage. The winning sperm can then combine its nucleus with the oocyte, forming the 46 chromosomes that will set the genetic basis of the new zygote (first stage of a baby).

To reach the egg, the sperm must travel over 20cm – beating its tail over 20,000 times. The probability that a certain sperm will fertilise the egg is 1 in 500,000,000.
Life starts under a near-zero probability condition.


(Full series here: https://jineralknowledge.com/tag/arkrepro/?order=asc)

Posted in Science & Nature

From Cell To Birth: Sex

Posted on by Jinavie

The two copulatory organs are the penis and the vagina. Both are designed to maximise the chance of a new life being conceived.

The penis is normally flaccid, but when stimulated through touch or erotic images and thoughts, it can become stiffened to eight times its original size. Contrary to certain slang words, the penis contains no bones – it is merely a sponge.
When the brain signals the penis to become erect, the sponge is relaxed, letting blood flood in, filling it like a balloon. This combined with two muscles and the sheath enclosing the penis achieves the erection which is critical in sex.

The vagina is shaped to perfectly accommodate an erect penis, and receives the sperm that will eventually fertilise the egg. As sex involves the piston movement of the erect penis within the vagina, it is bound to suffer chafing. So nature developed Bartholin’s glands that produce a lubricant, smoothing the process.
The clitoris actually shares its origin with the penis, and thus swells when sexually excited. It is also extremely sensitive.

The goal of sex is simple – excite the penis enough for the man to achieve an orgasm (note that female orgasm is optional, but ideal, for conception). When a threshold is reached, the brain sends out strong signals to squeeze sperm out from the epididymis, and seminal fluid from the prostate and seminal vesicles. The combined fluid (semen) shoots through over half a metre of tube until it is ejaculated out.
The semen collects in the vagina, where the cervix laps up the semen and transports it into the uterus. From here, the sperm’s adventure begins, facing many troubles to conceive the egg at the end of the line.

(Full series here: https://jineralknowledge.com/tag/arkrepro/?order=asc)

Posted in Psychology & Medicine

From Cell To Birth: A Man And A Woman

Posted on by Jinavie

Organisms have the amazing ability to beget new life. In bacteria, this can be as simple as splitting itself in two. In humans, however, this process is much more complex.
As a sexually reproducing animal, both a man and a woman are required for the creation of a new person. The process, as complex as is it, is so intricately designed by nature that it could possibly be considered as one of the greatest abilities of the human body.

A man contributes sperm, providing half of the genetic material the future baby. The sperm also decides the sex, depending on whether it carries the X or Y chromosome. Note that gametes only carry half the number of chromosomes (which are usually paired) of a normal cell.
Sperm is made in the testes. Here, under the guidance of hormones such as testosterone and nurturing cells, they grow from a small stem cell, into a plump, round spermatocyte, until it is streamlined to become the sleek spermatozoa that people are more familiar with. All of this occurs as the cell journeys from the outside of the seminiferous tubule to the centre where it is released altogether with its fellow batch.
The sperm is still immature, the equivalent of a high-school graduate. It is expelled into the epididymis, a 4-metre-long tube packed full of concentrated sperm, acting as the “boot camp”. Here, the sperm is drained of extra baggage it is carrying, while learning how to swim effectively. It is stored until the time comes.

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A woman contributes an egg, carrying the other half of the genetic material required. It is significantly bigger than a sperm, and as such is produced in much fewer numbers. A woman, unlike a man, has a limit to how many eggs she can produce, and the moment her reservoir runs out is called menopause. Until then, she produces one (or more sometimes) egg every month according to her menstrual cycle.
An egg is developed within a follicle, that acts as a house and oestrogen factory until the egg is released. To get to this stage, it needs to defeat its competitors first. To prevent multiple pregnancies, the ovaries kill all secondary follicles except one dominant follicle. The follicle then ovulates, wherein the oocyte (egg) is expelled almost explosively, caught by the finger-like fimbriae, and then transported towards the uterus via the fallopian tube.
If the egg is not fertilised within a day, it dies and is later expelled with the endometrium, in what every woman knows as a period.

This is only the beginning of the long journey until the miraculous birth of a child.

(Full series here: https://jineralknowledge.com/tag/arkrepro/?order=asc)

Posted in Science & Nature

Man Versus Ant

Posted on by Jinavie

Human:
A mammal that ranges in size from about 1 to 2 metres.
Weighs between 30kg and 100kg. Females are pregnant for 9 months. 
Omnivorous diet. Population estimated around 7 billion.

Ant:
An insect that ranges in size from about 0.01 to 3 centimetres.
Weighs between 0.001mg and 1g. Can produce eggs endlessly given there is an ample sperm reservoir.
Omnivorous diet. Population estimated around 1 quintillion (1 billion times 1 billion).

(from The Encyclopaedia of Relative and Absolute Knowledge by Bernard Werber)

Posted in Psychology & Medicine

Madness

Posted on by Jinavie

“As mad as a hatter” – this is a well-known English idiom, particularly famous after Lewis Carroll created the Mad Hatter character in his work Alice in Wonderland. However, what is less known is the fact that this idiom is based on actual events.

In the 18th and 19th centuries, hatters used mercury to treat felt (traditionally made from rabbit fur or the more luxurious beaver fur). Unfortunately, mercury is a highly toxic heavy metal, which causes severe damage in the human body. In the case of mercury poisoning (also known as Minamata disease or the Mad Hatter disease), it infiltrates neurons to cause severe neurological symptoms. For example, it can impair vision and hearing, cause paresthesia (pins and needles), anxiety, depression, tremors and hallucinations. The famous physicist, Isaac Newton, also suffered from Mad Hatter disease.

Another mad character from Alice in Wonderland is the March Hare. As one may deduce from his name, he is modelled after a normal hare. The reason why the March Hare is mad is that March is around the time when rabbits enter their mating season, and male hares are in heat. They then have only one thing in mind: sex. 

Maybe, as the Cheshire Cat explains, “we’re all mad down here”.

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Posted in Science & Nature

Parthenogenesis

Posted on by Jinavie

It is often believed that for complex organisms such as animals, sexual reproduction is a must to produce offspring. Asexual reproduction is common in bacteria and (some) plants, but even tiny beings such as insects use sexual reproduction to produce a mass of variable offspring. 

However, it has been found that despite having two clear sexes – male and female – there are cases where a certain species is able to reproduce without the need of sexual intercourse. In these cases, the female’s eggs spontaneously divides to form a new offspring without being fertilised by a sperm. Furthermore, sometimes the egg either fuses with another egg or undergo several genetic mixing and mutation to produce some variety in genetic pool, thus avoiding the issue of asexual reproduction (where the entire population can be wiped out by a single disease due to identical genetic makeup). This is known as parthenogenesis, and it has been documented in many insects, fish, reptiles and even birds. 

Obviously, as there is no donation of a Y chromosome, every offspring born from parthenogenesis is female. Because of this, some species such as the New Mexico whiptail, a lizard that is capable of both sexual reproduction and parthenogenesis, the population has completely rid itself of males, making it a completely female species. Curiously, they still engage in “mock sex”, giving them the nickname “lesbian lizards”.

Although parthenogenesis has never been documented in mammals in nature, it has been induced artificially in mice, rabbits and monkeys. However, they all developed severe developmental issues due to the numerous mutations in the egg required for parthenogenesis. 
But if in the future, human parthenogenesis is perfected, it is possible that humanity too could end up as an all-female society.