cell – Jineral Knowledge

Ship Of Theseus

Posted on 09/01/201322/11/2021 by Jinavie

An ancient Greek philosopher named Plutarch pondered this scenario. Imagine that the Greek hero Theseus was to repair his ship after a long journey by replacing broken parts with new timber. If he was to embark on so many journeys and repair his ship so much that all of the original material that made the ship were replaced, is that ship still the same ship of Theseus?

This is an interesting philosophical question where some may argue that the ship is still, by definition, the “ship of Theseus” while some may argue that it is no longer the same ship Theseus once owned, but merely a replacement.

Although it is hard to grasp the significance of this question when using an analogy of ancient Greek heroes and ships, it comes closer to home in the field of biology. It is a known fact that the human body is under constant change; cells divide to produce a new lineage of fresh cells while shedding away old, dead cells. Different cells turnover at different rates; skin is almost completely replaced every 4~6 weeks, the lining of the gut is turned over every 4~6 days, while brain cells are almost never replaced (but contrary to popular belief, they can regenerate). If this is the case, are you the same “you” as you were a year ago when the majority of your skin and gut cells were technically “different” (but genetically identical) cells to what they are now? Or are you simply a replacement shell for your brain?

A simpler way of thinking about this would be to consider the case of clones: are clones the “same” as their originals?

The paradox of the ship of Theseus can be extended into a larger scale. Consider a large city like New York. If we were to assume that all of the inhabitants of a city are replaced over a hundred years, then is that city still “New York”? Not only would it looks different because of its new buildings and whatnot, but the people that make up the culture and substance of the city would be completely changed. However, New York is still called “New York” just as it was in the early 1900’s. So is the modern day New York still New York or New New York?

Science & Nature

Mitochondrial Eve

Posted on 05/12/201114/02/2019 by Jinavie

We were all born from our parents. Our parents were all born from our grandparents. Everyone has a family tree and a root. If so, is it possible to find the beginning of mankind – our true “root”?

Our cells have an organelle (a part of the cell) called mitochondria. Mitochondria act as the cell’s engine and allow the cell to generate energy through respiration. An interesting fact about them is that they are not originally “ours”. About 1.5 billion years ago, there was an event where a prokaryote (cells without a nucleus, like a bacteria) invaded (or was eaten by) a eukaryote (cells with nuclei, like our cells). The prokaryote and the cell began a symbiosis and the prokaryote became a part of the cell.

Due to the external origin of mitochondria, they have a different genome to us. This is called mitochondrial DNA, shortened to mtDNA, which allows mitochondria to divide and synthesise proteins without the help of the host cell. It used to be a completely independent organism, but it has lost some of its functions to the cell.

mtDNA is inherited in a different way to normal DNA. Normally we receive half of our mother’s and half of our father’s genes, but we only inherit our mother’s mtDNA. This is because sperm keeps mitochondria in the tail which is lost during fertilisation, meaning our father’s mitochondria cannot be inherited. The only way to gain mitochondria is from those in the cytoplasm (the material that fills cells) of our mother’s egg. This is known as maternal inheritance.

Using this information, scientists compared a large sample of people’s mtDNA to turn back the clock. Knowing that a child and its mother share the same mtDNA and the mother and grandmother share the same mtDNA, we can analyse mtDNA to find the origin of mankind, or our first common female ancestor – also called Mitochondrial Eve.

Mitochondrial Eve is estimated to have lived 200,000 years ago in Africa, thus she is also known as African Eve. Her mtDNA is an ancient heirloom passed along generation after generation to us, as evidence of evolution. Every living person on the face of the Earth is a descendant of her. So in some ways, it could be said that we truly are one big family.

Science & Nature

From Cell To Birth: Growth

Posted on 01/05/201112/02/2019 by Jinavie

After implantation, the embryo quickly grows from a ball of cells into what will be a fully-formed baby. However, it first needs a way to feed: the placenta.
It is an organ that actively takes nutrients and oxygen from the mother’s blood, exchanging it for the embryo’s waste products. It is extremely effective in keeping the fetus alive and protects it from infections or the mother’s immune system.
The blood is carried by the umbilical cord, which plugs into the belly button. This cord is the lifeline throughout term, and disrupting the blood supply will lead to permanent brain damage or even death.

In the first 10 weeks, the blastocyst develops into a very primitive disk-like object that shares no resemblance to a person. It keeps growing and differentiating at a rapid rate (almost doubling in size per week) until it forms an embryo that is more familiar, roughly about week 6. Interestingly, a human embryo looks almost identical to embryos of rabbits, chickens, turtles and fish, showing how all animals shared a common ancestor in the course of evolution. At this stage, the embryo has features such as gills, a tail and a fish-like appearance.

After 10 weeks, the embryo has grown to about 5~8cm (almost 10~20 times the size at week 6), and is now called a fetus. It begins to properly grow organs, and resembles a miniature baby with primitive features.
It continues to grow for the next 30 weeks, continuously relying on the mother for nutrition and life support.

Many different factors contribute to premature birth and IUGR (intrauterine growth restriction), which leads to the birth of a small baby. This may result in less developed organs (especially the lungs) and may affect the health of the newborn throughout its life. There are also many poisons known to harm the development of the embryo/fetus, such as alcohol, nicotine, cocaine, heroin and much more. These should be avoided from a few weeks before conception onwards (even after birth while breastfeeding).

By about 38 weeks, the lungs (the last organs to fully mature) are ready and the fetus is upside down. It is ready to leave the womb, and thus sends a signal to the mother, known as labour. This is when the arduous process of childbirth begins.

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Science & Nature

From Cell To Birth: Implantation

Posted on 30/04/201112/02/2019 by Jinavie

When an egg is fertilised by the sperm, it is called a zygote. This zygote immediately starts to divide at an exponential rate, to achieve the feat of transforming from a single cell to a 3kg baby. The division and growth happens as the zygote slowly drifts towards the uterus, where it can secure itself.

30 hours after fertilisation, the zygote is now 2 cells.
72 hours, the zygote is now 16 cells.
96 hours, the zygote is now a ball of over 60 cells, and now called a morula.
108 hours, the morula has a cavity inside, and is called a blastocyst.
The blastocyst, essentially a shell of cells with a mass of cells at one point, hatches out of the zona pellucida as it is now much bigger.
To gain the massive amount of energy required for development, the zygote eats up simple sugars in the fallopian tube during its travel.

As mentioned before, when pregnancy does not happen, the endometrium is shed and flushed out. To prevent this, the blastocyst secretes something called βhCG, keeping the corpus luteum alive, which secretes progesterone to maintain the endometrium. As the endometrium is the fertile “soil” where the embryo will grow, this is a vital step (βhCG is the hormone tested in a pregnancy test).
When the blastocyst reaches the uterus, it finds a safe spot on the endometrium, with the inner cell mass facing the wall. This is where implantation begins.

After clinging tightly to the endometrial cells, the blastocyst fuses some of its cells into a digging tool that can eat away at the endometrium. As it digests away the cells, the blastocyst slowly burrows in until it is completely embedded inside. Cells invade the hollowed space, firmly securing the blastocyst while destroying blood vessels and glands to release nutrients, securing a supply line. Now, it can start its rapid development into an embryo as it leeches away the mother’s nutrients.

A foreign body latching on to the host’s cells, digesting away tissue and leeching blood and nutrients – an embryo acts exactly like a parasite, to ensure that it can safely survive the 40-week gestation. In fact, an embryo can implant itself almost anywhere in the body, such as the fallopian tubes, ovaries or even the gut, as long as there is a secure blood supply. This is called an ectopic pregnancy, and can be an extremely dangerous scenario to both the mother and developing fetus.

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Science & Nature

From Cell To Birth: Fertilisation

Posted on 29/04/201112/02/2019 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.

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Science & Nature

From Cell To Birth: Sex

Posted on 28/04/201112/02/2019 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.

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Psychology & Medicine

From Cell To Birth: A Man And A Woman

Posted on 27/04/201112/02/2019 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.

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.