TWINS - What are they?


Twins are 2 offsprings produced in the same pregnancy. 

Twins can be monozygotic and dizygotic.

Or in simple terms, identical and fraternal.

But.

Twins can also be conjoined.
*Dun dun duuun*

Find out more about twins in the next few posts!


Identical Twins


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Introduction

Identical twins are the result of a single fertilized egg splitting into two separate embryos after conception. They share a single placenta. Since identical twins come from the same fertilized egg, they have the exact same DNA, gender and blood type.
Even though identical twins have the same DNA, it can be expressed in different ways. This is affected by environment that the twins are exposed to (in the womb or out of the womb). As a result, identical twins usually have different fingerprints. 
How are they formed? 

Naturally


Fertilisation: One sperm fertilises the ovum and the nuclei of the sperm and ovum fuse.
Day 1: The fertillized egg, now called a zygote, divides once, resulting in two cells.
Day 4 to 5: Each zygote develops, and has about 100 cells by the end of Day 5.
Day 6:  The zygotes implant themselves in the endometrium (uterine lining). They will continue to develop for approximately nine months (like normal zygotes).

    Here's a self-made video to illustrate the process!
If the zygote divides after Day 13 of conception, then the cells forming the two embryos do not entirely separate and the twins are born conjoined (read more about conjoined twins in this post).


Artificially

  • Artificial embryo twinning uses the same approach as how identical twins develop naturally, but it occurs in a Petri dish instead of in the mother's body.
  • An early embryo is split into two individual cells.
  • Each cell is allowed to divide and develop on its own.
  •  A surrogate mother will then carry and deliver the embryos. Since all the cells come from the same embryo, the twins will be genetically identical. 




Fraternal Twins

Introduction

Fraternal twins usually occur when 2 fertilized eggs are implanted in the uterus wall.

Unlike identical twins, fraternal twins have 2 placentas (which provide vital substances to the baby during pregnancy). As they have their own placentas, fraternal twins are not at risk for some of the risky conditions that affect identical twins, such as TTTS or monoamniotic twins.

Fraternal twins usually do not look alike as they have a very small chance of having the same chromosome profile. It is quite common to have twins of different genders. Some people may even regard fraternal twins as, simply, siblings of the same age.


 How are they formed? 

The 2 most common ways fraternal twins develop are superfecundation and superfetation.
                                                                                 
Superfecundation:
  • A female ovulates more than 1 egg during her cycle (a process called hyper ovulation). Different sperms fertilize each of the 2 eggs.
  • The female may have inherited a gene that causes hyperovulation, thus the common conception that "twins are hereditary". Many fertility drugs cause a female to hyperovulate too.

Superfetation:
  • A female ovulates and releases another egg when she is already pregnant. A different sperm then fertilizes the second egg.
  • While the twins have different conception dates the babies will most likely be delivered at the same time, although they may be of different size and at different developmental stages.

In-vitro fertilization - where fertilized eggs are implanted in the female - is a form of superfetation. Fertility treatments, whether by drugs like Clmid or by procedures like in-vitro fertilization, sometimes produce twins or multiples, and in majority of these births, the twins are fraternal.

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      Here's a self-made video showing the formation of 
     fraternal twins.

Sources


Conjoined Twins

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Introduction 
Conjoined twins are extremely rare. They occur in approximately once every 200 000 births. As the term 'conjoined' suggests, these identical twins are joined together. 

There is a very low chance of survival, with the survival rate capped at only 25%. 40 to 60% are delivered stillborn, meaning that they die even before leaving their mother's womb. 35% survive only one day.

Conjoined twins are also known as Siamese twins.




How are they formed? 
There are two contradicting theories.

1. Fission
When the fertilized egg splits into two, identical twins are formed. However, if the split occurs more than twelve days after conception, the embryos will not fully divide, resulting in conjoined twins.


2. Fusion
The fertilized egg initially splits into two, forming identical twins. However, while the twins lie side-by-side in the uterine wall, the embryos fuse together. 

How is this possible? Well, a human embryo in its earliest stages consists of three layers of cells. These cells "seek out" cells of the same type and bond together to form individual organs. However, in the case of conjoined twins, when the two identical embryos lie side-by-side, very close to each other, the signals may get mixed. The cells of Twin A attach themselves to the cells of Twin B and although the cells are of the same type, they will end up sharing the same organ and become conjoined. 

Here's an analogy to make it clearer. If the liver cells of Twin A attach themselves to the liver cells of Twin B, the twins will share the same liver and end up becoming conjoined! 
Some Types of Conjoined Twins 


Thoraco-omphalopagus: This is the most common type of conjoined twins.
Both bodies are fused from the upper chest to the lower chest. The twins usually share a heart and liver.
(Or part of the digestive system).
Thoracopagus: The twins’ bodies are fused from the upper thorax to the lower belly. In these cases, the heart is almost always conjoined.
Omphalopagus: In this case, the bodies are fused at the lower chest. The heart is never involved. However, the twins share a liver, digestive system, diaphragm and other organs.
Parasitic twins: Twins that are asymmetrically conjoined, resulting in one twin that is smaller, less formed, and dependent on the larger twin for survival.
Craniopagus:  The twins have fused skulls but separate bodies. They can be conjoined at the back or front of the head, or the side of the head. However, they are not conjoined on the face or the base of the skull.




Cephalopagus: Two faces on opposite sides of a single, conjoined head.  Only the upper portion of the bodies is conjoined. These twins generally cannot survive due to severe malformations of the brain.
Parapagus: The twins are fused side-by-side with a shared pelvis.



Credits
Diagrams of different types of conjoined twins: http://www3.telus.net/tyee/multiples/4conjoined.html
http://www.conjoinedtwins.med.sa/twins_type.php

 Information: 
http://www.mayoclinic.com/health/conjoined-twins/DS00869
http://en.wikipedia.org/Conjoined_twins
http://multiples.about.com/od/conjoinedtwinsfaq/f/conjoinedtwin_b.htm
http://www.umm.edu/conjoined_twins/facts.htm
http://www.phreeque.com/conjoined_twins.html

Case Study on Conjoined Twins: Hensel Twins



Who Are They?



Abigail and Brittany Hensel were born on March 7, 1990 in Minnesota in the United States. Doctors and nurses predicted that they would not last through the night, but the twins have proved them wrong with their development as healthy children. 

 The 22-year-olds are still conjoined now, and they are leading a perfectly normal life: learning how to drive and even travelling around Europe with their friends!

In what way are they conjoined? 
A diagram showing the organs that Abigail and Brittany share
The Hensel girls are the rare form of conjoined twins (parapagus). 

 From the waist down, all organs are shared, such as the large and small intestines, the pelvis and the reproductive organs. Other organs are also shared above the waist: the ribcage and liver. The twins also have 3 kidneys, 4 lungs(two of which are joined), a shared circulatory system and partially shared nervous systems. They were originally born with 3 arms but had one removed during an operation.

 How they lead their (not-so-normal) lives 

Brittany - the left twin - can’t feel anything on her right, while Abigail - the right twin -can’t feel anything on her left. However, their limbs move automatically as if co-ordinated by one person!
The twins have different personalities. Abigail is the domineering one while Brittany is more reserved. However, they seldom argue and even help each other. When Brittany was sick, Abigail even volunteered to take her medicine as her sister could not swallow the pill.

Find out more!

 Click on http://abcnews.go.com/Health/conjoined-twins-abby-brittany-hensel-astound-doctors-normalcy/story?id=17021596 for more information! 
Adapted from:
http://gawker.com/5933247/two-heads-one-body-a-billion-questions-the-gawker-guide-to-tlcs-newest-stars-abby--brittany-hensel
http://www.dailymail.co.uk/news/article-2185896/Extraordinary-conjoined-twins-expected-survive-infancy-star-reality-TV-charting-graduation-travels-Europe.html
Video taken from Youtube (Hensel Girls 22 Years Young)

Case Study 2 on Conjoined Twins: Zainab and Jannat Rahman


Who Are They???  


Jannat and twin Zainab Rahman
The girls when they were babies

 

Zainab and Jannat Rahman were born at Homerton Hospital in East London on December 1, 2002. Doctors were extremely pessimistic about their condition and predicted that they would only have a one-in-a-million chance of survival.

But they were successfully separated!

In what way were they conjoined? 

Zainab and Jannat are omphalopagus twins. They were conjoined from chest to abdomen and shared a liver.

Their separation 

Their parents call them 'miracle princesses' and it is no wonder. They were extremely fortunate that they had individual hearts and that the only organ they shared was the liver - the only organ in the human body capable of regenerating - which enabled a separation. However, Jannat had a hole in her heart which needed to be operated on.

They were separated on 15 January 2003 in only 45 minutes. There was a huge risk as Jannat had 40% of the liver and the join was less than a centimetre from their hearts! Two days later, Jannat had to be operated on again due to the hole in her heart and suffered an infection and a twisted bowel. Fortunately, both survived and were only the third set of conjoined twins to survive in Britain!

How they lead their normal lives 

The twins have recently celebrated their tenth birthdays.

They have different personalities, but sometimes they switch. One week Zainab can be caring and another week Jannat would be the caring one. Although they often fight like normal siblings, they are extremely close. Well, this is probably due to the power of twins! 



Little beauties: Jannat and Zainab
The girls now
Find out more! 

For more info,  click on:

http://www.mirror.co.uk/news/real-life-stories/siamese-twins-whose-survival-amazed-1510402
http://www.telegraph.co.uk/news/uknews/1420167/Separated-twins-are-miracle.html

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