Introduction to Twinning
There are two types of twins: dizygotic and monozygotic. So how are each type formed?
Dizygotic twins, also known as fraternal twins, result from two eggs being simultaneously released, one egg coming from each ovary. Normally, the female releases one egg at a time, but releasing multiple eggs at once seem to be genetically influenced. Two different sperm will fertilize the two eggs around the same time. Two embryos will form, in separate placenta and with different genetic information.
Monozygotic twins, also known as identical twins, result from the splitting of a single egg that was released from one of the two ovaries and later fertilized by a single sperm. It is theorized that the egg splits around the time of the development of the embryonic disc. Depending on how early the egg splits, monozygotic can either have separate placentas or share one. For either occurrence, both of the monozygotic twins are genetically the same, since they came from the same fertilized egg. The exact cause of why the egg splits is unknown, but it’s possibly due to genetic and/or environmental factors.
Twinning Statistics
Dizygotic twins: 10-15 in 1000 live births
Monozygotic twins: 4 in 1000 live births
Twinning in general: 1 in 90 births
Introduction to Conjoined Twinning
Conjoined twinning can arise from the event of monozygotic twinning. Though the exact cause of conjoined twinning is unknown, there are two theories that researchers have come up with: fusion vs. fission. Both theories deal with the development of the embyro during the first week of gestation. This is the period where the fertilized egg is capable of splitting into two individual embryos.
The first theory is that the egg splits into two embyros, and then rejoins during this early stage of development. This process is known as fusion.
The second theory, and the most commonly believed, is that the egg does not completely split. This problem in fission results in the joining of the two embyros at a certain site. Researchers believe that the division of the embryo occurs during the embryological disc phase of development, some time before or during neuralation. If the embryo divides later in development, it may not fully separate, leading to the formation of conjoined twins.
It is thought that conjoined twinning results from the formation of more than one organizing center, or primitive streak, in a single embryonic disc during gastrulation. Each primitive streak determines their own axes of development, with corresponding determination of paraxial tissues. Lateral folds for on both sides of each primitive streak, leading to the development of two separate embryos.
The theory of duplicated primitive streaks is illustrated in the three diagrams below. The two primitive streaks are shown in the first diagram. The formation of the lateral folds on both sides of each primitive streak can be seen in the second diagram. In the third diagram, the two separate embryos are developing.
Using the second theory as a basis, researchers have come up with a few contemporary models of conjoined twinning. One of the models is Spencer's contemporary model, shown right below.
Note: Please refer to the "Twin Types" link on the menu bar for small descriptions of the different types of conjoined twinning.
The arrangement, orientation, and proximity of the primitive streaks to one another helps predict the type of conjoined twinning that will occur. According to Spencer's contemporary model of conjoined twinning, Parapagus and omphalopagus both result from parallel primitive streaks. But as shown, the closer proximity is what makes parapagus different from omphalopagus. Ischiopagus twins result from tail-to-tal arrangements of the streaks, while cephalopagus and thoracopagus twins result from head-to-head arrangements but differ due to proximity.
Conjoined Twinning Statistics
Conjoined twins: 1 in 50,000 live births
Delivered stillborn: 60%
Survival after separation: 64%
Female:Male ration = 3:1
Below is a video with a quick overview on conjoined twins.