monozygotic twins Flashcards
how do monozygotic twins arise?
from fertilization of a single by a single sperm
frequency of monozygotic twins?
3/1000 with no significant variation in different populations or maternal age
what results in monozygotic twinning?
a split of embryonic cells, most likely of the inner cell mass. complete monozygotic twins reflect the regulative versus mosaic development of higher vertebrates.
what do extraembryonic and embryonic structures that are shared by developing monozygotic twins reflect?
the stage and extent of splitting of the original embryonic cell mass or developing embryo
what does split at 2 cell stage result in?
separate amnions, chorions, and placentas, which may later fuse
what does a split of the ICM result in?
separate amnions but a common chorion and placenta
what does an incomplete split of the ICM or a split at later stages (e.g bilaminar disc stage) result in?
common amnion, chorion and placenta. Incomplete splitting of the bilaminar disc may result in conjoined twins.
what is the frequency of conjoined twins?
1-2/50,000 births and 1/600 sets of twins. a higher frequency occurs in female twins than male twins.
what do specific conjoined regions depend on?
the region of the ICM or bilaminar disc that did not completely separate
What is Bateson’s rule?
shared, duplicated organs are mirror images of each other.
