From Zygote To
Blastocyst
Zygote
With
fertilisation the oocyte and spermatozoon combine to become a zygote. The
zygote is the simplest form of the new animal, and will begin to split and
divide into new cells that will become organised, specialised and form shapes
and new structures as it becomes more complex.
Around 24
hours after fertilisation the zygote begins to increase its number of cells by
rapid mitosis, but without increasing its size. The cells become smaller with
each cell division. The number of cells doubles with each division. This
is cleavage (Fi he cells of the zygote are called blastomeres.
Cells beco
ly squashed
together. From around the 12‐cell stage the ball of cells becomes called
the morula (Figure 11.2), derived from the Latin word for
mulberry, which it now resembles.
The cells of
the morula will not only give rise to the cells of the embryo, but also to many
of its supporting structures, such as part of the placenta.
By this
stage the cells are communicating with each other and becoming organised and
ready for the next stage.
The
blastomeres in the middle of the morula become the inner cell
mass or embryoblast. These cells will directly form the
embryo. The blastomeres on the outside of the morula become the outer
cell mass or trophoblast. These cells will form some of
the supporting structures for the embryo.
The morula
passes into the uterus around 4 days after fertilisation (Figure 11.3).
Trophoblast
cells pull luminal fluid from the uterine cavity into the centre of the morula
(Figure 11.2). The fluid‐filled space that forms is called the blastocoel (or
blastocyst cavity). The cells of the inner cell mass are pushed to one end of
the cavity and become called the embryonic pole. The morula is now
called a blastocyst.
Around 5
days after fertilisation the blastocyst loses the zona pellucida.
By doing this it becomes able to grow in size and interact with the uterine
wall. The blastocyst attaches to the endometrial epithelium lining the uterus,
triggering changes to the trophoblast and to the endometrium in preparation for
the implantation of the blastocyst into the uterine wall (see Chapter 13).
Twinning can
occur in different ways. Two separate blastocysts may form from fertilisation
by different sperm of two different ova released from an ovary simultaneously.
These twins would not be identical twins, and they would have separate
placentas (dichorionic), separate amniotic sacs (diamniotic) and may even be of
different sexes (Figure 11.4). These would be dizygotic twins (or
fraternal or non‐identical twins).
A zygote may
split during cleavage, or later, when the inner cell mass has formed, or later
still, when the embryo has become more complicated and formed a bilaminar
embryonic disc (see Chapter 13). If the zygote splits during cleavage each
blastocyst will implant separately. If the zygote splits at a later stage the
two embryos may share the same chorion, amnion or placenta (Figure 11.5).
If a single
zygote splits identical twins will grow. These twins would come from the same
ovum and spermatozoon, so would be genetically identical. These would be monozygotic
twins (or iden- tical twins). This is rarer. It is common for
monozygotic twins to share a placenta (monochorionic), but have separate
amniotic sacs (diamniotic). This situation arises from cleavage of the
blastocyst 4–8 days after fertilisation. A small number of monozygotic twins
share their amnion (monoamniotic), and this occurs if the division of the
zygote occurs later than 9 days after fertilisation. The more tissues shared
between twins the greater the risk to the embryos. Hence, dizygotic twins have
the lowest mortality risk. Conjoined twins are at significant risk. This
situation arises when the zygote splits incompletely later than 12 days after
fertilisation.
Clinical relevance
It is
thought that blastocyst abnormalities are common and not compatible with life.
Most probably do not implant into the uterus, show no signs of pregnancy and
therefore often the pregnancy is not detected.
Twins are
more likely to be born prematurely, resulting in low birth weights and the
associated complications.
In
vitro fertilisation (IVF) treatments can result in multiple zygotes because of
the drugs used to encourage ovulation. Clomid is a drug that blocks oestrogen
receptors, so the body perceives low, more FSH is released and more follicles
mature in he ovary to be released and fertilised.
