From Zygote To Blastocyst
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.
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.
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.