Human Anatomy
Second Week of Development
Second week of development for MBBS: cleavage, blastocyst, trophoblast, implantation and the bilaminar disc, mapped to NMC codes AN78.1 to AN78.5.
MedNext Academy | 3 min read
Second Week of Development
Second week of development for MBBS: cleavage, blastocyst, trophoblast, implantation and the bilaminar disc, mapped to NMC codes AN78.1 to AN78.5.
This chapter follows the second week of development, when the blastocyst implants, the trophoblast differentiates and the bilaminar embryonic disc forms with its cavities. It covers the abnormal sites of implantation and the basis of the pregnancy test.
High-yield: Second Week of Development
- Cleavage is the series of mitotic divisions of the zygote that increase cell number without increasing overall size.
- The solid ball of cells formed by cleavage is the morula, which enters the uterus about three days after fertilisation.
- The morula develops a fluid-filled cavity to become the blastocyst around the fifth day.
- The blastocyst has an outer trophoblast, an inner cell mass or embryoblast, and a cavity called the blastocoele.
- The inner cell mass gives rise to the embryo and the trophoblast contributes to the placenta.
- Implantation into the endometrium begins about the sixth day and is usually complete by the end of the second week.
- The trophoblast differentiates into an inner cytotrophoblast and an outer syncytiotrophoblast.
- The syncytiotrophoblast invades the endometrium and erodes maternal vessels to begin the uteroplacental circulation.
- The inner cell mass forms a bilaminar disc of epiblast and hypoblast during the second week.
- The amniotic cavity forms within the epiblast and the primary yolk sac is lined by the hypoblast.
- The extra-embryonic mesoderm forms between the trophoblast and the exocoelomic membrane and later splits to form the chorionic cavity.
- The prochordal plate is a thickening of the hypoblast that marks the future head end and the site of the mouth.
- The common abnormal sites of implantation include the uterine tube, the ovary and the cervix, giving ectopic pregnancy.
- The rule of twos of the second week describes two trophoblast layers, two germ layers, two cavities and two extra-embryonic mesoderm layers.
- Early pregnancy is confirmed by detecting human chorionic gonadotrophin, secreted by the syncytiotrophoblast.
The rule of twos of the second week
- **Trophoblast:** Two layers: cytotrophoblast and syncytiotrophoblast. The syncytiotrophoblast invades the endometrium.
- **Embryonic disc:** Two germ layers: epiblast and hypoblast, forming the bilaminar disc.
- **Cavities:** Two cavities appear: the amniotic cavity and the primary yolk sac.
- **Implantation:** Normally in the uterine wall; ectopic sites include the tube, ovary and cervix.
NMC competencies in this chapter
- **AN78.1:** Cleavage and formation of the blastocyst
- **AN78.2:** Development of the trophoblast
- **AN78.3:** Implantation and its common abnormal sites
- **AN78.4:** Formation of extra-embryonic mesoderm and coelom, bilaminar disc and prochordal plate
- **AN78.5:** Abortion, decidual reaction and the pregnancy test
Frequently Asked Questions
What is a blastocyst?
It is the stage that forms around the fifth day, consisting of an outer trophoblast, an inner cell mass that becomes the embryo, and a fluid-filled cavity called the blastocoele.
What are the two layers of the trophoblast?
The inner cytotrophoblast, which retains cell boundaries, and the outer syncytiotrophoblast, which is multinucleate and invades the endometrium to establish the uteroplacental circulation.
What is the bilaminar disc?
It is the two-layered embryonic disc of the second week, made of the epiblast facing the amniotic cavity and the hypoblast facing the yolk sac.
Where can abnormal implantation occur?
The commonest ectopic site is the uterine tube, but implantation may also occur in the ovary, the cervix or the peritoneal cavity, each carrying a risk of haemorrhage.
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