In the second week the embryo nestles in the wall of the uterus. The tissue on the periphery, the nutritive trophoblast, grows fast. From the inner cell mass, the embryoblast, the embryonic disc will be formed, and this disc becomes largely disconnected from the trophoblast. Differences in growth rate between the periphery and the centre are responsible for this process.
Implantation
At the end of the first week the embryo, now a blastocyst, has come out of its rigid zona pellucida and has arrived in the uterus. It can now grow, and it implants itself into the wall of the uterus with the side of the embryoblast first. Enzymes of the embryo digest the maternal tissue of the uterine lining. The embryo invades. It eats into the wall of the uterus, and in this sense its behaviour can be called aggressive.
The trophoblast grows so fast that it forms a proliferating tissue with many nuclei and without cell membranes between them. This tissue is called the syncytiotrophoblast (syn = together, cyto = cell). A layer of ordinary trophoblast cells with their own cell membranes, the cytotrophoblast, remains between the syncytiotrophoblast and the embryoblast.
In the syncytiotrophoblast gaps arise, called lacunae, through which maternal blood starts to flow. Maternal blood and embryonic tissue stay separated, but the barrier between them is thin, and substances are exchanged across it. Embryonic tissue also grows around the capillaries and the glands of the uterine lining. In this way the embryo is supplied with oxygen and nutrients, and waste products are carried away. The barrier is not a complete filter: many substances carried in the maternal blood can pass it as well.
The embryo eats into the maternal tissue. On the other hand, the mother gives room to the embryo in her own tissue. She allows a creature that is genetically partly foreign to grow in her own body, since the embryo arose from the fusion of an egg and a sperm cell. Normally the body defends itself against foreign tissue, which makes this acceptance a remarkable process. The syncytiotrophoblast produces the hormone hCG (human chorionic gonadotropin), which keeps the corpus luteum in the ovary active so that the uterine lining is maintained. How the maternal immune system comes to tolerate the embryo involves several further mechanisms that are still being studied.
Around day 10 the embryo lies completely inside the maternal tissue. The small defect in the surface of the uterine lining is first closed by a plug of fibrin and then covered again by epithelium, so that the wall is closed. Around the embryo lie the trophoblast, which will later be called the chorion (= skin), then the uterine tissue and the uterine wall.
Development of the embryonic disc
Flat, square cells separate from the embryoblast on the side facing the blastocoele (Fig. 12 to 15). These cells are called the hypoblast (hypo = under). The hypoblast also spreads over the inside of the trophoblast and lines it. The blastocoele, lined in this way, is now called the yolk sac.
In the embryoblast a small cavity develops, the amniotic cavity; its roof is called the amnion (= sheep skin). The cells next to the hypoblast become tall, cylindrical or elongated cube-shaped. These cells form the epiblast (epi = upper).
Through the formation of the yolk sac and the amniotic cavity, a round, flat embryonic disc, or germ disc, arises. It consists of two layers, epiblast and hypoblast. At the end of the week the prochordal plate, a spot of differently shaped cells, appears in the embryonic disc. It marks the side where the head will develop.
Origin of the chorionic cavity and the connective stalk
From day 9 on, the tissue between the trophoblast on one side and the embryonic disc, amnion and yolk sac on the other side thickens (Fig. 13). This tissue is called extra-embryonic mesoderm (extra = outside). The name can confuse, because the tissue lies inside the conceptus. It only indicates that the tissue lies outside the embryonic disc.
The syncytiotrophoblast and the cytotrophoblast grow much faster than the germ disc. As a result, crevices and cavities arise in this mesoderm tissue, together called the extra-embryonic coelom (coelom = cavity). From day 12 on they unite and form the chorionic cavity. The trophoblast, together with the mesoderm that lines it, is now called the chorion (= skin or leather).
At first the germ disc with amnion and yolk sac is connected to the chorion over a broad area on the posterior side (day 12). This attachment moves towards the place where the tail will develop and becomes narrower. It is called the connective stalk.
The round, flat germ disc, the amniotic cavity and the yolk sac are attached to the connective stalk like two halves of a ball. Together they hang freely in a large spherical space, the chorionic cavity, with the chorion as its surrounding wall.
Size
The embryo grows. At the beginning of the second week its size is approximately 0.3 mm, at the end 3 to 3.5 mm, so in one week it grows to about ten times its size. The germ disc itself stays small: about 0.5 mm.
The week of twos
Textbooks of embryology often call the second week the week of twos, and the name is a useful way to remember its structures. The trophoblast has split into two layers, the syncytiotrophoblast and the cytotrophoblast. The embryoblast has become a disc of two layers, the epiblast and the hypoblast. Two cavities have formed, the amniotic cavity above the disc and the yolk sac below it. The extra-embryonic mesoderm, too, splits into two sheets around the chorionic cavity: one lining the trophoblast and one covering the amnion and the yolk sac.
This doubling is not a detail. Each of these pairs has a place in what follows. The epiblast will give rise to all three germ layers of the third week, the amnion will surround the whole body, and the chorion will develop into the fetal part of the placenta.
Twins
About one in 90 human births is a twin birth. Twins may grow from one or two eggs. Dizygotic twins result from two fertilised eggs. They do not look more alike than other brothers and sisters and can be of the same or of different sex. They make up about two thirds of all twins. The two embryos develop as described above, and each has its own amnion and chorion (Fig. 16).
Monozygotic twins arise from one fertilised egg, usually because two embryoblasts develop in the blastocyst during the first week. These embryos each have their own amnion and share the chorion and placenta (Fig. 17).
A small number of twins arises in the second week, after the amnion has formed, by division of the embryonic disc itself. They lie together in the same amnion and also share the same chorion and placenta (Fig. 18).
Characteristics
In the first week the embryo developed inside the shell of the zona pellucida. The single-celled zygote divided into many small and similar cells, creating the morula and the blastocyst. At the end of that week the trophoblast and the embryoblast arose. Because the embryo breaks out of the zona pellucida, the second week shows a reversal: the embryo starts to grow. The cells no longer become smaller with each division; they can grow now. The trophoblast in particular grows so fast that no cell membranes are formed in the syncytiotrophoblast.
The focus of growth lies at the periphery, in the trophoblast. The growth of the centre, of the embryoblast, lags behind, as the tearing of the extra-embryonic mesoderm and the formation of the chorionic cavity show. In the centre there is differentiation: into amnion and yolk sac, into epiblast and hypoblast.
There is now exchange with the environment. The isolation of the first week behind the zona pellucida has ended. The embryo gives off hormones and enzymes and takes up nutrients, and so it can grow and gain a place in the uterine wall. The shelter of the zona pellucida is replaced by the shelter of the nourishing uterine wall.
The centre, the embryonic disc, is a flat round disc of two cell layers. It has an upper and a lower side, but no left and right. The disc has no content, as both layers lie on the outside. A first orientation in space appears only at the end of the week, with the formation of the connective stalk and the prochordal plate.
Monozygotic twins can arise in the first week and at the beginning of the second week. After that the embryonic disc can no longer be divided. The embryo has become in-dividual, which literally means not divisible.
The development of the first week is similar in form and timing in all mammals. From now on that is no longer the case: both the form and the duration of the stages depend on the species.
| first week | second week | |
|---|---|---|
| development | divisions | growth and proliferation |
| exchange with the environment | none | intensive |
| duration | independent of species | dependent on species |
| tissues | no differentiation | differentiation |
| shell | zona pellucida | uterine wall |
The plant-man
In the phenomenological reading of embryology, the characteristics of this week are seen as a reversal of those of the first week, and as those of the plant, a comparison worked out on the page plant and animal:
- There is a huge growth on the outside.
- The embryonic disc is flat and round, so the centre has no left and right and no content.
For this reason the embryo of the second week is described in this approach as having the characteristics of the plant, and it is called the "plant-man" (Fig. 19). A plant is committed to the periphery: from its growing points it spreads in all directions into space, and its form is the result of that outward movement rather than of an inner centre.
According to Rudolf Steiner, in the second week the human being is not yet inside the embryo, but floats around it; in his view the spirit lives in the periphery, in the tissue of the trophoblast. This is a statement from the anthroposophical tradition, not a result of embryological research, and it is given here as the way that tradition interprets the image of the plant-man.
Questions
What happens to the human embryo in the second week?
The embryo implants in the wall of the uterus and becomes completely enclosed by maternal tissue. The outer trophoblast grows very fast, while the inner cell mass forms a flat embryonic disc of two layers, the epiblast and the hypoblast, together with the amniotic cavity and the yolk sac.
What is the syncytiotrophoblast?
It is the outer part of the trophoblast, a tissue with many nuclei but without cell membranes between them. It grows into the uterine wall, opens lacunae in which maternal blood flows and produces the hormone hCG. Between it and the embryoblast lies a layer of ordinary cells, the cytotrophoblast.
When is implantation complete?
Around day 10 the embryo lies entirely inside the uterine wall. The small opening in the epithelium through which it entered is first closed by a plug of fibrin and then covered again by the epithelium.
What is the bilaminar embryonic disc?
It is the flat, round disc that forms in the second week from two cell layers: the tall cells of the epiblast facing the amniotic cavity and the flat cells of the hypoblast facing the yolk sac. It has an upper and a lower side, but no left and right yet.
Why is the extra-embryonic mesoderm called extra-embryonic?
The name only means that the tissue lies outside the embryonic disc. It fills the space between the trophoblast and the amnion and yolk sac, and cavities in it later merge into the chorionic cavity.
What is the connective stalk?
It is the band of extra-embryonic mesoderm by which the embryonic disc, amnion and yolk sac stay attached to the chorion. It moves towards the future tail end and later becomes part of the umbilical cord, as described on the page about the fourth week.
What does hCG do in early development?
Human chorionic gonadotropin is made by the syncytiotrophoblast. It keeps the corpus luteum in the ovary active, so that progesterone continues to be produced and the uterine lining in which the embryo lies is maintained.
How do identical and non-identical twins differ in their membranes?
Non-identical twins come from two fertilised eggs and each has its own amnion and chorion. Identical twins come from one fertilised egg; depending on when they separate they share the chorion only, or share both chorion and amnion.
Why is the second-week embryo called the plant-man?
In the phenomenological reading of embryology the strong growth in the periphery and the flat, round centre without content resemble the gesture of a plant, which grows outwards into space. The term is an image, not a biological classification.
What is meant by the week of twos?
Embryology textbooks use this phrase because many structures come in pairs in week two: two layers of trophoblast, two layers in the embryonic disc and two cavities, the amniotic cavity and the yolk sac.