In the second week of life (or the fourth week of pregnancy, if pregnancy is measured from the last menstrual period), the processes of the pre-embryonic stage continue. By the end of the second week this stage is complete and an environment has been created in the uterus that can provide a continuous source of nourishment and protection for the development of the embryo.
At the end of the first week the food supply to the blastocyst becomes inadequate. More direct access to the oxygen and nutrients in the maternal blood supply is therefore an immediate necessity. The uterus has been preparing for this moment since ovulation, under the controlling influence of theoestrogen and progesterone. By the time the blastocyst attaches itself to the uterine wall and begins to embed itself, the uterine lining (endometrium) is thick and well supplied with tiny blood vessels.
Occasionally, the blastocyst does not complete the journey to the uterus but implants instead in the Fallopian tube. When this occurs the pregnancy is called anpregnancy.
Approximately one in every 150 pregnancies are ectopic and can be potentially life-threatening for the mother if not detected early enough. Diagnosis of the condition can be difficult, however, if the Fallopian tube does not burst. Rarely, an infant survives to be delivered by Caesarean section; in most cases, however, the embryo dies within a few weeks and is absorbed back into the mother’s body.
In other cases the tube ruptures to cause severe abdominal pain and bleeding, necessitating prompt surgical treatment.
Future pregnancies will probably require careful medical monitoring.
It is the trophoblast, the outer layer of the blastocyst, that carries out the process of implantation. It erodes and digests the surface cells of the uterine wall to form a smallin which the blastocyst rests, usually in the upper part of the uterus. At the same time as the cells of the trophoblast digest the lining they also grow into it, putting out small finger-like projections (villi). Thus the trophoblast gradually attaches itself to the lining of the uterus as well as obtaining nourishment from it. The part of the blastocyst that is embedded most deeply in the uterine wall is usually that containing the inner cell mass. By the end of the second week the villi, which develop into the placenta, surround the trophoblast like a hairy ball a few millimetres across, and implantation is complete.
During implantation a new hormone, the pregnancy hormone, or human chorionic gonadotropin (HCG), is produced by the trophoblast. This hormone, which can be detected in the urine sometimes as early as two weeks after conception, is vital to the continuation of pregnancy because it causes the corpus luteum to continue functioning. The corpus luteum is the structure formed in the ovary at the site where an ovum has- been released. It is responsible for the production of the hormones oestrogen and, most important of all, progesterone. These hormones keep the lining of the uterus thick and spongy to facilitate the embedding of the trophoblast and to provide nutrition for the embryo. They also prevent menstruation, which would qtherwise cause the blastocyst to be shed from the mother’s body. Under the influence of the pregnancy hormone, the corpus luteum persists until the placenta is ready to take over the production of oestrogen and progesterone, in about the twelfth week.
Specialization and differentiation
All the cells of the implanted blastocyst are derived from one fertilized ovum. They therefore all have the same genetic ‘blueprint’. This does not mean, however, that every cell will carry out every instruction of the blueprint. Each cell will follow only a specific part of the total set of genetic instructions. By the second week it has already been determined which cells will develop into skin tissue. Cells at other sites, already`know’ that they have to develop into, for example, bladder or lung tissue. This process is called differentiation and specialization.
Before the inner cell mass of the blastocyst develops into an embryo, two fluid-filled cavities form around it. These are derived from small trophoblast cells that have a specialized task: to provide supportive tissue. The most important of the two cavities is the amniotic cavity. It is filled with a straw-coloured liquid called amniotic fluid. As the embryo becomes bigger the amnion will completely surround it. The other cavity is the yolk sac, which eventually shrivels away. At birth a remnant of it can sometimes be seen in the umbilical cord. The fluid-filled cavities surrounding the embryo allow space for the growth and movement of the eventual, and serve to protect and buffer it from the second week of life until birth.
Soon after implantation the cells that will form the embryo arrange themselves into two layers. Because this cell mass assumes the shape of a disc, the two layers are called embryonic discs. The cells of one layer, the ectodermal layer, develop into the surface tissues of the body: the skin, hairs and nails. The nervous system also develops from these cells. The other, innermost layer is called the endoderm, from which epithelial tissues develop that line the digestive system, bladder and lungs. The tissues that will develop into muscles, bones, the circulatory system of the heart and blood vessels and other internal organs are in essence all connective tissue. Such cells are derived from a third, middle layer of the embryonic disc, called the mesoderm, which is formed after the two other layers. By the end of the second week, the embryo is implanted firmly in the uterus, and develops rapidly.
Because a large quantity of blood is attracted to the site of implantation, a little blood is sometimes lost from the uterus. The woman may not realize that she is pregnant and would normally be expecting her period at this time so that such an implantation bleeding can give rise to confusion. If a pregnancy test is performed, it will be clearly demonstrated that the woman is pregnant.