5.7Reproduction and Development of Mammals

Although most vertebrates lay eggs that develop independently of the mother, mammals develop in their mother's body, on which they are dependent. Most vertebrates other than mammals develop using nutrients stored in the egg, whereas mammals develop using nutrients and oxygen supplied by the mother's body.
During the development of mammals, germ-cell precursor cells migrate within the developing embryo to the region where gonads are formed. As the ovaries in females differentiate, the germ cells grow until they become primary oocytes—the precursors of egg cells. At this point, growth suddenly stops. After birth, the body grows, sets out to take on secondary sex characteristics, and begins its estrous cycle. Then, the oocytes, whose growth has been arrested, grow into a fixed number of mature egg cells over each estrous cycle and are discharged.
The female estrous cycle is controlled by follicle-stimulating hormone and luteinizing hormone secreted from the anterior pituitary gland. Mature cells are ovulated from the ovaries into the fallopian tubes, through which they travel toward the uterus. The egg is fertilized by the sperm in the fallopian tube and is then carried to the uterus while undergoing rapid cleavage. By the time the embryo reaches the uterus, it develops into a blastula. The embryo soon implants itself in the uterine membrane, burrows into the membrane, and continues to develop (see Fig. 5-1B).
In humans, the word "embryo" is used until the 8th week after fertilization. From then until birth, the word "fetus" is used. After becoming implanted in the uterine membrane, the embryo grows for about 10 months until it is ready for birth. During that time, nutrients and oxygen are supplied from the mother to the fetus through the placenta, which connects the mother and the fetus. A barrier in the placenta prevents bacterial infections from spreading from the mother to the fetus and prevents the mother's immune system from attacking it. Ingestion of large amounts of alcohol or drugs that can pass through this barrier may have a severe effect on the development of the fetus.
Today, a field called "reproductive medicine" (see Column at the bottom) has been established for applications in developmental medicine, and research and clinical applications are also advancing in the field of regenerative medicine (see Section 11 chapter 5).


Reproductive Medicine

Reproductive technology and medical treatment includes assisted reproductive technology, such as in vitro fertilization, and diagnosis of fertilized eggs and fetuses. Here we will mainly discuss assisted reproductive technology (see Chapter 11 for a discussion of diagnosis).
Assisted reproductive technology is a method for infertility. Artificial insemination and in vitro fertilization are used when the problem is abnormalities in egg and sperm function. For artificial insemination, sperm is prepared outside the body and injected into the body. For in vitro fertilization, an egg is removed from the body, fertilized in a Petri dish, and placed in the uterus.
When pregnancy is not achieved even when these methods are administered, a method of providing an egg, sperm, or embryo from a donor can be used. The method of insemination using a donor's sperm is called "artificial insemination by donor" (AID); this method has been used in Japan since the 1950's. "In vitro fertilization by donor" is prohibited by The Japan Society of Obstetrics and Gynecology, but in some cases, this method has been used in Japan. There is controversy over whether this can be called therapeutic intervention. When the uterus is lost due to sickness or does not function properly, one solution is the method of surrogate conception, in which one person gives birth to another person's child.
Furthermore, since it is possible to cryopreserve sperms for a long time, if the partner's sperm is cryopreserved, a child can be created with him even after he dies. Regarding this kind of assisted reproductive technology that involves a third party or a person no longer living, the "right of parents who want a child" is asserted, but from the standpoint of "the welfare of the child to be born," this technology is strongly criticized. In the past, The Japan Society of Obstetrics and Gynecology voluntarily restrained these procedures, but the restrictions were lax. There are cases where the legal status of born children has been disputed in court.
Therefore, with legislation in view, these issues are under review at the national level. Until now, in addition to whether these technologies are right are wrong, the right of the children to know his or her origins and the legal status of children have been discussed.

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