At about 30 to 36 hours after fertilization, the zygote divides by mitosis—a process called cleavage—into two smaller cells. The rate of cleavage is thereafter accelerated. A second cleavage, which occurs about 40 hours after fertilization, produces four cells. A third cleavage about 50 to 60 hours after fertilization produces a ball of eight cells called a morula (= mulberry). This very early embryo enters the uterus 3 days after ovulation has occurred (fig. 20.43).
Continued cleavage produces a morula consisting of thirty-two to sixty-four cells by the fourth day following fertilization. The embryo remains unattached to the uterine wall for the next 2 days, during which time it undergoes changes that convert it into a hollow structure called a blastocyst (fig. 20.44). The blastocyst consists of two parts: (1) an inner cell mass, which will become the fetus, and (2) a surrounding chorion, which will become part of the placenta. The cells that form the chorion are called trophoblast cells.
On the sixth day following fertilization, the blastocyst attaches to the uterine wall, with the side containing the inner cell mass positioned against the endometrium. The trophoblast cells produce enzymes that allow the blastocyst to "eat its way" into the thick endometrium. This begins the process of implantation, or nidation, and by the seventh to tenth day the blastocyst is completely buried in the endometrium (fig. 20.45). Approximately 75% of all lost pregnancies are due to a failure of implantation, and consequently are not recognized as pregnancies.
Blastocyst mm tiÊù
Sperm cell nucleus
Fertilization cell nucleus
Sperm cell nucleus
■ Figure 20.43 Fertilization, cleavage, and the formation of a blastocyst. A diagram showing the ovarian cycle, fertilization, and the events of the first week following fertilization. Implantation of the blastocyst begins between the fifth and seventh day and is generally complete by the tenth day.
■ Figure 20.44 Scanning electron micrographs of preembryonic human development. A human ovum fertilized in a laboratory (in vitro) is seen at (a) the 4-cell stage. This is followed by (b) cleavage at the 16-cell stage and the formation of (c) a morula and (d) a blastocyst.
Progesterone, secreted from the woman's corpus lu-teum, is required for the endometrium to support the implanted embryo and maintain the pregnancy. A drug developed in France, and recently approved for use in the United States, promotes abortion by blocking the progesterone receptors of the endometrial cells. This drug, called RU486, has the generic name mifepristone. When combined with a small amount of a prostaglandin, which stimulates contractions of the myometrium, RU486 can cause the endometrium to slough off, carrying the embryo with it. Sometimes called the "abortion pill," RU486 has generated bitter controversy in the United States. A recent study found mifepristone followed by prostaglandin treatment to be 96% to 99% effective at terminating pregnancies of 49 days or less.
Only the fertilized egg cell and each of the early cleavage cells are totipotent, a term that refers to their ability to create the entire organism if implanted into a uterus. The nuclei of adult somatic cells, however, can be reprogrammed to become totipotent if they are transplanted into egg cell cytoplasm. Through such somatic cell nuclear transfer, the cloning of an entire adult organism (often called reproductive cloning) is possible, and indeed has
Blastocyst cavity Trophoblast
Inner cell mass Embryonic pole
Amniotic cavity Cytotrophoblast Syncytiotrophoblast
■ Figure 20.45 Implantation of the blastocyst. (a) A diagram showing the blastocyst attached to the endometrium on about the sixth day. (b) Implantation of the blastocyst at the ninth or tenth day.
been accomplished in sheep, cattle, cats, and other animals. The possible use of this technique to clone humans has been widely condemned by scientists and others for many reasons, including the low probability of producing healthy children.
This differs from the possibility of nuclear transplantation to produce stem cells, sometimes referred to as therapeutic cloning, for the purpose of growing specific tissues for the treatment of diseases. For example, nerve tissue produced by therapeutic cloning holds promise for the treatment of Parkinson's disease, multiple sclerosis, stroke, and spinal cord injury; cloning of islet of Langerhans beta cells may help treat diabetes mellitus; and other cloned tissues might offer new treatments for many other maladies. When nuclear transplantation is performed for the purpose of developing stem cells (therapeutic cloning), rather than for the purpose of reproductive cloning, the totipotent cell is not implanted into a uterus but is rather allowed to develop in vitro to the blastocyst stage.
Stem cell research uses cells that can be described as pluripotent or multipotent. Cells obtained from the inner cell mass of a blastocyst—termed embryonic stem (ES) cells—are pluripotent. Pluripotency refers to the ability to give rise to all tissues except the trophoblast cells of the placenta. This contrasts with adult stem cells, which have been described as multipotent because they can give rise to a number of differentiated cells. For example, neural stem cells (chapter 7) give rise to neurons and different types of glial cells, and hematopoietic stem cells (chapter 13) give rise to the different types of blood cells. There is also research suggesting that neural stem cells might be able to form blood and muscle cells, and that stem cells from the skin can be induced to develop into neurons, glial cells, smooth muscle cells, and adipocytes. The ability of adult stem cells to differentiate into such different tissue types, however, is incompletely understood and currently controversial.
In a recent report, scientists obtained neurons from cultured mouse ES cells and used these to reverse symptoms of Parkinson's disease in rats. However, the use of ES cells in this way does present some potential problems: the transplanted neurons derived from ES cells will likely be immunologically rejected by the host, and ES cells that are transplanted develop benign tumors containing different types of cells.
In another exciting recent report, scientists isolated what may be pluripotent stem cells from bone marrow cultures taken from adult humans, mice, and rats. When they injected the cells into mouse embryos, the descendants of those cells developed into almost every tissue type. It is not currently known if these cells normally exist in the bone marrow, or were created in the process of tissue culture. The scientists, hesitant at present to call these cells pluripotent, have named them "multipotent adult progenitor cells (MAPCs)." The potential health benefits of therapeutic cloning using ES cells, adult stem cells, and MAPCs have engendered excitement, hope, and ethical controversy.
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