II. Imprinting; cloning.

III. Variations in development.

Terms

A. The term development refers to the events that occur in the transition from a one-cell zygote to a multi-cell complex organism that has many different types of tissue. The generation of the different tissues and organs is also called differentiation.

B. The embryo forms from the inner cell mass that forms in a blastocyst. At first a bilayer of cells, it then develops into a trilayer (ectoderm, endoderm, and mesoderm). Each layer further differentiates into specialized tissues and organs. At about 8 weeks gestation, the major organs and tissues have appeared, and the embryo is now designated a fetus.

C. Embryonic development is under the control of many genes, although we know about only a few of them. So-called developmental genetics is a very active area of research. Many of the genes found to be important in early development of the fruit fly appear also to be important in mammalian development.

D. The stages of pregnancy are often divided into three equal trimesters. During the first trimester, the embryo goes from a zygote to a form in which major differentiation has occurred. In the second trimester, organs complete their formation and maturation of tissues occurs. The last trimester is largely characterized by increase in size.

E. The trophoblast is the outer layer of cells of the blastocyst that gives rise to nonembryonic tissues (chorion, amnion, placenta).

A. Although most genes function irrespective of parental origin, a few are active only when transmitted by the mother, and others are active only when transmitted by the father. In other words, the genes "remember" which parent they came from. This phenomenon is called imprinting. It means, for example, that normal development can occur only if the zygote receives a complement from a male and a female parent.

B. A clone is a population of cells or individuals that descended from a single cell and therefore are genetically identical.

1. Each of us is a clone of genetically identical cells derived from a zygote.

2. Familiar examples of clones are monozygotic twins, plants that have been grown from cuttings or other nonsexual means of reproduction, and colonies of bacteria that originated from a single cell.

3. Examples of artificial cloning include the following:

a. Dolly, the famous sheep from Scotland, was derived from taking a nucleus from a cell culture of mammary tissue from an adult sheep, inserting the nucleus into an enucleated ovum, and inserting the reconstructed ovum into a foster mother. The significance of Dolly is the demonstration that the differentiation that occurs during development can be reversed. Mice and cattle have now also been cloned from differentiated cells.

b. Cattle embryos can be fertilized in vitro, allowed to go through several cell divisions, and then divided into several embryos. At this stage each of the embryos has the potential for complete, normal development. When implanted into foster mothers, the resulting offspring are a clone.

A. Twins occur through two mechanisms:
1. Monozygotic twins result from splitting of an early embryo into two genetically identical embryos capable of normal development.

2. Dizygotic twins occur when two ova are released and fertilized. They have the same genetic relationship to each other as any pair of siblings. So-called fertility pills act by increasing release of multiple ova.

B. Dispermy (double fertilization) occurs when an ovum is fertilized by two sperm. The resulting embryo is triploid. Approximately 1% of pregnancies result from dispermy. Such fetuses are aborted spontaneously most of the time, and in the rare instances when they survive to birth, the child is stillborn or dies soon after birth.

C. Parthenogenesis refers to development of an ovum without fertilization. It occurs fairly commonly in humans, but the product is a type of tumor called an ovarian teratoma. The teratoma has elements of various tissues but they are not organized into a normal embryo.

D. Complete hydatidiform moles are abnormal intrauterine structures that originate as a fertilized ovum that has lost the maternal pronucleus, i.e. the nucleus is entirely paternal in origin. It consists largely of placental-like structures with no evidence of an embryo.

E. Teratogens are agents that interfere with normal development and cause malformations. Although some also cause mutations in genes, THIS IS NOT THE MECHANISM FOR CAUSING MALFORMATIONS. Rather, they interfere with development by altering the delicate interactions among tissues. Embryos are especially sensive to such agents. There are many examples, of which the fetal alcohol syndrome is a major problem. Radiation during development of the brain causes reduction in mental abilities and a smaller head. Deficiency of the B vitamin folic acid causes neural tube defects, such as spina bifida and anencephaly.

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