43.6 Fertilization and Early Embryonic Development
There are two ways to end an existing pregnancy.
Spontaneous abortion happens very early in the pregnancy, usually within the first few weeks.
This happens when the fetus can't develop properly.
An abortion is voluntary.
Fetal viability is one of the criteria that varies between states when it comes to regulating abortion.
Sexually transmitted diseases, which can cause scarring of the reproductive tubes in either men or women, are one of the 75 percent of causes of infertility that can be identified.
Delayed menstruation can be caused by inadequate nutrition.
Stress can cause infertility.
Short-term stress can affect hormones, while long-term stress can affect puberty and menstrual cycles.
Tobacco smoking, marijuana use, gonadal injuries, and aging are some of the factors that affect fertility.
Several assisted reproductive technologies are available to aid conception if infertility is identified.
In IVF, an egg and sperm are combined outside the body and placed in the uterus.
Eggs are obtained from the woman after extensive hormonal treatments that prepare mature eggs for fertilization and prepare the uterus for implantation of the fertilized egg.
The sperm are obtained from the man and they are combined with the eggs and supported through several cell divisions to ensure viability of the zygotes.
When the embryo reaches the eight-cell stage, one or more are implanted into the woman's uterus.
A procedure that injects the sperm into an egg can be used if fertilization is not accomplished by simple IVF.
A surplus of fertilized eggs can be frozen and stored for future use.
Multiple births can result from the procedures.
A sperm is injected into an egg for fertilization.
A multi-cellular organism develops from a single-celled zygote.
Ensuring the fitness of the organisms is dependent on the early stages of embryonic development.
One set of chromosomes is contained in the egg and sperm.
The sperm nucleus is transferred into the ovum through the fusion of the two membranes.
The two haploid genomes condense to form a diploid genome when the nuclear membranes of the egg and sperm break.
To ensure that no more than one sperm fertilizes the egg, once the acrosomal reactions take place at one location of the egg, the egg releases proteins in other locations to prevent other sperm from fusing with the egg.
The embryo dies within a few days.
The blastula is formed from a single-celled zygote, which undergoes rapid cell division.
Multiple rounds of cell division are called cleavage.
There is a depiction of savage in Figure 43.24a.
The embryo is called a blastula after it has produced over 100 cells.
The blastoderm is a spherical layer of cells surrounding the blastocoel.
One large single-celled zygote divides into multiple smaller cells during cleavage.
Each cell within the blastula is called a blastomere.
The amount of yolk in the eggs can affect the type of cleavage.
In mammals where food is provided by the mother's body, the eggs have a small amount of yolk.
Birds have a lot of yolk in the egg to nourish the embryo during development.
The embryo will be formed by a mass of cells in the inner cell mass.
The embryo will be nourished by the trophoblast.
The Endowment for Human Development has an interactive that shows the stages of embryo development, including micrographs and rotating 3-D images.
A blastula is a ball of cells.
The body plan is the next stage in development.
Three layers of cells are formed by the cells in the blastula.
The blastula folds into three layers of cells during gastrulation.
Each germ layer is a different organ system.
The nervous system and the epidermis are the result of the ectoderm.
The muscles and tissue in the body are created by the mesoderm.
Columnar cells are found in the digestive system and many internal organs.
Different cell types can be found in the animal body.
The logo from the Second International Eugenics Conference in New York City in September of 1921 shows how eugenics tried to combine several fields of study with the goal of producing a genetically superior human race.
The production of "designer babies" with desirable characteristics was once a topic restricted to science fiction.
Science fiction is now related to science fact.
Many options for offspring are already available, with many more likely to be available in the not too distant future.
There is a lot of debate within the worldwide medical community about which traits should be selected.
Modern reproductive technologies could lead to a new form of eugenics, as the ethical and moral line is not always clear or agreed upon.
Information and technology can be used to improve the genetic makeup of the human race.
The goal of creating genetically superior humans in several countries during the early 20th century was controversial, but fell into disrepute when Nazi Germany developed an extensive eugenics program in the 1930s and 40s.
The Nazis killed tens of thousands of institutionally disabled people as part of a program to develop a genetically superior race of Germans known as the Aryans.
Eugenic ideas have not been publicly expressed, but there are still people who promote them.
The use of donated sperm from men with desired traits has been done in the past.
Robert Klark Graham established a sperm bank in 1980 that only had samples from donors with high IQs.
The "genius" sperm bank didn't capture the public's imagination and closed in 1999.
PgD is a procedure that has been developed in recent times.
The screening of human embryos is part of the process of in vitro fertilization, during which embryos are conceived and grown outside the mother's body for some period of time before they are implanted.
The term is usually used to refer to both the diagnosis and the selection of the embryo.
In the least controversial use of PGD, embryos are tested for the presence of alleles which cause genetic diseases, in which a single disease-causing allele or pair of alleles has been identified.
The disease is prevented and the unused embryos are either donated to science or thrown away.
The ethics of this type of procedure is rarely questioned by the worldwide medical community, as it allows people who are scared to have children because of their genes to do so successfully.
The procedure's expense is the main limitation.
Not usually covered by medical insurance and out of reach financially for most couples, only a small percentage of live births use such complicated methodologies.
Not everyone agrees with the morality of these types of procedures, even if the ethical issues seem clear-cut.