Stageofa Baby Using a Diagram of Cells of Multicellular Organization at Ifferent Levels

Chapter 24. Animal Reproduction and Development

24.six. Fertilization and Early on Embryonic Development

Learning Objectives

By the terminate of this department, yous volition be able to:

• Discuss how fertilization occurs
• Explicate how the embryo forms from the zygote
• Discuss the role of cleavage and gastrulation in animal evolution

The procedure in which an organism develops from a single-celled zygote to a multi-cellular organism is circuitous and well-regulated. The early stages of embryonic evolution are also crucial for ensuring the fitness of the organism.

Fertilization

Fertilization, pictured in Figure 24.23 a is the process in which gametes (an egg and sperm) fuse to class a zygote. The egg and sperm each comprise ane fix of chromosomes. To ensure that the offspring has simply one complete diploid set of chromosomes, only one sperm must fuse with one egg. In mammals, the egg is protected past a layer of extracellular matrix consisting mainly of glycoproteins called the zona pellucida. When a sperm binds to the zona pellucida, a serial of biochemical events, chosen the acrosomal reactions, take place. In placental mammals, the acrosome contains digestive enzymes that initiate the degradation of the glycoprotein matrix protecting the egg and allowing the sperm plasma membrane to fuse with the egg plasma membrane, as illustrated in Effigy 24.23 b . The fusion of these two membranes creates an opening through which the sperm nucleus is transferred into the ovum. The nuclear membranes of the egg and sperm interruption downwards and the two haploid genomes condense to class a diploid genome.

Figure 24.23.  (a) Fertilization is the procedure in which sperm and egg fuse to grade a zygote. (b) Acrosomal reactions help the sperm degrade the glycoprotein matrix protecting the egg and allow the sperm to transfer its nucleus. (credit: (b) modification of work by Mariana Ruiz Villareal; scale-bar information from Matt Russell)

To ensure that no more than than 1 sperm fertilizes the egg, once the acrosomal reactions have place at one location of the egg membrane, the egg releases proteins in other locations to prevent other sperm from fusing with the egg. If this mechanism fails, multiple sperm tin can fuse with the egg, resulting in polyspermy. The resulting embryo is not genetically viable and dies within a few days.

Cleavage and Blastula Stage

The development of multi-cellular organisms begins from a unmarried-celled zygote, which undergoes rapid cell division to class the blastula. The rapid, multiple rounds of jail cell division are termed cleavage. Cleavage is illustrated in (Figure 24.24 a ). After the cleavage has produced over 100 cells, the embryo is chosen a blastula. The blastula is usually a spherical layer of cells (the blastoderm) surrounding a fluid-filled or yolk-filled cavity (the blastocoel). Mammals at this stage course a structure called the blastocyst, characterized by an inner jail cell mass that is distinct from the surrounding blastula, shown in Figure 24.24 b . During cleavage, the cells divide without an increase in mass; that is, one large single-celled zygote divides into multiple smaller cells. Each prison cell within the blastula is chosen a blastomere.

Figure 24.24.  (a) During cleavage, the zygote rapidly divides into multiple cells without increasing in size. (b) The cells rearrange themselves to class a hollow ball with a fluid-filled or yolk-filled cavity chosen the blastula. (credit a: modification of work past Grayness'due south Anatomy; credit b: modification of work by Pearson Scott Foresman, donated to the Wikimedia Foundation)

Cleavage can have identify in two ways: holoblastic (total) cleavage or meroblastic (partial) cleavage. The type of cleavage depends on the amount of yolk in the eggs. In placental mammals (including humans) where nourishment is provided by the mother'due south trunk, the eggs accept a very small amount of yolk and undergo holoblastic cleavage. Other species, such as birds, with a lot of yolk in the egg to nourish the embryo during development, undergo meroblastic cleavage.

In mammals, the blastula forms the blastocyst in the next stage of development. Here the cells in the blastula arrange themselves in two layers: the inner cell mass, and an outer layer called the trophoblast. The inner prison cell mass is as well known as the embryoblast and this mass of cells will go along to form the embryo. At this stage of development, illustrated in Figure 24.25 the inner prison cell mass consists of embryonic stalk cells that will differentiate into the different cell types needed past the organism. The trophoblast will contribute to the placenta and nourish the embryo.

Figure 24.25.  The rearrangement of the cells in the mammalian blastula to ii layers—the inner jail cell mass and the trophoblast—results in the formation of the blastocyst.

Concept in Activity

Visit the Virtual Homo Embryo project at the Endowment for Homo Development site to step through an interactive that shows the stages of embryo development, including micrographs and rotating three-D images.

Gastrulation

The typical blastula is a brawl of cells. The side by side phase in embryonic development is the formation of the trunk plan. The cells in the blastula rearrange themselves spatially to form three layers of cells. This procedure is called gastrulation. During gastrulation, the blastula folds upon itself to form the three layers of cells. Each of these layers is called a germ layer and each germ layer differentiates into different organ systems.

The three germs layers, shown in Figure 24.26, are the endoderm, the ectoderm, and the mesoderm. The ectoderm gives rise to the nervous system and the epidermis. The mesoderm gives rising to the muscle cells and connective tissue in the body. The endoderm gives rise to columnar cells found in the digestive system and many internal organs.

Effigy 24.26.  The three germ layers give rise to dissimilar cell types in the animal body. (credit: modification of piece of work by NIH, NCBI)

Are Designer Babies in Our Future?

Effigy 24.27.  This logo from the Second International Eugenics Briefing in New York Metropolis in September of 1921 shows how eugenics attempted to merge several fields of study with the goal of producing a genetically superior human race.

If y'all could preclude your child from getting a devastating genetic disease, would you lot do it? Would y'all select the sexual practice of your child or select for their attractiveness, strength, or intelligence? How far would you go to maximize the possibility of resistance to disease? The genetic engineering of a human child, the production of "designer babies" with desirable phenotypic characteristics, was once a topic restricted to science fiction. This is the instance no longer: science fiction is now overlapping into science fact. Many phenotypic choices for offspring are already available, with many more probable to be possible in the not too distant future. Which traits should be selected and how they should be selected are topics of much debate within the worldwide medical community. The ethical and moral line is not ever clear or agreed upon, and some fright that modern reproductive technologies could lead to a new class of eugenics.

Eugenics is the use of data and technology from a diverseness of sources to improve the genetic makeup of the human race. The goal of creating genetically superior humans was quite prevalent (although controversial) in several countries during the early 20^thursday century, merely fell into disrepute when Nazi Deutschland developed an extensive eugenics program in the 1930'due south and 40'south. Every bit part of their program, the Nazis forcibly sterilized hundreds of thousands of the and so-called "unfit" and killed tens of thousands of institutionally disabled people as function of a systematic program to develop a genetically superior race of Germans known as Aryans. Ever since, eugenic ideas have not been as publicly expressed, merely there are even so those who promote them.

Efforts have been made in the by to command traits in human children using donated sperm from men with desired traits. In fact, eugenicist Robert Klark Graham established a sperm banking company in 1980 that included samples exclusively from donors with loftier IQs. The "genius" sperm depository financial institution failed to capture the public'due south imagination and the operation closed in 1999.

In more than recent times, the procedure known as prenatal genetic diagnosis (PGD) has been developed. PGD involves the screening of human embryos as function of the procedure of in vitro fertilization, during which embryos are conceived and grown outside the mother's body for some period of fourth dimension before they are implanted. The term PGD ordinarily refers to both the diagnosis, selection, and the implantation of the selected embryos.

In the to the lowest degree controversial utilize of PGD, embryos are tested for the presence of alleles which cause genetic diseases such as sickle cell affliction, muscular dystrophy, and hemophilia, in which a single disease-causing allele or pair of alleles has been identified. By excluding embryos containing these alleles from implantation into the female parent, the disease is prevented, and the unused embryos are either donated to science or discarded. There are relatively few in the worldwide medical community that question the ethics of this type of procedure, which allows individuals scared to have children considering of the alleles they behave to do then successfully. The major limitation to this procedure is its expense. Not ordinarily covered past medical insurance and thus out of reach financially for near couples, only a very small percent of all live births apply such complicated methodologies. Yet, even in cases like these where the ethical issues may seem to exist clear-cut, not anybody agrees with the morality of these types of procedures. For example, to those who have the position that human life begins at formulation, the discarding of unused embryos, a necessary effect of PGD, is unacceptable nether whatsoever circumstances.

A murkier ethical situation is found in the selection of a child's sex, which is easily performed by PGD. Currently, countries such as Great Britain have banned the selection of a child's sex activity for reasons other than preventing sex-linked diseases. Other countries allow the process for "family balancing", based on the desire of some parents to have at to the lowest degree one child of each sex. Nevertheless others, including the United States, have taken a scattershot approach to regulating these practices, essentially leaving it to the individual practicing dr. to decide which practices are acceptable and which are not.

Fifty-fifty murkier are rare instances of disabled parents, such as those with deafness or dwarfism, who select embryos via PGD to ensure that they share their inability. These parents usually cite many positive aspects of their disabilities and associated civilization every bit reasons for their pick, which they see as their moral right. To others, to purposely crusade a disability in a kid violates the basic medical principle of Primum non nocere, "first, do no impairment." This process, although non illegal in most countries, demonstrates the complication of ethical issues associated with choosing genetic traits in offspring.

Where could this process lead? Will this technology become more affordable and how should it exist used? With the ability of technology to progress rapidly and unpredictably, a lack of definitive guidelines for the use of reproductive technologies before they arise might go far difficult for legislators to proceed pace once they are in fact realized, assuming the process needs any regime regulation at all. Other bioethicists contend that we should only deal with technologies that exist now, and not in some uncertain hereafter. They argue that these types of procedures will e'er be expensive and rare, then the fears of eugenics and "chief" races are unfounded and overstated. The debate continues.

Summary

The early stages of embryonic development brainstorm with fertilization. The process of fertilization is tightly controlled to ensure that only i sperm fuses with one egg. After fertilization, the zygote undergoes cleavage to form the blastula. The blastula, which in some species is a hollow brawl of cells, undergoes a process called gastrulation, in which the three germ layers form. The ectoderm gives rise to the nervous system and the epidermal skin cells, the mesoderm gives rise to the muscle cells and connective tissue in the body, and the endoderm gives rise to columnar cells and internal organs.

Exercises

1. Which of the post-obit is simulated?
   1. The endoderm, mesoderm, ectoderm are germ layers.
   2. The trophoblast is a germ layer.
   3. The inner jail cell mass is a source of embryonic stalk cells.
   4. The blastula is often a hollow ball of cells.
2. During cleavage, the mass of cells:
   1. increases
   2. decreases
   3. doubles with every cell division
   4. does not change significantly
3. What do you think would happen if multiple sperm fused with one egg?
4. Why exercise mammalian eggs have a small concentration of yolk, while bird and reptile eggs have a large concentration of yolk?

Answers

1. B
2. D
3. Multiple sperm tin fuse with the egg, resulting in polyspermy. The resulting embryo is not genetically feasible and dies inside a few days.
4. Mammalian eggs do non demand a lot of yolk because the developing fetus obtains nutrients from the mother. Other species, in which the fetus develops outside of the mother's body, such as occurs with birds, require a lot of yolk in the egg to nourish the embryo during development.

Glossary

acrosomal reaction
series of biochemical reactions that the sperm uses to intermission through the zona pellucida

blastocyst
structure formed when cells in the mammalian blastula split up into an inner and outer layer

gastrulation
process in which the blastula folds over itself to grade the three germ layers

holoblastic
consummate cleavage; takes place in cells with a pocket-size amount of yolk

inner cell mass
inner layer of cells in the blastocyst

meroblastic
partial cleavage; takes place in cells with a big amount of yolk

polyspermy
condition in which one egg is fertilized past multiple sperm

trophoblast
outer layer of cells in the blastocyst

zona pellucida
protective layer of glycoproteins on the mammalian egg

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Source: https://opentextbc.ca/biology/chapter/24-6-fertilization-and-early-embryonic-development/

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