43.6: Fertilización y Desarrollo Embrionario Temprano

Habilidades para Desarrollar

• Discutir cómo ocurre la fertilización
• Explicar cómo se forma el embrión a partir del cigoto
• Discutir el papel de la escisión y la gastrulación en el desarrollo animal

El proceso en el que un organismo se desarrolla de un cigoto unicelular a un organismo multicelular es complejo y bien regulado. Las primeras etapas del desarrollo embrionario también son cruciales para garantizar la aptitud del organismo.

Fertilización

La fertilización, que se muestra en la Figura\(\PageIndex{1}\) a, es el proceso en el que los gametos (un óvulo y esperma) se fusionan para formar un cigoto. El óvulo y el esperma contienen cada uno un conjunto de cromosomas. Para asegurar que la descendencia tenga solo un conjunto diploide completo de cromosomas, solo un espermatozoide debe fusionarse con un óvulo. En los mamíferos, el huevo está protegido por una capa de matriz extracelular que consiste principalmente en glicoproteínas llamadas zona pelúcida. Cuando un espermatozoide se une a la zona pelúcida, se producen una serie de eventos bioquímicos, llamados reacciones acrosómicas. En los mamíferos placentarios, el acrosoma contiene enzimas digestivas que inician la degradación de la matriz de glicoproteínas protegiendo al óvulo y permitiendo que la membrana plasmática espermática se fusione con la membrana plasmática del óvulo, como se ilustra en la Figura\(\PageIndex{1}\) b. que el núcleo espermático se transfiere al óvulo. Las membranas nucleares del óvulo y el esperma se descomponen y los dos genomas haploides se condensan para formar un genoma diploide.

Figura\(\PageIndex{1}\): (a) La fertilización es el proceso en el que espermatozoides y óvulos se fusionan para formar un cigoto. (b) Las reacciones acrosómicas ayudan al esperma a degradar la matriz de glicoproteínas protegiendo al óvulo y permiten que el esperma transfiera su núcleo. (crédito: (b) modificación de obra de Mariana Ruiz Villareal; datos de barra de escala de Matt Russell)

Para garantizar que no más de un espermatozoide fertilice el óvulo, una vez que las reacciones acrosómicas tienen lugar en una ubicación de la membrana del óvulo, el óvulo libera proteínas en otras ubicaciones para evitar que otros espermatozoides se fusionen con el óvulo. Si este mecanismo falla, múltiples espermatozoides pueden fusionarse con el óvulo, resultando en poliespermia. El embrión resultante no es genéticamente viable y muere en pocos días.

Escisión y Estadio de Blastula

El desarrollo de organismos multicelulares comienza a partir de un cigoto unicelular, que se somete a una rápida división celular para formar la blástula. Las rondas rápidas y múltiples de división celular se denominan escisión. La escisión se ilustra en (Figura\(\PageIndex{2}\) a). Después de que la escisión haya producido más de 100 células, el embrión se llama blástula. La blástula suele ser una capa esférica de células (el blastodermo) que rodea una cavidad llena de líquido o llena de yema (el blastocoel). Los mamíferos en esta etapa forman una estructura llamada blastocisto, caracterizada por una masa celular interna que es distinta de la blástula circundante, mostrada en la Figura\(\PageIndex{2}\) b. Durante la escisión, las células se dividen sin aumentar la masa; es decir, un cigoto unicelular grande se divide en múltiples células más pequeñas . Cada célula dentro de la blástula se llama blastómero.

a

b

Figura\(\PageIndex{2}\): (a) Durante la escisión, el cigoto se divide rápidamente en múltiples células sin aumentar de tamaño. b) Las células se reorganizan para formar una bola hueca con una cavidad llena de líquido o yema llamada blástula. (crédito a: modificación de obra de Gray's Anatomy; crédito b: modificación de obra de Pearson Scott Foresman, donada a la Fundación Wikimedia)

La escisión puede tener lugar de dos maneras: escisión holoblástica (total) o escisión meroblástica (parcial). El tipo de escisión depende de la cantidad de yema en los huevos. En los mamíferos placentarios (incluidos los humanos) donde la nutrición es proporcionada por el cuerpo de la madre, los huevos tienen una cantidad muy pequeña de yema y sufren escisión holoblástica. Otras especies, como las aves, con mucha yema en el huevo para nutrir al embrión durante el desarrollo, experimentan una escisión meroblástica.

En los mamíferos, la blástula forma el blastocisto en la siguiente etapa de desarrollo. Aquí las células de la blástula se disponen en dos capas: la masa celular interna y una capa externa llamada trofoblasto. La masa celular interna también se conoce como el embrioblasto y esta masa de células pasará a formar el embrión. En esta etapa de desarrollo, ilustrada en\(\PageIndex{3}\) la Figura, la masa celular interna consiste en células madre embrionarias que se diferenciarán en los diferentes tipos celulares que necesita el organismo. El trofoblasto contribuirá a la placenta y nutrirá al embrión.

Figura\(\PageIndex{3}\): El reordenamiento de las células en la blástula de mamífero a dos capas, la masa celular interna y el trofoblasto, da como resultado la formación del blastocisto.

Enlace al aprendizaje

Visite el proyecto Virtual Human Embrion en el sitio de Endowment for Human Development para dar un paso a través de un interactivo que muestra las etapas del desarrollo del embrión, incluyendo micrografías e imágenes giratorias en 3D.

Gastrulación

La blástula típica es una bola de células. La siguiente etapa en el desarrollo embrionario es la formación del plan corporal. Las células de la blástula se reorganizan espacialmente para formar tres capas de células. Este proceso se llama gastrulación. Durante la gastrulación, la blástula se pliega sobre sí misma para formar las tres capas de células. Cada una de estas capas se llama capa germinal y cada capa germinal se diferencia en diferentes sistemas de órganos.

Las tres capas de gérmenes, mostradas en la Figura\(\PageIndex{4}\), are the endoderm, the ectoderm, and the mesoderm. The ectoderm gives rise to the nervous system and the epidermis. The mesoderm gives rise 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.

Figure \(\PageIndex{4}\): The three germ layers give rise to different cell types in the animal body. (credit: modification of work by NIH, NCBI)

Everyday Connection: Are Designer Babies in Our Future?

Figure \(\PageIndex{5}\): This logo from the Second International Eugenics Conference in New York City 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 you could prevent your child from getting a devastating genetic disease, would you do it? Would you select the sex 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 case no longer: science fiction is now overlapping into science fact. Many phenotypic choices for offspring are already available, with many more likely 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 always clear or agreed upon, and some fear that modern reproductive technologies could lead to a new form of eugenics.

Eugenics is the use of information and technology from a variety 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 20th century, but fell into disrepute when Nazi Germany developed an extensive eugenics program in the 1930's and 40's. As part of their program, the Nazis forcibly sterilized hundreds of thousands of the so-called "unfit" and killed tens of thousands of institutionally disabled people as part 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, but there are still those who promote them.

Efforts have been made in the past to control traits in human children using donated sperm from men with desired traits. In fact, eugenicist Robert Klark Graham established a sperm bank in 1980 that included samples exclusively from donors with high IQs. The "genius" sperm bank failed to capture the public's imagination and the operation closed in 1999.

In more recent times, the procedure known as prenatal genetic diagnosis (PGD) has been developed. PGD involves the screening of human embryos as 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 PGD usually refers to both the diagnosis, selection, and the implantation of the selected embryos.

In the least controversial use of PGD, embryos are tested for the presence of alleles which cause genetic diseases such as sickle cell disease, 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 mother, 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 because of the alleles they carry to do so successfully. The major limitation to this procedure is its expense. Not usually covered by medical insurance and thus out of reach financially for most couples, only a very small percentage of all live births use such complicated methodologies. Yet, even in cases like these where the ethical issues may seem to be clear-cut, not everyone agrees with the morality of these types of procedures. For example, to those who take the position that human life begins at conception, the discarding of unused embryos, a necessary result of PGD, is unacceptable under any 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 for reasons other than preventing sex-linked diseases. Other countries allow the procedure for "family balancing", based on the desire of some parents to have at least one child of each sex. Still others, including the United States, have taken a scattershot approach to regulating these practices, essentially leaving it to the individual practicing physician to decide which practices are acceptable and which are not.

Even 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 disability. These parents usually cite many positive aspects of their disabilities and associated culture as reasons for their choice, which they see as their moral right. To others, to purposely cause a disability in a child violates the basic medical principle of Primum non nocere, "first, do no harm." This procedure, although not illegal in most countries, demonstrates the complexity 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 be 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 make it difficult for legislators to keep pace once they are in fact realized, assuming the process needs any government regulation at all. Other bioethicists argue that we should only deal with technologies that exist now, and not in some uncertain future. They argue that these types of procedures will always be expensive and rare, so the fears of eugenics and "master" races are unfounded and overstated. The debate continues.

Summary

The early stages of embryonic development begin with fertilization. The process of fertilization is tightly controlled to ensure that only one sperm fuses with one egg. After fertilization, the zygote undergoes cleavage to form the blastula. The blastula, which in some species is a hollow ball 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.

Glossary

acrosomal reaction

series of biochemical reactions that the sperm uses to break through the zona pellucida

blastocyst

structure formed when cells in the mammalian blastula separate into an inner and outer layer

gastrulation

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

holoblastic

complete cleavage; takes place in cells with a small amount of yolk

inner cell mass

inner layer of cells in the blastocyst

meroblastic

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

polyspermy

condition in which one egg is fertilized by multiple sperm

trophoblast

outer layer of cells in the blastocyst

zona pellucida

protective layer of glycoproteins on the mammalian egg