Development Begins with Fertilization

Fertilization is the union of a haploid sperm and a haploid egg to produce a diploid zygote. Fertilization does more, however, than just restore a full complement of maternal and paternal genes. The entry of a sperm into an egg activates the egg metabolically and initiates the rapid series of cell divisions that produce a multicellular embryo. Also, in many species, the point of entry of the sperm creates an asymmetry in the radially symmetrical egg. This asymmetry is the initiating event that enables a bilateral body plan to emerge from the radial symmetry of the egg. We will describe the mechanisms of fertilization in Chapter 43. Here we take a closer look at the cellular and molecular interactions of sperm and egg that result in the first steps of development.

The sperm and the egg make different contributions to the zygote

Nearly all of the cytoplasm of the zygote comes from the egg (Figure 20.1). Egg cytoplasm is well stocked with nutrients, ribosomes, and a variety of molecules, including mRNAs. Because the sperm's mitochondria degenerate, all of the mitochondria (and therefore all of the mitochondrial DNA) in the zygote come from the mother. In addition to its haploid nucleus, the sperm makes one other important contribution to the zygote in some species: a centriole. This centriole be-

Human Fertilization
20.1 Sperm and Egg Differ Greatly in Size This artificially colored micrograph of human fertilization illustrates the size difference between the two types of gametes in mammals.The large egg (blue) contributes more cytoplasm to the zygote than the much smaller sperm (yellow).

comes the centrosome of the zygote, which produces the mitotic spindles for subsequent cell divisions.

It had long been assumed that the one thing that sperm and egg contributed equally to the zygote was their haploid nuclei. However, we now know that even though they are equivalent in terms of genetic material, mammalian sperm and eggs are not equivalent in terms of their roles in development. In the laboratory, it is possible to construct zygotes in which both haploid nuclei come from the mother or both come from the father. In neither case does development progress normally. Apparently, in mammals at least, certain genes involved in development are active only if they come from a sperm, and others are active only if they come from an egg. This phenomenon, called genomic imprinting, was described in Chapter 17.

Fertilization causes rearrangements of egg cytoplasm

The entry of the sperm into the egg stimulates changes in and rearrangements of the egg cytoplasm that establish the polarity of the embryo. The nutrients and molecules in the cytoplasm of the zygote are not homogeneously distributed, and therefore, they are not divided equally among all daughter cells when cell divisions begin. This unequal distribution of cytoplasmic factors sets the stage for the signal transduc-tion cascades that orchestrate the sequential steps of development: determination, differentiation, and morphogenesis. Let's examine these earliest developmental events in the frog, an organism in which they have been well studied.

The rearrangements of egg cytoplasm in some frog species are easily observed because of pigments in the egg cytoplasm. The nutrient molecules in an unfertilized frog egg are dense, and they are therefore concentrated by gravity in the lower half of the egg, which is called the vegetal hemisphere. The haploid nucleus of the egg is located at the opposite end of the egg, in the animal hemisphere. The outermost (cortical) cytoplasm of the animal hemisphere is heavily pigmented, and the underlying cytoplasm has more diffuse pigmentation. The vegetal hemisphere is not pigmented.

The surface of the frog egg has specific sperm-binding sites located only in the animal hemisphere, so sperm always enter the egg in that hemisphere. When a sperm enters, the cortical cytoplasm rotates toward the site of sperm entry. This rotation reveals a band of diffusely pigmented cytoplasm on the side of the egg opposite the site of sperm entry. This band, called the gray crescent, will be the site of important developmental events (Figure 20.2).

The cytoplasmic rearrangements that create the gray crescent bring different regions of cytoplasm into contact on opposite sides of the egg. Therefore, bilateral symmetry is imposed on what was a radially symmetrical egg. In addition to the up-down difference of the animal and vegetal hemispheres, the movement of the cytoplasm sets the stage for the creation of the anterior-posterior and left-right axes. In the frog, the site of sperm entry will become the ventral (belly) region of the embryo, and the gray crescent will become the dorsal (back) region. Since the gray crescent also marks the posterior end of the embryo, these relationships specify the anterior-posterior and left-right axes as well.

Rearrangements of egg cytoplasm set the stage for determination

The molecular mechanisms underlying the first steps in frog embryo formation are beginning to be understood. The

Animal cortical cytoplasm (pigmented)

Animal pole

The cortical cytoplasm rotates relative to the inner cytoplasm.

Animal cortical cytoplasm (pigmented)

Animal pole

The cortical cytoplasm rotates relative to the inner cytoplasm.

Cortical Cytoplasm

Sperm entry point

Vegetal pole

-Vegetal cortical cytoplasm (unpigmented)

The gray crescent is created by the rotation

Sperm entry point

Vegetal pole

20.2 The Gray Crescent create the gray crescent.

-Vegetal cortical cytoplasm (unpigmented)

The gray crescent is created by the rotation

Rearrangements of the cytoplasm of frog eggs after fertilization sperm centriole rearranges the microtubules in the vegetal hemisphere cytoplasm into a parallel array that presumably guides the movement of the cortical cytoplasm. Organelles and certain proteins from the vegetal hemisphere move to the gray crescent region even faster than the cortical cytoplasm rotates.

As a result of these movements of cytoplasm, proteins, and organelles, changes in the distribution of critical developmental signals occur. A key transcription factor in early development is P-catenin, which is produced from maternal mRNA and is found throughout the cytoplasm of the egg. Also present throughout the egg cytoplasm is a protein kinase called GSK-3, which phosphorylates and thereby targets P-catenin for degradation. However, an inhibitor of GSK-3 is segregated in the vegetal cortex of the egg. After sperm entry, this inhibitor is moved along microtubules to the gray crescent, where it prevents the degradation of P-catenin. As a result, the concentration of P-catenin is higher on the dorsal side than on the ventral side of the developing embryo (Figure 20.3).

Evidence supports the hypothesis that P-catenin is a key player in the cell-cell signaling cascade that begins the process of cell determination and the formation of the embryo in the region of the gray crescent. But before there can be cell-cell signaling, there must be multiple cells, so let's turn first to the early series of cell divisions that transforms the zygote into a multicellular embryo.

Was this article helpful?

0 0
Pregnancy Guide

Pregnancy Guide

A Beginner's Guide to Healthy Pregnancy. If you suspect, or know, that you are pregnant, we ho pe you have already visited your doctor. Presuming that you have confirmed your suspicions and that this is your first child, or that you wish to take better care of yourself d uring pregnancy than you did during your other pregnancies; you have come to the right place.

Get My Free Ebook


Responses

  • Priamus
    Do sperms always enter at animal pole?
    8 years ago
  • medhanit
    Does sperm always enter the animal hemisphere?
    8 years ago
  • leila
    What do both the sperm and egg contribute to the zygote equally?
    8 years ago
  • Lenora
    Does the egg or sperm contribute more mitochondrial dna to the zygote?
    8 years ago
  • william
    Why is it important that the egg contributes more mitochondrial dna to the zygote?
    8 years ago
  • fnan russom
    Why is the gray crescent of frog embryos so important?
    8 years ago
  • Randy
    What are changes to the egg resulting from fertilization that set the stage for determination.?
    8 years ago
  • jari
    Are the cytoplasmic contents of the egg homogeneously distributed for mammals?
    7 years ago
  • negisti massawa
    Does the cytoplasm of a zygote distribute equally?
    7 years ago
  • Beato
    Which contributes more than half material to zygote oocyte or sperm?
    7 years ago
  • imelda schiavone
    Can sperm enter the egg on the vegetal pole?
    5 years ago
  • furuta
    What contribution do the shortterm and egg cell make to the cytoplasm of the zygote?
    5 years ago
  • Zufan Nuguse
    Why does the sperm enter through the animal pole?
    3 years ago
  • genet kifle
    When does doest he mitochondria degenerate in fertilization?
    2 years ago

Post a comment