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Chapter 4

Egg Differentiation and Genetic Abnormalities

The egg is a differentiated cell type. It is specialized in many ways: to receive the sperm during fertilization, to supply the majority of the cytoplasm for early development, to provide half of the genome for the zygote and to provide information to initiate the events of early development. The egg specializes early and subsequently undergoes unequal cell divisions during meiosis (releasing smaller polar bodies) so that this differentiated egg cytoplasm is not greatly diminished. One aspect of egg structure is the presence of various "envelopes" that surround it. As we will see, these cellular and non-cellular (extracellular matrix) components are critical to the survival and fertilization of the egg.

The Egg is a Differentiated Cell

Compared to other cells in the body, the egg is a very large, essentially round cell. The growth and differentiation phases occur simultaneously during prophase I of meiosis. At this stage the nucleus is called a germinal vesicle. The germinal vesicle is a very specialized nucleus. For example it contains a minimal version of "lampbrush chromosomes", common to other species, that are actively involved in gene transcription. The egg itself is surrounded by egg coats which consists of cells (cumulus oophorus; corona radiata) and zona pellucida (protein “shell”) as will be detailed in Chapter 6 when we discuss its role in fertilization. The egg also contains many specialized organelles in its cytoplasm. Cortical granules (see Chapter 6) align adjacent to the egg cell membrane in anticipation of fertilization. Adjacent to the nucleus are the annulate lamellae. The annulate lamellae consist of parallel stacks of nuclear envelope-like membranes that lie adjacent to the nucleus which may give rise to them. Evidence indicates that the annulate lamellae are essential for the formation of the pronuclei during fertilization.

Many animals have large store of yolk but this is not the case in humans. The human egg has a minimal amount of yolk. Interestingly, yolk proteins made in liver and are transported to egg via the blood. Yolk proteins are taken into growing oocytes via receptor-mediated endocytosis. The small amount of yolk is due to the fact that the developing embryo only needs internal nutrients until implantation. At that time it obtains nutrients from the maternal body via the placental relationship.

Meiotic Divisions

Human gametes, like those of mammals and most other animals, are haploid (i.e., contain only half the amount of somatic DNA). This is because the diploid state will be re-established at fertilization when the haploid sperm and haploid egg fuse to produce the diploid zygote. The goal of meiosis is to reduce the diploid state to haploid via two meiotic divisions. The problem with meiosis during oogenesis is the egg has gone through a period of significant growth and differentiation. During normal meiosis, the cytoplasmic volume is also reduced. If this occurred in oogenesis, this would be a significant waste of energy since all the work that was done to make all the egg components would be reduced at each stage of meiosis. As a result, the egg has mechanism that reduces the genetic complement while not significantly reducing its cytoplasmic volume.

The nucleus of the egg is off-center, so that when meiotic cell divisions occur, the division of the egg is unequal. The eccentric nucleus leads to large secondary oocyte (meiosis I) or ovum (meiosis II) plus the release of much smaller polar bodies. Thus meiosis in total produces a single fertilizable egg plus 2 or 3 polar bodies (if the polar body itself divides) rather than the normal number of four meiotic cells. Little research has been done on the importance and fate of polar bodies. The consensus is polar bodies do not play a role in fertilization and may be reabsorbed by the egg or embryo or simply die off.

Figure 4.1. A fertilized egg showing two polar bodies (false colored blue) and the fusing pronuclei (false colored green).

As shown in Figure 4.1., the fusing haploid pronuclei (false coloured green in right panel) are seen in the centre of the egg while the two released pronuclei (false coloured blue in right panel) that resulted from the meiotic divisions are situated outside the egg. Sometimes a third polar body is observed which is due to the division of the first polar body. To reiterate, the polar bodies have no known developmental function other than to reduce the genetic complement of the egg without causing a large reduction in the egg cytoplasm. The zona pellucida (dark wide circular material outside the egg) surrounds the egg and polar bodies.

Meiosis and Genetic Abnormalities: Non-Disjunction

Non-Disjunction: With non-disjunction, the meiotic chromosomes don’t pull apart. As a result one cell gets both sister chromatids (that each will become a chromosome) while the other cell doesn't get that chromosome. Non-disjunction can occur during either meiosis I or meiosis II as illustrated in the following graphic (Figure 4.2). This figure summarizes how gametes can end up with one more or one less chromosome when non-disjunction occurs during meiosis.

Figure 4.2. Non-disjunction during meiosis leads to abnormal chromosome numbers in developing eggs.

The following points summarize the issue of non-disjunction:

•Non-disjunction leads to abnormal chromosome numbers in eggs.

•If one daughter gamete has an extra chromosome (i.e., 24 rather than the normal 23) the resulting syndrome is called trisomy (3 copies of the same chromosome will exist after fertilization) e.g., Down Syndrome—trisomy 21 is due to an extra copy of chromosome 21.

•If the daughter gamete has 1 less chromosome (i.e., a total of 22 rather than the normal 23) it results in monosomy (e.g., Turner Syndrome—lacks a sex chromosome). Most cases of monosomy are not viable.

Most cases of non-disjunction are due to the “maternal age effect”. Older women have higher incidences of abnormal chromosome numbers in their eggs due to those eggs being held in a meiotic block (prophase of meiosis I) for an extended period of time. Similarly older men can also produce sperm with abnormal chromosome numbers. This “paternal age effect” however cannot be due to meiotic block since it doesn’t occur in males.

Trisomy 21—Down Syndrome

Down syndrome is probably the most well known result from non-disjunction. As a result, Down syndrome individuals have an extra chromosome 21, hence the syndrome is also known as trisomy 21. Trisomy 21 affects the development of brain, immune system, heart and skeleton. Mental retardation is the constant hallmark of Down syndrome and individuals will typically develop early onset Alzheimer’s disease. Physically Down syndrome individuals have a broad face with a flat nasal bridge, wide set eyes and epicanthic folds (a skin fold on the upper eyelid). They also show a transverse crease in the palms of their hands. Often called a “simian crease”, the term is falling out of favor since “simian” refers to monkey or ape. Down syndrome occurs about once in every 700 births making it a common occurrence.

Down Syndrome at the Molecular Level

Trisomy 21 or Down syndrome is due to an extra copy of chromosome 21. Since these individuals have extra copies of certain genes this usually results in an excess production of certain proteins. The extra genes on chromosome 21 have many other functions and effects that are not all related to Down syndrome. However, a partial understanding of Down syndrome has come from studies of the genes encoded on chromosome 21. One of these gene is DSCR1 (Down Syndrome Critical Region 1) which encodes an inhibitory protein. While the role of the inhibitory protein DSCR1 in neuronal development is just one aspect of Down syndrome it does serve to exemplify how our understanding of this syndrome as well as many diseases is progressing

Figure 4.3. The role of DSCR1 in regulating gene expression during brain neuron development.

Figure 4.3 summarizes the role of DSCR1 in neuronal development. NFATc (Nuclear Factor of Activated T cells) is phosphorylated in the cytoplasm of nerve cells. In this state, NFATc cannot enter the nucleus. During the normal development of neurons, calcineurin (CN), a protein phosphatase, removes phosphate groups (i.e., dephosphorylates) NFATc. Dephosphorylated NFATc now can enter the nucleus to regulate genes required for normal neuron development. DSCR1 protein is an inhibitor of CN which exists in normal neurons but at a level where it doesn’t negatively affect the normal events of NFATc dephosphorylation.

As discussed previously, the gene DSCR1 is found on chromosome 21. The extra copy of chromosome 21 leads to over-expression of the CN inhibitor DSCR1 in developing brain cells (neurons) of Down syndrome individuals. These increased levels of DSCR1 lead to the inhibition of CN which in turn prevents it from dephosphorylating the critical transcription factor NFATc. Since phosphorylated NFATc can't enter the nucleus it can’t regulate specific genes required for normal brain development. As a result, neuronal development is inhibited in the brains of Down syndrome persons resulting in reduced cognitive ability.

Non-Disjunction: Monosomy—Turner Syndrome

While comparatively less common and less well-studied, Turner syndrome serves as a good example of monosomy. Individuals with Turner syndrome lack an X or Y sex chromosome due to non-disjunction. As a result these individuals have an XO genotype after fertilization. With only the X chromosome present, they exhibit a female phenotype. However due to the lack of certain genes, they are sterile. Physically these individuals have a short stature, webbed neck and high arched palate in their mouths. They also suffer from cognitive defects (i.e., affects learning). The syndrome is occurs once in about 2,500 individuals.

The Life of the Human Egg

The human egg goes through a series of starts and stops on its way to becoming prepared for fertilization and subsequently beginning development. The following diagram shows these events as they occur for a single egg (Figure 4.4). During embryonic development the PGCs increase in number by mitosis and migrate into the genital ridge where further mitosis occurs. As the embryo develops, the eggs will begin meiosis progressing to prophase I where they become arrested. At puberty, under the influence of hormones, some of the eggs will re-initiate meiosis and will be ovulated or will degenerate. The ovulated oocyte is arrested at metaphase II and will not continue meiosis unless activated by a sperm cell.

Figure 4.4. The life of a human egg from primordial germ cell to zygote.

Fertilization results in the completion of meiosis producing the female haploid pronucleus that will then fuse with the sperm pronucleus. The mechanisms whereby meiosis is arrested and restarted in other animals such as mice and frogs is an exciting area of research that involves signal transduction pathways and various kinases. Little has been done on this subject in human eggs. Remember, the majority of eggs remain locked in prophase I of meiosis during a female’s life. Each month only a few eggs are stimulated to move to meiosis II the rest remain in prophase I.