Oogenesis
Oogenesis
Early germ cells in the ovary, called
oogonia, increase in number by ordinary
mitosis. Each oogonium contains
the diploid number of chromosomes.
After the oogonia cease to increase in
number, they grow in size and become primary oocytes (Figure 7-10). Before
the first meiotic division, the chromosomes
in each primary oocyte meet in
pairs, paternal and maternal homologues,
just as in spermatogenesis.
When the first maturation (reduction)
division occurs, the cytoplasm is divided
unequally. One of the two daughter
cells, the secondary oocyte, is large
and receives most of the cytoplasm; the
other is very small and is called the first
polar body (Figure 7-10). Each of these
daughter cells, however, has received
half of the chromosomes.
In the second meiotic division, the secondary oocyte divides into a large ootid and a small polar body. If the first polar body also divides in this division, which sometimes happens, there are three polar bodies and one ootid (Figure 7-10). The ootid develops into a functional ovum. The polar bodies are nonfunctional, and they disintegrate. Formation of nonfunctional polar bodies is necessary to enable the egg to dispose of excess chromosomes, and the unequal cytoplasmic division makes possible a large cell with the cytoplasm containing sufficient yolk for the development of the young. Thus a mature ovum has the N (haploid) number of chromosomes, the same as a sperm. However, each primary oocyte gives rise to only one functional gamete instead of four as in spermatogenesis.
In most vertebrates and many invertebrates the egg does not actually complete all the meiotic divisions before fertilization occurs. The general rule is that development is arrested during prophase I of the first meiotic division. Meiosis resumes and is completed either at the time of ovulation (birds and most mammals) or shortly after fertilization (many invertebrates, teleost fishes, amphibians, and reptiles). In humans, the ova begin the first meiotic division at about the thirteenth week of fetal development, then their development arrests in prophase I as the primary oocyte until puberty, at which time one of these primary oocytes typically develops each menstrual month into a functional egg. Meiosis II is completed only when the ovum is penetrated by a spermatozoon.
The most obvious feature of egg maturation is the deposition of yolk. Yolk, usually stored as granules or more organized platelets, is not a definite chemical substance but may be lipid or protein or both. In insects and vertebrates, all having more or less yolky eggs, yolk may be synthesized within an egg from raw materials supplied by surrounding follicle cells, or preformed lipid or protein yolk may be transferred by pinocytosis from follicle cells to the oocyte.
The result of the enormous accumulation of yolk granules and other nutrients (glycogen and lipid droplets) is that an egg grows well beyond the normal limits that force ordinary body (somatic) cells to divide. A young frog oocyte 50 m in diameter, for example, grows to 1500 m in diameter when mature after 3 years of growth in the ovary, and its volume has increased by a factor of 27,000. Bird eggs attain even greater absolute size; a hen egg will increase 200 times in volume in only the last 6 to 14 days of rapid growth preceding ovulation.
Thus eggs are remarkable exceptions to the otherwise universal rule that organisms are composed of relatively minute cellular units. This creates a surface area-to-cell volume ratio problem, since everything that enters and leaves the ovum (nutrients, respiratory gases, wastes, and so on) must pass through the cell membrane. As the egg becomes larger, the available surface per unit of cytoplasmic volume (mass) becomes smaller. As we would anticipate, the metabolic rate of an egg gradually diminishes until, when mature, an ovum is in suspended animation awaiting fertilization.
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| Figure 7-10 Oogenesis. Early germ cells (oogonia) increase by mitosis during embryonic development to form diploid primary oocytes. After puberty, each menstrual month a diploid primary oocyte is divided in the first meiotic division into a haploid secondary oocyte and a haploid polar body. If the secondary oocyte is fertilized, it enters the second meiotic division. The double-stranded chromosomes separate into a large ootid and small second polar body. Both ootid and second polar body now contain the N amount of DNA. Fusion of the haploid egg nucleus with a haploid sperm nucleus produces a diploid (2N) zygote. |
In the second meiotic division, the secondary oocyte divides into a large ootid and a small polar body. If the first polar body also divides in this division, which sometimes happens, there are three polar bodies and one ootid (Figure 7-10). The ootid develops into a functional ovum. The polar bodies are nonfunctional, and they disintegrate. Formation of nonfunctional polar bodies is necessary to enable the egg to dispose of excess chromosomes, and the unequal cytoplasmic division makes possible a large cell with the cytoplasm containing sufficient yolk for the development of the young. Thus a mature ovum has the N (haploid) number of chromosomes, the same as a sperm. However, each primary oocyte gives rise to only one functional gamete instead of four as in spermatogenesis.
In most vertebrates and many invertebrates the egg does not actually complete all the meiotic divisions before fertilization occurs. The general rule is that development is arrested during prophase I of the first meiotic division. Meiosis resumes and is completed either at the time of ovulation (birds and most mammals) or shortly after fertilization (many invertebrates, teleost fishes, amphibians, and reptiles). In humans, the ova begin the first meiotic division at about the thirteenth week of fetal development, then their development arrests in prophase I as the primary oocyte until puberty, at which time one of these primary oocytes typically develops each menstrual month into a functional egg. Meiosis II is completed only when the ovum is penetrated by a spermatozoon.
The most obvious feature of egg maturation is the deposition of yolk. Yolk, usually stored as granules or more organized platelets, is not a definite chemical substance but may be lipid or protein or both. In insects and vertebrates, all having more or less yolky eggs, yolk may be synthesized within an egg from raw materials supplied by surrounding follicle cells, or preformed lipid or protein yolk may be transferred by pinocytosis from follicle cells to the oocyte.
The result of the enormous accumulation of yolk granules and other nutrients (glycogen and lipid droplets) is that an egg grows well beyond the normal limits that force ordinary body (somatic) cells to divide. A young frog oocyte 50 m in diameter, for example, grows to 1500 m in diameter when mature after 3 years of growth in the ovary, and its volume has increased by a factor of 27,000. Bird eggs attain even greater absolute size; a hen egg will increase 200 times in volume in only the last 6 to 14 days of rapid growth preceding ovulation.
Thus eggs are remarkable exceptions to the otherwise universal rule that organisms are composed of relatively minute cellular units. This creates a surface area-to-cell volume ratio problem, since everything that enters and leaves the ovum (nutrients, respiratory gases, wastes, and so on) must pass through the cell membrane. As the egg becomes larger, the available surface per unit of cytoplasmic volume (mass) becomes smaller. As we would anticipate, the metabolic rate of an egg gradually diminishes until, when mature, an ovum is in suspended animation awaiting fertilization.
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