Eggs (eggs, egg cells) are the reproductive cells of female organisms. Animals and seed plants can lay eggs. In higher organisms, eggs are produced by ovaries. All mammals have immature eggs in their ovaries at birth, and the number of eggs will not increase after birth. The egg and * * * are fertilized to form a fertilized egg, which is the beginning of a new life. Some animals (such as birds) are fertilized in vivo, while others (such as most fish) are fertilized in vitro.
2 The structure of the egg cell The egg cell is spherical in appearance, the nucleus is covered by yolk membrane, and the main component is glycoprotein, which is secreted by the egg cell or other cells. In mammals, this coat is called zona pellucida, and its function is to protect eggs and prevent alien species from entering. Many eggs have a layer of secretory vesicles under the zona pellucida (cortex), called cortical granules. When fertilization, cortical granules are released outward, which can cause structural changes of zona pellucida, form fertilization membrane and prevent other * * * from entering. The release of cortical granules during fertilization can cause structural changes of zona pellucida, form fertilization membrane, and prevent other * * * from entering.
Eggs (eggs, egg cells) are the reproductive cells of female organisms. Animals and seed plants can lay eggs.
In higher organisms, eggs are produced by ovaries. All mammals have immature eggs in their ovaries at birth, and the number of eggs will not increase after birth. The egg and * * * are fertilized to form a fertilized egg, which is the beginning of a new life. Some animals (such as birds) are fertilized in vivo, while others (such as most fish) are fertilized in vitro.
For human reproduction, counting eggs is essential. The egg is the largest cell in the human body, unique to women, and also the mother cell that produces new life.
3. Egg production 3. 1 Egg cells in plants The process of egg formation in higher plants occurs in the ovule of the pistil ovary. There is a blastocyst mother cell in the ovule. After meiosis, the embryo sac mother cell produces four cells, one of which develops into an embryo sac cell and three smaller cells disintegrate, which has no development prospect. The nuclei of embryo sac cells undergo three consecutive mitoses to form a "seven-cell eight nuclear" embryo sac. The number of chromosomes of these eight nuclei is the same as that of embryo sac cells, which is half of that of embryo sac mother cells. Directly related to germ cells are an egg cell near the micropyle and two polar nuclei in the center of embryo sac. After fertilization, one egg cell forms a fertilized egg, and after fertilization, two polar nuclei form a fertilized egg polar nucleus. This fertilization method is called double fertilization, which only exists in angiosperms. Where did * * * come from? There are many pollen mother cells in the anther of stamens, and each pollen mother cell undergoes meiosis to produce four pollen cells, which are called pollen for short, and their chromosomes are only half of the original ones. When pollen germinates on the stigma of pistil, its nucleus will undergo a mitosis to form a reproductive nucleus and a vegetative nucleus, and the reproductive nucleus will undergo another mitosis to form two * * *. [ 1]
Mammalian egg cell
The egg of most animals is a large single cell, which stores a lot of nutrients needed for embryo development. The diameter of an egg is about 0. 1mm for humans and sea urchins, 12mm for fish and amphibians, and several centimeters or even ten centimeters for birds. Egg cytoplasm is rich in protein, lipids and polysaccharides, which is called yolk. They usually exist in yolk granules. In some species, yolk granules are surrounded by membranes. In the species hatched in vitro, the volume of yolk can reach more than 95% of the egg. There is a capsule outside the egg, and its composition is mainly glycoprotein, which is secreted by egg cells or other cells. In mammals, this coat is called zona pellucida, and its function is to protect eggs and prevent alien species from entering. Many eggs have a layer of secretory vesicles under the zona pellucida (cortex), called cortical granules. When fertilization, cortical granules are released outward, which can cause structural changes of zona pellucida, form fertilization membrane and prevent other * * * from entering. In fact, an egg cell is an oocyte that stays in the first or second meiosis stage and does not complete meiosis until after fertilization.
3.2 Since puberty, male and female human eggs have matured or ovulated from their sexual organs-testicles or ovaries. Most normal women's ovaries discharge a mature egg every month, and a batch of * * * can be matured in about 3 days, and hundreds of millions of * * * can be discharged at a time. Ovulation period is usually before the next menstruation 14 days. When the egg is discharged from the ovary, the egg is taken at the umbrella-shaped end of the fallopian tube. Under the action of cilia of epithelial cells, they enter the junction of the far and middle part of fallopian tube (i.e. ampulla of fallopian tube) within a few minutes, and stay there for 2~3 days waiting for fertilization. During this sexual life, * * is discharged into * * *, and within 2~3 hours, * * swims quickly to the fallopian tube through the cervical canal and uterine cavity to meet the egg. A large number of * * * compete to enter the egg, but as long as one * * * first breaks through the peripheral tissue of the egg and enters the egg cell, other * * * cannot enter the same egg. After the head of the introduced * * * comes into contact with the egg, their cell membranes rupture, the nuclei begin to fuse, the outer zona pellucida hardens, and then the pronucleus of sperm and egg fuse to become pregnant eggs. At this time, the 23 chromosomes brought in by them were fused into 23 pairs.
Eggs are produced by what we usually call female gonads-ovaries, which are about 0.2 mm in diameter. The main function of ovaries is to produce eggs in addition to secreting sex hormones necessary for women. When a girl is pregnant for about 3 ~ 6 weeks, the embryonic form of the ovary has been formed. Before birth, millions of oocytes are formed in the ovary. After childhood and adolescence, there are only 65438+ 10,000 oocytes left in adulthood. Oocytes are encapsulated in primordial follicles. Under the action of sex hormones, only one primordial follicle matures every month, and the mature egg is discharged from the ovary to the abdominal cavity. Generally speaking, there are about 300 to 400 mature eggs in a woman's life, and the rest will die. About 1.5 ~ 20 primordial follicles begin to grow and develop at the same time every month since puberty. Usually, only 1 ~ 2 follicles can develop into dominant mature and ovulate, and others develop into atresia follicles.
Follicle development can be divided into three stages: follicular stage, ovulation stage and luteal stage.
? The egg is composed of cystic follicles and cysts beside the ovary. When ovulating every month, the cystic follicle will appear red and become prominent, and the eggs surrounded by colloidal substances will be discharged at the end of the cystic follicle.
An egg can live for about 48 hours after it is excreted, and wait for it to meet and combine with * * * within these 48 hours. If the egg cannot meet with * * * for various reasons to form a fertilized egg, it will die naturally after 48 ~ 72 hours. If we lose this chance of fertilization, we have to wait until 1 month later, when another egg matures and is expelled, and repeat the same process. The left and right ovaries usually ovulate in turn, and in a few cases, two or more eggs can be expelled at the same time. If combined with * * * respectively, there will be fraternal twins and multiple eggs.
Among the causes of female infertility, ovarian factors account for about 15% ~ 25%, and ovarian anovulation is one of the important reasons.
4 the process of egg migration the main link of egg operation is the role of the umbrella end of fallopian tube. According to direct observation in some animals, eggs do not swim a long distance in the abdominal cavity after ovulation. Due to the coordination of contraction activities of oviduct muscles, mesangium and ovarian proper ligament, the umbrella-shaped end of oviduct is very close to the ovulation site of ovary. In humans, bilateral fallopian tubes often appear around the back of the uterus during surgery. It is estimated that the function of human oviduct in catching eggs may be similar to that of mammals.
The egg enters the fallopian tube mainly because of the picking-up function of the umbrella end of the fallopian tube. In recent years, it has been observed under direct vision that the ovulated follicle does not rupture violently, and the egg is washed into the abdominal cavity, but the follicular fluid slowly flows out through the ovulation point with the secondary oocyte of cumulus cells. After ovulation, due to the effect of progesterone, the umbrella-shaped end of fallopian tube is widely dispersed and congested, and the contraction intensity of fallopian tube increases. In addition, the umbrella tip is close to the ovulation point, a large number of cilia swing at the umbrella tip, and the eggs are quickly sent to the ampulla within a few minutes. The velocity of oviduct fluid in tubal stenosis is faster, but it is slower in ampulla, which is convenient for eggs to stay in ampulla and fertilize here. After the egg is discharged from the ovary, the fertilization effect is the best within 15 ~ 18 hours, and it begins to degenerate if it is not fertilized within 24 hours.
* * * meets the egg * * *' s head enters the egg, and the egg swims to the uterus along the narrow fallopian tube.