概念:它是如何工作的
成为 怀孕了,必须执行以下步骤:
- 精子的运输. The sperm must be deposited and transported to the site of fertilization.
- 鸡蛋的运输. Ovulation must occur and the egg must be "picked up" by the tube.
- 受精与胚胎发育. 精子和卵子必须结合.
- 植入. The embryo must implant and begin to grow in the uterus.
下面描述了这些步骤.
精子的运输
精子的运输取决于几个因素:
- The sperm must be capable of propelling themselves through the environment of the female vagina and cervix.
- 这种环境下, 哪一种受周期性荷尔蒙控制, must be favorable to admit the sperm without destroying them.
- The sperm must possess the capability of converting to a form that can penetrate the cell membrane of the egg (capacitation).
射精后, the semen forms a gel that protects it from the acidic environment of the vagina. The gel is liquefied within 20 to 30 minutes by enzymes from the prostate gland. This liquefaction is important for freeing the sperm so transportation may occur. 精浆留在阴道内.
The protected sperm with the greatest motility travel through the layers of cervical mucus that guard the entrance to the uterus. 在排卵, 这个屏障变薄并改变了它的酸度, 为精子创造一个更友好的环境. The cervical mucus acts as a reservoir for extended sperm survival.
一旦精子进入子宫, contractions propel the sperm upward into the fallopian tubes. The first sperm enter the tubes minutes after ejaculation. The first sperm, however, are likely not the fertilizing sperm. Motile sperm can survive in the female reproductive tract for up to five days.
鸡蛋的运输
鸡蛋的运输 begins at ovulation and ends once the egg reaches the uterus. Following ovulation, the fimbriated, or finger-like, end of the fallopian tube sweeps over the ovary. 纤毛上的粘附部位, 哪些位于菌毛表面, are responsible for egg pickup and movement into the tube. 导管内的纤毛, and muscular contractions resulting from the movement of the egg, 创造一个向前的运动. 通过管道运输大约需要30个小时.
比如盆腔感染和 子宫内膜异位 can permanently impair the function of the fallopian tubes, 由于结疤或损坏的毛.
受精与胚胎发育
Following ovulation, the egg is capable of fertilization for only 12 to 24 hours. 卵子和精子之间的接触是随机的.
Once the egg arrives at a specific portion of the tube, 叫做壶腹峡交界处, 它再休息30个小时. Fertilization — sperm union with the egg — occurs in this portion of the tube. The fertilized egg then begins a rapid descent to the uterus. The period of rest in the tube appears to be necessary for full development of the fertilized egg and for the uterus to prepare to receive the egg.
Defects in the fallopian tube may impair transport and increase the risk of a tubal pregnancy, 也叫异位妊娠.
A membrane surrounding the egg, called the zona pellucida, has two major functions in fertilization. First, the zona pellucida contains sperm receptors that are specific for human sperm. Second, once penetrated by the sperm, the membrane becomes impermeable to penetration by other sperm.
Following penetration, a series of events set the stage for the first cell division. 单细胞胚胎称为受精卵. 在接下来的七天里, the human embryo undergoes multiple cell divisions in a process called mitosis. 在这个过渡时期结束时, 胚胎变成一团非常有组织的细胞, 叫做囊胚. 现在人们认为,随着女性年龄的增长, this process of early embryo development is increasingly impaired due to diminishing egg quality.
植入
一旦胚胎进入囊胚期, 大约在受精后5到6天, it hatches out of its zona pellucida and begins the process of implantation in the uterus.
In nature, 50 percent of all fertilized eggs are lost before a woman's missed menses. 在体外受精(IVF)过程中也是如此, an embryo may begin to develop but not make it to the blastocyst stage — the first stage at which those cells destined to become the fetus separate from those that will become the placenta. 囊胚可以植入,但不能生长, or the blastocyst may grow but stop developing before the two week time at which a pregnancy can be detected. The receptivity of the uterus and the health of the embryo are important for the implantation process.
加州大学旧金山分校健康 medical specialists have reviewed this information. It is for educational purposes only and is not intended to replace the advice of your doctor or other health care provider. We encourage you to discuss any questions or concerns you may have with your provider.