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Blastocyst at the time of implantation

The nidation ( implantation , according to Latin nidus , Nest ' ), or implantation ( implantation , by latin plantatus planted' ) is the implantation of the fertilized egg cell in the stage of blastocyst in the uterine lining . Nidation starts with people on the fifth or sixth day after fertilization of the egg. The implanting blastocyst secretes the hormone human chorionic gonadotropin (hCG), which prevents the uterine lining from being rejected during menstruation .

Fertilization and implantation in humans

Schematic structure of the blastocyst

The fusion of egg and sperm takes place 12 to 24 hours after ovulation ( ovulation ) in the oviduct ( fallopian tube ). The fertilized egg, as zygote referred divides and is carried to the cilia of the fallopian tubes cells blow direction uterus moved. At around the 12-16 cell stage, the zygote - known as the morula at this point - reaches the uterine cavity on the third day after fertilization. The blastocyst emerges from the morula through further divisions and structural changes. This is characterized by the fact that the cells from which the embryo emerges ( embryoblast ) accumulate on one side of the blastocyst. The wall of the blastocyst is made up of flattened cells called a trophoblast . The trophoblast encloses the blastocyst cavity.

Nidation begins with the attachment of the blastocyst to the lining of the uterus ( endometrium ) between the 5th and 6th day. Then those trophoblast cells that are located at the pole of the blastocyst where the embryoblast lies penetrate between the surface cells ( epithelial cells ) of the uterine lining. To do this, the trophoblast cells release enzymes that dissolve structural proteins of the connective tissue of the mucous membrane ( proteolytic enzymes ). The cells of the lining of the uterus also help the blastocyst to implant. The whole process of implantation continues until the end of the second week of development. The blastocyst has then completely penetrated the upper layer of the uterine lining and is covered by newly formed epithelium. So it is actually an implantation in the uterus and not just an attachment.

The implantation involves the opening of some blood vessels in the lining of the uterus, so that there may be a slight bleeding, the so-called implantation bleeding . Since the next, now absent, menstrual bleeding would also be due at this point in time, the implantation bleeding can be confused with this. The woman can therefore mistakenly assume at this point that she is not pregnant.

Place of nidation

In humans, nidation usually takes place on the back wall of the uterus , very rarely also outside the uterus, if there are passages in the area of ​​the fallopian tube . In these cases, an ectopic pregnancy arises that in 99% of the cases has the fallopian tube itself as its location, which is known as an ectopic pregnancy . In the remaining cases, the embryo / blastocyst embeds itself in the abdominal cavity , usually in the Douglas space or the mesentery . Such pregnancies outside the uterus are also called ectopic (from the Greek ek 'outside' , and topos 'place' ) because they are outside the natural place for a pregnancy.

Ectopic pregnancy can seriously endanger the mother's life, especially because blood vessels can rupture as the embryo grows. Nevertheless, in rare cases an ectopic child is born.

Inhibition of nidation

For some methods of contraception , inhibition of implantation is discussed as a possible mechanism. In particular in the case of intrauterine devices (“coil”), emergency contraception (“ morning-after pill ”) and the mini-pill , inhibiting implantation could contribute to the contraceptive effect. As a rule, the so-called "spiral afterwards" is used to perform an interception (preventing the egg cell from implanting after fertilization). There is no evidence of an anti-nidation effect of the methods mentioned. Because of the possible anti-nidation effects, these methods are used by some churches, e.g. B. the Orthodox Churches and the Catholic Church , rejected, because in their opinion human life begins with the fusion of egg and sperm cells.

According to German criminal law, pregnancy does not begin until after implantation. Measures that take effect before the completion of the implantation are not considered to be an abortion and are therefore not punishable as an abortion .

Literature and Sources

  • T. Sadler: Medical Embryology. 10th edition. Thieme, Stuttgart 2003, ISBN 3-13-446610-4 , pp. 25-44.

Individual evidence

  1. Keith L. Moore, T. Vidhya N. Persaud: Embryology. Stages of development, early development, organogenesis, clinic. 5th edition. Urban & Fischer, Munich 2007, ISBN 978-3-437-41112-0 , p. 123.
  2. R. Rivera, I. Yacobson, D. Grimes: The mechanism of action of hormonal contraceptives and intrauterine contraceptive devices. In: Am J Obstet Gynecol. 181 (5 Pt 1), 1999 Nov, pp. 1263-1269. PMID 10561657 .
  3. ^ WL Larimore, JB Stanford: Postfertilization effects of oral contraceptives and their relationship to informed consent. In: Arch Fam Med. 9 (2), 2000 Feb, pp. 126-133. PMID 10693729
  4. Timothy Ware: The Orthodox Church. Penguin, ISBN 0-14-014656-3 , p. 296.
  5. Section 218, Paragraph 1, Sentence 2 of the Criminal Code

Web links

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