Implantation is the stage in embryonic development in which the blastocyst hatches as the embryo, and adheres to the wall of the uterus. Once this adhesion is successful, the female is considered to be pregnant and the embryo will receive oxygen and nutrients from the mother in order to grow.

We can learn this lesson under the following topics.

  1. Preimplantation
  2. Implantation
  3. Hormonal Aspect of Implantation
  4. Hypophyseal – ovarian relationship
  5. Abnormal implantation sites


After a successful fertilization, which takes place in the ampullary part of the fallopian tube, the embryo migrates through the tube into the uterine cavity. This migration takes six days. Along the way, the zygote divides several times, initially without increasing its volume because it is still enveloped by the pellucid zone. Daughter cells are engendered and one speaks now of the blastomere stage. After around 16 cells (morula) the compaction occurs in which the outer cells, the trophoblasts, form a compact epithelial structure. 


Implantation occurs at the 6th day after fertilization and is completed about the 11th day.

The normal site is the endometrium of the posterior wall of the fundus of the uterus in or near the middle line. The endometrium after implantation is called decidua.

In the human, the trophoblast cells over the embryonic pole of the blastocyst penetrate the epithelial cells of the uterine mucosa at about day 6 or 7 or about 20 days after the beginning of the last menstruation

1. As invasion of uterus proceeds, the trophoblast differentiates into 2 layers

2. The finger like processes of the syncytial trophoblast grow into the endometrial epithelium and invade the endometrial stroma

3. By the end of week 1, the blastocyst is superficially implanted in the compact layer and the conceptus derives its initial nourishment from endometrial tissues. Later it receives nutrients directly from maternal blood.

4. At the time of implantation, the uterine mucosa is at day 21 of the menstrual cycle and is richly vascularized, oedematous, and secreting mucus and glycogen – all favouring implantation of the blastocyst

6. As the blastocyst is implanting, early differentiation of the inner cells mass occurs, forming bilaminar Embryo (Human development about day 8 to 9 )

Hormonal Aspect of Implantation


Hypophyseal-ovarian relationship

1. HYPOPHYSIS: endocrine activity of the ovary is under the control of the anterior lobe of the pituitary gland. The latter, in the human, secretes 2 gonad-stimulating hormones called gonadotropins or gonadotropes

2. THE OVARY: the endocrine activity of the ovary, under the influence of the hypophyseal gonadotropins, is diphasic

Abnormal Implantation Sites

1.THE HUMAN BLASTOCYST normally implants in the endometrium along the posterior wall of the body of the uterus, where it becomes attached between the openings of the endometrial glands or occasionally in the mouth of a glandular duct

2.NOT INFREQUENTLY, THE BLASTOCYST IMPLANTS IN ABNORMAL LOCATIONS outside the uterine body. This usually leads to the death of the embryo and severe hemorrhage of the mother during the second month of pregnancy. Such an implantation is called an extrauterine or ectopic pregnancy and may occur in the abdominal cavity, the ovary, the uterine tube or pelvis. Rarely does an extrauterine embryo come to full term

  1. Tubal pregnancy is the most frequent ectopic sit The tube usually ruptures during the second month of pregnancy, resulting in severe internal hemorrhaging
  2. Abdominal pregnancy: the peritoneal lining of the rectouterine cavity is the most frequent implantation sit Also on peritoneum of the intestinal tract or omentum

3.OCCASIONALLY, IMPLANTATION IN THE UTERUS ITSELF may lead to serious complications, particularly if implantation occurs near the internal os (low uterus). The placenta then bridges the os and we have what is called placenta previa which results in severe bleeding in the latter or second part of pregnancy and during delivery


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