By the 13th day of development, the surface defect in the endometrium has usually healed. Occasionally, however, bleeding occurs at the implantation site as a result of increased blood flow into the lacunar spaces. Because this bleeding occurs near the 28th day of the menstrual cycle, it may be confused with

Figure 3.7 Section through the implantation site of a 13-day embryo. Note the amniotic cavity, yolk sac, and exocoelomic cyst in the chorionic cavity. Most of the lacunae are filled with blood.

normal menstrual bleeding and, therefore, cause inaccuracy in determining the expected delivery date.

The trophoblast is characterized by villous structures. Cells of the cy-totrophoblast proliferate locally and penetrate into the syncytiotrophoblast, forming cellular columns surrounded by syncytium. Cellular columns with the syncytial covering are known as primary villi (Figs. 3.6 and 3.7) (see Chapter 4).

In the meantime, the hypoblast produces additional cells that migrate along the inside of the exocoelomic membrane (Fig. 3.4). These cells proliferate and gradually form a new cavity within the exocoelomic cavity. This new cavity is known as the secondary yolk sac or definitive yolk sac (Figs. 3.6 and 3.7). This yolk sac is much smaller than the original exocoelomic cavity, or primitive yolk sac. During its formation, large portions of the exocoelomic cavity are pinched off. These portions are represented by exocoelomic cysts, which are often found in the extraembryonic coelom or chorionic cavity (Figs. 3.6 and 3.7).

Meanwhile, the extraembryonic coelom expands and forms a large cavity, the chorionic cavity. The extraembryonic mesoderm lining the inside of the cytotrophoblast is then known as the chorionic plate. The only place where extraembryonic mesoderm traverses the chorionic cavity is in the connecting stalk (Fig. 3.6). With development of blood vessels, the stalk becomes the umbilical cord.

CLINICAL CORRELATES Abnormal Implantation

The syncytiotrophoblast is responsible for hormone production (see Chapter 6), including human chorionic gonadotropin (hCG). By the end of the second week, quantities of this hormone are sufficient to be detected by radioimmunoassays, which serve as the basis for pregnancy testing.

Because 50% of the implanting embryo's genome is derived from the father, it is a foreign body that potentially should be rejected by the maternal system. Recent evidence suggests that a combination of factors protects the conceptus, including production of immunosuppressive cytokines and proteins and the expression of an unusual major histocompatibility complex class IB molecule (HLA-G) that blocks recognition of the conceptus as foreign tissue. If the mother has autoimmune disease, for example systemic lupus erythe-matosus, antibodies generated by the disease may attack the conceptus and reject it.

Abnormal implantation sites sometimes occur even within the uterus. Normally the human blastocyst implants along the anterior or posterior wall of the body of the uterus. Occasionally the blastocyst implants close to the internal opening os (opening) (Fig. 3.8) of the cervix, so that later in development,

Figure 3.8 Abnormal implantation sites of the blastocyst. 1, implantation in the abdominal cavity. The ovum most frequently implants in the rectouterine cavity (Douglas' pouch) but may implant at any place covered by peritoneum. 2, implantation in the am-pullary region of the tube. 3, tubal implantation. 4, interstitial implantation, that is, in the narrow portion of the uterine tube. 5, implantation in the region of the internal os, frequently resulting in placenta previa. 6, ovarian implantation.

Rectouterine Cavity


Figure 3.8 Abnormal implantation sites of the blastocyst. 1, implantation in the abdominal cavity. The ovum most frequently implants in the rectouterine cavity (Douglas' pouch) but may implant at any place covered by peritoneum. 2, implantation in the am-pullary region of the tube. 3, tubal implantation. 4, interstitial implantation, that is, in the narrow portion of the uterine tube. 5, implantation in the region of the internal os, frequently resulting in placenta previa. 6, ovarian implantation.

Uterine tube



Placental Lacunar
Figure 3.9 Tubal pregnancy. Embryo is approximately 2 months old and is about to escape through a rupture in the tubal wall.

the placenta bridges the opening (placenta previa) and causes severe, even life-threatening bleeding in the second part of pregnancy and during delivery

Occasionally, implantation takes place outside the uterus, resulting in ex-trauterine pregnancy, or ectopic pregnancy. Ectopic pregnancies may occur at any place in the abdominal cavity, ovary, or uterine tube (Fig. 3.8). However, 95% of ectopic pregnancies occur in the uterine tube, and most of these are in the ampulla (Fig. 3.9). In the abdominal cavity, the blastocyst most frequently attaches itself to the peritoneal lining of the rectouterine cavity, or Douglas' pouch (Fig. 3.10). The blastocyst may also attach itself to the peritoneal covering of the intestinal tract or to the omentum. Sometimes the blastocyst develops in the ovary proper, causing a primary ovarian pregnancy. In most ectopic pregnancies, the embryo dies about the second month of gestation, causing severe hemorrhaging and abdominal pain in the mother.

Abnormal blastocysts are common. For example, in a series of 26 implanted blastocysts varying in age from 7.5 to 17 days recovered from patients of normal fertility, nine (34.6%) were abnormal. Some consisted of syncytium only; others showed varying degrees of trophoblastic hypoplasia. In two, the embryoblast was absent, and in some, the germ disc showed an abnormal orientation.

It is likely that most abnormal blastocysts would not have produced any sign of pregnancy because their trophoblast was so inferior that the corpus luteum could not have persisted. These embryos probably would have been

Douglas Pouch
Figure 3.10 Midline section of bladder, uterus, and rectum to show an abdominal preg nancy in the rectouterine (Douglas') pouch.

aborted with the next menstrual flow, and therefore, pregnancy would not have been detected. In some cases, however, the trophoblast develops and forms placental membranes, although little or no embryonic tissue is present. Such a condition is known as a hydatidiform mole. Moles secrete high levels of hCG and may produce benign or malignant (invasive mole, choriocarcinoma) tumors.

Genetic analysis of hydatidiform moles indicates that although male and female pronuclei may be genetically equivalent, they may be different functionally. This evidence is derived from the fact that while cells of moles are diploid, their entire genome is paternal. Thus, most moles arise from fertilization of an oocyte lacking a nucleus followed by duplication of the male chromosomes to restore the diploid number. These results also suggest that paternal genes regulate most of the development of the trophoblast, since in moles, this tissue differentiates even in the absence of a female pronucleus.

Other examples of functional differences in maternal and paternal genes are provided by the observation that certain genetic diseases depend on whether the defective or missing gene is inherited from the father or the mother. For example, inheritance of a deletion on chromosome 15 from a father produces Prader-Willi syndrome, whereas inheritance of the same defect from the mother results in Angelman syndrome. This phenomenon, in which there is differential modification and/or expression of homologous alleles or chromosome regions, depending on the parent from whom the genetic material is derived, is known as genomic imprinting. Imprinting involves auto-somes and sex chromosomes (in all female mammals, one X chromosome is inactivated in somatic cells and forms a chromatin-positive body [Barr body]) and is modulated by deoxyribonucleic acid (DNA) methylation. Certain diseases, such as Huntington's chorea, neurofibromatosis, familial cancer disorders (Wilms' tumors, familial retinoblastoma), and myotonic dystrophy, also involve imprinting. Fragile X syndrome, the leading cause of inherited mental retardation, may be another example of a condition based on imprinting (see Chapter 1).

Preimplantation and postimplantation reproductive failure occurs often. Even in some fertile women under optimal conditions for pregnancy, 15% of oocytes are not fertilized, and 10% to 15% start cleavage but fail to implant. Of the 70% to 75% that implant, only 58% survive until the second week, and 16% of those are abnormal. Hence, when the first expected menstruation is missed, only 42% of the eggs exposed to sperm are surviving. Of this percentage, a number will be aborted during subsequent weeks and a number will be abnormal at the time of birth.


At the beginning of the second week, the blastocyst is partially embedded in the endometrial stroma. The trophoblast differentiates into (a) an inner, actively proliferating layer, the cytotrophoblast, and (b) an outer layer, the syncytiotrophoblast, which erodes maternal tissues (Fig. 3.1). By day 9, lacunae develop in the syncytiotrophoblast. Subsequently, maternal sinusoids are eroded by the syncytiotrophoblast, maternal blood enters the lacunar network, and by the end of the second week, a primitive uteroplacental circulation begins (Fig. 3.6). The cytotrophoblast, meanwhile, forms cellular columns penetrating into and surrounded by the syncytium. These columns are primary villi. By the end of the second week, the blastocyst is completely embedded, and the surface defect in the mucosa has healed (Fig. 3.6).

The inner cell mass or embryoblast, meanwhile, differentiates into (a) the epiblast and (b) the hypoblast, together forming a bilaminar disc (Fig. 3.1). Epiblast cells give rise to amnioblasts that line the amniotic cavity superior to the epiblast layer. Endoderm cells are continuous with the exocoelomic membrane, and together they surround the primitive yolk sac (Fig. 3.4). By the end of the second week, extraembryonic mesoderm fills the space between the trophoblast and the amnion and exocoelomic membrane internally. When vacuoles develop in this tissue, the extraembryonic coelom or chorionic cavity forms (Fig. 3.6). Extraembryonic mesoderm lining the cytotrophoblast and amnion is extraembryonic somatopleuric mesoderm; the lining surrounding the yolk sac is extraembryonic splanchnopleuric mesoderm (Fig. 3.6).

The second week of development is known as the week of twos: The trophoblast differentiates into two layers, the cytotrophoblast and syncytiotro-phoblast. The embryoblast forms two layers, the epiblast and hypoblast. The extraembryonic mesoderm splits into two layers, the somatopleure and splanchnopleure. And two cavities, the amniotic and yolk sac cavities, form. Implantation occurs at the end of the first week. Trophoblast cells invade the epithelium and underlying endometrial stroma with the help of proteolytic enzymes. Implantation may also occur outside the uterus, such as in the rectouter-ine pouch, on the mesentery, in the uterine tube, or in the ovary (ectopic pregnancies).

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  • jose
    Can placenta previa be hereditary?
    8 years ago

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