This chapter gives a day-by-day account of the major events of the second week of development; however, embryos of the same fertilization age do not necessarily develop at the same rate. Indeed, considerable differences in rate of growth have been found even at these early stages of development.
At the eighth day of development, the blastocyst is partially embedded in the endometrial stroma. In the area over the embryoblast, the trophoblast has differentiated into two layers: (1) an inner layer of mononucleated cells, the cytotrophoblast, and (2) an outer multinucleated zone without distinct cell boundaries, the syncytiotrophoblast (Figs. 4.1 and 4.2). Mitotic figures are found in the cytotrophoblast but not in the syncytiotrophoblast. Thus, cells in the cytotrophoblast divide and migrate into the syncytiotrophoblast, where they fuse and lose their individual cell membranes.
A 7.5-day human blastocyst, partially embedded in the endometrial stroma. The trophoblast consists of an inner layer with mononuclear cells, the cytotrophoblast, and an outer layer without distinct cell boundaries, the syncytiotrophoblast. The embryoblast is formed by the epiblast and hypoblast layers. The amniotic cavity appears as a small cleft.
Figure 4.2.Section of a 7.5-day human blastocyst (×100).
Note the multinucleated appearance of the syncytiotrophoblast, large cells of the cytotrophoblast, and slit-like amniotic cavity.
Cells of the inner cell mass or embryoblast also differentiate into two layers: (1) a layer of small cuboidal cells adjacent to the blastocyst cavity, known as the hypoblast layer, and (2) a layer of high columnar cells adjacent to the amniotic cavity, the epiblast layer (Figs. 4.1 and 4.2).
Together, the layers form a flat disc. At the same time, a small cavity appears within the epiblast. This cavity enlarges to become the amniotic cavity. Epiblast cells adjacent to the cytotrophoblast are called amnioblasts; together with the rest of the epiblast, they line the amniotic cavity (Figs. 4.1 and 4.3). The endometrial stroma adjacent to the implantation site is edematous and highly vascular. The large, tortuous glands secrete abundant glycogen and mucus.
Figure 4.3. 9-day human blastocyst.
The syncytiotrophoblast shows a large number of lacunae. Flat cells form the exocoelomic membrane. The bilaminar disc consists of a layer of columnar epiblast cells and a layer of cuboidal hypoblast cells. The original surface defect is closed by a fibrin coagulum.
The blastocyst is more deeply embedded in the endometrium, and the penetration defect in the surface epithelium is closed by a fibrin coagulum (Fig. 4.3). The trophoblast shows considerable progress in development, particularly at the embryonic pole, where vacuoles appear in the syncytium. When these vacuoles fuse, they form large lacunae, and this phase of trophoblast development is thus known as the lacunar stage (Fig. 4.3).
At the abembryonic pole, meanwhile, flattened cells probably originating from the hypoblast form a thin membrane, the exocoelomic (Heuser’s) membrane that lines the inner surface of the cytotrophoblast (Fig. 4.3). This membrane, together with the hypoblast, forms the lining of the exocoelomic cavity, or primitive yolk sac.
Days 11 and 12
By the 11th to the 12th day of development, the blastocyst is completely embedded in the endometrial stroma, and the surface epithelium almost entirely covers the original defect in the uterine wall (Figs. 4.4 and 4.5). The blastocyst now produces a slight protrusion into the lumen of the uterus. The trophoblast is characterized by lacunar spaces in the syncytium that form an intercommunicating network. This network is particularly evident at the embryonic pole; at the abembryonic pole, the trophoblast still consists mainly of cytotrophoblastic cells (Figs. 4.4 and 4.5).
Figure 4.4.Human blastocyst of approximately 12 days.
The trophoblastic lacunae at the embryonic pole are in open connection with maternal sinusoids in the endometrial stroma. Extraembryonic mesoderm proliferates and fills the space between the exocoelomic membrane and the inner aspect of the trophoblast.
Figure 4.5.Fully implanted 12-day human blastocyst (×100).
Note maternal blood cells in the lacunae, the exocoelomic membrane lining the primitive yolk sac, and the hypoblast and epiblast.
Concurrently, cells of the syncytiotrophoblast penetrate deeper into the stroma and erode the endothelial lining of the maternal capillaries. These capillaries, which are congested and dilated, are known as sinusoids. The syncytial lacunae become continuous with the sinusoids, and maternal blood enters the lacunar system (Fig. 4.4). As the trophoblast continues to erode more and more sinusoids, maternal blood begins to flow through the trophoblastic system, establishing the uteroplacental circulation.
In the meantime, a new population of cells appears between the inner surface of the cytotrophoblast and the outer surface of the exocoelomic cavity. These cells, derived from yolk sac cells, form a fine, loose connective tissue, the extraembryonic mesoderm, which eventually fills all of the space between the trophoblast externally and the amnion and exocoelomic membrane internally (Figs. 4.4 and 4.5). Soon, large cavities develop in the extraembryonic mesoderm, and when these become confluent, they form a new space known as the extraembryonic cavity, or chorionic cavity (Fig. 4.4). This space surrounds the primitive yolk sac and amniotic cavity, except where the germ disc is connected to the trophoblast by the connecting stalk (Fig. 4.6). The extraembryonic mesoderm lining the cytotrophoblast and amnion is called the extraembryonic somatic mesoderm; the lining covering the yolk sac is known as the extraembryonic splanchnic mesoderm (Fig. 4.4).
Figure 4.6.A 13-day human blastocyst.
Trophoblastic lacunae are present at the embryonic as well as the abembryonic pole, and the uteroplacental circulation has begun. Note the primary villi and the extraembryonic coelom or chorionic cavity. The secondary yolk sac is entirely lined with endoderm.
Growth of the bilaminar disc is relatively slow compared with that of the trophoblast; consequently, the disc remains very small (0.1 to 0.2 mm). Cells of the endometrium, meanwhile, become polyhedral and loaded with glycogen and lipids; intercellular spaces are filled with extravasate, and the tissue is edematous. These changes, known as the decidua reaction, at first are confined to the area immediately surrounding the implantation site but soon occur throughout the endometrium.
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 normal menstrual bleeding and, therefore, may cause inaccuracy in determining the expected delivery date.
The trophoblast is characterized by villous structures. Cells of the cytotrophoblast 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. 4.6 and 4.7) (see Chapter 5).
Figure 4.7.Section through the implantation site of a 13-day embryo.
Note the amniotic cavity, yolk sac, and chorionic cavity. Most of the lacunae are filled with blood.
In the meantime, the hypoblast produces additional cells that migrate along the inside of the exocoelomic membrane (Fig. 4.4). 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. 4.6 and 4.7). 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. 4.6).
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. 4.6). With development of blood vessels, the stalk becomes the umbilical cord.
The syncytiotrophoblast is responsible for hormone production (see Chapter 8), 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, similar to rejection of a transplanted organ. A pregnant woman’s immune system needs to change in order for her to tolerate the pregnancy. How this occurs is not well understood, but it appears that a shift from cell-mediated immunity to humoral (antibody mediated) immunity occurs and that this shift protects the embryo from rejection. However, the immune system alterations place pregnant women at increased risk for certain infections, such as influenza, which explains the increased risk of death from these infections in pregnant women. In addition, manifestations of autoimmune disease may change during pregnancy. For example, multiple sclerosis and rheumatoid arthritis, primarily cell-mediated conditions, show improvement during pregnancy, whereas women with systemic lupus erythrematosis (a predominantly antibody-mediated immune disorder) are more severely affected when pregnant.
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 os (opening) (Fig. 4.8) of the cervix, so that later in development, the placenta bridges the opening (placenta previa) and causes severe, even life-threatening bleeding in the second part of pregnancy and during delivery.
Figure 4.8.Abnormal implantation sites of the blastocyst.
1, implantation in the abdominal cavity (1.4%; the ovum most frequently implants in the rectouterine cavity [pouch of Douglas; Fig. 4.10] but may implant at any place covered by peritoneum); 2, implantation in the ampullary region of the tube (80%); 3, tubal implantation (12%); 4, interstitial implantation (0.2%; e.g., in the narrow portion of the uterine tube); 5, implantation in the region of the internal os, frequently resulting in placenta previa (0.2%); and 6, ovarian implantation (0.2%).
Occasionally, implantation takes place outside the uterus, resulting in an extrauterine pregnancy, or ectopic pregnancy. Ectopic pregnancies may occur at any place in the abdominal cavity, ovary, or uterine tube (Fig. 4.8). Ninety-five percent of ectopic pregnancies occur in the uterine tube, however, and most of these are in the ampulla (80%; Fig. 4.9). In the abdominal cavity, the blastocyst most frequently attaches itself to the peritoneal lining of the rectouterine cavity, or pouch of Douglas (Fig. 4.10). 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. Ectopic pregnancies occur in 2% of all pregnancies and account for 9% of all pregnancy related deaths for the mother. In most ectopic pregnancies, the embryo dies about the second month of gestation and may result in severe hemorrhaging in the mother.
Figure 4.9.Tubal pregnancy.
Embryo is approximately 2 months old and is about to escape through a rupture in the tubal wall.
Figure 4.10. Midline section of bladder, uterus, and rectum shows an abdominal pregnancy in the rectouterine (Douglas) pouch.
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, 9 (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 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 although 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, because 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, a microdeletion on chromosome 15 inherited from a father causes Prader-Willi syndrome (a condition characterized by hypotonia, intellectual disability, hypogonadism, and obesity), whereas inheriting the same deletion from the mother causes Angelman’s syndrome (a condition characterized by seizures, little to no speech, paroxysms of laughter, and severe intellectual disability). 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. Less than 1% of human genes are believed to be imprinted (see Chapter 2).
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 (1) an inner, actively proliferating layer, the cytotrophoblast, and (2) an outer layer, the syncytiotrophoblast, which erodes maternal tissues (Fig. 4.1). 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. 4.6). 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. 4.6).
The inner cell mass or embryoblast, meanwhile, differentiates into (1) the epiblast and (2) the hypoblast, together forming a bilaminar disc (Fig. 4.6). Epiblast cells give rise to amnioblasts that line the amniotic cavity superior to the epiblast layer. Hypoblast cells are continuous with the exocoelomic membrane, and together they surround the primitive yolk sac (Fig. 4.4). 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. 4.6). Extraembryonic mesoderm lining the cytotrophoblast and amnion is extraembryonic somatic mesoderm; the lining surrounding the yolk sac is extraembryonic splanchnic mesoderm (Fig. 4.6).
The second week of development is known as the week of 2’s:
- The trophoblast differentiates into 2 layers; the cytotrophoblast and syncytiotrophoblast
- The embryoblast forms 2 layers; the epiblast and hypoblast
- The extraembryonic mesoderm splits into 2 layers; the somatic and splanchnic layers
- 2 cavities form; the amniotic and yolk sac cavities.
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 rectouterine pouch, on the mesentery, in the uterine tube, or in the ovary (ectopic pregnancies).