The Human Placenta: an overview and some highlights

 

Oops. Wrong species.

 

 

Placenta of Homo sapiens sapiens

 

 

 

 

 

The biochemical and physical duet of the mother and the fetus in the formation of the placenta is one of the most carefully orchestrated phenomena in fetal development. It represents the cooperation of two distinct individuals to form a single structure that protects one and enables the genes of the other to live on. This marvel is not a miracle, but an example of evolution at its finest.

The fetal-maternal interface in all mammals consists of the placenta, a derivative of the trophoblast. The trophoblast separates from the inner cell mass as the fertilized zygote travels down the uterine tube, before implantation in the uterine lining. The other extraembryonic membranes &emdash;the amnion, yolk sac, allantois, and extraembryonic mesoderm&emdash;are all derivatives of the inner cell mass itself. The placenta is of special interest because it is directly in contact with the tissue of the mother. The placental complex, which consists of the placenta and the chorion, is formed by a cooperative effort between the extraembryonic tissues of the embryo and the endometrial tissues of the mother. This symbiosis of two separate organisms is amazing, both developmentally and immunologically.

 

Trophoblast differentiation

The first differentiation event in the formation of the placenta is the formation of trophoblasts, specialized epithelial cells, that ultimately form a physical connection between the embryo and the uterus. Allocation of cells to the trophoblast lineage is dependent on their position at the morula stage. The cells begin to develop epithelial-like characteristics. The remaining cells migrate to one pole of the embryo, forming the inner cell mass (ICM), which does not begin to differentiate until the first placental structure has begun to form. Trophoblasts close to the ICM continue to proliferate; those that are removed from the ICM stop dividing and become primary trophoblast giant cell (Cross et. al., 1994).

Several transcription factors have been implicated in trophoblast differentiation. Members of the basic helix-loop-helix (bHLH) family of transcription factors, that are involved in the determination of many cell types, have been identified as important in trophoblast differentiation. Mash-2 , and Hxt, have been shown to cause differentiation of non-committed blastocysts into trophoblasts, in sheep and mice. Both maternal and paternal chromosomes are important in this process (Cross, et. al., 1994).

 

Adhesion and invasion of the trophoblast

Six to seven days after fertilization, the embryo, now known as the blastocyst, begins to adhere to the uterine endometrium. Estrogen and progesterone begin to prepare the uterus for implantation (Cross, et. al., 1994). At this point trophoblast cells give rise to a second population of cells that divide in the absence of cytokines. They form a multinucleated syncytiotrophoblast (Gilbert, 1994) which surrounds the entire embryo, and forms projections between the cells of the uterine epithelium. It then spreads along the surface of the basal lamina to form the trophoblastic plate (Carlson, 1994).

The original type of cell in the trophoblast constitutes a layer known as the cytotrophoblast, which adheres to the uterine wall through a series of adhesion molecules. In humans, the cells use proteolytic enzymes to enter the uterus and remodel blood vessels to enable maternal blood and fetal blood vessels to come into contact with each other (Gilbert, 1994). The invasive method of the trophoblasts is so similar to that of a tumor that it has been termed "psuedomalignant". Trophoblast cells cross the basement membranes of uterine epithelial cells and vasculature. Invasion is probably brought about by proteases bound to the trophoblast cell surface, or present in the extracellular environment. The uterus must be receptive to this invasion, yet be able to control it to prevent the formation of malignant chorionic carcinomas (Strickland & Richards, 1992). The uterine blood vessels eventually contact the syncytiotrophoblast. As this happens, mesodermal tissue from the embryo joins the trophoblastic extensions and forms blood vessels. This piece of extraembryonic mesoderm eventually forms the umbilical cord.

In the second week of pregnancy, cytotrophoblastic projections called primary villi begin to form. The extraembryonic mesoderm begins to project into this, creating a secondary villus. When blood vessels begin to enter the mesodermal core, the villus is known as a tertiary villus. By the third week of pregnancy, the newly formed tertiary villi undergo branching, increasing the surface area of the chorion that is exposed to maternal blood. The end of each villus consists of cells of the cytotrophoblast, known as the cytotrophoblastic cell column, covered by a thin layer of syncytiotrophoblast. The cell column expands distally, and penetrates the syncytiotrophoblast layer. The penetrating cells spread over the maternal decidual cells (whose formation is discussed in the next paragraph), forming the cytotrophoblastic shell. Therefore, these villi that connect mother with fetus are known as the anchoring villi. The villi and the outer surface of the chorionic plate of this hemochorial placenta are constantly bathed in maternal blood (Gilbert, 1994). Studies suggest that maternal perfusion of the placenta not only supplies blood to the fetus, but also creates an environment favorable to trophoblast differentiation to become more invasive (Cross et.al., 1994).

Thus, trophoblast tissue and extraembryonic mesoderm form an organ called the chorion, which fuses with the uterine wall to create the placenta. In the deciduous placenta of humans (and most other mammals), the tissues are integrated to the point that they cannot be separated without damaging the mother and the fetus (Gilbert, 1994). This is accompanied by a transformation of the stromal cells of the uterus, known as the decidual reaction. The stromal cells accumulate glycogen and lipid droplets and swell, becoming tightly adherent and forming a cellular matrix surrounding the embryo (Carlson, 1994). They are now known as decidual cells. The decidual tissue directly in contact with the chorion overlying the embryo is the decidua capsularis, the decidua between the chorion and the uterine wall is the decidua basalis. The decidual tissue on the sides of the uterus that are not occupied by the embryo is the decidua parietalis.

The chorion begins to become two distinct parts &emdash; the region containing the chorionic villi, known as the chorion frondosum, and the rest of the chorion, which becomes smooth, known as the chorion laeve. The growth of the chorion causes the decidua capsularis to the pushed further and further away from the endometrial blood vessels, until it begins to atrophy. As it begins to disappear, the chorionic leave comes into direct contact with the decidua pareitalis, until the two fuse by mid-pregnancy. As this happens, the distinction between the chorion frondosum and chorion leave becomes clearer, and the placenta can now be defined. The fetal component of the placenta is the chorion frondosum, consisting of the chorionic plate and its corresponding villi. The maternally derived part of the placenta is the decidua basalis, covered by the fetal cytotrophoblastic shell. The space in between the fetal and maternal placental components contains freely circulating maternal blood (Carlson, 1994).

 

Control of placental development

How does the human uterus regulate and encourage the development of the placenta? One molecule that has been implicated in this process is colony stimulating factor-1 or CSF-1. Normally required for the proliferation and differentiation of mononuclear phagocytes, a 10,000 fold elevation of this molecule was found in the uteri of mice during pregnancy. The trophoblast cells in humans and mice express c-fms mRNA, which encodes the CSF-1 receptor. The fetally derived trophoblast cells are thought to be stimulated to enter DNA synthesis, and thus proliferate, by maternal CSF-1. In situ hybridizations show that the trophoblast cells themselves do not produce CSF mRNA (Pollard et. al., 1987). This is, therefore, an example of a maternal protein inducing fetal tissues. Unlike maternal effect genes in drosophila, the maternal protein does not act after entering the cytoplasm, but stimulates the offspring through a receptor.

CSF-1 presented on the cell surface of uterine epithelial cells in the form of the transmembrane CSF-1 precursor may enable CSF-1-receptor-bearing trophoblast cells to attach to the uterine epithelium. Interestingly, intrauterine CSF-1 concentration is regulated by the action of oestradiol and progesterone, female sex hormones (Pollard et. al., 1987). This may have implications for the normal development of the placenta in the presence of exogenous substances that mimic sex hormones or bind to their receptors. Also, the presence of a substance that mimics progesterone or oestradiol, thereby stimulating the production of CSF-1, may result in the formation of choriocarcinomas, by increasing the invasiveness of trophoblast cells and creating a malignant tumor.

Invasiveness of the trophoblast may also be regulated in another way, by endogenous retroviral sequences in the human genome. As much as 0.1 to 0.6% of the genome is derived from retro-virus sequences. It is unlikely that these would persist in the genome without a positive selecting factor, conferring a better chance of survival on individuals with the viral insert.

Pleiotropin is a secreted heparin binding polypeptide growth factor that induces the release of proteolytic enzymes from endothelial cells. The upregulation of pleiotropin in trophoblast cells would inhance their capability to penetrate into the uterine epithelium. A human endogenous retrovirus type C (HERV-C) is inserted into the pleiotropin gene, between the 5' untranslated end and the coding region. This generates an additional promoter with activity specific to trophoblast cells, and activation is thought to occur early in trophoblast formation. This novel tissue specific promoter is not present in the murine and primate pleiotropin genes, which descended from a common ancestor of the human gene. The resultant increased transcription of the pleiotropin gene in the human trophoblast appears to be responsible for its aggressive and invasive growth into the uterine lining. This increase in invasiveness may also be responsible for chorionic carcinomas (Schulte, et. al., 1996).

For more information on HERVs, check out this website

The low density lipoprotein receptor related protein (LRP) is another molecule that has been implicated in trophoblast invasiveness. Disruption of the LRP gene in mice has been found to block development at the implantation stage. LRP is thought to be involved in clearing protease inhibtor complexes, which would block blastocyst implantation. It binds complexes containing urokinase-type plasminogen activator (uPA), secreted by giant trophoblast cells, which turns plasmin into a potent protease. In the absence of LRP, functional uPA levels decrease. uPA attaches to its receptor on the leading edge of the cell surface, allowing the trophoblast to invade the uterus. The protease is then inactivated by plasminogen activator inhibitor (PAI). LRP then binds to this complex, including the uPA receptor, endocytosing it. Within endosomes in the cell, the complex dissociates, enabling the uPA receptor to be recycled back to the leading edge of the cell where it can bind an active molecule of uPA and continue the cell's invasive process (Herz et. al., 1992).

The invasiveness of cytotrophoblasts is also affected by the production of epidermal growth factor, perhaps because it increases the motility of the cells (Bass et. al., 1994). This is a second example of a maternal factor influencing the development and differentiation of fetal cells. It reiterates the importance of not underestimating the effect of the maternal environment on the fetus. Maternal factors have been shown to play an important role in placental development; we have to be attentive to the possible adverse effects of unnatural chemicals or altered maternal factors fetal development in general, particularly in trying to understand birth defects and the causes of miscarriage.

At implantation, the previously non-adhesive surface of the trophectoderm becomes adhesive. This is mediated by a number of molecules, all of which are not well defined at this time. Thought to play a role are carbohydrate lectin interactions, stabilized by binding of integrins to their extracellular matrix ligands. Trophoblasts also express proteoglycans and other carbohydrate structures thought to be critical in binding (Cross et. al., 1994).

 

Immunology of the placenta

The fetally derived placental tissue is not recognized as foreign and rejected by the maternal immune system. There are several possible factors contributing to this phenomenon, including secretion of factors to suppress local immune responses, selective expression of antigens, and impaired responses to immune activating cytokines in the placental bed of the uterus.

Trophoblasts do not express MHC class II antigens, which are recognized by T-cells. However, if they did not present any MHC at all, they would be destroyed by natural killer cells. Therefore, they do express an MHC class I molecule, HLA-G, without expressing classical class I molecules. The gene encoding HLA-G exhibits limited polymorphism, unlike that of other types of MHC, and therefore, the fetal HLA-G does not differ significantly from the mother, is recognized as self by the NK cells, and the trophoblast cells are left alone. The mechanism for lack of classical MHC expression in trophoblasts is not known, but is thought to be related to a non functional cytokine enhancer for these genes.

The endogenous retroviruses make another appearance in placental immunology. The human endogenous retrovirus-3 (ERV-3) is not expressed in undifferentiated cytotrophoblast cells. However, when differentiation of the syncytiotrophoblast begins to take place, ERV-3 expression is upregulated. It is thought that the ERV-3 protein contains a homologue of p15E, a viral envelope protein expressed by many retroviruses (Boyd, et. al. , 1993). p15E is a transmembrane protein, and has been shown to inhibit lymphocyte responses to alloantigens (Ciancolo, et. al., 1985). The immunosuppression associated with viral infections may, therefore, be in effect in the human placenta, preventing maternal lymphocytes from responding to paternal antigens on the fetus, thus protecting the fetus from rejection by the maternal immune system.

Mouse placental macrophages have been shown to have a decreased ability to present antigen. These fetal cells, which are found in relative abundance in the utero-placental region, exhibit a decreased ability to present whole protein antigens to T-cells. This may reflect an alteration in the antigen-presenting pathway, such as modification in the class II molecules. This may serve as a mechanism to prevent the allogenic response of maternal T cells (Chang, et. al., 1993).

 

Anomalies of placental development

Several anomalies of placental development occur. Preeclampsia, which occurs only in humans, is the result of dysfunctional trophoblast invasion. It is characterized by shallow cytotrophoblast invasion and little uterine arteriole invasion. Although the precise mechanism of preeclampsia is unknown, studies have suggested the action of trophoblasts as the cause. In preeclamptic pregnancies, trophoblasts produce the fibronectin receptor and ECM components at normal levels, but do not up-regulate the integrin that binds to the fibronectin receptor. This restricts their invasive abilities, result in the incomplete trophoblast invasion of preeclampsia (Cross et. al., 1994).

Sometimes, the female pronucleus is absent, but the egg is fertilized and undergoes implantation. A diploid chromosomal number can be derived from the duplication of the male pronucleus. The chorion forms, but the villi have nodular swellings, and the embryo may not develop, or if it does develop to a point, is not viable. There is no vascularization of the villi, and this condition, known as a hydatiform mole is non-invasive (Carlson, 1994).

 

Choriocarcinomas (Picture 1 , Picture 2 )are malignant tumors derived from cytotrophoblast and syncytiotrophoblast. The invasive nature of these tissues can sometimes get out of hand, resulting in a tumor. Most choriocarcinomas contain only paternally derived chromosomes, like hydatiform moles (Carlson, 1994).

The placenta dies when the fetus is born, but although it lives for a brief nine months, it is critical in determining the continuing viability and health of the offspring. An understanding of placental development and the maternal-fetal interactions thereof is useful in understanding the mechanisms of immunity, and gives us clues as to the formation of certain kinds of cancer. More directly, understanding placental development will help us prevent disorders like pre-eclampsia and spontaneous abortions of fetuses. And, if it were not for all these compelling reasons of medical value, the study of the placenta would be valuable solely for curiosity's sake.

To learn about how nicotine and cocain make it across the placental barrier to affect the fetus, look at Haejin's web page.

 

 

References

Bass, K., D. Morrish, I. Roth, D. Bhardwaj, R. Taylor, Y. Zhou, S.J.Fisher. Human cytotrophoblast invasion is up-regulated by epidermal growth factor: evidence that paracrine factors modify this process. Developmental Biology, 164, 550-561(1994).

Boyd, M. T., C.M.R. Bax, B. E. Bax, D. Bloxam, R. Weiss. The human endogenous retrovirus ERV-3 is upregulated in differentiating placental trophoblast cells. Virology, 196, 905 909 (1993).

Carlson, Bruce M. 1994. Human Embryology and Developmental Biology. Mosby-Year Book, Inc., St. Louis, MO.

Chang, M. Y., J. W. Pollard, H. Khalili, S.M. Goyert, & B. Diamond. Mouse placental macrophages have a decreased ability to present antigen. Proc. Natl. Acad. Sci. USA, 90, 462-466 (1993).

Cianciolo, G. J., T.D. Copeland, S. Oroszlan, R. Snyderman. Inhibition of lymphocyte proliferation by a synthetic peptide homologous to retroviral envelope proteins. Science, 230, 453-455 (1985).

Cross, J. C., Z. Werb, S.J. Fisher. Implantation and the placenta: key pieces of the developmental puzzle. Science , 266, 1508-1518 (1994).

Gilbert, Scott F. Developmental Biology. Sinauer Associates, Inc., Sunderland, MA (1994).

Herz, J., D.E. Clouthier, & R. E. Hammer. LDL receptor-related protein internalizes and degrades uPA-PAI-1 complexes and is essential for embryo implantation. Cell, 71, 411-421 (1992).

Pollard, J.W., A. Bartocci, R. Arcreci, A. Orlofsky, M.B. Ladner, &E.R. Stanley. Apparent role of the macrophage growth factor, CSF-1, in placental development. Nature, 330, 484 -486 (1987).

Schulte, A., S. Lai, A. Kurtz, F. Czubayko, A.T. Riegel, A. Wellstein. Human trophoblast and choriocarcinoma expression of the growth factor pleiotropin attributable to germ-line insertion of and endogenous retrovirus. Proc. Natl. Acad. Sci. USA, 93, 14759-14764 (1996).

Strickland, S, & W. G. Richards. Invasion of the trophoblasts. Cell, 71, 355-357 (1992).