Chorionic Gonadotropin

One of the first hormonal signals from the embryo to the mother prevents mensuration and sustains an endometrial lining that is receptive for implantation. This is achieved by the secretion of a gonadotropin-like glycoprotein hormone, chorionic gonadotropin (CG, designated hCG in humans), produced by the trophoblastic cells of the early embryo. hCG acts on the corpus luteum (CL) to prevent luteolysis and maintain steroidogenic function. This causes progesterone levels to be sustained. Progesterone is required to maintain the endometrium in the secretory state. In nonconceptive cycles, the CL usually regresses at about the second week after ovulation, and the subsequent decline in progesterone leads to menstruation. For pregnancy to be established, CL regression and the associated decrease in systemic progesterone must be prevented. Thus one of the first endocrine interactions between the conceptus and the mother involves signaling by the early embryo to promote intrauterine conditions that will allow implantation and the establishment of pregnancy. This is referred to as the maternal recognition of pregnancy and involves extending the functional lifespan of the CL.

As with other glycoprotein hormones, such as luteinizing hormone (LH) and follicle stimulating hormone (FSH), hCG is a heterodimer composed of α- and β-subunits. The α-subunits of hCG, LH, and FSH are identical, whereas the β-subunits differ, conferring specificity to each hormone. hCG is biologically and immunologically similar to pituitary LH. The α-subunit of hCG is produced by cytotrophoblast cells, especially as they differentiate during implantation. The β-subunit of hCG is primarily produced by the syncytiotrophoblast, but can also be detected in mature cytotrophoblast cells just before they fuse to form the syncytiotrophoblast. The syncytiotrophoblast produces both subunits and is the principal source of functional hCG.

Multiple variants of hCG have been identified in the human placenta. In addition to native hCG produced by the syncytiotrophoblast, extravillous invasive cytotrophoblasts produce hyperglycosylated hCG. The three hCG variants have distinct physiologic functions. Like LH, native hCG is a potent luteotropin, and as such, it stimulates progesterone secretion by the CL. The hyperglycosylated hCG and the monomeric β-subunit are thought to exert primarily extragonadal actions and are associated with malignancies. The actions of all hCG variants are mediated through interaction with the LH receptor.

In an evolutionary context, production of hCG may provide a basis for selection, as pregnancy will not occur if the embryo cannot gain control of the CL. The capacity of the embryo to produce large amounts of hCG may represent a selective test of the embryo’s endocrine competence. The maternal level of resistance to the embryo’s efforts to control the CL would select embryos with more robust endocrine function.

During the first 5 to 7 weeks of pregnancy, the majority of progesterone is produced by the CL in response to hCG. Consequently, the ovaries are obligate organs for pregnancy maintenance during this time, and abortion rapidly ensues if they are removed. However, after weeks 6 to 7 of pregnancy, the placenta begins producing progesterone, and at around the same time, progesterone production by the CL decreases. Thereafter the placenta supplies the bulk of progesterone for the remainder of pregnancy. The transition in the source of progesterone is referred to as the luteal-placental shift .

In normal pregnancies, hCG is detectable 9 to 11 days after the midcycle LH peak, which is around 8 days after ovulation and only 1 day after implantation. Therefore pregnancy can be detected before the first missed menstrual period. This has clinical utility when it is important to determine the presence of pregnancy at an early stage. In early pregnancy, there is an approximate doubling of circulating hCG levels every 2 to 3 days, and concentrations of hCG rise to peak values by 60 to 90 days of gestation. Thereafter hCG levels decrease to a plateau that is maintained during the remainder of the pregnancy ( Fig. 11.3 ). Assayable LH and FSH levels in the maternal blood are virtually undetectable throughout pregnancy.

Schematic representation of concentrations of human chorionic gonadotropin (hCG) and placental lactogen (hPL) throughout gestation.

Note differences in the magnitude of the concentrations of the two hormones in early and late gestation. LMP , Last menstrual period.

The actions of hCG may not be limited to maintaining progesterone production by the CL. Much of the increased thyroid activity that occurs in pregnancy has been attributed to hCG, which binds specifically to thyroid cell membranes and displaces thyroid-stimulating hormone (TSH). hCG also influences the development and function of the fetal adrenals and testes. In addition, hCG may have actions on the maternal reproductive tract, including the decidual response, relaxin production by the CL, and relaxation of uterine smooth muscle. hCG receptors have been detected in fetal membranes and in vitro hCG inhibits spontaneous contractility in strips of human myometrium.

Production of hCG by the invading blastocyst may contribute in a paracrine manner to the implantation process. In vitro and in vivo studies in nonhuman primates demonstrate that hCG promotes decidualization of the endometrial stroma. The hormone appears to have marked effects on endometrial physiology at the implantation site before its levels are detectable in the circulation. Studies suggest that hCG produced by the blastocyst prolongs the window of implantation by inhibiting endometrial insulin-like growth factor binding protein-1 (IGFBP-1) production, augmenting angiogenesis at the implantation site by increasing VEGF expression, modulating local cytokine and chemokine expression, augmenting local protease activity, and, via an autocrine effect on the trophoblasts themselves, promoting differentiation and augmenting invasive potential.

Placental trophoblasts may have an autoregulatory mechanism for hCG synthesis that involves intrinsic hormonal axes analogous to those operating in the hypothalamic-pituitary-gonadal (HPG) axis. Regulators of pituitary gonadotropins can also influence placental hCG production (at least in vitro). These regulators include progesterone, inhibin, activin, and gonadotropin-releasing hormone (GnRH).

Activin and Inhibin

Activin and inhibin are disulfide-linked homo- and heterodimeric proteins belonging to the transforming growth factor-β (TGF-β) superfamily. Inhibin is a heterodimer composed of an α-subunit and one of two β-subunits, βA or βB. Inhibins (αβA and αβB) derive their name from their ability to preferentially inhibit pituitary FSH secretion. In contrast, activins, composed of βA or βB homodimers (βA-βA and βB-βB), stimulate FSH production. Both hormones affect target cell function via specific cell surface receptors. The bioavailability of activin is restricted by follistatin that binds to activin and prevents it from interacting with its receptor on target cells.

Inhibins are produced by the human placenta. All three subunits are expressed in the syncytiotrophoblast, and the levels of expression do not change with advancing gestation. Activin-A is also produced by the CL, decidua, and fetal membranes during human pregnancy. The placenta also produces follistatin. These factors are secreted into the maternal and fetal circulations and amniotic fluid, and their production varies with stage of gestation.

Although the exact function of the inhibin-activin system in human pregnancy is not known, several studies indicate their involvement in the pathogenesis of gestational diseases. Levels of inhibin-A and activin-A in the maternal circulation can be indicative, albeit with relatively weak predictive value, of pathologies such as placental tumors, hypertensive disorders of pregnancy, intrauterine growth restriction, fetal hypoxia, Down syndrome, fetal demise, preterm delivery, and intrauterine growth restriction.

Because inhibin and activin are synthesized by cytotrophoblast cells, they may be involved in autoregulating placental hCG production by modulating local GnRH activity. In vitro studies have shown that inhibin decreases GnRH-stimulated hCG production by placental cell cultures, whereas inhibin antiserum increases GnRH release and causes a parallel rise in hCG secretion. Thus it is possible that inhibin exerts an autocrine/paracrine effect on placental hCG secretion by suppressing GnRH action. In contrast, activin augments the GnRH-induced release of hCG in cultured trophoblast cells, an effect that can be reduced by the addition of inhibin. Thus, at least in vitro, activin and inhibin, via their paracrine effect on placental GnRH production, contribute to the regulation of hCG secretion in a manner similar to their effect on hypothalamic-pituitary gonadotropin secretion.

Gonadotropin-Releasing Hormone

The human placenta produces GnRH, which is identical to that produced by the hypothalamus. Levels of GnRH in the circulation of pregnant women are highest in the first trimester and correlate closely with hCG levels. The close relation between GnRH and hCG suggests a role for GnRH in regulating hCG production. GnRH stimulates the production of both the α-subunit and β-subunit of hCG in placental explants, and specific GnRH-binding sites are present in the human placenta. Thus there appears to be autoregulation of hCG production within the placenta, suggesting that placental GnRH is essential for the establishment and maintenance of early pregnancy.

Corticotrophin-Releasing Hormone and Urocortin

First identified in the hypothalamus, the corticotrophin-releasing hormone (CRH) is a 41-amino acid peptide that stimulates the expression and processing of proopiomelanocortin (POMC) by pituitary corticotropes and the secretion of adrenocorticotropic hormone (ACTH). The human placenta, fetal membranes, and decidua also express CRH that is identical to that produced by the hypothalamus. Expression of placental CRH can be detected from the seventh week of pregnancy and increases progressively until term, rising more than 20-fold in the last 5 to 7 weeks of pregnancy. Actions of CRH are mediated by two CRH receptors (CRH-Rs): CRH-R1 and CRH-R2. These receptors and various subtypes within each group exhibit tissue-specific expression and possibly contribute to differential actions of CRH on different cell types.

Placental CRH is released mainly into the maternal compartment. Levels of CRH in the maternal circulation can be detected as early as 15 weeks of gestation and then increase through gestation reaching maximum levels of 1 to 10 ng/mL at term. Remarkably, this is about 1000-fold higher than peripheral CRH levels in nonpregnant women.

A binding protein (BP) for CRH also exists, and for most of pregnancy, it is present in excess of CRH in the maternal circulation. As the CRH-BP binds CRH with greater affinity than the CRH receptor, it is thought to suppress CRH activity. Thus, for most of pregnancy, the bulk of the placental CRH is considered to be sequestered and rendered inactive by CRH-BP. However, during the last 4 weeks of pregnancy, CRH-BP levels decrease markedly. This coincides with the exponential increase in placental CRH production, which could result in a dramatic increase in CRH biological activity ( Fig. 11.4 ).

Levels of corticotropin-releasing hormone (CRH) and CRH binding protein (CRH-BP) in the maternal circulation during human pregnancy.

(From McLean M, Bisits A, Davies J, Woods R, Lowry P, Smith R: A placental clock controlling the length of human pregnancy. Nature Med 1:460, 1995.)

Despite the elevated concentrations of CRH during pregnancy, maternal ACTH secretion does not increase concordantly. In fact, pituitary ACTH levels remain low throughout pregnancy. The lack of CRH stimulation could be due to inhibition by the CRH-BP. However, maternal ACTH production remains low late in gestation when CRH increases and CRH-BP decreases. In vivo studies have shown that responsiveness of the maternal pituitary to CRH is markedly attenuated during pregnancy, and in vitro studies have shown that CRH downregulates expression of its receptor in pituitary corticotropes.

In vitro studies indicate that agents that increase CRH production by the hypothalamus also increase CRH production by placental cells. These agents include PGs E ₂ and F _2α (PGE ₂ and PGF _2α ), norepinephrine, acetylcholine, vasopressin, angiotensin-II, oxytocin (OT), interleukin-I, and neuropeptide-Y (NPY). In contrast, progesterone and nitric oxide donors inhibit placental CRH expression in vitro.

Interestingly, expression and secretion of CRH by the placenta are increased by glucocorticoids. This is in contrast to hypothalamic CRH expression, which is decreased by glucocorticoids. This stimulatory action has been observed in vivo in women who receive glucocorticoid treatment during the third trimester and in vitro in cultured placental cells. The stimulation of placental CRH production by glucocorticoids may result in a positive feedback endocrine loop. Placental CRH may stimulate ACTH production by the fetal pituitary, which would increase cortisol secretion by the fetal adrenals. Fetal adrenal cortisol could then further stimulate placental CRH production. The marked rise in placental CRH during the last 10 weeks of pregnancy could be due to such a positive feedback interaction. This endocrine loop may be involved in the process of parturition (discussed later in the chapter).

CRH also may influence fetal adrenal steroidogenesis by directly increasing dehydroepiandrosterone sulfate (DHEA-S) production. Interestingly, CRH is as effective as ACTH in stimulating DHEA-S production, but 70% less potent than ACTH in stimulating cortisol production in primary cultures of midgestation human fetal adrenal cortical cells ( Fig. 11.5 ). The capacity for CRH to act as an adrenal androgen secretagogue also has been demonstrated in vivo in adult men. The preferential induction of adrenal androgen synthesis indicates that placental CRH indirectly augments placental estrogen synthesis (discussed later in the chapter). Placental CRH and fetal adrenal DHEA-S increase concordantly during the third trimester, suggesting that production of these two hormones is related. This endocrine axis may play a key role in the regulation of human parturition (discussed later in the chapter).

Effect of corticotropin-releasing hormone (CRH) on cortisol and DHEA-S production in cultured human fetal (midgestation) adrenal cortical cells.

ACTH , Adrenocorticotropic hormone.

(Modified from Smith R, Mesiano S, Chan EC, Brown S, Jaffe RB: Corticotropin-releasing hormone directly and preferentially stimulates dehydroepiandrosterone sulfate secretion by human fetal adrenal cortical cells. J Clin Endocrinol Metab 83:2916–2920, 1998.)

Several actions have been ascribed to placental CRH in the control of human pregnancy. CRH may serve an autocrine-paracrine function within the placenta by regulating expression and processing of POMC. Placental CRH may be part of the fetal-placental stress response mechanism. The placenta is comparable to the hypothalamus in its production of CRH in response to stress. Neurotransmitters and neuropeptides activated in response to stress stimulate placental CRH release in vitro. The physiologic implications of this are that the fetus may mount a stress response via placental CRH. This may be critical in conditions such as preeclampsia, placental vascular insufficiency, and intrauterine infection.

The CRH-family of neuropeptides also includes a group of structurally and functionally related proteins known as urocortins that have a high affinity for both CRH receptors. The urocortins are expressed by syncytiotrophoblast and extravillous trophoblasts, and by the decidua and fetal membranes. Circulating levels of urocortins are low compared with CRH during pregnancy, suggesting that the peptide acts mainly as a paracrine regulator within the gestational tissue. Urocortin stimulates ACTH and PG production by trophoblast cells and causes vasodilation of the uteroplacental vasculature via activation CRH-R2. In women with decreased uterine artery blood flow during midgestation, circulating urocortin levels are reduced in proportion to the increase in uterine artery resistance. Interestingly, placentas from women with preeclampsia have reduced responsiveness to urocortin, suggesting that a principal role of urocortin is to facilitate vasodilation in the fetoplacental circulation. Studies in animal models also show the vasodilatory effects of urocortin on placental blood flow and suggest that one of its main functions is to protect the fetus from hypoxic insults. Studies of term myometrial cells show that urocortin acting via CRH-R2 increases contractility, suggesting that it plays a role in parturition.

Proopiomelanocortin and Proopiomelanocortin-Derived Hormones

Human placenta expresses POMC. In pituitary corticotropes, this 31-kDa glycoprotein is the precursor for the ACTH-endorphin family of peptides. POMC is enzymatically cleaved into several peptide hormones, including ACTH, β-lipotrophic hormone (β-LPH), α-melanocyte-stimulating hormone (α-MSH), and β-endorphin (β-EP). These neuroendocrine hormones play major roles in the physiologic response to stress and the control of behavior. Each of these peptides, including full-length POMC, has been detected in the human placenta.

The syncytiotrophoblast expresses POMC in a transcriptional pattern, similar to that of extrapituitary tumors. However, the processing of POMC in the placenta is different than that in the pituitary. Although some placental POMC is cleaved, a significant amount of intact POMC is secreted by the placenta into the maternal circulation. In contrast, POMC is processed completely in the pituitary and in nonpregnant adults is undetectable in the circulation. However, during pregnancy, maternal circulating POMC levels are readily detectable by the third month and then increase steadily until midgestation, reaching a plateau of around 300 U/mL between 28 weeks and term. Soon after birth, POMC returns to undetectable levels. Unlike its pattern of secretion by the anterior pituitary, POMC produced by the placenta has no diurnal variability, and it is not inhibited by glucocorticoids. Interestingly, during the third trimester, maternal POMC levels closely correlate with plasma CRH levels but do not correlate with plasma ACTH or cortisol levels.

The physiologic role, if any, of placental ACTH and other POMC-derived proteins in the control of human pregnancy remains to be elucidated. With regard to fetal adrenal growth, placental ACTH plays a negligible role, because it is not sufficient to prevent adrenal hypoplasia in fetal hypopituitarism, due to anencephaly. However, placental ACTH may influence maternal physiology and could be responsible for the relative resistance to negative feedback suppression of pituitary ACTH by glucocorticoids during pregnancy.

Other POMC products are produced by the human placenta. Immunoreactive β-EP in the maternal circulation remains relatively low throughout pregnancy and rises during labor and delivery, indicative of the stress of parturition. Factors that increase pituitary ACTH (e.g., hypoxia and acidosis) also increase β-EP production. Endogenous opioids, enkephalins, and dynorphins are also produced by the placenta. Immunoreactive methionine-enkephalin has been found in the human placenta and is chemically identical to the native molecule. Circulating levels of methionine-enkephalin do not change appreciably throughout pregnancy. Three forms of dynorphin have been found in the human placenta. The amount of dynorphin in the placenta at term is similar to that found in the pituitary gland and brain. Relatively high concentrations of dynorphin are found in amniotic fluid and umbilical venous plasma, and maternal plasma levels in the third trimester, and at delivery are higher than in nonpregnant women. Dynorphin binds to kappa opiate receptors, which are abundant in the human placenta. Dynorphin receptor agonists stimulate the release of human placental lactogen (hPL), suggesting that dynorphin exerts local regulatory effects on hPL production.

Human Placenta Lactogen

hPL is a single-chain polypeptide of 191 amino acids, with 96% homology with hGH. hPL can be detected in the placenta from around day 18 of pregnancy and in the maternal circulation by the third week of pregnancy. Low levels of hPL (7 to 10 ng/mL) are present in the maternal circulation by 20 to 40 days of gestation. Thereafter, hPL levels in the maternal circulation increase exponentially, reaching levels of 5 to 10 µg/mL at term. In contrast to the elevated levels of hPL in the maternal circulation, concentrations of hPL in the fetal circulation range from 4 to 500 ng/mL at midgestation and only 20 to 30 ng/mL at term. Thus hPL is preferentially secreted into the maternal compartment.

In normal pregnancy, hPL is first synthesized by the cytotrophoblasts of the developing placenta during the first 6 weeks of pregnancy. Thereafter expression switches to the syncytiotrophoblast, which eventually becomes the exclusive source of hPL. The extent of hPL expression by the syncytiotrophoblast does not change during the course of pregnancy, although total placental production increases substantially. Therefore the rise in hPL production is thought to be due to the increase in placental mass, as maternal hPL levels rise concordantly with the amount of syncytiotrophoblast tissue as gestation progresses ( Fig. 11.6 ). After removal of the placenta, the half-life of the disappearance of circulating hPL is 9 to 15 minutes. To maintain circulating concentrations, this would imply placental production of between 1 and 4 g of the hormone per day at term. Thus production of hPL represents one of the major metabolic and biosynthetic activities of the syncytiotrophoblast. hPL is expressed by all types of trophoblastic tissue. It has even been detected in the urine of patients harboring trophoblastic tumors, in men with choriocarcinoma of the testis, and in the serum and urine of women with molar pregnancies.

Levels of placental lactogen (hPL) in the maternal circulation in relation to placental weight (Pl wt) .

(From Selenkow HA, Saxena BM, Dana CL, Emerson K, Jr: Measurement and pathologic significance of human placental lactogen. In Pecile A, Fenzi C, editors: The foeto-placental unit , Amsterdam Excerpta Medica, 1969, Elsevier Science Publishers, pp 340–362.)

Factors that regulate hPL production have been assessed in cultured cytotrophoblast cells. Insulin and growth hormone-releasing factor (GRF) stimulate hPL secretion, whereas somatostatin (SS) inhibits its secretion. The presence of hPL, human placental growth hormone (hPGH), SS, and GRF in the same cell suggests that another autoregulatory loop analogous to the hypothalamic-pituitary axis may operate within the placenta. In the third trimester, maternal hPL and GRF levels are closely correlated. Interestingly, SS expression is maximal in early pregnancy and decreases during the second and third trimesters, a pattern opposite to that of hPL secretory activity. Thus locally produced GRF and SS may regulate placental hPL expression. Several studies have demonstrated changes in maternal hPL levels in response to metabolic stress. Specifically, prolonged fasting at midgestation and insulin-induced hypoglycemia raise maternal hPL concentrations. However, hPL levels do not change in association with normal metabolic fluctuations during a typical 24-hour period.

Human Placental Growth Hormone

Two forms of hPGH have been identified, both of which are expressed in the syncytiotrophoblast. The smaller, 22-kDa form is almost identical to pituitary GH, differing by only 13 amino acids. The larger 26-kDa hPGH is a splice variant that retains intron 4. The extent of hPGH production is significantly less than that of hPL, and hPGH is not secreted into the fetal compartment. Consequently, circulating levels of hPGH are approximately 1000-fold less than hPL and can be detected in the maternal circulation later in gestation (between 21 and 26 weeks). During the third trimester, maternal hPGH levels increase exponentially, in concert with hPL, and reach a maximum of approximately 20 ng/mL by term. In the first trimester, pituitary GH is measurable and secreted in a highly pulsatile manner. However, pituitary GH production decreases progressively from about week 15, and by 30 weeks cannot be detected. During the same period, nonpulsatile secretion of hPGH by the placenta increases markedly and becomes the GH of pregnancy.

Biological Actions of hPL and hPGH

Studies of hPL and hPGH deficiency have revealed their potential roles in human pregnancy. In all cases of complete hPL deficiency (lack of detectable hPL in the mother’s blood or in the placenta), pregnancy and fetal development were normal. Thus, despite the remarkably high levels of hPL, it is not essential for normal pregnancy. However, deficiency of both hPL and hPGH due to a mutation in the GH/PL gene cluster results in severe fetal growth retardation, but an otherwise normal pregnancy. Pregnancies in which only hPGH is deficient have not been identified. These experiments of nature indicate that hPL is not necessary for normal pregnancy, whereas normal fetal growth is dependent on either hPGH or hPL, and that these hormones comprise a redundant system.

The initial identification of hPL was based in its lactogenic activity in bioassays, suggesting that hPL acts as a lactogen in human pregnancy. Indeed, hPL binds to the prolactin receptor with relatively high affinity (Kd 0.1 nM). In contrast, its affinity for the GH receptor is low (Kd 770 nM). Thus hPL may function mainly as a lactogen during pregnancy, with only minimal activity as a somatogen (promotes growth). However, administration of hPL to nonpregnant women in sufficient quantities to mimic pregnancy levels did not induce lactation. This does not rule out hPL as a lactogen, as its actions on the mammary gland may be dependent on other factors in the endocrine milieu of pregnancy (e.g., estrogen and progesterone). Nonetheless, the in vivo lactogenic properties, if any, of hPL in human pregnancy remains to be established. It should be noted that maternal prolactin levels increase significantly in the later stages of pregnancy and, together with estrogen and progesterone, are likely sufficient to induce mammary growth and lactation.

As hPGH does not enter the fetal compartment, its principal action is likely on the mother. In contrast, hPL is present in the mother and the fetus. However, deficiency in hPL alone has no effect on fetal growth, whereas deficiency in both hPL and hPGH is associated with severe fetal growth retardation. Taken together, these observations indicate that hPL and hPGH influence fetal growth through effects on the mother. This is consistent with the thesis that hPL and hPGH modulate maternal metabolism to meet fetal energy requirements.

Maternal food intake and intestinal calcium absorption increase during the first trimester, and insulin secretion immediately after feeding almost doubles. Because maternal sensitivity to insulin is maintained in the first half of pregnancy, the increased insulin increases lipogenesis and maternal fat deposition. However, as pregnancy progresses, the fetus increases its nutritional requirements, which leads to an increased functional role for hPL and hPGH. In the second half of pregnancy, maternal cells become increasingly resistant to insulin (i.e., a diabetogenic state), which is thought to be promoted by the combined GH-like and contra-insulin activity of hPGH and hPL. This decreases glucose uptake and increases free fatty acid release. The net effect is increased availability of free fatty acids, glucose, and amino acids for fetal consumption. The decreased maternal glucose consumption would ensure a steady supply of glucose for the fetus, which is especially important for the fetal brain, which uses glucose exclusively as an energy source. Free fatty acids can cross the placenta, and the increased ketones induced by their metabolism are an important energy source for the fetus. Thus, during the second half of pregnancy hPGH and hPL, direct maternal metabolism toward mobilization of maternal energy resources to furnish the needs of the developing fetus. Soon after birth, insulin resistance reverts to the normal nonpregnant state, suggesting that maternal glucose homeostasis is influenced by hormonal factors produced by the fetus-placenta.

This fetal-maternal hormonal interaction represents an example of fetal-maternal conflict whereby the fetus, through natural selection, acquires traits (e.g., placental somatotropin production) that favor the extraction of resources from the maternal organism. Conversely, mothers have evolved mechanisms to counteract fetal demand. Disorders on either side of this equation lead to pathophysiologies of pregnancy. For example, in women with gestational diabetes, insulin secretion is insufficient to balance the decrease in insulin sensitivity, and consequently blood glucose levels increase, leading to an oversupply of glucose to the fetus resulting in macrosomia.

Insulin-like Growth Factors

The human placenta produces insulin-like growth factor-I (IGF-I), expressed by the syncytiotrophoblast, and IGF-II, expressed by cytotrophoblasts, syncytiotrophoblast, and extravillous trophoblasts, from as early as week 8 of human pregnancy.

Gene deletion studies in mice have shown that IGF-I and IGF-II are key regulators of placental and fetal growth. The studies suggest that IGF-I is important for fetal and postnatal growth, whereas IGF-II is essential for placental and fetal growth. Pups of IGF-II knockout mice exhibit fetal growth restriction but normal growth after birth, whereas pups of IGF-I knockout mice have poor postnatal growth and die before reaching adulthood. Specific inhibition of placental IGF-II expression inhibits placental and fetal growth, demonstrating the key role of IGF-II during fetal development.

IGFs in the circulation are bound to six GF binding proteins (IGF-BPs) that control IGF function by sequestering circulating IGFs and controlling their bioavailability to interact with receptors on target cells. During pregnancy, IGFBP-1 is produced by the decidua, and all of the IGFBP-1 in amniotic fluid is maternally derived. Overexpression of decidual IGFBP-1 inhibits placental and fetal growth, primarily by sequestering placental IGF-II. Interestingly, IGF-II is an imprinted gene that is expressed only by the paternal allele. In general, genes expressed only by the paternal allele promote fetal/placental growth, whereas those expressed only by the maternal allele suppress fetal growth. Loss of IGF-II imprinting is associated with fetal growth restriction that is thought to be secondary to placental dysfunction. In Beckwith-Wiedemann syndrome, abnormal methylation of the imprinting centers on chromosome 11 leads to additional expression of IGF-II from maternal allele, which is normally suppressed. The extra IGF-II produced in this condition causes fetal macrosomia.

The finding that a paternal gene promotes placental growth supports the concept of genetic conflict between the maternal and fetal genomes. The size and ultimate health of the fetus depends greatly on the size of the placenta. Growth factors that increase placenta size are an advantage to the fetus because they allow it to more efficiently extract resources from the mother. Passage of paternal genes to the next generation is favored if nutrient supply to the fetus is maximized. Maternal genes, on the other hand, not only must survive to the next generation, but they also must ensure that the current pregnancy does not compromise the mother’s future reproductive capacity. Maternal genes (e.g., IGF-BP1) would therefore be selected to oppose the effects of paternally imprinted genes such as IGF-II.

Epidermal Growth Factor Family

The EGF family hormones include EGF, heparin-binding EGF-like growth factor (HB-EGF), amphiregulin, betacellulin, epiregulin, and TGFα. HB-EGF is expressed by the syncytiotrophoblast and cytotrophoblast cells, and this is relatively high in first-trimester and decreases with advancing gestation. EGF-family growth factors are thought to promote trophoblast invasion during implantation and deficiency in EGF expression, and/or signaling is associated with preeclampsia and fetal growth restriction.

Vascular Endothelial Growth Factor Family

The VEGF family of peptides comprises VEGF-A (referred to as VEGF), VEGF-B, VEGF-C, VEGF-D, VEGF-E (generated by alternative splicing from a single gene), and four splice variants of a factor produced by trophoblast cells known as placenta growth factor (PlGF). These peptides are mitogenic and antiapoptotic for endothelial cells and promote the formation of new blood vessels via angiogenesis and vasculogenesis, and increase tissue perfusion via vasodilatation and increased microvascular permeability. Actions of VEGFs and PlGFs are mediated by two tyrosine kinase receptors FMS-like tyrosine kinase-1 (FLT-1) and kinase insert domain receptor (KDR). A soluble form of FLT-1 (sFLT-1) also exists, which is present in the circulation and binds VEGFs and PlGFs to neutralize their angiogenic activity.

Remodeling of the uterine spiral arterioles, and continued angiogenesis through pregnancy to match placental growth, with appropriate maternal vascular supply, are critical for the success of pregnancy. These events are affected by VEGFs and PlGFs expressed by cytotrophoblasts, the syncytiotrophoblast and villous stromal cells in the placenta, and VEGF/PlGF receptor expressed by endothelial cells at the placental/maternal interface. The placental VEGF/PlGF system is linked to pregnancy complications, especially preeclampsia, which is caused by placental factors affecting maternal hemodynamic homeostasis. The etiology of preeclampsia is thought to involve placental hypoxia, due to decreased invasion of trophoblasts into the maternal spiral arteries early in pregnancy, leading to compromised placental perfusion later in gestation, which is thought to induce compensatory mechanisms involving the VEGF/PlGF system to increase placental perfusion by modulating maternal hemodynamics. One mechanism for this is that hypoxia increases placental expression of VEGF, which, via KDR, increases trophoblast expression of sFLT1, leading to increased levels of sFLT1 in the maternal circulation. In pregnancies complicated by preeclampsia, circulating sFLT1 levels in the mother are increased compared with normal pregnancies and sFLT1 levels have biomarker potential to assess risk for preeclampsia and discriminate between normal pregnancies and those that will eventually develop preeclampsia. Excess sFLT1 in the maternal circulation may disrupt maternal endothelium function, leading to the symptoms of preeclampsia, especially hypertension and proteinuria. Thus the placental VEGF/PlGF system appears to play a key role in maintaining maternal vascular function to provide placental perfusion necessary for normal fetal/placental development and the compensatory mechanisms in response to inadequate placental perfusion that can affect the maternal vasculature, leading to complications such as preeclampsia.

Fibroblast Growth Factor Family

Like VEGF, the FGF family (there are 23 members designated FGF-1 to -23) of polypeptides are mitogens that affect endothelial cell migration and promote formation of blood vessels. Effects of FGFs are mediated by four specific high-affinity cell surface FGF receptors (FGFR-1 to -4). The human placenta expresses mainly FGF-2 and FGF-10 that localizes to villous trophoblasts and connective tissue stroma of mesenchymal villi. Interestingly, each of the FGFRs are expressed by Hofbauer cells, mononuclear phagocytes that play a key role in tissue injury repair, inflammation, atherogenesis, and tumor development. FGF induces Hofbauer cells to produce multiple growth factors and cytokines involved in tissue repair. Activation of Hofbauer cells by FGF may promote placental growth by facilitating the outgrowth of syncytiotrophoblast buds. It is also possible that FGF via the placental Hofbauer cells contributes to tissue repair within the placenta during pregnancy.

Placental Adipokines

Adipokines are factors produced by adipose tissue that affect metabolic homeostasis, satiety, and reproduction. Currently known adipokines include leptin, adiponectin, resistin, and ghrelin. Adipokines produced by the placenta regulate the maternal metabolic adaptation to pregnancy, especially increased insulin resistance.

Leptin, a 146-amino acid protein produced primarily by adipocytes, is a key regulator of satiety and body mass index, and its levels are thought to reflect the amount of energy stores and nutritional state. Leptin decreases food intake and body weight via its hypothalamic receptor. In the reproductive system, leptin is thought to coordinate body mass status with reproductive function. In general, leptin acts as a permissive factor. Pulsatile hypothalamic GnRH secretion does not occur unless leptin levels reach a threshold value. Such a mechanism may ensure that energy stores are sufficient to support a pregnancy.

The placenta is the principal source of leptin during pregnancy. Leptin produced by the placenta is secreted mainly into the maternal circulation, and as a consequence leptin levels are elevated during pregnancy. In the first trimester, maternal plasma leptin levels double nonpregnant values and continue to increase during the second and third trimesters, during which time they are also expressed by the chorion and amnion. Leptin levels decline to normal nonpregnant values within 24 hours of delivery.

The influence of placental leptin on maternal biology is unclear. Abnormally high placental leptin production is associated with maternal diabetes mellitus and hypertension, and umbilical leptin levels correlate with fetal adiposity. Interestingly, leptin levels during pregnancy do not correlate with body mass index as they do in the nonpregnant state. Pregnancy appears to be a state of hyperleptinemia and leptin resistance, with uncoupling of leptin effects on eating behavior, satiety, and metabolic activity. Leptin is lipolytic and favors fatty acid mobilization from adipose tissue. It also may act in the liver, pancreas, and muscle to decrease insulin sensitivity and mobilize glucose. The human placenta expresses leptin receptors, and therefore leptin can act in a paracrine manner to modulate placental function. Leptin induces hCG production in trophoblast cells and is also thought to increase placental growth by augmenting mitogenesis, amino-acid uptake, and extracellular matrix synthesis.

Adiponectin, resistin, and ghrelin are also produced by the placenta and secreted into the maternal and fetal compartments. As with leptin, these adipokines appear to affect maternal metabolic homeostasis in favor of pregnancy and the supply of nutrients to the fetus.

Progesterone

The human placenta produces large amounts of progesterone throughout pregnancy. It does this mainly by converting low-density lipoprotein cholesterol, extracted from the maternal circulation, to pregnenolone, which is then converted to progesterone. Cholesterol is converted to pregnenolone in the mitochondria of trophoblast cells by the cytochrome P450scc enzyme. This is the rate-determining step for placental progesterone production. The conversion of pregnenolone to progesterone also occurs in the mitochondria and is catalyzed by type-1 3β-hydroxysteroid dehydrogenase (3βHSD-I). The human placenta is incapable of converting progesterone to 17α-hydroxyprogesterone. Production of progesterone approximates 250 mg/day by the end of pregnancy, at which time circulating levels are on the order of 130 ng/mL.

As its name implies, progesterone is a progestation hormone and has been aptly called the “hormone of pregnancy” because it is essential for the establishment and maintenance of pregnancy. In all viviparous animals examined so far, disruption of progesterone synthesis or action during pregnancy induces pregnancy termination or labor and delivery. The role of progesterone in the maintenance of pregnancy and the mechanism by which its actions are withdrawn to trigger parturition are discussed later in this chapter.

Estrogens

The principal role of estrogens in human pregnancy is to stimulate uterine growth and increase uterine blood flow. Estrogens also affect breast development in preparation for lactation. At parturition, estrogens oppose the actions of progesterone by augmenting uterine contractility and inducing cervical softening.

Estrogens are synthesized by the human placenta from C19 steroids. The principal precursor used for placental estrogen formation is DHEA-S, supplied mainly by the fetal adrenal glands. Because the placenta has an abundance of the sulfatase (sulfate-cleaving) enzyme, DHEA-S is rapidly converted to free (unconjugated) DHEA, which is then converted to androstenedione by 3βHSD-I. The human placenta also expresses high levels of the aromatase enzyme, which converts androstenedione estrone. The 17β hydroxysteroid dehydrogenase (17βHSD) enzymes then interconvert estrone and estradiol ( Fig. 11.8 ).

Biosynthesis of progesterone and estrogens by the human placenta.

Progesterone is produced mainly from maternal cholesterol. P450c17 is not expressed in the human placenta, and therefore, progesterone cannot be converted to C19 androgens. Instead, estrogens are biosynthesized from C19-androgen precursors (mainly DHEA-S) provided by the maternal and fetal adrenals. ACTH , Adrenocorticotropic hormone; CRH , corticotrophin-releasing hormone; DHEA-S , dehydroepiandrosterone sulfate; HSD , hydroxysteroid dehydrogenase; OH-DHEA-S , hydroxydehydroepiandrosterone sulfate.

(From Mesiano S: Roles of estrogen and progesterone in human parturition. Front Horm Res 27:86, 2001.)

The major estrogen formed during human pregnancy is estriol, which has an additional hydroxyl group at position 16. Estriol constitutes more than 90% of the estrogen in pregnancy urine, into which it is excreted as sulfate and glucuronide conjugates. Estriol production by the placenta increases with advancing gestation, and ranges from approximately 2 mg/24 hours at 26 weeks to 35 to 45 mg/24 hours at term. At term, the concentration of estriol in the maternal circulation is 8 to 13 ng/dL. In contrast, ovarian production of estriol in nonpregnant women is barely detectable.

Placental estriol is formed by a biosynthetic process unique to human (and higher primate) pregnancy, that begins with DHEA-S from the fetal adrenal glands. When DHEA-S of either fetal or maternal origin reaches the placenta, estrone and estradiol are formed. However, little of either is converted to estriol by the placenta. Instead, some of the DHEA-S undergoes 16α-hydroxylation to form 16α-hydroxyDHEA-S, primarily in the fetal liver and, to a limited extent, in the fetal adrenal. In the placenta, that sulfatase enzyme converts 16α-hydroxyDHEA-S to 16α-hydroxyDHEA, which is then aromatized to form estriol. Thus levels of estriol in the maternal blood reflect the steroidogenic activity of the fetal HPA. In the mother, estriol is conjugated to form estriol sulfate and estriol glucosiduronate in the liver and excreted via the urine. The function of estriol in pregnancy has attracted much speculation. In most biological systems, estriol is a weak estrogen, with approximately 1% the potency of estradiol and 10% that of estrone. However, in pregnancy, the uterus estriol is as effective as the other estrogens in increasing uteroplacental blood flow.

Another placental estrogen derived from a fetal precursor is estetrol, formed after 15 and 16-hydroxylation of DHEA-S. Its function is not known. Relative levels of progesterone and estrogens (estrone, estradiol, estriol, and estetrol) in the maternal circulation during human pregnancy are shown in Fig. 11.9 .

Schematic depiction of maternal progesterone and estrogen (estradiol, estrone, and estriol) concentrations through human pregnancy compared with average levels in normal cycling women.

Production of estrogens by the human placenta is extraordinarily high, and circulating levels during pregnancy are several orders of magnitude higher than physiologic levels in nonpregnant women. A key question, therefore, is why does the human placenta make so much estrogen? Conditions in which placental estrogen synthesis is markedly decreased (e.g., anencephaly, congenital adrenal lipoid hyperplasia, placental aromatase deficiency, and placenta sulfatase deficiency) have provided some insight into the role of the placental estrogens in human pregnancy. Although pregnancy was prolonged in some cases of placental sulfatase deficiency and anencephaly, in most cases, a marked decrease in placental estrogen synthesis had little effect on fetal and placental development and the timing of parturition. Likewise, pregnancies with lowered estrogen levels due to placental aromatase deficiency appear normal, although mothers and female fetuses exhibited virilization signs due to androgen excess. Thus high levels of estrogens of placental origin may not be essential for normal pregnancy and parturition, but rather a critical role of placental aromatase is to metabolize circulating androgens to protect the female fetus and mother from virilization.

These observations do not exclude a role for estrogen in the control of human pregnancy. Although levels of maternal estrogens in pregnancies with placental estrogen synthesis defects were low compared with normal pregnancies, they were still in a physiologically significant range (1 to 1.6 nmol/L) and comparable with levels reached in the midcycle and luteal phase of the menstrual cycle (0.6 to 2 nmol/L). Thus, despite the inability of the placenta to produce estrogens in certain abnormalities such as aromatase deficiency, the estrogen target tissues were still exposed to moderately high levels of estradiol. This suggests that a minimal level of estrogen is necessary for human pregnancy, and that the excessive levels produced by the placenta are redundant.

Placental Glucocorticoid Metabolism

For most of gestation the placenta expresses the 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD-2) enzyme, which inactivates cortisol by catalyzing its conversion to cortisone. As maternal cortisol levels are 3 times those of the fetus, this biochemical barrier serves to prevent excess cortisol from entering the fetal compartment. This is important because exposure of the fetus to high levels of cortisol not only could interfere with the fetal hypothalamic-pituitary axis, but also is associated with decreased birth weight and hypertension.

Studies in the baboon placenta suggest that estrogen upregulates 11β-HSD-2 expression in the placenta. Because estrogen synthesis by the placenta requires fetal adrenal androgen precursors (produced in part by fetal pituitary ACTH stimulation), the estrogen regulation of placental 11β-HSD-2 represents a regulatory loop that ensures that maternal cortisol does not affect the fetal HPA axis.

Gonadotropin-Releasing Hormone

The human fetal hypothalamus produces immunoreactive and bioactive GnRH by 10 weeks of gestation. Between 10 and 22 weeks, the concentration of GnRH in the fetal hypothalamus remains constant (0.27 to 13.1 pg/mg) and is not different between the sexes. GnRH release from human fetal hypothalamic explants is pulsatile. In male rhesus monkey fetuses, the pituitary-gonadal axis is active during the last third of gestation. The pituitary produces LH in response to GnRH, and the testes produce testosterone in response to LH. A similar situation likely exists in the human male fetus. Whether this also is the case for the female fetus is unresolved.

Thyrotropin-Releasing Hormone

Significant levels of immunoreactive TRH are found in the human fetal hypothalamus early in gestation. As with GnRH, fetal hypothalamic TRH levels do not correlate with sex or gestational age. The presence of TRH in the fetal hypothalamus in early gestation and midgestation suggests its role in the regulation of TSH, and possibly prolactin (PRL), secretion.

Growth Hormone-Releasing Hormone and Somatostatin

GHRH can be detected in fetal hypothalamic neurons and fiber tracts at 18 weeks, with increasing levels found up to 30 weeks. The simultaneous detection of GHRH in neuron cell bodies and fiber tracts indicates its release into portal vessels by midgestation. Immunoreactive SS can be identified in the hypothalamus of human fetuses from 10 to 22 weeks. In contrast to GnRH and TRH, fetal hypothalamic SS increases with advancing gestational age.

Corticotropin-Releasing Hormone and Arginine Vasopressin

CRH is a potent secretagogue for ACTH and β-endorphin release by the human fetal pituitary gland. Arginine vasopressin (AVP) also directly stimulates ACTH secretion by the fetal pituitary and can synergize with CRH. CRH-immunoreactive fibers can be detected in the median eminence between 14 and 16 weeks. CRH immunoactivity and bioactivity and AVP immunoactivity have been detected in human hypothalamic extracts from 12 to 13 weeks.

Hypothalamic CRH and AVP increase with gestational age, and the CRH bioactivity of fetal hypothalamic extracts, measured in isolated rat anterior pituitary cells, is augmented by AVP. Thus the human fetal hypothalamus has the capacity to regulate pituitary ACTH production from early in the second trimester. The extent to which fetal hypothalamic CRH and AVP and placental CRH interact in regulating fetal ACTH release remains uncertain.

Catecholamines and Dopamine

Catecholamines in cells projecting from the arcuate nuclei to the internal and external layers of the median eminence appears during the interval from 12 to 16 weeks. Dopamine is present in the fetal hypothalamus at weeks 11 to 15 at a concentration twice that of the adult. Hypothalamic dopamine inhibits PRL release from fetal the pituitary during this time.