J Clin Pathol: first published as 10.1136/jcp.s1-7.1.31 on 1 January 1976. Downloaded from J. clin. Path., 30, Suppl. (Ass. Clin. Path.), 7, 31-35 Secretion of pituitary-like peptides by the human placenta LESLEY H. REES From the Department of Chemical Pathology, St Bartholomew's Hospital, London ECIA 7BE Evidence for the secretion of protein hormones by trophoblast. The latter consists of an outer syncytial the placenta is conclusive only for human chorionic layer, the syncytiotrophoblast, and an inner layer, gonadotrophin (hCG) and human placental lactogen the cytotrophoblast, which disappears late in (hPL). Nevertheless, there is some evidence that pregnancy. The syncytiotrophoblast is differentiated material resembling pituitary thyroid stimulating into thin areas overlying fetal capillaries and thicker hormone (TSH) and a peptide resembling adreno- areas not adjacent to capillaries which are probably corticotrophin (human chorionic corticotrophin) responsible for the synthetic functions of the may also be secreted. placenta. Studies using fluorescent antibodies have shown that hCG is probably synthesised and secreted Human chorionic gonadotrophin by the syncytiotrophoblast (Midgley and Pierce, 1962). Thiede and Choate (1963) suggested that hCG STRUCTURE synthesis may begin in the cells of the cytotropho- Ascheim and Zondek (1927) first demonstrated the blast before their transition into the syncytium, but presence of gonadotrophic substances in the urine in a more recent study of placentas obtained between copyright. of pregnant women. Such urine injected into 6 and 41 weeks gestation the hormone was demon- infantile mice induced premature oestrus with follicle strated only in the syncytial layer and a few cells of ripening and luteinisation. hCG is a glycoprotein the amniotic epithelium (de Ikonicoff and Cedard, structurally related to the pituitary hormones TSH, 1973). FSH, and LH and comprises two subunits (Reichert The concentration of hCG in maternal serum et al., 1970; Pierce and Liao, 1970). The a-subunit reaches a peak around the 8th to the 10th week of (92 amino-acids) shows extensive structural sequence gestation and the normal trophoblast secretes around http://jcp.bmj.com/ homology with FSH, LH, and TSH, and the f- subunit (145 amino-acids) is similar to that of LH apart from an extra 30 C-terminal amino-acids Ancestral hormone (Swaminathan and Bahl, 1970). The carbohydrate content comprises over 20% of the molecule (Diczfalusy and Troen, 1961) and is essential for biological activity (Whitten, 1948). Isolated a- and fl-subunits possess very little on September 27, 2021 by guest. Protected intrinsic biological activity but, because of the close c(-type chain ,8-type chain structural similarities, an a-subunit of hCG can be combined with the fl-subunit of LH or TSH with restoration of a high degree of biological activity 3 -qgonad r w n appropriate to the fl-subunit (Pierce, 1971). In- | thyrotropin,8- n terestingly, the fl-subunits of all the glycoprotein hormones show a significant degree of homology, Common suggesting that they evolve from a common ancestral cC-chain P-LH ,-FSH P -TSH gene (Pierce. 1971; Acher, 1976) (Fig. 1). Fig. 1 Hypothetical evolution of the dimeric hormones luteinising hormone (LH), follicle stimulating hormone SITE OF SYNTHESIS (FSH), and thyrotrophin (TSH). Duplication ofan In the early placenta maternal blood circulates in ancestral structural gene and subsequent differentiation the intervillous space and bathes the chorionic villi. led to an a-type chain and a P-type chain. Successive The chorionic villus comprises a core of fetal duplications of the P-gene and selective evolution gave mesenchyme and fetal capillaries outside which is the the specific P-chains of three hormones (Acher, 1976). 31 J Clin Pathol: first published as 10.1136/jcp.s1-7.1.31 on 1 January 1976. Downloaded from 32 Lesley H. Rees 1.4 x 10-2 IU hCG/day. The trophoblast neoplasm, .. choriocarcinoma, has much reduced hCG synthetic capability both in vivo and in vitro. The introduction of highly sensitive and specific radioimmunoassays and radioreceptor assays for hCG (Vaitukaitis et al., 1972) have clearly demon- strated secretion of the hormone within a few days of conception (the basis of an early pregnancy test) and in-vitro studies using placental slices have shown that isolated a- and fl-subunits are secreted as well as intact hCG (Franchimont et al., 1972). BIOLOGICAL FUNCTIONS OF hCG . : .. -.. The biological role of hCG is unknown. Various W 4 8k 12 .1.6. 21... 4 hypotheses put forward include stimulation of the WW;s ofpregna,ncy corpus luteum to maintain progesterone output, a Fig. 2 Immunoreactive HCG, testosterone, and LH in possible direct effect on the fetal testes causing sexual the fetal circulation in relationship to gestational age. differentiation, and depression of maternal immunity preventing rejection of the fetus. Early progesterone secretion by the maternal hyperthyroidism, however, does occur in women ovary is believed to prevent expulsion of the ovum with trophoblastic neoplasia (Odell et al., 1963) by 'damping down' spontaneous uterine contractility thus providing indirect evidence for a thyroid through a direct membrane effect rendering the stimulating role of hCG. Kenimer et al. (1975) have uterine muscle insensitive to circulating endogenous shown that on a molecular basis hCG contains about oxytocin (progesterone block) (Bengtsson and 1/4000 of the thyrotrophic activity of human pitui- Schofield, 1963). Thus in some species such as the tary TSH. copyright. sheep progesterone levels fall precipitously before the onset of labour. This fall may facilitate an CLINICAL USES OF hCG increase in concentration of uterus-stimulating Over the years hCG has had important therapeutic prostaglandins, particularly PGF2a, into the utero- and diagnostic roles in reproductive endocrinology. ovarian vein blood (Liggins et al., 1973). Such a Thus injection of purified hCG has been used to clear cut relationship does not exist in man, however, induce ovulation in anovulatory women, and the and an inhibiting effect of progesterone has not been specific measurement of serum hCG by radioim- http://jcp.bmj.com/ observed (Fuchs and Stakemann, 1960). Nevertheless, munoassay can now be used to diagnose pregnancy Csapo et al. (1971) found significantly lower pro- within a few days of fertilisation. Sensitive specific gesterone levels in women whose labour was short. hCG measurements can also be used to monitor the The evidence suggests that progesterone, like mater- treatment of choriocarcinoma (Bagshawe, 1969), nal oestradiol, is more likely to play a facilitatory and this, in combination with advances in chemo- role in labour than to be the initiating agent (Turn- therapeutic regimes, has dramatically reduced bull et al., 1977). This whole field has been the subject mortality of this otherwise uniformly lethal disease. on September 27, 2021 by guest. Protected of a recent Ciba Foundation Symposium (No. 47, Finally, the possibility remains, albeit remote, that New Series). hCG has been implicated in the sexual immunisation against hCG could be used to control differentiation of the male fetus but the peak of fetal human fertility (Stevens, 1975). plasma testosterone does not coincide with the hCG peak or with the fetal pituitary gonadotrophin peak, Human placental lactogen the latter occurring between the 16th and the 24th week of gestation (Fig. 2). STRUCTURE hCG also has TSH-like activity and high con- Josimovich and MacLaren (1962) found a substance centrations can be shown to interfere with the bind- in human placenta and in fetal and maternal serum ing of labelled TSH to human and animal thyroid which had lactogenic activity in vivo and in vitro and membranes and thus, by inference, to bind to the they showed its partial immunological identity with TSH receptor or a closely allied site (Hall, 1977). It human growth hormone (hGH) of pituitary origin. has not so far been shown to activate adenylate This substance was therefore called human placental cyclase in the human thyroid and it has not been lactogen (hPL). However, both its somatotrophic and possible to administer it in sufficiently high doses to lactogenic actions are very weak (Florini et al., 1966) examine the other aspects of TSH activity. Clinical although about 80% of the amino-acid residues of J Clin Pathol: first published as 10.1136/jcp.s1-7.1.31 on 1 January 1976. Downloaded from Secretion ofpituitary-like peptides by the human placenta 33 hPL and hGH are identical, with close similarities 10 in their sequences. These two hormones also show considerable homology with human prolactin, which 9 suggests that all three hormones evolve from a common ancestral polypeptide. 8 SITES OF SYNTHESIS AND SECRETION 7 Josimovich and Atwood (1964) showed that hPL could be extracted from placental tissue but not b from pituitary tissue of pregnant and non-pregnant 5 women or from the fetus. hPL was later localised by a- immunofluorescent techniques to the syncytiotropho- _J blast (Sciarra et al., 1963), though some doubt has 4 been cast on these findings (Gau and Chard, 1976). Studies using cultured fragments of placentas taken at various stages of gestation show that hPL is present in the supernatant medium (Grumbach and 2 Kaplan, 1964), and specific incorporation of 14C_ labelled amino-acids into hPL by placental frag- ments taken at term has been clearly shown by radio- immunoelectrophoresis (Gusdon and Yen, 1967). 16 20 24 28 32 36 46 44 In the last trimester of pregnancy the amount of Weeks of gestation hPL synthesised by the placenta is of the order of 1 Fig. 3 Immunoreactive human placental lactogen levels g per day (Kaplan et al., 1968), a rate of production throughout normal pregnancy showing the mean and greatly in excess of that of any other peptide hor- two standard deviations. Note that the distribution is mone. Thus a woman in the last month of pregnancy skewed, being greater above the mean than below it copyright.