GESTATION 



997 



(Fig. 16.30). The value rose from a pre- 

 pregnancy level of 20 enzyme units (E.U.) 

 per gm. of fresh tissue to a maximum of 100 

 E.U. per gm. at approximately the 8th day 

 of pregnancy. This level was maintained 

 until the 12th day and then declined to 

 approximately the prepregnancy level by 

 about the 20th day. Examination of the 

 placentas at this time revealed marked ac- 

 tivity, 68 E.U. per gm. of maternal placenta 

 and 25 E.U. per gm. of fetal placenta. The 

 curve for the concentration of carbonic 

 anhydrase in the uterine mucosa during 

 pscudopregnancy is essentially the same 

 as that seen during pregnancy, although 

 some minor differences exist. 



It is obvious from the above data and 

 from the evidence involving the increased 

 concentration of carbonic anhydrase in the 

 uterine mucosa following treatment with 

 progesterone, that the enzyme is probably 

 under the control of the luteoid hormone. 

 Indeed, an excellent correlation has been 

 shown between the degree of {progestational 

 proliferation in the uterus and the concen- 

 tration of carbonic anhydrase. In the ewe, 

 however, the carbonic anhydrase of the 

 uterus is independent of the ovary. A pos- 

 sible explanation for this discrepancy be- 

 tween the two species has been offered on 

 the basis of differences in the blood level 

 of progesterone. However, no explanation is 

 forthcoming for the failure to maintain 

 the carbonic anhydrase level throughout 

 pregnancy in the rabbit, even though the 

 circulating progesterone remains liigh. 



The significance of this enzyme in the 

 physiology of reproduction is still unknown. 

 From the data on the rabbit, it miglit be 

 inferred that the carbonic anhydrase con- 

 tributes to the maintenance of bicarbonate 

 in the blastocyst fluid. The universal pres- 

 ence of the enzyme in placental tissue could 

 also lead to the assumption that carbonic 

 anhydrase is involved in fetal metabolism. 

 Lutwak-Mann (1955) indicates that the 

 enzyme might be involved in the trans- 

 mission of calcium across the placenta. 

 ^^'hether carbonic anhydrase is essential for 

 fetal (Icvelopment and successful pregnancy 

 is still unanswered. Treatment with car- 

 bonic anhydrase inhibitors (Diamox) 

 failed to affect adversely the pregnancy or 

 fetuses in pregnant rats even though no 



Fid. 16.30. Carhonic anhydrase activity in the 

 uteiu.s of the rabbit during pregnancy, i),seudo- 

 pregnancy, large doses of gonadotrophin, and 

 pregnant mare's serum (PMS). Pregnancy, 

 • •; i^seudopregnancy, O O; gonado- 

 trophin, D D: PMS, x" X. (From C. Lut- 



\val<-Mann. .1. Kndocrinol.. 13, 26. 1955.) 



enzyme acti\'ity was present either in the 

 matei'nal blood or placenta. 



XI. Factors in the Maintenance 

 of Gestation 



A. THYROID GLAND 



Several recent reviews have pointed out 

 that the extract role of the thyroid gland in 

 reproductive physiology is still in need of 

 elucidation (Peterson, Webster, Rayner and 

 Young, 1952; Reineke and Soliman, 1953). 

 Numerous investigations over the past half 

 century have definitely indicated that the 

 thyroid gland is involved in reproduction 

 but the site and manner of action are still 

 not well known. In addition, contradictory 

 reports indicate that each species and even 

 each strain may have to be studied in- 

 dependently (Alaqsood, 1952). Some evi- 

 dence foi- the involvement of the thyroid 

 gland in gestation has already been con- 

 sidered. The increase in FBI at the onset of 

 jiregnancy and the incidence of miscarriage 

 in the human female when the FBI fails to 

 rise tend to involve the thyroid hormone in 

 the maintenance of pregnancy. Habitual 

 abortion in women is usually associated 

 with t'ithcr hypo- or hyperthyroidism 

 (Litzenberg, 1926). Litzenberg and Carey 

 (1929 » I'eported that in 70 married women 

 with low basal metabolic rates appi'oxi- 

 mately 45 per cent had one or more abor- 



