Book - Physiology of the Fetus 7
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Windle WF. Physiology of the Fetus. (1940) Saunders, Philadelphia.
1940 Physiology of the Fetus: 1 Introduction | 2 Heart | 3 Circulation | 4 Blood | 5 Respiration | 6 Respiratory Movements | 7 Digestive | 8 Renal - Skin | 9 Muscles | 10 Neural Genesis | 11 Neural Activity | 12 Motor Reactions and Reflexes | 13 Senses | 14 Endocrine | 15 Nutrition and Metabolism | Figures
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Chapter VII The Fetal Digestive System
As we find elsewhere in studying the nature of vital activities before birth the new individual is prepared for postnatal functions far in advance of the time of need. Nowhere is this better illustrated than in the digestive System. Motor activities associated with obtaining and assimilating food are present in prematurely delivered human infants and in late fetal life of many other animals. The digestive secretions appear to be supplied at this time although information concerning them is meager. 0n the other hand much has been learned about fetal swallowing and gastrointestinal movements in recent years.
That amniotic fluid is swallowed by the fetus and that it may even serve some useful function during intrauterine life is not a new idea. Preyer 1 summarized the view held in 1885 when he wrote: «That amniotic fluid is a food for the fetus is certain but if it is not swallowed plentifully it contributes little to nourishment beyond mere water feeding." swallowing was inferred be— cause of the presence of squamous epithelial cel1s, lanugo hair and vernix caseosa in the digestive tract before birth.
Preyer reviewed experiments which had been performed to determine the source of amniotic Iiuid but which incidentally provided some evidence of fetal swallowing. certain chemicals administered by mouth or by injection into the maternal blood stream appeared later in the amniotic fluid although not in the fetal tissues. However, .urine samples col1ected from the infants after birth often contained these substances. It was assumed that the fetus had swallowed the amniotic fluid.2-3
Others have demonstrated that rabbit and dog fetuses swallow actively and that absorption of the swallowed fluid takes place readily through the gastric and intestinal epithelia. 6 Two or three hours after injecting calcium ferrocyanide into the amniotic cavity it was possible to induce the prussian blue reaction in all the fetal tissues, especially in the stomach, intestines, kidneys and skin. More recent1y injections of colloida1 dyes into the amniotic cavity in guinea pigs and cats have demonstrated that absorption talces place through the gastroäntestinal mucosa and respiratory epithelium as well as through the amnion itselfJ Although one must conclude that the fetuses did swallow the amniotic Huid it was not proved that swallowing is a normal physiologic function of the fetus. The experiments were conducted under general anesthesia and under conditions which may have caused a.certain amount of anoxemia in the fetuses which in turn could have induced the swallowing.
An ingenious method for treating patients suffering from the efkects of the distention of the abdomen attendant upon polyhydramnios has been described.8 The fetus was apparently induced to swallow more actively by« sweetening the amniotic Huid. This was accomplished under local anesthesia by injecting saccharine solution directly through the maternal abdomen into the amniotic cavity. Girth of the abdomen diminished, the disagreeable symptoms of the mother gradually subsided and normal in— fants were borne by all but one patient. In this one instance in which treatment had been ineffective the child was born alive but a congenital atresia of the esophagus had prevented it from swallowing its amniotic Huid. Polyhydramnios associated with teratoma of the neclc which occluded the esophagus has been reported by others.9
In the experiments described above saccharine was present in the umbilical vein blood and in the first urine of the infants which had swallowed it. Moreover, by catheterizing the mothers and collecting urine samples regularly after injecting methylene blue together with saccharine it was possible to obtain evidence that the fetus swallowed intermittently. The patients reported an increased incidence of fetal movements at times coinciding with the appearance of dye in their urine. The conclusion reached was that the child apparently sleeps for many hours in utero and, becoming wakefuL begins to move and drinlc the sweet amniotic fluid. 0ther evidence of fetal swallowing in humans has been ob tained by injecting materials impervious to Iorays into the amniotic sac after withdrawing an amount of Huid equal to that injected. The first investigator 10 to pursue this type of experimentation used solutions of strontium iodide which were swal— lowed by the fetus near term. Others have employed "diodrast." 11 Much more striking results have been obtained recently in specimens of Hve and six months’ gestation by means of "thorotrast." 12, 13 Very clear shadows of the fetal stomach were seen in recent roentgenograms of the uterus and its contents taken after hysterectomy 15 hours or more following« injections From the experiments employing direct injection into the amnion we may conclude that the human fetus can svvallow its amniotic fluid at least as early as the 5th month. How much earlier is not known. swallowing and gastrointestinal activity have been studied from the developmental standpoint in the guinea pig.14 Colloidal solutions of non-absorbabl»e and relatively inert thorium dioxide (thorotrast) and thorium hydroxide (thorad) were injected through the intact abdominal wall into the amniotic cavity in small amounts (o.6 cc. to 1.o cc.) after withdrawing equivalent volumes of amniotic iluid at various times during the gestation period. No anesthesia was used, no surgical procedures were necessary and it was therefore possibles to maintain practically normal physiologic conditions. The guinea pig fetuses began to swallow the thorium compounds mixed with amniotic tluid about the forty-second day of gestation, for at that time the first stomach shadows appeared in roentgenograms such a shadow can be seen in Fig. 39, a roentgenogram talcen on the 48th day. The rapidity with which the material reached the fetal stomach in— creased with age, talcing about 36 hours at 42 days but only about 2 hours in fetuses near term (66 to 67 days) . so much fluid was drunlc that all the thorium dioxide or hydroxide was flushed out of the amniotic cavity. It took nearly 4.5 days for this to be accomplished in the early part of the 3rd quarter of fetal life but only about 18 hours near term. Fetal swallowing seemed to be increased when the mother guinea pigs were subjected to conditions of anoxemia for brief intervals. That swallowing begins at about the time the fetus is growing most rapidly was apparent. The relation of this function to absorption and possibly to fetal water utilization will be discussed later.
Fig. 39. Pregnant guinea pig on the 48th day of gestation Colloidal thorium hydroxide (0.6 cc.) was injected into one amniotic sac on the 46th day; some of it: has been swallosved and is in the stomach and intestines (Becker, et al.: Burg. Gyn. 8c Obst. Vol. 70, 1940.)
Fetal Gastric Motility
Movement of the stomach musculature of human fetuses may be implied from the presence of amniotic constituents in the intestines as early as the 4th or 5th month. Furthermore, several investigators have observed gastric motility directly in mammalian fetuses but the extent to which asphyxia occasioned by the experimental procedures induced or influenced the movements is not known. 15, 16 Cat fetuses delivered with placental circulation intact, but of course not functioning as eföciently as normally, showed marked peristaltic movements of the stomach by about the middle of prenatal life, at which time they were only 35 mm. long.I7 After they had grown to 70 mm. the behavior of the stomach appeared to be no different from that in unanesthetized lcittens a day or two old (120-140 mm.). Peristalsis was very active and rhythmica1, began on the fundic side of the py1oric antrum and spread over the pylorus. Emptying of the stomach contents into the duodenum could be seen, especially when the specimens were a1lowed to swallow a little air.
X-ray studies in guinea pigs have revealed gastric movements in a few of the fetuses« Furthermore, it was found that stomach emptying time decreased greatly between the 42nd day and the time of birth. Experiments in incubating chicks substantiate these observationsss By injecting lycopodium powder into the amnion at various times and examining stomach and intestinal contents 1ater, it was found that swallowing began about the gth day and it toolc Io hours for the material to reach the intestines. At 12 days it required 8 hours and at 16 days, only 2 hours.
Rhythmical waves of movement directed toward the cardiac end of the stomach have been observed in cat fetuses at hyster— otomy on a number of occasionsss We have seen regurgitation of stomach contents and even of bile colored iluid in cat fetuses during the last third of prenatal life. This seemed to be associated with strong respiratory efforts and perhaps it was caused by com— pression of the stomach by the abdominal muscle contractions.
Hunger contractions have been studied in newborn infants and in puppies delivered 8 to 1o days prematurelyIY They were found to be vigorous and more frequent than in the adult.
Fetal Intestinal Movements
some of the earliest experiments in fetal intestinal activity are those of Preyer, reviewed in his book in 1885. Between that time and the present only one systematic developmental study has been made. YanaseAs «« described intestinal movements which he observed in freshly lcilled fetuses of the guinea pig and human. He saw the earliest peristalsis in guinea pigs on about the 26th or 27th day (19 mm. long) at which time the longitudinal muscle of the intestine had been laid down. Earlier (15 mm.) only circular muscle was present -and he could elicit only local constrictions by pinching and by faradic stimulation. Peristalsis could not be observed in human fetuses before the iith weelc although longitudinal muscle and its nerves had been present since the 7th weelc. Faradic stimulation elicited local contractions during the 6th weelc when the circular layer alone was present. It was believed that the early intestinal movements were of neurogenic origin. Yanase’s observations tell us little about what actually takes place in the normal fetus in utero because he studied asphyxiated materiaL
We have examined cat fetuses delivered by hysterotomy with placental circu1ation intact.17- 19 The cats had been decerebrated previously by the anemia method and required no anesthetic during the experiments with their fett1ses. Local constrictions of the intestinal wall were induced in embryos 18.5 mm. long (about 1 gram) by mechanical and faradic sti1nulation. Movements appeared spontaneously upon opening the abdomens of 25 to 35 mm. specimens (2-3 gms.) at about the middle of the gestation period. There seemed to be no well established directional control of the early gut movements. Ikrequently waves of contraction progressed both orally and aborally from one point of constriction. sometimes a movement passed in one direction around a loop of intestine and then reversed itself. strength of move— ments increased rapidly after midfetal life and by 7o 1nn1. (18 grams) the behavior of the intestine Evas very much like that seen in unanesthetized kittens a day or two old (1 1o—12o gms.) . Po— larity became better established with advancement in age.
Intestinal peristalsis of the digestive type involved seginentap tion and propagation of contents. In sotne instances spastic rhythmical segmentatioti of the intestine replaced the« digestivse type of n1ovement. All specimens vvere studied under conditions involving a certain amount of anoxemia even though the placentas were intact. It was found that cligestive peristalsis tended to prevail when the unibilical vein blood contained on the average 6.5 volumes of oxygen per 1oo cc. and that when this decreased to about 2.1 volumes perscent the rhythmical segmentations were more apt to occur. When the fetuses were deeply asphyxiated some time after clamping the umbilical cord, tonus of intestinal musculature diminished and agonal, pendulous writhing movements were manifested. similar changes in activity of the in— testines were observed under profound asphyxia in lcittens one or two days old.
It would appear from the roentgenologic study in the guinea pig14 that intestinal peristalsis is a perfectly normal function of the fetus, but its incidence before swallowing occurs has not been proved in the intact animal. The rate of passage of material along the fetal intestine increases with age. Near term it toolc only one— jifth the time necessary -early in the third prenatal quarter for material to pass from stomach to large intestine. The x-ray Iilms demonstrated segmentations of the intestines very clearly and it was possible to determine that they are more vigorous in late fetal life than they are earlier. Movement of intestinal contents will be seen in Figs. 40 and 41.
Fig. 40. Same guinea pig on the 57th day of gestatiom Most of the thorium hydroxide has been swallowetL has passed out of the stomach and is concentrated in the intestines. Intestinal constrictions indicating activity can be seen. (Becker, et al.: Burg. Gyn. 8c 0bst., Vol. 7o, 1940.)
Fig. 41. Same guinea pig on the day of birth. Defecation in amnio is occur— rings; a pool of meconium is seen in the aniniotic sxic at the arrow. (Becl(er, et al.: surg. Cyn. sc Obst» Vol. 7o, 1940.)
Absorption in the Fetal Digestive Tract
That absorption of chemicals and dyes can occur throughout the gastrointestinal tract of mammalian fetuses was well estab— lished by the experiments to which reference was made on page 99.S- 7 It was proved by the x-ray studies of guinea pig fetuses that large quantities of fluid from the amniotic sac are actually absorbed under normal physiologic conditions34 Only a small amount of thorotrast or thorad, usually less than o.8 cc., was added to many times its volume of amniotic fluid with which it mixed thoroughly. After having been swallowed this dilute solution be— came concentrated in the stomach and intestines until it cast a darlc shadow of these Organs. In roentgenograms of one human experiment at 6 months of gestation,I2 it may be seen that a high degree of concentration had been elfected in the stomach alone in 15 hours time, for little material passed into the small intestine and yet the amniotic sac was nearly clear. Otherss have suggested that much of the absorption takes place in the fetal stomach and our own experiments bear this out. Material traverses the duodenum so rapidly that little absorption can occur there; but other parts of the small and especially the large bowel retain their contents for long periods during prenatal life. considerable absorption undoubtedly takes place in them.
The significance of fetal swallowing, gastrointestinal movements and absorption is not entirely c1ear. It has been suggested however that these activities serve an important function in fetal life. Some of the water may be of real use to the fetus. ReynoldsV has shown that the volume of fetal Huid reaches a pealc at about the 24th day of gestation in the rabbit and then drops olf rapidly until birth, which occurs on the send day. At the time the iluids begin to diminish precipitously the fetal growth curve is rising rapidly. Concurrently, placental growth rate and efkiciency of maternal blood llow through the placenta are declining. Thesö facts suggest that some of the water needed for metabolism begins to be talcen by mouth from the amniotic reservoir at this point and that during further growth the fetus draws more and more upon this source as it becomes less easy to get sufkicient fluids from the placental blood stream. This concept was suggested by Dr. Reynolds who, however, has pointed out that complete occlus sion of the human fetal esophagus is not necessarily accompanied by po1yhydramnios. There are not enough data on the guinea pig24- 23 to allow accurate comparison with the rabbit. Fetal growth does begin to increase rapidly about the time (42nd day) swallowing can be demonstrated. Until approximately the 46th or 48th days the volume of amniotic fluid increases steadily. Thereafter conflicting data are encountered.
Defecation and Meconiopeagy In Amnio
The contents of the fetal intestines are derived from the amniotic fluid swallowed by the fetus. from various secretory products and from desquamated epithelium, both external and internal. They form a darlc greenish, viscid, odorless substance passed from the bowel at birth, to which the name meconium has been given. The green color is due largely to the presence of bile pigments and has been reported to be laclcing in rare cases of congenital obliteration of the bile ducts, meconium being then light gray or yellow instead of green. Meconium is regularly observed in the fifth month and we have seen it as early as the fourth. It is of lighter color than in later months because biliverdin is not formed in significant amount before the sixth month. 26
Concerning defecation in amnio there are few observations save our recent ones. Some« hold that it occurs occasionally in the human fetus before birth. Meconium is passed at birth or soon thereaften but the amniotic kluid is usually said to be clear except after prolonged labor or in cases of asphyxia neonatorum. Some evidence has been presented that passage of meconium at birth is due to an increase in the carbon dioxide tension consequent upon lowering of the blood pI—I and not to a simple accumulation of carbon dioxide.27 Perhaps it· may be related to the estab1ishment of a general viscera1 motor hyperactivity lilce that ·which closes the ductus arteriosus and starts splenic contractions. Although active peristalsis was commonly seen in the large in— testine of cat fetuses with placental circulation intact» the act of defecation was never observed while the intestines were exp0sed. The passage of meconium by intact cat fetuses has been seen in a few instances. The amniotic Auid of many species of animals is found to contain meconium after anesthesia has been used. It has been observed at the time of hysterotomy under local anesthesia in human fetuses of 4 months gestation.
Defecation in amnio is a normal physiologic activity in the mature fetus of the guinea pig, occurring about a week before term. Furthermore, the xsray studies demonstrate that meconium mixes with the amniotic fluid and is swallowed by the fetus« The same material passes through the gastrointestinal tract again and again during the "last weelc of prenata1 life. Defecation in amnio on the last day of gestation is i11ustrated in Fig. 41. Meconiophagy has not been« observed in man.
The Fetal Digestive Glands - Enzymes
The subject of functional activity in prenatal digestive glands is poorly understood in spite of the fact that many investigators have discussed it. For the most part experiments have been un— controlled, methods inadequate and results discordant and confused. For a more complete review of the principal studies on enzymes in the various portions of the gastrointestinal tract the reader is referred e1sewhere.29
Secretion of the salivary glands has been studied extensively at the time of birth, but little is known about their prenatal function. Ptyalin has been identitied as early as the fourth month in the human fetus. Mucous glands develop later than serous and contribute little secretion to the saliva of the newborn infant. The nervous mechanism for salivary seeretion seems to be at least partially in order at birth.
Lack of agreement concerning presence or absence of hydros chloric acid in the fetal stomach prevails in man as well as in other mammals. The human gastric glands seem to be capable of forming acid about as soon as they diikerentiate in the fourth month.30 The presence of free acid has been denied even at the time of birth.31 Spontaneous seeretion appears to be taking place in guinea pig fetuses near terms« and elaboration of free acid seems to occur in the stomach of rabbit fetuses, the quantity increasing with age. 33
several gastric ferments have been identiiied in the fetal stomach of man and other animals. Pepsin may be present during the fourth or iifth months, rennin appears a little later and amylase has been found at birth. The question whether the fetal stomach actually performs any chemical digestive function or not has been debated. Some have held that the presence of protein curds indicates activity and have made the suggestion that some little nutriment may be gained from the contents of the amnion which are swallowed. 1, 2, 29, 30
The fetal intestinal glands are said to secrete. Many enzymes have been declared to be present by some investigators and to be absent by others. Maltase, invertase, lactase, erepsin and trypsin have been identiiied before the lifth month in man; rennin and enterokinase are said to be present by the sixth month and others at full term. The present state of our lcnowledge is too insecure to justify anything like a complete consideration of intestinal secretions here.29 However, there is little. doubt that the fetal small intestines are prepared for some of the chemical as well as the mechanical digestive processes well in advance of the time they will be called upon to employ them.
The large size of the fetal liver and the fact that it is the only fetal organ which receives undiluted the highly oxygenated blood from the umbilical vein suggests that it performs some important functions before birth. Its hemopoietic role in early fetal life has been mentioned in the chapter on fetal blood. Its relationship to prenatal carbohydrate aisid lipid metabolism will be deferred to the last chapter. It begins its secretory activity early. Between the third and fifth months of gestation the human gall bladder contains a thin mucoid material which is practically colorless, but at about the lifth month the fetal bile begins to take on a yellow appearance. The pigment bilirubin is·said to be present at that time and formation of biliverdin is thought to start about a month laterks During the last four months of fetal life the bile pigments are produced extensively and the contents of the intestines become deep green in consequence.
The production of bilirubin in the human fetus is related to the commonly encountered pathologic condition known as icterus neonatorum. The mechanism of formation of the pigment and its escape into the blood stream t·o give rise to hyperbilirubinemia34 is not understood. Any thorough discussion of this subject would lead us to questions of possible placental function in the capacity of a uterine liver. 35 Fetal iron metabolism will be considered briefly in the last chapter.
It is well known that the pancreas elaborates secretions in late fetal life. In the calf its, proteolytic activity is slight szbefore the üfth or sixth monthskss s« Nevertheless some have been able to identify enzymes in the human fetal gland much earlier than this. Trypsinogen, lipase and amylase are present near term and the first two are said to have been detected as early as the fourth month.29
. Preyer, W. i885. specielle Physiologie des Embryo» Grieben, Leipzig. . Fehlingz H. i879. Arch. Gynäk., i4: sei.
. Zuntz N. i878. Piliigeks Arch., is: 548.
. Wiens-r, M. i88i. Arch. Gynäk., i7: 24.
Icrukenberz G. i884. Ibid., es: i.
. Wiens-r, M. i883. ZentralbL Gynäk., 7: 4o9.
. Wislockh G. B. i92o. contr. Emb., ii: 47.
De sn00- K· 1937- M0natschr. Geburt-h. Gynäk.. ioz: 88.
Wilson, J. se. G. i939. J. Obst. Gyn. Brit. Emp., 46: 44.
MERMIS« T— O» J. D. Miller 8c L. E. I-Iolly. i93o. Am. J. Roenh Rad.
Thier» 24: 363. . case, J. T. i933. In A. H. curtis’ 0bstetrics and Gyneco1ogy, z: 762, saunders Philadelphizh . Ehrhardh K. i937. Mund-i. nied- Wchnschr., 84: i699.
Zcp you— Ietzt-do- «- s. III T
sM UND-Ists- ItsZOFPCPPEIOM AS«
. Reitkerscheich W. sc R. schmjemann. . Becher, R. F» W. F. Winde, E. E. Barth sc M. D. Schule. 194o. Sorg.
. Gundobin, N. P. 19z9. 2entra1bl. Gynäk., 63: 146.
Gyn. Obst» 7o: 6o3.
. Tani, K. 1927. Jap. J. Obst. Gynsp to: e. . Friedman, M. I-I. F. 1936. Proc. soc. Expetx Bio1. sc Med., 34: 495. . Windle, W. F. sc C. L. Bishop.
1939 Ibid., 4o: L.
Vrbitcly s. 1924. cotnpt. Rend. soc. Bio1., g« 6o4 Becken R. F. sc W. F. Wind1e. 194o. Unpublished
Car1son, A. J. sc H. Ginsburg. 1915. Am. J. Physiol-« 382 TO· Yanase, J. 19o7. Pllügeks Arch., 117: 345.
Yanase, J. 19o7. Ibid., no: 451.
Reyno1ds, s. R. M. 1939. Physiology ok the Uterus- HOODOII N« YIbsen, I-I. L. 1928. J. Exp. Zool» Hi: Hi.
. Drapen R. L. 192o. Anat. Rec., is: z6g.
Feldman, W. M. 192o. Ante—nata1 and Posvnatal child Physiologzg Longtnans, Green sc Co» London.
. Noguchh M. 1937. Jap. J. Obst. Gyn., so: 248.
Ibrahim, J. 19o9. Biochenx Zucht» es: 24.
Needhanx J. Igzn chemical Embryologzz catnbridge Und-s. Press. 1912 Die Besonderheiten des Kinder-sahen. Allgemeine med. Verlag» Berlin.
. schmidh R. 1g14. Biochem Ztschr., 63: 287. . sutherlanch G. F. 1921. Am. J. Physiol» 55: 3g8. . Van Puteren, M. 188o. cited by Needhanh tgzh
Ylppö, A. 1913. Ztschxx Icinderh1k., g: 208 (1914, Obern. Abst., s: 941) .
. Schick, B. 1921. Ztschr. Kinderhllsp 27: est. . Banting, F. G. sc c. I-I. Best. 1922. J. Lab. clin. Mec- 7t 464·
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