Book - Physiology of the Fetus 9
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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 IX The Fetal Muscles
Tissue Cultures and In-Vitro Experiments
Histogenesis of muscle tissue has been carefully investigatedI and a few observations have been made from the standpoint of functionx Although it is diHicult to make physiologic studies on the earliest embryonal muscles in vivo, it has been possible to obtain growth from small pieces of muscle in tissue culture and to study the behavior of new cells in this altered environment. Thus, valuable information concerning the activity of very young cells of smooth, cardiac and slceletal muscle has been gained.
At one time discussion centered about the question whether or not muscle tissue can contract in the absence of nerve iibers supplying it. We now know that it can, and it does so under normal physiologic conditions in some locations. For example the amnion of the chiclc embryo contains sheets of smooth muscle in which nerve fibers have never been demonstrated? This tissue functions actively to churn the embryo about in a rhythmical fashion. Then too, the embryonic heart begins to beat well in advance of the time nerves grow into it. Tissue cultures give conclusive proof that nerves are not essential. Recent histologic studies of amphibian and mammalian heart primordia -have demonstrated that specialized cytologic structures, iibri11ae, and cross striations, are not formed until after contractility has become well established. 3,4
Nerve-free cultures of both cardiac muscle· and amniotic smooth muscle from chiclc embryos demonstrate spontaneous rhythmical contraction in isolated cells as well as in groups of cells.5- C« simi1arly, pieces from the legs of 4 to 1o day chiclcs, grown in Lewis-Loclce so1ution, contain functional myoblasts and young slceletal muscle übers? All these young isolated elements do not contract at any one time and the period of the rhythm varies in different ones, some slceletal iibers contracting as often as 120 times a minute, some in slower rhythms and others only once in one to ten minutes.
Interesting difkerences have been observed in the nature of the contractions of smooth, cardiac and skeletal muscle cells in tissue cultures. Individual contractions of amniotic smooth muscle cells are slow and seem to be produced by currents of protoplasm flowing toward a center of active change in the cell. This results in a 1ocal pi1ing up of the protoplasm at the center of active change. In embryonic cardiac muscle cells the contractions appear as quiclc rhythmical beats, the cell bellying out at its center with each beat. Individual cells as well as groups showed this phenomenon. contractions of skeletal muscle Hbers and myoblasts resemble straight twitches rather than the flowing and beating movements of the smooth and cardiac e1ements. In no type of muscle could myotibrils be demonstrated in the living cellsk Recent studies have shown that it is impossible to relate the genesis of contractility in rat embryonic slceletal muscle in vivo to any intrinsic structural change in the cell.8
The cause of the spontaneous contractions of embryonic muscle cells and myoblasts in cultures is not known, but autochthonicity is not necessarily assumed. Many cells are inactive while one contracts. Activity may depend upon some change in the tissue it— self or in the surrounding medium. A certain Optimum balance of sodium, potassium and calcium ions (Lewis-Locke solution) has proved to be favorable. slight changes in this ionic balance and dilution of the medium sometimes stimulates an increase in activity for»a brief period but soon leads to degeneration of the growth.
In the amniotic smooth muscle a laclc of oxygen or an accumulation of carbon dioxide appea«red to bring about cessation of spontaneous rhythmical activity in vitrok This seems to be equally true of adult smooth muscle, although some believe that carbon dioxide may act as a stimulus to rhythmical spontaneous activity.9- I« In the intact egg of the chick the amnion becomes progressively more active during the course of incubation until about the izth day, after which contractions are less frequent. From analyses of the atmosphere within the air—space of the incubating eggII we know that cessation of amniotic activity coincides with the time a physioIogic partial anoxemia becomes well established.
Spontaneous Activity of Intact Skeletal Muscle
Skeletal muscle Ebers and myob1asts begin to develop in 1 1 mm. to 12 mm. cat embryos«about 23 days after insemination. 0ne should expect to encounter functional activity at that time and, to be sure, faradic stimulation with micro-electrodes does result in contractions.32 However, spontaneous movements have never been observed in small embryos when studied with their placental circulation intact, and we may conclude that the early skeletal muscle cells possess no autochthonicity in vivo. Furthermorek re— mova1 of the specimens from the uterus with consequent asphyxia does not bring about automatic muscular activity. Embryos only a litt1e larger than these (13.5 mm.) do show spontaneous move— ments of the fore1imbs under certain unnatura1 conditions. When removed from the uterus they remain perfectly inactive so long as the surrounding medium contains sodium, potassium and calcium ions in the proportion found in« Locike’s solution. When an ionic unbalance is introduced by substituting solutions deficient in ca1cium or potassium or both, the forelimbs begin to move rhythmically in a waving manner.I3-14-I5 In larger specimens the rhythmical movements occur first in the tail and limbs, structures from which diffusion can take place most readily. We may conclude that spontaneous contractility of new muscle übers is stimulated by ionic unbalance in the young tissues. But since nerve libers are already present in the intact specimens, one does not know whether the muscle itself responds to direct stimulation, whether the nervous elements are stimulated or whether some elementary nervous inhibitory control over the new muscle has been removed by the change of medium, thus al1owing the muscle übers to exhibit contractility which is their inherent property and which never manifests itself in the presence of nerves.
Small mammalian embryos stand in marked contrast with those of lower vertebrates in respect to the absence of observable contractions of intact skeletal muscles. some investigators have re— ported muscle twitching and even rhythmical contractions before external stimulation elicits responses of a reflex natura. 16 ln some experiments activity of the nervous centers seems to have been ex— cluded, the movements being entirely aneural but occurring only upon direct stimulation.17, 18
Faradic And Mechanical Stimulation
Development of contraction of skeletal muscles in response to stimu1ation has been studied in several species of animals. That contractility can be induced before a fetal reflex mechanism can activate it has been well established in mammals.12s19«22 The first muscu1ar responses were obtain-ed in cat embryos only 11 mm. long (Fig. 44) . These were brought about by faradic stimu1ation wich micro—e1ectrodes applied to points directly over developing muscle masses. The forelimb is scarcely more than a limb bud at this time but it did move outward, forward or backward, depending on the position of the electrodes. These forelimb movements were the first responses which could be obtained. soon the head could be caused to Hex to one side or the other, and at the 12 mm. stage separate movements of the proximal and distal parts of the forelimb were induced. At this time contractions of the entire trunk and even the tail muscles appeared. Further development was very rapid and soon a large number of muscles could be brought into activity by properly placed stimuli. It is apparent from these studies that in a general way the course of physiologic development proceeds caudally and rostrally from the shoulder region and distally and ventrally from the dorsal part of the trank.
Fig. 44. Cat embryo 11 mm. C. R. length. The first muscle contractions were elicited by stimulating the region marked with the asterisk (*).
Faradic stimuli were more eifective than other kinds during the earIiest stages of muscle deve1opment. strong mechanical stimuli produced contractions, but they could not be app1ied with as much linesse and the passive movement brought about by such means made it diflicult to see muscle contractions. As de— velopment proceeds, the strength of stimulus needed to induce a response decreases. several factors other than strength of stimu1i determine whether or not responses to direct musc1e stimulation will be obtained. Movements were at their best in specimens whose p1acenta1 circulation was intact and which had been delivered into a saline bath at body temperature. Death of the embryo abo1ished responses only after a considerable length of time. Contractions of the embryonic muscles persisted after 1ow— ering the bath temperatures 10 or 15 degrees Centigrade but were abolished quiclcly by raising it less than 5 degrees above normal body temperature.
Fig. 45. Photomicrograph of nerve übers ending upon myoblasts in the shoulder region of a 7-week human embrytx Pyridine-silver stainx X 450.
The induced embryonic muscIe contractions possess certain characteristics unlilce those of reiiexes A minima1 stimu1us gives rise to a quick, sharp, immediate contraction moving the forelimb, let us say, outward. Upon cessation of the stimulation, the 1imb tends to maintain this contraction momentarilzg returning more slowly to the original Position (see «fetal tetanic reactionsk below). A second stimu1ation following closely upon this induces another response, for there appears to be no grossly perceptible fatigue period. The direction of movement can be controlled by placing the electrodes in different positions. This indicates that the stimuli were well localized and suggests that rather small groups of muscle Ebers were being stimulated. Histologically it was found that a few primitive nerve über terminations are already present in the embryonic muscles at the time early contractions can be obtained (Fig. 45) . Whether or not these nerve endings have anything to do with the nature of responses to direct stimulation is undetermined.
The Fetal Tetanic Reaction
A most interesting study of fetal muscle physiology was made by MinlcowslciW who tested the excitability of nerves and of muscles to galvanic and faradic stimulation in 2o human fetuses 15 mm. to 350 mm. crown-heel length after removal from the Uterus. By applying galvanic stimuli directly to the muscles, he found only two specimens which did not react. In four others the anodal closing contractions were sharp and were followed by slow, often incomplete relaxation while the current was still passing. Brealcing the circuit produced a new sharp contraction. Fourteen fetuses responded to closing the circuit both at the anode and cathode with a quiclc contraction of the muscle; upon reach— ing a certain magnitude, they remained contracted so long as the current llowed. There followed a slower relaxation after brealcing the circuit. To this phenomenon the name "fetal tetanic reaction" was given. Faradic stimulation, as has been observed in young infra-human fetuses, produced a similar quiclc contraction followed by a slow relaxation.
Minlcowski was not the first to make a study of the electrical responses in fetal muscles but his emphasis of the tetanic reaction was new. 0thers24 experimented with human fetuses of about 7 months gestation and reported no tetanic reaction. However, Bichat25 recognized that slceletal muscles of fetal guinea pigs can be induced to contract in response to electrical stimulation and found that the more mature the fetus, the better and more rapid the reaction. Preyer 19 spolce of tonic contraction of the muscles of rabbit fetuses in which he had stimulated the spinal cord with faradic shocks.
1n the newborn rabbit, cat and dog, excitability to electrical stimulation is less than insadultsks The muscle ok the newborn acts like that ok a katigued animal, as may be seen in Fig. 46. Although it toolc 7o stimuli to bring about tetanus in the adult, only 16 were required to do so in the newborn, conditions being the same. These studies have been confirmed in human infants.27- 23
Fig. 46. Myograms in the cat- a, adultx b, 7-day-o1d kirren; c, neu-horn- Time in second-s. Arrow indicates direction ok movement ok the paper. (Soltmann: Jahrb. Kinderhlk. Vol. 12, 1878.)
The character of the myogram was studied in one healthy premature inkant weighing 1,260 grams and comparisons were made with normal new»borns and with inkants sufkering from several diseases.29 We are interested in the results obtained in the healthy individuals and have reproduced them in the following Table 20.
|Age||Latent period (sigma)||Maximum contraction reached (sigma)||Duration of construction (sigma)||Height of contraction (mm)|
1t will be seen that the latent period was longer, the contraction a little slower and its duration much greater in the one premature inkant than in the newborn; but no signiticance should be placed on a study ok one individual. According to another investigatorso the latent period ok muscle in young rat ketuses is betwen 5oo and 1,ooo sigma. How this was determinexl is not stated.
Excitation of Fetal Muscle By Nerve Stimulation
Fetal muscle responds to direct stimulation, as we have said, in advance of the time it can be activated by stimulating nerves. All investigators agree upon this point. In cat» rat and sheep fetuses, responses to nerve stimulation can be elicited soon after the first direct muscle contractions. In the human it has been reported to be about a month later.31 Contractions following human nerve stimulation were not obtained until about the fourth month of gestatioxy at which time the responses were less lively and less constant than those following direct muscle stimulation. They should be manifested much earlier than this« It has been found that asphyxia prejudices the results obtained from stimulation of the nervous systemFs since all of the human fetuses were studied under asphyxial conditions it is doubtful if results with indirect activation of the fetal muscles can be of great significance.
The type of motor response obtained by stimulating the spinal motor centers of small cat fetuses differs from that which results from stimulating the muscle itself with faradic shoclcs. VVhen the point of a line dental broach is passed into the spinal cord or when a faradic shoclc is applied to the cord by microselectrodes the movements which occur are quiclcer and their tetanic or maintained characteristic is not so pronounced as it is when the muscle itself is stimulated. The direction of movement is less easily molded than it is when the electrodes are shifted about over the surface of muscle masses. Furthermore, fatigue enters in when motor centers are stimulated directly, malcing a second response diflicult to elicit after obtaining the first one.
Effects of Curare
Recently an attempt was made to curarize small rat fetuses of 17 days gestation delivered by experimental Caesarean Section« It was reported that three minutes after injecting a minute amount of a one per cent solution of this drug, reflexes were abolished but direct muscle stimulation still produced contractions. These results are very different from those obtained 50 years earlier in guinea pig fetuses. 19 At that time it was reported that a dosage adequate to curarize an adult guinea pig in 1o minutes failed to akfect the fetuses when injected into them in utero until 52 minutes had elapsedz complete feta1 curarization did not come about for 8o minutes. Furthermore, the drug passed through the placenta1 barrier from fetal to maternal side and killed the mother before alfecting the fetus. It seems doubtful if the observations on such small fetuses as were used in the recent experiments can mean more than that reflexes had died out be— cause of asphyxial conditions prevailing cluring the experiments
Fetal Rigor Mortis
There has been discussion from time to time as to whether or not fetuses dying in utero exhibit the phenomenon of rigor mortis. Ballantyne 35 reported several cases and gave references to the early literature on this subject. From what is known of the chemistry and physiology of fetal skeletal Inuscle there is no reason to doubt that rigor does take place but it may not be as pronounced as in the adult. The phenomenon has been described in kittens dying in uteroIs In no case was the degree of rigidity as marked in the fetus as in the mother cat.
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