Anatomico-histological characteristics of myometrium at the end of pregnancy and during labor

It is important to note that studies aimed at studying the architectonics of myometrium and anatomo-histological studies of the structure of the uterus showed that the uterus, by the end of the pregnancy, increases along the length to 36 cm, its width reaches 25 cm, the thickness (anterior-posterior diameter) of the body to 24 cm .

A powerful layer of smooth muscles that makes up the middle layer of the uterus along the direction and direction of the fibers is represented by three layers: the outer and the inner layers - the longitudinal and the middle - the annular layer. The same layers continue into the neck, gradually thinning, and the ring-like layer is especially thinned.

It has been established that starting from the second month of pregnancy, the lumen of the isthmus begins to develop gradually, taking part in the formation of the egg cavity, and this expansion is already completed by the beginning of the 5th month and from this moment until the end of pregnancy (in the absence of labor) zev forms a boundary between the embryo, in which the lower uterine segment also participates, and the neck, while the length of the isthmus reaches 7 cm by the end of pregnancy. The uterus muscles, starting from the 4th month of pregnancy, both in the body region and in the region the lower segment of the uterus are parallel plates and until the end of pregnancy the musculature of the lower segment differs little from the musculature of the body, although in the latter it is still thicker. With the onset of contractions, the lower uterine segment progressively becomes thinner and stretches. The stretching zone of the uterus reaches the place of a tight attachment of the peritoneum to the anterior wall of the organ. At the height of this place is located what is called the "contraction ring". Despite the stretching, the musculature of the lower segment of the uterus during and after childbirth is actively shrinking. Tissues of the cervix during pregnancy change significantly, she herself turns into a cavernous body. Isthmus, which turns during pregnancy into the lower uterine segment, is an independent segment of the uterus with certain boundaries, both macro- and microscopic definite anatomical and functional features. The upper border of the lower segment of the uterus corresponds to the place of tight attachment of the peritoneum to its walls. There is a significant functional difference between the properties of the muscle cells of the body of the pregnant uterus and its lower segment. It is believed that the muscle cells of these two segments belong to two different differentiated types and sees in it a certain functional parallelism with the data of anatomical studies. Muscular cells of the lower segment, at least some of them, possess the contractile capacity inherent in the muscular cells of the uterus body.

A number of interesting regularities have been revealed, which find confirmation in the clinical observations of modern authors. It is shown that the round ligament has the form of a triangular ribbon and is a muscle layer 5-7 mm thick in the nonpregnant state, with the ligament unfolding when approaching the uterus, dressing the front surface of its body almost completely, ie, starting immediately below the attachment pipes and ending with the place where the peritoneum leaves from the anterior surface of the uterus and which serves as the lower boundary of the body of the uterus. Muscle bundles have a longitudinal direction in relation to the ligament.

If we further trace the distribution of the most superficial bundles that have passed from the ligament to the anterior surface of the uterus, then it can be seen that these beams pass to the anterior surface of the uterus, being located in the transverse direction to the long axis of it. On the middle line of the uterus, the muscle bundles of the ligaments of the one and the other side, when meeting each other, bend mostly downwards and lie next to each other. From this, along the middle line of the anterior surface of the uterus, a large median fascicle appears, projecting above the level of transversely moving bundles that have passed from the ligament.

In the peripheral parts of the right and left sides of the uterine body, the main direction of the bundles coming from the outer layer of the front wall from front to back, perpendicular to the axis of the uterus. At the same time, the muscular tufts of the uterus body that are close to the cervical boundary keep this direction with the greatest constancy, it is here that these transverse bundles are the thickest, strongest and longest, so that they even enter the back wall of the cervix.

The same transversely extending muscle tufts are visible on a considerable thickness of the uterine sides, in particular they abound above the border between the body and the neck.

A special feature of the arrangement of muscle beams in the neck is that the main mass of muscle beams in the neck is a direct continuation of the muscular bundles of the outer and vascular layers of the uterine body and the entire complex of muscle beams occupying almost the entire thickness of the neck goes straight down. From this complex, the muscle bundles depart one by one, inward, towards the mucous membrane, and they make bends, changing their direction to a more horizontal one, and this departure of the individual muscle beams inside is observed throughout the cervix from top to bottom. Bent muscular tufts approach the mucosa in a direction perpendicular to it, wherever its surface is facing.

Due to this arrangement of the muscle beams during labor, the opening of the cervical canal is first marked and the muscle beams serve as real dilators of the cervix. At the same time, the submucosal layer of muscles, according to the author, is so weak that it can not with its reduction counteract the tensile action of the described muscular system. It becomes clear why the cervix is gradually opening from the top, begins with the internal pharynx - the upper fascicles are short and less curved, the first effect of the contraction of this group of muscles begins. As the cervix opens, the curved lower fascicles gradually straighten out, and only afterwards their straightening begins their tensile action. Such straightening of muscles occurs consistently, beginning with the upper ones and ending with the lowest ones, which open the outer yawn. The author makes a very important conclusion that there is no division into the contracting active (upper) and stretchable passive parts.

It is believed that all parts of the uterus are active during labor: and with the opening of the cervix, and with the forward movement of the fetus, the muscles of the lower segment and cervix should take the most active part; the effect of the action of muscles depends on the direction of the muscle bundles. In the peripheral layers of the uterine flanks, a thick layer of transversely extending strong muscle bundles, with its contraction, narrows the uterine cavity in a direction transverse to its axis, and since the thickest and longest muscle beams are at the level of the transition of the body into the cervix, and therefore the strongest action of this a group of muscles must be immediately above the neck.

The work of modern authors has shown that the uterine muscle is a complexly organized system of functionally ambiguous smooth muscle cells and it is necessary to take into account the possibility of its functional heterogeneity. From these positions, the study of the dependence of the functional state of the smooth muscle beam on its spatial orientation in a separate layer is of particular interest, given that each layer of the female's myometrium is represented by a three-dimensional network of muscle bundles. It is believed that obstetrician clinicians and up to the present time insufficiently assess the condition of the lower segment and cervix during pregnancy and childbirth, and three layers have different spontaneous activity. In this case, the inner and middle layers have a similar activity, but in most cases the spontaneous activity of the inner layer is higher than that of the outer layer. It was found that oxytocin exerts an effect by increasing the spontaneous uterine activity of all three layers. At the same time, the inner and middle layers (experiments on rats) shrink at a high frequency and with a lower intensity than the outer layer. This difference between the inner and middle layer from the outer one is due to the fact that the outer layer has another embryological origin. Based on these data, the author emphasizes that spontaneous uterine activity, including prenatal contractions of the uterus of the Brexton Gyx type, initially begin in one or more areas of the inner layer and then transform to other layers.

The latest studies on the physiology of the lower segment of the uterus during pregnancy, childbirth, the study of the histological pattern of neurovegetative receptors of the vaginal cervix, the relationship between the structure of the myometrium and the effect of stimulation and suppression of uterine contractile activity, showed that Isthmus undergoes progressive hypertrophy and lengthening and its sphincter remains very tight contraction ring, at least until the end of the 24th week of pregnancy. In this case, the lower segment of the uterus is formed entirely from prolonged and hypertrophic Isthmus. The upper isthmic sphincter begins to undergo relaxation long before the lower sphincter and this is the result of the gradual unfolding of the isthmus from above downward. At the same time, in most of the first-pregnant, the upper sphincter becomes completely relaxed approximately 3-4 weeks before delivery. In re-pregnancy it is not observed until the I period of childbirth and the head goes deep into the entrance to the pelvis, as soon as the upper sphincter is completely relaxed. There are also changes during childbirth: smoothing of the cervix depends on the relaxation of the lower sphincter and abnormal contractions of the isthmus, there is a slow progression of the head and a slow opening of the uterine throat. In this case, the constricting ring - cervical dystocia - is the cause of local abnormal contractions of the upper or lower sphincter.

At present, the division of the myometrium into 3 layers is common : subserosal from longitudinal beams, medium of circular beams and submucosal from longitudinal bundles. The notion of the direction of the movement of muscle bundles in individual layers of the myometrium has changed somewhat in recent years. Thus, some authors indicate that the submucosal (internal) muscle layer consists of circular (rather than longitudinal) bundles, and the middle (vascular) - from muscle beams that go in a variety of directions. Other authors did not find any regularity in the direction of muscle fibers in the uterine wall.

The study of the electromyographic activity of the cervix showed that its highest activity was recorded at the time of contraction, basal activity immediately after amniotomy and into the active phase of labor. At the least mature neck, the maximum electromyographic activity after amniotomy was noted, and there were no electromyographic discharges recorded in the uterine body. With the appointment of oxytocin, these discharges are grouped, their intensity increases, they are synchronized with the onset of contractions. The ratio of discharges of the cervix and the body of the uterus is more than one for an immature neck and less than one for a mature neck. As the delivery progresses, the electromyographic activity of the uterus body begins to predominate. At the beginning of labor after amniotomy, the cervix is most active.

There are also two mechanisms for opening the cervix during labor:

• longitudinal contraction of the walls of the uterus, causing an increase in intrauterine pressure;
• Radial tension as the head moves along the cervix.

Prior to the present study, there was no method of separate measurement of intrauterine pressure and radial strain. The authors constructed a voltage converter that minimally reacted to the growth of intrauterine pressure. A probe with 4 such transducers was placed between the fetal head and the mother's uterine neck along the long axis of the fetus. At the end of the probe, the intrauterine pressure transducer made it possible to measure the amniotic pressure at the same time. The possibility of radial tension in the opening of the cervix during labor is established.

Biochemical, biophysical, electron microscopic and X-ray structural characteristics of the contractile apparatus of the uterus at the end of pregnancy and in childbirth

The study of the main structural and functional substrate - uterine myocytes showed that, compared with the end of pregnancy (38-40 weeks), with normal labor, myocytes are significantly increased in size, "light" and "dark" cells are presented in the same amount.

An increase in the activity of respiratory enzymes succinate dehydrogenase, cytochrome C oxidase and total nucleic acid content in mitochondria indicates a high level of oxidation-reduction processes in myometrium cells in normal labor activity, as well as the possible participation of these organelles in enhancing the biosynthesis of cellular proteins.

The increase in activity of creatine phosphokinase found in our studies in homogenate of uterus muscle during normal labor indicates the presence of this enzyme in myometrium and its role during labor. The increased activity of creatine phosphokinase in the mitochondria of the uterus muscle can indicate the place of application of the action of this enzyme in a complex system of regulation of the processes of contraction of the myometrium in childbirth.

We followed the changes taking place in the contractile apparatus of the uterus muscles on glycerinized muscle models and found that the beams of glycerinized cells under the influence of ATP develop the highest voltage.

When studying the regulation of myosin smooth muscle indicates that the phosphorylation of light chains of smooth muscle myosin is the key reaction necessary for the development of stress. Native thin filaments are involved in the regulation of actomyosin interaction. Phosphorylation of myosin begins with an increase in the intracellular Ca ²⁺ concentration , which is mediated through a system of secondary mediators.

To determine the structural features of the contractile apparatus of the uterine muscle with its various functional state (end of pregnancy, normal labor, weakness of labor, rhythm-stimulating therapy), we used the X-ray diffraction analysis method, which has high information content and allows us to judge interatomic and intermolecular distances in matter. Our study of the roentgenogram of beams of glycerinized cells prepared from the uterine muscle during normal labor activity showed the presence of weak (due to a significant low content of myosin in the smooth muscle), but pronounced traces of the meridian arc of the corresponding periodicity of 5.1 A and seals or spots on the equator arc periodicity of 9.8 A, which indicates the existence of orientation of fibrillar proteins in the contractile apparatus of myometrium cells, with which the development of these cells by high apryazheniya under the influence of ATP, and the uterus as a whole - marked contractile activity. At the end of term pregnancy, these beams of glycerinized cells indicate the disorientation of fibrillar proteins in the contractile apparatus of myometrium cells, which appears to be one of the factors responsible for the absence of high voltage developed by the bundles of these cells under the influence of ATP and the pronounced contractile activity of the uterus during this period of pregnancy .

From the standpoint of perinatal fetal protection in treating the weakness of labor, a special place belongs to the study of the structure and function of the placenta. A separate direction deserves the development of the problem of insufficiency of the placenta.

Our electron microscopic examination of the placenta with normal labor showed that its ultrastructure is not much different from that at the end of a full term pregnancy. In the homogenate and mitochondria of placental tissue in normal labor activity, in comparison with full term pregnancy, the activity of succinate dehydrogenase, cytochrome C oxidase, creatine phosphokinase and the total content of nucleic acids were increased. Consequently, the directionality of these changes in the placenta corresponds to those in the uterine muscle.

The increase in the total content of nucleic acids in the blood can be explained by their increased formation in the myometrium and placenta during pronounced labor activity. For the same reason, the activity of creatine phosphokinase increases, the latter, apparently, also due to increased production of this enzyme in the tissue of the myometrium and the placenta and its entry into the bloodstream.

When the labor activity is weak in the fine structure of the uterine myocytes and in the mitochondrial fraction of these cells, a number of changes take place, the dominant ones being the signs of disorganization, mainly in myofilaments and especially in mitochondria, against which the changes in the activity of the studied enzymes and the content of nucleic acids are detected.

Thus, the uterine myocytes swell, they have a "light" appearance. The intercellular spaces are expanded and filled with bundles of collagen fiber fibrils and heterogeneous amorphous material of various electron-optical density. Several enlightened main membrane in some areas swollen and fragmented. There was a pronounced swelling of the sarcoplasm in the near-sarcolement area, accompanied by the appearance of empty spaces filled with edematous fluid in the zone of myocyte contraction. In the same zone, edema, swelling and disorientation of myofilaments with a higher electron-optical density are most pronounced.

In most of the myocytes, the number of organelles has been reduced, and disorganization phenomena are predominant in the surviving ones. The membrane of the sarcoplasmic reticulum is sharply fragmented. Ergastoplasma is degranulated, unfixed ribosomes are rare. The Golgi complex is not found in most of the cells. Most mitochondria have only the remains of cristae with blurred or granular contours.

Thus, the changes observed in the ultrastructure of the uterine myocytes, in the mitochondrial fraction of these cells, indicate the existence (in case of weakness of the labor activity) of the disorientation of myofilaments and the disturbance of the mitochondrial structure, the substrate of oxidative phosphorylation occurring in the crystals and elementary particles of these organelles.

With the weakness of labor activity, the bunches of glycerinized cells of the uterus muscle under the influence of ATP develop significantly less tension than with normal labor. Radiographs of these resemble those of the cells of the uterine muscle at the end of a full term pregnancy. Such a change in the X-ray diffraction pattern may indicate either a disruption in the structure of the molecules themselves, or a disorder in the mutual orientation of the molecules with each other.

Consequently, the disorientation of molecules or cells relative to one another can lead to a change in muscle contractility and a decrease in the tension developed by the muscle model prepared by glycerinization. This was revealed by us at the end of full term pregnancy and in violation of the contractile capacity of the uterus with weakness of labor.

Electron microscopic studies of the placenta in the case of weakness in labor activity reveal changes in the uterus of the same type as the myocytes and they reduce to the following: a flattening of the plasmodial trophoblast, basal membrane and capillaries develops. Reduces the number of microvilli with their characteristic thickening and clavate expansion. In the cytoplasm of the plasmodiotrophoblast, the number of mitochondria is significantly reduced, the metrics of which become darker. In the syncytiotrophoblast, the amount of amorphous substance increases. The cells of Langerhans increase in size, but the number of mitochondria decreases in them, and the metric becomes more enlightened. Basal membrane considerably thickens. In all cellular elements, the endoplasmic reticulum is presented in the form of small vesicles coated with granules, RPN. Detection of young villi and hyperplasia of capillaries can be considered a characteristic sign of the development of compensatory reactions.

In the mitochondrial fraction, the placenta of the organelles varies in size, from small to large. As in mitochondria of myocytes, only in some of them the remains of crista are preserved and occasionally homogeneous inclusions are found.

Consequently, a number of stereotyped changes occur in the placenta, among which characteristic, along with disorganization-functional shifts, is the presence, to a greater or lesser extent, of compensatory-adaptive reactions.

Determination of enzyme activity and nucleic acid content showed that in the homogenate and in the mitochondrial fraction of the uterus and placental tissue, with weakness of labor activity compared to the norm, there was a decrease in the activity of creatine phosphokinase, succinate dehydrogenase, cytochrome C oxidase and total nucleic acid content indicating a decrease the level of oxidative processes, the inhibition of tissue respiration and protein biosynthesis in myometrium and placenta.

In the blood of parturient women, a pronounced metabolic acidosis is determined, a slight decrease in the calcium and sodium content in the blood plasma, as well as a significant increase in oxytocinase activity, a decrease in the activity of creatine phosphokinase, and a decrease in the total content of nucleic acids.

During the treatment (application of uterotrophic drugs in case of weakness of labor activity), the ultrastructure of the uterine myocytes does not undergo significant changes according to the accepted methods and intravenous administration of oxytocin.

Most of the myocytes have a "light" appearance and are unequally large in size. The intercellular spaces remain expanded due to the proliferation of fibrils of collagen fibers and an amorphous substance. In most of the cells, fuzzy contours of the sarcoma remain in the edematous, loosened basal membrane adjacent to it. The disordered myofilaments are narrowed or swollen. Ergastoplasma on a significant space is degranulated. The Golgi complex in most of the myocytes is absent. Unlike the uterine myocytes in women with untreated weakness of labor activity in mitochondria, in rare cases a preserved composition of several hypertrophied cristae and single osmiophilic inclusions are found. The size of the nucleus of myocytes is somewhat increased, and their scalloped membranes have sufficiently distinct contours.

The mitochondrial fraction is dominated by organelles with an enlightened structure, indistinct, granularly degenerate contours of cristae. Vacuum-like mitochondria with a lack of internal structure are found less frequently.

Consequently, with the use of uterotrophic drugs in order to stimulate labor in the ultrastructure of the uterine myocytes and their mitochondrial fraction, the picture typical for the untreated weakness of labor activity remains basically. However, against the background of disorganization, after the marked treatment, compartments with sufficient safety of myofilaments, membranes of the sarcoplasmic reticulum and mitochondria are detected more often, which, apparently, is associated with the action of estrogens, and may indicate some improvement in the course of oxidative processes in them.

Investigation of the x-ray structure of the beams of glycerinized myometrium cells (contractile models) also showed some improvement in the degree of orientation of the molecules of fibrillar proteins compared with those in the untreated weakness of labor activity.

In the ultrastructure of the placenta, a flattening of the plasmodio-trophoblast with a cytoplasm filled with a large amount of structureless substance is noted. In some of them, there are separate mitochondria with no cristae and an enlightened matrix. The number and size of mitochondria in the cells of Langerhans has been slightly increased and the amount of structureless material in the basal membrane has been reduced.

In the mitochondrial fraction, organelles with complete absence of cristae are also found, and in some mitochondria, the cristae are located near the inner membrane, and their matrix contains dense, osmiophilic inclusions.

Thus, during the drug-induced stimulation of labor in the placenta, the disorganization-functional changes that we observed in the untreated weakness of labor activity persist. However, the revealed differences, although they are not particularly significant, may indicate some improvement in compensatory-adaptive reactions and the course of oxidative processes in the placenta, apparently also associated with the action of extrogens included in the treatment of weakness in labor.

It can be assumed that with the same factor (the effect of estrogen hormones) there is a tendency to improve oxidation-reduction processes in the parturient women, which is primarily manifested by an increase in the content of the total amount of nucleic acids in the mitochondria of the uterine muscle and an increase in the activity of creatine phosphokinase in mitochondria and placenta homogenate , while maintaining a picture of pronounced metabolic acidosis in the blood of the parturient women.

A comparative analysis of the results of electron microscopy studies of uterine myocytes and placental tissue showed that the rhodostimulation by intravenous drip of oxytocin on the buffer solution leads to the greatest increase in the number and size of myocytes in which organelles, especially mitochondria and a sarcoplasmic network with distinct membrane contours are found. In addition, myofilaments are located in them more in parallel, and in a number of cases, there is an increase in the number of non-fixed grains of ribosomes and even in rosettes of polyrnbosomes.

The mitochondrial fraction is dominated by organelles of increased size with preserved, but somewhat disorderly located cristae. As for the ultrastructure of the placental tissue, it does not show flattenings of the basal membrane and capillaries. The cytoplasm contains osmisfil granules, ribosomes, and the plasmodiotrophoblast has a denuclearized and flattened-nuclear zone. Langerhans cells contain the Golgi apparatus with an increased number of mitochondria in them, etc. In the cytoplasm of the endothelial cells of the capillaries, ribosomes, the Golgi complex, and mitochondria appear.

In the mitochondrial fraction of the placenta, organelles of especially large size are less common, and in most of them the structure of the crista is retained.

In the homogenate and mitochondria of the uterine muscle and placental tissue, an increase in the activity of creatine phosphokinase, succinate dehydrogenase, cytochrome C-oxinase and total nucleic acid content is detected, which in turn indicates the functional usefulness of the uterus myocytes, placenta cells and their mitochondria, against the background of the existing this time in the blood of women giving birth to a picture of compensated metabolic acidosis and an increase in the activity of creatine phosphokinase and the total content of nucleic acids.

The results of the experimental studies also showed that intravenous injection of the buffer solution to animals, even with uncompensated metabolic acidosis, leads to normalization of the acid-base and electrolyte balance in the blood, the activity of mitochondrial respiratory enzymes and the total content of nucleic acids in myometrium, and together with oxytocin significantly increases the severity of contractions of the uterine horns, whereas intramuscular administration of estrogen hormones, as well as intravenous - oxytocin on 5% glucose solution does not lead to the restoration of the studied parameters and a marked increase in the contractility of the myometrium. In addition, it was found that the sodium succinate, which is part of the buffer solution, increases the activity of mitochondrial enzymes succinate dehydrogenase and cytochrome C oxidase, which is apparently due to the high efficiency and productivity of this acid in maintaining the cell's energy potential. Inclusion of succinic acid in the metabolic cycle activates not only energy but also plastic processes, since the four-carbon skeleton of this acid is used for the synthesis of all kinds of oxidative systems of the cell on the porphyrin base (cytochromes, catalase, peroxidase, etc.). Such a mechanism explains and adaptation to hypoxia - an increase in the regeneration of mitochondria with the intake of succinic acid.

A comparative analysis of the results of the study of the X-ray structure of the contractile apparatus of the uterine muscle of women showed that after intravenous administration of oxytocin on the buffer solution the most pronounced increase in the degree of order of fibrillar proteins was observed, an improvement in the degree of orientation of the contractile protein molecules, and the approach of the diffraction pattern of these muscle models to those prepared from muscle womb of women with normal labor.

Thus, during rhodostimulating therapy with intravenous droplet administration of oxytocin on a buffer solution, we found a dramatic increase in the repair of membranes (crista) of mitochondria and other membrane structures in uterine myocytes and placental cells, which may indicate an increase in the intensity of oxidative phosphorylation processes, -reducing processes. This is confirmed by the discovery of an increase in the activity of creatine phosphokinase, succinate dehydrogenase, cytochrome C oxidase and an increase in the total content of nucleic acids in the homogenate and mitochondria of the myometrium and placenta, and in the contractile apparatus of the uterus muscle - by the existence of a pronounced orientation of the molecules of fibrillar proteins, by the action of ATP by beams of glycerinized myometrium cells.

Our new data on the structural and functional characteristics of the contractile apparatus of the uterine muscle and the subcellular formations of the myometrium and placental tissue made it possible to establish new, previously unknown aspects of the pathogenesis of the weakness of labor activity and to substantiate a new complex method of rhythm-stimulating therapy by intravenous administration of oxytocin on a buffer solution, pathology of labor, metabolic processes in the organism of parturient women and fetuses.

[1], [2], [3], [4]