Functional Morphology of Uterus During Labor

Myometrium. The basis for the myometrial uterus system is a myocyte that is a large smooth muscle cell functioning as a fundamental contracting unit. We have not managed to detect any morphological differences between the myocytes of myometrium located in different sections of humane uterus during labor (bottom – body - lower segment). It is the first fundamentally important proposition.

Myocytes are united into primary cylindrical beams with the help of a connective-tissue skeleton in which each smooth muscle cell occupies its own place. Hexagonal “packing” and the lineal arrangement of myocytes all the way along a beam, strict fixation in their own connective-tissue cells inside the beam and the presence of an individual connective-tissue shell outward allow for identifying this morphological lump as a primary cylindrical beam. As the distance between some of the cells may be less than 20 nanometers, some of the myocytes (but not all of them) form a system of the gap junction in its stratum. The external connective-tissue of the primary cylindrical beam is more than 20 nanometers thick, that’s why smooth muscle cells located in different, even in close beams are never in direct contact. It is the second fundamentally important proposition.

If a separate smooth muscle of the retracting uterus can be considered a fundamental contracting unit, a primary cylindrical beam can be considered a fundamental contracting unit of myometrium.

Primary cylindrical beams of the smooth muscle sells of myometrium are united into large cylindrical and tabular beams consisting of a few dozens of primary cylindrical beams “packed” all the way along this large lump with the diameter of up to 1.5-2.0 mm. The basis for the myometrium during labor is provided by the large cylindrical beams, with its central axis occupied by small cylindrical beams in a spiral way. Tabular beams – whose number is considerably smaller, are located only on the uterus surface under peritoneum. They are in the form of a belt with its width exceeding the thickness. Each of the large cylindrical beams has its own connective-tissue shell that absolutely excludes the possibility of a direct contact between the myocites of different beams.

We are still the only authors that studied the morphology of myometrium of human uterus in a stretched state during labor, when myometrium beams are in the state of in situ. What did we find out?

Firstly, large cylindrical and tabular beams of myometrium of the uterus during labor are fundamental contracting units.

Secondly. Each of the fundamental contracting units of myometrium has anatomic points of fixation defining”the beginning” and “the end” of each of the large cylindrical or tabular beams. Thus, all morphological lumps that can be surely identified as a anatomic point of fixation of the large beams, were located whether in the area of lower-third of the lower segment, istmic-cervical fistula, or uterus ligaments – round, cardinal, sacrouterine and vesicocervical. We did not find out any lumps of this kind in the stratum of myometrium located in the bottom, body of the uterus and the two upper thirds of the lower segment. It means that every single fundamental contracting unit of myometrium of uterus during labor can simultaneously be located not only in various uterus sections in a vertical way, but also in different layers of its muscle shell “all the thickness way” and in various “parts” of the organ. It is the third fundamentally important proposition.

Nevertheless, the very fact that each or the majority of fundamental contracting units belong to different sections of the uterus does not mean that myometrium during labor can be considered “the huge functional syncytium”. From the myoarchitectonics point f view the myometrium of the parturient female is a three-dimensional grid lump in all the planes and organ sections: bottom, body and two upper thirds of the lower segment. The uterus during labor does not have either purely longitudinal nor diametrical (circular) smooth muscle beams. On separate parts of its anatomic length each of the fundamental contracting units is located whether oblique longitudinally or oblique diametrically and can turn into direction close to horizontal only in case this part of the beam is positioned in the area of the porporeal-istmic fistula (“contraction ring”). Large cylindrical beams can be divided into 3 types according to their positioning in myometrium. The first type is referred to the act when the large cylindrical beam spirally passes the part of the myometrium of the uterus body, starting in one of the ligaments of uterus and ending in another. This type is characterized by the majority of the beams positioned in the shallow layer of the uterus body. This type also includes the large tabular beams that are positioned in the shallow muscle layer of the body of the uterus.

The second type of functional contracting units contains only large cylindrical beams that pass from one uterus sector to another, for instance, from the body to the lower segment, initially starting in one of the ligaments of uterus. At the same time, they necessarily form a semiring in the area of the corporeal-istmic fistula. The third type contains those functional contracting units (large cylindrical beams) that does not go outside the uterus body, but necessarily take part in “building” the contraction ring. Moreover, both the second and third types of functional contracting units of the uterus during labor are characterized by their presence in various sections of myometrium as well as all along thickness. That is, these large cylindrical beams can be positioned now in deeper, now more in shallow sections of myometrium.

There are two formal morphological criteria that allow to single out two layers from the myometrium of retracting uterus– shallow (external) and deep (internal). The first criterion is based on the fact that the majority of cylindrical beams and all the tabular ones with their anatomic fixation points located in the uterus ligaments form the “shirt of uterus” tightly fixed to the pelvic bones. The second criterion is the unique abilities of development of the vascular system in deep layers of myometrium, that allow to clearly single out the internal layer of myometrium of the uterus body which was given the name of “vascular” long ago.

Thus, from the point of view of the formal morphology it can be stated that:

1. In morphological respect all the muscular shell of uterus during labor consists of totally identical elementary contracting unit s- hypertrophic smooth muscle cells that unite in initial, small cylindrical beams used in their turn as a basis for building large cylindrical and tabular beams. They are considered the main functional contracting unit of myometrium. Tabular beams exist only in the shallow layers of myometrium under the uterus serose and are only positioned in its body. Large cylindrical beams serve as a base for the contracting system of myometrium and can simultaneously have their separate parts present in various uterus sections – bottom, body and lower segment.

2. All the anatomical fixation points of large cylindrical and tabular beams are positioned in either uterus ligaments, or lower third of the lower segment, in the area of istmic-cervical fistula.

3. Smooth muscle cells that are part of large cylindrical and tabular beams are in no morphologically identified contact with the smooth muscle cells located in other beams. A definite number of the cells located in small primary cylindrical beams are united in delivery by high-permiable gap junctions.

Mioarchitectonics of uterus during labor is largely defined by the fact that myometrium during the first period of labor has an optimally stretched state (2l₀). Functional contracting units make up a three-dimensional grid structure, in which each of the elements is of a spiral course against the surface of the uterus shell. That’s why in each of the definite point of the body shell and the lower segment of uterus every single contracting unit is located in relation to the longitudinal axis of uterus whether oblique- longitudinally or oblique- diametrically. The only exception is the area of the retraction ring, where the definite part of the contracting unit is included in the semi-half-loop system and has almost a horizontal position. Transition of the fundamentall contracting unit from a oblique- longitudinal or oblique- diametrical position into a horizontal one results in the anatomical deformation of the beam. I t’s a twisting in the area of beam’s entering into the contraction ring structure from the uterus body as well as in the area of exiting it into the lower segment tissue, which returns it back into the oblique- longitudinal or oblique- diametrical position. It is the fourth fundamentally important proposition.

Cervix

Basing on the results of our own research as well as on the research made by others we state that the matured cervix has a connective-issue vascular structure and is totally stripped of any organized smooth muscle system. By the moment of delivery a venous depot of unique volume and structure is developed in the cervix tissue. It consists of thin-walled venous sinus, part of which has straight arteriolar-venous shunts. By the moment of delivery the cervix looks like corpus cavernosum, which has been lately clearly proved by morphological and ultrasonic study. For instance, according to Yannaeva N. the vascularization index of cervix rises at the period of 38 weeks up to 0,89±0,5%, at the period of 40 weeks up to 9,98±2,72%, with the vascular-flow index from 0,40±0,22 to 2,70±0,82% respectively. As a result of these processes a short time before the delivery the cervix looks like corpus cavernosum when studied by ultrasonography in the energy doppler regime.

In phisiological conditions together with the development of this huge venous depot, if compared with the volume of cervix, changes take place also in the connective-tissue matrix that influnce the modulus of elasticity of its tissue. The uterus tissue becomes more prone to deformation. Professor Zabozlaev F. – a representative of one the most influencial native morphological schools headed by Professor Milovanov A. – introduced a new morphological term “cavernous-like transformation of celvix”, with its degrees and peculiarities not only serving as criteria for maturation of cervix, but also defining its phisiological and pathological course. These data have also been proved in the works of native researchers.

So, mature celvix is a vascular connective-tissue lump that is totally stripped of smooth muscles which could take part in the dilatation process. It is the fifth fundamentally important proposition.

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