Pregnancy and fertilization

Most doctors consider the first day of your last menstrual period to be the beginning of pregnancy. This period is called the "menstrual age," and it begins approximately two weeks before fertilization. Here is some basic information about fertilization:

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Ovulation

Each month, one of a woman's ovaries begins to develop a certain number of immature eggs in a small fluid-filled sac. One of the sacs completes maturation. This "dominant follicle" suppresses the growth of the other follicles, which stop growing and degenerate. The mature follicle ruptures and releases eggs from the ovary (ovulation). Ovulation usually occurs two weeks before a woman's next menstrual period.

Development of the corpus luteum

After ovulation, the ruptured follicle develops into a formation called the corpus luteum, which secretes two types of hormones – progesterone and estrogen. Progesterone helps prepare the endometrium (the lining of the uterus) for the implantation of the embryo by thickening it.

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Release of the egg

The egg is released and travels into the fallopian tube, where it remains until at least one sperm enters it during fertilization (egg and sperm, see below). The egg can be fertilized within 24 hours of ovulation. On average, ovulation and fertilization occur two weeks after the last menstrual period.

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Menstrual cycle

If the sperm does not fertilize the egg, it and the corpus luteum degenerate; the elevated hormone levels also disappear. The functional layer of the endometrium is then shed, leading to menstrual bleeding. The cycle repeats.

Fertilization

If a sperm reaches a mature egg, it fertilizes it. When a sperm reaches an egg, a change occurs in the protein coat of the egg, which no longer allows sperm to enter. At this point, the genetic information about the child is laid down, including its sex. The mother gives only X chromosomes (mother=XX); if a Y sperm fertilizes the egg, the child will be male (XY); if an X sperm fertilizes, the child will be female (XX).

Fertilization is not just the summation of the nuclear material of the egg and sperm - it is a complex set of biological processes. The oocyte is surrounded by granulosa cells called the corona radiata. Between the corona radiata and the oocyte, the zona pellucida is formed, which contains specific receptors for sperm, preventing polyspermy and ensuring the movement of the fertilized egg along the tube to the uterus. The zona pellucida consists of glycoproteins secreted by the growing oocyte.

Meiosis resumes during ovulation. Resumption of meiosis is observed after the preovulatory LH peak. Meiosis in the mature oocyte is associated with the loss of the nuclear membrane, the bivalent assembly of chromatin, and the separation of chromosomes. Meiosis ends with the release of the polar body during fertilization. A high concentration of estradiol in the follicular fluid is necessary for the normal process of meiosis.

Male germ cells in the seminiferous tubules as a result of mitotic division form first-order spermatocytes, which undergo several stages of maturation similar to the female egg. As a result of meiotic division, second-order spermatocytes are formed, containing half the number of chromosomes (23). Second-order spermatocytes mature into spermatids and, no longer undergoing division, turn into spermatozoa. The set of successive stages of maturation is called the spermatogenic cycle. In humans, this cycle is completed in 74 days and the undifferentiated spermatogonium turns into a highly specialized spermatozoon, capable of independent movement, and having a set of enzymes necessary for penetration into the egg. The energy for movement is provided by a number of factors, including cAMP, Ca ²⁺, catecholamines, protein motility factor, protein carboxymethylase. Spermatozoa present in fresh semen are incapable of fertilization. They acquire this ability when they enter the female genital tract, where they lose the membrane antigen - capacitation occurs. In turn, the egg cell secretes a product that dissolves the acrosomal vesicles covering the head nucleus of the sperm, where the genetic fund of paternal origin is located. It is believed that the fertilization process occurs in the ampullar section of the tube. The funnel of the tube actively participates in this process, tightly adjoining the section of the ovary with the follicle protruding on its surface and, as it were, sucks in the egg cell. Under the influence of enzymes secreted by the epithelium of the fallopian tubes, the egg cell is released from the cells of the corona radiata. The essence of the fertilization process consists of the unification, fusion of female and male reproductive cells, separated from the organisms of the parent generation into one new cell - a zygote, which is not only a cell, but also an organism of a new generation.

The sperm introduces into the egg mainly its nuclear material, which combines with the nuclear material of the egg into a single zygote nucleus.

The process of egg maturation and fertilization are provided by complex endocrine and immunological processes. Due to ethical issues, these processes in humans have not been sufficiently studied. Our knowledge is mainly obtained from experiments on animals, which have much in common with these processes in humans. Thanks to the development of new reproductive technologies in in vitro fertilization programs, the stages of human embryo development up to the blastocyst stage in vitro have been studied. Thanks to these studies, a large amount of material has been accumulated on the study of the mechanisms of early embryo development, its movement through the tube, and implantation.

After fertilization, the zygote moves along the tube, undergoing a complex development process. The first division (the stage of two blastomeres) occurs only on the 2nd day after fertilization. As it moves along the tube, the zygote undergoes complete asynchronous cleavage, which leads to the formation of a morula. By this time, the embryo is freed from the vitelline and transparent membranes, and at the morula stage, the embryo enters the uterus, representing a loose complex of blastomeres. Passage through the tube is one of the critical moments of pregnancy. It has been established that the relationship between the hometa/early embryo and the epithelium of the fallopian tube is regulated by an autocrine and paracrine pathway, providing the embryo with an environment that enhances the processes of fertilization and early embryonic development. It is believed that the regulator of these processes is gonadotropic releasing hormone, produced by both the preimplantation embryo and the epithelium of the fallopian tubes.

The epithelium of the fallopian tubes expresses GnRH and GnRH receptors as messengers of ribonucleic acid (mRNA) and proteins. It turned out that this expression is cycle-dependent and mainly appears during the luteal phase of the cycle. Based on these data, a group of researchers believes that tubal GnRH plays a significant role in the regulation of the autocrine-paracrine pathway in fertilization, early embryo development and implantation, since in the uterine epithelium during the period of maximum development of the "implantation window" there are significant amounts of GnRH receptors.

It has been shown that GnRH, mRNA and protein expression are observed in the embryo, and it increases as the morula turns into a blastocyst. It is believed that the interaction of the embryo with the epithelium of the tube and the endometrium is carried out through the GnRH system, which ensures the development of the embryo and the receptivity of the endometrium. And again, many researchers emphasize the need for synchronous development of the embryo and all mechanisms of interaction. If the transport of the embryo can be delayed for some reason, the trophoblast can show its invasive properties before entering the uterus. In this case, tubal pregnancy can occur. With rapid movement, the embryo enters the uterus, where there is no receptivity of the endometrium and implantation may not occur, or the embryo is retained in the lower parts of the uterus, i.e. in a place less suitable for further development of the ovum.

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Egg implantation

Within 24 hours of fertilization, the egg begins actively dividing into cells. It remains in the fallopian tube for about three days. The zygote (fertilized egg) continues to divide, slowly moving down the fallopian tube to the uterus, where it attaches to the endometrium (implantation). The zygote first becomes a clump of cells, then a hollow ball of cells, or blastocyst (embryonic sac). Before implantation, the blastocyst emerges from its protective covering. As the blastocyst approaches the endometrium, hormonal exchanges promote its attachment. Some women experience spotting or light bleeding for a few days during implantation. The endometrium thickens and the cervix is sealed with mucus.

Over the course of three weeks, the blastocyst cells grow into a cluster of cells, forming the baby's first nerve cells. The baby is called an embryo from the moment of fertilization until the eighth week of pregnancy, after which it is called a fetus until birth.

The implantation process can only occur if the embryo entering the uterus has reached the blastocyst stage. The blastocyst consists of the inner part of the cells - the endoderm, from which the embryo itself is formed, and the outer layer of cells - the trophectoderm - the precursor of the placenta. It is believed that at the preimplantation stage, the blastocyst expresses preimplantation factor (PIF), vascular endothelial growth factor (VEGF), as well as mRNA and protein to VEGF, which enables the embryo to very quickly carry out angiogenesis for successful placentation and creates the necessary conditions for its further development.

For successful implantation, it is necessary that all the required changes in the differentiation of endometrial cells appear in the endometrium for the appearance of the "implantation window", which is normally observed on the 6-7th day after ovulation, and that the blastocyst reaches a certain stage of maturity and proteases are activated, which will facilitate the advancement of the blastocyst into the endometrium. "Endometrial receptivity is the culmination of a complex of temporal and spatial changes in the endometrium, regulated by steroid hormones." The processes of the appearance of the "implantation window" and the maturation of the blastocyst must be synchronous. If this does not happen, implantation will not occur or the pregnancy will be interrupted in its early stages.

Before implantation, the surface epithelium of the endometrium is covered with mucin, which prevents premature implantation of the blastocyst and protects against infection, especially Muc1 - episialin, which plays a kind of barrier role in various aspects of the physiology of the female reproductive tract. By the time the "implantation window" opens, the amount of mucin is destroyed by proteases produced by the embryo.

Blastocyst implantation into the endometrium includes two stages: stage 1 - adhesion of two cellular structures, and stage 2 - decidualization of the endometrial stroma. An extremely interesting question is how the embryo identifies the implantation site, which still remains open. From the moment the blastocyst enters the uterus until implantation begins, 2-3 days pass. It is hypothetically assumed that the embryo secretes soluble factors/molecules that, by acting on the endometrium, prepare it for implantation. Adhesion plays a key role in the implantation process, but this process, which allows two different cellular masses to be held together, is extremely complex. A huge number of factors are involved in it. Integrins are believed to play a leading role in adhesion at the time of implantation. Integrin-01 is especially significant; its expression increases at the time of implantation. However, integrins themselves lack enzymatic activity and must be associated with proteins to generate a cytoplasmic signal. Research conducted by a group of researchers from Japan has shown that the small guanosine triphosphate-binding proteins RhoA convert integrins into active integrin, which is able to participate in cell adhesion.

In addition to integrins, adhesion molecules include proteins such as trophinin, bustin, and tastin.

Trofinin is a membrane protein expressed on the surface of the endometrial epithelium at the site of implantation and on the apical surface of the blastocyst trophectoderm. Bustin and tustin are cytoplasmic proteins that form an active adhesive complex in association with trophinin. These molecules participate not only in implantation but also in the further development of the placenta. Extracellular matrix molecules, osteocanthin and laminin, participate in adhesion.

An extremely important role is given to various growth factors. Researchers pay special attention to the role of insulin-like growth factors and proteins binding them, especially IGFBP, in implantation. These proteins play a role not only in the implantation process, but also in modeling vascular reactions and regulating myometrium growth. According to Paria et al. (2001), heparin-binding epidermal growth factor (HB-EGF), which is expressed both in the endometrium and in the embryo, as well as fibroblast growth factor (FGF), bone morphogenic protein (BMP), etc., play a significant place in the implantation processes. After the adhesion of the two cellular systems of the endometrium and trophoblast, the trophoblast invasion phase begins. Trophoblast cells secrete protease enzymes that allow the trophoblast to “squeeze” itself between the cells into the stroma, lysing the extracellular matrix with the enzyme metalloprotease (MMP). Insulin-like growth factor II of the trophoblast is the most important growth factor of the trophoblast.

At the time of implantation, the entire endometrium is permeated with immunocompetent cells, one of the most important components of the trophoblast-endometrium interaction. The immunologic relationship between the embryo and mother during pregnancy is similar to that observed in graft-recipient reactions. It was believed that implantation in the uterus is controlled in a similar way, through T cells recognizing fetal alloantigens expressed by the placenta. However, recent studies have shown that implantation may involve a new allogeneic recognition pathway based on NK cells rather than T cells. The trophoblast does not express HLAI or class II antigens, but it does express the polymorphic HLA-G antigen. This paternally derived antigen serves as an adhesion molecule for the CD8 antigens of large granular leukocytes, which increase in number in the endometrium in the mid-lutein phase. These NK cells with CD3- CD8+ CD56+ markers are functionally more inert in the production of Th1-associated cytokines such as TNFcc, IFN-y compared to CD8- CD56+ decidual granular leukocytes. In addition, the trophoblast expresses low-binding capacity (affinity) receptors for the cytokines TNFa, IFN-y and GM-CSF. As a result, there will be a predominant response to fetal antigens caused by the response through Th2, i.e. there will be predominantly production of not pro-inflammatory cytokines, but, on the contrary, regulatory ones (il-4, il-10, il-13, etc.). The normal balance between Th 1 and Th2 promotes more successful trophoblast invasion. Excessive production of pro-inflammatory cytokines limits trophoblast invasion and delays normal placental development, due to which the production of hormones and proteins decreases. In addition, T cytokines enhance prothrombin kinase activity and activate coagulation mechanisms, causing thrombosis and trophoblast detachment.

In addition, the immunosuppressive state is influenced by molecules produced by the fetus and amnion - fetuin and spermine. These molecules suppress the production of TNF. Expression on trophoblast cells HU-G inhibits NK cell receptors and thus also reduces immunological aggression against the invading trophoblast.

Decidual stromal cells and NK cells produce cytokines GM-CSF, CSF-1, aINF, TGFbeta, which are necessary for trophoblast growth and development, proliferation and differentiation.

As a result of the growth and development of the trophoblast, hormone production increases. Progesterone is especially important for immune relations. Progesterone locally stimulates the production of placental proteins, especially protein TJ6, binds decidual leukocytes CD56+16+, causing their apoptosis (natural cell death).

In response to the growth of trophoblast and invasion of the uterus to the spiral arterioles, the mother produces antibodies (blocking), which have an immunotrophic function and block the local immune response. The placenta becomes an immunologically privileged organ. In a normally developing pregnancy, this immune balance is established by 10-12 weeks of pregnancy.

Pregnancy and hormones

Human chorionic gonadotropin is a hormone that appears in the mother's blood from the moment of fertilization. It is produced by the cells of the placenta. It is a hormone that is detected by a pregnancy test, however, its level becomes high enough to be detected only 3-4 weeks after the first day of the last menstrual period.

The stages of pregnancy development are called trimesters, or 3-month periods, because of the significant changes that occur during each stage.