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In the armory of medical technology available for alleviation of disease and quality of life enhancement, there is nothing to match the unique contribution of assisted reproductive technology (ART). There is no other life experience that matches the birth of a baby in significance and importance. The responsibility of nurturing and watching children grow and develop alters the appreciation of life and health, with a resulting long-term impact upon individuals, families and, ultimately, society. Thus, the combination of oocyte and sperm to create an embryo with the potential to develop into a unique individual cannot be regarded lightly, as merely another form of invasive medical technology, but must be treated with the respect and responsibility merited by the most fundamental areas of human life.
Every cell in an individual has a unique chromosome complement, with 20 000–25 000 genes coded into a DNA sequence of 3 billion base pairs, packed into 23 pairs of chromosomes: a total of 46 chromosomes in each diploid human cell. All of these cells have the same genetic information, copied during mitotic divisions by replicating the DNA during each cell cycle. The pattern of gene activity in each cell (gene expression/transcription) dictates its function and fate, enabling different cells to differentiate and carry out distinct functions.
The first live births following frozen-thawed embryo transfer were reported in 1984 and 1985 by groups in Australia, the Netherlands and the United Kingdom. Since that time, the original protocols have been modified and simplified such that cryopreservation with successful survival of sperm, oocytes and embryos is now an essential component of every routine IVF program. Pregnancy and live birth rates after frozen embryo transfer contribute significantly to cumulative conception rates after fresh transfer. In recent years, traditional methods of freezing and thawing have been increasingly replaced by protocols for vitrification/warming. For both slow freezing and vitrification, an understanding of the basic principles of cryobiology involved is essential to ensure that the methodology is correctly and successfully applied, in order to minimize cell damage during the processes of freezing/vitrification and thawing/warming.
After a blastocyst has implanted in the uterus and begins to differentiate into the three primary germ layers, a special population of cells develops as primordial germ cells (PGCs). These are destined to become the gametes of the new individual: future reproduction of the organism is absolutely dependent upon the correct development of these unique populations of cells. They originate immediately behind the primitive streak in the extraembryonic mesoderm of the yolk sac; toward the end of gastrulation they move into the embryo via the allantois, and temporarily settle in the mesoderm and endoderm of the primitive streak. In humans, PGCs can be identified at about 3 weeks of gestation, close to the yolk sac endoderm at the root of the allantois.
At least 50% of couples referred for infertility investigation and treatment are found to have a contributing male factor. Male factor infertility can represent a variety of defects, which result in abnormal sperm number, morphology or function. Detailed analysis of sperm assessment and function are important for accurate diagnosis, and are described in detail in numerous textbooks of practical andrology and semen analysis. A comprehensive review of semen analysis is beyond the scope of this book, and only details relevant to assisted conception treatment will be described here.
Gametogenesis, embryo development, implantation and in-vitro culture involve numerous complex pathways and interactions at the cellular and molecular level; a true understanding of their significance requires fundamental knowledge of the underlying principles. This chapter therefore provides a condensed overview and review of basic terminology and definitions, with particular emphasis on aspects relevant to reproductive biology and in-vitro fertilization.
During the transition from morula to blastocyst the embryo enters the uterus, where it is sustained by oxygen and a rich supply of metabolic substrates in uterine secretions. The subsequent sequence of events that lead to implantation is a crucial milestone in mammalian embryo development. Carefully orchestrated programs are set into action, which establish diverse cell lines, specify cell fate and major remodeling that will generate the embryo and its extraembryonic tissues: during gastrulation, the three primary germ layers that lead to body formation are formed. The critical conditions that are created in this early stage will pave the way to a successful pregnancy.
Every individual treatment cycle involves a number of different stages and manipulations in the laboratory, and each case must be assessed and prepared for in advance; the afternoon prior to the procedure (the day after hCG administration) is a convenient time to make the preparations. The laboratory staff should ensure that all appropriate consent forms have been signed by both partners, including consent for special procedures and storage of cryopreserved embryos. Details of any previous assisted conception treatment should be studied, including response to stimulation, number and quality of oocytes, timing of insemination, fertilization rate, embryo quality and embryo transfer procedure, and judgments regarding whether any parameters at any stage could be altered or improved in the present cycle can be assessed. The risk of introducing any infection into the laboratory via gametes and samples must be absolutely minimized: screening tests such as human immunodeficiency virus (HIV 1 and 2: Anti-HIV 1, 2) and hepatitis B (HbsAg/Anti-HBc) and C (Anti-HCV-Ab) should be confirmed, as well as any other tests indicated by the patients’ history (e.g., HTLV-I antibody, RhD, malaria, Trypanosoma cruzi, Zika virus). If donor gametes are to be used, additional tests for the donor are required: chlamydia, cytomegalovirus and a validated testing algorithm to exclude the presence of active infection with Treponema pallidum for syphilis testing.
Biologists and physiologists began to micromanipulate cells during the last century, using a variety of manipulator systems to dissect or record from cells. The earliest attempt to inject sperm was recorded in 1914, when G.I. Kite injected sperm cells into starfish oocytes, but with inconclusive results (Lillie, 1914). Experiments in which sperm were injected into eggs around the mid-1960s were primarily designed to investigate the early events of fertilization, i.e. the role of membrane fusion, activation of the oocyte and the formation of the pronuclei. Two series of early experiments by independent groups demonstrated major species differences. Hiramoto showed that microinjection of spermatozoa into unfertilized sea urchin oocytes did not induce activation of the oocyte or condensation of the sperm nucleus (Hiramoto, 1962), whereas others demonstrated the opposite in frog oocytes. Ryuzo Yanagimachi and his group later demonstrated that isolated hamster nuclei could develop into pronuclei after microinjection into homologous eggs, and a similar result was obtained after injecting freeze-dried human spermatozoa into a hamster egg (reviewed by Yanagimachi, 2005). These experiments indicated that membrane fusion events can be bypassed during activation of mammalian oocytes, without compromising the initiation of development. The experiments not only provided information on the mechanism of fertilization, but also led to a new technique in clinical embryology.
After completing fertilization with fusion of the pronuclei during syngamy, the zygote now has a diploid complement of chromosomes, undergoes its first mitotic division and then continues to divide by mitosis into a number of smaller cells known as blastomeres. In humans, the first few cleavage divisions take place in the oviduct, before the embryo reaches its site of implantation in the uterus (Figure 5.1).
From the beginning of the twenty-first century onwards, laboratories that offer human ART treatment or are involved in the handling of human gametes and/or tissue became subject to increasing demands and precepts set down by regulatory and legislative authorities. The regulations differ from country to country, and some directives (e.g., the EuropeanTissue Directive 2004/23/EC-2006/86/EC) are subject to interpretation according to legislation or guidelines set down by national authorities in individual countries. In many countries, it has become necessary to obtain accreditation and/or certification by a national or international body that will carry out in-depth assessments and inspections to ensure that all aspects of facilities and treatment meet a required standard.
Mature human gametes ready for fertilization differ in their state of nuclear maturation: the spermatozoon has completed meiosis and the oocyte is arrested at metaphase II. However, both gametes must also undergo a process of cytoplasmic maturation before they are capable of fertilization. This involves a complex series of biochemical, physiological and structural events that occur in a carefully orchestrated temporal and spatial pattern in parallel with, but independent from, nuclear maturation. Cytoplasmic and nuclear maturation are often asynchronous (Dale, 2018a): therefore, a cohort of human metaphase II oocytes collected after controlled ovarian hyperstimulation in an IVF program may appear to be similar with regards to the nuclear apparatus, but they are in fact at various stages of cytoplasmic maturation. This may partly explain the different developmental capabilities of embryos generated from a single cohort of oocytes.
Synchrony is essential for gametogenesis and correct embryo development, and a basic knowledge of reproductive endocrinology is fundamental to understanding synchrony in reproductive physiology. Although sexual arousal, erection and ejaculation in the male are obviously under cerebral control, it is less obvious that the ovarian and testicular cycles are also coordinated by the brain. For many years after the discovery of the gonadotropic hormones follicle-stimulating hormone (FSH) and luteinizing hormone (LH), the anterior pituitary gland was considered to be an autonomous organ, until animal experiments in which lesions were induced in the hypothalamus clearly demonstrated that reproductive processes were mediated by the nervous system. The hypothalamus is a small inconspicuous part of the brain lying between the midbrain and the forebrain; unlike any other region of the brain, it not only receives sensory inputs from almost every other part of the central nervous system (CNS), but also sends nervous impulses to several endocrine glands and to pathways governing the activity of skeletal muscle, the heart and smooth muscle (Figure 2.1). Via a sophisticated network of neural signals and hormone release, the hypothalamus controls sexual cycles, growth, pregnancy, lactation and a wide range of other basic and emotional reactions. Each hypothalamic function is associated with one or more small areas that consist of aggregations of neurons called hypothalamic nuclei. In the context of reproduction, several groups of hypothalamic nuclei are connected to the underlying pituitary gland by neural and vascular connections.
Preimplantation genetic diagnosis (PGD) was developed in the late 1980s to help couples who are at risk of transmitting an inherited disease to their offspring, as an alternative to prenatal diagnosis during pregnancy. Prenatal diagnosis has the disadvantage that if the diagnosis shows the fetus to be affected, the couple must decide whether they wish to terminate the pregnancy or continue with the knowledge that their child is going to be affected by the genetic disease. PGD offers some of these couples an alternative, as the diagnosis is performed on the preimplantation embryo, and only embryos assessed as being unaffected by the genetic disease are transferred to the patient. The pregnancy is therefore initiated with the knowledge that the fetus is free from the disease, at that moment in time.
This extensively updated new edition provides an indispensable account of modern in-vitro fertilization practice, building upon the popularity of previous editions. The authors initially give a comprehensive review of the biology of human gametes and embryos, before outlining basic to advanced IVF techniques. New developments in practical techniques and understanding are discussed, including in-vitro maturation, vitrification, preservation of fertility for cancer patients, stem cell technology, preimplantation genetic testing, and the role of epigenetics and imprinting. The revised introduction also incorporates a 'refresher' study review of fundamental principles of cell and molecular biology, now updated with current knowledge of meiosis in human oocytes, embryo metabolism and basic principles of genome editing. With high-quality illustrations and extensive, up-to-date reading lists, it is a must-have textbook for trainee and practising embryologists, as well as clinicians who are interested in the scientific principles that underpin successful IVF.