Preface

Embryology is a vast discipline concerned with the study of embryogenesis. It is the branch of animal biology that studies the prenatal development of gametes (sex cells), fertilization, and development of embryos and fetuses. Additionally, embryology encompasses the study of congenital disorders that occur before birth, known as teratology. Human embryology studies began by using human embryo samples derived from maternal deaths, abortion, or surgery in the 19th century. However, human embryology has progressed little during the last 100 years because of ethical aspects and limited human materials. Since the middle of last century, embryo in vitro culture technology has advanced with developments in assisted reproductive technology (ART), including in vitro fertilization (IVF) and its related procedures, including intracytoplasmic sperm injection (ICSI), embryo cryopreservation and frozen embryo transfer, and preimplantation genetic testing. Also, recent studies using both nondestructive and destructive imaging techniques, such as time-lapse imaging, have allowed many morphological measurements of embryos. Embryology has been enriched and developed greatly in terms of its contents and forms. This book summarizes some novel observations and developments in embryology. It includes five sections.

Section 1 discusses gamete collection and cryopreservation technology and theory. The beginning of life starts with the combination of an egg and sperm. The first step in current in IVF practice is the collection of an oocyte and sperm. Important to note is that an individual's body weight has a significant effect on their fertility. Thus, Chapter 1 examines the impact of body mass index (BMI) on oocyte retrieval number in IVF. In the study presented, obese patients had fewer oocytes retrieved.

The cryopreservation of gametes including sperm and oocytes has been widely used in animal breed resource preservation and human IVF clinic practice. Typically, two freezing methods, slow freezing and vitrification, have been used in ART practice. Slow freezing is a conventional cryopreservation method that is based on a slow cooling rate and the use of a low concentration of cryoprotectants. This leads to less toxicity to cells/tissues; however, it can also decrease their survival rates. Nowadays, vitrification has replaced slow freezing to obtain a higher survival rate for sperm, oocyte, and embryo cryopreservation. Vitrification, as a significant stressor, appears to have a significant impact on epigenetic modifiers and maternal transcripts of the oocyte, which ultimately results in lower developmental potential. Thus, Chapter 2 reviews the recent progress of human sperm vitrification technology. Then, Chapter 3 discusses the impact of oocyte vitrification on epigenetic modification and maternal transcripts, providing a theoretical basis for the optimization and improvement of vitrification-warming technology.

ART has been extensively applied for treating human infertility for more than 40 years, but IVF success rates could be further improved.

 Section 2 includes two chapters that discuss how to increase IVF outcomes by both enhancing IVF laboratory quality control and improving embryo technique. These approaches include improving the air quality inside the lab to ensure volatile organic compound (VOC)-free air, use of a tri-gas incubator, adequate control of pH and osmolarity of culture media, use of embryo-tested devices and plastics, and strict quality control that allows adequate development of the embryos until the blastocyst stage. Other strategies to improve embryo quality during in vitro culture include volume reduction of drop culture media and individual or group culture of embryos. Embryo transfer is the last step of a series of events in IVF and poor management can jeopardize the entire process. Thus, Chapters 4 and 5 review some key techniques of embryo transfer.

This book provides a comprehensive overview of embryology and presents basic theories and techniques for human IVF clinic practitioners and embryologists.

**Bin Wu, Ph.D. HCLD (ABB)**

Tucson, Arizona, USA

Arizona Center for Reproductive Endocrinology and Infertility,

I would like to thank all the contributing authors for their excellent chapters.

Preimplantation embryo development refers to the maturation of a fertilized ovum to a blastocyst. This process is highly regulated and required for proper implantation of the blastocyst into the endometrium. During this phase, several tasks must be accomplished. The differentiated zygotic genome must undergo reprogramming back to totipotency to generate all the different types of tissue that make a human being. Next, certain cells begin to differentiate to prepare for implantation, which occurs at approximately day 7 post-fertilization. This progression is a result of the careful interplay between maternally persistent RNA transcripts and the activation of the zygotic genome. After the embryonic genome activation, blastomere differentiation begins to occur and cellular polarity has been shown to be the signal transduction that initiates this differentiation. Chapter 6 discusses the molecular and cellular mechanisms regulating preimplantation embryo development and its real effect in ART.

Thousands and thousands of human embryos have been produced by IVF and how to use these embryos has become a heated argument all over the world. The scientific associations of human reproduction experienced fundamental change in the twentieth century, with the development of in vitro fertilization for the treatment of infertility. The separation of sex and assisted reproductive technology treatments led to a revolution in gender selection and similarity relations, while embryo diagnostics led to a shift from scheduling families to planning a child. Furthermore, a fertilized egg outside the womb is a new form of human life that can be conserved and manipulated. The embryo in a petri dish in a laboratory has become the entity of the reproductive market, driven by clients and their claim to a right to reproductive choice. These reproductive technological improvements encounter deep-set moral sensitivities of human self-respect and the relation of human beings to their own nature.

Section 4 (chapter 7) presents these moral approaches, especially regarding interventions in preimplantation-stage embryos in the laboratory, suggesting environmentally suitable laboratory conditions for this entity. Additionally, not only do new suggestions for legitimately suitable regulations takes care of embryo's right, but also embryology laboratory personnel, clinics and parents.

After an embryo is transferred into a uterus, it will begin to grow rapidly and form early organs. With x-ray and ultrasonographic imaging technology innovation, the organogenesis of early embryos has obtained some novel observations and knowledge. Section 5 includes two chapters (chapters 8 and 9) that described some new discoveries in the hepatic fetal venous system and dental crown formation during early embryo development.

This book provides a comprehensive overview of embryology and presents basic theories and techniques for human IVF clinic practitioners and embryologists.

I would like to thank all the contributing authors for their excellent chapters.

**Bin Wu, Ph.D. HCLD (ABB)** Arizona Center for Reproductive Endocrinology and Infertility, Tucson, Arizona, USA

**1**

Section 1

Gamete Collection and

Vitrification for Freezing
