In vitro maturation (IVM) systems have been developed to support the growth and development of oocytes, from their most immature stages to developmentally competent oocytes. The IVM technique has been in veterinary practice for a long time. The first reported study of IVM in humans dates back to 1965; however, the first pregnancy resulting from IVM in humans was only reported in 1991 and involved the use of donated oocytes from unstimulated ovaries from women undergoing gynecological surgery. A 1994 report described a pregnancy in an anovulatory woman with polycystic ovary syndrome (PCOS) after IVM of her oocytes.

Our genetic information, sometimes described as the “Book of Life,” can be envisaged as a compilation containing two “sets” of information, with one originated from the female and the other from the male. Each set includes 23 “volumes” (chromosomes), and each of these chromosomes contains many “chapters and pages” (clusters of genes). Each gene encodes for a specific message to the cell cytoplasm, where ribosomes translate the codes to specific proteins. The genetic code is made up of combinations of three out of four possible essential nucleotides (A, T, G, and C). These nucleotides are the chemical molecules, components of deoxyribonucleic acid (DNA), creating the genetic information that can be further transcripted and translated by different cell types of machinery to produce the essential cellular proteins. A trinucleotide code (three basic nucleotides), called a codon, encodes a specific amino acid, a basic brick of protein structure.

In vitro fertilization (IVF) is a complex series of techniques used to help with fertility or prevent genetic problems and assist with the conception. During IVF, oocytes are collected from ovaries and fertilized by spermatozoa in a laboratory. The fertilization can be done using the patient’s oocytes and the partner’s sperm or donor oocytes and partner’s/donor sperm. All gametes (spermatozoa and oocytes) should be correctly prepared and selected before the initiation of the fertilization process.

The corpus luteum (CL) is a transitory endocrine gland that develops from the postovulatory ruptured follicle during the luteal phase. Human chorionic gonadotropin (hCG), produced by the embryo, maintains the secretory activity of the CL due to its structural similarity to luteinizing hormone (LH) and subsequent activation of the same receptor. It maintains and stimulates the CL to produce estradiol (E2) and progesterone (P4). Luteal P4 is involved in the transition of the endometrium from a proliferative to a secretory type, with increasing decidualization – an essential facilitator of implantation [1] – and relaxation of the uterine muscle. Preparation of the endometrium lining the uterus for implantation of the embryo begins toward the end of a proliferative phase and extends throughout the luteal phase. This is important for the implantation process and maintenance of pregnancy until the placenta takes over steroid hormone production at approximately 7 weeks.

The male reproductive system consists of organs that function to produce, transfer, and introduce mature sperm cells into the female reproductive tract, where fertilization can occur (Figure 3.1). The initial development of the male reproductive organs begins before birth when the reproductive tract differentiates into the male form. Several months before birth, the immature testes descend behind the parietal peritoneum into the scrotum, guided by the fibrous gubernaculum. The testes and other reproductive organs remain in an immature form. They remain incapable of providing reproductive function until puberty when levels of reproductive hormones stimulate the final stages of their development (Figure 3.2). Prepubertal boys have no spermatogenesis; however, spermatogonia preserve in their testicles. Sexual maturity and ability to reproduce are reached at puberty. A gradual decline in hormone production and testicular cell count during adulthood may decrease sexual desire and fertility.

In vitro fertilization (IVF) strives to ensure an in vitro environment that closely mimics the physiological environment of gametes. However, the providing of such conditions is limited by the lack of our knowledge on the actual natural levels of oxygen concentration, pH, and temperature within the reproductive tract.

The female and male reproductive tracts originate from the same embryonic/fetal tissue. The gonads and internal and external genitalia begin as bipotential tissues. The indifferent gonad consists of a medulla and cortex. Human female and male embryos develop in the same way for the first 6 weeks, regardless of genetic sex (46,XX or 46,XY karyotype) (Figure 1.1). The one way to tell the difference between 46,XX and 46,XY embryos during this time period is by looking for a Barr body (“inactive” one X chromosome) or a Y chromosome. The medulla of the XY embryo will develop into the testes and the cortex will regress. In the XX embryo, the ovary will originate from the cortex and the medulla will decline. A complete 46,XX chromosomal complement is necessary for normal ovarian development. The second X chromosome contains elements essential for ovarian development.

Cryopreservation involves the cooling of cells and tissues to subzero temperatures to stop all biological activity and preserve them for future use. Initial cryopreservation methods were ineffective as simple cooling techniques led to cellular damage due to the altered concentration of solutes within the cells, ice formation, and excessive dehydration.

Historically, the absence or a small number of sperm cells in the ejaculate often precluded men from fathering their genetic progeny and relegated couples to the use of donor spermatozoa insemination or adoption or childlessness. With the development of intracytoplasmic sperm injection (ICSI), men with azoospermia (absence of sperm in the ejaculate) or severe oligozoospermia (less than 5 × 106 spermatozoa in the ejaculate) are able to father a child following single sperm cell injection into the cytoplasm of a single oocyte. In the years after the development of ICSI, it was discovered that sperm retrieved directly from testicular tissue can also be used for oocyte fertilization and enable healthy embryo development.

Evidence of infertility complications dates back to biblical times, when the foremother, Sarah, failed to conceive. Many centuries later, in 1667 the Danish scientist Anton van Leeuwenhoek, a glazier by profession, serendipitously invented a magnifying glass and in 1674 Nicolaas Hartsoeker, a Dutch scientist, assisted by Leeuwenhoek, found sperm cells in the seminal fluid under magnification (Figure 0.1). Observing human sperm through a microscope, Hartsoeker believed that he saw tiny men inside the sperm, which he called Homunculus. The theory of Homunculus (1694) claimed that a tiny, formed child exists in the head of the sperm cell, which becomes engulfed in the uterus, where it grows as if in an incubator until the moment of birth (Figure 0.2). It was then that the idea of in vitro fertilization (IVF) was born, i.e., to grow a child in a laboratory flask instead of in the mother’s womb. The understanding that the embryo is formed by fertilization of the oocyte by the sperm cell came only later.

At the beginning of the menstrual cycle, there is an increase in bioactive follicle-stimulating hormone (FSH) levels, a stimulus for the growth and differentiation of follicular granulosa cells (GCs). GC steroidogenic enzymes are also inducible by FSH and are necessary for the production of estradiol (E2) and progesterone (P4), as well as expression of luteinizing hormone (LH) receptors on theca cells (TCs). LH then stimulates theca cells to produce androgens, which are metabolized to E2 by GCs under the influence of FSH. Elevated levels of E2 then inhibit FSH secretion, providing a negative feedback effect. Growth of the leading follicle continues owing to elevated levels of FSH receptors, whereas secondary follicles with fewer FSH receptors undergo atresia. Taken together, FSH and LH work in concert, as depicted by the classic two-cell (TC and GC), two-gonadotropin (FSH and LH) theory.

Cervical mucus is a regulator of the sperm transfer from the vagina to the uterine cavity. Estradiol (E2) stimulates the production of large amounts of thin, watery, alkaline acellular cervical mucus with ferning, spinnbarkeit (crystallization), and sperm receptivity. Progesterone (P4) inhibits the secretory activity of cellular mucus and produces low spinnbarkeit and absence of ferning, which is impenetrable by spermatozoa. In midcycle, the cervix softens progressively, the os of the cervical canal dilates, and clear, profuse mucus exudes from the external os. In a few days after ovulation, the cervix becomes firm, and the os closes. The cervix is the first barrier for the sperm to overcome.

The physiological importance of the female reproductive system is the production of offspring. The female produces gametes that can be fertilized by the male gamete to form the first cell of the offspring. The sequence of events is tightly dependent on the proper functionality of the endocrine system.

Much of the endocrine system is governed by rhythms, some of which are intrinsic, while others are influenced by the environment. Rhythms that are longer than 24 hours, the infradian rhythms, include the seasonal breeding patterns in some animals and the female menstrual cycle. Circadian or 24-hour rhythms include the sleep–wake cycle and the increase in gonadotropin secretion seen at night in adolescents. Finally, cycles of less than 24 hour, the ultradian cycles, include the pulsatile release of luteinizing hormone (LH), follicle-stimulating hormone (FSH), growth hormone, and prolactin.