Fertilization

Frank Longo

• Publisher: Garland Science
• Year: 2020
• Print ISBN: 9780412563508
• eBook ISBN: 9781000142792
• Edition: 1
• Topic: Medicine
• Subtopic: Gynecology, Obstetrics & Midwifery

This edition provides the reader with an introduction to this subject. During the past five years there has been a virtual explosion of information on the different phases of fertilization.This book should be of interest to advanced undergraduates and graduate students in developmental biology, zoology and cell biology; researchers entering the field.

1 General considerations of fertilization: a definition

The interaction of the spermatozoon and the egg initiates a series of transformations involving the nuclear and cytoplasmic components of both gametes. These transformations constitute the process of fertilization, which commences with the interaction and subsequent fusion of the gametes and culminates in the association of the corresponding groups of chromosomes derived from two pronuclei, one of maternal and the other of paternal origin (Wilson, 1925). In almost all cases investigators have pointed out that the essential aspects of fertilization are: (1) the association of the maternal and paternal genomes - biparental heredity - and (2) the activation of both the sperm and the egg - a series of events which alters the metabolism of both gametes and leads to the cleavage and differentiation of the fertilized egg or zygote. This mode of bisexual reproduction emerged during evolution and has been maintained in most metazoans (Margulis et al., 1985). Although the reasons for this are not clearly understood, fertilization may accelerate the rate of adaptation while avoiding the accumulation of detrimental mutations in a changing environment (Michod and Levin, 1987).

Meiosis, the cell division that reduces by half the number of chromosomes, is similar to and derived from mitosis (Margulis et al., 1985). The resultant cells, sperm and eggs, with half of the chromosomes' complement, fertilize, thereby restoring the diploid number of chromosomes and initiating the process of development. According to Margulis et al. (1985) this process flourished not because of its tendency to mix genes from separate sources and to generate genetic variation, but because it became fixed in the life cycles of a rapidly evolving group - the first animals.

Although isogamy, reproduction resulting from the union of two gametes that are identical in size and structure, occurs in some groups, particularly the protozoa, heterogamy, the union of two gametes of differing size and structure, is generally the rule among most groups of animals. Bell's (1988) discussion of the evolution of germ cells provides interesting insights into the basis of gamete dimorphism. Using the Volvocales as a model system, he shows that it is better to produce small gametes, as more can be generated from a given mass of material. However, if the viability of a zygote increases with size, small gametes will continue to be an advantage because they can be produced in great numbers, whereas large gametes will also be favored because they give rise to large and highly viable zygotes. The net effect of these two factors may then create a selection for gamete dimorphism.

Activation of the egg can be initiated by processes other than fertilization. Parthenogenetic activation by chemical and/or physical stimuli may lead to complete development or to the initiation of processes that simulate fertilization, but in most cases does not give rise to viable offspring. In either case, these observations indicate that the egg is endowed with the essential machinery and information to initiate developmental processes when suitably stimulated. Furthermore, activation changes that occur at fertilization and parthenogenesis do not require immediate gene action, implying that the genetic activity required for establishing the fertilization response occurs during oocyte maturation. Metabolic changes in the egg evoked by fertilization ultimately affect new gene expression and differentiation during later stages of embryogenesis. Hence, fertilization in the scheme of a biparental organism serves as a point of transition between gamete and embryonic development.

In addition to the processes of fertilization that characterize most bisexual organisms, there are groups of unisexual animals that have evolved unique mechanisms for perpetuation of their species (Bogart et al., 1989).

1. Some genera of female lizards reproduce by parthenogenesis.
2. Females of some species of fish reproduce by gynogenesis, i.e. sperm from a sympatric male activates the egg but does not contribute its genome to the embryo.
3. In hybridogenesis maturing eggs of some fish and frogs eliminate an entire genome which is retrieved in fertilization by the sperm of a sympatric male.

The loss of biparental reproduction in such cases has been the subject of controversy (Margulis et al., 1985).

Investigations of fertilization date well before the turn of the century under the leadership of cytologists such as van Beneden, Flemming, Strasburger, Boveri and Wilson whose observations on germ cells were closely affiliated with theoretical writings of Nageli, Weissmann, Hertwig, Roux and de Vries (Wilson, 1925). The remarkable observations of these investigators and their formulations provided the intellectual framework for the chromosome theory of inheritance. More contemporary research on fertilization tends to serve a dual role, one being its application to fertility control and the other as a model system to study basic processes of cells in general.

Much of the research in fertilization has employed the gametes of invertebrates, particularly echinoderms, mollusks and ascidians, as well as nonmammalian vertebrates such as frogs. The reasons for the popularity of these animals include their availability and minimal requirements for maintenance. In addition, because both sexes are separate and relatively large quantities of gametes can be obtained, which can be fertilized externally and develop in synchrony, specific processes of fertilization may be analyzed using a wide variety of techniques. Consequently, important insights have been gained into the causal chain of events that occur during fertilization in these animals, which are relevant to the study of fertilization in higher organisms. In contrast, the number of eggs available from mammals is generally small and, because they normally undergo internal fertilization, working with the gametes of such organisms is much more involved. Despite numerous difficulties, there have been considerable advances in our knowledge of fertilization in mammals, and in some ways we have greater insights into processes of mammalian fertilization than in invertebrates and lower vertebrates (Yanagimachi, 1994).

The timing of many of the events comprising fertilization in sea urchins is given in Table 1.1. Studies using the gametes of other invertebrates and vertebrates indicate that a comparable sequence of events is

Table 1.1 Timing of fertilization events in the sea urchin, Lytechinus pictus (incubated at 16-18°C); tinning of these events depends markedly upon species and temperature (See Whitaker and Stein-hardt, 1982)

Membrane potential
Ca^2+-Na^+ activation potential	Before 3 s
Na^+ activation potential	3-120 s
K^+ conductance increase	500-3000 s
Intracellular calcium release	40-120 s
Cortical granule reaction	40-100 s
NAD kinase activation	40-120 s
Reduced nicotinamide nucleotides increase	40-900 s
Acid efflux	1-5 min
Intracellular pH increase	1-5 min
Oxygen consumption increase	1-3 min
Protein synthesis initiation	5 min onwards
Amino acid transport activation	15 min onwards
DNA synthesis initiation	20-40 min

initiated in both groups of animals; sperm-egg attachment leads to the 'turning on' of new synthetic activities and developmental programs (Metz and Monroy, 1985).

Since metabolic and morphological changes of echinoderms, mollusks, ascidians, amphibians and mammalian gametes during fertilization have been well characterized experimentally, studies employing these organisms provide the major thrust of what is discussed herein. The two principal events of fertilization considered are (1) the initial interaction and activation of the sperm and egg and (2) the concluding events, involving pronuclear development and association that eventually lead to cleavage.

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Contents
• Preface to the first edition
• Preface
• 1 General considerations of fertilization: a definition
• 2 The spermatozoon
• 3 Sperm-egg binding
• 4 Gamete fusion and sperm incorporation
• 5 Egg activation
• 6 Blocks to polyspermy and the cortical granule reaction
• 7 Alterations in the egg cortex and cytoskeleton
• 8 Resumption of meiotic maturation
• 9 Metabolic alterations at egg activation
• 10 Development of the male pronucleus
• 11 Aspects regulating pronuclear development
• 12 Fate of incorporated sperm mitochondria, flagellum and perinuclear structures
• 13 Centrosome expression, sperm aster assembly and pronuclear migration
• 14 Association of the male and female pronuclei: the concluding events of fertilization
• 15 Manipulation and in vitro fertilization of human and other mammalian gametes
• References
• Index