© Springer International Publishing Switzerland 2017 1
F. Pelegri et al. (eds.), Vertebrate Development, Advances in Experimental
Medicine and Biology 953, DOI 10.1007/978-3-319-46095-6_
Chapter 1
Egg Activation at Fertilization
Zoltan Machaty, Andrew R. Miller, and Lu Zhang
Abstract Fertilization is the union of gametes to initiate development of a new
individual. The female gamete is formed during oogenesis. The process begins
when, in the early embryo, primordial germ cells arise and subsequently colonize
the genital ridges. They differentiate into oogonia, start meiosis, and become pri-
mary oocytes. The cell cycle of the primary oocytes then becomes arrested in mid-
meiosis for an extended period of time. Prior to ovulation the oocytes undergo a
growth phase and their sizes increase significantly. A hormonal cue then triggers
oocyte maturation that involves the resumption of meiosis, the completion of the
first meiotic division, and, as a result, the reduction in the diploid chromosome
number. The cell cycle then stops again; in vertebrates this arrest occurs at the meta-
phase stage of the second meiotic division. Meiosis resumes at fertilization, when
the sperm activates the egg, i.e., it causes a series of changes that are required for the
initiation of embryo development. This is achieved by triggering an elevation in the
egg’s intracellular free calcium concentration. In response, the fertilized egg com-
pletes meiosis and enters the first embryonic cell cycle.
Keywords Fertilization • Oocyte • Egg • Activation • Sperm • Calcium • Signal
transduction
1.1 Introduction
Sexual reproduction evolved 1.2 billion years ago (Butterfield 2000 ). Around that
time some organisms stopped simply dividing into two and started to reproduce
sexually; fossil records of red algae clearly show traces of specialized sex cells
(spores). During sexual reproduction, the genetic material from two individuals
come together through the unification of their haploid sex cells, and the arising
progeny will have genes from both parents. Sex cell production is associated with
Z. Machaty (*) • A.R. Miller • L. Zhang
Department of Animal Sciences, Purdue University, Lilly Hall of Life Sciences,
915 W. State Street, West Lafayette, IN 47907, USA
e-mail: [email protected]; [email protected]; [email protected]