BIOL/ BMB 430
DEVELOPMENTAL BIOLOGY CLASS NOTES
Fertilization and early embryonic cleavage
Learning goals: Be able to:
Describe, in a general way, the process of fertilization and the different features of sperm and
Explain how the orientation of cleavage planes is determined.
Explain the consequences of cleavage planes on establishment of cell fate.
Describe the onset and consequences of the "mid-blastula transition", and compare its importance
in different organisms.
Reading Chapter 9, pp 342-348
Fertilization involves some spectacular cell biology, but in terms of development it simply
provides the stimulus to start the process. It serves three major developmental functions: bringing in the
paternal genome, bringing in a new centrosome, and re-starting the cell cycle in the arrested oocyte.
Contact of sperm with egg initiates a sequence of pre-programmed responses of the two gametes to each
other. These result in: invasion of the egg by the sperm, transmembrane signals that block further sperm
entry to prevent polysomy and activate the egg's metabolism, triggering of lipid and macromolecular
synthesis in preparation for cleavage, re-initiation of the cell cycle, and finally fusion of sperm and egg
nuclei to produce a diploid zygote with a combination of maternal and paternal genetic information.
Interestingly, in almost all animals, the process of ooctye meiosis is not complete when the sperm
enters the egg. In amphibians, fish, and most mammals, the oocyte is arrested in metaphase of meiosis II.
Accordingly, the oocyte nucleus still has two copies of each chromosome when the sperm nucleus
penetrates the egg! The oocyte has to complete meiosis, shunting off those chromosomes into the last
polar body before fusing with the sperm nucleus. The fertilized egg then goes immediately into mitosis--
the chromosomal material from the egg and sperm replicate separately, and then all the chromosomes line
up along the metaphase plate, and go on to complete the first mitotic division of the embryo. Finally, at
the now 2-cell stage, each cell has an integrated complete set of information from each parent, and
development can proceed.
Reading Chapter 8, pp 295-8
Cleavage is the first stage of embryogenesis. The processes that occur during cleavage accomplish several
partition the contents of the zygote into many cells (called blastomeres), with no increase in total
begin to establish different cell identities and separate certain cells from each other;
form a hollow ball (or disc) of cells called the blastula, and
shift control of development from maternally derived mRNA's and proteins to embryonically encoded
gene products. Also during cleavage, the axes of the embryo (A/P, D/V, L/R) become established; we
will discuss this process in more detail later. Embryos of different species differ in the relative timing
of these events as well as in their patterns of cleavage, but the developmental functions of cleavage are