DEVELOPMENTAL BIOLOGY CLASS NOTES
Fertilization and early embryonic cleavage
: Chp 7: 175-180, 205-206--skim rest of chapter if you want;
cleavage: Chp 8: 211-215, 243-
246; Chp 10:291-295.
You may also read about cleavage in chick and mammals if you like (Chp 11:
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.
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
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 (7.5). In amphibians, fish, and most mammals, the oocytes is arrested in metaphase of
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 (7.32) 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.
Cleavage is the first stage of embryogenesis. The processes that occur during cleavage
accomplish several things:
1) partition the contents of the zygote into many cells (called blastomeres), with no increase in
2) begin to establish different cell identities and separate certain cells from each other;
3) form a hollow ball (or disc) of cells called the blastula, and
4) 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