Dr. Morris Maduro, UC Riverside
Biology 168 – 10F – Lecture 16, page 1
– Mesoderm induction, gastrulation
pp. 32-35; 67-76; 89-105; 125-133.
Some figures in these notes are redrawn from
Wolpert et al.
Formation of the blastocoel and mesoderm induction
At the blastula stage, a fluid-filled cavity in the animal half of the embryo is formed (the blastocoel). The
origin of this cavity has been traced to the first cell division: The cleavage furrow widens in the animal
hemisphere and over the next cell divisions fills with water, becoming the blastocoel. Water enters the
blastocoel because NaCl is actively pumped into the space by the cells surrounding it. Ions are not free to
diffuse because the cells are connected by tight junctions – hence, osmotic pressure causes water to
enter the blastocoel. What purpose does the blastocoel serve?
Nieuwkoop and others performed experiments to address the importance of the blastocoel by examining
the fates adopted by cells as if the blastocoel were not present:
If the cells at the very animal part of the embryo (called the
) are removed and allowed to
develop in isolation (an
experiment), ectoderm fates result. Similarly, ventral and dorsal
) cells give rise to ventral and dorsal mesoderm, respectively. Ventral mesoderm = blood,
mesenchyme; Dorsal mesoderm = notochord, muscle. Similarly,
give rise to
undifferentiated vegetal tissue.
If the presumptive ectoderm cells are recombined with vegetal cells, they give rise to mesoderm instead.
If ventral vegetal cells are used, ventral mesoderm results; if dorsal vegetal cells are used, dorsal
Properties: There must be a certain amount of animal cap cells (about 200) present for mesoderm
induction to occur (the
), which suggests that a factor must accumulate to a certain
threshold for induction of mesoderm genes to occur. The animal cap is able to respond to the induction
) up to 11 hours after fertilization, or up to the time gastrulation starts. Contact need only
occur for a short time (~2h) to induce some mesoderm, with full induction at 5h. The signal for induction
must be a secreted molecule -- if filters with pores too small for cells to develop junctions are used
between the animal cap and vegetal cells, induction still occurs.
The conclusion is that vegetal cells can
the specification of mesoderm in adjacent cells (
), and furthermore, that this induction is different depending on
(prospective ventral or dorsal) are in contact with the ectoderm. Hence, the blastocoel isolates a subset of