Gametes unite to form the zygote. The zygote
has all the genetic information for the formation of different parts of an
individual. The zygote divides to form the embryo. The embryo develops as per
the genetic information into an organism. The organism has different types of
cells and tissues.
Each cell and tissue has different functions
e.g. muscle cells have protein, actin and myosin for muscle contraction, the
goblet cells produce mucous and oxyntic cells produce HCl. So different cells
function in different ways having the same genetic material as it was present
in zygote. The question is that, how the same genetic material function
differently in different types of cells?
Evidence has suggested that there are three
factors, which act together in various ways to bring out differentiation. These
are:
a) Nuclei
b) Cytoplasm
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| Acetabularia |
The Role of
the Nucleus
Danish biologist Joachim Hammerling performed
experiment on Acetabularia in 1943. It is a single celled marine green alga.
Individuals have distinct foot stalk and cap regions. The nucleus of this cell
is located in the foot. It grows to a length of 6 to 9 centimeter.
Hammerling selected individuals from two
species of the genus in which caps looked very different from each other.
Acetabularia mediterranea, which has a disk shaped cap and Acetabularia
crenulata which has a branched flower like cap.
Experiment: The cap was removed from one
species and thrown away. Next, the cytoplasmic stalk was cut off and grafted to
the base of other species, whose stalk and cap had already been cut off and
thrown away. A whole new alga grew from the joined pieces. The new organism
becomes complete with cap, stalk and foot.
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| Acetabularia Grafting Experiment |
Result: The first cap regenerated may be intermediate
in type, but when it was removed, the next cap formed always had the
characteristic of the species supplying the nucleus.
Conclusion: The nucleus of each grafted
algal cell must exert its influence through the alien cytoplasm in determining
the form of cap, so there is an evidence of nuclear control of the development
processes forming a new cap.
Nuclear
Equivalence
1. Spemann placed minute ligatures of human
hair around salamander zygotes just as they were about to divide, constricting
them until they were almost, but not quite, separated into two halves. The
nucleus lay in one half of the partially divided egg, the other side was
anucleate, containing only cytoplasm. The egg then completed its first cleavage
division on the side containing the nucleus, the anucleated side remained
undivided. Eventually, when the nucleate side had divided into about sixteen
cells, one of the cleavage nuclei would wander across the narrow cytoplasmic
bridge to the anucleate side. Immediately this side began to divide.
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| Spemann's delayed nucleation experiments |
Two kinds of experiments were performed.
(A) Hair ligature was used to constrict an
uncleaved fertilized new egg. Both sides contained part of the gray crescent.
The nucleated side alone cleaved until a descendant nucleus crossed the
cytoplasmic bridge. Then both sides completed cleavage and formed to complete
embryos.
(B) Hair ligature was placed so that the nucleus
and gray crescent were completely separated. The side lacking the grey crescent
became an unorganized piece of belly tissue, the other side developed normally.
Cytoplasmic
Influence on Development
(A) A frog's egg has anterior/posterior and
dorsal/ventral axes that correlate with the position of the gray crescent.
(B) The first cleavage normally divides the
gray crescent in half, and each daughter cell is capable of developing into a
complete tadpole.
(C) But if only one daughter cell receives the
gray crescent, then only that cell can become a complete embryo. This shows
that the chemical messengers are not uniformly distributed in the cytoplasm of
frog's eggs.
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| Cytoplasmic influence on development |
(A) A frog's egg has anterior/posterior and
dorsal/ventral axes that correlate with the position of the gray crescent.
(B) The first cleavage normally divides the
gray crescent in half, and each daughter cell is capable of developing into a
complete tadpole.
(C) But if only one daughter cell receives the
gray crescent, then only that cell can become a complete embryo. This shows
that the chemical messengers are not uniformly distributed in the cytoplasm of
frog's eggs.
2. Spemann performed another experiment. He
separated the two halves of embryo, both of them contained nuclei. Both these
halves developed into complete embryos. He also observed that from a 16-cell
embryo, even if a single cell is separated, it contains a complete set of genes
and form a complete embryo cell and thus, the nuclei were equivalent.
Spemann also observed that sometimes it may
happen that the nucleated half can develop into abnormal ball of cells. Later
studies proved that development depend on gray crescent. Gray crescent is the
pigment free area that appears at the time of fertilization. So in the half
lacking gray crescent, no further development can take place.
Influence of
Cytoplasm on Nucleus during Development
Experiment No. 1: If the early embryos of Sea
urchin are placed in sea water from which the calcium ions have been removed,
the cells tend to separate. Thus, it is possible to isolate the two cells
formed by the first cleavage or the four cells formed by the second cleavage.
Each of the cell continues to develop and becomes in time, a small but complete
sea urchin larva. This experiment was perfumed in 1892 by Hans Dietrich.
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| Influence Of Cytoplasm On Nucleus During Development |
Experiment No. 2: Instead
of dividing a sea urchin egg along the natural planes of cell division, we cut
across the axis of an unfertilized egg. Now, we have two halves of an egg,
sometimes the nucleus is located in the upper half and sometimes in the lower
half.
Both the halves heal, each forming an
apparently normal cell. Then we see sea urchin sperms. A sperm will enter each
half of the egg cell. If the half has a nucleus, the half will be diploid. If
the half is without nucleus, then it will have only the monoploid number of
chromosomes contributed by the sperm. Both the upper halves develop into a
hollow ball of cells with many cilia, but it forms no internal tissues. It
swims around for several days and then dies. The lower half too is very
abnormal and incomplete and dies before long. Neither of the half embryos is
then able to develop into a normal embryo, or even to survive for long.
Conclusion: In the experiment all the
four or two cells are getting same type of nucleus so all the cells fertilized
developed into embryo. In the second experiment there is different cytoplasm in
the lower and upper cut cells as the cytoplasm is lacking some of the material,
so after fertilization the embryos did not develop completely. This, it can be
concluded that the materials indeed affect and limit what genes in the nucleus
are above to do in controlling the path of the development.
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