Embryology - Biology 104, Spring 2006 - Albert Harris and Corey Johnson
OUTLINE OF NINTH LECTURE: Feb 3, 2006, by Corey JohnsonCleavageCleavage is the division of cells following fertilization. Egg activation is characterized by a great increase in metabolic activity. The activity of cells during cleavage stage is geared toward DNA synthesis and mitosis rather than growth or developmental specialization. Cells are continually dividing the existing cytoplasm of the egg into smaller and smaller cells without increasing the size of the embryo. In most species, cleavages are synchronous. The 2 cells simultaneously divide to form 4. The 4 simultaneously divide to form 8, and so on. Around the 16 cell stage, the embryo is called a morula and cleavages become less synchronous. Around this stage and into the blastula stage, specialization begins to take place. That is, cells begin to take on different characteristics, express different genes and the axes of the embryo are fixed/determined. Cytokinesis occurs after karyokinesis (nuclear division) in the same plane as the metaphase plate. Physically manipulating the position of the mitotic apparatus can cause the formation of the cleavage furrow in different areas. What ever region of the plasma membrane is closest, cleaves first. The centrioles derived from the sperm migrate 90 degrees on either side of the zygote's nucleus. After nuclear division, each cell gets one of the centrioles (which duplicate, for subsequent cell divisions). The centrioles in the new cells migrate 90 degrees and this process continues on and on. See the figures from lecture: The yolk can often displace the nucleus toward the animal pole (amphibians). This guides cytokinesis to begin at the animal pole and progress toward the vegetal in such embryos.
Oligolecithal/microlecithal - little yolk
Telolecithal - yolk displaced to one side of egg
Holoblastic cleavage separates blastomeres completely
What combination of the three catagories describes the animals we've discussed?
Make a chart:
Sea urchin cleavageCell cycle is very short. Divisions about every 30 minutes! S-M-S-M-S etc.Only later is there a M-G1-S-G2-M-G1-etc First 2 cleavages are meridional (like longitude); third cleavage is equatorial (latitude) Fourth cleavage. Animal pole: symmetric; Vegetal pole: asymmetric, resulting in macromeres and micromeres. Micromere formation depends on cytoplasmic determinants form the vegetal pole. If vegetal cytoplasm is removed from the zygote, micromeres will not form Driesch formulated the principle that the prospective potency of a blastomere is greater than its prospective fate. He called such blastomeres (like the early urchin blastomere), harmonious equipotential systems. We consider it, regulatory development. 8 cell urchins lose their equivalent potentials. The vegetal 4 cells can give rise to a slightly abnormal but complete larva. The animal 4 cells form as far as a blastula but no further. The vegetal cells contain the "ingredients" for gastrulation. So a regulatory system becomes mosaic after the 3rd cleavage
Amphibian cleavageBlastula forms in about 24hr. The first cleavage bisects the gray crescent.Why? Hint: it has to do with the sperm entry point and the migration of the centrioles. 1st and 2nd cleavages are meridional and perpendicular to one another. The 3rd is equatorial (lattitudinal is probably better since the plane of cleavage is displaced toward the animal pole.
Cleavages in the animal pole are synchronous, but the vegetal cells divide much more slowly. Blastocoel formation can be traced back to the 2-cell stage! Na+ is pumped by a small segment of plasma membrane surrounded by tight junctions. Hans Spemann temporarily separated a newt zygote (with a hair from his child's head) into a region with a nucleus and a region without. The region with a nucleus began forming a normal embryo. He then loosened the hair allowing one nucleus to pass into the undeveloped side. That side then began to undergo normal embryonic development. This demonstrated that even at a later stage in development, the nucleus can still direct development. However! Something must have ÒtoldÓ the nucleus that he was now in a zygote's cytoplasm. Likewise, a nucleus from and adult frog was inserted into an enucleated zygote producing a normal frog embryo. Nuclear transplantation... just like Dolly the sheep (only in 1952).
***Please read about avian and mammalian cleavage below. Discoidal cleavage. 1st, 2nd, and 3rd cleavages are synchronous and perpendicular to the previous. The 4th cleavage is circumferential, dividing 8 cells into 8 inner and 8 outer cells. After the 32 cell stage is reached, a cleavage plane separates cells into upper and lower cells. More layers are added in this way. At about 100 cells, the blastoderm is several layers thick and one layer thick at the perimeter. The subgerminal cavity has a lower pH and an electric potential exists across the blastoderm. Reversal of the pH or electrical gradient results in the reversal of the dorsal/ventral axis The A-P axis of the blastoderm is determined by gravity (anterior end forms closes to gravity). The posterior end migrates in forming hypoblast, after cells are shed from the ventral surface of the blastoderm.
Mammalian cleavage How are the inside and outside cells specified? As usual there are two lines of thought. 1) cytoplasmic determinants 2) micro-environment determines fate... those that "end-up" on the outside become trophoblasts.
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