EMBRYOLOGY Lec.5 ــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــــ Cleavage refers to the stereotyped pattern of early mitotic divisions that divides up the large volume egg cytoplasm. The early zygote is unique in being so large. Most cells undergo a period of growth between cycles of mitosis, but this is not true for early cleavage stage blastomeres. With each division the cells get smaller. PATTERNS OF EMBRYONIC CLEAVAGE Pattern of embryonic cleavage is determined both by the position of the mitotic spindles and by the amount and distribution of yolk. The types of eggs based on yolk characteristics are described as: Isolecithal: sparse evenly distributed yolk, eg., sea urchin, mouse Mesolecthal: moderate amount of yolk, often unevenly distributed, eg., frog Telolecithal: dense yolk concentrated at one end, eg., bird, reptile Centrolecithal:yolk concentrated at the middle of the egg,eg.fly There are several types of cleavage symmetry seen in nature: radial (echinoderms, amphibians), spiral (mollusks, annelids), Bilateral (ascidians,tunicates), Rotational (mammals). The two figures below show examples of holoblastic and meroblastic cleavage symmetries. 1 2 3 Sea urchin cleavage Sea urchins also have radial holoblastic cleavage, but with some interesting differences. First cleavage is meridional. Second cleavage is meridional. Third cleavage is equatorial Fourth cleavage is meridional, but while the four animal pole cells split equally to give rise to eight equal sized animal blastomeres termed MESOMERES, the vegetal cells divide asymmetrically along the equatorial plane to give 4 large MACROMERES and 4 much smaller MICROMERES at the vegetal pole. Fifth division the MESOMERES divide equatorially to give two tiers of eight MESOMERES an1 and an2 , the MACROMERES divide meridionally forming a tier of eight cells below an2, the MICROMERES divide to give a cluster of cells below the veg1 layer. The sixth divisions are all equatorial, giving a veg2 layer. The seventh divisions are all meridional giving a 128 cell blastula. 4 AMPHIBIAN CLEAVAGE Cleavage in many amphibians is holoblastic with radial symmetry, however the large volume of yolk (its mesolecithal) interferes with cleavage. At the animal pole first cleavage proceeds proceeds cleavage egg the at about 1mm/min, while through the vegetal pole is 50-100 times slower (.02mm/min). While the first is still incomplete in the yolky vegetal region of the second meridional cleavage begins to take place. WHAT FUNCTION DOES THE BLASTOCOEL SERVE? The blastocel spatially separates cells so they do not touch one another. Cells at the roof of the blastocoel normally become ectoderm. 5 MAMMALIAN CLEAVAGE The mammalian egg is released from the ovary into the oviduct where it is fertilized. First cleavage begins about a day after fertilization within the oviduct. In sharp contrast to most animals, cleavage in mammals can be very slow---1/day. 6 Additionally, the cleavage planes are somewhat different from other animals. First cleavage is meridional just like sea urchin and frog. However, the second cleavage division sees one of the blastomeres dividing meridionally and the other equatorially! This type of cleavage is called ROTATIONAL HOLOBLASTIC CLEAVAGE. MEROBLASTIC CLEAVAGE In telolecithal and centrolecithal eggs the large dense yolk prevents cleavage. Telolecithal eggs are characteristic of birds, fishes, and reptiles while centrolecithal eggs are characteristic of insects. Telolecithal eggs result in meroblastic discoidal cleavage. Cleavage is restricted to the blastodisc at the animal pole of the egg. At early cleavages, because cleavage cannot 7 proceed through the yolk, the blastomeres are continuous at their vegetal margins. 8