Lecture 7&8 Notes - Vandan Desai BIOL...

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Vandan Desai BIOL 442—Developmental Biology 1 Lecture #7 & #8—Cleavage I. Basics of Cleavage :: A. Cleavage—a series of rapid mitotic divisions that divide the enormous volume of the egg into numerous smaller cells called blastomeres i. In most systems, rate of cell division and placement of blastomeres relative to each other is under control of RNA & proteins stored in the egg Mammals are the exception ii. During cleavage, cytoplasmic volume of egg doesn’t increase ; instead, zygote cytoplasm is divided into increasingly smaller cells B. Cleavage divisions are very rapid : i. Examples: Drosophila = 10 min per division Xenopus (frogs) = 35 min per division ii. Humans are an exception = 24 hours for first division iii. Rapid division achieved by leaving growth phases out of cell cycle Normal cell cycle: M => G 1 => S => G 2 => M… Cleavage cell cycle: M => S => M… C. In figure on left – midblastula transition @ hour 40 i. Period b/w end of cleavage and beginning of gastrulation ii. See transition back to normal cell cycle : rate of divisions ↓, G 1 and G 2 phases restored, and cytoplasmic volume ↑ D. Cleavage Cell Cycle— i. No growth: embryonic volume remains the same ii. No RNA synthesis: maternal stores provide most of needed materials iii. Cells are non-motile during cleavage (do see motility in gastrulation) iv. In many organisms, cleavages are synchronous (all cells divide at once) II. Body Axes & Cleavage Patterns and Factors that Influence Them (Yoke) :: A. Body axes—a body has a top ( anterior ) and a bottom ( posterior ), a back ( dorsal ) and a front ( ventral ), a left and a right, & you can cut it and look at it in different planes B. Factors that Influence Cleavage Patterns— 2 major factors determine cleavage planes that divide fertilized egg into blastomeres: i. Factors in the egg cytoplasm determine the orientation of the mitotic spindles and the timing of their formation ii. Amount and distribution of yolk Yolk gives an egg an axis Yolk-rich end = Vegetal Pole ( BOTTOM )
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Vandan Desai BIOL 442—Developmental Biology 2 Yolk-free or lower yolk end = Animal Pole ( TOP ) Yolk inhibits cleavage If one pole contains less yolk , then cleavage will occur preferentially there or will progress more rapidly there, influencing number and size of blastomeres Yolky side = less cleavage => larger size and smaller # C. 2 Types of Cleavages— i. Holoblastic = cleavage furrow passes completely through the egg ii. Meroblastic = incomplete cleavage due to yolk; cleavage furrow is either slowed or cannot pass through the yolk D. 6 Patterns of Cleavages— radial , bilateral , spiral , rotational , discoidal , and superficial ( Mesolecithal = slowed b/c yolk at vegetal; smaller & more rapid divisions in animal pole b/c less yolk)
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Vandan Desai BIOL 442—Developmental Biology 3 ( Centrolecithal = nuclear division first; migrate to periphery) E. 2 Cleavage Axes— i. Meridional Cleavage = top/bottom; animal/vegetal axes ii. Equatorial Cleavage = around middle III. Cleavage Patterns of Representative Animals :: A. Sea Urchin— very simple radial holoblastic
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