Cleavage

Group 3: Rose Ann Esteban Karyl Sabrina Javellana Methence Tayuba

Submitted on: August 14, 2006

LABORATORY ACTIVITY CLEAVAGE AND BLASTULA 1. Study slides and models showing cleavages and blastula stages. 2. Draw and label observed stages as seen in x.s. and w.m. preparations.

3. Classify and draw eggs on the basis of: (a) yolk content

(b) Cleavage pattern

4. Compare and contrast appearance of frog cleavage and blastula with Amphioxus, bird/reptile, and mammalian embryo at same stages. Draw and label. Frog Frog embryo undergoes modified holoblastic cleavage. It has much yolk and its diameter is 2mm. Because the vegetal yolk impedes cleavage, the second division begins in the animal region of the egg before the first division has divided the vegetal cytoplasm. (C) Frog cleavage is known to be holoblastic. The third division is displaced toward the animal pole. (D–H) The vegetal hemisphere ultimately contains larger and fewer blastomeres than the animal half. The frog embryo contains a blastocoel in the animal hemisphere of the embryo. H represents a cross section through a midgastrula stage embryo.

Amphioxus The Amphioxus embryo also undergoes holoblastic cleavage. It has little yolk and egg diameter is 0.1mm. Cells of the vegetal hemisphere are but slightly larger than those of the animal hemisphere, the wall of the blastula is one cell layer in thickness, and the blastocoel is large.

Mammal Mammalian embryo undergoes holoblastic cleavage. It practically has no yolk and has an outer trophoblast and inner cell mass formed in the blastula. 1st cleavage is meridional (N-S), 2nd cleavage is also meridional, but at right angles to the first cleavage plane. 3rd cleavage is equatorial. 1st and 2nd cleavages produce equal sized blastomeres, but the 3rd cleavage produces two size classes of cells. The 4th cleavage results in three cell size classes, the Mesomeres, the large Macromeres and the tiny Micromeres. The blastomeres of an eight cell embryo are packed tightly to one another. This phenomenon is called compaction. The blastula is known as the ‘blastocyst’.

Bird Bird embryo undergoes meroblastic cleavage. Large quantities of yolk present. It has a diameter or 30mm. Only the germinal disk cleaves, forming a disk of cells – the blastodermresting on the yolk. A slight slit between this and the yolk represents the blastocoel. The blastocoel is smaller and displaced dorsally, due to the thickness of the ventral wall.