| superficial cleavage | Meroblastic cleavage with the divisions limited to the peripheral (surface) cytoplasm of a centrolecithal egg. (05 Mar 2000) |
|---|---|
| determinate cleavage | Cleavage resulting in blastomeres each capable of developing only into a particular embryonic structure. (05 Mar 2000) |
| discoidal cleavage | Meroblastic cleavage limited to the small cap (animal pole) of protoplasm of large-yolked eggs, such as the telolecithal eggs of birds. (05 Mar 2000) |
| incomplete cleavage | Incomplete separation of the blastomeres, with the divisions being limited to the nonyolked portion of the egg. Synonym: incomplete cleavage. (05 Mar 2000) |
| indeterminate cleavage | Cleavage resulting in blastomeres of similar developmental potencies, each capable, when isolated, of producing an entire embryonic body. (05 Mar 2000) |
| thioclastic cleavage | The splitting of a bond in fashion analogous to hydrolysis or phosphorolysis except that the elements of a substituted hydrogen sulfide (usually coenzyme A) are added across the break. (05 Mar 2000) |
| total cleavage | Cleavage in which the blastomeres are completely separated; the entire egg participates in cell division. Synonym: complete cleavage, total cleavage. (05 Mar 2000) |
| enamel cleavage | The splitting of enamel in a plane parallel to the direction of the enamel rods. (05 Mar 2000) |
| equal cleavage | Cleavage producing blastomeres of like size. (05 Mar 2000) |
| equatorial cleavage | Cleavage in which the plane of cytoplasmic division is at right angles to the axis of the ovum. (05 Mar 2000) |
| yolk cleavage | Segmentation of the vitellus. (05 Mar 2000) |
| unequal cleavage | Cleavage producing blastomeres of different sizes at the two poles. (05 Mar 2000) |
| freeze cleavage | Method of specimen preparation for the electron microscope in which rapidly frozen tissue is cracked so as to produce a fracture plane through the specimen. The surface of the fracture plane is then shadowed by heavy metal vapour, strengthened by a carbon film and the underlying specimen is digested away, leaving a replica that can be picked up on a grid and examined in the transmission electron microscope. The great advantage of the method is that the fracture plane tends to pass along the centre of lipid bilayers and it is therefore possible to get en face views of membranes that reveal the pattern of Integral membrane proteins. The E face is the outer lamella of the plasma membrane viewed as if from within the cell, the P face the inner lamella viewed from outside the cell. Fracture planes also often pass along lines of weakness such as the interface between cytoplasm and membrane, so that outer and inner membrane surfaces can be viewed. Further information about the structure can be revealed by freeze etching. Extremely rapid freezing followed by deep etching has allowed the structure of the cytoplasm to be studied without the artefacts that might be introduced by fixation. (18 Nov 1997) |