Cleavage Patterns
Cleavage is the rapid series of cell divisions that occur after fertilization of an egg, leading to the formation of a multicellular embryo. The pattern of cleavage depends on the type of egg and the species. Here are some common cleavage patterns:
1. Radial Cleavage:
In radial cleavage, the cell divisions are symmetrical and occur perpendicular or parallel to the embryo's polar axis. This pattern is typically seen in deuterostomes, including echinoderms and chordates (including humans). The daughter cells sit atop each other, creating a tiered appearance.
2. Spiral Cleavage:
Spiral cleavage involves diagonal and unequal cell divisions. As the cells divide, they twist around the embryo, creating a spiral arrangement. This pattern is common in protostomes, including many mollusks and annelid worms.
3. Bilateral Cleavage:
Bilateral cleavage is a type of spiral cleavage where the divisions are consistently diagonal, creating a left and a right half. This pattern is seen in some annelids and mollusks.
4. Rotational Cleavage:
In rotational cleavage, the divisions are unequal and alternating. Daughter cells are smaller and larger in a predictable sequence. This type of cleavage is observed in some marine invertebrates.
5. Meroblastic Cleavage:
Meroblastic cleavage occurs in eggs with a large amount of yolk that is not evenly distributed. The cleavage furrows only penetrate the part of the egg without yolk, leaving the yolk-filled region intact. This type of cleavage is common in reptiles, birds, and monotreme mammals.
6. Holoblastic Cleavage:
Holoblastic cleavage occurs in eggs with little or moderate amounts of yolk, where the cell divisions divide the entire egg into smaller cells. This type of cleavage is seen in amphibians, most fish, and mammals.
7. Superficial Cleavage:
In superficial cleavage, cell divisions occur only in the outer layer of the egg, leaving a central, yolk-filled cavity. This pattern is observed in some insects, such as fruit flies.
8. Blastula Formation:
After cleavage, a blastula is formed, which is a hollow ball of cells. The blastula undergoes further developmental processes, including gastrulation, to form the germ layers that give rise to different tissues and organs.
These cleavage patterns are a
reflection of the specific developmental strategies of different organisms.
They are influenced by factors such as the amount and distribution of yolk in
the egg and the evolutionary history of the species.