Stages and Terminology
The process begins with the formation of a 2-cell embryo. The cells generated during this period are called blastomeres. This term is used throughout the entire cleavage stage, even before a fully formed blastula structure appears.
During these divisions, the embryo passes through two main stages:
- Morula — a compact cluster of blastomeres resembling a mulberry. At this stage, an internal cavity within the embryo is absent.
- Blastula — the stage that follows cavitation (the appearance of a cavity). Inside the cell mass, the blastocoel (central cavity) forms, and the outer cell wall is termed the blastoderm.
Classification of Cleavage Types
The exact pattern of zygote cleavage depends directly on the type of oocyte: the amount of yolk and its distribution within the cytoplasm. Several main types are distinguished:
- Holoblastic equal cleavage. Found in invertebrates and primitive chordates (e.g., amphioxus, sea urchin, Ascaris) with alecithal or oligo-/isolecithal eggs. The zygote divides entirely, blastomeres are equal in size, and divisions occur synchronously.
- Holoblastic unequal cleavage. Characteristic of amphibians with moderately telolecithal eggs. The entire zygote divides, but due to yolk accumulation at the vegetal pole, divisions there proceed more slowly, resulting in larger cells. At the animal pole (where yolk is sparse), smaller cells form. The process is asynchronous.
- Meroblastic (incomplete) cleavage. Observed in birds, reptiles, and teleost fish (strongly telolecithal eggs). There is so much yolk that it does not divide at all. Only the small apical region containing the nucleus (animal pole) undergoes division.
- Mammalian cleavage. Placental mammals (including humans) have secondary oligolecithal eggs. Cleavage is holoblastic, somewhat unequal, and strictly asynchronous (stages with 3 or 5 blastomeres may occur). Smaller cells divide faster than larger ones.
Types of Blastulae
Each cleavage type corresponds to a specific morphological form of the blastula:
- Coeloblastula — the result of holoblastic equal cleavage. It is a single-layered spherical vesicle with a central cavity (blastocoel) and a wall of uniform thickness.
- Amphiblastula — forms via holoblastic unequal cleavage. The blastoderm is multilayered, and the blastocoel is displaced toward the animal pole. The roof consists of small micromeres, while the floor consists of large macromeres.
- Discoblastula — the outcome of meroblastic cleavage. A flat embryonic disc lies atop the undivided yolk. The blastocoel appears as a narrow cleft between the disc and the yolk.
- Blastocyst — forms in mammals. It has a unique structure: the outer single-layered wall is the trophoblast (source of the chorion), the inner cell mass is the embryoblast (source of the embryo proper and remaining extraembryonic membranes), and the cavity between them is the blastocoel.