Features of the cell cycle
Cleavage differs significantly from classic somatic cell division. These differences ensure a high rate of the process:
- Ultrashort interphase. The cycle lacks presynthetic ($G_1$) and postsynthetic ($G_2$) phases. Because of this, cells physically do not have time to grow between divisions, and the total volume of the embryo remains equal to the volume of the original zygote.
- Specific S-phase. DNA synthesis starts already at the end of the telophase of the previous mitosis. DNA replication proceeds simultaneously and synchronously in all replicons. Although the movement speed of the replication fork itself is standard, there are many more initiation sites in the DNA of blastomeres than in regular cells.
- Totipotency. Blastomeres do not yet know what tissues they will become in the future. At this stage, differentiation is completely absent.
Morphology: from morula to blastula
Cleavage goes through two main stages:
- Morula. At the initial stage (16 or more blastomeres), the embryo resembles a mulberry. This is a dense cluster of cells closely adjacent to each other.
- Blastula. Gradually, the cells move to the periphery, forming the outer wall — the blastoderm. Inside the embryo, a fluid-filled cavity appears — the blastocoel.
Having completed the blastula stage, multicellular organisms proceed to the next stage of development — gastrulation (formation of germ layers).
Classification: Holoblastic cleavage
The type of cleavage is directly determined by the amount and distribution of yolk in the ovum. Yolk is inert and mechanically inhibits cytoplasmic division. If there is little yolk, the ovum divides entirely. This type of cleavage is called holoblastic.
- Equal holoblastic. Found in alecithal and oligolecithal ova (e.g., in amphioxus). Cleavage furrows run strictly along the meridians and equator. All resulting blastomeres are of equal size. The outcome is a coeloblastula.
- Unequal holoblastic. Characteristic of mesolecithal ova (e.g., in amphibians). Due to the accumulation of yolk at the vegetal pole, division there is slower. As a result, small cells (micromeres) form at the animal pole, and large cells (macromeres) form at the vegetal pole. The blastocoel is small and displaced toward the animal pole. The outcome is an amphiblastula.
Classification: Meroblastic cleavage
If there is a lot of yolk in the ovum (polylecithal, strongly telolecithal, centrolecithal ova), cleavage furrows cannot divide it completely. Such cleavage is called meroblastic or partial.
- Incomplete discoidal. Characteristic of reptiles and birds. Only the region at the animal pole, free of yolk, divides. A blastodisk is formed. The blastocoel looks like a narrow slit beneath the blastoderm. The outcome is a discoblastula.
- Incomplete superficial. Found in arthropods (insects) with centrolecithal ova. Nuclei divide repeatedly in the center and then migrate to the periphery, where there is no yolk. There, cytoplasm separates around the nuclei, forming a single layer of blastomeres. The central part (blastocoel) remains filled with yolk. The outcome is a periblastula.
Dependence on blastomere arrangement
In addition to the completeness of division, cleavage is classified by the spatial arrangement of blastomeres relative to each other:
| Type of symmetry | Description | Examples of animals |
|---|---|---|
| Radial | Cleavage planes are perpendicular, blastomeres lie symmetrically | Amphioxus, echinoderms, amphibians |
| Spiral | Symmetry is disrupted due to the spiral displacement of cells | Annelids, mollusks |
| Bilateral | Only one plane of symmetry can be drawn through the embryo | Ascaris, tunicates, cephalopods |
| Rotational | Alternating direction of furrows | Mammals, nematodes |
Briefly: answers to topical questions
What cell populations does the mammalian (human) blastocyst consist of?
The mammalian blastocyst consists of two cell populations:
- Trophoblast — the outer layer of cells providing implantation and nutrition; the chorion develops from it, and it participates in the formation of the chorion and then the placenta.
- Embryoblast (inner cell mass, germ nodule) — a group of embryonic cells adjacent to the trophoblast from the inside and displaced by fluid to one of the poles. The embryo proper and some extraembryonic organs develop from it.
At what time (on which day) does cleavage of the human zygote occur in the fallopian tubes?
Cleavage of the human zygote begins 24 hours after fertilization. It takes place in the pre-implantational period during the transport of the cleaving zygote from the ampullary end of the fallopian tube into the uterine cavity.
Developmental dynamics:
- By 96 hours from the moment of fusion of the sperm and egg nuclei, the embryo reaches the morula stage and consists of 16–32 blastomeres.
- The blastocyst forms on the 4th–5th day after ovulation.
- The pre-implantational period ends with the implantation of the blastocyst into the decidua of the uterus on the 5th–6th day after fertilization.
Class and exam questions
Does the embryo increase in size during cleavage?
No, the overall size of the embryo by the end of cleavage does not exceed the size of the initial zygote. This is due to the fact that the cell cycle of blastomeres lacks growth periods ($G_1$ and $G_2$).
What are macromeres and micromeres?
These are cells of different sizes formed during unequal holoblastic cleavage (e.g., in the frog). Micromeres are small cells at the animal pole (without yolk), while macromeres are large cells at the vegetal pole (rich in yolk, which inhibits division).
How does a morula differ from a blastula?
The morula is an early stage representing a dense clump of 16 or more cells. The blastula is the next stage, when cells move to the periphery (forming the blastoderm), and a cavity — the blastocoel — appears inside.
What else to study on this topic
- Structural features of ova (alecithal, telolecithal, centrolecithal)
- Gastrulation as the next stage of ontogenesis
- Specifics of rotational cleavage in mammals
- Molecular mechanisms of cell cycle regulation during cleavage
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