Stages and terminology
The process begins with the formation of a 2-cell embryo. The cells formed during this period are called blastomeres. This term is used throughout the entire cleavage stage, even before the appearance of a fully formed blastula structure.
During divisions, the embryo goes through two main stages:
- Morula — a dense cluster of blastomeres, outwardly resembling a mulberry. At this stage, there is no cavity inside the embryo.
- Blastula — the stage that occurs after the process of cavitation (the appearance of a cavity). Inside the cell mass, the blastocoel (central cavity) is formed, and the outer cell wall is called the blastoderm.
Classification of cleavage types
The exact way the zygote undergoes cleavage depends directly on the type of oocyte: the amount of yolk in it and its distribution in the cytoplasm. Several main types are distinguished:
- Holoblastic equal cleavage. Found in invertebrates and primitive chordates (amphioxus, sea urchin, Ascaris) with alecithal or oligo-/isolecithal oocytes. The zygote divides entirely, blastomeres are of equal size, and divisions occur synchronously.
- Holoblastic unequal cleavage. Characteristic of amphibians with moderately telolecithal oocytes. The entire zygote divides, but due to the accumulation of yolk at the vegetal pole, divisions are slower there, and the cells are larger. Smaller cells are formed at the animal pole (where there is little yolk). The process proceeds asynchronously.
- Meroblastic (incomplete) cleavage. Observed in birds, reptiles, and teleost fish (sharply telolecithal oocytes). There is so much yolk that it does not divide at all. Only the small apical part with the nucleus (animal pole) undergoes division.
- Mammalian cleavage. Placental mammals (and humans) have secondary oligolecithal oocytes. Cleavage is holoblastic, partly unequal, and strictly asynchronous (stages with 3 or 5 blastomeres may occur). Smaller cells divide faster than larger ones.
Types of resulting blastulae
Each type of cleavage corresponds to its own morphological form of blastula:
- Coeloblastula — the result of holoblastic equal cleavage. It is a single-layered spherical vesicle with a central cavity (blastocoel) and a wall of equal thickness.
- Amphiblastula — formed during holoblastic unequal cleavage. The blastoderm is multilayered, and the blastocoel is displaced toward the animal pole. The roof consists of small micromeres, and the floor consists of large macromeres.
- Discoblastula — the outcome of incomplete cleavage. A flat blastodisc lies on the undivided yolk. The blastocoel appears as a narrow slit between the disc and the yolk.
- Blastocyst (germinal vesicle) — formed 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 body and other extraembryonic organs), and between them is a cavity (blastocoel).
Briefly: answers to topical questions
On which day of human embryogenesis does the embryo reach the morula and blastocyst stages?
The human embryo reaches the morula stage on days 3–4 of embryogenesis, and the blastocyst stage on days 4–6.
- Morula — formation occurs on days 3–4 (72 to 96 hours after fertilization). At this stage, the embryo is a dense cluster of cells without a cavity.
- Blastocyst — formation occurs on days 4–5.5 (completed by days 5–6). The embryo becomes a fluid-filled vesicle.
What types of intercellular junctions provide the compaction of blastomeres at the morula stage?
The connection of embryonic cells during cleavage is provided by tight and gap junctions.
- Tight junctions — they connect smaller, light blastomeres that divide faster and are located at the periphery.
- Gap junctions — they are formed by large, dark blastomeres that divide more slowly and are localized in the center of the embryo.
Class and exam questions
Does the embryo grow during cleavage?
No, this is the main feature of the stage. The cells divide mitotically, but there is no growth between divisions, so the total volume of the zygote remains unchanged.
What is the difference between a morula and a blastula?
The morula is a dense clump of cells without an internal cavity. The blastula forms later when a cavity—the blastocoel—appears inside the cell mass.
How does the embryoblast differ from the trophoblast in the blastocyst?
The trophoblast forms the vesicle wall and gives rise to the chorion. The embryoblast is the inner cell mass from which the embryo body itself and other provisional organs will develop.
What else to study on this topic
- Mechanisms of cavitation and blastocoel formation
- Relationship between oocyte type and cleavage pattern
- Spatial organization of the coeloblastula
- Dynamics of mammalian cell division
- Prospective potencies of the trophoblast and embryoblast
Master the entire topic in the "Histology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Cleavage" and 3,485 other pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 137 video lessons on the subject
Other topics in the section "Germ cells, fertilization, cleavage, blastula"
Free video lessons: histologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.