Classification by yolk volume
In the course of evolution, species evolved along the pathway of increasing yolk reserves in the cytoplasm. Depending on the amount of yolk, four types of oocytes are distinguished:
- Alecithal (from Greek a — without). Yolk is practically or completely absent. Characteristic of flatworms.
- Oligolecithal (from Greek olygos — small). Contain a small amount of yolk, but more than alecithal ones. Found in amphioxus and mammals.
- Mesolecithal (from Greek mesos — middle). Have an intermediate nutrient reserve. Examples: amphibians and sturgeon fish.
- Polylecithal (from Greek polys — many). Distinguished by a large volume of yolk. Characteristic of reptiles, birds, and teleost fish.
Classification by yolk distribution
In addition to volume, how yolk is localized within the cell is of critical importance for embryonic development.
- Isolecithal (or homolecithal, from Greek isos — equal). Yolk is distributed evenly throughout the cytoplasm. Usually, these are cells with a moderate or small amount of yolk (mammals, amphioxus).
- Telolecithal (from Greek telos — end). Yolk is distributed unevenly. They are divided into two subgroups:
- Moderately telolecithal: yolk is immersed in the cytoplasm and not segregated from it (amphibians, sturgeons).
- Extremely telolecithal: yolk is completely separated from the active cytoplasm. Found in amniotes — birds and reptiles.
- Centrolecithal (from Greek kentron — center). Yolk is concentrated in the center of the cell, while the cytoplasm is displaced to the periphery, forming a thin surface layer. A classic example is arthropods.
Polarity and ooplasmic segregation
The uneven distribution of cytoplasmic components leads to the emergence of internal heterogeneity of the egg — ooplasmic segregation. As a result, a pronounced polarity is formed:
- Animal pole: the region to which the active, yolk-free cytoplasm is displaced.
- Vegetative pole: the opposite region of the oocyte, where the main yolk reserve accumulates.
Polarity is outlined as early as the stage of yolk accumulation (during the period of rapid oocyte growth) and is finally fixed at the moment of polar body release.
Chemical composition of the cytoplasm
The oocyte cytoplasm accumulates substances that ensure the autonomy of early embryogenesis without the intake of external nutrients.
Main groups of substances:
- Components of template processes: enzymes, ribosomes, mRNA, tRNA, and their precursors. Necessary for transcription, translation, and replication.
- Specific regulatory substances: coordinate the stored components. Include various factors: nuclear envelope breakdown factor (initiates prometaphase I of meiosis), chromosome condensation factor, the factor transforming the sperm nucleus into a pronucleus, cytokinesis factor, and the meiotic arrest factor at metaphase II (released only upon fertilization).
- Yolk: consists of proteins, phospholipids, neutral fats, and mineral salts. Serves as the main energy resource for the developing embryo.
Briefly: answers to topical questions
What are cortical granules of the oocyte and what is their function?
Cortical granules (granula corticalia) are membrane-bound vesicles containing specific enzymes and located in the cortical layer of the oocyte directly beneath the plasmalemma.
Their main function is participation in the cortical reaction during fertilization, which prevents polyspermy (the penetration of additional spermatozoa).
Mechanism of action:
- Upon penetration of the first spermatozoon, the granules release the enzyme ovoperoxidase into the perivitelline space.
- Hardening of the zona pellucida (zona pellucida) and formation of the hyaline layer occur.
- The oocyte membrane transforms into the fertilization membrane, creating an insurmountable mechanical barrier for the remaining male gametes.
To memorize names by yolk distribution: ISO (equal) — yolk is distributed evenly throughout the cell; TELO (end) — yolk is displaced to one pole; CENTRO (center) — yolk is located strictly in the middle.
Class and exam questions
Why do mammalian egg cells contain so little yolk?
Although mammals evolved from reptiles, in the course of evolution they secondarily lost yolk reserves. This occurred due to the emergence of intrauterine development, in which the embryo receives nutrients from the maternal organism.
How does the oocyte cytoplasm regulate the early stages of cleavage?
During the cleavage period, the embryo relies not on the genetic information of blastomere nuclei, but on the chemical composition of the cytoplasm. mRNA, tRNA, ribosomes, and regulatory factors ensuring division are pre-stored within it.
What is the meiotic arrest factor?
This is a specific regulatory substance in the cytoplasm of oocytes of many vertebrates that arrests maturation at the metaphase II stage. Release from this arrest occurs exclusively at the moment of fertilization.
What else to study on this topic
- Mechanisms of ooplasmic segregation
- Specific regulatory factors of the oocyte
- The process of transforming the sperm nucleus into a pronucleus
- Significance of template process components in ontogenesis
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