Internal structure and nucleo-cytoplasmic ratio
Inside the egg cell are all typical cellular organelles and a morphologically distinct nucleus that stores the genetic material.
A crucial distinguishing feature is the nucleo-cytoplasmic ratio. Unlike most somatic cells, in the egg cell this ratio is heavily shifted toward the cytoplasm. This occurs due to the massive accumulation of a specialized nutritional substrate — yolk, which is necessary to sustain the life of the developing embryo at early stages.
Yolk: composition and dependence on developmental conditions
Yolk is a complex chemical cocktail. It includes fats, carbohydrates, ribonucleic acids (RNA), minerals, as well as proteins in the form of lipoproteins and glycoproteins.
The amount of nutrients directly depends on the conditions under which the embryo will develop:
- Development outside the maternal organism (birds, reptiles): yolk is the primary and only source of nutrition, which is why it accounts for more than 95% of the total egg volume.
- Development within the organism (mammals): since the embryo's nutrition starts coming from the maternal organism almost immediately, the yolk volume is less than 5%.
Classification of egg cell membranes
Egg cell membranes are generally divided into three main types based on the source of their formation:
- Primary (vitelline) membrane. Formed by the surface layer of the immature gamete itself (oocyte). It is found in all animals. The membrane is penetrated by numerous microvilli and processes of follicular cells through which nutrient transport is carried out. In mammals, this structure becomes part of the inner portion of the zona pellucida.
- Secondary membrane. Produced exclusively by the ovarian follicular cells surrounding the egg cell. An interesting feature is observed in insects: their secondary membrane contains a special channel — the micropyle — through which the sperm penetrates inside.
- Tertiary membrane. Formed as the egg passes through the oviduct due to the secretions of its glands. It lacks a cellular structure and performs primarily a protective function. It is well developed in birds and reptiles (for example, in birds it is represented by the albumen, shell membranes, and the eggshell).
Structural features of the mammalian egg cell
In mammals, the primary and secondary membranes merge into a single complex structure — the zona pellucida (zona pellucida). By composition, it is a complex extracellular glycoprotein matrix.
Between the zona pellucida and the cell membrane of the egg itself lies the perivitelline space. The polar body is also located within this space.
On top of the zona pellucida lies a layer of follicular cells that remain "stuck" to the egg after its release from the follicle. This layer is named the corona radiata (corona radiata).
Functions of the zona pellucida
The mammalian zona pellucida undertakes a number of critically important tasks:
- Protective function: mechanical protection of the internal contents.
- Species specificity: the glycoprotein complex guarantees that only a sperm of the same biological species can penetrate the egg.
- Cortical reaction (polyspermy block): as soon as the first sperm penetrates inside, the cortical granules located beneath the membrane instantly release the enzyme ovoperoxidase into the perivitelline space. This leads to the chemical hardening of the zona pellucida, permanently blocking the path for other sperm.
- Pre-implantation protection: the membrane persists around the developing embryo until the moment of its implantation into the uterus. This prevents embryos from sticking to one another and hinders their premature adhesion to the oviduct walls, which is the main natural prevention of ectopic pregnancy.
Briefly: answers to topical questions
What types are egg cells divided into depending on the amount and distribution of yolk?
Depending on the amount and distribution of yolk, egg cells are classified into several types. According to the amount of yolk, the following types are distinguished:
- Alecithal — no yolk.
- Oligolecithal — contain a small amount of nutrients.
- Mesolecithal — moderate amount of yolk.
- Polylecithal — contain a large amount of yolk.
According to the distribution of yolk, egg cells are subdivided into:
- Isolecithal (homolecithal) — yolk is distributed evenly.
- Telolecithal — yolk is displaced toward the vegetal pole.
- Centrolecithal — yolk is located in the center.
What structures make up the tertiary egg membrane in birds?
The tertiary egg membrane in birds lacks cellular structure, is formed from the secretions of the oviduct wall glands, and performs a predominantly protective function. It consists of the following structures:
- Albumen layer — provides water reserves and protection.
- Shell membrane — performs a protective function.
- Eggshell — performs a protective function.
At what stage of meiosis is the mammalian egg cell at the moment of ovulation?
At the moment of ovulation, the female gamete is at the metaphase stage of the second meiotic division. Before ovulation, the first meiotic division is completed, resulting in the formation of a secondary oocyte. After this, the second meiotic division begins, reaching the metaphase stage and arresting (meiotic arrest). In this state, the oocyte leaves the ovary into the fallopian tube. Further maturation and completion of meiosis occur only upon fertilization.
For quick memorization of membrane origins, use this logic: The Primary is built by the cell itself (from the inside), the Secondary by its "neighbors" (follicular cells), and the Tertiary by the "road" (glands of the oviduct walls).
Class and exam questions
Why is the nucleocytoplasmic ratio shifted in the egg cell?
Unlike ordinary cells, the egg cell needs to accumulate a massive reserve of nutrients (yolk) in its cytoplasm to provide energy for the initial stages of embryonic development, which is why the cytoplasmic volume vastly exceeds the nuclear volume.
What is a micropyle and what is its purpose?
A micropyle is a specialized channel in the secondary membrane of egg cells in certain animals (e.g., insects). Through this opening, the sperm gains the ability to penetrate inside the cell for fertilization.
How does the polyspermy block work in mammals?
Immediately upon fusion with the first sperm, the cortical reaction is triggered. Cortical granules release the enzyme ovoperoxidase, which makes the zona pellucida hard and impassable for the remaining sperm.
What role does the zona pellucida play in preventing ectopic pregnancy?
The zona pellucida surrounds the embryo during its movement through the oviduct toward the uterus, physically preventing it from "sticking" to the oviduct walls until normal implantation occurs.
What else to study on this topic
- Detailed chemical composition of the nutritional substrate (role of RNA, lipoproteins, and glycoproteins)
- Molecular mechanisms of the cortical reaction and ovoperoxidase release
- Structure and functions of the corona radiata
- Stages of tertiary membrane formation using birds as an example (albumen, shell membranes, eggshell)
- The process of zona pellucida shedding prior to implantation (hatching)
Master the entire topic in the "Biology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Egg Cell Structure" and 3,793 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 129 video lessons on the subject
Other topics in the "Gametogenesis" section
Free video lessons: BiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.