Main phases of oogenesis
The maturation process of female gametes includes three key phases:
- Multiplication phase. Initial diploid cells of the ovarian oogenic tissue actively divide by mitosis. This stage results in the formation of diploid oogonia.
- Growth phase. This stage corresponds to interphase prior to subsequent division. Cells undergo DNA replication, and prophase I of meiosis begins (the dictyotene stage). Oogonia increase significantly in size due to the accumulation of nutrients (yolk), transforming into primary oocytes.
- Maturation phase. Includes two meiotic divisions. Before ovulation, the first division is completed: the primary oocyte yields one large cell—the secondary oocyte—and a small first polar (reduction) body containing a nucleus and minimal cytoplasm. The second division proceeds to metaphase and halts. It is completed only upon fertilization, yielding a large ovum and a second polar body. The first polar body may also divide. Ultimately, all resulting polar bodies degenerate, leaving only a single functional gamete.
Chronology of development
Unlike continuous spermatogenesis in males, oogenesis has a strict timeline with periods of dormancy:
- Embryonic period: the female fetus completely traverses the multiplication and growth phases, and the maturation stage begins.
- Neonatal period: a newborn girl's ovaries contain hundreds of thousands of primary oocytes. They are in a state of specific arrest—stopped at the dictyotene stage (prophase I of meiosis). At this point, chromosomes assume a characteristic "lampbrush" appearance.
- Puberty (sexual maturation): meiosis resumes under hormonal stimulation. Each month, a single oocyte reaches the metaphase II stage.
- Completion: if fertilization occurs, meiosis goes to completion. If a spermatozoon fails to penetrate the cell, the secondary oocyte dies and is eliminated.
Changes in genetic material
Cell ploidy changes at each stage of oogenesis, ensuring the transition from a diploid to a haploid set:
- Oogonium: 2n2c (result of mitosis).
- Primary oocyte: 2n4c (preparing for reductional division—meiosis I).
- Secondary oocyte: n2c (result of the first meiotic division, with simultaneous formation of the first polar body).
- Ovum: nc (result of equational division—meiosis II, secondary polar bodies are formed).
Folliculogenesis
The development of the female germ cell is closely linked to follicular maturation in the ovary. This process goes through a series of stages:
- Primary (growing) follicle.
- Primordial follicle.
- Secondary (maturing) follicle.
- Tertiary (mature) follicle, or Graafian follicle.
- Ovulation — release of the contents into the fallopian (uterine) tube.
- Corpus luteum.
- Corpus albicans (scar).
Structure of the Graafian follicle: The mature follicle is a complex structure. Inside is a cavity filled with follicular fluid and the secondary oocyte itself. The cell is surrounded by the zona pellucida (Zona pellucida) and the corona radiata (Corona radiata). Externally, it is lined by the granulosa layer, part of which forms the cumulus oophorus (Cumulus oophorus).
Briefly: answers to topical questions
Which hormones of the hypothalamus, pituitary, and ovaries regulate folliculogenesis and ovulation?
Folliculogenesis and ovulation are regulated by a complex of hormones within the hypothalamic-pituitary-ovarian axis.
- Hypothalamic hormones — gonadotropin-releasing hormone (GnRH), which stimulates pituitary gonadotropin release in a pulsatile manner.
- Pituitary hormones — follicle-stimulating hormone (FSH), which stimulates follicular growth, and luteinizing hormone (LH), which peaks at the time of ovulation.
- Ovarian hormones — estrogens, whose concentration peaks prior to ovulation (they can suppress GnRH and FSH production via negative feedback), as well as progesterone and inhibins.
Additionally, ovulation is locally mediated by prostaglandins, oxytocin, relaxin, and proteolytic enzymes.
What are all the main differences between oogenesis and spermatogenesis?
Main differences between oogenesis and spermatogenesis:
| Sign | Oogenesis | Spermatogenesis |
|---|---|---|
| Stages | 3 stages: multiplication, growth, maturation | 4 stages: multiplication, growth, maturation, differentiation |
| Localization | In the ovarian cortex; development occurs within follicles, one germ cell per follicle. Oogenesis begins in the ovaries; a secondary oocyte is released at ovulation; the ovum forms in the uterine tubes after fertilization. | In the walls of the seminiferous tubules; the process takes place entirely within the testes. |
| Temporal organization | Begins during the embryonic period, proceeds intermittently, and is tied to menstrual cycles. Multiplication and growth phases are completed during fetal life; primary oocytes are arrested in dictyotene of prophase I. Maturation resumes at puberty and completes after fertilization. | Begins at puberty and continues almost until death; the total duration of spermatogenesis is 64 days. |
| Cells by stage | Multiplication — oogonia; growth — primary oocyte; maturation — secondary oocyte and polar body, followed by ovum and polar bodies. | Multiplication — spermatogonia; growth — primary spermatocyte; maturation — secondary spermatocyte and 4 spermatids; differentiation — spermatids transform into spermatozoa. |
| Outcome per single cell | From 1 primary oocyte, 1 ovum and a maximum of 3 polar bodies are formed; polar bodies discard "excess" genetic material while preserving nutrient reserves. | From 1 primary spermatocyte, 4 spermatozoa are formed. |
| Productivity | Typically, one follicle reaches full development every 28 days; 250–600 secondary oocytes over a lifetime. At the fifth month of embryogenesis, there are 6–7 million germ line precursor cells, dropping to approximately 100,000 oocytes by the start of the reproductive period. | During his reproductive lifespan, a man produces at least 500 billion spermatozoa. |
| Process characteristics | Cell growth, onset of meiosis up to diplotene of prophase I, formation of the zona pellucida and corona radiata; the second meiotic division finishes immediately after fertilization. | During the differentiation of spermatids into spermatozoa, chromatin condenses, a flagellum forms from the centrioles, the acrosome develops from the Golgi apparatus, mitochondria localize at the base of the tail, and excess cytoplasm is phagocytosed by cells. |
Class and exam questions
What is the main difference between oogenesis and spermatogenesis?
Oogenesis is initiated and begins during the prenatal period and proceeds intermittently, arresting at the prophase I (dictyate) and metaphase II stages.
What is the first polar body and what is its purpose?
This is a small cell formed after the first meiotic division along with the secondary oocyte. It takes half of the chromosomes (reduction of genetic material) but leaves almost all the cytoplasm to the future oocyte, and subsequently disintegrates.
Under what condition does meiosis complete in oogenesis?
The second meiotic division is completed exclusively upon fertilization. Without it, the secondary oocyte remains arrested at metaphase until its demise.
What else to study on this topic
- Hormonal regulation of oogenesis by progesterone
- Structure and functions of the zona pellucida
- Morphology of lampbrush chromosomes in the dictyate stage
- The process of ovulation and capture of the oocyte by the fallopian tube
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