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Secondary Oocyte
A secondary oocyte is the haploid female germ cell formed when a primary oocyte completes meiosis I. It begins meiosis II, becomes arrested at metaphase II, and is the cell normally released from the ovary during ovulation.
Region-
SystemReproductive System
A secondary oocyte is the female germ cell produced when a primary oocyte completes the first meiotic division. In humans, the secondary oocyte is the cell normally released from the ovary during ovulation. It contains a haploid chromosome set, begins meiosis II, and becomes arrested at metaphase II until fertilization occurs.
The secondary oocyte represents a brief but critical stage of oogenesis. Unlike the primary oocyte, which may remain arrested for decades, the secondary oocyte normally exists for a relatively short period. If fertilization occurs, sperm entry triggers completion of meiosis II and formation of the mature ovum and second polar body. If fertilization does not occur, the secondary oocyte degenerates.
Definition
A secondary oocyte is a haploid female germ cell formed by completion of meiosis I in a primary oocyte.
It immediately enters meiosis II but normally becomes arrested at metaphase II before being released from the ovary.
Position in Oogenesis
The secondary oocyte occupies a late stage in the sequence of female gamete development:
- Primordial germ cell
- Oogonium
- Primary oocyte
- Secondary oocyte
- Ovum after completion of meiosis II
The transition from primary to secondary oocyte occurs shortly before ovulation.
Origin
The secondary oocyte develops from a primary oocyte within a mature ovarian follicle.
Primary oocytes originate during fetal life and remain arrested in prophase I. During reproductive life, selected follicles mature, and the primary oocyte within the preovulatory follicle can resume meiosis.
Primary Oocyte
A primary oocyte is a diploid germ cell that has entered meiosis I. It remains arrested in prophase I for an extended period.
Shortly before ovulation, hormonal changes associated with the midcycle luteinizing hormone surge permit the primary oocyte to resume and complete meiosis I.
Resumption of Meiosis
Meiotic resumption occurs within the mature preovulatory follicle. The primary oocyte progresses beyond its prolonged prophase I arrest and proceeds through the remaining stages of meiosis I.
Homologous chromosomes separate during this division, reducing the chromosome complement of the resulting secondary oocyte.
Completion of Meiosis I
Completion of meiosis I produces two cells with markedly unequal amounts of cytoplasm.
The large cell is the secondary oocyte, while the much smaller cell is the first polar body.
First Polar Body
The first polar body receives one set of chromosomes during the first meiotic division but receives very little cytoplasm.
This asymmetric division allows the secondary oocyte to retain nearly all of the cytoplasmic resources accumulated during growth of the primary oocyte.
Asymmetric Cytokinesis
Female meiosis is characterized by asymmetric cytokinesis. Chromosomes are separated between daughter cells, but the cytoplasm is distributed unequally.
This differs from many ordinary mitotic divisions, in which daughter cells receive approximately equal amounts of cytoplasm.
Chromosome Number
The human secondary oocyte contains 23 chromosomes and is therefore haploid.
However, immediately after meiosis I, each chromosome still consists of two sister chromatids. The sister chromatids do not separate until meiosis II is completed.
Chromosome and DNA Content
| Cell Stage | Chromosome Number | Chromosomal State |
|---|---|---|
| Oogonium before DNA replication | 46 | Diploid, chromosomes unreplicated |
| Primary oocyte | 46 | Diploid, chromosomes replicated |
| Secondary oocyte | 23 | Haploid, chromosomes still contain sister chromatids |
| Ovum | 23 | Haploid after separation of sister chromatids |
Haploid State
The secondary oocyte is described as haploid because it contains one member of each homologous chromosome pair.
The presence of two sister chromatids per chromosome does not make the cell diploid. Ploidy refers to the number of homologous chromosome sets rather than the number of chromatids.
Meiosis II
After meiosis I is completed, the secondary oocyte rapidly begins meiosis II.
Unlike the first meiotic division, meiosis II separates sister chromatids and therefore resembles a mitotic division in its chromosome-segregation pattern.
Metaphase II Arrest
The secondary oocyte progresses through meiosis II until metaphase II, where it becomes arrested.
This is the normal meiotic stage of the human female germ cell at ovulation.
What Is Released at Ovulation?
The cell normally released from the human ovary during ovulation is a secondary oocyte arrested in metaphase II.
Although the term ovum is frequently used informally for the ovulated female gamete, a mature ovum is technically produced only after meiosis II is completed.
Structure
The secondary oocyte is a large spherical cell surrounded by specialized extracellular and cellular layers.
Important structures associated with the ovulated secondary oocyte include:
- Oocyte plasma membrane
- Cytoplasm
- Metaphase II chromosomes
- Cortical granules
- Perivitelline space
- First polar body
- Zona pellucida
- Corona radiata
Size
The human secondary oocyte is approximately 0.1 mm in diameter and is one of the largest cells in the human body.
Its large volume reflects the extensive accumulation of cytoplasm and cellular components during growth of the primary oocyte.
Oocyte Plasma Membrane
The secondary oocyte is enclosed by a plasma membrane, sometimes historically referred to as the oolemma.
This membrane participates in sperm-oocyte recognition and fusion during fertilization.
Cytoplasm
The secondary oocyte contains abundant cytoplasm containing organelles, proteins, messenger RNAs, metabolites, and regulatory molecules.
These maternal components support fertilization and the earliest stages of embryonic development before the embryonic genome becomes fully active.
Mitochondria
The oocyte contains a large population of mitochondria required to support its metabolic demands and those of the early embryo.
Human mitochondrial inheritance is overwhelmingly maternal because the established mitochondrial population of the embryo is derived primarily from the oocyte.
Cortical Granules
Cortical granules are secretory vesicles located beneath the plasma membrane of the secondary oocyte.
After sperm-oocyte fusion, cortical granule contents are released and modify the zona pellucida as part of the mechanisms preventing additional sperm from successfully fertilizing the oocyte.
Zona Pellucida
The zona pellucida is a glycoprotein-rich extracellular coat surrounding the secondary oocyte.
It develops during follicular growth and has important roles in sperm interaction, fertilization, protection of the oocyte and early embryo, and regulation of interactions with surrounding tissues.
Perivitelline Space
The perivitelline space is located between the oocyte plasma membrane and the zona pellucida.
The first polar body is typically located within this space after completion of meiosis I.
Corona Radiata
The corona radiata consists of granulosa cells that remain immediately external to the zona pellucida when the secondary oocyte is ovulated.
These cells originate from the cumulus oophorus of the mature ovarian follicle.
Cumulus Oophorus
Before ovulation, the oocyte is supported within the follicle by a mound of granulosa cells called the cumulus oophorus.
During ovulation, the oocyte is released together with associated cumulus cells, including the corona radiata.
Layers Surrounding the Secondary Oocyte
| Structure | Location or Function |
|---|---|
| Corona radiata | Granulosa cells surrounding the zona pellucida |
| Zona pellucida | Extracellular glycoprotein coat |
| Perivitelline space | Space between zona pellucida and plasma membrane |
| First polar body | Small product of meiosis I, usually within the perivitelline space |
| Plasma membrane | Boundary of the secondary oocyte |
| Cytoplasm | Contains maternal organelles and molecular resources |
Relationship to the Mature Follicle
The secondary oocyte forms within the mature preovulatory follicle shortly before ovulation.
Before completion of meiosis I, the germ cell within the follicle is still a primary oocyte. The transition to a secondary oocyte therefore occurs relatively late in follicular maturation.
Preovulatory Follicle
The mature preovulatory follicle contains a large fluid-filled antrum. The oocyte is located eccentrically within the cumulus oophorus.
Following the LH surge, the primary oocyte completes meiosis I and becomes a secondary oocyte before follicular rupture.
Luteinizing Hormone Surge
The midcycle surge of luteinizing hormone triggers a series of changes within the dominant follicle associated with meiotic maturation, follicular rupture, and luteinization.
These events include resumption of meiosis by the primary oocyte and formation of the secondary oocyte.
Ovulation
Ovulation is the release of the secondary oocyte from the mature ovarian follicle.
The oocyte leaves the ovary surrounded by its zona pellucida and associated cumulus cells.
Ovarian Surface
As ovulation approaches, the mature follicle protrudes toward the ovarian surface. Local tissue remodeling contributes to rupture of the follicular wall and overlying ovarian surface.
The secondary oocyte and surrounding cumulus-oocyte complex are then expelled from the follicle.
Entry into the Peritoneal Cavity
The ovary is not directly connected to the lumen of the uterine tube. The ovulated secondary oocyte therefore enters the peritoneal cavity near the fimbrial end of the uterine tube.
It must subsequently enter the abdominal opening of the uterine tube.
Fimbriae
The fimbriae are finger-like projections extending from the infundibulum of the uterine tube.
Their movements, together with fluid currents and ciliary activity, help direct the ovulated secondary oocyte toward the tubal opening.
Infundibulum
The infundibulum is the funnel-shaped lateral end of the uterine tube.
It opens into the peritoneal cavity and receives the ovulated oocyte into the tubal lumen.
Transport Through the Uterine Tube
After entering the uterine tube, the secondary oocyte is transported by a combination of ciliary activity, smooth muscle contractions, and movement of tubal fluid.
These mechanisms move the oocyte toward the ampullary region and ultimately toward the uterus.
Ampulla
The ampulla is the longest and widest portion of the uterine tube and is the usual site of fertilization.
It is therefore the typical location where the secondary oocyte encounters sperm.
Fertilization
Fertilization normally occurs when a sperm successfully traverses the investments surrounding the secondary oocyte and fuses with its plasma membrane.
Sperm entry activates the oocyte and triggers completion of meiosis II.
Sperm and the Corona Radiata
Sperm approaching the secondary oocyte first encounter the cells of the corona radiata and surrounding cumulus matrix.
They must pass through these cellular investments before interacting directly with the zona pellucida.
Sperm and the Zona Pellucida
The zona pellucida participates in sperm interaction during fertilization.
Successful progression through the zona allows the sperm to reach the perivitelline space and oocyte plasma membrane.
Acrosome Reaction
The sperm acrosome contains substances involved in the fertilization process. Appropriate sperm interactions with the oocyte investments are associated with the acrosome reaction.
This reaction contributes to the sperm's ability to progress through the zona pellucida.
Sperm-Oocyte Fusion
After reaching the secondary oocyte, the sperm plasma membrane and oocyte plasma membrane undergo fusion.
This event initiates oocyte activation and a series of intracellular changes required for completion of fertilization.
Oocyte Activation
Oocyte activation includes intracellular signaling events triggered by fertilization. Changes in intracellular calcium are central to this response.
Activation leads to cortical granule exocytosis, resumption of meiosis II, and preparation for early embryonic development.
Cortical Reaction
Following sperm-oocyte fusion, cortical granules release their contents into the region surrounding the oocyte.
This cortical reaction modifies the zona pellucida and contributes to mechanisms that prevent polyspermy.
Prevention of Polyspermy
Normal human fertilization requires incorporation of the genetic contribution of a single sperm.
Changes involving the zona pellucida after oocyte activation help prevent additional sperm from successfully fertilizing the same oocyte.
Completion of Meiosis II
Fertilization causes the secondary oocyte to resume and complete meiosis II.
Sister chromatids separate, producing the mature ovum and the second polar body.
Second Polar Body
The second polar body is the small cell produced during completion of meiosis II.
As in the first meiotic division, cytokinesis is asymmetric so that the mature female gamete retains nearly all of the cytoplasm.
Formation of the Ovum
Strictly speaking, the mature ovum forms when the secondary oocyte completes meiosis II following fertilization.
The maternal chromosome set then becomes organized into the female pronucleus.
Female Pronucleus
Following completion of meiosis II, the maternal haploid chromosome set forms the female pronucleus.
The paternal genetic material delivered by the sperm forms the male pronucleus.
Zygote Formation
The maternal and paternal chromosome sets contribute to formation of the diploid genome of the zygote.
The zygote represents the first cellular stage of the new developing organism and subsequently begins cleavage divisions.
If Fertilization Does Not Occur
If the secondary oocyte is not fertilized, it does not normally complete meiosis II.
Instead, it degenerates after a limited period within the reproductive tract.
Secondary Oocyte versus Primary Oocyte
| Feature | Primary Oocyte | Secondary Oocyte |
|---|---|---|
| Ploidy | Diploid | Haploid |
| Chromosome number | 46 | 23 |
| Meiotic division | Meiosis I | Meiosis II |
| Typical arrest | Prophase I | Metaphase II |
| Formation | During fetal development | Shortly before ovulation |
| Normally ovulated | No | Yes |
Secondary Oocyte versus Ovum
| Feature | Secondary Oocyte | Ovum |
|---|---|---|
| Meiotic stage | Meiosis II incomplete | Meiosis II completed |
| Typical arrest | Metaphase II | No meiotic arrest after completion |
| Timing | Present at ovulation | Forms following fertilization-induced meiotic completion |
| Second polar body | Not yet formed | Produced during completion of meiosis II |
Secondary Oocyte versus Sperm
| Feature | Secondary Oocyte | Sperm |
|---|---|---|
| Ploidy | Haploid | Haploid |
| Size | Large | Small |
| Motility | Non-motile | Motile |
| Cytoplasm | Abundant | Minimal |
| Sex chromosome | X | X or Y |
| Major contribution | Maternal genome, cytoplasm, organelles and molecular resources | Paternal genome and sperm-derived cellular components |
Sex Chromosome
A normal secondary oocyte contains an X chromosome.
A sperm may contain either an X or Y chromosome, so the sperm contributes the variable sex chromosome in the conventional XX or XY chromosomal combinations.
Maternal Cytoplasmic Contribution
The secondary oocyte provides nearly all of the cytoplasm of the future zygote.
This includes mitochondria, ribosomes, proteins, messenger RNAs, metabolites, and regulatory molecules needed during the earliest stages of development.
Maternal Mitochondrial Inheritance
Mitochondrial inheritance in humans is overwhelmingly maternal.
The secondary oocyte contains the mitochondrial population that gives rise to the established mitochondria of the early embryo.
Clinical Identification in Assisted Reproduction
In assisted reproductive procedures, oocytes retrieved from ovarian follicles can be evaluated for their stage of nuclear maturation.
A secondary oocyte arrested in metaphase II is generally regarded as a mature oocyte suitable for fertilization procedures.
First Polar Body as a Maturity Marker
The presence of the first polar body within the perivitelline space provides evidence that meiosis I has been completed.
It is therefore an important morphological feature used to identify a metaphase II secondary oocyte in assisted reproductive laboratories.
Intracytoplasmic Sperm Injection
Intracytoplasmic sperm injection is an assisted reproductive technique in which a sperm is introduced directly into the cytoplasm of a mature secondary oocyte.
This bypasses several natural steps involving sperm passage through the cumulus cells and zona pellucida.
Oocyte Cryopreservation
Secondary oocytes at metaphase II can be cryopreserved for potential future reproductive use.
The meiotic spindle, cellular membranes, cytoplasm, and intracellular structures must remain sufficiently intact for successful survival and subsequent fertilization.
Meiotic Spindle
The meiotic spindle organizes chromosomes during meiosis and is essential for accurate chromosome segregation.
In the secondary oocyte, the metaphase II spindle maintains chromosomes in preparation for completion of meiosis following fertilization.
Meiotic Nondisjunction
Nondisjunction occurs when chromosomes or chromatids fail to segregate normally during meiosis.
Errors during meiosis I can produce a secondary oocyte with an abnormal chromosome complement, while errors during meiosis II can occur when sister chromatids separate after fertilization.
Aneuploidy
Aneuploidy is the presence of an abnormal number of individual chromosomes.
Meiotic segregation errors in the female germ-cell lineage are an important mechanism by which aneuploid conceptions arise.
Maternal Age
Increasing maternal age is associated with a higher frequency of chromosomal segregation abnormalities in oocytes.
This relationship is connected in part to the prolonged developmental history of primary oocytes before they eventually become secondary oocytes.
Developmental Timeline
- Primordial germ cells migrate to the developing ovary.
- They differentiate into oogonia.
- Oogonia proliferate by mitosis.
- Oogonia enter meiosis I and become primary oocytes.
- Primary oocytes arrest in prophase I during fetal life.
- Primary oocytes become enclosed within ovarian follicles.
- Selected follicles develop during reproductive life.
- The primary oocyte in the preovulatory follicle resumes meiosis.
- Meiosis I is completed.
- A secondary oocyte and first polar body are produced.
- The secondary oocyte begins meiosis II.
- It arrests at metaphase II.
- The secondary oocyte is released during ovulation.
- It enters the uterine tube.
- Fertilization usually occurs in the ampulla.
- Sperm entry triggers completion of meiosis II.
- The mature ovum and second polar body are produced.
- The maternal and paternal genetic contributions establish the zygotic genome.
Key Anatomical and Developmental Relationships
- The secondary oocyte develops from a primary oocyte.
- It forms when meiosis I is completed.
- The first polar body is produced at the same division.
- The secondary oocyte contains 23 chromosomes and is haploid.
- Each chromosome initially retains two sister chromatids.
- The secondary oocyte immediately enters meiosis II.
- It becomes arrested at metaphase II.
- The secondary oocyte is the cell normally released during human ovulation.
- It is surrounded by the zona pellucida and corona radiata.
- The first polar body is usually located within the perivitelline space.
- The ovulated oocyte enters the peritoneal cavity before being captured by the uterine tube.
- The ampulla of the uterine tube is the usual site of fertilization.
- Sperm entry triggers completion of meiosis II.
- Completion of meiosis II produces the mature ovum and second polar body.
- If fertilization does not occur, the secondary oocyte degenerates without normally completing meiosis II.
Clinical Importance
The secondary oocyte is particularly important in reproductive anatomy because it is the female germ cell actually released during normal human ovulation. Distinguishing it from the primary oocyte and mature ovum prevents a common misunderstanding of oogenesis. The primary oocyte completes meiosis I shortly before ovulation, while the resulting secondary oocyte begins meiosis II and arrests at metaphase II.
The secondary oocyte is also the stage at which natural fertilization normally occurs. After entering the uterine tube, it typically encounters sperm within the ampulla. Successful sperm-oocyte fusion activates the cell, triggers completion of meiosis II, promotes the cortical reaction, and leads to formation of the mature female pronucleus.
Recognition of the metaphase II secondary oocyte is also fundamental in assisted reproduction. Its first polar body, zona pellucida, cytoplasmic appearance, and meiotic status are relevant when evaluating oocyte maturity for procedures such as in vitro fertilization, intracytoplasmic sperm injection, and oocyte cryopreservation. Understanding the secondary oocyte therefore provides a foundation for studying oogenesis, meiosis, ovulation, fertilization, ovarian follicles, early embryology, aneuploidy, and assisted reproductive technology.
Published on October 17, 2026
Last updated on October 17, 2026
Last updated on October 17, 2026