Ovum

The ovum is the mature female gamete produced through the process of oogenesis. It contains a haploid set of maternal chromosomes and, after fusion with a male gamete, contributes half of the nuclear genetic material of the resulting zygote. The ovum also provides nearly all of the cytoplasm and cytoplasmic organelles of the early embryo.

In human reproductive biology, the term ovum is often used broadly for the female reproductive cell. Strictly speaking, however, the cell released from the ovary during ovulation is a secondary oocyte arrested in metaphase II. It becomes a mature ovum only after sperm entry triggers completion of meiosis II.

Female Gamete

The ovum represents the final product of female gametogenesis. It is specialized for fertilization and early embryonic development.

Its organization differs markedly from that of the sperm. The ovum is large, non-motile, and contains abundant cytoplasm, whereas the sperm is small, highly specialized for motility, and contains relatively little cytoplasm.

Developmental Sequence

The ovum develops through a sequence of germ-cell stages:

1. Primordial germ cell
2. Oogonium
3. Primary oocyte
4. Secondary oocyte
5. Ovum
6. Zygote after fusion of the maternal and paternal genetic material

Much of this process begins before birth and remains incomplete until fertilization.

Origin from Primordial Germ Cells

Female germ-cell development begins with primordial germ cells. These cells migrate into the developing gonadal ridges during embryonic development.

Within the developing ovary, primordial germ cells differentiate into oogonia.

Oogonia

Oogonia are diploid female germ cells that proliferate by mitosis during fetal development.

Many oogonia subsequently enter meiosis I and become primary oocytes, while large numbers of germ cells undergo degeneration during fetal life.

Primary Oocyte

A germ cell is classified as a primary oocyte after it enters meiosis I. Primary oocytes begin meiotic development during fetal life.

They progress through prophase I and then enter a prolonged period of meiotic arrest.

Prophase I Arrest

Primary oocytes remain arrested in the dictyate stage of prophase I from fetal life until they are recruited during later reproductive cycles.

This means that an oocyte ovulated in adulthood may have begun meiosis many years earlier during fetal development.

Primordial Follicle

A primary oocyte surrounded by a single layer of flattened follicular cells forms a primordial follicle.

Primordial follicles are concentrated within the ovarian cortex and collectively contribute to the ovarian reserve.

Follicular Development

During reproductive life, groups of follicles are recruited into growth. Follicular cells become granulosa cells, the oocyte enlarges, and surrounding stromal cells differentiate into the thecal layers.

Progressive follicular development produces primary, secondary, antral, and eventually preovulatory follicles.

Completion of Meiosis I

In the follicle selected for ovulation, the primary oocyte resumes meiosis and completes the first meiotic division shortly before ovulation.

This division is highly asymmetric and produces a large secondary oocyte and a small first polar body.

Secondary Oocyte

The secondary oocyte contains a haploid set of chromosomes, although each chromosome still consists of two sister chromatids immediately following meiosis I.

The secondary oocyte rapidly begins meiosis II but becomes arrested at metaphase II.

Metaphase II Arrest

The secondary oocyte remains arrested in metaphase II when it is released from the ovary during ovulation.

It remains in this state unless fertilization occurs.

What Is Released During Ovulation?

The cell released during human ovulation is technically not a fully mature ovum. It is a secondary oocyte arrested in metaphase II.

This distinction is important when describing the precise sequence of human gametogenesis.

Formation of the Mature Ovum

If a sperm successfully interacts with and enters the secondary oocyte, the oocyte resumes and completes meiosis II.

Completion of meiosis II produces the mature female gamete, or ovum, and a second polar body.

Chromosome Number

The mature ovum is haploid. In humans, it contains 23 chromosomes.

Fusion of maternal and paternal haploid chromosome sets during fertilization restores the diploid chromosome complement in the resulting zygote.

Chromosomal Changes During Oogenesis

Cell	Chromosome Sets	Developmental State
Oogonium	Diploid	Mitotically proliferating germ cell
Primary oocyte	Diploid	Arrested in prophase I after DNA replication
Secondary oocyte	Haploid	Arrested in metaphase II
Ovum	Haploid	Produced after completion of meiosis II
Zygote	Diploid	Formed following fertilization

Size

The human oocyte is one of the largest cells in the body and is approximately 0.1 mm in diameter.

Its relatively large size reflects the abundance of cytoplasm and cellular machinery required to support the earliest stages of development.

Structure

The female gamete consists of a plasma membrane surrounding abundant cytoplasm and the maternal genetic material. During ovulation and fertilization, it is also surrounded by specialized extracellular and cellular layers.

Important structures associated with the ovulated secondary oocyte include:

• Oocyte plasma membrane
• Cytoplasm
• Zona pellucida
• Perivitelline space
• First polar body
• Corona radiata

Oocyte Plasma Membrane

The plasma membrane encloses the oocyte cytoplasm and participates in interactions associated with sperm-oocyte fusion.

After sperm fusion, changes within the oocyte help prevent additional sperm from successfully fertilizing the same cell.

Cytoplasm

The oocyte contains abundant cytoplasm compared with the sperm. This cytoplasm contains organelles, proteins, messenger RNAs, metabolites, and other molecules needed during fertilization and early embryonic development.

Before activation of the embryonic genome, early development depends heavily on maternal components accumulated within the oocyte.

Maternal Organelles

The oocyte contributes most of the cytoplasmic organelles of the zygote. In humans, mitochondrial inheritance is overwhelmingly maternal.

The sperm contributes important nuclear and cellular components during fertilization, but the bulk of the zygotic cytoplasm comes from the oocyte.

Zona Pellucida

The zona pellucida is a specialized extracellular glycoprotein layer surrounding the oocyte.

It develops during follicular growth and plays important roles in sperm interaction, fertilization, protection of the early embryo, and prevention of inappropriate implantation during early transport.

Perivitelline Space

The perivitelline space lies between the oocyte plasma membrane and the zona pellucida.

After meiosis I, the first polar body is usually located within this space.

Polar Bodies

Polar bodies are small cells produced by the asymmetric meiotic divisions of oogenesis. They contain chromosome sets that are separated from the developing female gamete while relatively little cytoplasm is allocated to them.

This arrangement allows the oocyte to retain most of its cytoplasmic resources.

First Polar Body

The first polar body is produced when the primary oocyte completes meiosis I.

It is considerably smaller than the secondary oocyte because the cytoplasm is divided unequally.

Second Polar Body

The second polar body is produced when the secondary oocyte completes meiosis II following fertilization.

The resulting mature ovum retains most of the cytoplasm.

Corona Radiata

The corona radiata consists of granulosa cells immediately surrounding the zona pellucida of the ovulated secondary oocyte.

These cells remain associated with the oocyte after ovulation and must be traversed by sperm during the fertilization process.

Cumulus Oophorus

Before ovulation, the oocyte is supported within the follicle by a mound of granulosa cells called the cumulus oophorus.

The innermost cumulus cells surrounding the zona pellucida constitute the corona radiata.

Layers Associated with the Ovulated Oocyte

Structure	Description
Corona radiata	Granulosa cells surrounding the oocyte
Zona pellucida	Extracellular glycoprotein coat
Perivitelline space	Space between zona pellucida and oocyte membrane
Plasma membrane	Cell membrane enclosing the oocyte
Cytoplasm	Contains maternal organelles and molecular resources

Ovulation

Ovulation is the release of the secondary oocyte from the mature ovarian follicle. The oocyte is released together with its zona pellucida and surrounding cumulus cells.

It enters the peritoneal cavity near the infundibulum of the uterine tube.

Relationship to the Ovary

The female gamete develops within an ovarian follicle in the ovarian cortex. As the dominant follicle enlarges, it approaches the ovarian surface.

At ovulation, rupture of the follicular and ovarian surfaces releases the secondary oocyte from the ovary.

Relationship to the Uterine Tube

The ovary is not directly continuous with the uterine tube. After ovulation, the secondary oocyte must cross a short region of the peritoneal cavity before entering the tubal infundibulum.

Fimbrial movement, tubal fluid currents, and ciliary activity assist its entry and subsequent transport.

Infundibulum and Fimbriae

The funnel-shaped infundibulum forms the lateral end of the uterine tube. Its margin bears fimbriae that lie close to the ovary.

These structures help direct the ovulated oocyte toward the abdominal opening of the uterine tube.

Transport Through the Uterine Tube

Once within the uterine tube, the oocyte is transported by a combination of ciliary activity, smooth muscle contractions, and movement of tubal fluid.

The ampulla is the usual site at which the oocyte encounters sperm and fertilization occurs.

Ampulla

The ampulla is the longest and widest portion of the uterine tube and is the usual site of fertilization.

If fertilization occurs, the developing conceptus subsequently travels through the isthmus toward the uterine cavity.

Fertilization

Fertilization is the process by which male and female gametes interact and ultimately establish the genetic complement of a new individual.

It normally occurs in the ampullary region of the uterine tube.

Sperm Interaction with the Corona Radiata

Before reaching the oocyte plasma membrane, sperm must move through the cells of the corona radiata and interact with the zona pellucida.

These interactions form part of a coordinated sequence leading to sperm-oocyte fusion.

Interaction with the Zona Pellucida

The zona pellucida participates in sperm recognition and binding during fertilization.

Successful sperm interaction with the zona contributes to events that allow the sperm to reach the oocyte membrane.

Acrosome Reaction

The sperm head contains an organelle called the acrosome. During fertilization, the sperm undergoes an acrosomal reaction that facilitates its passage through the extracellular investments surrounding the oocyte.

This process is one component of the complex interaction between sperm and the zona pellucida.

Sperm-Oocyte Fusion

After traversing the zona pellucida, the sperm reaches and fuses with the oocyte plasma membrane.

This interaction activates the secondary oocyte and initiates cellular changes that include completion of meiosis II.

Cortical Granules

Cortical granules are secretory vesicles located beneath the oocyte plasma membrane.

Following sperm-oocyte fusion, their contents are released and modify the zona pellucida as part of the mechanisms that prevent additional sperm from successfully fertilizing the oocyte.

Block to Polyspermy

Normal development requires fertilization by a single sperm. Entry of additional sperm would produce an abnormal chromosome complement.

Post-fertilization changes involving the oocyte and zona pellucida contribute to the block against polyspermy.

Completion of Meiosis II

Sperm entry stimulates the secondary oocyte to complete meiosis II. This produces the mature ovum and the second polar body.

The maternal chromosomes then become organized within the female pronucleus.

Female Pronucleus

After completion of meiosis II, the maternal haploid chromosome set becomes enclosed within the female pronucleus.

The paternal genetic material forms a corresponding male pronucleus within the fertilized cell.

Male Pronucleus

After sperm entry, the sperm nucleus undergoes structural changes and forms the male pronucleus.

The maternal and paternal chromosome sets subsequently participate in establishment of the diploid genome of the zygote.

Zygote Formation

The zygote is the single-cell stage produced following fertilization and establishment of the combined maternal and paternal chromosome complement.

It contains a diploid chromosome set and begins a series of mitotic divisions known as cleavage.

Ovum versus Secondary Oocyte

Feature	Secondary Oocyte	Ovum
Meiotic stage	Arrested in metaphase II	Meiosis II completed
Timing	Present at ovulation	Forms after fertilization triggers meiotic completion
Chromosome state	Haploid chromosomes with sister chromatids	Haploid maternal chromosome complement after meiosis II
Second polar body	Not yet produced	Produced during completion of meiosis II

Ovum versus Sperm

Feature	Female Gamete	Sperm
Size	Large	Small
Motility	Non-motile, transported by reproductive tract mechanisms	Motile
Cytoplasm	Abundant	Minimal
Chromosome set	Haploid	Haploid
Sex chromosome	X chromosome	X or Y chromosome
Major contribution to zygote	Nuclear genome, cytoplasm, organelles, maternal molecular components	Paternal nuclear genome and other sperm-derived components

Sex Chromosome

A normal human ovum contains an X chromosome. A sperm can normally contain either an X or a Y chromosome.

The sex chromosome contributed by the fertilizing sperm therefore determines whether the resulting conventional chromosomal combination is XX or XY.

Maternal Mitochondrial Inheritance

The mitochondria of the early embryo are inherited overwhelmingly from the oocyte.

This pattern results from the enormous cytoplasmic contribution of the female gamete and the elimination or failure of paternal mitochondria to contribute significantly to the established embryonic mitochondrial population.

Asymmetric Cytokinesis

One of the defining features of oogenesis is asymmetric cytokinesis. Meiotic divisions distribute chromosomes while retaining most of the cytoplasm within a single large female gamete.

The remaining chromosome sets are packaged into much smaller polar bodies.

Why the Ovum Is Large

The large size of the female gamete allows it to contain substantial cytoplasmic resources required immediately after fertilization.

These include mitochondria, ribosomes, proteins, messenger RNAs, regulatory molecules, and other cellular components required before embryonic gene expression becomes fully established.

Duration After Ovulation

The ovulated secondary oocyte remains capable of fertilization for a relatively limited period after ovulation.

If fertilization does not occur, it degenerates rather than completing meiosis II and forming a mature ovum.

Oocyte Aging

Human primary oocytes can remain arrested in meiosis for decades. This unusually prolonged meiotic arrest is an important feature of female reproductive biology.

Increasing maternal age is associated with an increased frequency of errors in meiotic chromosome segregation.

Meiotic Nondisjunction

Nondisjunction occurs when chromosomes fail to segregate normally during cell division.

Errors during oocyte meiosis can produce gametes containing an abnormal number of chromosomes and can contribute to embryonic aneuploidy after fertilization.

Aneuploidy

Aneuploidy refers to an abnormal number of individual chromosomes rather than an exact multiple of the haploid chromosome set.

Abnormal chromosome segregation during maternal meiosis is an important mechanism by which aneuploid conceptions can arise.

Clinical Relevance in Assisted Reproduction

Oocytes can be retrieved from ovarian follicles for use in assisted reproductive procedures. Their developmental maturity can be assessed before fertilization procedures are performed.

A metaphase II oocyte represents the stage normally considered mature for fertilization in assisted reproductive settings.

Oocyte Retrieval

During assisted reproductive treatment, follicular development can be stimulated hormonally and oocytes retrieved from mature follicles.

The retrieved cells are evaluated for maturity and may subsequently undergo conventional insemination or intracytoplasmic sperm injection.

Intracytoplasmic Sperm Injection

Intracytoplasmic sperm injection involves introduction of a sperm directly into the cytoplasm of a mature oocyte.

The procedure bypasses several natural steps required for sperm to traverse the cumulus cells and zona pellucida.

Oocyte Cryopreservation

Mature oocytes can be preserved by cryopreservation for potential future use.

The large size, high water content, meiotic spindle, and complex cellular organization of the oocyte are important considerations in successful cryopreservation techniques.

Developmental Timeline

1. Primordial germ cells migrate to the developing ovary.
2. Primordial germ cells differentiate into oogonia.
3. Oogonia proliferate by mitosis during fetal life.
4. Oogonia enter meiosis and become primary oocytes.
5. Primary oocytes arrest in prophase I.
6. Primary oocytes become enclosed within primordial follicles.
7. Selected follicles develop during reproductive cycles.
8. The primary oocyte completes meiosis I before ovulation.
9. A secondary oocyte and first polar body are produced.
10. The secondary oocyte begins meiosis II and arrests at metaphase II.
11. The secondary oocyte is released during ovulation.
12. Fertilization triggers completion of meiosis II.
13. The mature ovum and second polar body are produced.
14. Maternal and paternal genetic material contribute to formation of the zygote.

Key Structural and Developmental Relationships

• The ovum is the mature female gamete.
• The female germ-cell lineage begins with primordial germ cells.
• Oogonia proliferate by mitosis during fetal life.
• Oogonia become primary oocytes when they enter meiosis I.
• Primary oocytes remain arrested in prophase I until recruited during reproductive life.
• Completion of meiosis I produces a secondary oocyte and first polar body.
• The cell released at ovulation is a secondary oocyte arrested in metaphase II.
• Fertilization triggers completion of meiosis II.
• The mature ovum is haploid and contains 23 chromosomes.
• The zona pellucida surrounds the oocyte.
• The corona radiata consists of granulosa cells surrounding the zona pellucida.
• The perivitelline space lies between the oocyte membrane and zona pellucida.
• The ampulla of the uterine tube is the usual site of fertilization.
• The oocyte supplies most of the cytoplasm and organelles of the zygote.
• Mitochondrial inheritance is overwhelmingly maternal.

Clinical Importance

Understanding the ovum requires distinguishing it from the secondary oocyte. Although the term ovum is commonly used for the cell released during ovulation, the ovulated human female gamete is technically a secondary oocyte arrested in metaphase II. The mature ovum exists only after fertilization initiates completion of the second meiotic division.

The unusual developmental history of the female gamete is clinically important. Oogenesis begins before birth, primary oocytes can remain arrested for decades, and the final meiotic division depends on fertilization. Errors in chromosome segregation during this process can produce abnormal chromosome numbers in the resulting conceptus.

The structural organization of the oocyte is also fundamental to reproductive biology. Its zona pellucida, corona radiata, cortical granules, abundant cytoplasm, maternal organelles, and meiotic machinery participate directly in fertilization and early development. These features provide the anatomical and cellular basis for understanding oogenesis, ovulation, fertilization, early embryology, reproductive aging, aneuploidy, and assisted reproductive technologies.