Egg Cell (Ovum): Structure, Oogenesis, Types, Functions and Disorders

Introduction to Egg Cell

The egg cell, also known as ovum, is female reproductive cell. Egg cell is released from ovaries in female reproductive system.

Egg cell is female gamete responsible for reproduction in female organisms.

• This cell is formed through process of oogenesis.
• Has potential to develop into new organism when it joins with sperm cell.
• Over time, egg cell matures. Some remain inactive for many years before maturing, while others never fully develop.

Egg cell contains central nucleus with female's genetic material. This, combined with genetic material from sperm during fertilization, determines characteristics of future child.

• These egg cells are characterized by haploid nature, containing half number of chromosomes found in parent cell.

Anatomy and Structure of Egg Cell

Egg cell is generally spherical and non-motile, varying in size across different animals. Size ranges from 10 millimeters to few centimeters in length.

External Layers Surrounding Egg Cell

Egg cell is surrounded by three layers externally:

• Corona radiata is outermost layer surrounding egg cell and consists of follicular cells. These cells play role in nourishing and protecting egg.
• Zona pellucida is thick, transparent, and non-cellular layer located beneath corona radiata. It is composed of glycoproteins and provides structural support to egg. It also plays crucial role in sperm-egg interaction during fertilization.
• Vitelline membrane is innermost layer, which is thin, transparent membrane secreted by egg cell itself. This membrane provides additional protection to egg.

Cytoplasm of Egg Cell

Cytoplasm is divided into outer exoplasm or egg cortex and inner endoplasm or ooplasm.

• Cortex contains cytoskeletal structures like microtubules and microfilaments.

• Holds pigment granules and cortical granules.

• Ooplasm contains cell organelles, tRNAs, histones, enzymes, etc.

• Cortical granules are found in outer cortex of egg cytoplasm and play important role in activation of egg upon sperm entry.

• Cortical granules modify egg coat to prevent multiple sperm from fertilizing egg.

• Egg cytoplasm also contains essential reserves in form of yolk which provides essential proteins, lipids, and other nutrients necessary for early development.

• Yolk is located in ooplasm.

Nucleus and Polarity

• Nucleus of ovum, called germinal vesicle, is large and contains genetic material in form of DNA, essential for development of embryo.
• Side of egg cell with nucleus and polar body is animal pole, while opposite side is vegetal pole.

Oogenesis - Development of Egg Cell

Egg cell development, known as oogenesis, progresses through several stages where primary egg cell matures into fully developed egg.

While process differs among species, general stages follow similar pattern.

Formation of Oogonia

• Oogenesis starts with formation of oogonia which are precursor cells of eggs.
• Formed from primordial germ cells which migrate into developing gonads during early stages of embryonic development.

Primary Oocyte Development

• Oogonia mature into primary oocytes.
• In mammals, this stage often begins before female is even born.
• By time of birth, all egg cells that will be released during reproductive years are already present as primary oocytes.
• Primary oocytes ready to begin meiosis I division.
• During meiotic division I, DNA replicates, forming two sister chromatids for each chromosome.
• Homologous chromosomes align and exchange genetic material.

Prophase I Arrest

• Primary oocytes then enter prophase I of meiosis, where they remain arrested for long period.
• During this phase, primary oocytes synthesize important components such as:
  • Protective coat
  • Cortical granules
  • Various nutrients essential for embryonic growth

Oocyte Maturation

• Upon reaching sexual maturity, oocyte maturation starts.
• Under influence of hormones, cell completes meiosis I.
• Chromosomes recondense, nuclear envelope disintegrates, and homologous chromosomes segregate.
• Forms small polar body and large secondary oocyte.

Meiotic Division II

• In division II of meiosis, secondary oocyte further divides, producing mature egg and another polar body.

Release and Fertilization

• Mature egg remains arrested at metaphase II until ovulation.
• Upon release from ovary, it travels through fallopian tubes, where it waits for potential fertilization by male sperm cells.

Types of Female Egg Cell

Various types of egg cells are classified on basis of amount and distribution of yolk.

Based on Yolk Amount

• Alecithal eggs - lack yolk or have very little yolk distributed throughout cytoplasm. Common in organisms like mammals, where developing embryo is sustained by mother’s placenta.
• Microlecithal eggs - contain small to moderate amount of yolk. Common in sea urchins and some marine invertebrates.
• Mesolecithal eggs - have moderate amount of yolk, typically concentrated near vegetal pole. Includes eggs from amphibians such as frogs, toads, and some fish.
• Polylecithal or Macrolecithal eggs - have large amount of yolk. Found in organisms like sharks, bony fishes, reptiles, birds, and insects.

Based on Yolk Distribution

• Isolecithal eggs - have moderate amount of yolk, uniformly distributed throughout egg. Such distribution observed in alecithal and microlecithal eggs.
• Telolecithal eggs - have most yolk near vegetal pole, with cytoplasm and nucleus situated near animal pole. Observed in mesolecithal and macrolecithal eggs.
• Centrolecithal eggs - have centrally located yolk. Certain insects have centrolecithal eggs.

Functions of Egg Cell

Female egg cells have several functions:

• Primary function is to combine with sperm cells during fertilization, forming zygote, that matures into new organism.
• Egg cells carry half of genetic material required to form new individual.
• Rich in cytoplasmic components, such as yolk, which provide essential nutrients for developing embryo.
• Release chemical signals and molecules that attract sperm cells and facilitate their entry into egg during fertilization.
• Upon fertilization, egg changes membrane potential and biochemical processes, activating zygote for cell division and embryonic development.
• Help to prevent fertilization by multiple sperm cells, ensuring only one sperm can successfully fertilize egg.

Intracytoplasmic Sperm Injection (ICSI) in Humans

• Intracytoplasmic Sperm Injection (ICSI) is assisted technique related to egg cell where sperm directly injected into egg.

Diseases and Disorders of Egg Cell

Polycystic Ovary Syndrome (PCOS)

Common hormonal disorder in women of reproductive age.

• Causes:
  • Irregular menstrual cycles
  • Elevated androgen levels
  • Excess body hair
  • Abnormal growths on ovaries, previously thought to be cysts but now understood as follicles that do not mature properly
• Often leads to fertility issues due to irregular ovulation.
• Exact cause unknown, but risk increased with:
  • Family history
  • Type 2 diabetes
• There is no cure but symptoms can be managed with lifestyle changes and medications.

Endometriosis

Characterized by growth of tissue similar to uterine lining outside uterus, often causing pain and fertility issues.

• Can form cysts on ovaries, hindering egg release.
• Common sites include:
  • Ovaries
  • Pelvic ligaments
  • Even organs like intestines and bladder
• Chronic condition without cure but various treatments can provide relief.
• Treatment options include:
  • Pain medications
  • Hormonal therapies
  • Surgery to remove endometrial tissue

Ovarian Cysts

Sacs filled with fluid that can develop in ovaries which are usually harmless.

• Generally, these cysts are functional and benign, disappearing on own without need for surgery.
• However, in some cases, cysts can grow large, potentially leading to complications such as:
  • Pelvic pain
  • Cyst rupture
  • Excessive bleeding
  • Ovarian torsion
• This situation requires immediate medical attention and surgical treatment to prevent complications.
• Important to remove large cysts to avoid these risks.

Diminished Ovarian Reserve (DOR)

Refers to decreased number or quality of eggs in ovaries.

• Can result in difficulty conceiving due to lower number of eggs available for ovulation.
• People may have no symptoms other than difficulty conceiving. Some may notice shorter menstrual cycles.
• Causes include:
  • Aging
  • Genetics
  • Medical treatments like chemotherapy
  • Autoimmune conditions
• Treatment options include:
  • Freezing eggs for later use
  • Attempting natural conception
  • IVF with own or donor eggs
• Lifestyle factors like weight and avoiding smoking may help egg health.

Ovarian Cancer

Type of cancer where cells in ovaries grow abnormally. These organs produce eggs during reproductive years.

• Symptoms include:
  • Pelvic pain
  • Changes in appetite
  • Abnormal bleeding
  • Bowel changes
• Can spread to other organs if untreated.
• Staging helps determine treatment options, which include:
  • Surgery
  • Chemotherapy
  • Targeted therapy
  • Hormone therapy
  • Rarely, radiation therapy
• Can impact function of ovaries, including production and release of eggs.
• Can affect overall health of ovary, disrupt hormone production, and interfere with normal processes of ovulation and egg maturation.

Primary Ovarian Insufficiency (POI)

Condition where ovaries stop functioning earlier than usual. Also known as premature ovarian failure.

• Ovaries produce and release eggs during ovulation, as well as important hormones for menstruation and pregnancy.
• Leads to low estrogen and decrease in number and quality of eggs, often resulting in infertility.
• Relatively rare, affecting about 1% of women between 15 to 44, more commonly in those over 30.
• Causes include:
  • Autoimmune disorders
  • Cancer treatments
  • Genetic disorders like Turner syndrome
  • Exposure to toxins
• Symptoms can vary but often include:
  • Irregular or absent periods
  • Trouble conceiving
  • Decreased libido
  • Other menopausal-like symptoms
• Treatment focuses on symptom relief and reducing risks associated with POI.
• Treatments like IVF and hormone therapy can help manage symptoms and risks, improving quality of life.

Summary

Egg cell or ovum is female gamete released from ovaries formed by oogenesis, haploid containing half chromosomes with central nucleus called germinal vesicle determining offspring characteristics. Structure is spherical non-motile 10 mm to centimeters with three external layers corona radiata of follicular cells, zona pellucida of glycoproteins for structural support and sperm interaction, and vitelline membrane, plus cytoplasm divided into outer cortex with microtubules, microfilaments, pigment and cortical granules preventing polyspermy and inner ooplasm with organelles, tRNAs, histones and yolk reserves. Animal pole contains nucleus and polar body opposite vegetal pole. Oogenesis starts from primordial germ cells forming oogonia, developing into primary oocytes arrested in prophase I before birth synthesizing protective coat and nutrients, completing meiosis I at sexual maturity forming secondary oocyte and polar body, then meiosis II forming mature egg arrested at metaphase II until ovulation in fallopian tubes. Types classified by yolk amount as alecithal in mammals, microlecithal in sea urchins, mesolecithal in amphibians, macrolecithal in birds reptiles fishes, and by distribution as isolecithal uniform, telolecithal vegetal, centrolecithal central in insects. Functions include fertilization forming zygote, providing yolk nutrients, attracting sperm via chemical signals, activating zygote via membrane potential change and preventing polyspermy. Disorders include PCOS with anovulation, endometriosis with extrauterine tissue and ovarian cysts, functional cysts risking torsion, DOR with reduced egg number, ovarian cancer affecting ovulation, and POI or premature ovarian failure in 1% of women causing low estrogen and infertility.

References

1. Gilbert SF. Developmental Biology. 11th Edition. Sinauer Associates. Chapter 2, Oogenesis and Egg Structure.
2. Alberts B, Johnson A, Lewis J, Morgan D, Raff M, Roberts K, Walter P. Molecular Biology of the Cell. 6th Edition. Garland Science. Chapter 21, Female Gamete.
3. Lodish H, Berk A, Kaiser CA, Krieger M, Bretscher A, Ploegh H, Amon A. Molecular Cell Biology. 8th Edition. W.H. Freeman. Chapter 21, Development of Egg.
4. Carlson BM. Human Embryology and Developmental Biology. 6th Edition. Elsevier. Chapter 2, Oogenesis.
5. Tortora GJ, Derrickson B. Principles of Anatomy and Physiology. 15th Edition. Wiley. Chapter 28, Female Reproductive System and Oogenesis.
6. Sadler TW. Langman's Medical Embryology. 14th Edition. Wolters Kluwer. Chapter 2, Gametogenesis and Egg Types.
7. Campbell NA, Urry LA, Cain ML, Wasserman SA, Minorsky PV, Reece JB. Biology. 12th Edition. Pearson. Chapter 46, Animal Reproduction and Egg.
8. Wassarman PM, Litscher ES. Mammalian Fertilization: Zona Pellucida Glycoproteins. Histology and Histopathology. 2001. Volume 16.
9. Balinsky BI. An Introduction to Embryology. 5th Edition. Saunders. Chapter 3, Types of Eggs.
10. Griffin DK, Ellis PJ. Human Ovarian Reserve and Egg Quality. Reproduction. 2018. Volume 155.
11. Rotterdam ESHRE. Revised 2003 Consensus on Diagnostic Criteria for PCOS. Fertility and Sterility. 2004. Volume 81.
12. Giudice LC, Kao LC. Endometriosis. Lancet. 2004. Volume 364.
13. Broekmans FJ, Soules MR, Fauser BC. Ovarian Aging: Mechanisms and Clinical Consequences. Endocrine Reviews. 2009. Volume 30.
14. Torre LA, Trabert B, DeSantis CE, Miller KD, Samimi G. Ovarian Cancer Statistics. CA Cancer Journal for Clinicians. 2018. Volume 68.
15. NCERT. Biology Textbook for Class XII. Reprint 2023-24. National Council of Educational Research and Training. Chapter 3, Human Reproduction and Oogenesis.