📘 Oogenesis

(Important for Anatomy & Physiology, OBG Nursing, GNM/BSc Nursing, NHM, AIIMS, NORCET, GPSC Staff Nurse Exams)

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✅ 1. Introduction / Definition:

Oogenesis is the process by which the female gametes (ova or eggs) are produced and matured in the ovaries.

• Begins during fetal life and continues until menopause
• Involves mitosis, meiosis I & II, and maturation of oocytes

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✅ 2. Location:

• Takes place in the ovarian cortex, within structures called ovarian follicles

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✅ 3. Phases of Oogenesis:

Oogenesis occurs in three major phases:

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🔹 A. Multiplication Phase (Fetal Life):

• Primordial germ cells (oogonia) multiply by mitosis in fetal ovaries
• Around 6–7 million oogonia formed by the 5th month of intrauterine life
• Most degenerate (atresia); only ~1–2 million remain at birth

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🔹 B. Growth Phase:

• Oogonia grow into primary oocytes
• Primary oocytes enter prophase I of meiosis I but get arrested in diplotene stage
• At birth: Female has ~400,000 primary oocytes arrested in prophase I
• These remain dormant until puberty

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🔹 C. Maturation Phase (Puberty to Menopause):

Each menstrual cycle:

• A few primary oocytes resume meiosis I
• One becomes dominant and completes meiosis I → forms:
  1. Secondary oocyte (large)
  2. First polar body (small, degenerates)
• Secondary oocyte starts meiosis II but is arrested in metaphase II

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✅ 4. Completion of Meiosis II:

• Occurs only if fertilization takes place
• Secondary oocyte completes meiosis II → forms:
  1. Mature ovum (functional gamete)
  2. Second polar body (non-functional)

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✅ 5. Summary of Oogenesis Process:

Stage	Cell Type	Process	Fate
Fetal life	Oogonia → Primary oocyte	Mitosis	Arrest in prophase I
Puberty	Primary oocyte → Secondary oocyte + 1st polar body	Meiosis I	Monthly after puberty
Fertilization	Secondary oocyte → Ovum + 2nd polar body	Meiosis II	Only if fertilized

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✅ 6. Differences from Spermatogenesis:

Feature	Oogenesis	Spermatogenesis
Time begins	Before birth	Puberty
Completion	Only if fertilized	Continuous
Output	1 ovum + 3 polar bodies	4 sperm
Arrest phases	Prophase I & Metaphase II	None

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✅ 7. Hormonal Regulation:

• FSH (Follicle Stimulating Hormone): Stimulates follicle development and oocyte maturation
• LH (Luteinizing Hormone): Triggers ovulation and resumption of meiosis

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✅ 8. Clinical Significance:

• Anovulation → absence of ovulation → infertility
• Chromosomal abnormalities in older oocytes → increased risk of Down syndrome, etc.
• Ovarian reserve → declines with age; measured via AMH (Anti-Müllerian hormone)
• Assisted Reproductive Techniques (ART) → stimulate multiple follicles

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✅ 9. Golden One-Liners for Quick Revision:

• Oogenesis starts before birth and continues until menopause
• Primary oocyte arrests in prophase I; secondary oocyte in metaphase II
• Fertilization is necessary to complete meiosis II
• Each cycle, 1 ovum + 3 polar bodies are formed
• Takes place in the ovarian follicles

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✅ 10. MCQs for Practice:

Q1. Oogenesis begins during:
a) Puberty
b) Embryonic life
c) Menopause
d) Ovulation
Correct Answer: b) Embryonic life
Rationale: Oogonia form and begin meiosis I during fetal development.

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Q2. The primary oocyte is arrested in which stage of meiosis?
a) Metaphase I
b) Anaphase II
c) Prophase I
d) Telophase I
Correct Answer: c) Prophase I
Rationale: Primary oocytes remain in prophase I until puberty.

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Q3. Completion of meiosis II occurs only if:
a) Menstruation occurs
b) LH surge happens
c) Fertilization occurs
d) Ovulation occurs
Correct Answer: c) Fertilization occurs
Rationale: Without fertilization, meiosis II is not completed.

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Q4. What is the product of the first meiotic division in oogenesis?
a) Ovum
b) Secondary oocyte and first polar body
c) Primary oocyte
d) Corpus luteum
Correct Answer: b) Secondary oocyte and first polar body
Rationale: Meiosis I yields one functional secondary oocyte and one polar body.

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Q5. Total number of ova formed from one primary oocyte is:
a) Four
b) Two
c) One
d) Three
Correct Answer: c) One
Rationale: Only one ovum is produced; others become polar bodies.

📘 Spermatogenesis

(Important for Anatomy & Physiology, Reproductive System, GNM/BSc Nursing, NHM, AIIMS, NORCET, GPSC Staff Nurse Exams)

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✅ 1. Introduction / Definition:

Spermatogenesis is the process of formation of mature male gametes (spermatozoa) from primitive germ cells (spermatogonia) in the seminiferous tubules of the testes.

It begins at puberty and continues throughout life.

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✅ 2. Site of Spermatogenesis:

• Occurs in the seminiferous tubules of the testes.
• Sertoli cells support and nourish the developing sperm.
• Leydig cells (outside tubules) produce testosterone, essential for this process.

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✅ 3. Phases of Spermatogenesis:

It occurs in 3 main phases, followed by spermiogenesis:

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🔹 A. Multiplication Phase:

• Spermatogonia (diploid, 2n) undergo mitosis
• Some remain as stem cells, others become primary spermatocytes

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🔹 B. Growth Phase:

• Primary spermatogonia grow and develop into primary spermatocytes (2n)

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🔹 C. Maturation Phase (Meiosis):

Meiosis I:

• Primary spermatocyte (2n) → divides to form 2 secondary spermatocytes (haploid, n)

Meiosis II:

• Each secondary spermatocyte (n) divides to form 2 spermatids (n)

✅ Final outcome: 1 primary spermatocyte → 4 spermatids

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🔹 D. Spermiogenesis:

• Spermatids (non-motile, immature cells) transform into mature spermatozoa (sperm)
• Changes include:
  • Formation of head, midpiece, tail
  • Development of acrosome
  • Condensation of nucleus
  • Shedding of cytoplasm

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✅ 4. Duration:

• Entire process takes ~64–74 days to complete
• Around 300–400 million sperm produced per day

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✅ 5. Hormonal Regulation:

Hormone	Source	Role
GnRH	Hypothalamus	Stimulates FSH and LH release
FSH	Anterior pituitary	Stimulates Sertoli cells for spermatogenesis
LH	Anterior pituitary	Stimulates Leydig cells → produce testosterone
Testosterone	Leydig cells	Promotes maturation of sperm
Inhibin	Sertoli cells	Inhibits FSH secretion

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✅ 6. Summary of Spermatogenesis:

Stage	Cell Type	Division	Ploidy
1	Spermatogonia	Mitosis	Diploid (2n)
2	Primary spermatocyte	Meiosis I	Diploid (2n)
3	Secondary spermatocytes	Meiosis II	Haploid (n)
4	Spermatids → Spermatozoa	Spermiogenesis	Haploid (n)

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✅ 7. Differences Between Spermatogenesis and Oogenesis:

Feature	Spermatogenesis	Oogenesis
Starts	Puberty	Fetal life
Duration	Lifelong	Ends at menopause
Product	4 sperm	1 ovum + 3 polar bodies
Meiosis completion	Continuous	Only after fertilization
Site	Testes	Ovaries

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✅ 8. Clinical Significance:

• Oligospermia – Low sperm count (<15 million/mL)
• Azoospermia – Absence of sperm
• Teratospermia – Abnormal morphology
• Asthenospermia – Poor motility
• Infertility – Due to hormonal imbalance or genetic defects

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✅ 9. Golden One-Liners for Quick Revision:

• Spermatogenesis starts at puberty and continues lifelong
• Occurs in seminiferous tubules of testes
• Sertoli cells → nourish developing sperm
• Leydig cells → secrete testosterone
• Each primary spermatocyte → forms 4 spermatids

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✅ 10. MCQs for Practice:

Q1. The site of spermatogenesis is:
a) Epididymis
b) Prostate gland
c) Seminiferous tubules
d) Vas deferens
Correct Answer: c) Seminiferous tubules
Rationale: Sperm formation occurs in the seminiferous tubules of the testis.

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Q2. One primary spermatocyte forms how many spermatozoa?
a) One
b) Two
c) Three
d) Four
Correct Answer: d) Four
Rationale: Meiosis I and II lead to 4 haploid spermatids.

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Q3. The hormone that stimulates Leydig cells to produce testosterone is:
a) FSH
b) LH
c) GnRH
d) Inhibin
Correct Answer: b) LH
Rationale: LH targets Leydig cells to secrete testosterone.

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Q4. The transformation of spermatids into spermatozoa is called:
a) Fertilization
b) Spermatogenesis
c) Spermiogenesis
d) Capacitation
Correct Answer: c) Spermiogenesis
Rationale: It is the maturation phase that gives sperm their motile structure.

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Q5. Which cells provide nutrition to developing sperm cells?
a) Leydig cells
b) Sertoli cells
c) Spermatogonia
d) Acrosomal cells
Correct Answer: b) Sertoli cells
Rationale: Sertoli cells are also called “nurse cells” of the testes.

📘 Fertilization

(Important for Anatomy & Physiology, Embryology, OBG Nursing, GNM/BSc Nursing, NHM, AIIMS, NORCET, GPSC Staff Nurse Exams)

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✅ 1. Introduction / Definition:

Fertilization is the process of fusion of a male gamete (sperm) with a female gamete (ovum) to form a zygote (fertilized egg).

It marks the beginning of pregnancy and combines the genetic material from both parents (23 chromosomes each) to form a diploid cell (46 chromosomes).

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✅ 2. Site of Fertilization:

• Most commonly occurs in the ampulla of the fallopian tube, which is the widest and most dilated part.

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✅ 3. Time of Fertilization:

• Usually occurs within 24 hours of ovulation.
• Sperm can survive in female reproductive tract for up to 3–5 days.

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✅ 4. Steps of Fertilization:

🔹 Step 1: Sperm Capacitation

• Biochemical changes that occur in the female reproductive tract to activate sperm and prepare it for penetration.
• Involves removal of glycoprotein coat and seminal proteins.

🔹 Step 2: Acrosomal Reaction

• Release of enzymes (e.g., hyaluronidase, acrosin) from the acrosome (a cap-like structure over the sperm head) to digest the zona pellucida (protective layer of ovum).

🔹 Step 3: Penetration of Zona Pellucida

• Only one sperm successfully penetrates the zona and reaches the oocyte membrane.

🔹 Step 4: Cortical Reaction (Block to Polyspermy)

• Once a sperm enters, the ovum releases cortical granules to prevent entry of other sperm.

🔹 Step 5: Fusion of Sperm and Ovum Nuclei

• Male and female pronuclei unite to form a zygote (46 chromosomes).
• This is known as syngamy.

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✅ 5. Result of Fertilization:

• Formation of a diploid zygote
• Determination of sex of the embryo (XX = female, XY = male)
• Initiation of cleavage and embryonic development

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✅ 6. Importance of Fertilization:

• Restores diploid chromosome number (46)
• Determines genetic sex
• Activates metabolic pathways in oocyte
• Starts embryogenesis

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✅ 7. Hormonal Influence:

• LH Surge: Triggers ovulation (release of ovum)
• Estrogen: Aids in cervical mucus thinning and sperm movement
• Progesterone: Prepares endometrium for implantation

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✅ 8. Clinical Relevance:

• Infertility: Due to failure in ovulation, sperm motility, or blocked tubes
• Ectopic pregnancy: Fertilized ovum implants outside uterus (commonly fallopian tube)
• In vitro fertilization (IVF): Fertilization outside the body (test tube baby)
• Contraceptives: Prevent fertilization by inhibiting ovulation or sperm entry

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✅ 9. Golden One-Liners for Quick Revision:

• Fertilization occurs in ampulla of fallopian tube
• Only one sperm fertilizes the ovum (monospermy)
• Acrosomal enzymes help in penetration
• Result: Zygote (diploid, 46 chromosomes)
• Fertilization restores diploid number and determines sex

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✅ 10. MCQs for Practice:

Q1. Fertilization usually occurs in which part of the female reproductive system?
a) Vagina
b) Cervix
c) Uterus
d) Ampulla of fallopian tube
Correct Answer: d) Ampulla of fallopian tube
Rationale: It provides the ideal environment for sperm-egg interaction.

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Q2. The enzyme that helps sperm penetrate the zona pellucida is:
a) Oxytocin
b) Hyaluronidase
c) Amylase
d) Inhibin
Correct Answer: b) Hyaluronidase
Rationale: It is released from the acrosome during the acrosomal reaction.

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Q3. The cortical reaction prevents:
a) Ovulation
b) Menstruation
c) Polyspermy
d) Cleavage
Correct Answer: c) Polyspermy
Rationale: It blocks additional sperm entry after fertilization.

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Q4. The union of male and female gametes results in the formation of:
a) Morula
b) Blastocyst
c) Zygote
d) Embryo
Correct Answer: c) Zygote
Rationale: The zygote is the first diploid cell formed after fertilization.

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Q5. Which hormone is responsible for ovum release from the ovary?
a) FSH
b) LH
c) Estrogen
d) Progesterone
Correct Answer: b) LH
Rationale: LH surge induces rupture of the Graafian follicle (ovulation).

📘 Principle Events of Embryonic and Fetal Development

(Important for Embryology, OBG Nursing, GNM/BSc Nursing, NHM, AIIMS, NORCET, GPSC Staff Nurse Exams)

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✅ 1. Introduction:

Embryonic and fetal development refers to the sequential biological events that occur from fertilization to birth. This process is divided into two main periods:

• Embryonic Period: From fertilization to 8 weeks (first 2 months)
• Fetal Period: From 9 weeks to birth

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🔶 I. EMBRYONIC DEVELOPMENT (Week 1 to Week 8):

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🔹 Week 1: Fertilization to Implantation

• Fertilization occurs in the ampulla of fallopian tube → forms zygote
• Cleavage: Rapid mitotic divisions → morula (solid ball of cells)
• Blastocyst formation: Hollow ball of cells with inner cell mass
• Implantation into endometrium begins ~Day 6–7

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🔹 Week 2: Bilaminar Embryo Formation

• Inner cell mass differentiates into two layers:
  • Epiblast (future embryo)
  • Hypoblast (contributes to yolk sac)
• Formation of:
  • Amniotic cavity
  • Yolk sac
  • Prechordal plate

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🔹 Week 3: Gastrulation – Trilaminar Embryo

• Formation of three germ layers:
  1. Ectoderm → skin, nervous system
  2. Mesoderm → muscles, bones, heart, kidneys
  3. Endoderm → GI tract, lungs, liver
• Formation of primitive streak, notochord, neural plate
• Beginning of neurulation

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🔹 Week 4: Body Folding & Organ Primordia

• Neural tube closes (future brain and spinal cord)
• Somites form (basis for vertebrae and muscles)
• Development of heart, pharyngeal arches, limb buds, primitive gut

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🔹 Weeks 5–8: Organogenesis

• Rapid development of:
  • Brain and cranial nerves
  • Heart chambers and circulation
  • Limb differentiation (digits form)
  • Eyes, ears, and facial features
• By the end of week 8, all major organ systems are established

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🔶 II. FETAL DEVELOPMENT (Week 9 to Birth):

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🔹 Month 3 (Weeks 9–12):

• External genitalia differentiate (sex can be determined)
• Urine formation begins
• Fetal movements may start (not felt yet)
• Heartbeat audible via Doppler (~10–12 weeks)

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🔹 Month 4–5 (Weeks 13–20):

• Quickening: Mother perceives fetal movements (~18–20 weeks)
• Lanugo (fine hair) and vernix caseosa appear
• Eyebrows, eyelashes, nails develop
• Sex clearly visible

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🔹 Month 6 (Weeks 21–24):

• Skin is translucent, red, wrinkled
• Surfactant begins forming in lungs
• Fetus may survive if born prematurely (~23–24 weeks)

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🔹 Month 7–9 (Weeks 25–40):

• Rapid weight gain and fat deposition
• Lung maturity improves
• Eyes open and close
• Term fetus (37–40 weeks):
  • Average weight ~3–3.5 kg
  • Length ~50 cm
  • Fully developed organ systems ready for birth

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✅ 3. Milestones Summary Table:

Event	Time
Fertilization	Day 0
Implantation	Day 6–7
Gastrulation (3 germ layers)	Week 3
Neurulation (neural tube forms)	Week 4
Heartbeat detectable by Doppler	Week 10–12
Quickening (first movement felt)	Week 18–20
Sex identifiable	Week 12–14
Surfactant begins	Week 24
Viability threshold	Week 24
Full term	Week 37–40

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✅ 4. Golden One-Liners for Quick Revision:

• Embryonic period = first 8 weeks (organ formation)
• Fetal period = Week 9 to birth (growth & maturation)
• Gastrulation = formation of 3 germ layers (Week 3)
• Neural tube closure = Week 4
• Quickening = Week 18–20
• Surfactant formation begins = Week 24
• Term baby = 37–40 weeks

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✅ 5. MCQs for Practice:

Q1. Fertilization usually occurs in the:
a) Uterus
b) Cervix
c) Vagina
d) Ampulla of fallopian tube
Correct Answer: d) Ampulla of fallopian tube
Rationale: It is the most common site of fertilization.

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Q2. Gastrulation and formation of three germ layers occurs in:
a) Week 2
b) Week 3
c) Week 4
d) Week 5
Correct Answer: b) Week 3
Rationale: Gastrulation begins in the third week.

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Q3. First fetal movement (quickening) is felt around:
a) 8 weeks
b) 12 weeks
c) 20 weeks
d) 28 weeks
Correct Answer: c) 20 weeks
Rationale: Quickening is typically felt between 18–20 weeks in primigravida.

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Q4. Surfactant production in the fetal lungs begins by:
a) 20 weeks
b) 24 weeks
c) 30 weeks
d) 36 weeks
Correct Answer: b) 24 weeks
Rationale: Surfactant begins forming around 24 weeks; essential for lung maturity.

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Q5. At the end of embryonic period (8 weeks), the embryo:
a) Has a beating heart only
b) Has all major organ systems formed
c) Cannot move
d) Has no limbs
Correct Answer: b) Has all major organ systems formed
Rationale: Organogenesis is completed by the 8th week.