TARGET DECK: MED::I::Morphology and Development::Embryology::01 - Human Embryology Introduction
Self Evaluation on Histology Lab
| Date | Time | Groups |
|---|---|---|
| 18 May | 14:00–17:00 | G1, G2, G3 |
| 19 May | 14:00–17:00 | G3, G4, G6 |
| Group | Arrival Time | Date |
|---|---|---|
| G1 | 14:00 | 18 May |
| G2 | 15:00 | 18 May |
| G5 | 16:00 | 18 May |
| G3 | 14:00 | 19 May |
| G4 | 15:00 | 19 May |
| G6 | 16:00 | 19 May |
Program
- Human Reproduction
- Transport of Gametes and Fertilization
- Cleavage and Implantation
- Formation of Germ Layers and Early Derivatives
- Establishment of the Basic Embryonic Body Plan
- Placenta and Fetal Membranes
- Molecular Basis for Embryonic Development
- Oogenesis, embryonic development and offspring from female iPSC-derived PGCLCs
- Self-Organization of the in vitro attached human embryo
- Ex utero mouse embryogenesis from pre-gastrulation to late organogenesis
Recommended Textbooks
- Carlson — Human Embryology & Developmental Biology
- Sadler — Langman’s Medical Embryology
- Moore & Persaud — The Developing Human: Clinically Oriented Embryology
- Schoenwolf et al. — Larsen’s Human Embryology
Embryology — Definition and Periods
Definition
Embryology is the branch of medicine that studies Human Development. Development begins at fertilization, approximately 14 days after the last normal menstrual period.
- The embryonic period covers the first 8 weeks of development.
- The fetal period begins in the 9th week.
- Most visible advances occur during the 3rd to 8th week.
Human Pregnancy Periods
Clinical (Obstetric) Subdivision — Trimesters
Physicians use trimesters: three-month periods starting from the onset of the last menstrual period (LMP).
| Trimester | Approximate Weeks from LMP |
|---|---|
| First | 0–13 weeks |
| Second | 14–26 weeks |
| Third | 27–40 weeks |
Embryological Subdivision — Periods
Human embryologists use three periods:
| Period | Timing |
|---|---|
| Period of the egg | Fertilization → end of 3rd week |
| Period of the embryo | Beginning of 4th week → end of 8th week |
| Period of the fetus | Beginning of 3rd month → birth |
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The embryonic period covers the first {1:8} weeks of development, after which the fetal period begins.
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In obstetric terminology, pregnancy is divided into {1:three} trimesters, each approximately {1:three} months long, starting from the {1:last menstrual period}.
Phases of Human Embryogenesis
Key Phases
Human embryologists identify the following sequential phases:
- Gametogenesis — formation of gametes (egg and sperm)
- Fertilization — joining of gametes to form the zygote
- Cleavage — rapid cell divisions forming first the morula (mulberry-like cluster), then the blastocyst (hollow ball of cells with a central cavity)
- Gastrulation — rearrangement of cells into three primary germ layers (ectoderm, mesoderm, endoderm) to form the embryonic disc
- Formation of the tube-within-a-tube body plan — body folding converts the embryonic disc into a C-shaped body:
- Outer ectodermal tube (future skin)
- Inner endodermal tube (gut tube)
- Mesoderm interposed between the two tubes
- Organogenesis — formation of organ rudiments and organ systems
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Cleavage results first in the formation of the {1:morula}, then in the {1:blastocyst}.
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Gastrulation forms three primary germ layers: {1:ectoderm}, {1:mesoderm}, and {1:endoderm}.
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The tube-within-a-tube body plan consists of an outer {1:ectodermal} tube (future skin) and an inner {1:endodermal} tube (gut tube), with {1:mesoderm} interposed between them.
Timetable of Prenatal Development
Info
Key early stages by approximate day post-fertilization:
| Stage | Event |
|---|---|
| Stage 2 begins | Early cleavage |
| Stage 3 begins | Morula |
| Stage 4 | Trophoblast differentiation |
| Stage 5 begins | Implantation begins |
| ~Day 7–12 | Implantation |
| Bilaminar embryonic disc | Prochordal plate visible |
Developmental Biology Frontiers
Historical Milestones
- 1995 — Edward B. Lewis, Christiane Nüsslein-Volhard, and Eric F. Wieschaus awarded the Nobel Prize in Physiology or Medicine for discovery of genes controlling embryonic development. Critical role of genes, signaling molecules, receptors, and molecular factors in regulating early embryonic development is being delineated.
- 1997 — Ian Wilmut and colleagues produced the first cloned mammal (Dolly the sheep) using somatic cell nuclear transfer (SCNT). Interest in human cloning has generated considerable debate due to social, ethical, and legal implications; concern exists about increased birth defects in cloned neonates.
- 2021 — Ex utero mouse embryogenesis from pre-gastrulation to late organogenesis demonstrated.
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The first cloned mammal, Dolly the sheep, was produced in {1:1997} by {1:Ian Wilmut} using the technique of {1:somatic cell nuclear transfer}.
Getting Ready for Pregnancy — The Reproductive Organs
Info
The reproductive organs produce and transport germ cells (gametes) from the gonads (testes or ovaries) to the site of fertilization, which usually occurs in the ampulla of the uterine tubes.
- Vagina — serves as excretory passage for menstrual fluid and forms the inferior part of the birth canal.
- Uterus — pear-shaped organ composed of two parts:
- Body — expanded superior two thirds
- Cervix — cylindrical inferior third
Uterus — Wall Layers
The wall of the body of the uterus consists of three layers:
| Layer | Description |
|---|---|
| Perimetrium | Thin external peritoneal layer |
| Myometrium | Thick smooth muscle layer |
| Endometrium | Internal layer (three sublayers) |
Endometrium Sublayers
| Sublayer | Description |
|---|---|
| Compact layer | Densely packed connective tissue around terminal ducts of uterine glands |
| Spongy layer | Highly vascularized connective tissue containing dilated uterine glands |
| Basal layer | Contains the blind ends of uterine glands |
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The three layers of the uterine wall from outer to inner are: {1:perimetrium}, {1:myometrium}, and {1:endometrium}.
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The endometrium is subdivided into a {1:compact} layer, a {1:spongy} layer, and a {1:basal} layer.
Gametogenesis
Definition
Gametogenesis is the formation of germ cells.
- Spermatogenesis in males
- Oogenesis in females
Sperms and oocytes are highly specialized gametes containing 23 chromosomes instead of 46, obtained through meiosis.
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Gametogenesis reduces the chromosome number from {1:46} to {1:23} via the process of {1:meiosis}.
Normal Gametogenesis
Important
- Sperm and oocytes each contain half the required chromosome number (23 instead of 46).
- Chromosome number is reduced during meiosis — a special type of cell division occurring only during gametogenesis.
- In males: spermatogenesis
- In females: oogenesis
Spermatogenesis
Overview
- Primordial germ cells (PGCs) derive from the epiblast (blastoderm) and remain dormant from the 6th week of embryonic development until puberty.
- Primordial sperms (spermatogonia) remain dormant in the seminiferous tubules of the testes.
- After puberty:
- Spermatogonia increase in number by mitosis
- They grow and transform into primary spermatocytes
- First meiotic division → two haploid secondary spermatocytes (half the size of primary)
- Second meiotic division → spermatids (half size)
- Spermatids differentiate into spermatozoa via spermiogenesis
Timeline and Storage
- When spermiogenesis is complete, sperms enter the lumen of the seminiferous tubules.
- They then move to the epididymis, where they are stored and become functionally mature.
- Spermatogenesis requires approximately 2 months for completion.
- Continues throughout the reproductive life of a male.
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After puberty, spermatogonia undergo mitosis to become {1:primary spermatocytes}, which then undergo the first meiotic division to form two haploid {1:secondary spermatocytes}.
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Mature sperms are stored and undergo functional maturation in the {1:epididymis}.
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Spermatogenesis requires approximately {1:2 months} for completion.
Primordial Germ Cells — Origin
Important
- Primordial germ cells (PGCs) can be identified during the 4th–6th week of gestation within an extraembryonic membrane called the yolk sac.
- Their lineage constitutes the germ line — a series of cells that form the gametes.
- One of the first events in the developing embryo is that the germ line is set aside for the next generation.
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Primordial germ cells (PGCs) can be identified during the {1:4th–6th} week of gestation within the {1:yolk sac}.
Mature Sperm Cell — Structure
Sperm Anatomy
- Acrosome — cap-like organelle on the anterior two thirds of the head; contains enzymes that facilitate sperm penetration during fertilization (penetration of the zona pellucida).
- Tail — provides motility; consists of three parts:
| Part | Length | Function |
|---|---|---|
| Middle piece | ~7 µm | Contains mitochondria; produces energy for lashing movements |
| Principal piece | ~40 µm | Main locomotor segment |
| End piece | 5–10 µm | Terminal segment |
- Hox genes influence microtubule dynamics at the molecular level, shaping the sperm head and forming the tail.
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The acrosome is located on the anterior {1:two thirds} of the sperm head and contains {1:enzymes} that facilitate penetration of the zona pellucida.
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The energy-producing organelles that fuel sperm motility are the {1:mitochondria}, located in the {1:middle piece} of the sperm tail.
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The three parts of the sperm tail are the {1:middle piece}, the {1:principal piece}, and the {1:end piece}.
At Puberty — Hormonal Control of Spermatogenesis
-
Testosterone secretion stimulates:
- Development of secondary sex characteristics
- Growth of the testes
- Maturation of seminiferous tubules
- Commencement of spermatogenesis → 150 to 275 million spermatozoa per day in humans
-
Sertoli cells differentiate into a system of seminiferous tubules.
-
Dormant PGCs resume development, divide by mitosis, then differentiate into spermatogonia.
-
Spermatogonia are located immediately under the basement membrane surrounding the seminiferous tubules.
-
Adjacent Sertoli cells are interconnected by tight junctions → establish the blood-testis barrier (testis is an immune-privileged site).
-
Developing spermatogonia reside within an immune-privileged site during development.
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Testosterone triggers the commencement of spermatogenesis, producing {1:150–275 million} spermatozoa per day in humans.
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The blood-testis barrier is established by {1:tight junctions} between adjacent {1:Sertoli cells}, making the testis an {1:immune-privileged} site.
https://unibo.smartzoom.com/s1241/course1776/f1815/i1823/
Major Functions of Sertoli Cells
| Function |
|---|
| Maintenance of the blood-testis barrier |
| Secretion of tubular fluid (10–20 µL/g of testis/h) |
| Secretion of androgen-binding protein |
| Secretion of estrogen and inhibin |
| Secretion of various proteins (growth factors, transferrin, retinal-binding protein, metal-binding proteins) |
| Maintenance and coordination of spermatogenesis |
| Phagocytosis of residual bodies of sperm cells |
What are the major functions of Sertoli cells?
- Maintain blood-testis barrier; 2. Secrete tubular fluid; 3. Secrete androgen-binding protein; 4. Secrete estrogen and inhibin; 5. Secrete growth factors and other proteins; 6. Maintain/coordinate spermatogenesis; 7. Phagocytose residual bodies of sperm cells.
Capacitation
Capacitation — Final Step of Sperm Maturation
Capacitation consists mainly of changes in the acrosome that prepare it to release the enzymes required to penetrate the zona pellucida (a shell of glycoprotein surrounding the oocyte).
- Capacitation takes place within the female genital tract.
- Requires contact with secretions of the oviduct.
- Spermatozoa used in IVF are artificially capacitated (using media of calcium ions, bicarbonate, and serum albumin).
- Spermatozoa with defective acrosomes may be injected directly into oocytes (ICSI) to assist reproduction.
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Capacitation, the final step of sperm maturation, involves changes in the {1:acrosome} and takes place within the {1:female genital tract}, requiring contact with secretions of the {1:oviduct}.
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In IVF, capacitation is achieved artificially using a medium containing {1:calcium ions}, {1:bicarbonate}, and {1:serum albumin}.
Practice Question — Acrosome Mutation
Exam Question
What would be a direct consequence of a single point mutation that modifies the proteins inside the acrosome?
- a. Inability to dissolve zona pellucida’s glycocalyx ✓
- b. Impairment of sperm capacitation
- c. None of them
- d. Impairment of sperm motility
- e. Failure to complete spermatogenesis
Oogenesis: From Oogonia to Oocytes
Timeline of Oogenesis
| Stage | Event |
|---|---|
| Fetal period | Oogonia proliferate by mitosis and enlarge to form primary oocytes |
| Birth | All primary oocytes have completed prophase I of the first meiotic division; arrested here until puberty |
| Just before ovulation | Primary oocyte completes first meiotic division with unequal cytoplasm division → 1 polar body (degenerates) + secondary oocyte (receives most cytoplasm) |
| Ovulation | Nucleus of secondary oocyte begins second meiotic division |
| Fertilization | Second meiotic division completed → second polar body formed |
Important
- The secondary oocyte is large and visible to the naked eye.
- Up to 2,000,000 primary oocytes are present in the ovaries of a neonate.
- Most regress during childhood; by puberty, no more than 40,000 remain.
- Of these, only ~400 oocytes mature into secondary oocytes and reach ovulation.
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At birth, all primary oocytes have completed {1:prophase I} of the first meiotic division and remain arrested there until {1:puberty}.
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A neonate has up to {1:2,000,000} primary oocytes; by puberty, only {1:40,000} remain, and only about {1:400} will ever be ovulated.
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The first meiotic division of the primary oocyte is completed just {1:before ovulation}, producing a {1:secondary oocyte} and a {1:first polar body}.
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Completion of the second meiotic division requires {1:fertilization}, which produces the definitive oocyte and the {1:second polar body}.
Comparison Between Male and Female Gametes
| Feature | Spermatozoa | Oocytes |
|---|---|---|
| Size | Small | Massive |
| Motility | Motile | Immotile |
| Cytoplasm | Scant | Abundant |
| Chromosome complement | 23, X or 23, Y | 23, X only |
| Types | Two types (X or Y-bearing) | One type |
Info
The difference in sex chromosome complement forms the basis of primary sex determination.
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Spermatozoa are of two types: those carrying {1:22 autosomes + X} and those carrying {1:22 autosomes + Y}. Secondary oocytes carry {1:23, X}.
Gametogenesis Differences: Males vs. Females
After PGCs Enter the Genital Ridge
After PGCs enter the genital ridge, they stop migrating, undergo 2–3 mitoses, and enter the premeiotic stage.
| Feature | Male | Female |
|---|---|---|
| PGC arrest | 6th week of embryonal development | Enter meiotic prophase as primary oocytes at 5th month of fetal development |
| Meiosis begins | At puberty | Fetal life (5th month) |
| Meiosis inhibitor | Yes — male meiosis inhibitor produced by Sertoli cells | No |
Key Concept — Cell Autonomy of Oogenesis
- If male PGCs (XY) are transplanted into female (XX) embryos, they follow the female PGC course, regardless of their chromosome constitution.
- PGCs that fail to reach the gonads also progress through meiosis as oocytes, regardless of genotype.
- All PGCs are programmed to be oocytes — this is cell autonomous and Tet1-dependent (a transcription factor that erases epigenetic marks in DNA).
- In the male genital ridge, a male meiosis inhibitor (produced by Sertoli cells) overrides this default.
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All PGCs are intrinsically programmed to become {1:oocytes}; this is a cell-autonomous process dependent on {1:Tet1}, which erases {1:epigenetic marks} in DNA.
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In males, a {1:male meiosis inhibitor} produced by {1:Sertoli cells} prevents PGCs from entering meiosis during embryonic development.
Male vs. Female — Full Comparison
| Feature | Male | Female |
|---|---|---|
| PGC dormancy | From 6th embryonic week until puberty | Undergo a few more mitoses → oogonia |
| Meiosis onset | At puberty | 5th month of fetal development (all oogonia begin meiosis → primary oocytes) |
| State of arrest | Not arrested after puberty | Meiotic arrest in prophase I until puberty; second arrest at metaphase II until fertilization |
| Production | Continuous, puberty to death | Cyclic; ~1 oocyte per month from puberty to menopause (~50 years) |
| Meiosis completion | Completed during spermatogenesis | Completed only if fertilization occurs |
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In females, meiosis begins at the {1:fifth month of fetal development}; oocytes are then arrested in {1:prophase I} until puberty, and in {1:metaphase II} until fertilization.
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In males, spermatozoa are produced {1:continuously} from puberty until death; in females, approximately {1:1} oocyte is ovulated per month, ceasing at {1:menopause}.
Sacrococcygeal and Oropharyngeal Teratomas
Info
Teratomas can arise from primordial germ cells that fail to complete normal migration to the gonads. Examples include:
- Sacrococcygeal teratoma (fetal)
- Massive oropharyngeal teratoma
Female Reproductive Cycles
Ovarian Cycle
Info
The ovarian cycle is the process that leads to the development of a mature follicle, followed by rupture and oocyte expulsion, ready for fertilization.
Ovarian Follicles — Development Stages
Four Stages of Follicular Development from Primordial Follicles
| Stage | Key Features |
|---|---|
| 1. Primordial follicle | Primary oocyte surrounded by flat follicular cells |
| 2. Unilaminar primary follicle | Follicular cells become cuboidal; oocyte grows to ~100–150 µm |
| 3. Multilaminar primary follicle | Follicular cells proliferate and stratify → granulosa cells; zona pellucida appears; theca folliculi develops |
| 4. Secondary (antral) follicle | Liquor folliculi accumulates among granulosa cells; antrum forms |
| 5. Graafian (mature) follicle | Fully mature; ready for ovulation |
FSH Dependence
- Development of primordial and primary follicles is independent of FSH — triggered by uncharacterized local ovarian factors.
- Secondary and later follicles are under the influence of FSH.
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The four stages of follicular development are: {1:primordial}, {1:unilaminar primary}, {1:multilaminar primary}, {1:secondary (antral)}, and {1:Graafian} follicles.
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Development of primordial and primary follicles is {1:independent} of FSH, whereas secondary follicles depend on {1:FSH} released by basophil cells of the anterior pituitary.
https://unibo.smartzoom.com/s1241/course1776/f1815/i2437/
Ovarian Follicles — Primary Follicles
Unilaminar Primary Follicle
- Primary oocyte grows to ~100–150 µm with enlarged nucleus.
- Follicular cells become cuboidal in shape.
Multilaminar Primary Follicle
- Follicular cells proliferate and stratify → now called granulosa cells.
- An amorphous substance — the zona pellucida — appears, separating the oocyte from surrounding follicular cells.
- Stromal or granulosa cells form:
- Theca interna — richly vascularized cellular layer
- Theca externa — mostly fibrous connective tissue
Estradiol Synthesis
Theca interna cells produce androstenedione (male sex hormone) → enters granulosa cells → converted by aromatase to estradiol (estrogen).
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In the multilaminar primary follicle, theca interna cells produce {1:androstenedione}, which is converted by {1:aromatase} in granulosa cells to {1:estradiol}.
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The {1:zona pellucida} is an amorphous acellular substance that appears during the primary follicle stage, separating the oocyte from surrounding follicular cells.
Ovarian Follicles — Secondary Follicles & Graafian Follicle
Secondary Follicle
- Similar to primary follicle but with accumulations of liquor folliculi among granulosa cells.
- Continued granulosa cell proliferation depends on FSH from basophil cells of the anterior pituitary.
Graafian (Mature) Follicle
- Fully mature, bulging on the surface of the ovary.
- LH surge allows discharge of the mature oocyte.
- After ovulation, the follicle undergoes atresia; granulosa cells → granulosa lutein cells; theca cells → theca lutein cells.
- Together they form the corpus luteum, which produces progesterone.
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After ovulation, the Graafian follicle transforms into the {1:corpus luteum}, which secretes primarily {1:progesterone}.
Follicular Development — Summary of Key Features
Development of an ovarian follicle is characterized by:
- Growth and differentiation of a primary oocyte
- Proliferation of follicular cells
- Formation of the zona pellucida
- Development of a connective tissue capsule — theca folliculi
Info
Thecal cells produce an angiogenic factor that promotes growth of blood vessels providing nutritive support for follicular development.
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The four hallmarks of follicular development are: growth of the {1:primary oocyte}, proliferation of {1:follicular cells}, formation of the {1:zona pellucida}, and development of the {1:theca folliculi}.
Ovulation
Process of Ovulation
- Follicular cells divide actively → antrum forms (containing follicular fluid) → follicle is now called a secondary follicle.
- Primary oocyte is surrounded by corona radiata projecting into the enlarged antrum.
- Follicle continues to enlarge → forms a bulge on the ovarian surface.
- A small oval avascular spot — the stigma — appears on the bulge.
- Before ovulation: secondary oocyte and some cells of the cumulus oophorus detach from the interior.
- Expelled secondary oocyte is surrounded by:
- Zona pellucida (acellular glycoprotein coat)
- Corona radiata (radially arranged follicular cells)
- Cumulus oophorus cells
https://unibo.smartzoom.com/s1241/course1776/f1815/i1827/
Ovulation — Hormonal Timeline
| Time (hours from LH surge) | Event |
|---|---|
| 0 | Ovulatory surge of LH and FSH |
| ~15 | Germinal vesicle breaks down |
| ~20 | Chromosomes in metaphase I; first meiotic division completed → secondary oocyte + first polar body |
| ~37 | Second meiotic metaphase arrest; ovulation occurs |
Important
- Ovulation follows within 24–36 hours of the LH surge.
- High estrogen level in blood elicits the LH surge (positive feedback from granulosa cells).
- LH surge causes the stigma to rupture, expelling the secondary oocyte with follicular fluid.
- Plasmins and matrix metalloproteinases (MMPs) also contribute to stigma rupture.
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Ovulation occurs within {1:24–36} hours of the {1:LH surge}, which is elicited by high levels of {1:estrogen} in blood.
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At ovulation, the secondary oocyte is arrested at {1:second meiotic metaphase} (metaphase II).
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Stigma rupture during ovulation is facilitated by {1:plasmins} and {1:matrix metalloproteinases (MMPs)}.
Corpus Luteum
Important
Under the influence of LH, the walls of the ruptured follicle develop into an endocrine glandular structure — the corpus luteum — which secretes primarily progesterone and some estrogen.
| Scenario | Outcome |
|---|---|
| Oocyte fertilized | Corpus luteum enlarges → corpus luteum of pregnancy; degeneration prevented by hCG (human chorionic gonadotropin) |
| Oocyte not fertilized | Corpus luteum degenerates 10–12 days after ovulation → corpus luteum of menstruation |
| After degeneration | Transformed into corpus albicans (white scar tissue) |
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If fertilization occurs, the corpus luteum is maintained by {1:hCG (human chorionic gonadotropin)}, enlarging to form the {1:corpus luteum of pregnancy}.
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If fertilization does not occur, the corpus luteum degenerates {1:10–12 days} after ovulation and is eventually replaced by the {1:corpus albicans}.
Why Is Folliculogenesis Selectively Stimulated in Only a Few Follicles Each Month?
Info
- Uncertain mechanism.
- Possibility 1: Follicles become progressively more sensitive to stimulating effects of FSH as they advance in development.
- Possibility 2: Selection is regulated by a complex feedback system between pituitary and ovarian hormones and growth factors.
Hormonal Profile — Menstrual Cycle
Info
The menstrual cycle is the period during which the oocyte matures, is ovulated, and enters the uterine tube. Estrogen and progesterone (produced by ovarian follicles and corpus luteum) cause cyclic changes in the uterine endometrium.
Viability of Oocytes and Sperms
| Gamete | Viability |
|---|---|
| Oocyte | Usually fertilized within 12 hours of ovulation; cannot be fertilized after 24 hours; degenerates shortly after |
| Spermatozoa | Most do not survive more than 24 hours in the female genital tract |
Info
- Some sperms are captured in folds of the cervical mucosa, gradually released into the cervical canal and through the uterus into the uterine tubes.
- Semen and oocytes can be frozen and stored for many years for use in assisted reproduction.
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Oocytes are usually fertilized within {1:12 hours} of ovulation and cannot be fertilized after {1:24 hours}.
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Most spermatozoa do not survive more than {1:24 hours} in the female genital tract.
Summary
- Gametogenesis — formation of gametes via meiosis
- Ovarian Cycle — folliculogenesis, ovulation, corpus luteum formation/degeneration
https://studentconsult.inkling.com/read/larsen-human-embryology-schoenwolf-5/videos/animation-1-1
Practice Questions
Question 1
Spermatogonia, derived from primordial germ cells, divide by mitosis during which period(s)?
✓ Continuously throughout postpuberty life
Question 2
Most oocytes become atretic and degenerate during which period(s)?
✓ Between the fifth and seventh fetal months
Question 3
In oocytes, the first meiotic division is completed during which period(s)?
✓ In response to the peak of FSH and LH during the menstrual cycle
Question 4
During the average menstrual cycle, LH and FSH levels are highest during which period?
✓ Immediately prior to ovulation
Question 5
Following ovulation, the corpus luteum is formed from which structure?
✓ The theca externa, theca interna, and granulosa
Question 9
The “definitive oocyte” (Haploid, 1N) is generated as a result of which of the following?
✓ In response to fertilization following ovulation
Question 10
Binding of sperm to integrin α6β1 mediates which of the following?
✓ Fusion of sperm and oocyte plasma membranes
[!tldr] TLDR — Full Summary
- Embryology studies human development from fertilization (~14 days after LMP) through birth; the embryonic period is weeks 1–8, the fetal period begins week 9.
- Pregnancy is divided into three obstetric trimesters or three embryological periods (egg, embryo, fetus).
- Phases of embryogenesis: gametogenesis → fertilization → cleavage (morula → blastocyst) → gastrulation (ectoderm, mesoderm, endoderm) → tube-within-a-tube body plan → organogenesis.
- Uterus has three wall layers (perimetrium, myometrium, endometrium); endometrium has compact, spongy, and basal sublayers.
- Gametogenesis reduces chromosome number from 46 to 23 via meiosis.
- PGCs originate in the yolk sac (weeks 4–6), migrate to the genital ridge; all PGCs are intrinsically programmed to become oocytes (Tet1-dependent cell-autonomous process); in males, Sertoli cell–derived meiosis inhibitor overrides this.
- Spermatogenesis: PGCs dormant from week 6 until puberty; at puberty testosterone drives spermatogonia → primary spermatocytes (meiosis I) → secondary spermatocytes → spermatids → spermatozoa (spermiogenesis); ~2 months; 150–275 million/day; stored in epididymis; continuous from puberty to death.
- Sperm structure: head (nucleus + acrosome with lytic enzymes) + tail (middle piece with mitochondria, principal piece, end piece).
- Capacitation: final maturation step occurring in the female genital tract (oviduct secretions); acrosome primed to release enzymes for zona pellucida penetration; artificially induced in IVF with Ca²⁺, bicarbonate, serum albumin.
- Oogenesis: oogonia → primary oocytes (5th fetal month, arrested at prophase I) → at puberty/LH surge, complete meiosis I → secondary oocyte + 1st polar body → arrested at metaphase II → fertilization completes meiosis II → second polar body; ~2,000,000 at birth → ~40,000 at puberty → ~400 ovulated over a lifetime.
- Follicular development (4 stages): primordial → unilaminar primary → multilaminar primary (zona pellucida, theca, granulosa cells, aromatase-driven estradiol synthesis) → secondary/antral (FSH-dependent) → Graafian.
- Ovulation: LH surge (triggered by high estrogen) → stigma rupture (plasmins, MMPs) → expulsion of secondary oocyte arrested in metaphase II; oocyte surrounded by zona pellucida, corona radiata, cumulus oophorus.
- Corpus luteum: formed from granulosa lutein + theca lutein cells post-ovulation; secretes progesterone (+estrogen); maintained by hCG if fertilization occurs (corpus luteum of pregnancy); degenerates at 10–12 days if not (corpus luteum of menstruation) → corpus albicans.
- Gamete viability: oocyte fertilizable for ~12 h (max 24 h); most spermatozoa survive <24 h in female tract.