Embryology Flashcards
Mitosis
A type of cell division where a parent cell divides into two genetically identical daughter cells.
Maintains the diploid chromosome number (46).
Phases:
- Prophase – Chromosomes condense, spindle fibers form, nuclear membrane dissolves.
- Metaphase – Chromosomes align at the equator, attach to spindle fibers.
- Anaphase – Sister chromatids separate and move to opposite poles.
- Telophase – Nuclear membrane reforms, cytokinesis occurs, two daughter cells form.
Significance:
✔ Growth, repair, and maintenance of tissues.
✔ Aids in wound healing and regeneration.
Meiosis
A special type of cell division occurring in gametes (sperm & ova).
Reduces chromosome number from diploid (46) to haploid (23).
Ensures genetic variation.
Stages:
✦ Meiosis I:
Prophase I: Crossing-over occurs, exchanging genetic material.
Metaphase I: Homologous chromosomes align.
Anaphase I: Homologous pairs separate (centromeres remain intact).
Telophase I: Two haploid cells form.
✦ Meiosis II (Similar to mitosis):
Sister chromatids separate → 4 genetically unique haploid cells.
Significance:
✔ Maintains chromosome stability across generations.
✔ Introduces genetic variation via crossing-over & independent assortment.
Clinical Correlation of Cell Division
Failure of chromosomes to separate properly, leading to:
Trisomy (47 chromosomes) – Extra chromosome (e.g., Down Syndrome - Trisomy 21).
Monosomy (45 chromosomes) – Missing chromosome (e.g., Turner Syndrome - 45, X).
Mitosis-Related Anomalies:
If nondisjunction occurs in zygote’s first cleavage, it causes mosaicism (some normal, some abnormal cells).
Meiosis-Related Anomalies:
Occurs in gametogenesis:
Meiosis I error → All 4 gametes abnormal (2 with 24 chromosomes, 2 with 22).
Meiosis II error → 2 normal (23), 1 with 24, 1 with 22.
✔ Leads to chromosomal disorders such as Klinefelter syndrome (XXY) & Edward syndrome (Trisomy 18).
Embryology
Study of formation and development of the embryo.
Stages:
Embryo: First 2 months.
Fetus: 3rd month onwards.
Gestation: 9 months (266 days).
Chromosomes
Karyotyping:
Classifies chromosomes based on total length and other characteristics.
Helps detect chromosomal abnormalities.
Genetic Material
Chromosomes contain DNA (Deoxyribonucleic Acid), which stores genetic information.
DNA directs cellular functions by synthesizing proteins.
Gametes & Fertilization
Sex organs (gonads) produce gametes:
Male: Spermatozoa (sperm).
Female: Ovum (egg).
Fertilization:
Fusion of sperm and ovum forms a zygote.
Spermatogenesis
The process of sperm formation in the testes.
Begins at puberty under the influence of hormones. Takes place in the seminiferous tubules of the testes.
- Spermatogonia (2n) → Primary Spermatocyte (2n) → Meiosis I.
- Primary Spermatocyte (2n) → Two Secondary Spermatocytes (n) → Meiosis II.
- Each Secondary Spermatocyte (n) → Two Spermatids (n).
- Spermatids (n) → Mature Spermatozoa (n) via Spermiogenesis.
Spermiogenesis
Spermatids mature into spermatozoa (sperm).
Involves condensation of the nucleus, acrosome formation, and tail development.
Spermiation
Fully developed sperm are released into the lumen of the seminiferous tubules.
Hormonal Regulation of Spermatogenesis
GnRH (Gonadotropin-releasing hormone) from the hypothalamus stimulates the pituitary gland.
FSH (Follicle-Stimulating Hormone): Stimulates Sertoli cells to support sperm development.
LH (Luteinizing Hormone): Stimulates Leydig cells to produce testosterone, which promotes sperm production.
Oogenesis
- Oogonia (2n) → Mitosis → Primary Oocyte (2n) (Arrested in Prophase I).
- At Puberty: Primary Oocyte (2n) → Meiosis I → Secondary Oocyte (n) + Polar Body (n).
- At Ovulation: Secondary Oocyte (n) Arrests in Metaphase II.
- If Fertilized: Secondary Oocyte (n) → Meiosis II → Mature Ovum (n) + Polar Body (n).
- If Not Fertilized: Secondary Oocyte Degenerates.
GnRH (Hypothalamus) → Stimulates the pituitary gland.
FSH (Follicle-Stimulating Hormone) → Stimulates follicular growth.
LH (Luteinizing Hormone) → Triggers ovulation.
Estrogen & Progesterone → Regulate the menstrual cycle and prepare the uterus for pregnancy.
Sperm
- Head
Pear-shaped (~4 µm), contains the nucleus and acrosome.
Nucleus: Carries 23 condensed chromosomes (haploid).
Acrosome: Contains enzymes (hyaluronidase, acrosin) for ovum penetration.
- Neck
Short region connecting head to body.
Contains centriole, which helps in zygote formation.
- Middle Piece
Provides energy for sperm motility.
Mitochondria form a spiral sheath to produce ATP.
Axial filament extends into the tail for movement.
- Tail (Principal & End Piece)
Longest part, responsible for sperm propulsion.
Contains axial filament, providing flexibility for movement.
Additional Features
Maturation (Epididymis): Sperm gain motility.
Capacitation (Female Genital Tract): Sperm undergo changes to fertilize an ovum.
Acrosome Reaction: Enzymes released to penetrate zona pellucida of ovum.
This structure enables successful fertilization by aiding mobility, energy production, and ovum penetration.
Ovum
The ovum is a large, immotile female gamete (~100–120 µm in diameter).
It is a secondary oocyte, arrested in metaphase II of meiosis at ovulation.
- Layers Surrounding the Ovum
Vitelline Membrane (Plasma Membrane): Innermost layer enclosing the cytoplasm.
Perivitelline Space: Space between the vitelline membrane and zona pellucida. Contains the first polar body.
Zona Pellucida:
A glycoprotein-rich layer essential for sperm binding and fertilization.
Prevents polyspermy (entry of multiple sperm).
Corona Radiata:
Outer layer of granulosa cells stuck to the zona pellucida.
Provides nutrients to the oocyte.
- Cytoplasmic Features
Large cytoplasm compared to other body cells.
Rich in organelles and mRNA to support early embryonic development.
Lacks centrioles (contributed by the sperm during fertilization). - Nuclear Features
Nucleus not visible at ovulation, as the nuclear membrane dissolves for completion of meiosis II.
Spindle Apparatus present, preparing for the second polar body release. - Maturation and Fate
If fertilization occurs: Meiosis II completes, forming a mature ovum and second polar body.
If not fertilized: The ovum degenerates within 12–24 hours.
Abnormality in gametes
- Abnormalities in Sperm Formation
Structural Defects
Giant or dwarf sperm (abnormal size).
Duplicated head, body, or tail.
Chromosomal Abnormalities
Nondisjunction (failure of chromosome separation during meiosis).
Leads to trisomy (extra chromosome, 47) or monosomy (missing chromosome, 45).
Common Syndromes
Klinefelter’s Syndrome (XXY): Male with underdeveloped testes, sterility, and gynecomastia.
Super Male (XYY): Taller than average, possible behavioral issues.
Super Female (XXX): Poor sexual development, menstrual irregularities, mental retardation.
- Abnormalities in Ovum Formation
Structural Defects
Enlarged nucleus or multiple nuclei.
Presence of two oocytes in one follicle.
Chromosomal Abnormalities
Turner’s Syndrome (45, XO): Female with underdeveloped ovaries, webbed neck, and sterility.
Trisomy 21 (Down Syndrome): Caused by an extra chromosome 21.
Triploidy (69 chromosomes): Usually results in miscarriage.
- Abnormalities Due to Faulty Meiosis
Nondisjunction in Meiosis I → Both homologous chromosomes move to one gamete.
Nondisjunction in Meiosis II → Sister chromatids fail to separate.
Leads to abnormal zygote formation when fertilization occurs.
Fertilization
Fertilization is the fusion of a haploid spermatozoon and haploid ovum, forming a diploid zygote. Both gametes undergo biological changes before actual fertilization.
Completion of Meiosis II:
Only one sperm penetrates the zona pellucida and enters the ovum.
This triggers the second meiotic division, leading to the extrusion of the second polar body.
Fusion of Pronuclei:
The ovum nucleus becomes the female pronucleus.
The sperm head transforms into the male pronucleus, while the middle piece and tail degenerate.
Fusion of 23 chromosomes from each forms a diploid set (46 chromosomes, 23 pairs).
Initiation of Mitosis: The zygote starts mitotic division, leading to the formation of a multicellular embryo.
Antithesis of Cell Division: Fertilization is the opposite of mitosis as it combines two haploid cells into one diploid cell, whereas mitosis divides a single cell into two.
Site of Fertilization: Occurs in the ampulla (lateral one-third of the uterine tube).
Stages of Fertilization:
Approximation of gametes (transport )
Contact and fusion of gametes.
Effects/results of fertilization
Effects/Results of Fertilization
Completion of Second Meiotic Division of the secondary oocyte.
Restoration of Diploid Chromosome Number (46).
Determination of Chromosomal Sex of the future individual (XX or XY).
Initiation of Cleavage (first mitotic division of the zygote).
Establishment of Polarity & Bilateral Symmetry in the embryo.
Genetic Diversity due to fusion of parental DNA.
Embryo inherits only maternal mitochondria (sperm mitochondria are discarded).
Early Development of Zygote
The two daughter cells remain inside the zona pellucida.
Each daughter cell is smaller than the original ovum.
With subsequent divisions, cells become progressively smaller until they reach normal body cell size.
Important Factors in Fertilization
Healthy reproductive organs in both male and female.
Chemotaxis (sperm movement toward the oocyte guided by chemical signals).
Proper coordination of fertilization processes:
Time between insemination and ovulation.
Time ovum remains fertilizable.
Time sperm retains fertilizing capacity.
Number of sperms reaching the uterine tube.
Time taken by sperm to reach the ovum.
Semen factors influencing fertilization success.
Semen
Semen (seminal fluid) contains spermatozoa and secretions from:
Seminal vesicles (60%) → Provides fructose for energy
Prostate (25%) → Contains ions, citric acid, acid phosphatase, fibrinogen
Bulbourethral glands (5%) → Acts as pre-ejaculatory lubricant
Spermatozoa (10%)
Semen pH: 7.2–7.6 (maintained by spermine)
Liquefaction of semen: Fibrinolysin liquefies semen within 30 minutes after ejaculation.
Semen quantity per ejaculation: 2–3 ml
Sperm count per ejaculation: 100 million/ml
Sperm Maturity, Motility & Transport
Sperm motility: Required for penetration of ovum barriers.
Transport of sperms:
Prostaglandins in semen stimulate peristaltic contractions of the female genital tract.
Time taken for sperm to reach the uterus: 5–45 minutes
Reduction in sperm count occurs due to constrictions in the female genital tract.
Sperm viability after ejaculation: 24–48 hours
Chemotaxis: Spermatozoa are attracted to ovum via chemicals released by follicular cells.
Capacitation
Final step in sperm maturation before fertilization.
Time required: 7 hours
Starts in the uterus and continues in the fallopian tubes.
Involves membrane changes, calcium influx, acrosomal enzyme release, and glycoprotein alterations.
Follicular fluid enhances capacitation.
Ovum transport
Structure at ovulation:
Secondary oocyte (23 unpaired chromosomes)
Surrounded by vitelline membrane, zona pellucida, and corona radiata
Transport to fallopian tube:
Fimbriae movement + ciliary beats + muscular contractions help move ovum to ampulla in 25 minutes.
Ovum viability: 24–48 hours; degenerates if not fertilized.
gamete fusion
Sperm must penetrate 3 barriers:
Corona radiata
Zona pellucida
Vitelline membrane
4 processes in penetration:
ACROSOME REACTION
Process in which a capacitated sperm establishes multiple contacts with the plasma membrane and outer acrosomal membrane, releasing enzymes to penetrate the oocyte’s barriers.
Coverings of Sperm Head:
Nuclear Envelope (Innermost)
Bilaminar Acrosomal Membrane (Contains enzymes for oocyte penetration)
Plasma Membrane (Outer layer)
Trigger: Occurs when sperm contacts the corona radiata of the oocyte.
Process:
Perforations develop in the acrosome.
Point fusions occur between the sperm plasma membrane and outer acrosomal membrane.
Acrosomal enzymes are released to help sperm penetrate the zona pellucida.
Acrosomal Enzymes Released:
Hyaluronidase – Breaks down hyaluronic acid in the corona radiata.
Acrosin (Protease enzyme) – Helps sperm penetrate zona pellucida.
Acid Phosphatase – Aids in enzymatic digestion of barriers.
Zona Pellucida Role: The glycoproteins (ZP3 & ZP2) induce the acrosomal reaction, allowing sperm to penetrate.
DISINTEGRATION OF BARRIERS
sperm must pass through three barriers, aided by enzyme reactions:
Corona Radiata (First Barrier)
Enzyme: Hyaluronidase (from sperm acrosome).
Additional help: Tubal mucosal enzymes and sperm tail movement.
Function: Digests corona radiata cells.
Zona Pellucida (Second Barrier)
Binding: Sperm glycoproteins bind to ZP3 & ZP2 receptors on the zona pellucida.
Enzyme: Acrosin digests the zona pellucida.
Zona Reaction: Prevents polyspermy by altering the zona pellucida and plasma membrane.
Vitelline Membrane (Third Barrier)
Fusion: Sperm plasma membrane fuses with the oocyte membrane.
Enzyme: Disintegrin peptide (from sperm) binds to integrin peptides (on vitelline membrane).
30 minutes for fusion.
CALCIUM WAVE IN OOCYTE
Depolarization of oocyte cytoplasm upon sperm contact.
Key Effects:
Resumption of second meiotic division (mature ovum formation).
Cortical reaction (Vitelline Block) – Lysosomal enzymes from cortical granules prevent polyspermy.
Zona Reaction – Alters zona pellucida to block additional sperm entry.
Metabolic Activation of the Egg – Prepares ovum for embryonic development.
NUCLEAR FUSION
1. Entry of Sperm into Oocyte Cytoplasm
Both head and tail of sperm (excluding plasma membrane) enter the oocyte cytoplasm.
Pronuclei Approximation: Male and female pronuclei move toward the center of the ovum.
- Completion of Second Meiotic Division
Triggered by sperm entry.
Ovum releases second polar body into perivitelline space.
Reconstitution of chromosomes → Formation of female pronucleus. - Male Pronucleus Formation
Sperm head rotates 180° inside the oocyte cytoplasm.
Sperm nucleus swells and transforms into the male pronucleus. - Zygote Formation (Fusion of Pronuclei)
DNA Replication: Each male and female chromosome (1N) replicates to form 2N DNA.
Centrioles appear, spindle formation occurs.
Nuclear membranes disappear, chromosomes align on the spindle equator.
Chromosomal segregation occurs → Two identical diploid cells (46 chromosomes each) form.
The ovum is now called a zygote.
Cleavage
Cleavage is a series of mitotic divisions of the zygote within the zona pellucida. It results in smaller cells called blastomeres.
Cleavage starts immediately after fertilization and continues as the zygote moves through the uterine tube toward the uterus.
This movement is aided by ciliary action of uterine epithelium and muscular contractions of the uterine tube.
Cleavage lasts for 6 days, continuing until the 7th day after fertilization.
Stages of Cleavage
-Stage of Compaction (8-cell stage)
-Starts at the third cleavage division (8-cell stage).
-Outer cells form tight junctions and show polarity. Inner cells form gap junctions for communication.
-Nutrients come from blastomere stores and breakdown products of tubal secretions.
Morula (16-cell stage)
Occurs at the fourth cleavage division (16 cells).
The embryo looks like a mulberry (hence the name morula).
The morula consists of:
Inner Cell Mass (Embryoblast) → Forms the embryo proper.
Outer Layer (Trophoblast) → Forms embryo coverings and helps in nutrition.
Blastocyst (4th–5th day, 32–64 cell stage)
Fluid enters the morula from the uterine cavity, partially separating:
Inner Cell Mass (Embryoblast) from
Trophoblast (Outer Layer)
As the fluid increases, the morula forms a cyst-like structure, called the Blastocyst.
The cavity inside is called the Blastocoele.
The side where the inner cell mass attaches is the Embryonic/Animal Pole.
The opposite side is called the Abembryonic Pole.
—-Divisions of the Trophoblast
Polar Trophoblast → Part in contact with Embryoblast.
Mural Trophoblast → Remaining part lining the Blastocyst wall.
Hatching of Blastocyst (4th–5th day)
Zona Pellucida starts thinning on the 4th day.
Completely disappears by the 5th day, allowing the Blastocyst to “hatch”.
This initiates Implantation → Trophoblast attaches to the uterine epithelium on 6th or 7th day.
Principal Effects of Cleavage
✔ Increase in cell number while cell size decreases.
✔ Zygote cytoplasm is partitioned among blastomeres.
✔ Increased protoplasmic motility helps in morphogenetic movements.
✔ Cells approximate in size to normal somatic cells.
✔ Restoration of the nuclear-cytoplasmic ratio.
✔ No increase in total protoplasmic volume due to high metabolic activity.
✔ Zygote genome activation occurs on the 2nd day after fertilization.
ZONE pellucida function
oocyte development
protection of oocyte during its growth
transport in female reproductive tract
spermatozoon
binding in fertilization
prevention of polyspermy
development of blastocyst preventing ectopic
premature implantation.
GERM LAYER
(till chorion and amnion)
- Formation of Germ Layers
The embryonic disc (also called the embryonic shield or germ disc) is formed at an early stage.
It consists of three germ layers:
Endoderm (inside)
Ectoderm (outside)
Mesoderm (middle)
All body tissues develop from these layers.
- Early Blastocyst Changes
Inner Cell Mass is eccentrically attached to the trophoblast.
The blastocyst is a spherical cyst lined by flattened trophoblast cells.
- Formation of Bilaminar Germ Disc
Hypoblast (Endoderm): Some inner cell mass cells flatten and line the lower surface.
Epiblast (Ectoderm): Remaining inner cell mass cells become columnar.
- Formation of Important Cavities
Amniotic Cavity:
Appears between the epiblast (below) and trophoblast (above).
Filled with amniotic fluid (liquor amnii).
Roof: Amniogenic cells (from trophoblast).
Floor: Epiblast.
Primary Yolk Sac: Formed by flattened hypoblast cells (or possibly trophoblast).
Heuser’s membrane lines this cavity.
- Extraembryonic Structures
Extraembryonic Mesoderm:
Forms from trophoblast.
Lies between trophoblast and endodermal cells of yolk sac.
Does NOT form embryonic tissues.
Extraembryonic Coelom (Chorionic Cavity):
Small cavities appear in extraembryonic mesoderm → merge into one large space.
Splits the extraembryonic mesoderm into two layers:
Parietal (Somatopleuric) Extraembryonic Mesoderm → Lines trophoblast & outside of the amniotic cavity (Forms Chorionic Plate).
Visceral (Splanchnopleuric) Extraembryonic Mesoderm → Lines outside of yolk sac.
Connecting Stalk:
Extraembryonic mesoderm that attaches embryo to trophoblast.
Later forms umbilical cord.
- Formation of Important Membranes
Chorion: Formed by parietal extraembryonic mesoderm (inside) + trophoblast (outside).
Amnion: Forms around the amniotic cavity. cells originate from the trophoblast.
The amnion is covered by parietal extraembryonic mesoderm and is connected to the connecting stalk.
- Role of Chorion and Amnion
Both structures play a crucial role in childbirth (parturition).
3. Formation of the Secondary Yolk Sac
Extraembryonic mesoderm and extraembryonic coelom cause the primary yolk sac to shrink.
The secondary yolk sac replaces it.
The lining cells change from flattened to cuboidal.
germ layer (yolk sac to intra embroyonic mesoderm)
- Circular Embryonic Disc Formation
The embryonic disc is initially circular and has two layers:
Epiblast (upper layer, towards amniotic cavity) – Columnar cells.
Hypoblast (lower layer, towards yolk sac) – Cuboidal cells.
No clear head or tail end at this stage.
- Formation of Prochordal Plate
A circular area near the disc’s margin where endodermal cells become columnar.
Determines the central axis of the embryo.
Helps distinguish the head and tail regions.
- Formation of Primitive Streak (15th Day)
Early sign of gastrulation (formation of three germ layers).
Appears near the tail end of the embryonic disc.
Initially a rounded swelling, then elongates into a linear structure.
Marks the beginning of body axis formation.
Changes the embryonic disc shape from circular to oval.
- Formation of Intraembryonic Mesoderm
Cells from the primitive streak move sideways between the epiblast and hypoblast, forming the intraembryonic mesoderm (secondary mesoderm).
Some primitive streak cells replace the hypoblast, forming the endoderm.
The remaining epiblast cells become the ectoderm.
- Extensions of Intraembryonic Mesoderm
The intraembryonic mesoderm spreads across the embryonic disc except at the prochordal plate.
At the prochordal plate, the ectoderm and endoderm remain in direct contact (no mesoderm present).
This region later forms the buccopharyngeal membrane (future mouth opening).
- Primitive Streak and Embryonic Disc Growth
The primitive streak elongates along the embryo’s central axis.
The embryonic disc elongates, changing from circular to pear-shaped.
germ layer (embroyonic disc)
- Connecting Stalk
The embryo is initially suspended from the trophoblast by the connecting stalk.
The connecting stalk is broad at first but becomes smaller and confined to the tail region as the embryo grows.
Some mesoderm extends into the connecting stalk, leaving an area caudal to the primitive streak where ectoderm and endoderm remain in contact (forming the cloacal membrane).
- Trilaminar Germ Disc
The embryo at this stage consists of three layers:
Ectoderm (outer)
Mesoderm (middle)
Endoderm (inner)
- Gastrulation
Gastrulation is the process where the primitive streak, endoderm, and intraembryonic mesoderm form.
It begins in the 3rd week of embryonic development.
- Preorganogenesis Period
(First 14 Days)
Covers development from fertilization to the bilaminar disc stage.
No organs are formed yet.
Teratogens (harmful substances) in this period usually result in embryo death, so anomalies from this stage are rarely seen in full-term babies.
- Embryonic Period
Begins with the formation of the primitive streak and intraembryonic mesoderm.
Marks the start of gastrulation (3rd week of development).
Development events
2 Embryo is at two cells stage
3 Morula is formed
4 Blastocyst is formed
8 Bilaminar disc is formed
14 Prochordal plate and primitive streak is seen
16 Intraembryonic mesoderm is formed/disc is now three layered
embryonic disc
- Shape Changes in the Embryonic Disc
2nd week: The embryonic disc changes from circular to oval.
3rd week: It further elongates and becomes pear-shaped. - Establishment of the Bilaminar Embryonic Disc (2nd Week)
The bilaminar disc consists of two germ layers:
Hypoblast
Epiblast
Two cavities are present:
Amniotic cavity
Yolk sac
The embryo is suspended by the connecting stalk. - Establishment of Body Axes
Cephalocaudal axis, dorsoventral axis, and right-left axis are established.
This occurs with the formation of the prochordal plate and migration of the connecting stalk.
The caudal epiblast cells remain pluripotent. - Pluripotent Ectodermal Cells Differentiate Into:
Definitive endoderm
Intraembryonic mesoderm
Definitive ectoderm
Notochord
Primordial germ cells (PGCs) - Primitive Streak Formation (15th Day of Gestation)
Pluripotent ectodermal cells actively migrate and invaginate between the ectoderm and endoderm.
This forms:
A narrow median groove
Raised lateral margins → Primitive streak
The primitive streak gives rise to:
Notochord
Intraembryonic mesoderm (3rd germ layer)
This leads to the formation of the trilaminar germ disc (early 3rd week).
Notochord
- Location and Development of the Notochord
The notochord is a midline structure that develops between:
The cranial end of the primitive streak
The caudal end of the prochordal plate
It undergoes several developmental stages before forming a solid rod. - Formation Process of the Notochord
Primitive Knot (Henson’s Node) Formation
The cranial end of the primitive streak thickens to form the primitive knot/node (Henson’s node).
A depression appears in its center → Primitive pit/blastopore.
Formation of the Notochordal Process
Cells from the primitive node proliferate and migrate cranially between ectoderm & endoderm.
These cells form a solid cord → Notochordal process (Head process).
Formation of the Notochordal Canal
The blastopore cavity extends into the notochordal process, forming a hollow tube → Notochordal canal.
Transformation into the Notochordal Plate
The floor of the notochordal canal intercalates with the endoderm.
The floor of the canal breaks down, creating a temporary connection between the amniotic cavity and yolk sac.
The remaining walls of the canal flatten → Notochordal plate.
Formation of the Definitive Notochord
The notochordal plate curves back into a tube.
Cell proliferation converts it into a solid rod → Definitive notochord.
The notochord completely separates from the endoderm.
- Function and Fate of the Notochord
The notochord acts as a structural and signaling center for development.
It induces neural tube formation.
It determines the cranio-caudal axis and right-left symmetry.
In humans, most of the notochord disappears, but some remnants persist as:
Nucleus pulposus of the intervertebral discs.
Apical ligament of the dens (Axis vertebra).
In some animals (e.g., Amphioxus), the notochord persists throughout life. - Fate of the Primitive Streak
The primitive streak induces:
Notochord formation
Intraembryonic mesoderm formation
It regresses at the end of the 3rd week and completely disappears by the 26th day.
Neural tube
The neural tube develops from the ectoderm overlying the notochord.
It extends from the prochordal plate to the primitive knot.
Neural tube derivatives:
Brain (cranial enlarged part).
Spinal cord (caudal tubular part).
The developing brain forms a large mass on the dorsal aspect in early embryos.
The process of neural tube formation is called neurulation.
SUBDIVISIONS OF INTRAEMBRYONIC
MESODERm
Formed by cell proliferation in the primitive streak.
It separates ectoderm and endoderm, except in these bilaminar regions:
Prochordal plate → Buccopharyngeal membrane (future mouth & pharynx junction).
Cloacal membrane → Anal & urogenital membranes (future openings for digestive, urinary, and genital systems).
Region occupied by the notochord (caudal to prochordal plate).
Mesoderm at the edges of the embryonic disc connects with extraembryonic mesoderm.
Divisions of intraembryonic mesoderm:
Paraxial mesoderm (thickened part beside the notochord).
Lateral plate mesoderm (thin outer layer).
Intermediate mesoderm (between paraxial and lateral plate mesoderm).
