Fertilization to Week 4
On this page 26 sections
FERTILIZATION AND EARLY DEVELOPMENT
Fertilization
Fertilization usually occurs in the ampulla of the uterine tube.
It is the fusion of male and female gametes to form a diploid zygote.
Main events
- Sperm penetrates the corona radiata.
- Sperm binds to and penetrates the zona pellucida.
- Fusion of sperm and oocyte membranes occurs.
- The oocyte activates and completes meiosis II.
- Cortical granule release helps prevent polyspermy.
- Male and female pronuclei form and their genetic material combines during the first mitotic division.
Results of fertilization
- Restores the diploid chromosome number.
- Combines maternal and paternal genetic material.
- Determines chromosomal sex at fertilization.
- Activates cleavage and embryonic development.
MCQ tip: The second polar body is produced after completion of meiosis II, following sperm entry.
Cleavage
Cleavage consists of rapid mitotic divisions of the zygote without an overall increase in the size of the conceptus.
| Stage | Key feature |
|---|---|
| Zygote | Single fertilized cell |
| Blastomeres | Cells formed by cleavage |
| Morula | Solid ball of cells, typically around day 3 |
| Blastocyst | Fluid-filled structure with inner cell mass and trophoblast |
| Hatching | Blastocyst escapes the zona pellucida before implantation |
Morula versus blastocyst
| Morula | Blastocyst |
|---|---|
| Solid ball of blastomeres | Contains a blastocyst cavity |
| Typically develops around day 3 | Usually forms around days 4–5 |
| No distinct inner cell mass/cavity organization as in the blastocyst | Has embryoblast and trophoblast |
Blastocyst Formation
The blastocyst consists of three major components.
| Component | Fate |
|---|---|
| Embryoblast (inner cell mass) | Embryo proper |
| Trophoblast | Major fetal contribution to placenta |
| Blastocyst cavity (blastocoel) | Fluid-filled cavity |
The blastocyst usually begins implantation around day 6–7.
Implantation
Implantation is the attachment and embedding of the blastocyst into the endometrium.
Trophoblast differentiation
| Layer | Function |
|---|---|
| Cytotrophoblast | Inner cellular layer; mitotically active |
| Syncytiotrophoblast | Outer invasive multinucleated layer; erodes maternal tissue and produces hCG |
Key events
- Blastocyst attaches to the endometrial epithelium.
- Syncytiotrophoblast invades the endometrium.
- Maternal tissue and blood spaces are progressively incorporated into the developing placental interface.
- hCG maintains the corpus luteum in early pregnancy.
Exam tip: The syncytiotrophoblast is the principal invasive trophoblastic layer and secretes hCG.
BILAMINAR DISC AND GASTRULATION
Bilaminar Germ Disc — Week 2
The embryoblast differentiates into two layers, forming the bilaminar embryonic disc.
| Layer | Position | Main significance |
|---|---|---|
| Epiblast | Dorsal/superior layer | Gives rise to all three definitive germ layers during gastrulation |
| Hypoblast | Ventral/inferior layer | Contributes to the lining of the yolk sac; does not form the definitive embryo's three germ layers |
Associated structures
- Amniotic cavity develops adjacent to the epiblast.
- The yolk sac is associated with the hypoblast.
- The trophoblast differentiates into cytotrophoblast and syncytiotrophoblast.
- Extraembryonic mesoderm develops and contributes to supporting structures.
MCQ tip: The epiblast forms ectoderm, mesoderm and endoderm during gastrulation.
Gastrulation — Week 3
Gastrulation converts the bilaminar disc into a trilaminar embryonic disc.
Primitive streak
- Appears on the surface of the epiblast.
- Establishes the embryonic body axes.
- Provides the route through which epiblast cells migrate inward.
- Helps establish the three definitive germ layers.
Formation of the germ layers
- Epiblast cells migrate through the primitive streak.
- The first migrating cells replace the hypoblast to form definitive endoderm.
- Additional cells spread between the layers to form intraembryonic mesoderm.
- Remaining epiblast forms ectoderm.
Germ layer derivatives
| Germ layer | Major derivatives |
|---|---|
| Ectoderm | Epidermis, central nervous system, peripheral nervous system and neural crest |
| Mesoderm | Muscle, bone, connective tissue, cardiovascular system, kidneys and gonads |
| Endoderm | Epithelial lining of the gastrointestinal and respiratory tracts, liver and pancreas |
Important exceptions and distinctions
- The notochord is derived from axial mesoderm.
- Neural crest cells originate from neuroectoderm at the neural plate borders.
- Most craniofacial connective tissues and much of the facial skeleton are derived from neural crest cells.
- The tongue's muscular component is primarily mesodermal, while much of its connective tissue and skeletal framework has neural crest contributions.
NEURULATION AND NEURAL CREST
Neurulation — Weeks 3–4
Neurulation forms the neural tube, which gives rise to the central nervous system.
Sequence
Neural plate → neural groove and neural folds → fusion of neural folds → neural tube.
- The notochord induces overlying ectoderm to form the neural plate.
- Neural folds elevate and fuse.
- Fusion progresses along the neural axis.
- Cranial and caudal neuropores close subsequently.
Neural tube versus neural crest
| Structure | Main derivatives |
|---|---|
| Neural tube | Brain and spinal cord |
| Neural crest | Peripheral ganglia, Schwann cells, melanocytes, adrenal medulla and much craniofacial tissue |
Neural crest derivatives — especially important for dentistry
- Odontoblasts.
- Much of the connective tissue of the face.
- Much of the cartilage and bone of the craniofacial skeleton.
- Dental pulp connective tissue.
- Periodontal ligament fibroblasts and other craniofacial connective tissue components.
- Schwann cells and sensory ganglia.
Remember: Odontoblasts are neural crest-derived ectomesenchymal cells.
Neural Tube Defects
Neural tube defects result from failure of normal neural tube closure.
| Defect | Key feature |
|---|---|
| Anencephaly | Failure of cranial neuropore closure; severe absence of cranial vault and major brain structures |
| Spina bifida occulta | Vertebral arch defect with intact skin; may be asymptomatic |
| Meningocele | Meninges protrude through the vertebral defect |
| Myelomeningocele | Meninges and spinal cord/neural tissue protrude |
| Myeloschisis | Open neural tissue without normal covering; severe defect |
Prevention
Periconceptional folic acid supplementation reduces the risk of neural tube defects.
MCQ tip: Cranial neuropore closure failure causes anencephaly; caudal neuropore closure failure is associated with spina bifida.
Notochord
The notochord is a midline axial structure that organizes surrounding embryonic development.
Functions:
- Induces neural plate formation.
- Establishes the embryonic midline.
- Helps organize axial development.
Its remnants contribute to the nucleus pulposus of intervertebral discs.
Exam tip: Notochord → nucleus pulposus.