Embryology overview

Overview

3 major germ layers form the initial developing embryo:

• Ectoderm
• Mesoderm
• Endoderm

Mesoderm differentiates into:

• Paraxial mesoderm
• Intermediate mesoderm
• Lateral plate mesoderm Ectoderm gives rise to 3 layers:
• Neuroectoderm
• Neural crest
• Epidermis

Early development

Key Terms and Timeframes

• Zygote: Fertilized egg (0–1 day).
• Morula: Solid cell cluster (2–3 days)
  • The zygote undergoes mitotic divisions to form a morula (solid ball of cells)
  • This travels down the uterine tube
• Blastocyst: Hollow structure prior to implantation (4–6 days)
  • The morula develops into a blastocyst, a hollow structure with two layers.
    • Trophoblast - Contributes to the placenta.
    • Inner cell mass - Becomes the embryo
      • This forms three layers - endoderm, mesoderm and ectoderm
      • Two cavities appear, the amnion and the yolk sac.
• Embryo: From implantation to 8 weeks.
  • By the end of 8 weeks most organs have differentiated and after that they just mature.
• Fetus: From 9 weeks until birth.

Endoderm

Development:

• The endoderm forms during gastrulation when cells migrate inward and establish the innermost layer of the embryo.
• It lines the primitive gut, which later develops into the gastrointestinal tract and associated structures.

Structures Derived from the Endoderm:

1. Digestive System:
   • Epithelium of the gastrointestinal tract (except parts of the mouth and anus, derived from ectoderm).
   • Liver and pancreas.
2. Respiratory System:
   • Epithelium of the trachea, bronchi, and lungs.
3. Endocrine System:
   • Thyroid gland (including parafollicular cells, derived from the ultimopharyngeal body).
   • Parathyroid glands.
4. Urinary System:
   • Epithelium of the urinary bladder and urethra.
5. Other Structures:
   • Epithelium of the tympanic cavity and auditory tube.
   • Epithelium of the tonsils and thymus.

Mesoderm

Development:

• The mesoderm forms as a middle layer during gastrulation, situated between the ectoderm and endoderm.
• It differentiates into subtypes: paraxial mesoderm, intermediate mesoderm, and lateral plate mesoderm, each giving rise to specific structures.

Structures Derived from the Mesoderm:

1. Musculoskeletal System:
   • Bones, cartilage, and skeletal muscles.
   • Connective tissues.
2. Cardiovascular System:
   • Heart, blood vessels, and blood cells.
3. Urogenital System:
   • Kidneys, ureters, gonads, and reproductive ducts.
4. Dermis and Subcutaneous Tissues:
   • Dermis of the skin and underlying connective tissues.
5. Other Structures:
   • Mesothelium lining body cavities (pleura, pericardium, peritoneum).
   • Adrenal cortex and parts of the spleen.

Ectoderm

Development:

• The ectoderm forms the outermost layer during gastrulation and gives rise to both surface and neural tissues.
• It differentiates into surface ectoderm and neuroectoderm.

Structures Derived from the Ectoderm:

1. Surface Ectoderm:
   • Epidermis, hair, nails, and sweat glands.
   • Enamel of teeth.
   • Lens and cornea of the eye.
2. Neuroectoderm:
   • Central nervous system (brain and spinal cord) from the neural tube.
   • Peripheral nervous system, including ganglia and nerves, from the neural crest.
   • Neural crest cells also contribute to melanocytes, craniofacial cartilage, and parts of the heart (e.g., aorticopulmonary septum).