Biology: Formation of Germ Layers

Formation of Germ Layers

• After fertilization, the ovum undergoes cell divisions, forming a mass called the morula in mammals
• The inner cell mass (embryoblast) separates into two layers: the epiblast and hypoblast
• The epiblast forms the embryo, while the hypoblast and other cells form supporting structures like the placenta
• Gastrulation is the process of forming the three germ layers: endoderm, mesoderm, and ectoderm

Formation of Primitive Streak and Germ Layers

• The epiblast cells divide and move through the primitive streak, replacing the hypoblast cells to form endoderm
• The cells remaining between the epiblast and endoderm form the mesoderm
• The cells remaining in the epiblast form the ectoderm, completing the formation of the three germ layers

Notocord Formation

• A depression in the center of the primitive knot forms a blastopore
• Prenotochordal cells pass cranially in the midline between the ectoderm and endoderm, forming a solid cord called the notochordal process
• The notochord provides a scaffold for neural tube formation and eventually becomes the vertebral column

Neural Tube Formation

• The ectoderm overlying the notochord thickens, forming the neural plate
• The neural plate undergoes a slipper-shaped thickening, forming neural folds
• The neural folds eventually form the neural tube

Neural Crest Cells

• Neural crest cells are multipotent cells that give rise to various cells, including odontoblasts, melanocytes, ganglia, and more
• The neural crest cells migrate to form the enamel organ, ectomesenchyme, and other structures in the head and neck region

Development of Pharyngeal Arches

• The gradual appearance of pharyngeal (branchial) arches contributes to the development of the face and neck
• Each arch has a skeletal element, artery, muscles, and nerve supply
• Ectodermal clefts and endodermal pouches between the arches give rise to various structures

Derivatives of Pharyngeal Arches

• First arch (mandibular arch): muscles of mastication, mylohyoid, anterior belly of digastric, Meckel's cartilage, and more
• Second arch (hyoid arch): muscles of facial expression, posterior belly of digastric, Reichert's cartilage, and more
• Third arch: stylopharyngeus muscle, greater horn and lower part of body of hyoid bone, common carotid artery, and glossopharyngeal nerve
• Fourth and sixth arches: crico-thyroid muscle, levator palatine, constrictor of pharynx, and more

Learn about the formation of germ layers in embryonic development, including gastrulation and the formation of endoderm, mesoderm, and ectoderm.