Human Development: Gastrulation Process

Gastrulation Overview

• Gastrulation is the formation of three germ layers in the embryo: ectoderm, mesoderm, and endoderm.
• It begins with the formation of the trilaminar disc and marks the start of the development of somites and the neural tube.

Key Terms

• Trophoblast is composed of cytotrophoblast and syncytiotrophoblast.
• Embryoblast is composed of epiblast and hypoblast (by day 8).
• Blastocyst is the stage present on day 7/8.

Primitive Streak Formation

• The primitive streak appears at the beginning of the third week.
• A primitive groove forms along the longitudinal midline of the epiblast.

Cell Migration and Germ Layer Formation

• The formation of the primitive streak begins.
• Epiblast cells migrate towards the primitive streak.
• Some cells displace the hypoblast, forming the embryonic endoderm.
• Others form the mesoderm (mesenchyme) between the hypoblast and newly-produced endoderm.
• The epiblast is renamed ectoderm.

Formation of Notochord and Mesoderm

• Notochord is the basis for the axial skeleton, formed by cells from the primitive node.
• Types of mesoderm include paraxial mesoderm, intermediate mesoderm, and lateral plate mesoderm.

Development of Paraxial Mesoderm

• Paraxial mesoderm develops into somitomers and somites.
• Somitomers are rounded structures in the head region.
• The first somites appear on day 9.
• Somites establish the body's segmental organization.

Germ Layers: Origin and Tissue Development

Ectodermal Derivatives

• Neuroectoderm derivatives include melanocytes, Merkel cells, medulla of adrenal gland, C cells of thyroid, ganglia, and parts of the nervous system.
• Neural Tube derivatives include olfactory epithelium, cornea, epidermis, and its derivatives, hypophysis (adenohypophysis), enamel organ, lens, parotid gland, and epithelium of inner ear.

Mesodermal Derivatives

• Paraxial mesoderm derivatives include sclerotome, myotome, and dermatome.
• Sclerotome produces mesenchyme, which forms bone, cartilage, blood, adipose tissue, spleen, smooth muscles, and endothelium.
• Myotome forms skeletal muscles.
• Dermatome forms dermis.
• Intermediate mesoderm develops into parts of the urinary system and reproductive system.
• Lateral mesoderm forms visceral and parietal layers of the intra-embryonic body cavity, cardiac muscles, and cortex of the adrenal glands.

Endodermal Derivatives

• Urinary and reproductive structures include urinary bladder, urethra, prostate gland, bulbourethral glands, and vestibule of the vagina.
• Digestive and respiratory epithelium derivatives include gall bladder, respiratory tract, and digestive tract epithelium.
• Glandular structures derivatives include thyroid and parathyroid gland, liver and pancreas, thymus, and submandibular and sublingual glands.
• Additional epithelial structures derivatives include epithelium of the tympanic cavity and Eustachian tube.