Bilaminar and Trilaminar Embryonic Disk

• Decidual reaction is a process where Progesterone induces endometrial cell conversion to secretory decidual.
• Implantation occurs between days 6-12 and involves the adhesion of blastocyst to endometrium, trophoblastic proliferation, secretion of hydrolytic enzymes by the synctiotrophoblast, breakdown of endometrium, and formation of extraembryonic tissues.
• Fertilization occurs in the Ampulla of uterine tube on day 0.
• Zygotic transcription begins on day 1.
• Cleavage, Morula, and Compaction occur on days 1-3.
• Transport to uterine cavity occurs on day 4 and involves relaxation of the uterotubal junction.
• Maturation of blastocyst and Hatching occur on days 5-6.
• Attachment or penetration of uterine stroma occurs on days 7-9.
• Invasion of uterine stroma occurs on days 9-11.
• Lacuna formation and erosion of spiral arteries occur during the second week.
• During the second week, the inner cell mass gives rise to the Epiblast and Hypoblast, which make up the Bilaminar Embryonic Disk. The Bilaminar Embryonic Disk develops into the Ectoderm, Mesoderm and Endoderm
• The Amniotic cavity, Amnion, Yolk Sac, Connecting stalk, Chorion, and Villi are formed during the second week.
• Formation of the primitive streak and notochord are important processes of gastrulation.
• Following the formation of the primitive streak, cells leave its deep surface and form a loose network of tissue called mesenchyme or embryonic connective tissue.
• Notochord is a cellular rod that develops from the notochordal process, a structure around which the vertebral column forms.
• By day 16, the extraembryonic mesoderm begins to proliferate and invade the center of each primary stem villus, forming secondary stem villi.
• The 3rd week of gastrulation, also known as the 3rd week of embryonic development, is characterized by the formation of the primitive streak and three germ layers.
• The primitive streak is a thickened band appearing on the epiblast, resulting from the proliferation and accumulation of epiblastic cells on the posterior part of the embryonic disc.
• By day 21, the mesodermal core differentiates into connective tissue and blood vessels, forming a tertiary stem villi.
• Gastrulation is a process by which the inner cell mass is converted into a trilaminar embryonic disc.
• A narrow primitive groove appears in the primitive streak.
• The cranial end of the primitive streak proliferates to form a primitive node.
• The most important event during the 3rd week of gastrulation is the formation of the primitive streak.
• A depression, known as the primitive pit, occurs at the primitive node.
• On day 12, the extraembryonic mesoderm becomes organized to form a layer coating the outside of Heuser’s membrane and a layer lining the inside of the cytotrophoblast.
• By the end of 3rd week, neural folds fuse and form NEURAL TUBE.
• In 18th day, neural plate invaginates to form NEURAL GROOVE.
• These cells form irregular masses on the ventrolateral side of neural tube.
• NEURAL FOLDS appear on each side of neural groove.
• NEURAL CREST CELLS appear on the ventrolateral side of neural tube.
• Notochord induces the overlying ectoderm to form NEURAL PLATE.
• Ganlionic cells, Schwann cells, Melanocytes, Adrenal medulla, Skeletal and muscular components of the head are found in Neural Crest Cells.
• During the separation of the neural tube from the surface ectoderm, some neuroectodermal cells around each neural fold loose their epithelial affinities and form irregular masses on the ventrolateral side of the neural tube. These masses are called neural crest cells.
• On day 12, a second wave of proliferation in the hypoblast causes a new membrane to migrate out over the inside of the extraembryonic mesoderm, pushing the primary yolk sac in front of it.
• This new layer becomes the endodermal lining of the definitive (secondary) yolk sac.
• The definitive yolk sac develops on day 13 and the primary yolk sac breaks up, reduced to a collection of vesicles in a cyst-like manner.
• At the end of gastrulation, the epiblast has become the ectoderm, mesoderm and endoderm and the hypoblast has become the endoderm
• The developing notocord induces the developing ectoderm to form the neural plate, which is a primordial structure of the Nervous System.
• A tubular structure grows in length as primitive node cells are added to its proximal end.
• No migration of mesenchyme occurs at the cloacal membrane because of the tight binding of ectoderm to endoderm.