Early Embryonic Development

Blastocyst: At about day 5 after implantation.

• Cells are divided into the inner cell mass and trophoblast.
• Inner Cell Mass: The cells on the inside.
• Trophoblast: The cells surrounding the outside.

Implantation: Blastocyst travels from ampulla ------> oviduct ------> uterus to implant in the uterus. Implantation occurs around day 5. Trophoblast forms two cell-layers.

• Syncytiotrophoblast: The cell mass at the embryonic pole (nearest the uterine wall).
• Cytotrophoblast: The cell mass jutting out from the pole. Will form the extraembryonic membranes.

Inner Cell Mass: Form two layers of cells, epiblast and hypoblast. A Bilaminar Embryonic Disc is formed.

• Epiblast Cells: From inner cell mass, will ultimately give rise to the three germ layers and the entire embryo.
• Amniotic Cavity: Forms as the space between the Epiblast and cytotrophoblast..
• Hypoblast Cells: These cells are the first to migrate and eventually disintegrate.

Progression of events after implantation: Each item represents simultaneous events.

• The epiblast cells grow deeper into the wall of the uterus.
• The amniotic cavity begins to form in the epiblast layer.
• Amnioblasts differentiate from the epiblasts and migrate to the inner amniotic layer.
• Heuser's Membrane is formed from hypoblasts (interior of cytotrophoblast layer). It will differentiate into the yolk sac.
• Trophoblastic Lacunae are openings that form in the syncytiotrophoblast.
• Maternal blood enters the lacunae.
• Extraembryonic Reticulum forms.
• Primary Yolk Sac: Epiblasts form mesoderm on the exterior of Heuser's Membrane.
• Definitive Yolk Sac: New Hypoblast cells migrate to displace Heuser's membrane, pushing back the primary yolks sac to form the new definitive yolk sac.
• Chorionic Cavity forms in the ECM.
• The Placenta is formed from a network of villi (trophoblast cells). Villi form from trophoblast in the following order of maturation:
• Primary Villus: Cytotrophoblast + a layer of syncytiotrophoblast grow into the lacunae.
• Secondary Villus: Extraembryonic mesoderm grows into the primary villus.
• Tertiary Villus: Extraembryonic mesoderm gives rise to blood vessels. Exchange of nutrients is now possible with mother.

Formation of Three Germ Layers:

• Form by migration of epiblast cells through the primitive streak.
• First Migration: Epiblasts migrate toward the midline and fold under, as they displace the hypoblast layer. This forms the true endoderm.
• NOTE: True Endoderm comes from migration of 1st epiblast layer -- not from the hypoblast!
• Second Migration: More epiblasts migrate to form mesoderm.
• The epiblasts that remain on the dorsal surface form the ectoderm.

Axis Determination:

• Buccopharyngeal Membrane: Forms in the future cephalic region, a tightly adhered region through which mesoderm cannot intrude. This is the future mouth.
• Cloacal Membrane: This is the future anus.

Notochord Formation:

• Notochordal Process forms from mesoderm.
• Neurenteric canal is a transient opening between the amniotic cavity and the yolk sac.
• Notochordal plate reforms a tube now known as the notochord.
• It is the dorsal remains of the notochordal canal.
• Endoderm once again lies dorsal to it, and the secondary yolk sac is again intact.

Concurrent Ectodermal Changes: Formation of the Neural Plate and Neural Crest Cells.

• Notochord induces ectodermal formation of the neural plate.
• As neural plate invaginates medially, forming the midline neural groove and laterally placed neural folds.
• Neural Tube forms as the fusion of the neural folds.
• Rostral Neuropore is leading cranial edge of this folding.
• Caudal Neuropore is caudal edge of this folding.
• The Amniotic Cavity repositions itself around the embryo, in preparation for flexion.
• Neural Crest Cells migrate outward from the neural tube to form the Dorsal Root Ganglia and Chain Ganglia.

Other Mesodermal Structures:

• Paraxial Mesoderm: Will form the somites.
• It is directly adjacent to the neural tube.
• It participates in formation of the axial skeleton.
• Intermediate Mesoderm: Forms the urogenital system.
• Lateral to the paraxial skeleton, but medial to the lateral plate mesoderm.
• Lateral Plate Mesoderm: It is the most lateral of the three. Forms primitive gut, and posterior and lateral body wall. It differentiates into two layers:
• Somatic Mesoderm: (Amniotic side)
• Splanchnic Mesoderm: (Yolk side)

Formation of the Intraembryonic Coelom: Forms as a division of the lateral-plate mesoderm.

• As opposed to the chorionic cavity (which is Extraembryonic coelom).
• It is horseshoe-shaped, with the base of the horseshoe extending cranial.
• The horseshoe is later divided into three segments:
• The Cranial Segment (base) becomes: Pericardial Coelom.
• Left and Right Caudal portions become the: Peritoneal Coeloms.
• The junction of the U-shapes coeloms is the pericardioperitoneal canal.
• Formation of Coelom creates two mesodermal layers now:
• Somatic Mesoderm -- parietal mesoderm.
• Splanchnic Mesoderm -- visceral mesoderm.

Somite Development: Somites establish the segmental nature of the body. They differentiate into three layers.

• Sclerotome: Forms the vertebral bone. This originates from paraxial mesoderm.
• As cells migrate outward, each sclerotome splits into inferior and superior halves. The inferior half of one sclerotome merges with the superior half of thenext sclerotome to form the respective vertebrata.
• Two somites from each side, for a total of FOUR DIFFERENT SOMITES, contribute to the formation of each vertebrata.
• When adjacent sclerotomes merge, the remaining notochord forms the nucleus pulposus. The rest of the notochord degenerates.
• Dermamyotome, consisting of myotome (medial part) and dermatome (most lateral part)

Early Somite Development:

• Chorioamnionic Membrane forms, merging the chorion and amnion cavities.
• Folding (flexion) of the embryo repositions the somites from the ventral-medial position to a dorsolateral position.

NEURAL CREST DERIVATIVES:

• 
  
• Dorsal Root Ganglia
• Autonomic Ganglia
• Melanocytes
• Chromaffin Cells of Adrenal Medulla
• Enterochromaffin Cells
• Pia
• Celiac Ganglion
• Schwann Cells (but not Oligodendrocytes)
• Odontoblasts
• Parafollicular cells of Thyroid

FETAL / ADULT EQUIVALENTS:

Category: Anatomy Notes