Embryology

Morula: The name of the embryo at the 16 to 32 cell stage.

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 is once again present 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 the

next 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

FETAL STRUCTURE ADULT STRUCTURES Umbilical Vein Ligamentum Teres Hepatis Umbilical Arteries Median Umbilical Ligament Ductus Arteriosus Ligamentum Arteriosum Ductus Venosus Ligamentum Venosum Foramen Ovale Fossa Ovalis Allantois, Urachus Median Umbilical Ligament