Embryonic Period Embryology Questions

Given a regular 28-day menstrual cycle, a woman who starts menses on, say, February 1 will
ovulate on February 14, and the secondary oocyte will be fertilized, if she becomes pregnant,
within 24 hours. So, the zygote undergoes week 1 of development from February 15 to 21. Week
2 of development is from February 22 to 28. On the next day, March 1, the woman would enter
her next menstrual cycle if she were not pregnant, but because she is pregnant, she does not
menstruate. Therefore, this first missed menstrual period corresponds with the start of week 3
of embryonic development. The embryonic period (week 3–week 8) is a time of high susceptibility to teratogens.

During week 3 of development, the process of gastrulation, which establishes the three primary germ layers (ectoderm, intraembryonic mesoderm, and endoderm), occurs. The origin of
all tissues and organs of the adult can be traced to one of these germ layers because these are
whence they “germinate.”

Gastrulation establishes the three primary germ layers during week 3 of development. Neurulation is the process by which neuroectoderm forms the neural plate, which eventually folds
to form the neural tube.

The embryo starts the embryonic period as a two-dimensional disk and ends as a threedimensional cylinder. This dramatic change in geometry is caused by formation of all the
major organ systems. As the organ systems gradually develop during the embryonic period,
the embryo appears more and more human-like; it has a distinct human appearance at the
end of week 8.

Intermediate mesoderm is a subdivision of intraembryonic mesoderm that forms a longitudinal dorsal ridge called the urogenital ridge from which the kidneys and gonads develop.

The formation of the primitive streak on the dorsal surface of the bilaminar embryonic disk
is the first indication of gastrulation.

Both the chromaffin cells of the adrenal medulla and the dorsal root ganglia are derived from
neural crest cells

The angioblasts are the mesenchymal cells that form blood vessels in embryonic development, as well as embryonic blood cells.

Both the kidneys and the gonads are derived from intermediate mesoderm. This longitudinal dorsal ridge of mesoderm forms the gonadal ridge, which is involved with the formation of
the future kidneys and gonads.

Hydrops fetalis is the most severe form of α-thalassemia, and is a direct result of the lack or
decreased synthesis of the α-globin chain of hemoglobin α2β2.

A hemangioma is a vascular tumor that can be present at birth in which the abnormal proliferation of blood vessels leads to a mass resembling a neoplasm. Hemangiomas are mesodermal in origin, in that they are formed by embryonic blood cells and the vascular endothelium
formed by angioblasts

The lateral mesoderm is a subdivision of intraembryonic mesoderm and initially is a solid
plate of mesoderm. The intraembryonic coelom forms in the middle of the lateral mesoderm,
thereby dividing it into the intraembryonic somatic mesoderm and intraembryonic visceral
mesoderm.

Approximately 35 pairs of somites form. They are derived from a specific subdivision of intraembryonic mesoderm called paraxial mesoderm. Somites differentiate into the components
called sclerotome (cartilage and bone of the vertebral column), myotome (epimeric and hypomeric muscle), and dermatome (dermis and subcutaneous area of skin).

The epiblast is capable of forming all three germ layers (ectoderm, mesoderm, and endoderm) during gastrulation. Epiblast cells migrate to the primitive streak and invaginate into a
space between the epiblast and hypoblast. Some of these epiblast cells displace the hypoblast to
form the definitive endoderm. Migrating epiblast cells also form the intraembryonic mesoderm.
The remaining epiblast cells form the ectoderm

The preponderance of sacrococcygeal tumors is found in female newborns. Because these
tumors develop from pluripotent cells of primitive streak origin, malignancy is of great concern,
and the tumor should be surgically removed by age 6 months. Occasionally, these tumors may
recur after surgery, demonstrating malignant properties.