L3: Third week of development (Gastrulation) Flashcards
What are the changes during the 3rd week?
β Changes in the structure of bilaminar embryonic plate:
ο² Development of prochordal plate
ο² Development of primitive streak (Groove)
ο² Development of notochord
β Formation of 3 germ layers:
ο² Gastrulation
prochordal plate
Site:
β near the cranial end in the midline
Cause:
β elongation (in height) of the primary endoderm (hypoblast): β’ It is the site of the oropharyngeal membrane
What is the site of the primitive streak?
β Opacity formed caudally in the midline (from caudal end to middle of the embryo)
Formation of the primitive streak
β proliferation and migration of the epiblastic cells:
β’ Primitive node: the proliferation of cranial end of the primitive streak
β’ Primitive pit: depression in the middle of the primitive node
What is the fate of the primitive streak?
β Diminishes in size and becomes an insignificant structure in the sacrococcygeal region of the embryo
β Disappears by the end of the fourth week (if it persists, it will lead to the formation of sacrococcygeal teratoma)
What is the definition of notochord?
β The core around which the vertebral column formed
What is the source of notochord?
β epiblast of primitive streak & nodes
What is the site of notochord?
β in the midline between ectoderm & endoderm
What are the stages of formation of notochord?
β Notochordal process: 17th day
β Notochordal canal
β Notochordal plate
β Notochord: 20th day
What is the function of notochord?
β Axis for the formation of the vertebral column
β Produce intraembryonic mesoderm
β Initiate (stimulate) neutralization (neural tube formation)
What is the fate of notochord?
β The cranial part: some parts of bones forming skull base
β Parts in the center of the bodies of vertebrae: Degenerate and disappear
β Parts in the intervertebral discs: Undergo mucoid degeneration to form nucleus bulbosus.
What is the definition of gastrulation?
β the process by which the bilaminar embryonic disc is converted into a trilaminar embryonic disc.
What is the time of gastrulation?
During the 3rd week
What are the steps of the process of gastrulation?
1) Cells from primitive streak migrate and are arranged between epiblast & hypoblast forming intraembryonic mesoderm.
2) The epiblast of primitive streak displaces the hypoblast, forming the embryonic endoderm
3) The cells remaining in the epiblast from the embryonic ectoderm
What is the first morphological sign of gastrulation?
β The first morphologic sign of gastrulation is the formation of the primitive streak
