Anatomy Chapter 3: Embryology: Events of the Embryonic and Fetal Periods

the embryonic period

- weeks 3-8

- begins with establishment of three primary germ layers

- by the end, main organ systems are developed

gastrulation

- week 3

- process by which cells from the epiblast migrate to form the primary germ layers

- starts with the formation of the primitive streak

- once all three primary layers are present the structure can be called an embryo

primitive node

cephalic end of the primitive streak

primitive pit

a depression in the primitive node

invagination

inward movement of cells from epiblast layer through primitive streak between epiblast and hypoblast layers

mesoderm

cells between the epiblast and hypoblast layers becoming primary germ layer, becomes bones and organs

endoderm

migrating cells that displace the hypoblast during gastrulation; becomes the inner linings

ectoderm

cells remaining in the epiblast during gastrulation; becomes the nervous system

notochord

derived from the mesoderm; important for signaling structure

neurulation

the formation of the neural tube from overlying cells; pinched off by misoderm

neural tube

formed from neurulation; pinched off by misoderm to eventually become the central nervous system

cephalocaudal folding

head and tail fold of the embryonic disc

transverse folding

lateral folding (like hug) of embryonic disc toward the midline

organogensis

organ development/formation; usually roughly completed by week 8

teratogens

any agent that can cause congenital malformations (birth defects); ex. alcohol, drugs

the fetal period

- begins week 9 and ends at birth

- characterized by maturation and growth of tissues and organs