Embryology

biological divisions of pregnancy

pre-embryonic, embryonic, fetal

zygote

first diploid cell formed by the fusion of male and female gametes

fertilization

fusion of male and female gametes that occurs at day 0 of pregnancy

blastomeres

smaller cells that result from the mitotic division of the zygote

morula

a clump of 16 or more blastomeres

blastocyst

a hollow sphere of 100 blastomeres

blastocoel

the internal cavity of a blastocyst

trophoblast

layer of cells that make up the blastocyst cell wall, which will become part of the placenta

embryoblast

cell mass on the side of the blastocyst that will become the embryo

implantation

blastocyst attachment to the endometrium that occurs 6 days after ovulation

embryogenesis

separation of the embryoblast from the trophoblast and formation of the embryonic disc that becomes the embryo

layers of the embryonic disc

epiblast, hypoblast

primitive streak

a groove that forms at the midline of the epiblast at the caudal end

cranial-caudal axis

defined by the primitive streak

gastrulation

formation of the three germ layers of the embryo in embryogenesis

beginning of gastrulation and mesoderm formation

formation of the primitive streak at the caudal end of the embryo

germ layers

ectoderm, mesoderm, endoderm

endoderm

epithelial tissue that becomes the lining of the GI and respiratory tracts

mesoderm

forms mesenchyme and becomes blood, muscle, and bone

mesenchyme

fetal connective tissue

primitive knot/Hensen’s node

the organizing center of the embryo that determines the anterior-posterior axis of the embryo

somite

segmented blocks of mesoderm that form on the cranial-caudal axis of the embryo

neurulation

formation of the neural tube that occurs during the later part of gastrulation

regions of mesoderm

chordamesoderm, paraxial mesoderm, intermediate mesoderm, lateral mesoderm

chordamesoderm

mesoderm region that becomes the notochord

paraxial mesoderm

mesoderm region that form somites

intermediate mesoderm

mesoderm region that forms the kidneys and gonads

lateral plate mesoderm

mesoderm region that forms smooth muscle, heart, and coelomic cavity

somite subdivisions

sclerotome, dermatome, myotome

sclerotome

part of the somite that becomes vertebrae and other bones

dermatome

part of somite that becomes the dermis of the skin

myotome

part of somite that becomes skeletal muscle