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50 terms

Embryology Terms

Johns Hopkins School of Medicine: Scientific Foundations of Medicine - Anatomy
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Zygote
earliest development stage of embryo; formed when sperm and egg fuse
Morula
compact ball of 16 cells formed on Day 3
Embryoblast
inner cell mass (ICM), aka pluriblast, which gives rise to endoderm, ectoderm, and mesoderm
Trophoblast
cells forming outer layer of blastocyst (Day 4); forms placenta
Blastocoele
blastocyst cavity
Blastocyst
forms on Day 4; consists of trophoblast surrounding blastocoele and embryoblast
Hypoblast
forms on Day 8 from inner cell mass; gives rise to extraembryonic endoderm (e.g. yolk sac); cuboidal cells
Epiblast
forms on Day 8 from ICM; dorsal (above) the hypoblast; columnar cells
Amniotic cavity
formed by end of Week 2 from epiblast cells
Yolk sac
formed by end of Week 2 from hypoblast cells (ventral); vestigial in humans, though perhaps early nutritional value
Gastrulation
embryonic development phase whereby bilaminar germ disc acquires a third germ layer. Sequence: (1) embryo becomes asymmetric, (2) primitive streak forms, (3) cells from epiblast at primitive streak undergo epithelial to mesenchymal transition
Trilaminar germ disc
ectoderm, endoderm, mesoderm (arise during gastrulation)
Primitive streak
formed in Week 3, initial event of gastrulation; establishes bilateral symmetry in embryo
Primitive node
cephalic (rostral) end of primitive streak
Ectoderm
forms CNS, PNS, sensory epithelia of eye (retina), nose and ear, and epidermis
Endoderm
forms gut epithelial lining, cloaca (future urethra and bladder), respiratory tract, tympanic cavity, parenchyma of thyroid, parathyroids, liver, pancreas, and reticular stroma of tonsils and thymus
Mesoderm
MESODERM (mnemonic):
Mesothelium (peritoneal, pleural, pericardial)/ Muscle (striated, smooth, cardiac)
Embryologic
Spleen/ Soft tissue/ Serous linings/ Sarcoma/ Somite
Osseous tissue/ Outer layer of suprarenal gland (cortex)/ Ovaries
Dura/ Ducts of genitalia
Endothelium
Renal
Microglia/ Mesenchyme/ Male gonad
Buccopharyngeal membrane
Thin membrane where ectoderm and endoderm come into direct contact; forms septum between primitive mouth and pharynx
Cloacal membrane
Thin membrane where ectoderm and endoderm come into direct contact; forms anal-urogentical opening
Prochordal plate
condensation of mesoderm cells extending rostrally from the primitive node in the midline
Cardiogenic plate
originally exists at most anterior end of trilaminar germ disk, but will fold ventrally and develop into the heart
Notochord
derived from mesoderm, notochord becomes vertebral column to provide axial support
Allantois
connecting stalk; helps embryo exchange gases and handle liquid waste; vestigial in humans (like yolk sac)
Cloaca
posterior opening that serves as opening for intestinal, reproductive, and urinary tracts
Lateral folding
transforms embryo from three flat germ layers to cylinder with tube of endoderm (gut) in the center
Cephalocaudul folding
cephalic and caudal ends of three-layered germ disc fold ventrally, most notably bringing heart region down to thoracic region
Neurulation
Week 4 process in which flat neural ectoderm rolls up to become neural tube; upon completion the brain and spinal cord have been formed
Neural tube
embryonic precursor to central nervous system
Neural ectoderm
aka neural plate, includes neural crest cells and neural tube
Placodes
surface ectodermal thickenings that contribute to formation of epithelium, ear and eye
Neural crest cells
specialized cells that form skin pigment cells, ganglia of autonomic nervous system, dorsal root ganglia, facial cartilage, spiral septum of developing heart, ciliary body of the eye, and adrenal medulla
Mesenchyme
middle germ layer composed of both mesoderm and ectoderm (e.g. the neural crest cells)
Paraxial mesoderm
formed during Week 4; organized into segmented units called somites
Somites
segmental units added on a cranial to caudal manner and are a good indicator of precise embryonic age
Sclerotome
somite that becomes future axial skeleton
Dermomyotome
somite that becomes future skeletal muscle and dermal components
Intermediate mesoderm
forms urogenital system; bridges paraxial and lateral plate mesoderm
Lateral plate mesoderm
two layers: somatic/parietal mesoderm and splanchnic or visceral mesoderm
Visceral mesoderm
layer continuous with mesoderm covering yolk sac; forms wall of gut
Parietal mesoderm
layer continuous with mesoderm covering the amnioin; forms parietal pleura and peritoneum
Coelemic cavity
potential space between visceral and parietal mesoderm
Vitelline duct
long, narrow tube that joins yolk sac to midgut lumen of developing fetus. Appears at end of fourth week and normally closes by week VI (VI telline)
Foregut
anterior part of alimentary canal, from mouth to duodenum (esophagus, stomach, duodenum, liver, gallbladder, pancreas, spleen)
Midgut
part of alimentary canal (forms intestines) between foregut (at opening of bile duct) and hindgut. Includes latter parts of duodenum, jejunum, ileum, cecum, appendix, ascending colon, hepatic flexure of colon, transverse colon (proximal 2/3)
Hindgut
posterior (caudal) part of alimentary canal; includes distal 1/3 of transverse colon, splenic flexure, descending colon, sigmoid colon and rectum
homeobox
highly conserved DNA region that codes for homeodomain proteins that are crucial for embryonic development
Hox combinational code
phrase used to describe association between segmentation in developing embryo and unique combinations of overlapping Hox gene expression
Teratogen
substance that causes abnormality in physiological development
Embryonic period
from conception (zygote formation) to the third month of development
Fetal period
from third month of development to birth (266 days post-pertilization): total of about 6 months