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Johns Hopkins School of Medicine: Scientific Foundations of Medicine - Anatomy


earliest development stage of embryo; formed when sperm and egg fuse


compact ball of 16 cells formed on Day 3


inner cell mass (ICM), aka pluriblast, which gives rise to endoderm, ectoderm, and mesoderm


cells forming outer layer of blastocyst (Day 4); forms placenta


blastocyst cavity


forms on Day 4; consists of trophoblast surrounding blastocoele and embryoblast


forms on Day 8 from inner cell mass; gives rise to extraembryonic endoderm (e.g. yolk sac); cuboidal cells


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


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


forms CNS, PNS, sensory epithelia of eye (retina), nose and ear, and epidermis


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 (mnemonic):
Mesothelium (peritoneal, pleural, pericardial)/ Muscle (striated, smooth, cardiac)
Spleen/ Soft tissue/ Serous linings/ Sarcoma/ Somite
Osseous tissue/ Outer layer of suprarenal gland (cortex)/ Ovaries
Dura/ Ducts of genitalia
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


derived from mesoderm, notochord becomes vertebral column to provide axial support


connecting stalk; helps embryo exchange gases and handle liquid waste; vestigial in humans (like yolk sac)


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


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


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


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


segmental units added on a cranial to caudal manner and are a good indicator of precise embryonic age


somite that becomes future axial skeleton


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)


anterior part of alimentary canal, from mouth to duodenum (esophagus, stomach, duodenum, liver, gallbladder, pancreas, spleen)


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)


posterior (caudal) part of alimentary canal; includes distal 1/3 of transverse colon, splenic flexure, descending colon, sigmoid colon and rectum


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


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

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