GLOSSARY
Allantois
Finger-like outpocketing of yolk sac forms the primitive excretory duct of the embryo; precursor to the urinary bladder
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Amnion
Transparent membranous sac that encloses the developing fetus and fills with amniotic fluid
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Amniotic cavity
Cavity that opens up between the inner cell mass and the trophoblast; develops into amnion
Blastocoel
Fluid-filled cavity of the blastocyst
Blastocyst
Term for the conceptus at the developmental stage that consists of about 100 cells shaped into an inner cell mass that is fated to become the embryo and an outer trophoblast that is fated to become the associated fetal membranes and placenta
Blastomere
Daughter cell of a cleavage
Chorion
Membrane that develops from the syncytiotrophoblast, cytotrophoblast, and mesoderm; surrounds the embryo and forms the fetal portion of the placenta through the chorionic villi
Chorionic membrane
Precursor to the chorion; forms from extra-embryonic mesoderm cells
Chorionic villi
Projections of the chorionic membrane that burrow into the endometrium and develop into the placenta
Cleavage
Form of mitotic cell division in which the cell divides but the total volume remains unchanged; this process serves to produce smaller and smaller cells
Conceptus
Pre-implantation stage of a fertilized egg and its associated membranes
Ectoderm
Primary germ layer that develops into the central and peripheral nervous systems, sensory organs, epidermis, hair, and nails
Ectopic pregnancy
Implantation of an embryo outside of the uterus
Embryo
Developing human during weeks 3–8
Embryonic folding
Process by which an embryo develops from a flat disc of cells to a three-dimensional shape resembling
a cylinder
Endoderm
Primary germ layer that goes on to form the gastrointestinal tract, liver, pancreas, and lungs
Epiblast
Upper layer of cells of the embryonic disc that forms from the inner cell mass; gives rise to all three
germ layers
Fetus
Developing human during the time from the end of the embryonic period (week 9) to birth
Gastrulation
Process of cell migration and differentiation into three primary germ layers following cleavage
and implantation
Gestation
In human development, the period required for embryonic and fetal development in utero; pregnancy
Human chorionic gonadotropin (hCG)
Hormone that directs the corpus luteum to survive, enlarge, and continue producing progesterone and estrogen to suppress menses and secure an environment suitable for the developing embryo
Hypoblast
Lower layer of cells of the embryonic disc that extend into the blastocoel to form the yolk sac
Implantation
Process by which a blastocyst embeds itself in the uterine endometrium
Inner cell mass
Cluster of cells within the blastocyst that is fated to become the embryo
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Mesoderm
Primary germ layer that becomes the skeleton, muscles, connective tissue, heart, blood vessels,
and kidneys
Morula
Tightly packed sphere of blastomeres that has reached the uterus but has not yet implanted itself
Neural plate
Thickened layer of neuroepithelium that runs longitudinally along the dorsal surface of an embryo and gives rise to nervous system tissue
Neural fold
Elevated edge of the neural groove
Neural tube
Precursor to structures of the central nervous system, formed by the invagination and separation
of neuroepithelium
Neurulation
Embryonic process that establishes the central nervous system
Notochord
Rod-shaped, mesoderm-derived structure that provides support for growing fetus
Organogenesis
Development of the rudimentary structures of all of an embryo’s organs from the germ layers
Placenta
Organ that forms during pregnancy to nourish the developing fetus; also regulates waste and gas exchange between mother and fetus
Placenta previa
Low placement of fetus within uterus causes placenta to partially or completely cover the opening of the cervix as it grows
Placentation
Formation of the placenta; complete by weeks 14–16 of pregnancy
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Pluripotent
Innately capable of developing into almost any cell type of the organism
Primitive streak
Indentation along the dorsal surface of the epiblast through which cells migrate to form the endoderm and mesoderm during gastrulation
Somite
One of the paired, repeating blocks of tissue located on either side of the notochord in the early embryo
Syncytiotrophoblast
Superficial cells of the trophoblast that fuse to form a multinucleated body that digests endometrial cells to firmly secure the blastocyst to the uterine wall
Trophoblast
Fluid-filled shell of squamous cells destined to become the chorionic villi, placenta, and associated
fetal membranes
Umbilical cord
Connection between the developing conceptus and the placenta; carries deoxygenated blood and wastes from the fetus and returns nutrients and oxygen from the mother
Yolk sac
Membrane associated with primitive circulation to the developing embryo; source of the first blood cells and germ cells and contributes to the umbilical cord structure
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https://embryology.med.unsw.edu.au/embryology/index.php/Somitogenesis
