NATS 1675 RN 12 Definitions

Blastomere

One of the small, nucleated cells that form during cleavage of a zygote

Gastrulation

The stage of embryonic development in which cells become arranged into primary tissue layers (germ layers); in humans, the layers are an inner endoderm, an intermediate mesoderm, and a surface ectoderm.

Germ layers

One of three primary tissue layers that forms during gastrulation and that gives rise to certain tissues of the adult body. Compare ectoderm (outer layer); endoderm (inner layer); mesoderm (middle layer).

Cell differentiation

The gene-guided process by which cells in different locations in the embryo become specialized

Morphogenesis

(“the beginning of form”) [Gk. morphe, form, and genesis, origin] Processes by which differentiated cells in an embryo become organized into tissues and organs.

Embryonic disk

  In early development, the oval, flattened cell mas that gives rise to the embryo shortly after implantation.

Neural tube

Embryonic forerunner of the brain and spinal cord.

Extraembryonic membranes

Membranes that form along with a developing embryo, including the yolk sac, amnion, allantois, and chorion

Yolk sac

1 of 4 extraembryonic membranes. Part becomes a site of blood cell formation and some of its cells give rise to the forerunners of gametes.

Amnion

1 or 4 extraembryonic membranes. It becomes a fluid-filled sac in which the embryo (and fetus) can grow, move freely, and be protected from sudden temperature shifts and impacts.

Allantois

1 of 4 extraembryonic membranes that from during embryonic development. In humans, it functions in early blood formation and development of the urinary bladder.

Umbilical cord

Structure containing blood vessels that connect a fetus to its mother’s circulatory system by way of the placenta.

Chorion

1 of 4 extraembryonic membranes; it encloses the embryo and the three other membranes. Absorptive structures (villi) that develop at its surface are crucial for the transfer of substances btwn the embryo and mother.

Placenta

Of the uterus, an organ composed of maternal tissues and extraembryonic membranes (the chorion especially); it delivers nutrients to the fetus and accepts wastes from it, yet allows the fetal circulatory system to develop separately from the mother’s.