Embryology

Embryo

The initial stage of development for a multicellular organism that begins just after fertilization of the female egg cells by the male sperm cell.

Fetus

Unborn mammalian offspring that develops from an embryo; characterized by the presence of all major body organs.

Blastocyst

A mammalian embryo at the early stage of development when it is a hollow ball of cells.

Embryoblast

Mass of cells inside the blastocyst that will eventually give rise to the structures of the fetus.

Epiblast

Columnar cells that form a layer of the embryoblast.

Hypoblast

Simple cuboidal cells that form a layer of the embryoblast.

Extraembryonic mesoderm

Layer of tissue that forms between the embryoblast and the cytotrophoblast and syncytiotrophoblast layers.

Gastrula

Early multicellular embryo that contains three layers of cells from which organs later derive.

Gastrulation

Process during embryonic development when the embryonic disc containing 2 layers changes into a structure that has 3 layers.

Mesoderm

Layer of the embryo (gastrula phase) that is situated between the ectoderm and the endoderm.

Endoderm

The innermost of the three layers of the embryo when it is a gastrula.

Ectoderm

The outermost layer of the three layers of the embryo when it is a gastrula.

Chorionic cavity

Fluid-filled cavity that surrounds the embryo, yolk sac, and amniotic sac (also called the extraembryonic coelom).

Coelom

Fluid-filled cavity.

Cephalic

Relates to the head.

Caudal

Relates to the bottom end of the spinal cord.

Cloaca

Common cavity at the caudal end that serves as the only opening for the digestive, reproductive, and urinary tracts.

Somatic mesoderm

The outer layer of the lateral plate of the mesoderm after it is divided into two parts by the intraembryonic coelom.

Splanchnic mesoderm

The inner layer of the lateral plate of the mesoderm after it is divided into two parts by the intraembryonic coelom.

Somatopleure

Layer of tissue in the embryo that is comprised of the ectoderm and the outer layer of the mesoderm (somatic mesoderm).

Splanchnopleure

Layer of tissue in the embryo that is comprised of the endoderm and the inner layer of the mesoderm (splanchnic mesoderm); gives rise to the gut and lungs.

Intraembryonic coelom

Cavity that forms in the lateral plate mesoderm; divides the lateral plate mesoderm into the somatic and splanchnic mesoderm layers.

Somites

Blocks of mesoderm that are located on either side of the neural tube in the developing embryo.

Dermatome

The lateral wall of the somite, gives rise to the skin.

Myotome

The dorsal portion of each somite; gives rise to muscle.

Sclerotome

The portion of the somite that gives rise to skeletal tissue.

Diencephalon

Region of the neural tube that gives rise to the thalamus, hypothalamus, and posterior pituitary.

Telencephalon

Anterior portion of the forebrain; gives rise to the cerebral hemispheres.

Bulbus cordis

Structure in the developing heart that contributes to the right ventricle, part of the outflow tracts and some portions of the left ventricle.

Truncus arteriosus

Structure in the developing heart that later divides into the aorta and the pulmonary trunk.

Atrioventricular canal

Junction between the primitive atrium and primitive ventricle in the embryo.

Endocardial cushions

Cells in the developing heart that contribute to the separation of the chambers of the heart.