The Second Week of Development

Vocabulary flashcards covering key embryonic structures, developmental events, and clinical applications from the second week of human development.

Bilaminar Germ Disc

A two-layered embryonic structure formed by the epiblast and hypoblast during the second week of development that gives rise to all tissues and organs of the body.

Cytotrophoblast

The inner layer of mononuclear cells of the trophoblast.

Syncytiotrophoblast

The outer multinucleated layer of the trophoblast without distinct cell boundaries that secretes proteolytic enzymes and Human Chorionic Gonadotropin (hCG).

Human Chorionic Gonadotropin (hCG)

A hormone secreted by syncytiotrophoblasts during week two, peaking at 8 to 10 weeks of pregnancy, that promotes the corpus luteum, uterine vascular angiogenesis, and immunosuppression.

Epiblast

The layer of high columnar cells of the inner cell mass facing the amniotic cavity, corresponding to embryonic ectoderm.

Hypoblast

The layer of small cuboidal cells of the inner cell mass facing the blastocyst cavity, corresponding to embryonic endoderm.

Amniotic Cavity

A cavity that develops within the epiblast and is lined by amnioblasts.

Amnioblasts

Epiblast cells that line the amniotic cavity to form the amniotic membrane.

Exocoelomic Membrane (Heuser's Membrane)

A membrane formed by migrating flat cells from the hypoblast that lines the inner surface of the cytotrophoblast.

Exocoelomic Cavity (Primitive Yolk Sac)

The space limited by the exocoelomic membrane and hypoblast, which allows movement of the embryonic disc.

Lacunae Stage

A stage occurring at day 9 of development characterized by vacuoles forming within the syncytiotrophoblast that fuse into intercommunicating networks called lacunae.

Uteroplacental Circulation

The vascular system established around days 11-12 when trophoblastic lacunae become continuous with eroded maternal sinusoids in the endometrial stroma.

Maternal Sinusoids

Enlarged maternal capillaries in the endometrial stroma that are eroded by syncytiotrophoblasts.

Extra-Embryonic Mesoderm

Primary mesodermal tissue derived from the yolk sac that fills the space between the cytotrophoblast and the inner amniotic and yolk sac membranes.

Chorionic Cavity (Extraembryonic Coelom)

A large cavity formed by the fusion of spaces within the extraembryonic mesoderm, surrounding the primitive yolk sac and amniotic cavity except at the connecting stalk.

Extraembryonic Somatopleuric Mesoderm

The portion of extraembryonic mesoderm lining the inner surface of the cytotrophoblast and covering the amniotic membrane.

Extraembryonic Splanchnopleuric Mesoderm

The portion of extraembryonic mesoderm covering the yolk sac.

Chorion

The wall of the chorionic sac, constituted by the extraembryonic somatic mesoderm, cytotrophoblast, and syncytiotrophoblast.

Decidua Reaction

Cellular and matrix changes throughout the endometrium during implantation, where stromal cells fill with glycogen and lipids to nourish the conceptus.

Decidua Basalis

The anatomical portion of the decidua located specifically at the site of implantation.

Decidua Capsularis

The anatomical portion of the decidua enclosing the conceptus.

Decidua Parietalis

The anatomical portion of the decidua comprising the remaining lining of the uterus.

Primary Chorionic Villi

Columns of cytotrophoblast cells that proliferate mitotically and extend into the surrounding syncytiotrophoblast layer.

Chorionic Villus Sampling

A clinical procedure in which cells from primary chorionic villi are removed for early genetic testing of the fetus.

Secondary (Definitive) Yolk Sac

A new cavity formed inside the exocoelomic cavity by a new cellular layer produced by the hypoblast.

Exocoelomic Cysts

Portions of the exocoelomic membrane that are pinched off during secondary yolk sac formation and float in the chorionic cavity.

Connecting Stalk

A band of extraembryonic somatic mesoderm connecting the germ disc to the trophoblast, serving as the primordium of the umbilical cord.

Hydatidiform Mole

An abnormal conceptus where a haploid sperm fertilizes an egg lacking a female pronucleus, leading to paternal genomic origin and trophoblast proliferation with little or no embryonic tissue.

Prader-Willi Syndrome

A genetic disease caused by a deletion on chromosome 15 inherited from the father.

Angelman Syndrome

A rare neurological disorder characterized by severe congenital mental retardation, unusual facial appearance, and muscular abnormalities, caused by a deletion on chromosome 15 inherited from the mother.

Week of Twos

A mnemonic describing the major events of the second week: trophoblast differentiates into 2 layers, embryoblast into 2 layers, primary mesoderm splits into 2 layers, and 2 cavities form.