Third Week of Development: Trilaminar Germ Disc

Vocabulary flashcards covering the biological processes, gene registrations, and clinical correlates associated with the third week of embryonic development and gastrulation.

Gastrulation

The most characteristic event occurring during the third week of gestation that establishes all three germ layers (ectoderm, mesoderm, and endoderm) in the embryo.

Primitive streak

A narrow groove with slightly bulging regions clearly visible on the surface of the epiblast in a 15- to 16-day embryo.

Primitive node

The cephalic end of the primitive streak consisting of a slightly elevated area surrounding the small primitive pit.

Invagination

The inward movement where epiblast cells become flask-shaped, detach from the epiblast, and slip beneath it upon reaching the primitive streak.

Fibroblast growth factor 8 (FGF8)

A growth factor synthesized by streak cells that controls cell migration and specification by downregulating E-cadherin and regulating BRACHYURY (T) expression.

E-cadherin

A protein that normally binds epiblast cells together.

Embryonic endoderm

The germ layer created by invaginating epiblast cells that displace the hypoblast.

Mesoderm

The germ layer formed by cells that come to lie between the epiblast and the newly created endoderm.

Ectoderm

The germ layer formed by cells remaining in the epiblast after gastrulation.

Prechordal plate

A structure formed between the tip of the notochord and the oropharyngeal membrane derived from the first cells migrating through the node; it is important for forebrain induction.

Oropharyngeal membrane

A region at the cranial end of the disc consisting of tightly adherent ectoderm and endoderm cells representing the future opening of the oral cavity.

Notochordal plate

A midline structure formed when prenotochordal cells become intercalated in the hypoblast so the midline consists of two cell layers.

Definitive notochord

A solid cord of cells that underlies the neural tube and serves as a signaling center for inducing the axial skeleton.

Neurenteric canal

A temporary connection between the amniotic and yolk sac cavities at the point where the primitive pit forms.

Cloacal membrane

A structure at the caudal end of the embryonic disc consisting of tightly adherent ectoderm and endoderm cells with no intervening mesoderm.

Allantois (allantoenteric diverticulum)

A small diverticulum appearing around the 16th day of development that extends from the posterior wall of the yolk sac into the connecting stalk.

Anterior visceral endoderm (AVE)

Cells at the cranial end of the endoderm layer that express genes (OTX2, LIM1, HESX1, cerberus, leftyl) essential for head formation and establishing the cranial end.

Node (the organizer)

The region that secretes chordin, noggin, and follistatin to antagonize BMP4 activity, thereby dorsalizing cranial mesoderm into notochord and paraxial mesoderm.

Bone morphogenetic protein (BMP4)

A member of the TGF-B family that, in the presence of FGF, ventralizes mesoderm into intermediate and lateral plate mesoderm.

PITX2

A homeobox-containing transcription factor known as the "master gene" responsible for establishing left-sidedness.

Serotonin (5-HT)

A neurotransmitter concentrated on the left side of the embryo that activates MAD3 to restrict NODAL expression to the left side.

Holoprosencephaly

A clinical condition where high doses of alcohol kill cells in the anterior midline of the germ disc, resulting in a small forebrain and closely set eyes (hypotelorism).

Caudal dysgenesis (sirenomelia)

A condition where insufficient mesoderm is formed in the caudalmost region, resulting in abnormalities of the lower limbs, urogenital system, and lumbosacral vertebrae.

Sacrococcygeal teratoma

The most common tumor in newborns ($1$ in $17,000$) containing tissues from all three germ layers, arising from remnants of the primitive streak.

Situs inversus

A condition in which the positioning of all internal organs is reversed in a mirror image arrangement.

Primary villi

Trophoblastic structures consisting of a cytotrophoblastic core covered by a syncytial layer appearing at the beginning of the third week.

Secondary villus

A structure formed when mesodermal cells penetrate the core of primary villi and grow toward the decidua.

Tertiary villus (definitive placental villus)

A villus where mesodermal cells in the core differentiate into blood cells and small blood vessels, forming the villous capillary system by the end of the third week.

Outer cytotrophoblast shell

A thin layer that surrounds the trophoblast entirely and attaches the chorionic sac firmly to the maternal endometrial tissue.

Connecting stalk

The structure by which the embryo is attached to its trophoblastic shell, which later develops into the umbilical cord.