Embryonic Genes and Embryogenesis Flashcards

Flashcards covering embryonic genes (SHH, FGF, Wnt-7a, HOX), limb patterning axes, and early embryogenesis stages from fertilization to gastrulation.

Sonic Hedgehog Gene (SHH)

An embryonic signaling protein gene responsible for CNS development (forebrain midline separation) and limb patterning along the anterior-posterior axis.

Holoprosencephaly

A developmental defect resulting from mutations in the SHH gene where the prosencephalon fails to cleave into two hemispheres, often leading to facial abnormalities like cyclopia or cleft lip.

Fibroblast Growth Factor (FGF)

A signaling molecule produced by the apical ectodermal ridge (AER) that stimulates the growth of underlying mesoderm and is critical for proximal-distal limb development.

Wnt-7a

A gene critical for dorsal-ventral limb development that "dorsalizes" the mesoderm by activating the LMX-1 gene.

Homeobox (HOX) Genes

A family of genes that code for transcription factors regulating the development of the anterior-posterior axis and the formation of body segments.

Homeosis

The transformation of one body structure into another, which can occur due to mutations in homeotic (HOX) genes.

Apical Ectodermal Ridge (AER)

The ectoderm overlying the mesoderm at the tip of the limb bud that is essential for proximal-to-distal limb growth.

Zone of Polarizing Activity (ZPA)

The region in the posterior limb (near the little finger) that influences the AER through SHH protein signaling to regulate anterior-posterior patterning.

Engrailed1

A gene expressed on the ventral side of the developing limb that represses Wnt-7a expression.

Polydactyly

A condition characterized by extra fingers or toes, which can result from rare mutations in certain HOX genes.

Syndactyly

The fusion of fingers or toes, often associated with abnormal limb formation due to HOX gene mutations.

Zygote

The single cell formed during fertilization when a haploid mature spermatozoon ($1N, 1C$) merges with a haploid ovum ($1N, 1C$) to create a diploid structure ($2N, 2C$).

Morula

A developmental stage consisting of a solid ball of cells formed by continuous division after the cleavage of the zygote.

Blastocyst

The human blastula which contains a fluid-filled cavity (blastocoel), an outer trophoblast layer, and an inner cell mass.

Trophoblast

The outer cells of the blastocyst that secrete fluid into the blastocoel and eventually contribute to the formation of the placenta.

Inner Cell Mass

The cluster of polarized inner cells in a blastocyst that gives rise to all tissues of the embryonic body and is the source of embryonic stem cells.

Implantation

The process where the blastocyst attaches to the uterus, typically occurring around day $6$ to $10$ after fertilization, which initiates the secretion of $\beta-hCG$.

Gastrulation

The process by which the bilaminar disc (epiblast and hypoblast) converts into a trilaminar structure consisting of the three germ layers: ectoderm, mesoderm, and endoderm.

Bilaminar Disc

The structure formed from the inner cell mass consisting of two layers, the epiblast and hypoblast, separated by a basement membrane.