Lecture 5 - 7 (Set C)

Study Notes

Repeated segments governed by HOX genes, which control body plan along cranio-caudal axis (anterior-posterior) and specify segment identity of tissues within the embryo
Age of embryo based on number of somites, which become hard to count, then determined by Crown-rump length (CRL) measured from vertex of skull to midpoint between apices of buttocks

Hensen's node factor 3 beta (HNF3β) maintains node and induces regional specificity in forebrain and midbrain areas
Goosecoid, chordin, noggin, follistatin, nodal antagonize and inhibit activity of BMP4 (bone morphogenetic protein 4), which favors dorsalization and cranial mesoderm dorsalization into notochord and somites
Nodal gene initiates formation of primitive streak, maintains PS, and regulates formation of dorsal and ventral mesoderm, as well as head and tail structures

BMP4 and FGF (Fibroblast growth factor) are secreted along embryonic disc, ventralizing mesoderm to form intermediate and lateral plate mesoderm
Node is the organizer, homologous to chordamesoderm in dorsal lip of blastopore
Nodal gene is homologous to goosecoid gene, coding for transcription factors regulating activity of genes at dorsal lip of blastopore and node

FGF8 secreted by node establishes expression of Nodal, which activates Lefty2 in lateral plate mesoderm
Nodal and Lefty2 activate PITX2 gene, coding for transcription factor controlling left-sidedness
SHH (sonic hedgehog) represses left-sided gene (PITX2) expression on the right
5HT (serotonin) neurotransmitter breakdown favored towards the left, activating FGF8 and regulating cascade of chain activation leading to left-sidedness

4 classes of HOX genes in vertebrates (A-D), responsible for determining general body plan
HOX genes control number of body segments, number of somites, and placement of appendages (forelimbs, hindlimbs) and animal head-tail directionality