General Embryology & Histology – Key Vocabulary

Key Terms

  • Embryology – study of development from fertilization to birth.

  • Histology – study of tissues and their organization into organs.

Prenatal Development Timeline

  • Germinal stage: 0−2 weeks0-2\,\text{weeks} (cleavage → morula → blastocyst → implantation)

  • Embryonic stage: 3−8 weeks3-8\,\text{weeks} (organogenesis, body plan set)

  • Fetal stage: 9 weeks9\,\text{weeks} to birth (growth & maturation)

Stem Cell Potency

  • Totipotent – can form all embryonic & extra-embryonic tissues (zygote, morula).

  • Pluripotent – forms all embryonic tissues (inner cell mass of blastocyst).

Core Transcription Factors

Factor

Key Developmental Role

Hox

Anterior-posterior body axis patterning

Pax

Sense organs & nervous system

Msx

Mesenchyme→epithelium transitions in limbs/face

Tbox

Limb outgrowth coordination

Fox

Multiple organ formation

Helix-Loop-Helix

Myogenesis- mucles formation

Wt1

Kidney & gonad formation

Dlx

Jaw & inner ear shaping

Principal Morphogens & Functions

Morphogen

Main Action

TGF\text{-}\beta

Mesoderm induction; myoblast proliferation

BMP

Broad structural formation by inhibiting alternate fates

Shh

Retina patterning; notochord, lung, hair, etc.

Wnt

Gastrulation movements; polarity establishment

Chordin/Noggin/Gremlin

BMP antagonists

Retinol (Vitamin A)

Hindbrain & pharyngeal neural-crest organization

Cell Signaling Architecture

  1. Ligand (morphogen) binds transmembrane receptor.

  2. Two core receptor responses:

    • Intrinsic kinase activity (e.g., receptor tyrosine kinase, RTK)

    • G-protein/second-messenger cascade (e.g., cAMP\text{cAMP})

  3. Signal transduction relays to nucleus → alters gene expression via transcription factors.

Major Developmental Pathways

  • Wnt pathway

  • Hedgehog pathway (Shh, Ihh, Dhh)

  • RTK pathway (FGF, EGF, VEGF, etc.)

  • Hippo pathway

  • Vitamin A (Retinoic acid) pathway

Signal Transduction Example (RTK → MAPK)

Ligand→RTK→RAS→RAF→MEK→ERK→TF phosphorylation→gene activation\text{Ligand} \rightarrow \text{RTK} \rightarrow RAS \rightarrow RAF \rightarrow MEK \rightarrow ERK \rightarrow \text{TF phosphorylation} \rightarrow \text{gene activation}

Exam Focus Points

  • Know stage-specific events (cleavage, implantation, organogenesis).

  • Match transcription factors & morphogens to their developmental roles.

  • Outline ligand-receptor types and downstream kinase vs. second-messenger signaling.

  • Recall key pathways (Wnt, Hedgehog, RTK) and their impact on cell fate & morphogenesis.