Introduction to Embryology Flashcards
Embryology Scope
Study of development of an organism from fertilization of the ovum through the period of organogenesis.
Embryonic period
From fertilization (from single cell) to 8th week of human development.
Organogenesis
Fetal Period
9th week to 38th week (Birth)
Karl Ernst Von Baer (XIX)
Father of modern embryology:
1. Described the oocyte and the cleaving zygote, blastocysts, and stages of embryonic development. “Established that mammals develop from eggs”
2. Determined that general characteristics precedes from specific ones: Phylotypic stage in embryonic development. Ancestor of vertebrates have a common basic vertebrate body
Hyppocrates
Father of medicine
Aristotle
Founder of embryology
Leonardo Da Vinci
Measurements of prenatal growth
Anton van Leeuwenhoek
Observed sperms under microscope
Regnier de Graaf
Eggs from the ovaries
Hans Spemann
Nobel Prize in Medicine 1935
Primary induction (Gastrulación)
Lewis, Nusslein-Volhard and Wieschaus
Novel Prize in Medicine 1995.
Discovery of genes that control embryonic development
Time line
Blastocyst
First two weeks.
This is a period of cell proliferation from the zygote to the morula, blastocyst, and bilaminar embryonic disc formation. Birth defects do not originate in this period because body systems and structures have not yet developed. Teratogens usually cause the loss of the entire conceptus.
Timeline
Embryonic Period
The third to Eighth week constitutes the dynamic period of gastrulation, folding of the embryo, and the formation of all the organ systems. Because this is the most active period of development and differentiation, the embryo in weeks 3 to 8 is most vulnerable to major birth defects.
Timeline
Fetal Period
The dominant theme in months 3 to 9 (full term) is growth of all major structures that have already appeared. Birth defects in this period are usually not as severe or obvious and include small size, mental retardation, and defects in the eyes, ears, teeth, and external genitalia.
Early Embryonic Developmental Stages
30 hours after fertilization
Repeated mitotic division
Blastomeres
Increased # of cells together with a decreased in size.
Morula
After the 8-cell stage, cell compactation occurs. (12-32 cells)
3 days after fertilization
Blastocyst
Fluid-filled blastula
Gastrula
Characterized by the formation of the 3 germ layers.
Ectoderm, mesoderm and endoderm
3rd week
Organizer: Primitive knot/node (Hensens node in humans)
Will induce the notochord
Neurula
Stage in which neurulation occurs (development of nervous system)
Neural tube formation
Notochord
Embryonic-specific structure that defines the primordial axis of the embryo.
Defines the phylum of chordates: urochordates and vertebrates
Gives some rigidity
Is the basis of the development of the axis skeleton
It disappears when the vertebral bodies form (4th week)
It degenerates and disappears, but it persists as the nucleus pulpous (NP) of each intervertebral disc.
Chardoma- tumors formed by vestigial remnants of the notochord
Pharyngeal (Branchial arches)
Condensation of mesoderm in the cranial region will give rise to transient structures to develop vertebrate head structures.
Example: 1st vertebral arch gives rise to the cartilage of the mandible
Induction
Process in which one group of cells or tissues causes another set of cells or tissues to change their fate.
One cell type or tissue is the inducer that produces a signal, and one is the responder to the signal.
Crosstalk between the two cell types or tissues is essential for differentiation
Notochord induces
overlying ectoderm to form neural plate (neuroectoderm)
FGF-B up regulation…
promotes neural plate formation
BMP-4 inhibition…
promotes epidermis formation
Intercellular comunication
paracrine
juxtacrine
cell-cell
Paracrine examples
morphogens, growth and differentiation factors
Juxtacrine examples
Receptor/membrane-bound ligand in adjacent cell
Cell-cell example
Gap junctions
Cell differentiation
Cells become specialized by means of cell signaling, environmental influences
Morphogenesis
Processes by which order the developing organism is created
Major morphogenetic processes in early embryology:
Cell division
Condensation
Cell Death
Migration
Matrix secretion and Degradation
Growth
Signaling pathways for differentiation of cell types during Embryogenesis
Transcription factors and gene expression
Morphogens
Notch/Delta
Transcription factors
Hox/Homebox proteins, Pax, Basic Helix-loop-Helix
Hox/Homebox proteins,
Hox genes discovered in Drosophila melanogaster.
In humans, Hox genes along the PA axis and chromosomal location are conserved as well.
Example: Defects in HOX1A impair human neural development
Morphogens
Diffusible molecules that specify which cell type will be generated at a specific anatomical location.
Morphogens also direct the migration of cells and their processes to their final destination.
Many morphogens are found in concentration gradients in the embryo and expressed in opposing gradients in the dorsoventral, anteroposterior and mediolateral axes.
Morphogens: Retinoic Acid
Derived from Vitamin A; important for the anteroposterior axis.
Normally acts “posteriorize” the body plan.
Insufficient RA will lead to more anteriorized structures.
Hox genes are crucial targets of RA receptors in early development.
RA are powerful teratogens, especially during the first trimester.
Morphogens: Hedgehogs
Sonic Hedgehog was the first mammalian orthodox of the Drosophila gene hedgehog.
The primary receptor for Shh is Patched (PTCH)
Shh in vertebrate development has a role in patterning the ventral neural tube. Shh is secreted at high levels by the notochord, thus providing high levels in the floor of the neural tube (promoting motor neurons).
Holoprosencephaly-fused cerebral hemispheres is a congenital disorder linked to mutations in Shh or its receptor.
Morphogens: Transforming Growth Factor
Important for migration and axonal guidance.
TGF-B binds to transmembrane kinase receptors to phosphorylate intracellular receptor-associated Smad Proteins. Smad complexes regulate target gene expression in the nucleus.
TGF-B signaling is a tumor suppressor factor and inhibits cell proliferation. Its dysregulation has been implicated in cancer.
Morphogens: Fibroblast Growth Factor
Influence morphogenesis in embryonic development.
Roles include cell migration, differentiation, survival, apoptosis, induction etc.
Important for formation of blood vessels
Morphogens : Wnt/B Catenin
Directs cell polarity, proliferation, apoptosis, cell fate and morphogenesis (patterning the CNS, the gut, the respiratory and circulatory systems)
Also play a role in tumor formation
Notch/Delta signaling pathway
Pathway is integral for cell fate determination, maintenance of stem cell niches (lateral inhibition), apoptosis, and differentiation.
Lateral inhibition- cells that are in the center differentiate meanwhile lateral cells are inhibited from differentiation and remain progenitor.
Cell Adhesion Molecules: Catherine Super Family
Calcium Dependent adhesion
Also involved in cell recognition, signaling, communication, morphogenesis, angiogenesis among others
Epithelial-Mesenchymal Transition
Acquisition of mesenchymal features from epithelial cells.
Occur during normal embryonic development (gastrulation)
Adult tissue regeneration
Cancer progression
Mesenchymal-Epithelial Transition
Metastatic tumor cells at distant sites undergo MET
Gestational age
Last normal menstrual period. Two weeks longer than the fertilization age.
