Chapter 32. Embryology
Recommended Article: 【Biology】 Table of Contents for Biology
1. Overview
h. Hydra Regeneration and Transplantation
1. Overview
⑴ Model Organisms for Developmental Research
① Fruit Fly (Drosophila melanogaster): Embryo develops outside the mother
② C. elegans
③ Xenopus laevis
④ Chick: Surgical removal is possible during embryo development
⑤ Mus musculus
⑥ Danio rerio: Transparent interior
⑦ Arabidopsis thaliana
⑵ Key Proteins
① Fibronectin
○ Located in the extracellular matrix (ECM).
○ Pulls the archenteron during its elongation.
○ Bound to integrins, which are integral membrane proteins.
② Cadherin
○ Adheres cells to one another to help form the blastocoel.
○ Also involved in separation of the neural tube.
③ Morphogens
○ Hh (Hedgehog): in Drosophila, patterns cells within ~30 μm.
○ Shh (Sonic Hedgehog): in mouse, patterns cells within ~100 μm; named after the video game Sonic the Hedgehog.
○ Ihh (Indian Hedgehog): in mouse, patterns cells within ~300 μm.
○ PTHrP
○ GDF5
○ BMP
○ WNT
④ Organizer-derived diffusible proteins (BMP inhibitors)
○ noggin, chordin, xnr3
⑤ Organizer-derived diffusible proteins (Wnt inhibitors)
○ cerberus, Fribee, dickkopf, Frzb
○ Note: Wnt is associated with deep invagination of the brain.
2. Genetic Identity
⑴ Evidence for genetic equivalence
① Totipotency in plants
② Nuclear transplantation in animals
○ Briggs & King’s experiment: concluded that as development proceeds substantially, nuclear activity changes.
○ Gurdon’s experiment: showed that nuclei from differentiated cells can still induce development.
③ Reproductive cloning in mammals
④ Differentiation capacity of stem cells
○ Stem cell: a cell with the capacity to differentiate into specific cell type(s).
○ Unipotency: the ability to differentiate into only one cell type.
○ Multipotency: the ability to differentiate into several cell types.
○ Pluripotency: the ability to differentiate into all cell types.
○ Totipotency: the ability to develop into a complete organism; also called “organism-forming potential.”
○ In other words, totipotency refers to the capacity to form the embryo, all adult tissues, and— in species that have them— the extraembryonic membranes.
⑵ Types of Stem Cells
① Type 1: Embryonic Cells
○ Definition: the fertilized egg itself
○ Consists of trophoblast cells + inner cell mass; totipotent
② Type 2: Embryonic Stem Cells (ESCs)
○ Definition: Cells obtained during the differentiation of embryonic cells.
○ Advantage: Pluripotency.
○ Disadvantage: Ethical issues (even though ESCs are considered only up to the blastocyst stage, ethical concerns remain).
○ Types: Fertilized-embryo–derived ESC, blastomere-derived ESC, parthenogenetic ESC, somatic cell nuclear transfer–derived ESC (SCNT-ESC).
○ Fertilized-embryo–derived ESC: Obtained by isolating the inner cell mass from an embryo.
○ Parthenogenetic embryonic stem cells: Oocytes undergo cleavage without sperm (parthenogenesis).
○ History
○ 1981: First established from mouse blastocysts.
○ 1998: Thomson et al. first established human embryonic stem cells (hESCs).
③ Type 3: Adult Stem Cells (ASC)
○ Definition: Stem cells found in adults
○ Advantages: No ethical concerns
○ Disadvantages: Multipotency, Unipotency
○ Hematopoietic Stem Cells (HSC)
○ Differentiates into white blood cells, red blood cells, platelets, etc.
○ Associated with malignant blood disorders, severe aplastic anemia, etc.
○ CD34+
○ Mesenchymal Stem Cells (MSC)
○ Obtained from connective tissues such as bone, cartilage, muscle, blood vessels, and fat, which are differentiated from the mesoderm.
○ Differentiates into skin dermis, muscle tissue, and even nerve tissue
○ Related to joint cartilage damage, bio-material production, and nervous system disorders
○ CD73+, CD90+, CD105+
○ Bone Marrow Stem Cells
○ Composed of hematopoietic stem cells: Differentiates into white blood cells, red blood cells, macrophages
○ Procedure takes 30 minutes
○ Normal daily activities possible immediately after the procedure
○ Neural Stem Cells
○ Also present in the adult nervous system
○ Skin Stem Cells
○ Cord Blood Stem Cells (CBC)
○ Derived from fetuses with undeveloped immune systems. Extracted from placenta and umbilical cord blood
○ No immune rejection
○ Active
○ Assists in regenerating damaged cartilage tissue
○ Type 1: Hematopoietic Stem Cells
○ Type 2: Mesenchymal Stem Cells
○ Low-grade GVHD
○ Amniotic stem cells
○ No immune rejection
④ Type 4. Induced Pluripotent Stem Cells (iPSCs)
○ Definition: Somatic cells reprogrammed into embryonic stem cell–like cells.
○ Advantage: No ethical issues, pluripotency.
○ Disadvantage: Very low success rate.
○ Yamanaka factors: A method of delivering reprogramming genes into somatic cells using viruses.
○ Oct4: Gene related to maintaining stem cell undifferentiated state.
○ Sox2: Gene regulated by Oct4.
○ c-Myc: Gene involved in phenotype maintenance and proliferation during in vitro culture.
○ Klf4: Gene involved in phenotype maintenance and proliferation during in vitro culture.
○ Japan has been heavily investing in iPSC research.
⑤ Type 5: Terminally Differentiated Cells
⑶ Determination: The time point at which the expression of specific genes drives cell differentiation (e.g., expression of MyoD1).
① Autonomous specification: Fate is fixed from the outset (e.g., mollusks, urochordata).
○ Example: Cytoplasmic determinants present in the egg become unevenly distributed after cleavage.
② Conditional specification: Fate is determined during development by influences from neighboring cells.
○ Differentiation can be induced by signaling molecules from surrounding cells.
3. Developmental Process
⑴ Overview
① Stage 1: Cell division
② Stage 2: Cell differentiation
③ Stage 3: Morphogenesis
③ Stage 4: Pattern formation
⑵ Oocyte activation
① Early oocyte activation: before fertilization
○ Because NADPH is used during biomembrane formation, NAD+ becomes activated/increased.
○ MAPK inactivation → transient arrest of cell division → preparation for full cleavage; initiation of DNA replication.
② Late oocyte activation: after fertilization
○ There is not enough time for de novo transcription → mRNAs are supplied to the cytoplasm → proteins needed for cleavage are produced → transcriptional repressors have no immediate effect.
○ Direct transcription begins only after the gastrula stage; therefore, transcriptional repressors become effective then.
○ Protein synthesis proceeds better at higher pH; the Na+/H+ exchanger raises cytoplasmic pH.
⑶ Cleavage
① Features
○ G1 and G2 phases are absent; only S and M phases occur.
○ As cleavage proceeds, the cytoplasm-to-nucleus volume ratio gradually decreases.
○ Among maternal-effect factors: a transcriptional inhibitor, actinomycin
② Classification by yolk
○ Yolk: nutrients required for development.
○ Type 1. Holoblastic cleavage: cleavage occurs throughout the entire egg.
○ Equal (isolecithal): occurs in eggs with little yolk evenly distributed (e.g., echinoderms—sea urchins—and mammals).
○ Unequal (mesolecithal): occurs in eggs with a moderate amount of yolk concentrated toward the vegetal pole (e.g., amphibians).
○ Type 2. Meroblastic cleavage: cleavage occurs only in specific regions.
○ Discoidal cleavage: in telolecithal eggs where abundant yolk occupies most of the egg (e.g., birds, reptiles, fish).
○ Superficial cleavage: in centrolecithal eggs where abundant yolk is centered (e.g., insects, Drosophila).
③ Classification by orientation
○ Radial cleavage: echinoderms (e.g., sea urchin), lancelets (amphioxus).
○ Spiral cleavage: annelids, mollusks, platyhelminths (flatworms).
○ Bilateral cleavage: cephalopods, amphibians.
○ Meridional cleavage: the animal–vegetal axis is vertical, and the cleavage plane is meridional (parallel to that axis); accordingly, the mitotic spindle is oriented horizontally.
○ Equatorial cleavage: the animal–vegetal axis is vertical, and the cytokinetic cleavage plane is horizontal (equatorial).
○ Rotational cleavage: mammals, nematodes.
④ Animal and vegetal poles
○ Animal pole: less yolk → cleavage proceeds readily; blastomeres are small.
○ Vegetal pole: more yolk → cleavage is impeded; blastomeres are large.
⑷ Blastula stage
① A blastocoel is present during the blastula stage.
② The blastocoel soon regresses and disappears.
⑸ Gastrulation: axis formation
① Gastrulation: invagination
○ Morphogen: a substance that diffuses to form a concentration gradient and specifies different cell fates according to the degree of exposure to that gradient.
② Organizers
○ Nieuwkoop center: induces the organizer.
○ Dorsal lip of the blastopore (Spemann organizer): amphibians.
○ Notochord: organizer/inducer of the neural tube.
○ Micromeres: form the archenteron; later give rise to skeletogenic spicules; observed in sea urchins.
○ Hensen’s node.
○ Limb bud.
③ Mechanisms of gastrulation
○ Large-scale cell movements.
○ The blastocoel regresses and disappears; the archenteron takes its place.
○ Formation of the germ layers.
⑹ Germ-layer theory
Figure. 1. Formation of the Germ Layers
① Ectoderm: epidermis, nervous system, brain, pigment cells, lens of the eye, adrenal medulla.
② Mesoderm: muscles, renal tubules (nephrons), notochord, heart, connective tissue, kidney, circulatory system, bones and skeletal muscle, adrenal cortex, dermis.
③ Endoderm: respiratory system, digestive system, endocrine glands, urinary bladder.
○ Note: digestive organs such as the liver, lung, and intestine are endoderm-derived; the kidney alone is mesoderm-derived.
④ Germ cells: sperm and egg; not part of any germ layer.
Input: 2015.07.15 08:11