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Chapter 32. Embryology

Recommended Article: 【Biology】 Table of Contents for Biology


1. Overview

2. Genetic Identity

3. Developmental Process


a. Neuroembryology

b. Drosophila Embryology

c. Sea Urchin Embryology

d. Fish Embryology

e. Amphibian Embryology

f. Avian Embryology

g. Mammalian Embryology

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

Xenopus laevis

Chick: Surgical removal is possible during embryo development

Mus musculus

Danio rerio: Transparent interior

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

Type 1: Embryonic Cells

○ Definition: the fertilized egg itself

○ Consists of trophoblast cells + inner cell mass; totipotent

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).

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

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.

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


image

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

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