○ 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.