Theories of Development and Early Embryology (Preformation vs Epigenesis)

A comprehensive set of flashcards covering the key concepts from the notes on theories of development, oogenesis, fertilization, embryogenesis, organogenesis, maternal contributions, genetics in Drosophila, metamorphosis, regeneration, and related topics.

What are the two historical theories of development?

Preformation and Epigenesis.

Fill in the blank: The theory that organisms develop from fully formed miniature versions is called __.

Preformation.

What is a homunculus in preformation theory?

A tiny, fully formed human believed to reside in the egg (ovism) or sperm (spermism).

What does encasement (Emboîtement) refer to in preformation?

The idea that all future generations are contained within the reproductive cells of the first humans (infinite nesting).

Define Epigenesis.

Organisms develop gradually from undifferentiated material through a series of steps, with progressive differentiation and emergent complexity.

What does ‘de novo formation’ mean in epigenesis?

Structures form anew during development rather than being preformed.

Name three historical supporters of epigenesis.

Aristotle, Caspar Friedrich Wolff, Karl Ernst von Baer.

What is a zygote?

A fertilized egg that contains genetic information from both parents.

What is oogenesis?

The process of female gamete (egg) formation in the ovaries.

What is vitellogenesis?

The process of yolk formation and deposition in the oocyte to nourish the embryo.

List the three modes of yolk formation.

Autosynthetic vitellogenesis, heterosynthetic vitellogenesis, mixosynthetic vitellogenesis.

Name the major yolk classifications by yolk content.

Alecithal (little/no yolk), Microlecithal (very little), Mesolecithal (moderate), Telolecithal (large), Centrolecithal (yolk in center).

What is the cortex of the egg and its role?

A specialized outer region of the cytoplasm containing determinants and cortical granules important for fertilization events.

What are cortical granules?

Secretory vesicles in the egg cortex that release enzymes after fertilization to modify the zona pellucida and prevent polyspermy.

Define morphogenetic determination.

Establishment of developmental fates in egg cytoplasm domains via localized determinants and morphogen gradients.

What are maternal effect genes? Give examples in Drosophila.

Genes deposited in the egg that control early development; examples include bicoid, nanos, oskar, and hunchback.

Explain the maternal-to-zygotic transition (ZGA).

Maternal factors govern early development until ZGA; the zygotic genome becomes transcriptionally active and directs subsequent development.

What is a fate map?

A diagram showing the future developmental fate of regions of the embryo.

What is Spemann’s organizer?

The dorsal lip of the blastopore that induces a secondary embryonic axis and nervous system; the primary organizer.

Name key morphogens and inhibitors involved in organizer signaling.

BMP antagonists (Noggin, Chordin, Follistatin), Shh, FGFs, Wnt pathways; gradients guide induction.

What is induction in embryology?

A process where one group of cells influences the development of another through signaling.

What is tissue competence in induction?

The ability of a tissue to respond to an inductive signal, depending on receptors and signaling context.

Describe the main steps of fertilization.

Sperm approach → acrosomal reaction → sperm entry → membrane depolarization (fast block) → cortical reaction (slow block) → pronuclear formation → pronuclear fusion.

What is egg activation?

Transition of the egg from dormancy to an actively developing embryo, triggered by fertilization and calcium signaling.

Which molecule initiates calcium waves during egg activation?

Sperm-delivered PLC-zeta, which releases IP3 and Ca2+ from intracellular stores.

List the major events during egg activation.

Metabolic activation, completion of meiosis, cortical granule exocytosis, maternal mRNA recruitment, pronuclear formation, DNA synthesis, and zygotic genome activation.

Differentiate external and internal fertilization.

External fertilization occurs outside the female body (e.g., fish, amphibians); internal fertilization occurs inside the female reproductive tract (e.g., mammals, birds, reptiles).

What is organogenesis?

The process by which the primary germ layers develop into organs and structures; involves epithelial and mesenchymal interactions.

What are EMT and MET traditionally in organogenesis?

Epithelial-Mesenchymal Transition and its reverse (Mesenchymal-Epithelial Transition), enabling tissue movement and organization.

Name derivatives of neural crest cells.

Peripheral nervous system components, melanocytes, craniofacial cartilage/bones, smooth muscle in arteries, adrenal medulla; Schwann cells and enteric neurons as examples.

What are the three germ layers formed in gastrulation?

Ectoderm, mesoderm, endoderm.

What brain development process marks neurulation?

Formation of the neural tube from the neural plate, followed by regional differentiation into brain regions.

Define metamorphosis in amphibians and its hormonal control.

A developmental transition from larval (aquatic) to adult (terrestrial) form, primarily regulated by thyroid hormones (T4/T3) and TSH, with corticosteroids contributing in some species.

What hormones drive amphibian metamorphosis?

Thyroid hormones (T4/T3), stimulated by TSH; corticosteroids may also contribute.

What is viviparity and its evolutionary advantages?

Live birth; advantages include embryo protection inside the mother, direct nutrient supply (placenta), and stable developmental conditions.

Name the four extra-embryonic membranes in amniotes and their roles.

Amnion (cushioning and protection), Chorion (gas exchange), Yolk sac (nutrient source and early blood formation in mammals), Allantois (waste collection and gas exchange; contributes to placenta in some species).

Describe placental types commonly found in mammals.

Discoidal, Cotyledonary, Zonary, Diffuse.

What is the cleidoic egg?

An amniotic egg with a protective shell and membranes enabling terrestrial reproduction; features include amniotic membrane, shell, chorion, and yolk.

What is the function of the mammalian placenta?

Facilitates exchange of gases, nutrients, and wastes between mother and fetus; supports pregnancy and provides hormonal signals.

Contrast totipotency and pluripotency.

Totipotent cells can form all embryonic and extra-embryonic tissues (e.g., zygote); pluripotent cells form all embryonic tissues but not extra-embryonic tissues.

What does epigenetics study?

Heritable changes in gene expression without changes to the DNA sequence, including histone modification and DNA methylation.

Name modern techniques shaping developmental biology (2015–2025).

CRISPR-Cas9 gene editing, single-cell RNA sequencing, organoid cultures, and studies of epigenetic regulation.

What is the Spemann organizer’s significance in induction?

Demonstrates that a specific embryonic region can instruct the development of an entire axis and neural tissue.

Explain germ plasm.

Localized cytoplasmic determinants in the egg that specify germ cell fate and early development.

Differentiate mosaic vs regulative development.

Mosaic: cell fates set by inherited determinants; regulative: cell fates shaped by cell interactions and environmental cues.

What is a blastula and a blastocoel?

A blastula is a hollow ball of cells; the blastocoel is the fluid-filled cavity inside.

What role do maternal contributions play in axis formation?

Maternal mRNAs and proteins establish anterior-posterior, dorsal-ventral, and left-right axes prior to zygotic transcription.

Give examples of maternal effect genes in Drosophila.

Bicoid (anterior), Nanos (posterior), Hunchback (anterior regulation), and Caudal (posterior regulation).