Cleavage, Yolk Influence, and Gastrulation – Developmental Biology Lecture

Vocabulary flashcards covering core terminology from the lecture on cleavage, yolk effects, and gastrulation.

Cleavage

A rapid series of synchronous cell divisions transforming the fertilized egg (zygote) into a multicellular embryo without overall growth.

Fertilization

The event that activates an otherwise dormant egg, producing a metabolically active zygote and initiating cleavage.

Cleavage furrow

The contractile indentation that forms parallel to the metaphase plate and separates daughter cells; its position is directed by asters interacting with the cell cortex.

Aster

Radial microtubule array emanating from the centrosome that contacts the cell cortex and dictates cleavage furrow positioning.

Mitotic apparatus

The spindle complex responsible for chromosome segregation; not the primary determinant of furrow placement, as shown by experimental displacement.

Blastomere

Any individual cell produced during cleavage divisions of the early embryo.

Morula

Solid ball of blastomeres formed after several cleavage rounds, preceding blastula formation.

Blastula

Early embryonic stage consisting of a hollow sphere (or disk) of cells surrounding a fluid-filled cavity called the blastocoel.

Blastocoel

The cavity inside the blastula; its size and shape vary with yolk distribution.

Oligolecithal (Isolecithal) egg

Egg containing little, evenly distributed yolk; undergoes complete (holoblastic) cleavage as in echinoderms and mammals.

Mesolecithal egg

Egg with a moderate amount of yolk concentrated toward the vegetal pole; cleavage remains holoblastic but slower (e.g., amphibians).

Telolecithal egg

Egg possessing a large yolk mass with cytoplasm confined to a small animal-pole disk; cleavage is meroblastic (incomplete) as in birds and reptiles.

Centrolecithal egg

Egg whose yolk is centralized, typical of arthropods; undergoes meroblastic cleavage producing a superficial blastoderm.

Holoblastic cleavage

Complete separation of blastomeres through the whole egg; characteristic of isolecithal and mesolecithal eggs.

Meroblastic cleavage

Partial division of the egg where yolk is not fully cleaved; typical of telolecithal and centrolecithal eggs.

Micromere

Smaller blastomere produced in yolk-poor regions (usually the animal pole) during uneven holoblastic cleavage.

Macromere

Larger blastomere rich in yolk, found toward the vegetal pole of mesolecithal embryos.

Maternal cytoplasm segregation

Partitioning of pre-existing egg cytoplasmic determinants among blastomeres during cleavage.

Maternal-to-embryonic transition (MBT)

Shift in control from maternal RNAs to zygotic genome transcription, coinciding with slower, asynchronous cell cycles.

Actinomycin D

A transcription inhibitor used experimentally to show that early cleavages proceed without new RNA synthesis.

Puromycin

A translation inhibitor demonstrating that protein synthesis from stored maternal ribosomes is essential during cleavage.

Morphogenetic movements

Coordinated cell migrations and shape changes during gastrulation that rearrange blastomeres into germ layers.

Gastrulation

Developmental phase where the blastula reorganizes to form ectoderm, mesoderm, and endoderm, establishing body axes.

Ectoderm

The outer germ layer formed during gastrulation, giving rise to epidermis and nervous system.

Endoderm

The inner germ layer produced in gastrulation, forming the gut lining and associated organs.

Mesoderm

Middle germ layer arising between ectoderm and endoderm; generates muscle, bone, and circulatory systems.

Embryonic axes

Primary spatial orientations (anterior-posterior, dorsal-ventral, left-right) fixed during cleavage and early gastrulation.

Inductive interaction

Process whereby one group of cells influences the fate of another, made possible by proximity established during gastrulation.

Lewis Wolpert quotation

"It’s not birth, death, or marriage that is the most important event in your life, but rather gastrulation," highlighting its developmental significance.

Yolk influence on cleavage

Variation in yolk quantity and distribution determines whether cleavage is holoblastic or meroblastic and shapes early embryo architecture.