Fertilization
Fertilization usually occurs in which part of the female reproductive tract?
Ampulla of the fallopian tube
Cleavage divisions result in smaller cells called ____?
Blastomeres
Gastrulation forms how many germ layers?
3
Which germ layer forms the notochord?
Mesoderm
What is the term for the reaction that prevents polyspermy?
Cortical reaction.
Is cleavage mitotic or meiotic division? And why?
Cleavage involves only mitosis, maintaining chromosome number while increasing cell number.
What structure marks the beginning of gastrulation?
Primitive streak.
Which germ layer produces the nervous system?
Ectoderm
What happens to the oocyte immediately after the sperm penetrates the zona pellucida?
The secondary oocyte (arrested in metaphase II) resumes division, forms an ovum and second polar body, creating the female pronucleus, which fuses later with the male pronucleus
What is compaction and when does it occur?
Around day 3, blastomeres flatten and form tight junctions, separating inner and outer cells → basis for inner cell mass & trophoblast differentiation.
What is invagination?
Inward movement of cells forming new layers.
What is the fate of endoderm formed during gastrulation?
Endoderm forms epithelial lining of GI tract, liver, pancreas, lungs, thyroid, and bladder.
What is syngamy
After sperm entry, male and female pronuclei migrate toward each other and fuse, restoring the diploid number (46 chromosomes) and forming the zygote.
What structure forms at day 4–5 during cleavage?
A fluid-filled cavity called the blastocoel forms, pushing inner cell mass to one side and forming trophoblast externally → now ready for implantation.
Define epithelial-to-mesenchymal transition (EMT) in gastrulation.
Epiblast cells lose tight junctions, gain motility, and migrate through the primitive streak to form mesoderm. EMT is essential for forming tissues like muscle, bone, and connective tissue.
How does the primitive streak disappear, and why is this important?
It regresses by week 4; persistence causes sacrococcygeal teratoma.
Explain capacitation and its importance.
Capacitation occurs in the female reproductive tract (uterus & tube). It removes cholesterol and glycoproteins from the sperm membrane, increasing motility and exposing receptors required for the acrosome reaction. Without capacitation, sperm cannot penetrate the zona pellucida.
How do the inner cell mass and trophoblast differ in fate?
Inner cell mass: forms epiblast & hypoblast → actual embryo, amnion, yolk sac.
Trophoblast: differentiates into cytotrophoblast & syncytiotrophoblast → implantation & chorionic villi.
This early differentiation is crucial for embryo formation and maternal–fetal interface.
Explain the role of Nodal and BMP in germ layer patterning.
Nodal signaling: promotes internalization of epiblast cells → mesoderm & endoderm formation.BMP inhibition (Noggin/Chordin): allows neural ectoderm formation.High BMP: epidermis; low BMP: neural tissue.This gradient is critical for body axis formation.
How does gastrulation establish body axes?
Anterior–posterior axis: Primitive streak (posterior) and node (organizer).Dorsal–ventral: Nodal, BMP, Wnt gradients.Left–right: Nodal flow and expression of PITX2 gene.Proper axis formation ensures correct organ placement and symmetry.