Oogenesis
What is an oogonium?
This is the diploid cell that begins the process of oogenesis before birth.
What is the ampulla?
Fertilization normally occurs in this specific part of the fallopian tube.
What is cleavage?
This term refers to the series of rapid mitotic divisions that transform a zygote into a multicellular embryo.
What is the endoderm?
This germ layer formed during gastrulation gives rise to the lining of the digestive and respiratory tracts.
what is an Allele?
A different version of a gene found at the same location (locus) on homologous chromosomes.
What is metaphase II?
During ovulation, the oocyte is released from the ovary arrested in this meiotic stage.
What is the zygote?
At the end of fertilization, the male and female pronuclei fuse to form this diploid structure, marking the start of a new individual.
What is a morula?
By the time the embryo reaches 16–32 cells, it is called this structure, which enters the uterus around day 4.
What is the trilaminar disc?
The process of gastrulation converts the bilaminar embryonic disc into this type of disc.
What is a dominant allele?
An allele that expresses its effect even when only one copy is present (in heterozygous form).
What is the luteinizing hormone (LH) surge?
This hormone surge triggers the completion of meiosis I and ovulation.
What is the acrosomal reaction?
This reaction, triggered when a sperm contacts the zona pellucida, releases enzymes from the acrosome to penetrate the egg’s outer layers.
What is the syncytiotrophoblast?
This outer layer of the trophoblast becomes invasive and releases enzymes that allow the blastocyst to burrow into the endometrium.
What does the trophoblast divide into?
Cytotrophoblast and synchiotrophoblast
what is Hemizygous?
Having only one copy of a gene instead of the usual two.
What is the zona pellucida?
This glycoprotein layer surrounding the oocyte plays a key role in sperm binding and preventing polyspermy after fertilization.
What is ZP3?
This glycoprotein in the zona pellucida specifically binds to sperm receptors, initiating the acrosomal reaction.
What is human chorionic gonadotropin (hCG)?
This hormone, secreted soon after implantation, maintains the corpus luteum during early pregnancy.
What is the notochord?
Cells that ingress through the primitive node and migrate cranially form this midline structure important for neural induction.
What is X-linked dominant?
A dominant allele on the X chromosome causes the trait or disorder. Both males and females can be affected, but females often show milder symptoms (since they have two Xs). Affected fathers pass it to all daughters (since daughters get his X) but no sons (sons get his Y). Example: Rett syndrome, Fragile X syndrome (some forms).
What are polar bodies?
Unequal cytokinesis during oogenesis produces one functional ovum and these small, nonfunctional cells.
Explain how the egg prevents polyspermy.
1. ⚡ Fast Block to Polyspermy
Occurs within seconds of sperm–oocyte fusion.
The oocyte plasma membrane depolarizes due to a Na⁺ influx, preventing additional sperm from fusing.
This is transient (lasts a few minutes) — not the main defense in mammals, but present in some species.
2. 💥 Slow Block (Cortical or Zona Reaction)
Triggered by a Ca²⁺ wave released inside the oocyte after sperm entry.
Causes cortical granules (vesicles beneath the oocyte membrane) to:
Release enzymes into the perivitelline space.
Modify the zona pellucida (especially the ZP3 and ZP2 glycoproteins), so it can no longer bind sperm.
This makes the zona impermeable to additional sperm — a permanent block.
How long does cleavage begins after fertilisation?
What is about 24–30 hours?
What is the paraxial mesoderm?
The somites, derived from this specific type of mesoderm, form the axial skeleton and skeletal muscles.
What is a Y-linked gene?
A mutation or gene on the Y chromosome causes the trait or disorder.