Developmental Biology Terms Starting With O
Developmental Biology Glossary: O
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Oocyte
/ OH-oh-syt / · Greek oion, egg; kytos, cell
Oocyte is an immature female germ cell that undergoes meiosis and accumulates maternal factors needed to support early embryonic development after fertilization.
An oocyte enters meiosis and arrests in prophase I as a primary oocyte, spending this arrested period accumulating mRNAs, proteins, nutrients, and morphogenetic determinants in its cytoplasm. Hormonal signals at ovulation trigger resumption of meiosis I, producing a secondary oocyte and a first polar body, followed by a second arrest at metaphase II. Fertilization by sperm releases the secondary oocyte from metaphase II arrest and triggers completion of meiosis II, yielding the mature egg nucleus and a second polar body.
Surrounding follicle cells supply growth factors including kit ligand and epidermal growth factor receptor ligands that coordinate oocyte growth throughout folliculogenesis.
In the surf clam (Spisula solidissima), oocytes arrest in prophase I and can be triggered to resume meiosis simply by adding potassium chloride to seawater, making this species a classic model for studying the biochemistry of meiotic resumption without the hormonal complexity of vertebrate systems.
Reproductive System Fun Facts →An oocyte is the same as a fertilized egg or zygote. An oocyte is unfertilized and has not yet completed meiosis II; only after sperm entry triggers the final division does the cell become a mature egg, and only after the two nuclei fuse does a zygote exist.
A human primary oocyte arrests in prophase I before birth and may remain in that state for 12 to 50 years before resuming meiosis at ovulation, making it one of the longest-arrested cell cycle states in human biology. During this arrest, the oocyte accumulates maternal mRNAs and proteins needed for early embryogenesis, including several thousand distinct transcripts stored in a translationally silenced state bound by CPEB protein; these are activated in sequence after fertilization without any new transcription. The extended prophase I arrest is maintained by high cyclic AMP levels generated by the GPR3 receptor in the oocyte membrane; removal of the oocyte from its follicle drops cAMP within minutes and triggers meiotic resumption, which is why isolated oocytes in culture spontaneously mature even without a hormone signal.
Oogenesis
/ oh-oh-JEN-eh-sis / · Greek oion, egg; genesis, origin
Oogenesis is the process by which a diploid oogonium undergoes meiosis, cytoplasmic growth, and accumulation of yolk, ribosomes, and maternal mRNAs to produce a large haploid egg ready for fertilization.
Oogenesis begins during fetal development when primordial germ cells migrate into the developing gonads, differentiate into oogonia, proliferate by mitosis, and then enter meiosis I. Most oocytes arrest in prophase I and remain as primary oocytes surrounded by follicle cells until puberty and beyond, a pause that can last decades in humans. Unlike spermatogenesis, which continuously produces four equal sperm from each meiotic cycle, oogenesis produces one large secondary oocyte and three small polar bodies, with the secondary oocyte receiving nearly all the cytoplasm and stored nutrients.
The first polar body usually degenerates, and the second polar body forms only if fertilization occurs and meiosis II is completed.
Human females are born with approximately 1 to 2 million oocytes, a number that declines to roughly 300,000 to 400,000 by puberty, yet only about 400 to 500 oocytes are ever ovulated during a lifetime of reproductive cycling.
Reproductive System Fun Facts →Oogenesis produces four equal eggs the way spermatogenesis produces four equal sperm. Each meiotic cycle in oogenesis yields one functional egg and up to three polar bodies that receive almost no cytoplasm.
Where Do Stem Cells Come From? →In the frog Xenopus laevis, oocytes arrested in prophase I can remain in that state for months while accumulating massive stores of yolk protein and maternal mRNA. A single fully grown Xenopus oocyte reaches about 1.2 millimeters in diameter, roughly 100,000 times the volume of a typical somatic cell, reflecting the enormous cytoplasmic investment required before fertilization.
What is Meiosis? →Organogenesis
/ or-GAN-oh-JEN-eh-sis / · Greek organon, instrument; genesis, origin
Organogenesis is the developmental stage during which the three primary germ layers give rise to the body's organs through coordinated cell migration, tissue folding, inductive signaling, and cell-type differentiation.
Organogenesis begins after germ layer formation is largely complete and involves coordinated folding, branching, cell migration, and cell differentiation within tissue primordia to generate functional organ structures. Organs form through epithelial folding that creates chambers and tubes, inductive signaling between tissues such as endoderm signaling to adjacent mesoderm, and differentiation of specialized cell types including vasculature and innervation. Different organs follow distinct developmental timetables: the heart begins to beat by week 3 to 4 in humans, while kidney nephrons continue forming into postnatal life in some species.
Fibroblast growth factors, transforming growth factors, and Wnt molecules coordinate cell behaviors across developing organs throughout this period.
Organogenesis in humans is largely complete by the end of the eighth week after fertilization, which is why this window is considered the period of greatest sensitivity to teratogens such as thalidomide, a drug that disrupted limb organogenesis in thousands of infants during the late 1950s and early 1960s.
Organs form only by cells multiplying in place. Organ formation also requires folding, directed migration, and tissue-to-tissue signaling that reshape primordia into three-dimensional structures.
During heart organogenesis in humans, paired endocardial tubes fuse by week 3, loop rightward by week 4 to position the future chambers correctly, and form septa between weeks 5 and 7 to separate the left and right sides of the heart. Disruption of any of these steps can produce congenital heart defects, which affect roughly 1 in 100 live births.