MCAT Biology Review - Chapter 3: Embryogenesis and Development Flashcards
Since all embryos are female, at what point does fetus undergo masculinization via the gene product of SRY, but also defeminization?
These processes occur (or don’t) around six to eight weeks post-fertilization. That said, prospective parents shouldn’t expect any answers before 16 to 17 weeks without a blood test, because ultrasonography equipment does not have high enough resolution to give answers earlier in pregnancy.
How many hours after the secondary oocyte is ovulated from the follicle around day 14 can it be fertilized in the widest part of the fallopian tube, called the ampulla?
Up to 24 hours after ovulation.
What happens when the sperm meets the secondary oocyte in the fallopian tube?
It binds to the oocyte and releases acrosomal enzymes which allows the head of the sperm to penetrate the corona radiata and zona pellucida.
What is the acrosomal apparatus and what does it allow?
It is a tube-like structure that extends to and penetrates the cell membrane.
It allows the pronucleus to then freely enter the oocyte once meiosis ll has come to completion.
Once the penetration of the sperm through the cell membrane of the oocyte is complete, the cortical reaction, a release of calcium ions, occurs. What is the role of these calcium ions?
They depolarize the membrane of the ovum, which serves two purposes:
- Depolarization prevents fertilization of the ovum by mutliple sperm cells
- Increased calcium concentration increases the metabolic rate of the newly formed diploid zygote.
What is the new name of the now depolarized and impenetrable membrane called?
Fertilization membrane
What are the two different mechanisms that twinning can occur?
Dizygotic (fraternal) twins and Monozygotic (identical) twins
Which twinning mechanism forms from fertilization of two different eggs released during one ovulatory cycle by two different sperm where each zygote will implant in the uterine wall, and develop its own placenta, chorine, and amnion?
Dizygotic twins (fraternal)
If the zygotes implant close together the placentas may grow onto each other. Fraternal twins are no more genetically similar than any other pair of siblings.
Which twinning mechanism forms when a single zygote splits into two and since the genetic material is identical, the genomes of the offspring will be too?
Monozygotic twins (identical).
If division is incomplete, conjoined twins may result, where the two offspring are physically attached.
How are Monozygotic twins classified as?
They can be classified by the amount of structures they share.
The more gestational structures that are shared, the more risks as the fetuses grow and develop.
Monochorionic/monoamniotic twins share the same amnion and chorion.
Dichorionic/diamniotic twins each have their own amnions and chorions.
What happens to the zygote within the fallopian tubes after it is fertilized?
Travels to the uterus for implantation.
Arriving too late, there will no longer be an endometrium capable of supporting the embryo.
As the zygote moves for implantation, what is the process of it undergoing rapid mitotic cell divisions? Why must this happen?
It is called cleavage.
The first cleavage officially creates an embryo, as it nullifies one of the zygote’s defining characteristics: unicellularity (the embryo is a multicellular eukaryote organism). After several rounds of mitosis occurs, the total size remains unchanged during the first few divisions.
The cells increase two ratios: The nuclear-to-cytoplasmic (N:C) ratio and the surface area-to-volume ratio. Thus, the cells achieve increased area for gas and nutrient exchange relative to overall volume.
What are the two types of cleavage and what do they result in?
Indeterminate cleavage results in cells that can still develop into complete organisms. In fact, monozygotic twins have identical genomes because they both originate from indeterminately cleaved cells of the same embryo.
Determinate cleavage results in cells with fates that are, as the term implies, already determined! In other words, these cells are committed to differentiating into a certain type of cell.
What do you call a solid ball of cells that results from many mitotic divisions later after embryonic fertilization?
A morula.
The term comes from the latin word for mulberry, which might help us grasp what an embryo at this stage looks like.
Describe what happens to the morula once it undergoes blastulation.
- The blastula forms, which is a hollow ball of cells with a fluid-filled inner cavity know as a blastocoel.
- The mammalian blastula is known as a blastocyst (two differentiated cell types vs a blastula) and consists of two noteworthy cell groups, the trophoblast and inner cell mass.
- The trophoblast cells surround the blastocoel and give rise to the chorion and later the placenta, whereas the inner cell mass protrudes into the blastocoel and gives rise to the organism itself.
Remmber that a BLASTula is an embryo with a BLASTed-out cavity
