Embryogenesis & Development Flashcards
What is ultrasonography?
A technique using echoes of ultrasound pulses to delineate objects or areas of different density in the body.
How is ultrasonography used to study the baby?
This radiographic technique is performed by placing a probe that emits high-frequency sound waves near the tissue to be examined. The Probe TRANSDUCEs an image onto a computer screen, which can be measured to determine the gestational age, screen for multiple pregnancy abnormalities or anomalies, and identify the baby’s sex.
When, during gestation, do babies begin to show their sex, and at what point can humans detect the difference?
All embryo’s begin FEMALE by default, so for a male fetus to develop, it must undergo masculinization via the gene product of SRY, as well as DEFEMINIZATION. These processes occur (or don’t) around six to eight weeks postfertilization. Humans can’t detect sex until 16 to 17 weeks because ultrasonography equipment does not have high enough resolution yet to give immediate answers.
Where does a secondary oocyte go once it is ovulated from the follicle?
The secondary oocyte travels into the fallopian tube, where it can be fertilized up to 24 hours after ovulation. Fertilization usually occurs in the widest part of the fallopian tube called the ampulla.
What happens when the sperm meets the secondary oocyte in the fallopian tube?
The sperm BINDS to the oocyte and releases acrosomal enzymes that enable the head of the sperm to penetrate through the corona radiata and zona pellucida.
Once the first sperm comes into DIRECT CONTACT with the secondary oocyte’s cell membrane, it forms a tubelike structure known as the ACROSOMAL APPARATUS, which extends to and penetrates the cell membrane.
What is the acrosome?
The acrosome is a vesicle positioned close to the plasma membrane at the tip of the sperm’s head. This vesicle contains soluble proteolytic enzymes and inner membrane proteins, such as bindin. The nucleus contains the genetic material that the sperm is trying to pass on, a haploid genome because it contains only one copy of each chromosome.
What does the acrosomal apparatus allow?
When the acrosomal apparatus is set up, the sperms pronucleus may then freely enter the oocyte once meiosis II has come to completion.
After penetration of the sperm through the cell membrane, the cortical reaction, a release of calcium ions, OCCURS. Theses calcium ions depolarize the membrane of the ovum.
What is the cortical reaction?
The cortical reaction is a process initiated during fertilization by the release of cortical granules from the egg which prevents polyspermy, the fusion of multiple sperm with one egg.
To create this barrier cortical granules, specialized secretory vesicles located within the egg’s cortex (the region directly below the plasma membrane), are fused with the egg’s plasma membrane, causing the contents of the cortical granules to be released outside the cell where they modify an existing extracellular matrix to make it impenetrable to sperm entry.
The trigger for the cortical granules to exocytose is the release of calcium ions from cortical smooth endoplasmic reticulum in response to sperm binding to the egg.
Upon binding of the sperm, what happens to the EGG?
Prior to fertilization, the egg is in a quiescent state, arrested in metaphase of the second meiotic division. Upon binding of a sperm, the egg rapidly undergoes a number of metabolic and physical changes that collectively are called egg activation. Prominent effects include a rise in the intracellular concentration of calcium, completion of the second meiotic division and the so-called cortical reaction.
The cortical reaction refers to a massive exocytosis of cortical granules seen shortly after sperm-oocyte fusion. Cortical granules contain a mixture of enzymes, including several proteases, which diffuse into the zona pellucida following exocytosis from the egg. These proteases alter the structure of the zona pellucida, inducing what is known as the zona reaction. Components of cortical granules may also interact with the oocyte plasma membrane.
What two purposes does the depolarization of the ovum by calcium ions serve?
- Depolarization prevents fertilization of the ovum by multiple sperm cells
- The increased calcium concentration increases the metabolic rate of the newly formed diploid zygote
Fertilization membrane
The now DEPOLARIZED and IMPENETRABLE membrane is called the FERTILIZATION MEMBRANE.
How do dizygotic twins form?
Dizygotic, or fraternal twins, form from fertilization of two different eggs released during one ovulatory cycle by two different sperm.
Each zygote will implant in the uterine wall, and each develops its own placenta, chorion, and amnion.
If the zygotes implant close together, the placentas may fuse. Fraternal twins are no more genetically similar than any other pair of siblings.
How do monozygotic twins form?
Monozygotic, or IDENTICAL twins, form when a single zygote splits into two. Because the genetic material is identical, so too will be the genomes of the offspring.
How do conjoined twins occur?
Monozygotic twins form when a single zygote splits into two; conjoined twins occur if the division is incomplete–the two offspring are physically attached at some point
How can monozygotic twins be classified?
Monozygotic twins can be classified by the number of structures they share.
