WEEK 1: PHYSIOLOGY OF PREGNANCY Flashcards
What is pregnancy?
When does pregnancy begin and when does it end?
Pregnancy is defined as the course of embryo and fetal growth and development in uterine.
It begins at the fertilization and ends at the delivery of the fetal and it’s attachment.
What is fertilization?
Fertilization is defined as the course of combination of the oocyte and sperm.
Brieflyoccur the hormonal and physical changes that occur during the menstrual cycle
Day 1: Start of menstruation (lasting about 2–6 days).
Days 1–14 (variable, see above): The follicular phase starts on the first day of menstruation.
The endometrium thickens to become prepared for the implantation of the fertilized ovum during the luteal phase, and about 20 ovarian follicles mature under the influence of FSH. One of these becomes the dominant follicle, which produces increasing quantities of estrogen.
Day 14 (variable): Ovulation. The amount of estrogens produced by the follicle increases rapidly between day 12 and 13.
The increased secretion of LH in response to higher levels of estrogen leads to ovulation.
Days 14–28: The luteal phase is characterized by the development of a corpus luteum which secretes progesterone, The endometrium is most responsive to progesterone around the 22nd day of the cycle, which is when nidation should occur if the ovum has been fertilized.
Menstruation The subsequent rapid decrease in the plasma concentrations of estrogens and progesterone results in constriction of endometrial blood vessels and ischemia causing menstruation.
How long does the menstrual cycle last?
A menstrual cycle can last 21–35 days.
The second half of the cycle (luteal phase = secretory phase) usually lasts 14 days.
The first half (follicular phase = proliferative phase) lasts 7–21 days.
Ovulation separates the two phases. If the cycle length varies by more than 2–3 days, ovulation generally does not occur.
Such anovulatory cycles account for 20% of allcycles in healthy females.
State the 4 phases of the menstrual cycle.
- Menstruation
- Follicular phase
- Ovulation Day
- Lacteal phase
Describe the events that happen during the menstrual phase.
- Menstruation (Days 1-5):
The menstrual cycle begins with menstruation, also known as the period. It typically lasts for 3 to 7 days.
*During this phase, the uterine lining (endometrium), which thickened in preparation for a potential pregnancy, is shed and expelled through the vagina.
*Hormones involved: Estrogen and progesterone levels are low during this phase.
Describe the follicular phase of the menstrual cycle.
- Follicular Phase (Days 1-13):
*Simultaneously with menstruation, the hypothalamus in the brain releases Gonadotropin-Releasing Hormone (GnRH).
*GnRH stimulates the anterior pituitary gland to release Follicle-Stimulating Hormone (FSH).
*FSH prompts the development of several ovarian follicles (tiny sacs) containing immature eggs.
*These follicles begin to produce estrogen.
*Rising estrogen levels trigger the thickening of the uterine lining again.
*The dominant follicle continues to mature and releases its egg during ovulation.
Hormones involved: FSH and increasing levels of estrogen.
Describe the ovulation phase of the menstrual cycle.
- Ovulation (Day 14):
Ovulation is the release of a mature egg (ovum) from the dominant ovarian follicle.
This usually occurs around the middle of the menstrual cycle.
*A surge in luteinizing hormone (LH) triggered by rising estrogen levels initiates ovulation.
*The released egg enters the fallopian tube, ready for potential fertilization.
Describe the lacteal phase of the menstrual cycle.
- Luteal Phase (Days 15-28):
*After ovulation, the remaining part of the ruptured follicle transforms into the corpus luteum.
*The corpus luteum produces high levels of progesterone, which helps prepare the uterine lining for possible implantation of a fertilized egg.
*If fertilization does not occur, the corpus luteum degenerates, causing a drop in progesterone and estrogen.
*The declining hormone levels trigger the shedding of the uterine lining, starting the next menstrual cycle.
*Hormones involved: High levels of progesterone and some estrogen.
What happens if the ovum is not fertilized?
Menstruation (if no pregnancy occurs):
If the released egg is not fertilized and no pregnancy occurs, progesterone and estrogen levels drop significantly, leading to the start of the next menstruation.
The cycle repeats, and the process continues as a monthly pattern.
Fertilization
What aids sperm transport within the female reproductive system?
Some sperms reach the ampulla of fallopian tubes within minutes
Sperm transport is aided by:
*Uterine and fallopian tube contraction because of seminal prostaglandin stimulation
*Oxytocin hormone released from the posterior pituitary gland of the female during her orgasm
State the time taken by the sperm to reach the following areas after ejaculation and % of ejaculated sperm.
Vagina
Cervical canal
Uterus
Fertilization site
Vagina: 0 min: 100%
Cervical canal: 1-3 min: 3%
Uterus: 10-20 min: 0.1%
Fertilization site (Ampulla): 30-60 min: 0.01%
What is ejaculation?
What is the role of ejaculation?
Ejaculation is the physiological process by which semen is forcefully expelled from the male reproductive system, typically through the urethra and out of the penis.
It is an essential step in the male reproductive system and is associated with sexual arousal and climax (orgasm).
Ejaculation serves the purpose of delivering sperm from the male’s reproductive organs into the female’s reproductive tract during sexual intercourse, allowing for the possibility of fertilizing an egg.
Describe the 4 steps of ejaculation.
The process of ejaculation involves several steps:
- Arousal: Sexual arousal and stimulation lead to an erection of the penis, where it becomes engorged with blood.
This is typically initiated by sensory or psychological stimuli.
- Emission: During this phase, sperm and seminal fluids are propelled from the testes, where sperm are produced, into the vas deferens (a duct that transports sperm) and then into the urethra.
The seminal vesicles, the prostate gland, and the bulbourethral glands contribute fluids to the semen, providing nourishment and transportation for the sperm.
- Expulsion: In response to increased sexual stimulation, the muscles in the pelvic region contract, forcing the semen through the urethra and out of the penis. This results in the ejaculation of semen.
- Resolution: After ejaculation, the male body typically enters a refractory period during which it is less responsive to sexual stimulation. The refractory period varies from person to person and can last from minutes to hours.
Ejaculation is a normal and natural part of the male reproductive process and is essential for the transfer of sperm for reproduction. It is also a pleasurable experience for many individuals and is closely associated with sexual satisfaction and climax during sexual activity.
What is capacitation?
Capacitation is a process that sperm cells undergo within the female reproductive tract to become fully competent and capable of fertilizing an egg (oocyte).
This process prepares the sperm to successfully penetrate and fuse with the egg during fertilization.
State the 2 Changes that happens to sperm cells during capacitation.
1) the previously regular wavelike beats of the sperm’s tail to be replaced by a more whiplike action that propels the sperm forward in strong lurches
(2) the sperm’s plasma membrane to become altered so that it will be capable of fusing with the surface membrane of the egg.
Describe the fertilization process.
- Zona Pellucida Binding:
Receptor on Sperm: ZP3 receptor (sperm bind to ZP3 glycoprotein in the zona pellucida).
- Acrosome Reaction:
*Receptor on Sperm: ZP3 receptor (sperm bind to ZP3 glycoprotein, triggering the acrosome reaction).
*Chemical Messengers: Calcium ions (Ca2+) play a significant role in initiating the acrosome reaction. Increased intracellular Ca2+ levels within the sperm are essential for the activation of enzymes in the acrosome.
*Upon binding to the zona pellucida, the sperm releases these enzymes, which help degrade and penetrate the glycoprotein matrix of the zona pellucida. The enzymes are mainly hyaluronidase and acrosin, which break down hyaluronic acid and other proteins in the zona pellucida.
- Zona Pellucida Penetration:
Receptor on Sperm: The sperm’s enzymatic action is necessary to create a path through the zona pellucida.
*Chemical Messengers: The acrosomal enzymes (e.g., hyaluronidase and acrosin) are responsible for breaking down the glycoprotein matrix of the zona pellucida.
- Egg Activation:
*Receptor on Egg: Sperm-binding to the egg’s zona pellucida and the subsequent release of sperm components can trigger a calcium wave in the egg, which is part of the egg activation process.
During egg activation, the fast block to polyspermy involves membrane depolarization and the entry of calcium ions into the egg. This inhibits the binding and fusion of additional sperm to the egg’s surface.
The slow block to polyspermy involves the cortical reaction, release of enzymes from cortical granules, and the alteration of the zona pellucida, preventing other sperm from binding to and penetrating the egg.
These mechanisms act as a multi-layered defense to ensure that only one sperm successfully fertilizes the egg.
- Completion of the Second Meiotic Division:
Egg activation also involves the completion of the second meiotic division in the egg. In human eggs, this division is arrested in metaphase II.
The increase in intracellular calcium levels, which occurs as a result of the binding and penetration of sperm, triggers the resumption and completion of meiosis.
This results in the formation of a second polar body and the release of a haploid egg nucleus.
- Formation of the Zygote:
Following the successful completion of the second meiotic division, the egg’s haploid nucleus is now ready to combine with the haploid nucleus of the sperm.
The fusion of these two nuclei results in the formation of a diploid zygote with a full set of 46 chromosomes, representing the unique genetic makeup of the future offspring.
Early stages of development.
What is the lifespan of an unfertilized ovum?
*Life span of unfertilized ovum is less than 20 hours following ovulation
Describe what happens once the sperm has entered the ovum.
- Oocyte completes the second meiotic division to form the mature ovum plus a second polar body that is expelled.
- In the fertilized ovum, the 23 unpaired chromosomes of the male pronucleus and the 23 unpaired chromosomes of the female pronucleus align themselves to re-form a complete complement of 46 chromosomes
From ovulation to implantation.
Describe the size of the
Day 0: Fertilization
Day 1: Zygote
Day 2: 2 cells
Day 3: 4 cells
Day 4: Morula
: Inner cell mass
Day 7: Blastocyst implants into the endometrium
How long does the fertilized egg require to move from the fallopian tube into the uterus to form a Morula?
What does it depend on?
On entering the uterus, the fertilized ovum reaches to the Blastocyst stage. How many cells does a blastocyst contain?
Requires 3-5 days from the fallopian tube into the uterus to form a Morula.
Depends on beating of cilia toward the uterus and the feeble fluid current.
On entering the uterus, the fertilized ovum reaches to the Blastocyst stage (32 and more cells)
Implantation of the blastocyst.
Name the cells present over blastocyst that secrete proteolytic enzymes that digest the adjacent cells of endometrium.
Trophoblast cells present over blastocyst secrete proteolytic enzymes that digest the adjacent cells of endometrium.
Describe the process of implantation.
- The blastocyst approaches the endometrial lining in a process known as apposition.
The outer trophoblast cells of the blastocyst adhere to the receptive area of the endometrial lining. This initial contact is mediated by adhesion molecules on both the trophoblast and the endometrial cells.
- Following adhesion, the trophoblast cells begin to invade the endometrial lining. They secrete enzymes that degrade the endometrial tissue, creating a space for the blastocyst to embed itself.
- The invasive trophoblast cells also fuse to form a layer called the syncytiotrophoblast, which is critical for establishing a connection between the embryo and the maternal blood supply.
