Fertilization: Sperm and Oocyte Union

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Questions and Answers

What happens to the sperm during fertilization?

They travel to the tubes and survive for 3-5 days.

They are attracted to the zona pellucida and penetrate the corona radiata.

Millions are killed due to acidity, blocked by cervical mucus, and killed by leukocytes. (correct)

They immediately fuse with the oocyte's nucleus.

What is the purpose of the acrosomal reaction?

To release enzymes to penetrate the corona radiata and zona pellucida. (correct)

To fertilize multiple oocytes at once.

To prevent polyspermy.

To help the sperm travel through the tubes.

What forms after fertilization to prevent polyspermy?

The zona pellucida

The corona radiata

The fertilization membrane (correct)

The capacitation membrane

What is the percentage of blastocysts that fail implantation?

50%-75% Signup and view all the answers

What is the function of the yolk sac?

To provide nourishment until the placenta is functional. Signup and view all the answers

During which week is the placenta complete?

Week 4-12 Signup and view all the answers

What is the purpose of the zona pellucida?

To facilitate sperm binding and penetration. Signup and view all the answers

What is the result of fertilization?

The formation of a zygote. Signup and view all the answers

What is the function of hCG?

To keep the corpus luteum healthy and secreting progesterone. Signup and view all the answers

What is the result of implantation?

The uterine mucosa rebuilding and enveloping the blastocyst. Signup and view all the answers

What is the primary method of transport for Oxygen and Carbon Dioxide across the placenta?

Simple diffusion Signup and view all the answers

During which weeks does the placenta take over the endocrine function from the corpus luteum?

Weeks 12-17 Signup and view all the answers

What is the function of the hormone Relaxin during pregnancy?

Loosening the pelvis and dilating the cervix Signup and view all the answers

When does sexual differentiation typically begin in a fetus?

Weeks 9-12 Signup and view all the answers

What is the function of the hormone Progesterone during pregnancy?

Inhibiting contractions Signup and view all the answers

What is the primary role of the hormone hCG during pregnancy?

Stimulating male gonads to produce testosterone Signup and view all the answers

Study Notes

Fertilization Process

Involves the combination of sperm and oocyte, resulting in a zygote with half genetic material from each parent.
Millions of sperm are killed immediately due to vaginal acidity.
Additional millions are blocked by thick cervical mucus, and thousands more are eliminated by phagocytic leukocytes.

Surviving Sperm

Surviving sperm reach the fallopian tubes within 30-120 minutes post-ejaculation.
Can survive for 3-5 days in the fallopian tubes if they do not encounter an oocyte.
Capacitation occurs, where the sperm’s membrane thins out in preparation for fertilization.

Oocyte Layers and Contact

Oocyte is surrounded by two layers: corona radiata and zona pellucida.
Sperm must penetrate the corona radiata to bind with receptors in the zona pellucida.
Only one sperm successfully penetrates both layers and fuses with the plasma membrane, initiating the acrosomal reaction.

Acrosomal Reaction

The enzyme-filled acrosome cap releases enzymes required to penetrate the corona radiata and zona pellucida.
Hundreds of sperm are needed to facilitate this enzyme release for fertilization.

Post-Fertilization Events

After fertilization, the plasma membrane of the oocyte depolarizes to block additional sperm entry.
Proteins between layers cause the release of excess sperm and destruction of receptors.
An impenetrable barrier called the fertilization membrane forms.

Zygote Development

Completion of meiosis occurs upon fertilization, with a polar body being ejected.
The oocyte transforms into an ovum, and fusion of DNA occurs.
Fraternal twins can arise if two oocytes are released and fertilized; identical twins result from zygote splitting.

Stages of Development

Pre-embryonic stage lasts for the first 2 weeks; embryo develops during weeks 3-8; fetus from week 9 until birth.
Conceptus includes the zygote and associated membranes moving to the uterus.

Embryonic Development

A 16-cell conceptus typically reaches the uterus within 3 days, where it organizes into an inner mass (embryo) and outer shell (placenta).
Inner cells remain totipotent for a few days, allowing for differentiation.

Implantation Process

50-75% of blastocysts fail to implant and are shed with menstruation.
Blastocysts must digest endometrial cells to successfully implant in the uterine wall.
Uterine mucosa rebuilds and envelops the blastocyst, while hCG is released to maintain the corpus luteum, sustaining progesterone production.

Complications of Implantation

Ectopic pregnancy occurs when implantation happens outside the uterus.
Placenta previa involves implantation in the inferior uterus covering the cervical opening.

Embryonic Membranes

During the second week, embryonic cells organize into layers: amnion, yolk sac, allantois, and chorion.
Amnion provides protection, yolk sac nourishes until the placenta is functional, allantois becomes part of the urinary bladder, and chorion surrounds other membranes.

Development of the Placenta

Placenta forms between weeks 4-12 and feeds the embryo by combining maternal and embryonic tissues.
Blood and waste travel via umbilical arteries, whereas oxygen and nutrients travel through a single umbilical vein.

Completion of the Placenta

The placenta is fully developed by weeks 14-16.
Substances crossing the placenta include O2 and CO2 through simple diffusion; active transport is needed for amino acids and iron.

Sexual Differentiation and Hormonal Effects

Sexual differentiation begins between weeks 9-12, with blood circulation through the heart adapting via foramen ovale and ductus arteriosus.
Hormonal influences include increased estrogen levels, which inhibit ovulation, support fetal development, and promote maternal tissue growth.
Relaxin loosens the pelvis and dilates the cervix, while progesterone inhibits contractions.

Key Developments in the Fetus

By weeks 9-12, sensory organs develop, hair grows, and fetal feces accumulate.
Weeks 13-16 see more powerful movements, skin coating with vernix caseosa, and significant weight gain.
The third trimester involves continued weight gain, nail growth, and preparation for birth.

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Description

Learn about the process of fertilization, where a sperm and an oocyte combine to form a zygote, and how genetic material from both parents is inherited. Discover the obstacles sperm face, including acidity, cervical mucus, and phagocytic leukocytes, and how the surviving sperm finally reach the fallopian tubes.