28.1 Fertilization
3 min read•june 18, 2024
Fertilization is a complex process where meets egg. It's a journey that starts with ejaculation and ends with the creation of a . Along the way, sperm face many challenges and undergo changes to prepare for their ultimate goal.
The fusion of sperm and egg is a carefully orchestrated dance. It involves sperm binding to the egg's outer layer, releasing enzymes to break through, and finally merging with the egg. This sets off a chain of events that prevents other sperm from entering and starts the development of a new life.
Fertilization
Journey of sperm to oocyte
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- Ejaculation releases sperm from the penis into the vagina
- Semen contains sperm and seminal fluid from accessory glands (, prostate gland, and )
- Sperm encounter the acidic and hostile environment of the vagina
- Most sperm die in the vagina, but some survive and enter the cervix
- is more alkaline and less hostile than the vaginal environment
- Filters out abnormal sperm and provides nutrients for surviving sperm
- Sperm travel through the cervix and enter the uterus
- Uterine contractions assist in propelling sperm towards the
- Sperm swim through the uterus and enter the fallopian tubes
- May take several hours to several days to reach the , the typical site of fertilization
- Only a small fraction of the original sperm reach the site of fertilization ()
- The journey of sperm is timed to coincide with , which releases a mature egg (oocyte) into the fallopian tube
Capacitation for fertilization preparation
- Maturation process sperm undergo in the female reproductive tract prepares them for fertilization
- Changes in sperm during include:
- Removal of coat and seminal proteins from sperm surface
- Increased membrane fluidity due to cholesterol removal
- Changes in ion permeability and influx of calcium ions
- Hyperpolarization of the sperm membrane
- enables sperm to:
- Respond to released by the oocyte and surrounding cells
- Undergo the , essential for penetrating the oocyte's outer layers
- Fuse with the oocyte plasma membrane during fertilization
- Time-dependent process usually occurs during sperm's journey through the uterus and fallopian tubes
Steps in sperm-oocyte fusion
- Sperm binding to the
- is a glycoprotein layer surrounding the oocyte
- Sperm receptors bind to specific glycoproteins () on the zona pellucida
- reaction
- Binding to zona pellucida triggers the reaction in the sperm
- Acrosome, a cap-like structure on the sperm head, releases enzymes () that digest the zona pellucida
- Enables the sperm to penetrate the zona pellucida
- Fusion with the oocyte membrane
- After penetrating the zona pellucida, sperm reaches the
- Sperm plasma membrane fuses with the oocyte plasma membrane
- Fusion triggers oocyte to complete II and release the second polar body
-
- Fusion triggers the in the oocyte
- release contents into the perivitelline space
- Enzymes from cortical granules modify the zona pellucida, making it impenetrable to other sperm (zona reaction)
- Prevents (fertilization by multiple sperm)
- Formation of the
- Sperm nucleus enters the oocyte cytoplasm and decondenses
- Male and female fuse, combining genetic material
- Fertilized oocyte is now called a zygote
Post-fertilization events
- After fertilization, the zygote begins to divide and develop
- The zygote becomes a , a hollow ball of cells, as it travels down the fallopian tube
- occurs when the attaches to the uterine wall
- Once implanted, the blastocyst continues to develop into an
Timing of fertilization
- Fertilization typically occurs within 24-48 hours after ovulation
- The is the period when an egg can be fertilized, usually lasting about 24 hours after ovulation
- Sperm can survive in the female reproductive tract for up to 5 days, increasing the chances of fertilization
Key Terms to Review (47)
Acrosin: Acrosin is a proteolytic enzyme found in the acrosome, a specialized organelle located at the tip of a mature sperm cell. It plays a crucial role in the fertilization process by helping the sperm penetrate the outer layers of the egg.
Acrosomal reaction: The acrosomal reaction is a process in which the enzyme-filled cap of a sperm, known as the acrosome, releases its enzymes to break down the outer layers of an egg's membrane, facilitating fertilization. This crucial step allows the sperm to penetrate the egg and merge its genetic material with that of the egg.
Acrosome: The acrosome is a cap-like structure that covers the anterior half of a sperm's head, containing enzymes essential for breaking down the outer membrane of an egg to allow sperm entry during fertilization. It plays a crucial role in ensuring successful fertilization by facilitating the sperm's penetration into the egg.
Acrosome: The acrosome is a cap-like structure located at the anterior end of a sperm cell, containing enzymes that are essential for penetrating the outer layers of an oocyte during fertilization. This organelle plays a critical role in the reproductive process, allowing sperm to successfully fertilize the egg by digesting the zona pellucida, which is the protective layer surrounding the oocyte. Understanding the acrosome's function is vital to grasping how fertilization occurs and how sperm interacts with the female reproductive system.
Acrosome reaction: The acrosome reaction is a crucial process during fertilization in which the acrosome, a cap-like structure on the head of a sperm, releases enzymes that help penetrate the outer layers of the egg. This reaction is vital for successful fertilization as it enables the sperm to reach and fuse with the egg's plasma membrane, allowing for the merging of genetic material. The acrosome reaction is triggered when the sperm binds to the zona pellucida, a glycoprotein layer surrounding the egg, initiating a cascade of events that ultimately leads to fertilization.
Ampulla: In the context of anatomy and physiology, an ampulla is a dilated portion of a tube or canal, such as those found in the reproductive and gastrointestinal tracts. Specifically, in the female reproductive system, it refers to the widened part of the fallopian tube where fertilization often occurs.
Ampulla: The ampulla is a small, dilated portion of a duct or tube in the body, particularly in the context of the female reproductive system and the process of fertilization. It serves as a key anatomical structure in these physiological processes.
Blastocyst: A blastocyst is an early stage embryo, approximately five to six days after fertilization, consisting of a sphere made up of an outer layer of cells (trophectoderm), a fluid-filled cavity (blastocoel), and a cluster of cells (inner cell mass) from which the embryo develops. It is the structure that implants into the uterine wall during the process of embryonic development.
Blastocyst: The blastocyst is a critical stage in early embryonic development, formed after the fertilized egg undergoes several cell divisions. It is a hollow, fluid-filled structure that contains an inner cell mass, which will eventually form the embryo, and an outer layer of cells called the trophoblast, which will form the placenta.
Bulbourethral glands: Bulbourethral glands, also known as Cowper's glands, are small glands located on either side of the urethra in males that produce a pre-ejaculate fluid. This fluid helps to lubricate and neutralize the acidity of the urethra before ejaculation occurs.
Bulbourethral Glands: The bulbourethral glands, also known as Cowper's glands, are a pair of small, pea-sized exocrine glands located beneath the prostate gland in the male reproductive system. These glands play a crucial role in the anatomy, physiology, and development of the male reproductive system, as well as in the process of fertilization.
Capacitation: Capacitation is a physiological process that spermatozoa undergo to gain the ability to penetrate and fertilize an egg. This process involves the removal of glycoproteins and other molecules from the surface of the sperm, enhancing its motility and changing its membrane to facilitate egg fusion.
Capacitation: Capacitation is a series of biochemical changes that occur in the sperm cell, preparing it for the process of fertilization. This process takes place within the female reproductive tract and is essential for the sperm to acquire the ability to fertilize the egg.
Cervical Mucus: Cervical mucus is a fluid secreted by the cervix, the lower part of the uterus, that plays a crucial role in the process of fertilization. This mucus undergoes cyclic changes throughout the menstrual cycle, providing an optimal environment for sperm transport and facilitating the journey of the sperm to the egg.
Chemoattractants: Chemoattractants are chemical signals that guide the movement and migration of cells, such as sperm cells, immune cells, and other specialized cells, towards their target. These substances create a chemical gradient that cells can detect and follow to reach their destination.
Corona radiata: The corona radiata is a layer of cells that surround the egg (oocyte) in mammals, providing essential support and nutrients to the developing egg. This layer remains attached to the oocyte even after ovulation and must be penetrated by sperm for fertilization to occur.
Cortical granules: Cortical granules are specialized secretory vesicles located in the oocyte's cortex that play a crucial role in fertilization. Upon fertilization, these granules undergo exocytosis, releasing their contents into the space between the oocyte membrane and the zona pellucida, which helps to prevent polyspermy and initiates changes in the egg's environment that are vital for successful embryo development.
Cortical reaction: The cortical reaction is a process that occurs immediately after a sperm cell successfully fuses with an egg's plasma membrane, during which the egg releases enzymes that prevent any other sperm from binding to the egg. This ensures that only one sperm can fertilize the egg, maintaining the correct number of chromosomes for human development.
Cortical Reaction: The cortical reaction is a rapid series of events that occurs in the egg cell immediately after fertilization. It is a critical process that helps prevent polyspermy, ensuring that only a single sperm fertilizes the egg and initiating the development of the zygote.
Embryo: An embryo is the early stage of a developing organism, formed from a fertilized egg and consisting of rapidly dividing cells. It is a crucial phase in the process of growth and development, particularly in the context of human reproduction and pregnancy.
Embryonic Development of the Axial Skeleton: The embryonic development of the axial skeleton is the process during which the central part of the skeleton, comprising the skull, vertebral column, ribs, and sternum, forms from specialized cells in an embryo. This intricate process involves the differentiation and organization of cells into specific structures that provide support and protection for the body's vital organs.
Fallopian Tubes: The fallopian tubes, also known as the uterine tubes or oviducts, are a pair of hollow, muscular structures that extend from the upper corners of the uterus to the ovaries. They play a crucial role in the female reproductive system by facilitating the transport of the egg from the ovary to the uterus, as well as providing the site for fertilization.
Fertilin: Fertilin is a protein complex found on the surface of mammalian sperm that plays a crucial role in the process of fertilization. It acts as a receptor that allows the sperm to recognize and bind to the egg, initiating the fusion of the two cells and the subsequent development of a new organism.
Fertilization Window: The fertilization window refers to the limited timeframe during a woman's menstrual cycle when she is most likely to become pregnant. This critical period is when the ovum (egg) is released from the ovary and is available for fertilization by a sperm cell.
Gametes: Gametes are the reproductive cells that fuse during fertilization to form a new organism. They are the specialized haploid cells produced by the male and female reproductive organs that contain half the normal number of chromosomes.
Glycoprotein: A glycoprotein is a macromolecule composed of a protein covalently bonded to one or more carbohydrate (sugar) chains. These complex molecules play crucial roles in various biological processes, including cell-cell interactions, immune system function, and fertilization.
Hyaluronidase: Hyaluronidase is an enzyme that breaks down hyaluronic acid, a key component of the extracellular matrix, which plays an essential role in tissue hydration and structure. This enzyme is primarily found in the body during fertilization, where it facilitates sperm penetration into the egg by degrading the hyaluronic acid in the zona pellucida, allowing sperm to reach and fertilize the oocyte more easily.
Implantation: Implantation is the process by which a fertilized egg (embryo) attaches itself to the wall of the uterus to begin pregnancy. This critical step occurs about one week after fertilization and marks the start of the embryo's growth within the uterine environment.
Implantation: Implantation is the process by which a fertilized egg (zygote) attaches to the wall of the uterus, marking the beginning of pregnancy. This critical event occurs about 6-12 days after fertilization and is a key step in the embryonic development process.
In Vitro Fertilization: In vitro fertilization (IVF) is a process where an egg is fertilized by a sperm outside of the body, in a laboratory setting. This technique is used to assist with fertility issues and help individuals or couples conceive a child when other methods have been unsuccessful.
Intracytoplasmic Sperm Injection: Intracytoplasmic sperm injection (ICSI) is a specialized assisted reproductive technology (ART) procedure in which a single sperm cell is directly injected into the cytoplasm of a mature egg (oocyte) to facilitate fertilization. This technique is used to overcome male infertility factors and improve the chances of successful in vitro fertilization (IVF).
Meiosis: Meiosis is a specialized cell division process that occurs in sexually reproducing organisms, resulting in the production of haploid gametes (such as sperm and eggs) from diploid parent cells. This critical process ensures genetic diversity and enables the fusion of two haploid cells during fertilization to restore the diploid state.
Oogonium: An oogonium is a type of cell found in the female reproductive system of many animals, including humans. It is the precursor to the egg cell, or oocyte, which is essential for the process of fertilization.
Ovulation: Ovulation is the process during the menstrual cycle when a mature ovarian follicle releases an egg (oocyte) into the fallopian tube, making it available for fertilization. This event is crucial for reproduction, as it marks the optimal time for sperm to meet the egg, leading to potential conception.
Perivitelline Space: The perivitelline space is the fluid-filled area between the cell membrane of an egg (oocyte) and the innermost layer of the egg's surrounding envelope, known as the zona pellucida. This space is a crucial feature in the process of fertilization.
Polyspermy: Polyspermy occurs when multiple sperm cells fertilize an egg cell, which usually leads to an unviable zygote due to the excess genetic material. This event is prevented in normal fertilization by rapid changes in the egg’s membrane after the first sperm enters.
Polyspermy: Polyspermy is a condition that occurs during fertilization when more than one sperm cell penetrates and enters the egg. This abnormal event can have severe consequences for the resulting embryo and is typically prevented by various mechanisms in the egg to block additional sperm from entering.
Pronuclei: Pronuclei are the nuclei of the sperm and egg cells that fuse during fertilization, combining the genetic material from both parents to form the first cell of a new organism.
Seminal Vesicles: The seminal vesicles are a pair of glandular pouches located behind the bladder in the male reproductive system. They contribute a significant portion of the fluid that makes up semen, the liquid that carries sperm during ejaculation.
Sperm: Sperm are the male reproductive cells that are responsible for fertilizing the female egg during the process of conception. They are produced in the testes and play a crucial role in the reproductive cycle.
Spermatogonium: A spermatogonium is a type of cell found in the seminiferous tubules of the testes. It is the precursor cell that undergoes a series of cell divisions and transformations to ultimately produce mature sperm cells through the process of spermatogenesis.
Syngamy: Syngamy is the fusion of the male and female gametes, or sex cells, during fertilization. It is a critical step in the process of sexual reproduction, where the genetic material from the sperm and egg cells combine to form a new, genetically unique individual.
Zona pellucida: The zona pellucida is a thick, glycoprotein layer surrounding the plasma membrane of mammalian oocytes. It plays a crucial role in fertilization by allowing only sperm of the same species to bind and facilitates sperm penetration during the fertilization process.
Zona Pellucida: The zona pellucida is a glycoprotein layer that surrounds the oocyte (egg cell) in mammals. It plays a crucial role in the process of fertilization and early embryonic development.
ZP3: ZP3 (Zona Pellucida Glycoprotein 3) is a glycoprotein that is a major component of the zona pellucida, the extracellular matrix surrounding the mammalian oocyte (egg cell). It plays a crucial role in the process of fertilization by serving as a receptor for the sperm during the initial stages of the fertilization process.
Zygote: A zygote is the initial cell formed when two gamete cells (sperm and egg) are joined through the process of fertilization. It contains all the genetic information necessary to form a new individual.
Zygote: A zygote is the initial cell formed when a sperm cell successfully fertilizes an egg cell. It is the earliest stage of embryonic development and marks the beginning of a new human life.