Fertilization and Pregnancy Notes

Fertilization Overview

• Involves both sperm and egg, each with distinct roles.

Sperm Roles

• Sperm are produced in the seminiferous tubules and stored in the epididymis.

• Initially at a low metabolic activity level.

• Small cells with minimal cytoplasm, lacking significant energy reserves.

• Require capacitation and maturation to become metabolically active.

Capacitation

• Occurs as sperm move through the male reproductive system.

• Enables the acrosome reaction and flagellum activation for swimming.

Maturation

• Completes in the female reproductive tract.

• Sperm are suspended in seminal fluids and mature at varying times post-ejaculation.

• Capacitation continues, and maturation begins in the female reproductive tract.

• Hundreds of millions of spermatozoa are introduced, but not all mature simultaneously.

Sperm Guidance

• Sperm lack independent egg detection capability.

• The egg emits a chemical gradient (chemotaxis) to guide sperm.

Preventing Polyspermy and Hybrid Formation

• Mechanisms ensure only one sperm fertilizes the egg (prevents polyspermy).

• Species-specific identification is required to avoid hybrid formation.

• The female immune system eliminates most sperm.

Oocyte Maturation

• The ovulated cell is a secondary oocyte, not yet a gamete.

• Meiosis II completion is contingent on fertilization.

• Only the sperm nucleus enters the egg.

Sequential Fertilization Process

Structures Surrounding the Egg

• Corona radiata

• Zona pellucida

• Vitelline membrane

Process

• Multiple sperm arrive, but only one fertilizes.

• The successful sperm contacts the zona pellucida, triggering reactions.

• Cortical and zonal reactions prevent other sperm from depositing nuclei.

• The egg completes meiosis II, producing the second polar body.

• The sperm nucleus (pronucleus) merges with the egg nucleus.

Syncarion Formation

• Male and female pronuclei join to form a single diploid nucleus.

• This fusion is termed synkaryon.

Key Terms

• Chemotaxis: Chemical gradient attracting sperm.

• Capacitation and Maturation: Processes enabling sperm motility and readiness.

• Acrosome Reaction: Release of enzymes to penetrate egg layers, facilitated by enzymes sometimes referred to as hydrolytic.

• Sperm Receptors: Species-specific receptors on the egg that must be activated.

• Cortical and Zonal Reactions: Mechanisms preventing polyspermy.

• Synkaryon: The fusion of sperm and egg nuclei.

Detailed Look at Egg Layers and Sperm

• The sperm contains an acrosome at its apical tip which releases proteolytic enzymes.

• Jelly coat: zona pellucida

• The egg cytoplasm contains the egg plasma membrane, which sits just inside the vitelline membrane.

• The vitelline membrane is attached to sperm receptors.

• The sperm penetrates through the plasma membrane and the vitelline membrane to deposit its nucleus in the egg cytoplasm.

Sperm Entry and Cortical Reaction

• Acrosome releases hydrolytic enzymes to digest a path through the zona pellucida.

• The sperm's cytoskeleton aids pronucleus entry.

• Cortical granules within the egg release substances upon sperm receptor activation.

• The cortical reaction makes the plasma membrane rigid, preventing further sperm entry.

Zonal Reaction

• The zona pellucida lifts away from the egg surface, creating a physical barrier.

• After these reactions, the sperm nucleus joins the egg nucleus, forming a zygote.

Post-Fertilization

• The zygote has a durable outer membrane.

• The size of the developing conceptus remains constant during initial cleavage.

Pregnancy and Development Phases

• Pregnancy occurs in three phases: pre-implantation, embryonic, and fetal development.

Cleavage

• The zygote undergoes cleavage (cell division) without size change.

• Divides into two, four, eight, and then sixteen cells.

Deuterostome Development

• Up to the four-cell stage, each cell is totipotent and can form a complete organism.

• At the eight-cell stage, cell fate becomes more restricted (indeterminate cleavage).

Morula and Blastocyst

• Cleavage leads to the formation of a morula (ball of cells).

• The morula develops into a blastocyst (approximately 100 cells).

Pre-Embryonic vs. Embryonic vs. Fetal Development

• Pre-embryonic development: Events prior to implantation.

• Embryonic development: From implantation until the placenta forms.

• Fetal development: From placenta formation to birth (parturition).

Blastocyst Structure

• The blastocyst contains the inner cell mass and the trophoblast.

Inner Cell Mass

• The inner cell mass becomes the future human (embryonic disc).

Trophoblast

• The trophoblast develops into the placenta.

Implantation

• The blastocyst finds a location within the uterus with sufficient conditions.

• The trophoblast digests the endometrium, allowing the blastocyst to burrow into the uterine wall.

• The endometrium grows over the blastocyst.

Extraembryonic Membranes

• Four membranes: allantois, amnion, chorion, and yolk sac.

• These create a supportive environment for the conceptus.

Amnion

• A thin, fluid-filled membrane surrounding the conceptus.

• Forms the amniotic cavity.

Chorion

• Tough, durable membrane surrounding the entire environment of the conceptus, including the placenta.

Allantois

• In other organisms, it serves as a repository for metabolic waste.

• In humans, it forms the umbilical cord, connecting the conceptus and placenta.

Yolk Sac

• In other organisms, it provides nutrients.

• In humans, it is present early in development, but the main source of nutrients is the maternal circulatory system via the placenta.

• Some cells migrate and become primordial germ cells in the gonads and parts of the digestive tract.

Placenta Functions

• The yolk sac, amnion, and chorion support early development.

Structure

• Incoming fetal blood via umbilical arteries forms chorionic villi.

• The chorion covers the surface of the placenta.

• The chorionic villi penetrate the maternal uterine wall.

Maternal and Fetal Exchange

• Fetal blood is contained within chorionic villi.

• Maternal arterioles create a sinus that surrounds the villi.

• Maternal venules take blood back into maternal circulation.

• Maternal and fetal blood exchange nutrients and waste via passive diffusion.

Placental Functions

• Digestive: Nutrients diffuse from maternal to fetal circulation.

• Respiratory: Gas exchange ($CO<em>2$ and $O</em>2$) occurs.

• Urinary (Excretory): Metabolic waste diffuses from fetal to maternal circulation.

• Endocrine: Hormone production.

Placental Endocrine Function

• Placenta produces hormones to maintain pregnancy.

Hormones Produced

• Initially, the corpus luteum provides estrogen and progesterone, maintained by hCG from the chorion.

• The placenta takes over estrogen and progesterone production, reducing the need for hCG.

• Human Chorionic Gonadotropin (hCG): Provided by the chorion of the conceptus.

• Human Chorionic Somatomammotropin (hCS): Stimulates mammary tissue maturation.

• Human Chorionic Thyrotropin (hCT): Increases maternal metabolic activity.

• Human Placental Lactogen (hPL): Inhibits maternal glucose use, increasing fetal glucose availability (may cause gestational diabetes).

• Relaxin: Causes connective tissues in joints to loosen, increasing pelvic floor dimensions.

Timeline of Fetal Development

• Embryonic development ends around eight weeks, transitioning to fetal development.