Fertilisation and pregnancy are vital stages in human reproduction. This section explores the mechanisms of fertilisation, stages of early embryonic development, implantation, the placenta's role, and hormonal changes throughout pregnancy.
Fertilisation
Sperm and Egg Interaction
- Ovulation: The ovary releases a mature egg, which enters the fallopian tube, guided by fimbriae.
- Sperm Migration: Sperm travels up the female reproductive tract to meet the egg.
- Fertilisation: Sperm penetrates the egg's outer layers, resulting in the fusion of genetic material.
Formation of the Zygote and Genetic Combination
- Zygote Creation: The fusion of sperm and egg forms a zygote with 46 chromosomes.
- Genetic Variation: Genetic recombination ensures the unique genetic identity of each individual.
Early Embryonic Development
Cleavage and Morula Formation
- Rapid Division: The zygote undergoes quick mitotic divisions without increasing in size.
- Morula Formation: A solid ball of cells forms within 3-4 days, consisting of identical cells.
Formation of the Blastocyst
- Blastocyst Development: A hollow structure with an inner cell mass and outer trophoblast layer.
- Implantation Preparation: The blastocyst prepares to implant into the uterine wall.
Gastrulation and Germ Layer Formation
- Three Germ Layers: Ectoderm (skin, nervous system), mesoderm (muscles, skeleton), endoderm (internal organs).
- Organogenesis: The formation of organs begins from these primary germ layers.
Implantation
Attachment to the Uterus and Embedding
- Blastocyst Adherence: The blastocyst attaches to the endometrial lining.
- Embedding: It embeds into the endometrium, aided by enzymes that digest endometrial cells.
- Chronic Villi Formation: Projections from the trophoblast invade the endometrium, forming the basis for placental development.
Formation of the Placenta and Amniotic Sac
- Placenta Development: A complex organ connecting the mother and foetus for nutrient and waste exchange.
- Amniotic Sac Formation: The fluid-filled sac protects and cushions the embryo.
Role of the Placenta
Nutrient Exchange and Hormone Production
- Blood Barrier: The placenta separates maternal and foetal blood, facilitating selective exchange.
- Nutrient Supply and Waste Removal: Oxygen, nutrients, and antibodies are supplied; waste is removed.
- Hormone Synthesis: Progesterone and oestrogen support pregnancy.
Hormonal Changes During Pregnancy
Human Chorionic Gonadotropin (hCG)
- Early Role: Maintains the corpus luteum, ensuring continuous progesterone production.
- Detection in Pregnancy Tests: Its presence in the urine indicates pregnancy.
Progesterone and Oestrogen Increases
- Endometrial Maintenance: Progesterone keeps the endometrial lining thick.
- Breast Preparation: Oestrogen and progesterone prepare the breasts for lactation.
Other Hormones
- Oxytocin: Induces labour contractions.
- Relaxin: Softens pelvic ligaments for childbirth.
- Prolactin: Stimulates milk production post-delivery.
FAQ
Chorionic villi are finger-like projections from the trophoblast layer that invade the endometrium during implantation. They contribute to the formation of the placenta by extending into the maternal blood supply. These villi allow for efficient nutrient and gas exchange between the maternal and foetal blood, making them vital for supporting and nourishing the developing foetus.
The amniotic sac surrounds the developing foetus, filled with amniotic fluid. It provides a cushioning effect that protects the foetus from physical shocks and injuries. Additionally, the fluid-filled environment allows free movement, which is crucial for normal musculoskeletal development.
Relaxin hormone, produced by the ovaries and placenta, softens and loosens the ligaments and joints in the pelvis during pregnancy. By making these structures more flexible, relaxin facilitates the widening of the pelvis during childbirth, allowing the baby to pass through the birth canal more easily.
The early embryo's rapid cell divisions without increasing in size, known as cleavage, increase the surface-to-volume ratio, improving nutrient absorption and waste removal. These divisions also help differentiate cells quickly into various types for specialized functions, laying the groundwork for organ development.
The blastocyst adheres to the endometrial lining through the action of specific adhesive molecules and receptors on both the blastocyst's trophoblast cells and the endometrial cells. This attachment allows the blastocyst to align properly with the mother's endometrium, facilitating subsequent embedding and connection with the maternal blood supply.
Practice Questions
The process of fertilisation begins with ovulation, where a mature egg is released into the fallopian tube. Sperm, having travelled up the female reproductive tract, meets the egg in the fallopian tube. Upon interaction, specific enzymes in the sperm's head facilitate penetration through the egg's outer layers. The nuclei of the sperm and egg fuse, forming a diploid zygote with 46 chromosomes. This union combines genetic material from both parents, resulting in genetic recombination and unique genetic identity.
The placenta plays a pivotal role in pregnancy as the connection between the mother and foetus. It facilitates the exchange of nutrients, oxygen, and antibodies from the mother's blood to the foetus, while simultaneously removing waste products. The placenta's selective barrier ensures harmful substances are filtered out. Additionally, the placenta synthesises hormones like progesterone and oestrogen that maintain the pregnancy. Progesterone sustains the endometrial lining, while oestrogen prepares the breasts for lactation. Overall, the placenta's multifunctionality supports the foetus's growth and protects it throughout pregnancy.
