Fertilization and Zygote Development in Mammals.

Internal Fertilisation: In mammals, fertilisation occurs inside the female's body, typically within the fallopian tube.

Sperm Capacitation: Before fertilising the egg, sperm undergo capacitation within the female's reproductive system. This process prepares sperm for fertilisation by removing glycoproteins and plasma proteins that were originally deposited by the epididymis and seminal fluid.

Acrosomal Reaction: When sperm reaches the egg, the acrosome (capsule on the sperm head) releases enzymes (hyaluronidase and acrosin) that help break down barriers like the corona radiata and zona pellucida, allowing the sperm to enter the egg.

Prevention of Polyspermy: After sperm entry, cortical granules release enzymes that alter the egg's membrane, preventing other sperm from fertilising the same egg.

Restoration of Diploid Chromosome Number: Fertilisation restores the diploid number of chromosomes in the zygote, as meiosis reduces the chromosome number by half during gametogenesis.

Genetic Variation: Fertilisation combines the genetic material of two parents, leading to variation in offspring, which enhances survival through adaptation.

Initiates Life: Fertilisation marks the beginning of the development of a new organism, which progresses through stages from zygote to embryo and then to a fully developed organism.

Cleavage: The zygote divides via mitosis, forming a ball of cells called a morula, which moves through the oviduct.

Blastulation: The morula forms a hollow blastocyst with a blastocoel (fluid-filled cavity). The outer layer of cells forms the trophoblast, which is involved in implantation and secretes Human Chorionic Gonadotropin (HCG) to maintain the corpus luteum.

Implantation: The blastocyst embeds itself in the uterine wall, with the trophoblast cells growing into the endometrium.

Gastrulation: The blastocyst undergoes cell rearrangement to form three germ layers:

• Ectoderm: Develops into the nervous system, skin, and sensory organs.
• Mesoderm: Develops into bones, muscles, circulatory system, and reproductive organs.
• Endoderm: Develops into digestive, respiratory systems, and glands.

Chorion: Forms villi that invade the endometrium and provide nutrients to the embryo.

Amnion: Surrounds the embryo with amniotic fluid that provides shock absorption and protection.

Yolk Sac: In mammals, plays a minimal role compared to other species but is important for early nutrition in some organisms.

Allantois: Develops into a structure containing blood vessels and eventually contributes to the formation of the placenta.

Formation: The placenta forms by the 12th week of pregnancy from the fusion of the chorion and allantois.

Nutrient Exchange: The placenta allows the exchange of nutrients, gases (oxygen and carbon dioxide), and waste products (urea).

Gas Exchange: Oxygen diffuses from the mother to the fetus, while carbon dioxide moves in the opposite direction due to differences in oxygen concentration and the fetus's higher affinity for oxygen.

Excretory and Immune Functions: The placenta removes excretory products from the fetus and provides antibodies to help protect the fetus from diseases.

Hormone Production: The placenta acts as an endocrine gland, secreting hormones like oestrogen, progesterone, HCG, and human placental lactogen (HPL) to support pregnancy.