Fertilization and Sperm Anatomy

• Fertilization is the union of a sperm and an egg, forming a zygote, which is the first cell of a new individual.

Parts of a Sperm Cell

• Consists of a tail (flagellum) that propels the sperm toward the egg.
• The middle piece contains mitochondria.
• The head contains the nucleus capped by an acrosome, which contains digestive enzymes.
• Only the sperm's nucleus contributes to the zygote; the zygote receives cytoplasm and organelles from the mother.

Steps of Fertilization

• The egg is surrounded by the zona pellucida and layers of follicular cells called the corona radiata.
• Several sperm cells will penetrate the corona radiata, though only one will enter the egg.
• The acrosome releases digestive enzymes to forge a pathway through the zona pellucida.
• The plasma membranes of the egg and sperm fuse, and the entire sperm enters the egg.
• The nuclei of the egg and sperm then fuse together.
• Polyspermy prevention occurs when a sperm contacts the egg, the egg's plasma membrane depolarizes to prevent binding of other sperm.
• Cortical granules release enzymes that change the zona pellucida into a fertilization membrane.
• This modification prevents sperms from binding to the zona surface as well as penetrating the zona matrix, thus preventing fusion with the egg.

Processes of Development

• Cleavage occurs immediately after fertilization, with the zygote dividing by mitosis, and cells not increasing in size.
• Growth takes place during embryonic development, where cell division is accompanied by increasing cell size.
• Morphogenesis is when the embryo changes shape.

Stages of Development

• Pre-embryonic development occurs during week 1.
• Embryonic development takes place from weeks 2 to 8.

Embryonic Development: Second Week

• Implantation occurs at the end of the first week, resulting in clinical pregnancy.
• Ectopic pregnancy involves implantation outside the uterus, typically in the uterine tubes, posing health risks.
• During implantation, the chorion secretes human chorionic gonadotropin (HCG).
• HCG secretion is the basis for pregnancy tests, and it also secretes progesterone to prevent menstruation.
• Gastrulation is when the inner cell mass becomes the embryonic disk.
• Cells develop in primary germ layers which include the ectoderm, mesoderm, and endoderm.

Primary Germ Layers

• Ectoderm (outer layer) forms the epidermis of the skin, epithelial lining of the oral cavity and rectum, and the nervous system.
• Mesoderm (middle layer) gives rise to the skeleton, muscular system, dermis of the skin, cardiovascular system, urinary system, reproductive system, and outer layers of the respiratory and digestive systems.
• Endoderm (inner layer) develops into the epithelial lining of the digestive and respiratory tracts, associated glands, and the epithelial lining of the urinary bladder.

Fourth and Fifth Weeks

• The future umbilical cord connects the embryo to the chorion via chorionic villi.
• The umbilical cord forms, connecting the developing embryo to the placenta.
• Limb buds appear and develop into arms and legs.
• The head enlarges, and eyes, ears, and nose become distinguishable.

Sixth Through Eighth Weeks

• The embryo begins to appear human
• The nervous system develops to allow for reflex actions.
• By the end of this period, the embryo measures about 38 mm (1.5 in.) long.
• All organ systems have been established.

Fetal Development

• This stage spans from the third to the ninth months.
• The fetus takes on a human appearance, increasing in size and gaining weight.
• The fetus is susceptible to environmental factors like chemicals and radiation.

Birth

• The uterus contracts throughout pregnancy
• Closer to the pregnancy's end, contractions strengthen and become more frequent.
• Braxton Hicks contractions are known as false labor.
• A positive feedback mechanism occurs during labor where uterine contractions are induced by the stretching of the cervix.
• Stretching of the cervix causes the release of oxytocin from the posterior pituitary gland.
• Oxytocin stimulates the uterine muscles to contract, pushing the fetus downwards and stretching the cervix further which stimulates the release of additional oxytocin, thus repeating the cycle until birth.