Ch 28, L1

Pregnancy

• Coitus refers to sexual union.
• Pregnancy encompasses the events from fertilization until birth.
• Conceptus refers to the developing offspring.
• The zygote is the newly fertilized ovum.
• The embryo develops from fertilization to the end of the 8th week.
• The fetus develops from the 9th week until birth.

Fertilization

• Fertilization occurs when a sperm's chromosomes combine with those of an egg to form a zygote.
• Sperm must navigate transport barriers and undergo capacitation to fertilize an egg.
• A sperm cell must breach the corona radiata and zona pellucida to fertilize an egg
• Sperm must bind to receptors on the zona pellucida before penetration
• Calcium channels open on the oocyte plasma membrane
• Entry of the sperm cell triggers the oocyte to complete meiosis II and shed other membrane receptors.
• Sperm binding to a membrane receptor triggers oocyte membrane block.
• There will be no place for other sperm to bind
• The oocyte completes meiosis II upon entry of sperm.
• An ovum pronucleus and a second polar body are formed.
• The male and female pronuclei subsequently fuse to produce a zygote.

Embryonic Development

• Cleavage is a period of fairly rapid cell division after fertilization.
• Mitotic divisions of a zygote result in daughter cells.
• At 36 hours the first cleavage occurs resulting in 2 daughter cells.
• At 72 hours the Morula stage is reached with 16+ cells.
• By day 3 or 4, the blastocyst, an embryo of about 100 cells, reaches the uterus.
• The blastocyst is a fluid-filled ball of cells with:
  • Flattened trophoblast cells and
  • A small cluster of rounded cells, known as inner mass cells.
• Trophoblast cells have immunosuppressive factors and participate in the placenta formation.
• The inner mass cells become the embryonic disc, which forms the embryo plus three of the embryonic membranes.

Implantation

• The blastocyst floats around the uterus for 2-3 days.
• Implantation begins 6-7 days after ovulation.
• The trophoblast adheres to a suitable site in the endometrium using proper receptors and chemical signals.
• An inflammatory-like response occurs in the endometrium during implantation.
• Trophoblasts consist of an inner cytotrophoblast and an outer synctiotrophoblast layer.
• Synctiotrophoblasts invade and digest the uterine wall (endometrium), ultimately contributing to placenta formation.
• Implantation is typically completed by the 12th day after ovulation.
• Menstruation must be prevented during pregnancy
• Human chorionic gonadotropin (hCG) is secreted by trophoblast cells, and later by the chorion. -hCG prompts the corpus luteum to continue secreting progesterone and estrogen.
• hCG prevents menstruation and promotes placental development.

Placentation

• Placentation involves both embryonic and maternal tissues which contribute to the placenta.
• The placenta is fully formed by the third month of pregnancy.
• Maternal and embryonic blood normally does not intermix.
• Embryonic placental barriers include membranes of chorionic villi and endothelium of embryonic capillaries.
• The placenta functions as a nutritive, respiratory, excretory, and endocrine organ.
• Sugars, fats, and oxygen diffuse from the mother to the fetus.
• Urea and carbon dioxide diffuse from the fetus to the mother.
• Syncytiotrophoblast cells in the placenta are responsible for hormone manufacture.

Embryonic Development

• Chorion = outermost; formation of placenta.
• Amnion = innermost; sac filled with amniotic fluid.
• Allantios = umbilical cord/bladder.
• Yolk Sac = part of gut/early circulatory development.
• Development spans from fertilization to week 8
• Three germ layers are responsible for tissue and organ development
  • Ectoderm
  • Mesoderm
  • Endoderm
• Gastrulation sets the stage for organogenesis, which is the formation of body organs and systems.
• By 8 weeks organs have typically been formed
• The embryonic period comes to an end.