A&P: CH.28 [Pregnancy]

• pregnancy: events that occur from the time of fertilization until the infant is born
• conceptus: pregnant woman's developing offspring
• gestation period: last menstural period until birth, 280 days aprox.
• embryo: fertilization to week 8 of conceptus
• fetus: from week 9 through birth of conceptus
• oocyte is viable for 12-24 hours after ovulation while sperm is viable for 24-48 hours after ejaculation
• for successful fertilization to occur coitus must occur no more than two days before ovulation and no later than 24 hours after
• sperm must be capacitated which enhances their motility while their membranes become fragile for hydrolytic enzymes in acrosomes to be released
• capacitation prevents acrosomal enzymes for spilling everywhere in the male reproductive, only occurring while in the female reproductive as it's secretions help to break down sperm membrane
• after breaching corona, sperm head binds to receptor in zona pellucida which opens a calcium channel and the calcium increase trigger acrosomal reaction which digests holes on the zona pellucida
• a sperm that comes along after hundreds of sperm have undergone acrosomal reactions to expose the oocyte membrane is in the best position to be the fertilizing sperm
• when a sperm binds to oocyte sperm receptors, the oocyte sheds remaining sperm receptors on surface of vesicles that line plasma membrane
• zona reaction: waves of calcium are released by oocyte's endoplasmic reticulum to cytoplasm, activating prep for second meiotic division
• the cortical granules fuse with plasma membrane releasing contents into extracellular space. these spilled enzymes called zonal inhibiting proteins destroy zona pellucida sperm-binding receptors
• polyspermy: entry of several sperm into an egg
• sperm penetration to zygote formation

  1. sperm penetrates secondary oocyte and oocyte completes meiosis II to form ovum and second polar body
  2. sperm and ovum nuclei swell to become pronuclei
  3. pronuclei DNA replicate and they approach each other while mitotic spindle forms between them
  4. chromosomes mix and fertilization accomplished becoming zygote
• cleavage: fairly rapid period of mitotic division of zygote to produce small cells with high surface-volume ratio to enhance nutrient uptake and waste disposal while also being building blocks for embryo; produces blastocyst and implantation of blastocyst
• morula: collection of 16 or more cells, occurring 72 hours after fertilization
• embryo transport to uterus still ongoing during cleavage process
• blastocyst: fluid-filled hollow sphere made of a single layer of trophoblast cells and inner mass cells clinging to the inside
• after blastocyst formation, trophoblast cells start to take part in placenta formation while also secreting immunosuppressive effects that protect the trophoblast and embryo
• inner cell mas becomes embryonic disc, forming embryo proper and four extraembryonic membranes
• implantation: the process by which a fertilized egg attaches to the endometrium

  • controlled by surging levels of ovarian hormones
  • uterine capillaries become more permeable and leaky
• trophoblast cells proliferate and form two distinct layers during implantation

  1. cytotrophoblast: inner layers of cells
  2. synctiotrophoblast: cells in outer layer that lose their plasma membranes, invade and digest endometrium so blastocyst can burrow into lining
• implantation is complete by the twelfth day after ovulation
• trophoblast cells secrete human chorionic gonadotropin which acts on corpus luteum to stay until placenta can support developing fetus
• implantation site on uterus depends on presence of proper receptors and chemical signals
• placentation: formation of placenta from embryonic and maternal tissue

  1. Embryonic tissues: mesoderm cells develop from inner cell mass and line trophoblast to form chorion and chorionic villi
  2. Maternal tissues: decidua basalis develops blood filled lacunae
• chorionic villi

  • grow into blood-filled lacunae
  • vascularized by umbilical arteries and veins
  • lie immersed in maternal blood
• maternal and embryonic blood supplies do not intermix
• embryonic placental barriers include membranes of chorionic villi and endothelium of embryonic capillaries
• decidua capsulari: part of endometrium at uterine cavity face of implanted embryo
• placenta secretes human placental lactogen, human chorionic thyrotropin, and relaxin
• placenta fully formed and functional as nutritive, respiratory, excretory, and endocrine organ by end of the month
• if placental hormones are inadequate when hCG levels wane, endometrium degenerates and pregnancy spontaneously aborts
• germ layers

  • during implantation blastocyst starts converting to gastrula
  • inner cell mass develops into embryonic disc
  • three primary germ layers and extraembryonic membranes develop
• amnion:epiblast cells form transparent sac filled with amniotic fluid

  • provides buoyant environment, protecting the embryo
  • helps maintain constant homeostatic temp
  • allows freedom of movement while preventing parts from fusing together
  • comes from maternal blood and fetal urine
• Yolk sac: sac that hangs from ventral surface of embryo

  • forms part of digestive tube (primitive gut)
  • source of earliest blood cells and blood vessels
• allantois: small outpocketing at caudal end of yolk sac

  • structural base for umbilical cord
  • becomes part of urinary bladder
• chorion: helps form placenta

  • encloses embryonic body and all other membranes