Reproductive

• Prenatal period


  Period from fertilization to birth (~38weeks)
• Prenatal period composed of three shorter periods

  • Pre-embryonic period
  • Embryonic period
  • Fetal period
• Pre-embryonic period


  From zygote (fertilized oocyte) up to implantation of blastocyst in uterus (Two weeks post fertilization)
• Embryonic period


  Major organs appear (Week 3-8 post fertilization)
• Fetal period


  Growth and complexity continue to advance (Week 9-38)
• Zygote


  Fertilized oocyte
• Morula

  Ball of cells
• Blastocyst


  Hollow ball of cells composed of Embryoblast (cells that form the embryo) and Trophoblast (outer ring of cells that form part of the placenta)
• Fertilization


  1. Sperm and oocyte fuse to form new diploid cell (Zygote)

  2. Restores the diploid chromosome number

  3. Determines the sex of the organism

  4. Initiates cleavage
• Capacitation


  Conditioning undergone by sperm to become capable of fertilization
• Millions of sperm are deposited in the vagina during intercourse, but only a few hundred have a chance at fertilization</b>
• Only one sperm is able to fertilize the oocyte
• Fertilization Phases


  1. Corona radiata penetration

  2. Zona pellucida penetration

  3. Fusion of sperm and oocyte plasma membranes and fusion of sperm and ovum pronuclei
• Acrosome reaction


  Release of digestive enzymes from acrosomes to allow sperm to penetrate zona pellucida
• Polyspermy


  If two sperm enter the oocyte simultaneously, it is immediately fatal with 23 triplets of chromosomes
• Ovum


  Secondary oocyte completing second meiotic division
• Pronuclei


  Nucleus of sperm and ovum that fuse to make a diploid nucleus (Zygote)
• Cleavage


  Zygote undergoes mitosis to increase cell number but not overall size of structure
• Morula


  16-cell stage
• Blastocyst


  Hollow ball of cells containing a blastocyst cavity, with Trophoblast (outer ring of cells surrounding cavity that will form the chorion) and Embryoblast (packed cells within one side that will form the embryo proper)
• Pluripotent


  Cells that can develop into any tissue
• Implantation


  1. Blastocyst burrows into the endometrium, beginning by ~day 7

  2. Trophoblast subdividing into Cytotrophoblast (inner layer) and Syncytiotrophoblast (outer layer)

  3. By day 9, blastocyst completely burrowed into uterine wall
• Human chorionic gonadotropin (hCG)


  Produced by syncytiotrophoblast, signals reproductive system that implantation occurred, promotes maintenance of corpus luteum, detected in urine by end of 2nd week
• Levels of hCG are high for the first 3 months of pregnancy, then decline as the placenta takes over estrogen and progesterone production
• Yolk sac


  First extraembryonic membrane to develop, important site for early blood cell and blood vessel formation
• Amnion


  Eventually encloses entire embryo in fluid-filled sac (amniotic cavity), specialized to secrete amniotic fluid bathing the embryo
• Chorion


  Outermost extraembryonic membrane, cells blend with functional layer of endometrium to form the placenta, site of nutrient exchange between embryo and mother
• Placenta


  Site of exchange of nutrients, wastes, and respiratory gases between maternal and fetal blood, transmits maternal antibodies, produces estrogen and progesterone, maintains and builds the uterine lining
• Connecting stalk


  Connects early embryo to placenta, precursor to future umbilical cord
• Gastrulation


  Critical period of development, occurs during third week, epiblast forms three primary germ layers (ectoderm, mesoderm, endoderm) from which all body tissues develop
• Primitive streak


  Thin depression on surface of epiblast, where cells detach and migrate to form the three primary germ layers
• Embryonic disc


  Flattened, disc-shaped 3-week embryo that starts to fold on itself during late 3rd and 4th week
• Derivatives of the three primary germ layers

  • Ectoderm: epidermis, sense organs, pituitary gland, adrenal medulla, enamel of teeth, lens of eye, nervous tissue
  • Mesoderm: axial skeleton, muscle, cartilage, dermis, connective tissues, kidneys, ureters, internal reproductive organs, spleen, adrenal cortex, cardiovascular system, serous membranes, heart
  • Endoderm: linings of GI, respiratory, urinary, and reproductive tracts, tympanic cavity, auditory tube, liver, gallbladder, pancreas, palatine tonsils, thyroid and parathyroid glands, thymus
• Organogenesis


  Organ development, begins once germ layers have formed and folding is complete, from week 4 to birth
• Teratogens


  Substances causing birth defects or death, cause the most harm during the peak development period of an organ system
• Examples of teratogens include alcohol, tobacco, drugs, and some viruses like thalidomide and Zika virus
• Fetal period


  From beginning of 3rd month to birth, features maturation of tissues and organs, rapid growth of body
• Pregnancy divided into trimesters

  • First trimester (first 3 months)
  • Second trimester (months 4 to 6)
  • Third trimester (months 7 to 9)
• Estrogen and progesterone


  Produced by corpus luteum during first trimester, then mostly by placenta in 2nd and 3rd trimesters, high levels suppress FSH and LH secretion, facilitate uterine enlargement, mammary gland enlargement, and fetal growth
• Relaxin


  Secreted by corpus luteum and placenta, promotes blood vessel growth in uterus