Embryology

1. Embryology Chapter 28

2. Stages of Development • Pregnancy is fertilization to birth • Gestation period is last menses to birth (~280 days) • Fertilization when sperm enters 2° oocyte  zygote • Conceptus is a developing offspring • Embryo till 8th week • Fetus is 9th week to birth • Infant once birthed

3. Accomplishing Fertilization • Viability of gametes • Egg 12 – 24 hours postovulation • Sperm 24- 72 hours postejaculation • Sperm must survive to meet 2° oocyte • Phagocytized in uterus • Leak out vagina • Destruction by acidic environment • May ‘sniff out’ • Sperm require capacitation • Weakened acrosome membranes and enhanced mobility • Female reproductive tract secretions contribute

4. Accomplishing Fertilization (cont.) • Sperm penetrates 2° oocyte • Weave through corona radiata • Multiple sperm must digest zonapellucida (acrosomal reaction) • PM’s of sperm and ova 2° oocyte fuse creating Ca2+ surge • Prepares 2° oocyte for cell division • Cortical reaction to prevent polyspermy • Ova granules release enzymes extraceullularly • Destroy sperm receptors and cleave attached sperm

5. Accomplishing Fertilization (cont.) • 2° oocyte completes meiosis II • Ova and 2nd polar body formed • Pronucleiexocytically release chromosomes to fuse = actual fertilization • Diploid cell, the 2° oocyte, completes 1st mitotic division

6. Zygote to Blastocyst • Zygote undergoes cleavage • Rapid mitotic division w/o growth • High SA to volume ratio • 2  4  8  16 (morula)  etc • Blastocyst after ~ 3 -4 days • Morula hollows out & released from zonapellucida • Outer single cell layer are trophoblasts • Contribute to placenta, secrete hCG  signals corpus luteum • Immunoprotection from mother’s cells • Inner cell cluster is inner cell mass • Becomes embryonic disc (embryo) • Forms 3 of 4 extraembryonic membranes http://www.utm.utoronto.ca/~w3bio380/lecture10.htm

7. Implantation and Placentation • ~ 6 – 7 days after ovulation • Trophoblasts interact w/ endometrium • Proliferation of 2 trophoblast layers • Outer layer erodes endometrium to create ‘burrow’ • Endometrial cells engulf blastocyst • Inner layer interacts w/ inner cell mass forming chorion • Form chorionic villi to join endometrial blood vessels = placenta • Placenta provides nutrient, gas, and waste exchange • Formed by ~ 3rd month • Expands uterus and then compresses except at umbilical cord

8. Blastocyst to Gastrula • Inner cell mass of blastocyst becomes 2 layers • Epiblast forms amnion w/ fluid sac between hypo- • Eventually surrounds fetus in aqueous environment • Hypoblast forms yolk sac • Forms part of GI tract and first blood cells/vessels • Caudal end becomes allantois, base of umbilical cord • Embryonic disc between forms germ layers • Gastrulation when epiblast cells invaginate • 1st displace hypoblast = endoderm • 2nd displace laterally = mesoderm • Special aggregation forms notocord (primitive vertebrae) • Remaining cells = ectoderm • Figs 28.9 and 28.10

9. Organogenesis • Completed by ~ 8 weeks • Ectoderm • Neurulation forms brain and spinal cord • Neural plate, neural groove, neural folds, neural tubes, neural crest cells • Primary vesicles ~ 4 weeks; flexures to secondary vesicles ~ 8 weeks • Mesoderm • Somite partitions to vertebrae and ribs (sclero-), dorsal skin (derma-), and trunk muscles (myotome) • Intermediate mesoderm becomes gonads and kidneys • Lateral plate mesoderm becomes ventral skin, limb muscles, bones, and parietal serosa (somatic) as well as cardiovascular system and visceral serosa (splanchnic) • Endoderm • Folds on part of yolk sac and pushes into amniotic sac • Folds fuse forming GI tract • Respiratory tract and glands from out pocketing

10. Fetal Development Landmarks • Weeks 9 – 38 • Recognize general events • Table 28.1

11. Anatomical Pregnancy Changes • Reproductive organs and breasts enlarge w/ blood engorgement • Uterus grows to occupy majority of abdominopelvic cavity • Center of gravity changes • Lordosis and back aches common • Pelvic ligaments relax, widen, and increase flexibility • Waddling gait results

12. Physiological Pregnancy Changes • Digestive system • Morning sickness due to elevated progesterone and estrogen levels • Heartburn from displacement of stomach and esophagus • Constipation due to decreased peristalsis • Urinary system • Increased production b/c increased metabolic rates, fetal waste, and bladder compression • Respiratory system • Tidal volume and respiratory rate increases due to increased demand • Cardiovascular system • Body water and blood volume increase to accommodate fetus and protect at birth • Increased pressure, cardiac output and heart rate from above and to increase circulation • Pressure in lower limbs cause edemas

13. Parturition • Initiation of labor • Elevated estrogen • Stimulates oxytocin receptor production • Inhibits progesterone = Braxton Hicks contractions begin • Fetal cells produce oxytocin and stimulate placental prostaglandin production • Encourage uterine contractions • Increased stress stimulates hypothalamic release of oxytocin • Stages • Dilation: from labor initiation to cervix dilates to 10cm • Contractions descend, push head into cervix, water breaks • Expulsion: dilation to delivery • Crowning when head reaches vulva • Episiotomies common • Placental: delivery of placenta and fetal membranes

14. Lactation • Elevated placental estrogen, lactogen, and progesterone stimulate prolactin release • Oxytocin stimulates milk let down • Colostrum • Little lactose and no fat • Protein, vitamin A, minerals, and IgA rich • True milk • Response to suckling • Ovulation and menses inhibited or irregular initially