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 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 zona pellucida (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
Pronuclei exocytically release chromosomes to fuse = actual fertilization
Diploid cell, the 2° oocyte, completes 1st mitotic division

6. Zygote to Blastocyst Zygote undergoes cleavage
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 zona pellucida
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

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 Mesoderm Endoderm
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 Stages 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