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Embryology
Chapter 28
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
Accomplishing Fertilization
• Viability of gametes
– Egg 12 – 24 hours postovulation
– Sperm 24- 72 hours postejaculation
• Sperm must survive to meet 2° oocyte
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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
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
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
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
http://www.utm.utoronto.ca/~w3bio380/lecture10.htm
– 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
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
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
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
Fetal Development Landmarks
• Weeks 9 – 38
• Recognize general events
– Table 28.1
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
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
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
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