Placental Mammals

Early Stages in the Oviduct

• Fertilization:  Ampulla of oviduct

• Cleavage and morula formation in oviduct

• Blastocyst implants in the uterus

Fertilization

• Acrosome reaction exposes IZUMO1

• IZUMO1 binds to egg specific protein JUNO

• Sperm carries phospholipase C to activate egg

• In primates, also carries centrioles

• Activates Ca++ oscillations

• Cortical reaction

• Meiosis resumes, forming 2^nd polar body

Cleavage

• Fertilization leads to 2nd
polar body formation
• Holoblastic cleavage
• Rotational

Compaction

• After 3rd division
• Cells upregulate E-cadherin
• Tight junctions form between
outside cells
• Compact together

Morula

• 16 cell state
• Inner cell mass: will become
embryo
• Outer cells: trophoblast, will
become chorion

Cavitation

• Trophoblast cells pump sodium
ions into center of morula
• Stimulated by oviduct cells
• Uses Na+/K+ and Na+/H+ pumps
• Draws water in, creating
blastocoel
• Embryonic and abembryonic
poles

Ectopic Pregnancy

 Occurs when blastocyst implants in
an oviduct prior to reaching the
uterus
 Zona pellucida usually prevents
adherence to the oviduct
 Fetus can grow to around 4-8 weeks
old
 Can cause hemorrhaging in mother
 Fetal growth must be stopped, and
fetus reabsorbed or removed

what causes Ectopic Pregnancy

• Usually caused by damage to oviduct or abnormal development of embryo

• Pelvic inflammatory disease, smoking, previous ectopic pregnancy

• Hormonal imbalances 

• Treatment

  • Monitoring

  • Methotrexate

  • Surgery

Escape from the Zona Pellucida

• Hatching

• Trophoblast cells secrete lysozyme and proteases to break hole in zona pellucida

• Caught by endometrial cells in uterus

• L-selectin binds integrins on endometrium

• Proteases digest ECM of endometrium

• Bury blastocyst in uterine wall

Decidual Reaction

• Endometrium of uterus

• Response to progesterone

• Differentiation into decidual cells

• Uterine wall becomes more vascularized 

• Angiogenesis

Development of Extra-Embryonic Tissues

• Allowed development of organisms away from water

• In mammals, allowed development of organisms inside mother

• Viviparous:  development inside of mother

Amnion

• Lines amniotic cavity, is filled with amniotic fluid

• Cushions the fetus

• Helps with movement and lung development

Yolk sac

• Early source of nutrients, gas exchange

• 1^st place that red blood cells form

Chorion

• Makes human chorionic gonadotropin (hCG), which supports pregnancy for the first three months

• Forms structures that will become part of placenta

Formation of Extra-Embryonic Membranes

• Immediately prior to gastrulation

• Hypoblast cells delaminates into uterine tissue

• Syncytiotrophoblast cells proliferate into uterine tissue

• Cytotrophoblast cells move into uterine tissue

• Coagulation plug forms

• Amnionic cavity forms

Gastrulation

• Starts with epiblast cells

• Primitive streak and primitive groove form, starting from caudal end

• Primitive node= organizer

• Cells invaginate through the primitive groove

• Epiblast cells displace hypoblast

• Form definitive endoderm of foregut

• Mesoderm forms between ectoderm and endoderm

• Streak retreats

Notochord Formation

• Neural tube develops: becomes brain and spinal cord

• Pharyngeal arches develop: become head and neck

• Somites (segments develop): bone, muscle, skin

Node

• Prechordal mesoderm, notochord, and gut endoderm

  • Secretes chordin and noggin:  Inhibit BMP

  • Expresses Dickkopf for head formation

AVE

• Head specifier

  • Migrating endoderm cells

  • Secretes Lefty and Cerebus:  Inhibit Nodal

  • Expresses Otx2 and Dickkopt:  Inhibit Wnt