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MCB 135E Discussion September 26, 2005 Mid-Term I Review Monday October 3rd 5-7 pm in 105 Northgate Topic List • • • • Early Embryonic Stages Placenta Fetal Circulation Nervous System Development Terminology • Germ Layer Theory – Embryo’s Method of sorting out its parts • Ectoderm – Outer covering of embryo • Entoderm (Endoderm) – Lies under the ectoderm and forms lining of the primitive gut cavity • Mesoderm – Develops between ectoderm and entoderm • Gastrulation – The process by which germ layers come to occupy their characteristic positions • Embryonic Stages – Phases of gastrulation; stages of organ and tissue differentiation Embryonic Stages • Early Gastrulation – Occurs at implantation when cells on ICM arrange into the entoderm – Remaining cells that are neither trophoblast or entoderm become a plate containing the progenitors of future ectodermal and mesodermal cells – Referred to as 2-layered embryo and characterized by embryonic disc Embryonic Stages • Late Gastrulation (second phase) – Segregation of mesoderm and notochord occurs • Notochord will for primitive vertebral column – Starts by end of second week and finishes by end of third week – 2-layered embryo becomes 3-layered embryo – Primitive streak appears upon upper surface of embryonic disc over which mesoderm spreads – At conclusion of this process, upper layer of embryonic disc becomes ectoderm Germ Layers and Their Systems • Ectoderm – – – – – – – Epidermis and lining cells of glands Appendages of skin Nervous system Posterior Pituitary Chromafin organs - adrenal medulla Anterior Pituitary Some Epithelium • Entoderm – – – – Epithelial lining of alimentary canal Lining cells of glands that open to alimentary canal Epithelium of most of the urinary bladder and urethra Epithelium of prostate • Mesoderm – – – – – – Remaining organs and tissues not made by Ectoderm or Entoderm Connective tissue Teeth Musculature Blood Adrenal Cortex Terminology • Placenta – Maternal-Fetal Membrane • Villus – small vascular process or protrusion • Sinus – recess, cavity or hollow space • Cotyledon – any one of the subdivisions of the uterine surface of a discoidal placenta Placenta Functions • • • • • Gas Exchange Nutrient Delivery Antibody Delivery Removal of Waste Secretion of hormones Placental Membranes • Fetal 1. Yolk Sac – – Center of blood formation in early embryonal life Facilitates transfer of nutrients from developing trophoblast to embryo 2. Allantois – – – Blood vessels develop around allantoic tube Tube eventually fuses with chorion By 6th week – 1 umbilical vein and 2 umbilical arteries 3. Amnion – – Membranous sac that surrounds embryo Fluid fills amnion that has protective role throughout pregnancy Placental Membranes • Maternal 1. Deciduas – Highly modified uterine endometrium 2. Basalis – Portion between ovum and uterine wall 3. Capsularis – Region of decidua where ovum is embedded 4. Parietalis – Lines remainder of uterus Hormones of Pregnancy • Endocrine Overview • Hormones and their functions • Hormone Synthesis Functions • Deliver Oxygen-rich blood and nutrients from placenta to fetus • Remove Carbon dioxide and metabolic waste from fetus • Umbilical Vein – PlacentaFetus • Umbilical Arteries – Fetus Placenta Challenges and Adaptations • Challenge – Oxygen and Nutrients are less in umbilical vein than in adult arteries • Adaptations – Establish priorities – Establish specific structures to supply priorities – Embryonal Hemoglobin Specific Structures 1. Ductus Venosus • Shunts Umbilical Vein to Inferior Vena Cava 2. Foramen Ovale • • Shunts between atria Establishes high pressure in right side instead of left 3. Ductus Arteriosus • Shunts pulmonary artery to aorta Nervous System • Major Functions – Communication with external/internal environment – Regulation of… • Major Components – – – – – – Neurons Neuroglia Mylenated nerve fibers Microglia Ground Substance Blood Vessels and CSF Neurulation • N.S. – Arises from ectoderm on dorsal portion of embryo • 3-4 Weeks – Cells proliferate along middle plate • 5-6 Weeks – Plate folds to form neural groove • 6-7 Weeks – Groove closes into neural tube – Brain develops from anterior portion – Spinal cord develops from the posterior portion Neural Epithelium • Neuroblast – Neuron • Spongioblast – Migratory Spongioblast • Oligodendria • Astrocytes – Astrocytes – Ependyma Energy Sources • Carbohydrates – Primarily maternal glucose – Stored as glycogen • Under influence of glucocorticoids • In fetus – Insulin levels high – Insulin sensitivity high – Hypoglycemic • Anaerobic Glycolysis – Glyceraldehyde-PDehydrogenase • Glycolitic enzyme • High in postnatal brain • During same period the oxidative enzyme – Succinic Dehydrogenase – Much lower Thyroid Hormone • Functions – Promotion of body growth – Development of CNS through: • Promotion of neorogenesis • Promotion of myelination • Promotion of brain metabolism – Stimulates oxygen consumption in all cells • Abnormalities – Hypothyroidism • Cretinism • Short stature, low metabolic rate, skin changes • Treatable if given Thyroxine at an early age Thyroid Hormones Functional Development • Neuronal Connections • Differential Development of N.S. – Neurotransmitter activity in different brain regions • Perinatal Behavior – Reflexes • Respiratory (17-24 weeks) • Gastrointestinal (24th week) • Startle (Presence of excessive activity after birth is an indicator of delayed development of certain brain centers) • Suckling - Postnatal • Education – Better educated appear to live longer with less disability – Several pieces of evidence discussed for this in class