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Placentation Embryonic development • Pre-implantation – Free-floating • Endogenous reserves • Nutrients from surrounding environment – Histotropic • Implantation and placentation – Formation of intimate but temporary relationship with uterus • Provision of nutrients • Protection • Placenta – Transient organ • Metabolic interchange between maternal and fetal systems • Endocrine organ – Production of steroids – Production of protein hormones – Composition • Chorion (fetal compartment) • Modified uterine endometrium (maternal compartment) • Contact between chorion and endometrium – Site of metabolic exchange – Hormone production Embryonic cellular differentiation • Blastcyst – Blastcoel • Fluid-filled cavity – Inner cell mass • Develops into fetus and part of placenta • Three distinct cell – Ectoderm – Mesoderm – Endoderm • Three embryonic cell layers – Ectoderm • Skin and hair • Nerve tissue • Part of chorion – Mesoderm • Muscles, bones, and organs • Part of chorion and allantois – Endoderm • Inner lining of the GI tract • Part of allantois and amnion Origin of placenta • Conceptus – Embryo – Extraembryonic menbrane • Extraembryonic membranes – Originate from trophoblasts – 3 compartments • Amnion (endoderm and ectoderm) • Chorion (ectoderm and endoderm) • Allantois (endoderm and mesoderm) Composition of placenta • Fetal component – Chorionic villus • Functional unit • Small, finger-like projections on the surface of chorion • Used for classification of placenta – Distribution Chorionic villi distribution • Diffuse – Villi distributed over the entire surface of the chorion • Pigs – Horse placenta • Specialized villi called microcotyledons (microzones) • Formation of endometrial cups (eCG secretion) • Cotyledonary placenta – Large discrete buttonlike structures • Cotyledons • Abundant blood supply – Formation of placentome • Cotyledons (fetal) • Caruncles (maternal) • Zonary placenta (dogs and cats) – Broad zone of villi • Exchange – Pigmented zone • Either end of the central region of the zone (blood clots) – Transparent zone • Distal ends • Discord – Humans and primates – One or two distinctive disks on the one end of the placenta Classification of placenta based on microscopic appearance • Number of placental layers separating the fetal blood from maternal circulation – Degree of intimacy – Prefix=maternal endometerium – Suffix=fetal membrane – Ranges anywhere from one to seven layers • Epitheliochorial placenta – Least amount of contact • Surface to surface contact • No invasion of the uterine endometrium by chorion • Pigs and horses • Epitheliochorial placenta – Some intimate between trophoblasts and endometrium • Ruminants – Syndesmochorial placenta • Formation of binucleated giant cells – Fusion of trophoblasts – Invasion and fusion with endometrium (trinucleated cells) • Endotheliochorial placenta – Complete erosion of endometrial epithelium and underlying interstitium – Exposure of maternal capillaries to the chorion • Embryos separated from actual maternal circulation • Hemochorial placenta – Primates • Actual contract between chorionic villi and maternal circulation – Fetal circulation separated from maternal circulation by three tissue layers • Direct exchange of materials from maternal circulation • Hemoendotherial placenta – Most intimate contact • Fetal circulation bathed in maternal blood Exchange of materials • Three methods – Diffusion • Small molecules • Gases – Facilitated diffusion • Glucose • Amino acids – Active transport • Ions (Na, K, Ca) • Acts as a barrier – Maternal proteins • Hormones • Exception – Antibodies/immunoglobulins in hemochorial and hemoendotherial placenta – Lipids and fat-soluble vitamins – Cannot prevent entrance of toxic materials and infectious materials • Alcohol, opium, and other drugs – Birth defects (teratogenic agents) • Virus and bacteria Placental lactogen • Some species – Type of placentation • Ruminants • Humans • Rodents • Produced by fused cells – Syncytiotropoblast – Binucleated/trinucleate d cells • Proteins related to pituitary GH and prolactin – Close to GH in humans – Close to prolactin in rodents – 50-50 in cows • Function – Regulation of metabolism • GH-like activity – Much weaker than pituitary GH – Development of mammary gland • Prolactin-like activity – Maintenance of CL function • Rodents during early stage of pregnancy • No known receptor(s) for placental lactogen – Interacts with GH receptor – Interacts with prolactin receptor • Pattern of secretion – Different between cattle and sheep • Degree of fusion between tropoblasts and endometrium • GH-Variant – Human only – Acts like GH • Tissue growth • Nutrient metabolism – Affects function of insulin • Pregnancy-induced diabetes Placental steroidogenesis • Cholesterol – Lipoproteins from circulation • No De Novo synthesis • Progesterone – Replace CL in some species • Maintenance of pregnancy • Precursor for fetal adrenal steroids • Estrogens – Limited production • Limited 17a-hydroxylase activity – Abundant in fetal adrenal gland – Androgens from fetal adrenal gland • Converted to estrogens in the placenta – Production of estriol rather than estradiol – Secretion of estrone • Majority of placental estrogen in some species
