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 button- like 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 17  -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