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/

trinucleated 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