Lecture 30 3/25/24 Flashcards
What are the two main roles of the placenta?
-transient organ of metabolic exchange
-transient endocrine organ
What are the two main components of the placenta?
-fetal component derived from chorion
-maternal component derived from modifications of uterine endometrium
What is the chorion?
outermost layer of placenta in direct contact with endometrium
What is the allantochorion?
fusion of allantois and chorion
What is the allantoic cavity?
fluid filled cavity containing liquid waste from the embryo that contributes to the umbilical cord
What is the yolk sac?
transient embryonic membrane the contributes to the umbilical cord
What is the amnion?
fluid filled inner layer that protects the embryo
Which structures give rise to the chorion and amnion?
-trophoblast
-primitive endoderm and mesoderm
What are the components of the blastocyst?
-inner cell mass
-trophoblast
-blastocoele
What happens to the yolk sac as the conceptus develops?
it regresses in size
What is the functional unit of the fetal placenta?
chorionic villus
What is the role of the chorionic villus?
to protrude away from the chorion and interdigitate with the uterine endometrium
What are the characteristics of diffuse placentas?
-many closely spaced villi distributed over the entire surface of the chorion
-uniform distribution
-pigs and mares
Why is the mare unique, despite being classified as having a diffuse placenta?
the chorionic villi are concentrated in specialized microzones known as microcotyledons
What are the characteristics of zonary placentas?
-band-like zone of chorionic villi
-prominent, broad transfer zone that allows for exchange
-pigmented zone consisting of small hematomas that allows for iron transport
-transparent zone with poor vascularity that allows for absorption of materials from uterine lumen
-dogs and cats
What are the characteristics of cotyledonary placentas?
-numerous button-like structures called cotyledons
-placentome consists of a fetal cotyledon and a maternal cotyledon from a caruncle
-ruminants
What are the characteristics of discoid placentas?
-form a regionalized disc
-one or two distinct adjacent discs
-primates and rodents
What are the characteristics of an epitheliochorial placenta?
-least intimate
-both endometrial epithelium and chorionic villi epithelium are intact
-six layers of tissue
-ruminants, sows, mares
How do ruminant epitheliochorial/syndesmochorial placentas differ?
-endometrial epithelium transiently erodes and regrows
-allows for intermittent exposure to maternal capillaries
-contain binucleate giant cells
What are the functions of the binucleate giant cells in ruminant placentas?
-transfer complex molecules from fetal to maternal placenta
-secrete placental lactogen and pregnancy specific protein B
-site of steroidogenesis
What are the characteristics of an endotheliochorial placenta?
-complete erosion of endometrial epithelium and underlying interstitium
-maternal capillaries are directly exposed to chorion epithelial cells
-five layers of tissue
-dogs and cats
What are the characteristics of a hemochorial placenta?
-chorionic epithelium in direct apposition to maternal pools of blood
-nutrients and gases exchanged directly from maternal blood
-three layers of tissue
-primates and rodents
Which methods of transport are utilized in placental exchange?
-simple diffusion
-facilitated transport
-active transport
-endocytosis
What is the major source of energy for the fetus?
glucose derived from maternal circulation
Which proteins are able to cross the placental barrier?
immunoglobulins
Which placenta types allow for the transfer of immunoglobulins?
-hemochorial
-endotheliochorial
Which compounds must the fetus synthesize from building blocks from the dam?
-proteins
-lipids
Which molecules can easily cross the placenta?
-large peptide hormones
-water-soluble vitamins
-nutrients
-toxic substances
-viruses
Why is progesterone important for early embryonic development?
-stimulates elevated secretion from endometrial glands
-inhibits myometrial contractions
What are the two sources of progesterone during pregnancy?
-CL first
-placenta later
When does the placenta produce estrogens?
second half of gestation
What are the characteristics of endometrial cups?
-found in mares
-trophoblastic and endometrial in origin
-produce equine chorionic gonadotropin/eCG
-sloughed into uterine lumen after day 60 of gestation
What are the roles of equine chorionic gonadotropin?
-provides a stimulus for maintenance of the primary corpus luteum
-responsible for controlling the formation and maintenance of supplementary corpora lutea
What are the characteristics of human chorionic gonadotropin?
-found in primates
-originates from trophoblastic cells
-secreted as soon as blastocyst hatches from zona pellucida
-maintains CL
-can cause ovulation in non-primate females
What are the characteristics of relaxin?
-originates from both ovary and placenta during parturition
-inhibits uterine contraction
-promotes rupture of fetal membranes
-relaxes cervix, vagina, and pelvic ligaments
What are the roles of placental lactogen/somatomammotropin?
-lactogenic activity/promotes mammary function
-somatotrophic action/promotes fetal growth
Why is the fetal hypothalamic-pituitary-adrenal axis obligatory for initiating parturition?
-fetus becomes stressed due to limited space in uterus
-stress causes fetal ant. pituitary to release ACTH
-ACTH stimulates fetal adrenal cortex to release corticoids
-corticoids decrease progesterone and remove myometrial “progesterone block,” allowing contractions
-corticoids increase estrogen and lead to increased repro tract secretions to lubricate birth canal
What is the importance of pressure on the cervix due to myometrial contractions?
-activates pressure-sensitive neurons that synapse with oxytocin-producing neurons in hypothalamus
-oxytocin facilitates myometrial contractility initiated by estradiol and PGF2a
What are the functions of estradiol in parturition?
-initiates secretory activity of repro tract/cervix to wash out cervical seal and lubricate birth canal
-stimulates myometrial contractions
What are the functions of PGF2a in parturition?
-abolish progesterone block
-stimulate myometrial contraction
What happens when the fetus becomes hypoxic in the birth canal?
-promotes fetal movement
-in turn, promotes further myometrial contraction
What are the three stages of labor?
1: initiation of myometrial contractions; fetus at cervix
2: active labor, expulsion of fetus
3: expulsion of fetal membranes
What are the characteristics of parturition stage 1?
-indistinct onset
-duration of 1-4 hours
-myometrial contractions increase
-fetus repositions
What are the clinical signs of parturition stage 1?
-sweating
-urination/defecation
-restlessness
-Flehmen response (mares)
What are the characteristics of parturition stage 2?
-dilation of cervix
-rupture of allantochorion and amnion at vulva
-abdominal contractions increase to maximum rate
-around 20 minutes
-abnormally long stage results in dystocia/difficult birth
What are the causes of dystocia?
-excessive fetal size
-failure of fetal rotation
-multiple births in monotocous species
What are the characteristics of parturition stage 3?
-requires chorionic villi to become dislodged from maternal side of placenta
-release is due to powerful vasoconstriction of arteries in the villi
How can pregnancy be diagnosed?
-palpation
-imaging
-endocrine testing
Why is it critical that the neonate ingest colostrum?
-contains high levels of immunoglobulins
-immunoglobulins are absorbed in gut mucosa and establish temporary passive immunity
Which type of placenta requires neonates to ingest colostrum in order to get immunoglobulins?
epitheliochorial
How does colostrum compare to normal milk?
-twice as much dry matter
-high lipid and protein
-rich in vitamins
-10x higher in immunoglobulins
