23 - Extra-embryonic membranes and placentation Flashcards
What is implantation/attachment
Joining of the extra-embryonic membranes to the uterine endometrium
What is placentation
organization and physical relationship of embryonic/fetal membranes to the uterine endometrium that provides a site of metabolic exchange between the dam and embryo/fetus
What is the placenta
Temporary organ of metabolic exchange between the embryo/fetus and the dam consisting of tissues of embryonic origin and maternal origin
Four steps in pre-attachment embryonic development**
- Development within the zona pellucida
- Hatching from the zona pellucida
- Maternal recognition of pregnancy
- Extra-embryonic membranes (preparation for placentation & attachment to endometrium)
How does development of extra-embryonic membranes occur
Following hatching, an “explosion” of embryonic tissue growth occurs to establish the extra-embryonic membranes prior to attachment
Slide 7 length in cattle, pigs
Formation of extra-embryonic membranes is necessary to…
allow the developing embryo to attach to the uterus
Name the extra-embryonic membranes
- yolk sac
- chorion
- amnion
- allantois
Three embryonic germ layers
Endoderm, mesoderm, ectoderm
What is the yolk sac*
“Transient” membrane that regresses as the conceptus develops. Formed from the primitive endoderm of the embryo. Contributes primordial germ cells & early RBC
What is the chorion*
Outermost membrane that develops villi (chorionic villi) which attach to the endometrium to form fetal sites of placental attachment. Formed from the trophoblast (ectoderm), primitive endoderm & mesoderm of the enmbryo.
What is the amnion*
Liquid filled membrane enclosing the fetus that serves to protect the embryo against mechanical damage and prevent tissue adhesions. Formed from the trophoblast (ectoderm), primitive endoderm & mesoderm of the embryo
What is the allantois*
Membrane that encloses a liquid waste storage reservoir for the developing embryo/fetus. Formed from the primitive gut of the developing embryo.
First steps in extra-embryonic membrane development (3)
ICM starts to differentiate
Primitive endoderm begins to form beneath the ICM & grows downward forming a lining on the inner surface of the trophoblast
Mesoderm begins to develop between the primitive endoderm and the embryo
Second steps in development of EE membranes
Yolk sac is formed once the primitive endoderm has completed its growth
Trophoblast is now referred to as the trophectoderm
Mesoderm continues to grow and extend away from the embryo between the trophectoderm and primitive endoderm of the newly formed yolk sac
Slides 10-16
Development of EE membranes, very long/complicated
Origin and function of the amnionic fluid
Origin: maternal circulation, secretions from fetal respiratory tract
Functions:
- protect fetus from mechanical damage
- prevents adhesions
- lubricates birth passage during parturition
Origin and functions of the allantoic fluid
Origin: fetal waste, secretions of allantoic membrane
Functions:
- positions allantochorion in contact with endometrium during attachment
- stores fetal waste
What is the allantochorion
Fetal contribution to the fetal/maternal interface of the placenta
What are chorionic villi
Small, finger-like projections on the surface of the allantochorion that serve as the interface between the embryonic/fetal and maternal systems
Placentas are classified according to…
the distribution of chorionic villi on their surface, as well as the number of tissue layers separating maternal and fetal blood
Different classifications of placentas **
Chorionic villi distribution:
- diffuse
- cotyledonary
- zonary
- discoid
Maternal-fetal layers:
- epitheliochorial (6)
- endotheliochorial (~5)
- hemochorical (3)
What is diffuse? Zonary?
Diffuse: uniform distribution of chorionic villi that cover the surface of the chorion (e.g. sows, mares)
Zonary: placenta has a band-like zona of chorionic microvilli (bitch)
What is discoid? Cotyledonary?
Discoid: placenta forms regionalized disc (e.g. rodents, primates)
Cotyledonary: placenta has numerous, discrete button-like structures referred to as cotyledons (cows, ewes)
What is epitheliochorial, endotheliochorial, and hemochorial? E.g.
Epi: 6 layers between fetal and maternal blood (e.g. sows, mares, ruminants)
Endo: 5 layers between fetal and maternal blood (bitches and queens)
Hemo: 3 layers between fetal and maternal blood (e.g. primates, rodents)
Slide 23
Classification of placentas**
Slides 24-29
Chorionic villi distribution on placentas
Six layers of the epitheliochorial
- Chorionic capillaries
- Chorionic interstitium
- Chorionic epithelium
- Endometrial epithelium
- Endometrial interstitium
- Endometrial capillaries
What is syndeschorial
Occasional contact of maternal capillaries with chorionic epithelium in ruminants because the endometrial epithelium periodically erodes and re-grows
What are binucleate giant cells? Roles?
Large cells with 2 nuclei formed in placental tissues of ruminants. Migrate from chorionic epithelium and invade endometrial epithelium.
Roles:
- transport complex molecules from the fetal to maternal placenta
- secretes steroids (progesterone)
Layers of endotheliochorial placenta
- Chorionic capillaries
- Chorionic interstitium
- Chorionic epithelium
- Endometrial interstitium
- Endometrial capillaries
Describe endotheliochorial placenta
Endometrial epithelium and interstitium are eroded
Direct exposure of the maternal capillaries to the chorionic epithelium
Layers of hemochorial placenta, describe it
- chorionic capillaries
- chorionic interstitium
- chorionic epithelium
Chorionic epithelium in direct contact with maternal blood
Nutrients and gases exchanged directly between maternal and fetal blood
Slide 34
Placental layers summary
Three mechanisms of maternal-fetal placental exchange
- Simple diffusion: gases and water move from high to low conc
- Facilitated diffusion: specific carrier proteins facilitate the diffusion of important metabolic substances such as glucose and aa
- Active transport: sodium, potassium and calcium moved across placenta by transport pumps
Metabolic exchange of glucose in placenta
- major source of energy for the fetus
- derived from maternal circulation
- glucose consumption is very high towards end of gestation and can drain the maternal system (some women get pregnancy diabetes, lack of glucose in late preg can cause abortion)
Metabolic exchange of proteins in placenta
- Maternal proteins generally do not cross the placenta
- Fetus uses maternal amino acids to synthesize proteins (cross in form of aa)
Metabolic exchange of lipids in placenta
- Lipids do not cross the placenta
- Placenta hydrolyses triglycerides and maternal phospholipids to be used by the fetus
Metabolic exchange of vitamins in placenta
- Fat soluble vitamins (D) do not cross placenta easily
- water soluble vitamins cross with relative ease (C, B)
Metabolic exchange of hormones in placenta
- large peptide hormones (LH, FSH) do not cross the placenta
- small molecular weight hormones can cross the placenta (steroids, thyroid hormones)
Metabolic exchange of viruses and bacteria in placenta
Many viruses & some bacteria can be transmitted from dam to fetus in utero
