Midterm 3 - Lecture 24 Flashcards
Implantation/Attachment
joining of the extra-embryonic membranes to the uterine endometrium
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
Placenta
temporary organ of metabolic exchange btw the embryo/fetus and the dam consisting of tissues of embryonic origin and maternal origin
4 steps in pre-attachment embryonic development
- developing within the zona pellucida
- hatching from the zona pellucida
- maternal recognition of pregnancy
- extra-embryonic membranes
- preparation for placentation and attachment to endometrium
Following hatching what happens prior to attachment?
“explosion” of embryonic tissue growth occurs to establish the extra-embryonic membranes
Why is the formation of the extra-embryonic membranes necessary?
to allow the developing embryo to attach to the uterus
What are the 4 extra-embryonic membranes?
- yolk sac
- chorion
- amnion
- allantois
Yolk sac
- a transient membrane that regresses as the conceptus develops
- formed from the primitive endoderm of the embryo
- contributes primordial germ cells and early RBC
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 and mesoderm of the embryo
Amnion
- liquid filled membrane enclosing the fetus the serves to protect the embryo against mechanical damage and prevent tissue adhesions
- formed from the trophoblast (ectoderm), primitive endoderm and mesoderm of the embryo
Allantois
- membrane that encloses a liquid waster storage reservoir for the developing embryo/fetus
- formed from the primitive gut of the developing embryo
Origin of amnionic fluid
- maternal circulation
- secretions from fetal respiratory tract
Functions of amnionic fluid
- protects fetus from mechanical damage
- prevents adhesions
- lubricates birth passage during parturition
Origin of allantoic fluid
- fetal waste
- secretions of allantoic membrane
Functions of allantoic fluid
- positions allantochorion in contact with endometrium during attachment
- stores fetal waste
Allantochorion
the fetal contribution to the fetal/maternal interface of the placenta
Chorionic villi
small, finger-like projections on the surface of the allantochorion that serve as the interface btw the embryonic/fetal and maternal systems
How are placentas classified?
according to the distribution of chorionic villi on their surface AND the number of tissue layers separating maternal and fetal blood
What are the 4 different placenta chorionic villi distributions?
- diffuse
- cotyledonary
- zonary
- discoid
What are the 3 different placenta maternal-fetal layers?
- epitheliochorial (6)
- endotheliochorial (~5)
- hemochorial (3)
Diffuse
uniform distribution of chorionic villi that cover the surface of the chorion
- sows and mares
Zonary
placenta has a band-like zone of chorionic microvilli
- bitch
Discoid
placenta forms regionalized disc
- rodents and primates
Cotyledonary
placenta has numerous discrete button-like structures referred to as cotelydons
- cows and ewes
Epitheliochorial
- 6 layers btw fetal and maternal blood
- sows, mares, and ruminants
Endotheliochorial
- 5 layers between fetal and maternal blood
- bitches and queens
Hemochorial
- 3 layers between fetal and maternal blood
- primate and rodents
Pig placenta
diffuse; epitheliochorial
Mare placenta
diffuse; epitheliochorial
Cow, sheep, goat placenta
cotyledonary; epitheliochorial
Dog, cat placenta
zonary; endotheliochorial
Primates, rodents placenta
Discoid; endotheliochorial
Epitheliochorial placenta
- both endometrial epithelium (maternal side) and epithelium of the chorionic villi (fetal side) are intact
- in ruminants the endometrial epithelium periodically erodes and then re-grows allowing for occasional contact of maternal capillaries with chorionic epithelium = syndeschorial
Binucleate giant cells
- large cells with 2 nuclei formed in the placental tissues of ruminants
- migrate from the chorionic epithelium and invade the endometrial epithelium
- roles:
1. transport complex molecules from the fetal to maternal placenta
2. secretes steroids (P4)
Endotheliochorial placenta
- endometrial epithelium and interstitium are eroded
- direct exposure of the maternal capillaries to the chorionic epithelium
Hemochorial placenta
- chorionic epithelium is in direct contact with maternal blood
- nutrients and gases are exchanged directly btw maternal and fetal blood
3 mechanisms of placental exchange
- Simple diffusion
- gases and water move from high to low concentration - Facilitated diffusion
- specific carrier proteins facilitate the diffusion of important metabolic substances such as glucose and amino acids - Active transport
- sodium, potassium and calcium are moved across the placenta by transport pumps
What does not cross the placenta?
- Proteins
- fetus uses maternal AA to synthesize proteins - Lipids
- placenta hydrolyses triglycerides and maternal phospholipids to be used by the fetus - Vitamins
- fat soluble vitamins do not, water soluble vitamins cross with ease - Hormones
- large peptide hormones do not; small molecular wt hormones can
What does cross the placenta?
- glucose
- major source of energy for the fetus
- derived from maternal circulation
- glucose consumption is very high towards the end of gestation and can drain maternal system - vitamins
- water soluble cross - Hormones
- small molecular wt hormones cross - viruses and bacteria
