exam 4 Flashcards
what are the developing stages of an embryo?
after fertilization…
1. zygote: single celled embryo, undergoes mitotic cleavage
2. morula: solid ball of cells, individual blastomeres can not be counted
3. blastocyst: contains a blastocoele and 2 distinct cell populations (ICM and trophoblast)
4. hatching of blastocyst: results in free-floating blastocyst
how does the blastocyst hatch out of the ZP?
pressure increases and the trophoblast releases proteolytic enzymes that weaken the ZP rupturing it
cells of blastocyst squeeze and now become free-floating (occurs in uterus)
what is the difference between the embryo and the fetus?
embryo: organism in the early stages of development and has not acquired species-specific anatomical form
fetus: potential offspring that can be recognized by species
yolk sac structure and function
structure: develops from the primitive endoderm
function: contributes primitive germ cells that migrate to the gonadal ridge
amnion structure and function
structure: fluid-filled sac for the developing fetus
function: protection of fetus, provides lubrication for parturition
allantois functions
- reservoir for nutrients and waste
- carries blood vessels to the fetus
chorion functions
- attach to the endometrium
- absorbs nutrients
- allows for maternal-fetal gas exchange
how is luteolysis prevented during embryo development?
maternal recognition of pregnancy: embryo signals to the dam of its presence to prevent luteolysis
sow maternal recognition of pregnancy =
estrogen
mare maternal recognition of pregnancy =
small unknown peptide and migration of conceptus to in the uterus (it bouncing around)
cow, ewe, and doe maternal recognition of pregnancy =
interferon-tau
why does the embryo release its embryonic signal during diestrus?
diestrus ends in luteolysis: signal must be sent out before luteolysis occurs in order to maintain pregnancy (signal prevents PGF2a pulses)
interferon-tau acts in a ______ manner on the endometrium epithelial to inhibit development of luteolytic mechanism
paracrine: type of cell signaling where a cell releases a signal to another cell adjacent
what two ways can a placenta be classified?
- by distribution of the chorionic villi
- # of tissue layers separating maternal and fetal blood
diffuse placenta species and description
species: mare, sow
description: chorionic villi are distributed uniformly (mare: endometrial cups and microcotyledons that produce eCG)
zonary placenta species and description
species: cat, dog
description: placentas have a band-like zone of chorionic villi
discoid placenta species and description
species: rodents, primates (humans)
description: placentas have a distinct disc shape
cotyledonary placenta species and description
species: ruminants
description: discrete button-like structures of caruncles and cotyledons
epitheliochorial placenta species and description
species: sow, mare
description: least intimate, 6 layers, complete layer of epithelium on maternal and fetal components
species: ruminants
description: 6 layers, BNGC transfer complex molecules from fetal to maternal placenta (20%)
endotheliochorial placenta species and description
species: dogs, cats
description: 5 layers, maternal capillaries are directly exposed to epithelial cells of chorion
hemochorial placenta species and description
species: rodents, primates (humans)
description: 3 layers, most intimate chorionic epithelium is directly exposed to maternal pools of blood
what is a placentome?
consists of the fetal cotyledon and the maternal caruncular and is the functional unit of the ruminant placenta
what are the three ways that molecules can be transported across the placenta?
- simple diffusion
- facilitated diffusion
- active transport
what are the stages of parturition?
- myometrial contractions
- expulsion of fetus
- expulsion of fetal membranes
why does P4 decrease during parturition?
“progesterone block” (myometrial contractions are inhibited) is removed by fetal cortisol converting P4 to E2 and stimulating the placenta to produce PGF2a
removed P4 block allows for concentrations to begin again and push the fetus towards the cervix
what initiates parturition?
the fetus
stress signals sent to the HPA (hypothalamo-pituitary-adrenal) axis of the fetus –>hypothalamus produces CRH –> anterior pituitary produces ACTH –> adrenal cortex produces corticoids –> sent to the dam
what may cause dystocia in a female?
- excessive size of fetus
- failure of fetal rotation
- multiple births in a female meant to have one offspring
BPQ: In the early stages of embryogenesis, there is a tremendous development of the new individual. Provide descriptions of the stages of a pre-attached embryo.
ootid: when male and female pronuclei can be observed
- enormous cytoplasmic volume relative to nuclear volume
zygote: single-celled embryo (immediately follows syngamy)
- undergoes series of mitotic divisions (cleavage)
blastomere: the cell that results from the first cleavage of zygote
- each blastomere will undergo numerous divisions
- cell mass is contained as the single-cell zygote (all in ZP)
morula: solid ball of cells, individual blastomeres cannot be accurately counted
- separate into inner and outer cells
- outer cells compacted more than center
- inner cells develop gap junctions (change permeability, fluid begins to accumulate inside)
early blastocyst: has distinct fluid-filled cavity (blastocoele)
- partitioning into two distinct cell populations (ICM-embryo and trophoblast-chorion, fetal component of placenta)
- as fluid accumulates, outer cells become flattened and the blastocoele is formed
hatching blastocyst: continues to undergo mitosis and fluid continues to fill blastocoele
- pressure increases
- proteolytic enzymes produced by the trophoblast
- ZP weakens and ruptures (blastocyst cells squeeze out, blastocyst now free-floating, survival depends on uterine environment)
BPQ: As the blastocyst continues to grow, it must hatch out of the ZP. Discuss the forces that govern the process of hatching.
3 forces:
- trophectoderm/trophoblast cells release enzymes that digest into the ZP
- contraction and expansion of the blastocyst (w/ modifications or additional forces on the ZP)
- blastocoele (fluid-filled cavity of the blastocyst) is going to have more and more fluid, increasing the pressure
BPQ: In order for the events of early embryogenesis to continue into an established pregnancy, luteolysis must be prevented.
1. define functional luteolysis
2. define structural luteolysis
3. understand maternal recognition in all females
- reducing the production of P4 or no longer producing P4
- breaking down the tissues that make up the CL; if SLCs and LLCs are no longer there, they can’t do their job
- sow: E2 being redirected into the lumen of the uterus where it will be metabolized (no longer going into utero-ovarian blood supply)
- mare: small unknown peptide and embryo moving around in the uterus
- cow, ewe, goat: interferon-tau is secreted by the embryo and ensures that PGF2a doesn’t cause luteolysis
BPQ: Parturition is a complex cascade of physiological events
1. What are the signals that stimulate the fetus that the conclusion of gestation is happening?
2. Name the endocrine signaling axis that is used in the fetus that relays the conclusion of gestation?
3. Once fetal corticoids cross the placentome what dramatic changes are initiated by the dam?
- placenta efficiency is reduced = not enough blood flow going back and forth for nutrition which stimulates stress —> tells hypothalamus to release CRH —> tells AP to release ACTH —> tells adrenal cortex to produce fetal corticosteroids (cortisol)
- the HPA axis (hypothalamus, pituitary, adrenal)
- decrease in P4, increase in E2 (stimulates myometrial contraction), increase in PGF2a (triggers uterine contractions, helps with cervical ripening, getting active birth control ready)
what are the four steps that must be achieved before the embryo can attach to the uterus?
- development within confines of the ZP
- hatching of blastocyst from ZP
- maternal recognition of pregnancy
- formation of extraembryonic membranes
what are the two types of twinning?
- dizygotic: results from ovulation of two oocytes during the same estrous cycle (more like siblings that twins)
- monozygotic: result from fertilization of a single oocyte (genetically the same)
what are the functions of the placenta?
exchange: gas, nutrients, waste products
endocrine: transient, E2, P4, HCG, eCG, PL
physical protection: shock absorber
chemical protection: immunosuppression, prevents infiltration by maternal defenses, filters toxins
how is implantation different from attachment?
what hormones does the placenta produce?
what are the stages of parturition? (detailed)
stage I: myometrial contractions
- initiated by fetus (fetal cortisol promotes the synthesis of three enzymes that convert P4 into E2) (fetal cortisol also stimulates the placenta to produce PGF2a)
- removal of progesterone block
- dilation of the cervix –> contracting uterus pushes fetus towards the cervix
stage II: expulsion of fetus
- entrance of fetus into birth canal –> movement promotes uterine and abdominal contractions
- oxytocin release –> maximum pressure = uterine and abdominal
- PGF2a stimulates relaxin to be produced (relaxin softens the cervix and CT)
- high levels of E2 –> increases the secretory activity of the repro tract
- amniotic sac ruptures during the end of the second stage (dramatic increase in lubrication)
- longer than 2 hrs considered to have dystocia
- amnion ruptures (2nd water bag) –> mucin lubricates vagina and vestibule
- fetus passes through vagina and vestibule
stage III: expulsion of fetal membranes
- myometrial contractions continue in stage II – yet not as strong
- chorionic villi become dislodged from maternal side of the placenta (high degree of proteolytic enzymes digest the fetal-maternal attachment) (powerful vasoconstriction –> cuts off blood flow)
- retention of membranes = retained placenta
how do you treat a retained placenta?
do not manually remove from the uterus –> uterine damage greater than infection of retained placenta
mare = do not pull placenta; vet will need to clean out her placenta is fully intact
cattle = treat with either PGF2a or oxytocin to expel; infuse uterus with tetracycline and systemic injections of penicillin until placenta passes
how do you prevent a retained placenta?
- vaccinate for brucellosis and leptospirosis
- maintain vit A, E, and Se at appropriate levels in diet
explain dilation of cervix in stage I of parturition
uterine contractions become coordinated and synchronized (E2 and PGF2a induced)
fetus pushed against the cervix –> amnionic sac dialates cervix and stimulates oxytocin
- pressure on cervix activates pressure sensing neurons –> ultimately synapse on oxytocin neurons
- oxytocin increases the forces of the contractions
chorioallantoic membranes may break (1st water bag)
where are the extramembryonic membranes located? what purpose do they serve?
yolk sac: under fetus, provides nutrient supply for early embryo
amnion: on top of fetus to protects it from injury, provides lubrication for parturition, reservoir for urine and waste
allantois: under fetus, carries blood vessels of placenta, reservoir for nutrients and wastes
chorion: around enrire conceptus, attaches to the uterus –> absorbs nutrients and allows maternal-fetal gas exchange
