Question 4 Flashcards
Describe the pre-attachment embryonic development from fertilization to hatching with great detail
During fertilization, syngamy (fusion of male and female pronuclei) occurs to produce a zygote that begins to cleave (mitotic division) to give rise to blastomeres. Cleavage of the blastomeres continues (2-celled -> 4-celled -> 8-celled embryo) until a ball of cells within the zona pellucida is formed, referred to as a morula. This is all occurring in the oviduct. Within the morula, two groups of cells form through the process of compaction (outer cells that create tight junctions and inner cells connected by gap junctions). Sodium is pumped into the intercellular spaces and water follows osmotically. Fluid accumulates in the morula which causes outer cells to flatten and the blastocoele forms. Two separate cellular components emerge: the inner cell mass (ICM) and trophoblast. The blastocyst eventually hatches from the zona pellucida, as the ZP limits the space the embryo has to grow. This is caused by three factors. First, growth and accumulation of liquid within the blastocyst. Second, enzymes from the trophoblast that degrade the zona pellucida. Third, contractions of the blastocyst that add pressure to the zona pellucida. Hatching occurs in the uterine horn.
Using a canine model, describe the maternal recognition process and type of placentation based on the distribution of the chorionic villi/layers in the fetal-maternal interface
Maternal recognition: there is no signal from the conceptus because the lifespan of the corpus luteum (CL) is longer than the gestation period. This means the CL will continue to produce progesterone (no luteolysis) up until parturition. No signal is required from the embryo to maintain the CL.
Type of placentation: the bitch has zonary distribution of chorionic villi. This means the placenta has a band-like zone of chorionic villi, resembling a “belt” around the fetus for metabolic exchange. She also has endotheliochorial placenta, meaning there are five layers separating the maternal and fetal blood. These five layers, from the maternal to fetal sides, are as follows: endometrial capillaries, endometrial interstitium, chorionic epithelium, chorionic interstitium, chorionic capillaries. This means there is direct exposure of the maternal capillaries to the chorionic epithelium.
Using a canine model, describe pregnancy length, timing of placental P4 takeover, and stages and duration of parturition.
Pregnancy length: two months
Timing of placental takeover: never occurs (CL remains primary P4 producer during entire pregnancy)
Stages/duration of parturition:
Stage 1: initiation of myometrial contractions (removal of P4 block): 6-12 hours
Starts with adrenal corticotropin being released by the fetus anterior pituitary when it is under stress due to space limitations in the uterus. ACTH stimulates cortisol release from adrenal gland which abolishes the P4 block: promotes conversion of P4 to E2 and promotes prostaglandin F2a synthesis by the placenta (luteolysis).
Stage 2: expulsion of fetus (strong contractions of myometrium): 6 hours (longer for larger litters)
Relaxin softens connective tissue of cervix, prepares birth canal for fetus expulsion
Oxytocin augments myometrial contractions initiated by estrogen that allow for expulsion of fetus / fetal membranes (secreted by posterior pituitary because of pressure on cervix from fetus)
Stage 3: expulsion of fetal membranes: pass with neonate
Look at pre-attachment embryonic development diagrams
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