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ASC 309 Early Embryogenesis and Maternal Recognition of Pregnancy Chapter 13 – Student Handout Embryogenesis Terminology Zygote Embryo Fetus Conceptus Early embryonic stage Pre-implantation stage Post-attachment stage Fertilization Four steps 1. 2. 3. 4. Fertilized Oocyte (Figure 13-1) Male and female pronuclei within the cytoplasm characterize a developmental stage of the newly fertilized oocyte. Ootid Single-celled embryo (zygote) Undergoes mitotic divisions called _______________. 1 First cleavage division Blastomere Blastomeres from the 2-, 4-, and 8-celled Totipotency Mitotic divisions of blastomeres 1) 2) 3) Morula stage Two distinct populations form: 1. Inner cells Gap junctions 2. Outer cells After Tight junctions tight junctions are formed, fluid begins to accumulate inside the embryo. Fluid filled cavity (Figure 13-2) Hatching Hatching of the blastocyst is governed by three forces. 1. 2 2. 3. Blastocyst (Figure 13-1, 13-3) Embryo with a distinct cavity is called a _____________. Due to tight junctions and gap junctions the embryo is partitioned into two cell populations. 1. Inner cell mass 2. Trophoblastic cells - Continues to undergo mitosis Concurrent The with growth and fluid accumulation blastocyst itself begins to contract and relax – Pressure pulses coupled with continued growth and enzymatic degradation cause ______________________________. A small crack or fissure in the zona pellucida develops Free-floating embryo within the lumen of the uterus Extra-embryonic Membranes After hatching, the conceptus undergoes massive growth. Due largely to development of a set of membranes 3 Pre-attachment Period Mammalian embryos are subdivided into two primary groups. Most domestic animals 1. Preattachment period Extensive extra embryonic membranes form Primates 1. Preattachment period Extraembryonic membranes form. Extraembryonic membranes of the preattached embryo consist of: 1) 2) 3) 4) Set of 4 anatomically distinctive membranes which originate from: (Figure 13-14) 1) 2) 3) 4) Specific membranes give rise to the following: Yolk sac - Chorion - Amnion - 4 Primitive endoderm (Figure 13-3) Yolk sac Chorion Amnion (Figure 13-4) Functions: 1) 2) Allantois Function: 1) Chorioallantoic membrane (Figure 13-4) Function 1) Biochemical Recognition The conceptus must provide a biochemical signal or pregnancy will terminate. Progesterone 5 The embryo enters the uterus between days 2 and 5 after ovulation, depending on the species. Maternal recognition of pregnancy Inadequate signal is not delivered (Figure 13-8) Maternal Recognition of Pregnancy Must occur prior to luteolysis CL produces oxytocin which stimulates PGF2 synthesis. Dependent Pulsatile on the number of oxytocin receptors. secretions of PGF2 occur and luteolysis follows (Figure 13- 8). A threshold number of oxytocin receptors is required for secretion of PGF2 and luteolysis follows (Figure 13-4). Clearly, this mechanism must be prevented if a successful pregnancy is to proceed. 6 Ewe and Cow In the ewe and cow the free-floating blastocyst produces specific proteins that prevent luteolysis. 1) Ovine trophoblastic protein (Figure 13-5) 2) Bovine trophoblastic protein Produced by o 1) Refered to as : o 1) o 2) Present in uterus Functions include: Bind to _________________and inhibits Binds to apical portion of uterine glands Sow (Figure 13-6) Two major differences 1. 2. Conceptus produces estradiol Estradiol Luminal PGF2 Two conceptuses are required 7 Precise mechanism for rerouting of PGF2 is not completely understood. Estrogen Prolactin Mare Presence of conceptus prevents luteolysis Endometrial Conceptus production of PGF2 is reduced. must migrate within the uterus. 8