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4/23/08 Reproductive System Chapter 26 – Day 4. 4/23/08 Conception  The purpose of reproduction – The sexual act (coitus) allows fertilization of the.

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Presentation on theme: "4/23/08 Reproductive System Chapter 26 – Day 4. 4/23/08 Conception  The purpose of reproduction – The sexual act (coitus) allows fertilization of the."— Presentation transcript:

1 4/23/08 Reproductive System Chapter 26 – Day 4

2 4/23/08 Conception  The purpose of reproduction – The sexual act (coitus) allows fertilization of the egg if it (coitus) occurs at the correct time of the ovarian cycle Conception  Secondary Oocyte is released – travels slowly down the uterine tube Timing is CRITICAL  After release from the ovary the oocyte survives for ONLY 24 hours  If sperm is available within this period of time, then fertilization can take place  Sperm remains in the female tract in a viable condition for approximately 72 hours

3 4/23/08 Conception  Millions of sperm are released because there are many challenges: ♦Acidic secretions ♦Fluid currents  When the sperm reaches the egg ♦Millions crowd the egg ♦As soon as on makes contact… ♦Changes in the egg block the other sperm  The acrosome contains hyaluronidase ♦This dissolves the outer wall of the ovum (zona pellucida) ♦The sperm then “enters the egg” – the membranes are fused  If fertilization occurs, the second polar body is expelled – the secondary oocyte/egg = ovum (mature gamete)

4 4/23/08 Conception  The membrane of the egg immediately thickens, forming a hard wall (fertilization membrane) ♦This prevents the entry of other sperm  An electrical reaction occurs – “cortical reaction” occurs after penetration ♦Enzymes destroy all sperm binding sites on the surface of the egg ♦The chromosome number is now diploid

5 4/23/08 Fate of the Fertilized Egg  At fertilization ♦The haploid nuclei of the ovum & sperm fuse to form a common diploid segmentation nucleus (the ZYGOTE) ♦Occasionally a single fertilized egg splits into two shortly after fertilization = monozygotic/identical twins Sometimes (very rare) the “split” is not complete, so they remain joined = siamese twins ♦If 2 ova are released and each gets fertilized separately by their own separate sperm = dizygotic/fraternal twins These are not genetically identical  After the formation of a fused nucleus – DEVELOPMENT BEGINS: 1.Mitotic cell division 2.Cell differentiation/specialization 3.Size increase (growth)

6 4/23/08 Initial Development Stages  Cleavage ♦Multiple mitotic divisions ♦The cell splits into 2 identical cells, these split, yielding 4, etc… ♦Each new cell = blastomere ♦Once there is a solid mass of cells = morula ♦The size of the mass remains the same as the original zygote Fig. 27.2

7 4/23/08 Initial Development Stages  Blastocyst Formation ♦4-5 days after fertilization ♦There is a rearrangement of the cells ♦Cells arrange along the edge of the morula– forms a hollow ball of cells = blastocyst ♦There are 2 distinct areas The trophoblast = surrounding cells The inner cell mass = cluster of cells at one pole  The embryo forms from the inner cell mass  The surrounding membrane forms from the trophoblast Fig. 27.2

8 4/23/08 Implantation  The blastocyst enters the uterus (free floating)  8 days after fertilization, the blastocyst becomes attached or implanted onto the uterine wall  Orientation of the blastocyst is important ♦The inner cell mass faces the uterine wall ♦It touches the wall ♦Upon contact: The trophoblast separates into 2 layers at the area of contact Fig. 27.3

9 4/23/08 Implantation  The trophoblast has separated into 2 layers:  The syncytiotrophoblast = outer layer ♦This layer invades the inner portion of the endometrium ♦Eventually it creates a region where maternal blood will be available for nourishment ♦Enzymes break down the uterine wall  The cytotrophoblast = inner layer, surrounding the inner cell mass ♦This layer forms the placenta along with the syncytiotrophoblast Fig. 27.2

10 4/23/08 Hormone Changes  At the onset of implantation ♦Trophoblast layers secrete HCG This maintains the corpus – which maintains levels of estrogen & progesterone Pregnancy tests detect HCG levels ♦Prostaglandins are released by endometrial cells This is triggered by the blastocyst Allows liquefaction of endometrial cells  Development up to the 8 th week = Embryo Development  Development from 8 th week to birth = Fetal Development Fig. 27.2

11 4/23/08 Embryo Development  Begins in second week:  Gastrulation ♦Inner cell mass forms primary germ layers Ectoderm = outer layer ◦Will become integument, nervous system Endoderm = innermost layer ◦Will become GI tract, respiratory tract & urinary system Mesoderm = middle layer ◦Will become connective tissue, bone, muscle, blood Fig. 27.4

12 4/23/08 Embryo Development – Weeks 2-5  Extraembryonic layers also form germ layers  Yolk sac= endoderm & mesoderm ♦basically a blastocoel layered with mesoderm ♦Blood cell formation  Amnion = mesoderm & ectoderm ♦Extension of the ectoderm = a chamber with amniotic fluid ♦Functions to cushion the baby  Allantois = endoderm & mesoderm ♦Forms near base of the yolk sac, close to endoderm ♦Extension towards uterine wall ♦Form the umbilical cord & urinary bladder  Chorion = mesoderm & trophoblast ♦Chorionic villi with blood vessels ♦Involved in placentation

13 4/23/08 Embryonic Development Fig. 27.5

14 4/23/08 Placentation  Placenta formation ♦Starts close to the beginning of the 4 th week ♦Is completed by the 5 th month of pregnancy  At the attachment point, the endometrium becomes the decidua (has 3 layers) = (mother/maternal placenta) ♦Parietalis = adjacent to the uterus wall ♦Capsularis = between the embryo & uterus wall ♦Basalis = adjacent to the chorion  On the fetal side: Chorion & Chorionic villi  Chorionic villi grow into the dicidua basalis, these contain fetal blood vessels originating from the allantois  As the embryo & later the fetus enlarges, the chorionic villi, along with blood vessels extend further towards the decidua basalis

15 4/23/08 Placentation  The space between the villi & the decidua basalis  Maternal blood vessels release blood into the space, this bathes the chorionic villi  This allows nutrients to diffuse in, but no mixing of blood  As the fetus extends away, the allantois develops into the umbilical cord with blood vessels leading to the chorionic villi

16 4/23/08 Chorionic villi and umbilical cord Fig. 27.5

17 4/23/08 Chorionic villi and umbilical cord Fig. 27.5

18 4/23/08 Function of the Placenta  Diffusion of O2 & nutrients from maternal blood to fetal blood  Diffusion of CO2 & wastes from fetal blood to maternal blood  Other items that can cross the placenta: ♦Hormones, antibodies, drugs, alcohol ♦Infectious agents: viruses like measles, rubella & aids  The placenta releases estrogen & progesterone to maintain uterus wall  At birth the placenta detaches from the uterus – it is expelled after birth  Umbilical cord is cut – leaves “umbilicus” = belly button

19 4/23/08 Diagnostics  Amniocentesis & Chorionic Sampling ♦Diagnose the “health” of the fetus – protein levels & genetic analysis ♦Done to rule out birth defects – offered only if risk is high  Amniocentesis (2 nd trimester, weeks): ♦Fetus is located by ultrasound ♦Needle is inserted into amnion ♦Fluid is withdrawn ♦Tissue can be cultured, karyotypes, sex determined ♦High levels of protein may indicate spina bifida  Chorionic villi sampling (as early as 8 weeks): ♦Catheter is inserted through cervix into uterus ♦Villi are collected for sampling ♦Cells are examined for genetic problems

20 4/23/08 Back to development  While those extraembryonic layers are developing, embryo development is also underway  Embryogenesis: ♦Starts at the 2 nd week ♦Folding: A head fold & tail fold develop ♦Orientation: dorsal vs. ventral sides become distinct (different)  Organogenesis = formation of organs

21 4/23/08 Table 27-1

22 4/23/08 Pregnancy  Duration = 3 trimesters of 3 months each  1 st trimester: ♦Extraembryonic layers are formed & develop as outlined ♦Embryogenesis ♦Organogenesis ♦By the 8 th week the fetus is formed ♦All major organs are formed by 3 months  2 nd trimester: ♦Fetal growth ♦Grows faster than the placenta ♦Amniotic membrane fuses with the chorion = amniochorionic membrane  3 rd trimester: ♦Next slide

23 4/23/08 Pregnancy  Duration = 3 trimesters of 3 months each  3 rd trimester: ♦Organ systems develop further towards functioning independently ♦The uterus expands in size In the process it pushes other maternal organs out of the way

24 4/23/08 Physical Changes in Mother (uterus) Fig

25 4/23/08 Pregnancy - Fetus  In the fetus: ♦Lung, kidneys, GI tract are all NOT functioning ♦Umbilical vein – brings oxygenated blood from placenta to the fetus ♦Blood reaches the fetus – mixed blood pumps through fetus ♦Blood returns to placenta via umbilical arteries  Full Term ♦At 9 months – the uterus has expanded extensively from its original size ♦The uterus now contains approximately 5 liters of fluid ♦The average fetus weighs 2.6 kg (7lb)

26 4/23/08 Physical Changes in Mother (uterus) Fig ♦It is a gradual process that reaches its max. at full term ♦increased smooth muscle contractions in uterus wall ♦Increased stretch ♦The process is controlled by hormone levels  HOW does the uterus stretch (look at Fig )?!?!

27 4/23/08 Stretching of Uterus - Hormones  Implantation: ♦HCG elevates progesterone ♦Progesterone inhibits stretching – OK for initial stages of pregnancy  As the placenta develops ♦Estrogen increases ♦Estrogen increases contractions in the uterus  As estrogen increases, oxytocin increases in response to estrogen levels  Usually at the time of delivery the very high estrogen levels will increase the force and frequency of contractions  As estrogen & oxytocin increase, prostaglandins increase in the endometrium = uterus stretches further


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