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

2. 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
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3. 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)
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4. 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
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5. 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:
Mitotic cell division
Cell differentiation/specialization
Size increase (growth)
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6. 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
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7. 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
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8. 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
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9. 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
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10. 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 8th week = Embryo Development
Development from 8th week to birth = Fetal Development
Fig. 27.2
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11. 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
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12. 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
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13. Embryonic Development
Bile capillaries, macrophages
Fig. 27.5
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14. Placentation Placenta formation
Starts close to the beginning of the 4th week
Is completed by the 5th 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
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15. 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
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16. Chorionic villi and umbilical cord
Bile capillaries, macrophages
Fig. 27.5
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17. Chorionic villi and umbilical cord
Bile capillaries, macrophages
Fig. 27.5
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18. 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
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19. 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 (2nd 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
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20. Back to development
While those extraembryonic layers are developing, embryo development is also underway
Embryogenesis:
Starts at the 2nd week
Folding: A head fold & tail fold develop
Orientation: dorsal vs. ventral sides become distinct (different)
Organogenesis = formation of organs
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21. Bile capillaries, macrophages
Table 27-1
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22. Pregnancy Duration = 3 trimesters of 3 months each 1st trimester:
Extraembryonic layers are formed & develop as outlined
Embryogenesis
Organogenesis
By the 8th week the fetus is formed
All major organs are formed by 3 months
2nd trimester:
Fetal growth
Grows faster than the placenta
Amniotic membrane fuses with the chorion = amniochorionic membrane
3rd trimester:
Next slide
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23. Pregnancy Duration = 3 trimesters of 3 months each 3rd 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
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24. Physical Changes in Mother (uterus)
Bile capillaries, macrophages
Fig
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25. Pregnancy - Fetus Full Term 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)
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26. Physical Changes in Mother (uterus)
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 )?!?!
Bile capillaries, macrophages
Fig
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27. 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
4/23/08