Transcript Reproductive System, Day 4 (Professor Powerpoint)

Reproductive System
Chapter 26 – Day 4
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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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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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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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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:
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1.Mitotic cell division
2.Cell differentiation/specialization
3.Size increase (growth)
Initial Development Stages
 Cleavage
♦
♦
♦
♦
♦
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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
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
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Fig. 27.2
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
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♦ 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
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
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Fig. 27.2
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
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Fig. 27.2
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
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Fig. 27.4
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
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♦ Chorionic villi with blood vessels
♦ Involved in placentation
Embryonic Development
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Fig. 27.5
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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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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Chorionic villi and umbilical cord
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Fig. 27.5
Chorionic villi and umbilical cord
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Fig. 27.5
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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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, 15-18 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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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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Table 27-1
Pregnancy
 Duration = 3 trimesters of 3 months each
 1st trimester:
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♦
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♦
♦
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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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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Physical Changes in Mother (uterus)
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Fig. 27.10
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)
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Physical Changes in Mother (uterus)
 HOW does the uterus stretch (look at Fig. 27-10)?!?!
♦ 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
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Fig. 27.10
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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