Chapter 28: Pregnancy and Fetal Development

Fertilization (definition)

The process in sexual reproduction that involves the union of a sperm an egg to form a diploid zygote

Fertilization Time Window

1. An oocyte is only viable for 24 hours (at most) after ovulation

2. Sperm can survive at most 5 days in female reproductive tract

Sperm making it to the Vagina

1. Millions lost immediately from vagina, more lost by acidic enviornment

2. If they make it to the uterus

   1. Phagocytes kill of many more

   2. Only a few thousand survives --> reverse peristalsis

   3. Sperm have various receptorss to ensure they travel in right direction

Sperm Capacitation

Problem is sperm are incapable of fertilizing the oocyte immediately after entering the vagina

• Motility is enhanced and weakens sperm cell membranes

Protective Structures of Oocyte

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1. Corona Radiata: Outer Layer
1. Function is protects and nourishes the oocyte it has been ovulated
2. Cells release chemicals to help guide sperm
2. Zona Pellucida: Inner Layer
1. Function is that protects secondary oocyte and is necesary for fertilization to occur
2. Produced in primary oocyte phase

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Sperm must be able to cross both structure of Oocyte

1. Sperm burrow through cells of corona radiata

2. At the zona pellucida, acrosomal reaction occurs

   1. Release of enzymes from the acrosome of the sperm

   2. Sperm bind zona pellucida --> Ca²⁺ levels in sperm rise

   3. Enzymes from acrosome digest holes in zona pellucida

Once Sperm Passes through Both Layers (after acrosomal reaction)

1. Sperm binds to sperm-binding receptors on surface of oocyte

2. Once bound, the two membranes fuse together and sperm contents enter the oocyte

Polyspermy

The entry of more than one sperm cell into the oocyte

• Preventing sperm is caused by oocyte, you dont want more than 1 genetic info to be binded at once

Blocking Polysperm (after 1 sperm cell has been bound)

1. Oocyte member blocks

   1. Sperm-binding receptors are shed from oocyte surfaces

   2. Sperm unable to bind oocyte surface and fertilize oocyte

2. Cortical Reaction

   1. Oocyte release Ca²⁺ 

   2. Causes zona pellucida to harden

Completion of Meiosis II in Oocyte

1. Sperm nucleus travels toward oocyte nucleus, as it travels sperm nucleus swells in size to form male pronucleus

2. Surge in Ca2+ from cortical reaction causes release of zinc from oocyte which causes completion of meiosis II --> forms mature ovum

Completion of Fertilization in Oocyte

1. After Meiosis II is formed --> female pronucleus forms

2. Two pronuceli membranes rupture and chromosomes are released

   1. 2 sets of chromosomes combine to form a diploid zygote

   2. THIS is fertilization

Monozygotic Twins

Occurs when a single oocyte is fertilized splits into 2 identical embryos

• Unknown why/how the zygot splits
• Twins usually share a placenta

Dizygotic Twins

Occurs when 2 oocytes are ovulated and both are fertilized

• Twins have their own placenta

Cleavage of Embryonic Development

"Rapid divisions of zygote

• The benefit of dividing so much --> is if there is a deficient/messed up one, we can just get rid of it
• Immunosuppresive hormones prevent cells being killed off by parents immune system since they are technically ""non-self""

First mitotic divisions produces 2 identical blastomere cells

• By 72 hours after fertilization, a morula is formed → contains around 16 cells

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Blasotocyte Layers

Morula cells contune to divide to form blastocyte
External Trophoblast

• Aids in embryo implantation, contributes to chorion formation/function, and has immunosuppressive effects

Internal Embryoblast (inner cell mass)

Oranogenesis

Formation of body organs and organ systems, begins with gastrulation

• Cells in 3 primary germ layers will differentiate to form organs and organ systems
• Endoderm, ectoderm, mesoderm
• Early process --> all organ systems are recognizable by week 8 

Endoderm

Forms epithelial lining of GI tract, respiratory tract forms from outpocketings

• Other glands formed form endothelial outpockets along the tract
• Opposite ends of the endoderm open

Ectoderm

Neurulation --> formation of brain and spinal cord, induced by chemicals released by notochord

• Results in formation of a neural tube sitting over the notochord
• Anterior portion becomes brain, remaining becomes spinal cord
• By week 8 --> cerebral hemispheres evident, brain waves can be recorded
• Remaining ectoderm forms epidermis

Mesoderm

Forms mesodermal blocks called somites that hug that notochord on either side

• 3 Functional parts of somites
• Sclerotome = Produce vertabrae and rib at each associated levels
• Dermatome = Forms dermis in dorsal part of body
• Myotome = forms skeletal muscle of neck, body trunk, and limbs
• Remaining mesoderm forms kidneys and gonads, connective tissues of limbs, heart and blood vessels, dermis and ventral body region

Cardiovascular Modifications of Umbilical Arteries and Vein

Although blood is transported via these structures, gas exchange (O2 and CO2) waste elimination, and various other processes occur in placenta

• Vein carries oxygenated blood TO fetus, arteries carry oxygen blood AWAY

Cardiovascular Modifications of Vascular Shunts

Redistrubutes blood to parts of body that need it most by bypass organs not yet used to developing fetus

Types of Vascular Shunts

Ductus Venosus

• Shunt that bypasses the liver, parent liver does all the work for fetus

Foramen Ovale

• Shunts blood from right atrium to left atrium
• Causes blood to bypass lungs

Ductus Arteriosus

• Shunts blood from pulmonary trunk to aorta
• Causes blood to bypass lungs

All Modifications are Closed After Birth

1. Umbilical blood vessels constrict and fibrose

   1. Arteries supply bladder and anchors bladder to umbilicus

   2. Vein remnant becomes round ligament of liver

2. Ductus venosus collapses and is converted to ligamentum venosum

   1. Venosum helps identity and anchor hepetic portal

   2. Venosum makes blood bypass liver if still active which means toxic unfiltered blood can go to the brain

3. Foramen ovale closes as pulmonary circulation becomes functional

   1. Forms the fossa ovalis (only one that doesnt have to close)

Effects of Pregnancy: Anatomical Changes

Uterine enlargement, at beginning of pregnancy uterus is about the size of a pear and at the end top of the uterus is even with xiphoid process

Pregnancy Effects: Weight gain

Around 28lbs, depends on weight at beginning of pregnancy

• Around 300 extra calories/day necessary to maintain healthy pregnancy

Placental Growth Hormone

Replaces growth hormone in pregnant women, stimulates lipolysis and glucose production for growing fetus

Human Placental Lactogen (hPL)

Stimulates maturation of breasts for lactation, promotes fetal growth, and is glucose-sparing

• Side effect is cells have to be less responsive to insulin which can cause gestational diabetes
• Glucose sparing is the parents body using less glucose for its need

Corticotroponin-releasing hormone (CRH)

Rises only toward end of pregnancy, as CRH rises parents ACTH and cortisol levels rise which helps during birth

• Cortisol leads to maturation of fetal organs
• help parent prepare for physical stress during birth

GI System Physiological Changes

Surge in hCH estroge, and progesterone can result in nausea/vommitting (called morning sickness)

• Uterus invading abdomen, pressing on other organs causing heartburn, constipation, etc

Urinary System Physiological Changes

Increased metabolic rate, higher blood volume and need to dispose of extra waste from fetus = more urine produced

• Uterine pushes on bladder --> increases frequency of urination

Respiratory System Physiological Changes

Respiratory rate is unaffected, increase in TV and decrease in RV

• Disposal of extra CO2 causes deeper breathes which means increase in TV and decrease in RV

Uterus pushes on diaphragm, can make breathing difficult by end of pregnancy

Initiation of Labor

The fetus determines its own birth date

• Increases in fetal cortisol levels leads to maturation of lung tissue (and increase in surfactant) and surge in estrogen from the placenta

Surge in Estrogen of Labor Effects

1. Increased synthesis of prostaglandis
1. Thin and soften cervical mucos
2. Increase in oxytocin and prostaglandin receptors in myometrium
1. Responsible for stimulating labor contractions
3. Addition of more actin, myosin, and other contractile elements
1. Increase gives stranger contractions, uterus is strongest of all smooth MT

Stimulation of Uterine Contractions

Stimulated by oxytocin released by posterior pituitary gland

• Uterine contractions push head of fetus into cervix
• Stretching of cervix by head stimulates more oxytocin release by the posterior pituitary
• Continues until infant is completely removed

Stages of Labor: Dilated Stage

Occurs from onset of labor until cervix is fully dilated, highly variable duration of time

• Dilation caused by force of babys head against cervix
• Full dilation is around 10 cm in diamete

Contractions begin as weak/short, becomes progressively stronger

• Push of head against cervix will break the amnion

Stages of Labor: Expulsion

Occurs from full dilation to delivery, contractions occur every 2-3 minutes and ladts 1 minute each

• Crowning occurs when the largest portion of infants head distends vulva

Infant should be in a vertex position since it dilates the cervix for the rest of the body ti pass easier, can suction mucus from oral/nasal passage for breathing to occur sooner

• Breech position: butt first, bad since it makes delivery very hard, can dilsocate or break bones of infant

Stages of Labor: Placental

Occurs after delivery of infant, occurs around 30 minutes after birth

• Parent must also deliver placenta and all of its structures (afterbirth)
• Uterine contractions compress blood vessels and tear placenta from uterine wall
• Must remove all afterbirth to prevent infection

Extrauterine Adjustments

After birth --> the infant must perform all physiological functions on its own, major adjustments that occur immediately after birth

• Taking the first breathe, umbilical blood vessels constrict which prevents removal of CO2 from infant

Lactation when not Pregnant

When not pregnant, mammary glands are not functional

• Lobules contain glandular alveoli which produce milk during lactation but not before

Lactation During Late Pregnancy and After Birth

Rising levels of progesterone, hPL, and estrogens leads to release of prolactin

• Glandular alveoli become active, which produce and release milk
• Prolactin only produced for few weeks after birth --> milk production stimulates by feeding infant
• Prolactin slows down ovaries which makes parent less likely to ovulate

Delay in milk production, colostrum is secreted by mammary glands

• Contains less lactose than milk, very little fat, but contains much more protein, minerals, vitamin A, and some antibodies

Benefits of Breast Milk For Infant

1. Various components of breast milk are more easily metabolized by infant

2. Various protective molecules, such as igA, complement, lysozymes, interferon, interleukins

   1. Interleukins important for activate immune system

3. Helps build healthy bacterial flora of infant

4. Serves as a natural laxative, cleanses infant bowels of meconium

Implantation of Blastocyst in Endometrium

Endometrium is receptive of implanting embryo due to high estrogen and progesterone levels

• Trophoblast attaches to endometrium, usually occurs high in uterus

• Hormones thickening endometrium allows embryo multiple chances to implant

Once Bound, Trophoblast Releases Growth Factors and Digestive Enzyme

Growth Factor Influence

• Endometrium thickens, and trophoblast proliferates and releases digestive enzymes to facilitate implantion

Digestive Enzyme Influence

• Erosion of endometrium around blastocyst --> blastocyt buries itself

• Surrounding endometrial cells proliferate shortly after to enclose the blastocyst in lining

Once Implantation is Complete

Embryo releases hormone human chorionic gonadotropnin (hCG)

• hCG maintains the corpus luteum for around 12 weeks and suppresses female immune system

• hCG can be filtered out by uterine, hormone that at home pregnancy tests uses

The Placenta

A temporary organ originating from ebryonic and maternal tissues

• Maintains pregnancy, exchange respiratory gases, provide nutrients to embryo/fetus, dispose of waste, etc, etc

Embryonic Contributions to Placenta Formation

Chorion membrane that surrounds fetus

• Chorion has chorionic villi which form umbilical vein and arteries

Parental Contributions to Placenta Formation

Functional layer of endometrium develops blood-filled lacunae

• Endometrium that lies underneath embryo becomes decidua basalis, which forms placenta with chorionic villi

• Endometrium found between embryo and lumen forms decidua capsularis

  • Chorionic villi here degenerate as pregnancy progresses to expand to accomodate growth of fetus

Extra-embryonic Membranes (definition)

Membranes formed during the first few weeks of development that support and nourish developing embryo/fetus during gestation

• Amnion

• Yolk Sac

• Allantois

• Chorion

Extra-embryonic Membrane: Amnion

Innermost layer that extends around the embryo, filled with amniotic fluid

• Provides buoyancy and protection

• Maintains consistent temperature

• Prevents developing parts of embryo from sticking together/fusing

• Allows movement of embryo/fetus

Extraenbryonic Membrane: Yolk Sac

Sac like structure, eventually forms digestive tube

• Forms earliest blood cells and precursor gamates

Extra-embryonic Membrane: Allantois

Helps form the umbilical cord later in fetal development

• Allows for gas exchange, waste disposal, and nutrient exchange

Extra-embryonic Membrane: Chorion

Enclose all other extra-embyronic membranes, allows exchange of gases, nutrients, and wastes

Gustrulation (definition)

Early developmental process where the blastocyst is reorganized into a 3-layered embryo

Three primary germ layers

1. Endoderm

2. Ectoderm

3. Mesoderm

The Process of Gulstrulation

1. Begins with the formation of primitive streak (a groove that will eventually form the long axis of embryo)

2. Cells migrate and enter the primitive streak, these cells form the inferior layer of endoderm

3. Other cells follow and push between cells of upper and lower layers, these cells form middle layer of mesoderm

   1. Mesodermal cells beneath primitive streak form the notochord

   2. Rubber rod that runs length of embryo, allows structures to it

4. Some cells remain at surface and form ectoderm