Chapter 12 - Conception & Fetal Development

Conception

Union of a single egg and single sperm

Oogenesis

• Egg (ovum) formation

  • begins during fetal formation in the female

  • degenerative process until menopause

Zygote

• united egg and sperm

How is a trisomy/down syndrome produced?

Gamete with 24 chromosomes unites with normal gamete with 23 chromosomes

How is monosomy produced?

Gamete with 22 chromosomes unites with a gamete with 23 chromosomes

Zona Pellucida

Thick, inner layer of the ovum

Corona Radiata

Outer layer of the ovum

How long is an ovum considered fertile after ovulation?

24 hrs

Average viable time for sperm to be viable in a woman’s vagina

2 to 3 days

Capacitation

• removal of protective coating from the head of the sperm

Hylauronidase

• enzymes that allow the sperm to penetrate the protective layers of the ovum before fertilization

Where does fertilization take place in?

In the outer third of the fallopian tube/ampulla

Cleavage

Mitotic cellular replication

Zygote destinations

• In the ampulla (outer third) of the tube for 1st 24hrs

• Propelled by ciliary action

• Travels thru tube into the uterus

Morula

• Solid ball of cells surrounded by the protective zona pellucida

• Further development as it flows freely within the uterus

Trophoblast

Gives rise to the placenta

Embryoblast

Gives rise to the embryo

Blastocyst

• Structure of the developing embryo

What happens to the zona pellucida at the implantation site?

It degenerates

Describe what goes during implantation?

• Trophoblast secrete enzymes to set into the endometrium until the blastocyst is covered

• Endometrial vessels erode →

  • Implantation bleeding

  • Spotting

  • Bleeding @ the time of 1st missed menses

Chorionic Villi

• Finger like projections out of the trophoblast

• Obtain oxygen and nutrients from maternal blood

• Dispose of waste & co2

Decidua

Name for the endometrium after implantation

Length of pregnancy (how is it measured)

From 1st day of LMP until day of birth

Intrauterine Developmental Stages

• Ovum Stage: conception to day 12

• Embryonic Stage: day 15 to 8 weeks

• Fetal Stage: 9 weeks until end of pregnancy

Embryonic Disk & Layers

• Endoderm

• Mesoderm

• Ectoderm

Ectoderm (Components)

• CNS

• Epideremis

• Glands

• Nails

Mesoderm (Components)

• Bones/teeth

• Muscles

• CV/Spleen

• Urogenital

Endoderm (Components)

• Respiratory

• Digestive

• Glandular Cells

• Roof of yolk sac?

Stage of the Embryo

• Around 3cm

• Critical time in the development of organogenesis & main external features

• Areas with rapid cell division at risk for malformation by env.teratogens

• At the end of 8 weeks = human igs

Chorion

• Develops from the trophoblast

• Increases in size and complexity

• Covers fetal side of placenta

• Contains major umbilical blood vessels

What happens to the chorionic villi?

Atrophy & degeneration → Smooth chorionic membrane

Amnion

• Inner cell membrane from the cells of the blastocyst

Amniotic Cavity

Cavity between inner cell mass and outer layer of cells (trophoblast)

What does the embryo do to the amnion?

Draws it around itself and forms a fluid filled sack

Volume of amniotic fluid

• Increases weekly

• Steadily decreases after term

Functions of Amniotic Fluid

• Fetal Thermoregulation

• Source of oral fluid & waste storage

• F&E balance

• Fetal movement

• Cushions from trauma

• Antibacterial factors

• Prevents membrane tangling

  • ✅ for symmetric growth

  • Otherwise, amputations/malformations

Oligohydramnios

• < 300mL of amniotic fluid

• Fetal renal abnormalities

Hydramnios/Polyhydramnios

• > 2L of amniotic fluid

• GI or other malformations

How can fetal lung maturity be determined?

By analysis of pulmonary surfactant

Yolk Sac

• Secondary cavity

  • Transfers nutrients & oxygen

  • Vessels form to aid transport

When does the embryo curve inward?

• By the 5th week

  • Stalk becomes compressed

  • Narrow cord formed

Umbilical Cord Components

• 2 arteries

• 1 vein

What happens when umbilical cords only have 2 vessels (1 artery:1 vein)?

• Congenital Malformation

  • Fetal CV/GI/Urinary Tract Anomalies

The umbilical cord..

• Rapidly increases in length

• False knots can compromise circulations

Whartons Jelly

• Surrounds umbilical vessels and prevents compression

Nuchal Cord

Cord wrapped around the fetal neck

Placenta & Metabolic Function

• Minimal, 2 layers make it too thick

• Permeability increases later on

• By the 8th week, genetic testing may done via aspiration biopsy

**Limb defects associated if done b4 10 weeks

Human Chorionic Gonadotropin (hCG)

• detected in urine 8 to 10 days after ovulation (used in preg.tests)

• ensures supply of estrogen and progesterone to maintain pregnancy

How does miscarriage occur?

• Corpus luteum stops functioning b4 placenta produces sufficient estrogen & progesterone

Human Placental Lactogen (hPL)

• Stimulates maternal metabolism to supply nutrients

Progesterone & Pregnancy

• Maintains endometrium

• Decreases uterus contractility

• Stimulates maternal metabolism

• Development of breast alveoli

Hormone produced by placenta and ovaries?

Placenta - Estriol

Ovaries - Estradiol

What does the placenta function as for the fetus?

As the lungs

Fetal concentration of glucose

• Lower than maternal glucose lvl (bc of the rapid metabolism of glucose by the fetus)

• Fetal glucose demand = high

Although many viruses can cross placenta

Bacteria & protozoa first infect placenta and then the fetus

Since only one layer separates the maternal & fetal blood…

• Breaks can occur and fetal blood leaks into maternal circulation

  • Mother may develop antibodies (isoimmunization)

Since placental functions depends on maternal BP…

HTN & cocaine use diminishes uterine blood flow

Why are pregnant woman told to not lay on their backs?

• Applies pressure to the uterus, vena cava & blood return to r.atrium is diminished

  • Otherwise, growth restrictions or small infants

Braxton Hicks Contractions

• Painless contractions after 1st trimester that ENHANCE blood flow

**prolonged contractions/short intervals between contractions REDUCE blood flow to placenta

During the fetal stage…

They are less vulnerable to teratogens except for those affecting functioning of CNS

Viability

Capability of the fetus to survive outside uterus

1st organ to function in the developing human

CV system

Alcohol can cause

• growth restrictions

• microcephaly

• fetal alc syndrome

Androgens can lead to

Masculinization of female

Organic mercury (ex.sushi) can cause

• cognitive impairment

• blindness

Formane ovale

• opening in l.atrium

• mixes with the deoxygenated blood

Fetal HR (Range)

110 - 160

Lecithin

Alveolar surfactant required for postnatal lung expansion

Once the L/S ratio is 2:1

Lungs are deemed mature

The prescense of Phosphatidylglycerol

Indicates VERY LOW risk for respiratory depression

A count > 50k lamellar bodies

Indicates lung maturity (mercer)

Gestational DM & chronic glomerulonephritis can

Inhibit lung maturity

Neonates born b3 39 weeks via cesarian

• Increased transient tachypnea risk

• Likely to have crackles first 24 to 48 hrs

Omphalocele

• intensines protrude from the umbilicus

Meconium

• fetal waste

• passed thru rectum within 24hrs of birth

How does the fetus prepare for extrauterine cold stress?

It lays down stores of brown fat

Since the placenta clears unconjugated billirubin

• Enzyme for conjugation is needed LESS

• Predisposes neonate to hyperbilirubinemia

Why is vitamin K given to neonates at birth?

• Their gut cannot produce vitamin K which is necessary for the synthesis of coagulation factors

@ birth, kidneys lack ability to concentrate urine

• Increasing risk for

  • Dehydration

  • Overhydration

Acoustic stimulation can evoke

Fetal HR response

Flavors from maternal diet are

Transferred to the fetus thru amniotic fluid

When the fetus swallows amniotic fluid in the 5th month

• sweeteners in fluid causes baby to swallow faster

• bitter substances results in decreased swallowing

• cold substances causes fetal hiccups

Congenital hypothyroidism if untreated can cause

Severe intellectual disability

(why we screen for hypothyroidism at birth)

Mothers with uncontrolled DM

• leads to fetal hyperglycemia

• hypperinsulemia

• islet cell hyperplasia

• large fetus

What does hyperinsulinemia do to the neonate?

Blocks lung maturation → respiratory distress & hypoglycemia

Sutures & fontanelles allow

Bones of skull to mold & enables head to pass thru birth canal

Vernix Caseosa

• protects fetal skim

  • thick at 24 weeks, scant by birth

Lanugo

• fine hairs that may disappear by term gestation

Fetus produced

IgM

Breast milk precursor, colostrum has large amount of

IgA

Hearing is fully developed

At birth

Fraternal/Dizygotic Twins

• same sex or different sex

• increased frequency with maternal age

• increased incidence following discontinuation of oral contraceptives

With conjoined twins,

Cleavage is incomplete and occurs late

Assistive Reproductive Therapy increases

incidence of monozygotic twinning

Multifetal gestation comes with

Increased perinatal complications & fetal anomalies

Since the brain has the longest developmental period

It is extremely vulnerable to teratogens & more defects can occur

Teratogens have greatest effect during

1st 3 months