M1 Prenatal Development

Definition of prenatal development

the continuous biological process in which a single-cell fertilized egg grows and transforms into a full-term baby inside the mother’s uterus over an average of 38 weeks

What are the four stages of prenatal development?

1. Fertilization-Zygote

2. Development of blastocyst

3. Development of the embryo

4. Development of the fetus and placenta

Process of spermatozoa development

Diploid (2n): Spermatogonium (to grow and mature)

Diploid (2n): Primary spermatocyte (then moves to Meiosis 1)

Haploid (n): Secondary spermatocyte (then moves to Meiosis 2)

Haploid (n): Spermatids (then moves to differentiation)

Haploid (n): Spermatozoa

Process of ovum development

Diploid (2n): Oogonium (to grow and mature)

Diploid (2n): Primary oocyte (then moves to Meiosis 1)

Haploid (n): Secondary oocyte (then moves to Meiosis 2) and first polar body

Haploid (n): Ootid (then moves to differentiation) and second polar body

Haploid (n): Ovum

Ovum release pathway

Ovary → Fallopian Tube → Uterus

What happens to the ovum if it is fertilized?

Conception occurs, forming a zygote

What happens to the ovum if it is not fertilized?

It disintegrates and gets flushed away with menstruation

What is the definition of zygote?

The union of the ovum and sperm in the fallopian tubes

What is the timeline of zygote development?

1. Zygote

2. Morula

3. Blastocyst

4. Blastocyst implantation

5. Embryo

What important developmental transition does a blastocyst represent?

From homogeneous cells to differentiated cell function

What two layers does the blastocyst differentiate into?

Trophoblast (outer layer) and Embryoblast (inner cell mass)

What is ICM?

Inner cell mass, part of the blastocyst that will give rise to an embryo

What is the purpose of a trophoblast?

Involved in implantation and finally form extra-embryonic tissues such as placenta and villi

When does the blastocyst attach itself to the lining of the uterus?

Between 5-8 days after fertilization

What is implantation?

An invasive activity where trophoblast cells helps to give the embryo access to deeper layers of the uterine wall

What two layers does the trophoblast differentiate itself into during implantation?

Syncytiotrophoblast and Cytiotrophoblast

What two layers does the ICM differentiate itself into during implantation?

Internal (ventral) hypoblast and external (dorsal) epiblast

When is implantation generally completed?

Day 9/10, the amnion and yolk sac are also formed

What are the development stages?

1. Embryo: first 8 weeks of human embryo development

2. Fetus: embryo after initial 8 weeks

3. Neonate: after birth

What is the hypoblast layer of the ICM?

a transient structure which contributes to the development of the extraembryonic mesoderm and yolk sac and plays a key role in signaling to establish axial patterning in the embryo itself

What is the epiblast layer of the ICM?

Gives rise to the embryo proper, forming all three germ layers (ectoderm, mesoderm, and endoderm) during gastrulation

What develops in the ectoderm layer?

Nervous system, skin, hair, nails, mouth, anus

“Atract-oderm”: Skin, hair are things people are attracted to

What develops in the mesoderm layer?

Muscoskeleton, circulatory system, gonads, adrenal cortex.

“Move-oderm”: Involved in moving things such as muscles, RBC, steroids.

What develops in the endoderm layer?

Endocrine glands, GI tract, respiratory tract, bronchi, bladder, stomach.

“In-doderm”: Things that are inside.

During the embryo phase, what is this stage characterized as?

The formation of most internal organs and external body structures

What is the timeline for organ formation during the first trimester?

Heart/blood vessels: formation = day 16, function = day 20

Most other other organs: complete formation = 10 weeks after fertilization

Brain/spinal cord: exception, as it continues to develop/form throughout the pregnancy

Most malformations (birth defects) occur during what period?

The first trimester during organ formation

• Embryo is most vulnerable to the effects of drugs, radiation, and viruses

What is the purpose of a placenta barrier?

A thin membrane separates the embryo's blood in the villi from the mother's blood that flows through the intervillous space

• allows materials to be exchanged between the blood of the mother and that of the embryo

• prevents the mother's immune system from attacking the embryo because the mother's antibodies are too large to pass through the membrane

Define some milestones of a fetus at 12 weeks of gestation?

Fetus fills entire uterus

Define some milestones of a fetus at 14 weeks of gestation.

Sex can be identified, developed extremities and internal organs (not functioning yet)

Define some milestones of a fetus at 16-20 weeks of gestation.

Fetus movement, covered in lanugo, heartbeat can be heard

Define some milestones of a fetus at 24 weeks of gestation.

High chance of survival outside uterus, breathing motions start, fat deposits to skin

Define some milestones of a fetus at 32 weeks of gestation.

Fetus turned head-down for birth, may react/jerk to noises, more fat deposits

The placenta develops from what two sources?

Fetal component: Chorionic frondosum

Maternal component: Decidua basalis

What does the fetal surface of the placenta look like?

• Covered by smooth and glistening amnion overlying the chorion'

• Umbilical cord is attached or near its center

• Makes up 4/5 of placenta

What does the maternal surface of the placenta look like?

• Rough and spongy

• Remanants of the decidua basalis gives it shaggy appearance

• Divided into 15-20 cotyledons by the septa

• Makes up 1/5 of placenta

When does a placenta start forming and is fully formed?

Start forming: 6 weeks

Fully formed: 18-20 weeks, but continues to grow throughout pregnancy

What is the structure of the placenta?

Outer layers:

• amniotic membrane on the fetal side

• decidua basal plate on the maternal

Inner Layer:

• intervillous space filled with maternal blood

• stem villi with their branches

What are the functions of the placenta?

BREEN

• Barrier (protects fetus from harmful agents in maternal blood)

• Respiratory (simple diffusion of O₂ and CO₂)

• Excretory (simple diffusion of urea, uric acid, creatinine)

• Endocrine (excretes hCG/enzymes)

• Nutritive (simple/active diffusion of glucose, amino acids, lipids, water, etc. from maternal blood)

What is amniotic fluid?

It is the fluid in the amniotic sac surrounding the fetus, originating from both fetus and mother

How does amniotic fluid vary with gestational age?

It increases:

• 12 weeks: 50 mL

• 20 weeks: 400 mL

• 36 weeks: 800-1000 mL

• At term: ~700 mL

What is amniotic fluid composed of?

• Organic

  • Nutrients (proteins, glucose, lipids)

  • Excretory waste (urea, uric acid, creatinine)

  • Hormones/enzymes (insulin, prolactin, renin)

• Inorganic

  • Na, K, Cl

• Suspended particles

  • Lanugo, cell sheddings, vernix caseosa

What is the color of amniotic fluid? any physical characteristics?

Clear or pale yellow, some turbidity, faintly alkaline

What is the function of amniotic fluid during pregnancy?

• Shock absorber

• Temperature maintenance

• Allows free movement/growth

• Prevents adhesion formation between the fetal parts and the amniotic sac

• Provides pool for the fetus to excrete urine

What is the function of amniotic fluid during labor?

• Forms hydrostatic wedge to help dilatation of cervix

• During uterine contractions , the amniotic fluid in the intact membranes prevents interference with placental circulation

• Protect the fetus from the ascending infections by its bactercidal action