Early fetal development pdf - dr. afraa

Fertilization, Implantation,

early fetal development &

placental development

Dr. Afraa Mahjoob Al-Naddawi

The human body has two types of cells:

1) Somatic cells: contain 23 pairs of chromosomes

(diploid), one of each pair originally is derived from
the mother and the other is from the father.

2) Germ cells: mature germ cells contain only one copy

of each chromosome, i.e. they are haploid in number
of chromosomes, these are found in the gonads
(testes in male, ovaries in female).

Fertilization is the fusion of ovum and sperm to form a

new  microorganism  (called  zygote),  this  process
usually  takes  place  in  the  ampullary  region  of
fallopian tube and it is composed of:

1. Penetration of

corona radiata.

2. Penetration of

zona pellucida

3. Sperm head attaches to ovum membrane.

4. The ovum reacts to sperm contact by zonal reaction

and  completion  of  the  second  meiotic  division  from
metaphase  II  and  expulsion  of  second  polar  body,
thus forming the female haploid pronucleus.

5. Once within the oocyte, the sperm tail degenerates

and the head enlarges to be transformed to male
pronucleus.

6. Male and female pronuclei approximate and fuse

forming the zygote which returns to diploid genetic
constitution.

• Fertilization of the ovum must occur a few hours and no

more than a day after ovulation

• 1 week postfertilization corresponds to approximately 3

weeks from the last menstrual period in women with regular
28-day cycles

Fertilization in the fallopian tube

zygote

a diploid cell with 46 chromosomes

blastomeres

morula

(solid mulberry-like ball of cells)

enters the uterine cavity about 3 days after fertilization

gradual accumulation of fluid between the cells of the

morula

early blastocyst

Cleavage of the zygote and

formation of the blastocyst.

A through D show various stages

of  cleavage.  The  period  of  the
morula  begins  at  the  12-  to  16-
cell  stage  and  ends  when  the
blastocyst  forms,  which  occurs
when  there  are  50  to  60
blastomeres present.

and

are

sections

blastocysts.  The  zona  pellucida
has  disappeared  by  the  late
blastocyst stage (5 days).

The polar bodies shown in A are

small, nonfunctional cells that
soon degenerate.

• In a 58-cell blastocyst,

– differentiates into: (4 to 5 days after fertilization)
– inner cell mass - five embryo-producing cells
– trophoblasts - 53 cells

Implantation: a process by which the blastocyst attaches

to the uterine wall and penetrates first the
endometrium then the circulation of the mother to form
the placenta.

It is divided into 3 phases:

1. Apposition.

2. Adhesion.

3. Invasion: penetration of the syncytiotrophoblast and

cytotrophoblast into the endometrium, inner third of the
myometrium and uterine vasculature.

• Occurs 6 or 7 days after fertilization
• Uterine receptivity limited to days 20-24
• Apposition – blastocyst loosely adheres to the

endometrial epithelial, most commonly on the upper
posterior wall of the uterus

• Integrins - mediate the adhesion of cells to

extracellular matrix proteins

First weeks of human development:
Blastocyst embedded in endometrium.

TROPHOBLAST DIFFERENTIATION

(8th DAY POSTFERTILIZATION)

• cytotrophoblasts

– inner layer of primitive mononuclear
– germinal cells for the syncytium
– primary secretory component within the placenta

• syncytiotrophoblast

– amorphous cytoplasm
– without cell borders
– multiple and diverse in size and shape of nuclei

FURTHER TROPHOBLAST DIFFERENTIATION (2 MAIN

PATHWAYS):

• Villous trophoblast

– gives rise to chorionic villi of the placenta
– transports O2 and nutrients between fetus and mother

• Extravillous trophoblast

– migrates into the decidua and myometrium
– penetrates maternal vasculature

CLASSIFICATIONS OF

EXTRAVILLOUS TROPHOBLAST

• Interstitial

– invades maternal deciduas  penetrates myometrium

 form placental bed giant cells

– Surrounds maternal spiral arteries

• Endovascular – penetrates the lumen of spiral arteries

EARLY TROPHOBLAST INVASION

• Day 10 – blastocyst totally encased in the endometrium
• Mechanisms leading to trophoblast invasion into the

endometrium

– similar to characteristics of metastasizing

malignant cells

• 7 ½ days after fertilization

– Inner cell mass

– embryonic disc
– Differentiate into a thick plate of primitive ectoderm

and an underlying layer of endoderm

• Chorionic vesicle – cavity completely lined with

mesoderm

• Chorion – chorionic vesicle membranes, composed of

trophoblasts and mesenchyme

• Body stalk – join the embryo to nutrient chorion, later

develops into umbilical cord

LACUNAE FORMATION WITHIN THE

SYNCYTIOTROPHOBLAST

• 12 days after conception

• Syncitiotrophoblast of the trophoblast shell is permeated

by a system of intercommunicating channels of
trophoblastic lacunae

• As embryo enlarges, the lacunae become filled with

maternal blood

DEVELOPMENT OF PRIMARY VILLOUS STALKS

• Primary villi – composed of a cytotrophoblasts core

covered by syncitium

• Complicated labyrinth – formed by joining of lacunae,

partitioned by solid cytotrophoblastic columns

Drawing of sections through implanted blastocysts.
A. At 10 days. B. At 12 days

after fertilization. The stage of development is characterized by

the

intercommunication of the lacunae filled with maternal blood. Note in B that

large

cavities have appeared in the extraembryonic mesoderm, forming the beginning of the
extraembryonic coelom. Also note that extraembryonic endodermal cells have begun to form
on the inside of the primitive yolk sac. (From

Moore, 1988.

)

CHORIONIC VILLI

• 12th day after fertilization – chorionic villi can first be

distinguished

• Secondary villi – formed by mesenchymal cords

invading the solid trophoblast columns

• Tertiary villi – secondary villi with angiogenesis

• 14th or 15th day after fertilization

• maternal arterial blood enters the intervillous space

• 17th day

• fetal blood vessels are functional
• Placental circulation is established

DECIDUAL SPIRAL ARTERY INVASION

• Extravillous trophoblast

• Interstitial trophoblast

• surround the maternal spiral arteries
• constitute a major portion of the placental bed
• penetrates the decidua and adjacent myometrium
• functions to prepare vessels to facilitate invasion by

the endovascular trophoblast

• Endovascular trophoblast

• penetrates the lumen of the spiral arteries

• Timing of the development of uteroplacental vessels

• 1st wave - before 12th weeks postfertilization
• 2nd wave - 12th-16th weeks postfertilization

DEVELOPMENT OF CHORION AND DECIDUA

• Chorion frondosum

• so-called leafy chorion
• fetal component of the placenta
• proliferation of the chorionic villi in contact with the

deciduas basalis

• Chorion Laeve

• smooth chorion
• Avascular fetal membrane that touches the decidua

parietalis

• Composed of cytotrophoblasts and fetal mesodermal

(mesenchymal) cells that survive in a relatively low-
oxygen atmosphere

• Chorion leave and amnion

• separated by exocoelomic cavity until the end of 3rd

month

• form an avascular amniochorion
• important sites of molecular transfer and metabolic

activity

• constitute an important paracrine arm of the fetal-

maternal communication system

• Decidua vera

• area where deciduas Capsularis and deciduas

Parietalis merge

Placental Cotyledons

• Certain villi of the chorion frondosum extend from

chorionic plate to the decidua and serve as anchoring
villi

• Each of the main stem villi(truncal) and their

ramifications (rami) constitute a placental cotyledon
(lobe)

• For each cotyledon, a 1:1:1 ratio of artery to vein to

cotyledon

The Placenta

The Amnion

Innermost fetal membrane and is contiguous with amnionic

fluid

Avascular structure

Provide almost all of the tensile strength of the fetal

membranes

• protect against rupture or tearing

FROM CONCEPTION TO BIRTH

Phases of Prenatal Development

• Period of the zygote: conception through

implantation

• Period of the embryo: 3rd-8th week, organ

formation, heart beat

• Period of the fetus: 9th week-birth

THE PERIOD OF THE ZYGOTE

Blastocyst: 60-80 cells

• Embryo – inner layer of blastocyst
• Protective/nourishing tissues – outer layer

Implantation:

• 7-10 days after conception
• Tapping mother’s blood supply through uterine

wall

PRIMARY GERM LAYERS

• Ectoderm
• Mesoderm
• Endoderm

THE PERIOD OF THE EMBRYO

Ectoderm (outer layer)

• Nervous system
• Skin
• Hair

Mesoderm (middle layer)

• Muscles
• Bones
• Circulatory system

Endoderm (inner layer)

• Digestive system
• Lungs
• Urinary tract
• Vital organs (pancreas, liver, etc.)

Developmental Milestones

• 3rd week – neural tube
• 4th week – heart beat
• 7th week – a rudimentary skeleton
• 7th-8th weeks – sexual development

• If male, the Y chromosome triggers a

reaction to produce testes, otherwise
ovaries result

•

Figure 4.3 A human embryo at 40 days.

THE PERIOD OF THE FETUS

Third Month

• Movement – cannot yet be felt by mother
• Digestive system and excretory systems

functioning

• Reproductive system contains immature ova or

sperm cells

Fourth-Sixth Months

• Sucking, swallowing, breathing
• Movements – felt by mother
• Heart beat can be heard with stethoscope
• Sweat glands functioning
• Vernix – protects skin from chapping
• Lanugo – fine hair helps vernix stick to skin
• Visual and auditory senses are functional

Seventh

– Ninth Months

• Age of viability - 22-28 weeks – survival outside

the womb is possible

• Weight is 4 pounds (at end of 7th month)
• 9th month – activity slows, sleep increases
• Birth occurs

GESTATIONAL AGE

Gestational age=Time since last menstrual period (LMP)

EDC, EDD, EDB

266 Days after fertilization

280 Days after onset of LMP

Fertilization

LMP

Days

266

280

Weeks

Calendar Months

8 3/4

Lunar Months

9 1/2

Nagele’s Rule
Add 7 days to the first day of the last normal menstrual

period, subtract 3 months and add 1 year.

Example:

1st day of LNMP=December 16 2006

add 7 days = Dec. 23 2006

subtract 3 months = Sept 23 2006

add 1 year =Sept 23 2007, estimated due date (EDD)

TRIMESTERS

Trimester

1st

week’s 1-13

2nd

week’s 14 - 26

3rd

week’s 27 and on

Thank you