Embryology overview

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What is embryology?

Study of origin & development of an organism

What is the time frame in which we study embryology?

Prenatal period! - (before birth)

Human gestation = ~ 38 weeks

Gynecologists time gestation from last menstrual period until birth - 40 weeks

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Embryonic vs foetal period of development

i) Embryonic period

First 8 weeks after conception

Relates to 1st trimester

All major organs formed at this stage

Basic body plan laid out

ii) Foetal period

Remaining 30 weeks after embryonic period until birth

Relates to 2nd & 3rd trimesters

Organs grow larger & become more complex

Foetus begins 'practicing' for life outside womb - breathing movements, moving limbs etc

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Ovulation, fertilisation & pre-implantation division

i) Ovulation

On approx day 14, a secondary oocyte + polar body are released from a mature Graafian follicle into the peritoneal cavity

The fimbriae of the oviduct 'capture' the released egg & send it down the Fallopian tube

(See GGE female reproduction lecture pg.3 ii) d) oocyte changes around ovulation)

ii) Fertilisation

12-24 hours after ovulation

Any later & the oocyte will die

Occurs when a sperm penetrates the oocyte

Zona pellucida

Cortical reaction

(See GGE fertilisation lecture pg.5 & 6)

iii) Cleavage

1.5 - 3 days post ovulation

Zygote undergoes rapid mitotic division (cleavage)

Each division occurs approx every 20 hours in cells called blastomeres

The number of cells doubles @ each division (exponential growth)

2...4...8...16...32....

Occurs whilst travelling down Fallopian tube

a) Morula

When the cell count reaches 16-32 cells... zygote becomes a morula

Morula = 'mulberry-shaped'

Zona pellucida limits the extent to which morula can grow

Morula leaves oviduct & enters uterine cavity 3-4 days after fertilisation

b) Blastocyst

4 days post-ovulation

Morula has entered uterine cavity

Cell division is still occurring...

However cell differentiation is beginning!

How does cell differentiation occur?

Cells decide what kind of body part they're going to be - whether that's an eye cell or a liver cell & everything inbetween

99% of the time this decision is final (cells can't keep changing their mind every 5 minutes)

Leads to formation of 2 layers in morula:

Trophoblast:

Outer shell layer located just underneath ZP

Will develop into placenta

Inner cell mass:

​​​​​​​'Lump' of pluripotent cells located underneath trophoblast

Parts of it will become embryo

(Division of this = monozygotic twins!)

The rest of the cavity becomes filled with fluid - it is called the blastocele

Expansion of the blastocele puts pressure on the ZP

Eventually the blastocyst will 'hatch' from ZP on ~6th day after fertilisation

Blastocyst can now grow & change shape

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Implantation & placenta formation

i) Implantation

5-6 days post ovulation

Trophoblast secretes an enzyme which erodes epithelial lining of uterus

Allows blastocyst to implant

hCG begins to be secreted by trophoblast cells

Stimulates corpus luteum to continue producing progesterone ( to maintain endometrium)

Trophoblasts will continue to engluf & destroy endometrial cells

Stimulate capillary growth

Begin formation of placenta

Implantation v similar to invasion of malignant tumours

Except implantation = controlled

ii) Ectopic pregnancies

Occur when blastocyst does not implant in uterus

Can continue for up to 16 weeks unnoticed

May be treated pharmacologically if caught early - otherwise surgery required

What locations can the blastocyst implant in other than the uterus?

See diagrams below

Implantation @ internal os = potential exposure of foetal membranes to vagina (microbes!)

Increased risk of pre-term birth

Breech birth

iii) Inner cell mass differentiation

~13 days post ovulation

Differentiates into 2 layers:

Epiblast

Will become embryo & amniotic cavity

Hypoblast

Will become yolk sac

Where the epiblast & hypoblast press against each other = bilaminar disc

​​​​​​​Splits the blastocyst sphere into 2 cavities

iv) Placenta formation

13 days post-ovulation

Chorionic villi 'fingers' form in the 'placenta'

Anchors embryo to uterus

Embryonic blood vessels start appearing in the villi

Yolk sac begins to produce haematopoeitic cells

Stalk formation also occurs at this time

Stalk will become umbilical cord

@ around day 18 post fertilisation maternal lacunae form around the villi

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Gastrulation

i) What happens during gastrulation?

Formation of the primitive streak

​​​​​​​Occurs day 15

Primitive streak cuts along the surface of the epiblast

​​​​​​​From the caudal end towards the rostral end (but doesn't completely transect epiblast - just cuts it a little bit)

Marks bilateral symmetry in embryo

Below image = superior view of epiblast

Cells migrate down the gap created by the primitive streak & 'wash' underneath it

Occurs ~ day 16

They fill the space beneath the epiblast & above the hypoblast

Creates a 'sandwich' of new cell layers:

Cells that washed down first & are closest to the hypoblast = endoderm

Cells that sit in the middle of the sandwich = mesoderm

Cells of epiblast that border the amniotic cavity = ectoderm

ii) Major derivatives of embryonic germ layers

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Neurulation

i) What happens during neurulation?

Occurs between days 16-23 post ovulation

Formation of the notochord in mesoderm

Also neural plate, neural tube & neural crest formed

(See CNS anatomy - Embryology lecture)

ii) Appearance of somites

19-21 days post ovulation

Somites originate in mesoderm

Appear on either side of neural groove

Will differentiate into muscle, cartilage, bone & dermis

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Further development

i) Other developments that occur until second trimester

Formation of heart

Limb formation

Skin formation

GIT formation

Brain blue-print laid out

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Developmental abnormalities

i) Congenital

Usually involve DNA mutations or chromosomal abnormalities

Includes disorders like Trisomy 21, Trisomy 18, haemophilia, sickle-cell disease

(See GGE genetics lectures for lots of examples!)

Occurance chromosomal abnormalities increases in older mothers

ii) Maternally derived

Relates to lifestyle, environment & nutrition

Excess alcohol (Foetal alcohol syndrome)

Viral infections at critical stages of development (Rubella)

Inadequate dietary folate intake (Spina bifida)

Effects of prescription drugs (Thalidomide)

Maternal endocrine function (Hypo/hyperthyroidism)

Smoking (Low birth weight, decreased height)

Illegal drug use (Heroin-addicted foetuses)

Intrauterine environment may have strong influence on postnatal health

Hypertension, coronary heart disease & Type II diabetes will also have an effect

iii) Teratogens & risks

Teratogens:

​​​​​​​Agent that causes structural abnormality in the foetus

Effect depends on dosage & time of exposure

Absolute risk = rate of occurrence of abnormal phenotype among individuals exposed to the agent

i.e. foetal alcohol syndrome

Relative risk = Ratio of rate of occurrence among exposed & non-exposed

Smokers risk of having low birth weight baby compared to non-smokers

High relative risk may indicate low absolute risk if condition rare