DEVELOPMENTAL STAGES OF EMBRYO

Stages

Fertilization

Cleavage formation

Gastrulation

Neurolation

Germ layers formation

Formation of extra embryonic cavities

Spermatogenesis

Oogenesis

FertilizationFertilization is

the process of

fusion of the

spermatozoon

with the mature

ovum.

Embryo

Stage of human

development

occurring between

the ovum and the

fetal stages, or from

2-8 weeks after

conception.

PREEMBRYONIC PERIOD (FIRST TO THIRD WEEK)

First week of life:

Day 1: Fertilization

Day 2 and 3: Cleavage

The original zygote divides about

30 hours after conception into two

daughter cells called

blastomeres.

Continued subdivisions of the

original cell result in increasing

numbers of blastomeres.

During cell division the dividing

cells decrease in size. This type

of cell division is called cleavage.

By the time the zygote is ready to

enter the uterus, it contains a

solid ball of 12 to 16 blastomeres

called the morula (from the latin

word for mulberry).

Day 4: Formation of the blastocyst

Fluid within the intercellular spaces

of the morula gradually increases,

and spaces on one side of the inner

cell mass come together, forming a

single cavity, the blastocele.

The outer layer organizes into the

trophoblasts, which give rise to the

placenta, and the inner layer of cells

form the embryo.

The cavity of the blastocele fills with

fluid, and the conceptus is now called

the blastocyst.

Preparation of the endometrium

Resting phase

Proliferative phase

Secretory phase

Attachment of the blastocyst The blastocyst attaches to the

uterine lining in the V-shaped.

When the trophoblast (the outer

cell layer) attaches to the

endometrium, it proliferates and

separates into an inner

cytotrophoblastic layer (fetal side)

and an outer syncytiotrophoblastic

(placental side).

The outer layer develops finger like

projections that proliferate and

superficially attach the blastocyst

to the endometrium within 6 days

after conception.

MAJOR EVENTS OF FIRST WEEK

NORMAL EVENTS POSSIBLE ABNORMAL

EVENTS

Fertilization and

formation of the

zygote (30hours).

Cleavage of the

zygote into 12 to 16

blastomeres- the

morula (day 2 and 3).

Formation of the

blastocyst ( day 5-8).

Abnormal

implantation

Maternal infection or

a genetic defect

Hydatidiform mole

Abortion

Ectopic implantation

Second week of life A slitlike amniotic cavity appears about

day 8, and the yolk sac appears as a

second cavity on day 12. Bilaminar

embryonic disc is formed in between

these two layers.

The endodermal disc becomes thicker

at it’s cephalad end, forming the

prochordal plate.

During early development of the

nervous system, the function of the

prochordal plate is to indicate the site of

the mouth and to form the membranes

of the mouth and throat.

The formation of the decidua, fetal

membranes, and placenta extends

beyond the second week, but their

development begins at this point.

THIRD WEEK OF LIFE

During the third week of life, the conceptus

develops rapidly. This period also coincides with

the first missed menstrual cycle of the mother.

The primitive streak is formed during the third

week, and three germ layers develop.

This periods from approximately day 15 to day

21, is called the “period of threes”; not only do

the three germ layers develop, but the primitive

streak, the notochord, and the neural tube are

formed.

GASTRULATION

Gastrulation is the process by

which the bilaminar embryo

becomes a trilaminar embryo.

On about day 15, the

cytotrophoblast cells proliferate

into the blastocyst to form the

extraembryonic mesoderm,

which later become the extra-

embryonic coelom.

The mesoderm lies between the

ectoderm and the endoderm,

completing the trilaminar disc of

the primitive streak. All tissues

and organs of the embryo are

developed from these three

layers.

Major derivatives of the embryonic germ layers

Notochord

Days 16-18

Primitive node

epiblast cells

invaginate and

migrate anteriorly with

some endoderm cells

Rod defining the body

axis is formed

Future site of the

vertebral column

Neurulation

Notochord signals overlying ectoderm

The neural tube is developed from the closure of the

neural plate and the neural fold- a process called

neurulation –at about 21 to 26 days.

Closure of neural tube: begins at end of week 3;

complete by end of week 4.

Extends cranially (eventually brain) and caudally

(spinal cord)

Neural crest, lateral ectodermal cells, pulled along and

form sensory nerve cells and other structures

DIVISON INTO SOMITES

About day 20, the mesoderm divides into paired bodies

called somites.

Located on either side of the developing neural tube, these

paired bodies give rise to the skeleton and muscle tissue.

During the somite period, day 20 to 30, 38 pairs of somites

develop.

Their total number eventually reaches 42 to 44 pairs, of

which 4 are occipital, 8 cervical, 12 thoracic, 5 lumbar, 5

sacral and 8 to 10 coccygeal.

Some of the somites – first occipital and the fifth to seventh

coccygeal –disappear, while the rest form the axial

skeleton.

By the end of the third week of life, the conceptus is about

1.0 mm in length.

fourth week of life

The embryo grows

dramatically during

the fourth week.

It more than triples its

length to 3.5mm and

its weight

approximates 5 mg.

Elongation of the embryo has

occurred, and it has become

curved pon itself with the formation

of a head and tail fold.

Lateral body folds develop making

the embryo tubular rather than flat

and disc shaped.

Closure of the neural tube begins

in the area of the occiput and

proceeds upward and downward

from that point.

Somites formed in a craniocaudal

sequence as the neural tube

closes, can be observed through

the ectoderm.

The pericardial sac

around the heart

enlarges, causing the

head region to elevate.

The larygotracheal

groove and lung buds,

which will become the

respiratory system, are

present.

The mandible and

maxilla of the jaw

become distinct, and

rudimentary forms of the

eyes, ears and nose are

The intestinal system is

formed from the yolk sac,

and differentiation of the

buds, which will become the

oesophagus, stomach, liver

and pancreas, is progressing.

The thyroid and thymus

glands are also developing.

The primitive circulatory

system is established, and

the heart is beating.

The budlike projections on

the surface of the embryo are

the beginning of the limbs.

FIFTH WEEKS OF LIFE

As the embryo and then the fetus grow during the

first half of pregnancy, it is measured by its crown –

rump length (CRL).

The fetus is measured during the last half of

pregnancy by its crown heel length (CHL), or

standing height. The CRL grows from 4 to 8 mm in

this week, and exceeds the growth.

The growth of the head is rapid and exceeds the

growth of the body during this week. The embryo

lengthens and bends into a C shape, while an

additional 42 to 44 pairs of somites are added to its

caudal end.

The umbilical cord is

formed from the union of

the amnion, the yolk, and

the connecting stalk. It now

contains two umbilical

arteries and one umbilical

vein.

The doubling of the size of

the heart makes it

prominent, and its atria and

ventricles are visible

through the ectoderm. The

embryo’s four limb buds

are most vulnerable to

teratogens at this time.

SIXTH WEEK OF LIFE The head has become larger than the

trunk and is bent over the heart

prominence.

Elevations in the facial ectoderm are

evident and the position of the eyes,

nose, mouth, and the groove that is to

become the external acoustic meatus is

established.

In the upper limbs, the elbows and wrists

are identifiable, and the hand plates

develop ridges called finger rays.

The lower limbs are not developed to the

same degree.

Changes are beginning to occur in the

genital region. (The CRL is 8 to 14 mm,

the weight 50 to 400mg).

SEVENTH WEEK OF LIFE

Cerebral hemispheres appear as the head enlarges

rapidly.

The eyes move from a lateral to a more frontal

position as the face elongates.

Prominences appear over the ventral body wall from

early because their function is vital to the maintenance

and survival of the embryo.

As the embryo continues to grow, the umbilical cord

shrinks.

The arm and hand of the upper limbs and the thigh,

leg, and foot segments of the lower limbs become

apparent.

The fingers develop, the their growth is critical at this

point (40 to 50 days).

EIGHTH WEEK OF LIFE

During this final week of the

embryonic period, the embryo

exhibits definite human

characteristics.

The cerebral hemispheres have

grown so rapidly that the head now

makes up 50% of the mass of the

embryo.

The face occupies the lower half of

the head, and the eyes continue to

move to a more frontal plane.

Eyelids folds develop. These will

become fused during the ninth week

and remain so until the seventh

The fingers lengthen, and the toes

are distinct by the end of the

eighth week.

The external ears are set low and

are taking on their final shape.

Sexual differences in the external

genitalia can now be seen by the

trained eye.

(The CRL is 21 to 30mm; the

weight 1000 to 3000 mg).

MAJOR EVENTS OF

fourth to eighth WEEK

NORMAL EVENTS POSSIBLE ABNORMAL

EVENTS

Conversion of the flat trilaminar embryonic

disc into a c- shaped cylindrical embryo.

Formation of the head, tail, and lateral folds.

Formation of the lateral and ventral body

walls.

Acquisition of an epithelial covering by the

umbilicus through the expansion of the

amnion.

Establishment of ventral position of the heart

and development of the brain in the cranial

region of the embryo.

Differentiation of the three germ layers into

various tissues and layers that will become

established as the major organ systems.

Appearance of the brain, limbs, ears, eyes,

Abnormalities of

the genes and

chromosomes.

Alterations of

maternal health,

such as infection

from rubella or

herpes.

Ingestion of

teratogenic

substances.

The risk of mortality

is greater than at

any other time of

life.

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