stages of embryonic development

 Embryonic development

After zygote formation, mitotic devision of zygote producing two blastomere.

16 cell stage called morula (solid mass of blastomere).


Two cell stage (called morula) reached approximately 30 hours after fertilization.

Each blastomere; equal cytoplasmic and chromosome number.

Morula after spending about 3 days in uterine tube enters uterine cavity through narrow uterine ostium on 4th day in the 16-64 cell stage.

Central cell of morula known inner cell mass (form embryo).

Peripheral cell called outer cell mass (form protective and nutritive membranes of embryo).


On 4th and 5th day morula in uterine cavity covered by film of mucus.

Fluid pass through canaliculi of zona pellucida; separate cells of morula called blastocyst.

Due to blastocyst enlargement zona pellucida become stretched, thinned and gradually disappears.

Lysis of zona pellucida and escape of embryo called zona hatching.

Outer side of morula become trophectoderm (adhesion of blastocyst to uterine epithelialium.

Inner side of morula become inner cell mass.

Changes in Blastocyst

Trophoblast cell adjacent to inner cell mass primarily involved in adhesion to endometrial cell.

Responsible for blastocyst attachment are;

Selectins, Integrins, trophinin, tasin, EGF (epidermal growth factor), proteoglycan, Heparin sulphate.


Implementation of blastocyst into endometrium occur 6-7 days after fertilization.

Implementation occurs in endometrium of anterior or posterior wall of body endometrium near fundus on 6th day after fertilization (20th day of regular menstrual cycle).

Endometrium in secretory phase crossponding to 20-21 days of cycle.

Microvilli on surface of trophectoderm integrate with decidual cells of endometrium.

Blastocyst differentiated into-

1. Trophoblast

a. Chorion (outer layer of embryo cell)

b. Placenta 

2. Inner mass cell

a. Amnion (inner layer of embryo cell)

b. Foetus

c. Umbilical cord

3. Blastocele


Implementation stages are;

Stage 1


Stage 2


Stage 3


Stage 4


1. Apposition

Pinopods; long finger like projection (microvilli) form endometrial cell surface.

Endometrial fluid secreted by endometrial gland cells, pinopods absorb endometrial fluid.

Endometrial fluid rich in glycogen and mucin provide nutrition to blastocyst.

2. Adhesion (Attachment)

Adhesion of blastocyst to endometrium occurs through adhesion molecules like integrin, selectin and cadherin (glycoprotein).

3. Penetration and invasion

Trophoblast cells of blastocyst attach to endometrial epithelium. Invasive trophoblast cells cross endometrial epithelial basement membrane and invade.

Uterine tissue breakdown and embryo grow.

Deep penetration of human blastocyst called interstitial implantation.

Decidua formation (blastocyst covered on all sides by endometrium (decidua).

Trophoblast (Trophectoderm)

Trophoblast is cell of blastocyst.

 Trophoblast differentiated into;

1. Cytotrophoblast or Langhan’s layer 

2. Syncytitrophoblast

Local cytokines regulate invasion of the cytotrophoblast in decidua.


Decidua is modified or specialized mucosal lining of endometrium during pregnancy.

Layers of Decidua

3 layers of decidua present;

1. Compact outer layer (stratum compactum)

Stratum compactum contain contain compact mass of decidual cells, gland ducts, dilated capillaries.

2. Intermediate spongy layer (stratum spongiosum)

Stratum spongiosum contains dilated uterine glands, decidual cells and blood vessels.

3. Thin basal layer (stratum basalis)

Stratum basalis is regeneration of mucus coat.

After the interstitial implantation of blastocyst into compact layer of decidua differentiated;

1. Decidua basalis or decidua serotina

Portion of decidua in contact with base of blastocyst.

2. Decidua capsularis

Covering the blastocyst.

3. Decidua parietalis or vera

Lining the rest of uterine cavity.

Growing ovum bulges towards uterine cavity space between decidua capsularis and decidua vera called decidual space gradually narrowed by 4th month.

Completely fusion of decidua capsularis with decidua vera.

Decidua basalis becomes maternal portion of placenta.

Function of Decidua

1. Site of implementation

2. Invasion of trophoblast

3. Supply nutrition to early implanted ovum.

4. Deeper penetration of trophoblast

5. Take part in formation of basal plate of placenta (maternal side of placenta).

Chorion and Chorionic Villi

Chorion is outermost fetal membrane around embryo.

After implantation trophoblast differentiated into; 

1. Outer layer Syncytitrophoblast

2. Inner layer Cytotrophoblast (Langhan’s layer)

Chorion layer made up of trophoblast and mesoderm.

Chorion villi surround developing ovum.

Villi overlying decidua basalis continue to grow and expand called chorion frondosum and form placenta.

Mesodermal tissue (connecting stalk) connects inner cell mass to chorion and will form umbilical cord later on.

Development of Inner Cell Mass

Development of inner cell mass differentiated into;

1. Amnion layer of embryo cell

2. Umbilical cord

3. Foetus

After implantation, 2 cavity appear in inner cell mass.

1. Amniotic cavity/ amniotic sac contains amniotic fluid

Amnion membrane forming fluid filled cavity (amniotic sac that enclose the embryo).

2. Yolk sac

Before placenta form, yolk cell provide nutrition and gas exchange between mother and developing embryo.

Inner Cell Mass 

Inner cell mass known as embryoblast.

Embryoblast development of embryo embryonic stem cell (ESCs) form bilaminar embryonic disc that differentiated into;

1. Epiblast

External layer

Primary ectoderm

Form amnion that surround amniotic cavity.

2. Hypoblast

Internal layer

Primary endoderm

Form yolk sac

Form amniotic cavity.

Note; Connective stalk form umbilical cord.

Embryo Differentiated as Human at 8th Week

1. Ectoderm layer

2. Mesodermal layer

3. Endoderm layer

1. Ectoderm layer 

Form central and peripheral nervous system.

Epidermis of skin

Pituitary gland

Chromaffin organs (adrenal glands)

Salivary gland

Mucus lining of nasal cavity

Paranasal sinus

Roof of mouth

2.  Mesoderm layer

Form bones, cartilage, muscles, cardiovascular system, kidney, spleen, mesothelial lining of pericardial, pleural and peritoneal cavity.

3. Endoderm layer

Epithelial lining of gastrointestinal tract, liver, gall bladder, pancreas, epithelial lining of respiratory tract, mucus membrane of urinary bladder, urethra, bulbo urethral.

Events in Fertilization

0 hour; fertilization (15th day from LMP)

30 hours; 2 cell stage (blastomeres)

40-50 hours; 4 cell stage

72 hours; 12 cell stage

96 hours; 16 cell stage (morula enters uterine cavity)

5th day; blastocyst

4-5th day; zona pellucida disappear

5-6th day; Blastocyst attachment to endometrial surface

6-7th day; differentiation of cyto and syncytitrophoblast layers.

10th day; Synthesis of hCG by syncytitrophoblast.

9-10th day; Lacunar network form 

10-11th; trophoblast invade endometrial sinusoids establishing utero-placental circulation.

Interstitial implantation completed with entire decidual coverage.

13th day; primary villi

16th day; secondary villi

21st day; Tertiary villi

21st -22nd day; fetal heart, feto-placental circulation