Embryo Development: Fertilization to Fetus

Fertilization is the first step of embryo development

Male provides spermatozoon (male gamete),
Female provides egg (female gamete)
Pregnancy or conception occurs in the uterus.

Reproduction process consists of 2 phases of Gametogenesis

  • Oogenesis
  • Spermatogenesis


Maturation of oocyte.
The promordial germ cells develop from the endoderm in the wall of yolk sac.
By 20th wk of IUL, 6-7 millions of 1˚oocytes are formed.
Ovary of a term fetus contain 2-4 million oocyte.
40000 at menarche.
After puberty, few oocyte are employed for maturation in each ovulatory cycle.


Spermatogenesis to Maturation process is about 2 months. Production of sperm begins at puberty continues throughout the adult life.
At each ejaculation, 2-4 ml of semen are deposited in the vagina.
The seminal fluid contains about 100 million sperm per ml, of which 20-25% are likely to be abnormal.
The remainder move at a speed of 2-3 mm per minute.
The individual spermatozoon has a head, a body & long mobile tail that lashes to propel the sperm along. The tip of the head is covered by an acrosome; this contains enzymes to dissolve the covering of the ovum in order to penetrate it.

Conception or pregnancy occurs when fertilized ovum embeds itself in the compact layer of secretory endometrium. Fertilization occurs in the outer part of fallopian tube when mature ovum fuses with spermatozoon to produce fertilized single mononucleated cell called Zygote.

What is Fertilization?

Meeting of ovum and the sperm in the ampullary part of fallopian tube.
Release of HYLURONIDASE from acrosome cap of sperm
Entry of sperms into perivitalline space.
One sperm entry cause vitalline block Dissolution of corona radiata & penetration of zona pellucida, Immobilizes other sperms in perivitelline space. When the ovum has been fertilized, it continues its passage through the uterine tube & reaches the uterus 3 or 4 days later.

Events after fertilization

  • 1st clevage after, 30 hrs.
  • 2nd clevage after, 40 hrs.
  • Morula, 50-60 hrs.
  • Formation of blastocyst, 4-5 days.
  • Implantation, 7th day: Embryo development starts after implantation
  • Completion of implantation, 14th day.

Zygote and Cellular Division

  • Zygote after 30 hrs, produces two blastomere
  • Formation of morula by binary division
  • Transportation of morula along the tube for next 3days
  • Entry to uterine cavity on 4th day (12-16 cell stage)


Morula covered by endometrial fluid and mucus, remains free in uterine cavity for 2 days. Formation of blastocyst from morula Trophoblast embeds into endometrium attaching blastocyst .


Occurs after 6th day of fertilization ( 20th day of menstrual cycle)
Histolytic action of blastocyst causes deeper penetration into decidua.
It begins to secrete substances that digest the endometrial cells, allowing the blastocyst to become embedded in the endometrium.
Embedding know as nidation (nesting).
Process completes by 10-11th day.


The endometrium during pregnancy is called decidua which helps in embryo development.

From the time of conception the increased secretion of oestrogens causes the endometrium to grow to four times its non-pregnant thickness.

Endometrial lining of uterus, sheds after delivery.

Layers of Decidua

The basal layer – this lies immediately above the myometrium.
The functional layer – it consist of tortuous glands. It rich in secretions.
The compact layer – it forms the surface of the decidua & is composed of closely packed stroma cells & the necks of the glands.

The blastocyst embeds within the spongy layer & the different areas of decidua are identified according to their relationship.

  1. Decidua Basalis: Embryo implants to form placenta. Maternal portion.
  2. Decidua Capsularis: Thin layer covering ovum.
  3. Decidua Vera/ Parietalis: Rest covering uterine cavity other than the site of implantation.

How decidua helps in embryo development?

Provides nidus for the implantation of fertilized ovum.
Nutrition ( glycogen & fat) to growing fetus.
Protective action.
Basal plate of placenta.


It differentiate into layers

  1. The outer Syncytiotrophoblast (Syncytium) – it is composed of nucleated protoplasm.
  2. The inner Cytotrophoblast – it produces a hormone Human Chorionic Gonadotrophin (HCG)
  3. Below this layer of Mesoderm – it consists of loose connective tissue.

The inner cell mass

The trophoblast in developing into the placenta, which will nourish the fetus, the inner cell mass is forming the fetus itself.
It differentiate into three germ layers, each of it will form particular parts of the fetus.
They are ectoderm, mesoderm, endoderm
The Ectoderm (Outer Layer): it mainly forms the skin & nervous system.

The Mesoderm ( Middle Layer): it forms bones, muscles, heart & blood vessels, including those in the placenta.
Certain internal organs also originate in the mesoderm.
The Endoderm (Inner Layer) : it forms mucous membranes & glands.
This there layers together known as Embryonic Plate.
Two cavities appear in the inner cell mass – one on either side of the embryonic plate.
They are the Amniotic Cavity & the Yolk Sac

The Amniotic Cavity

It lies on the side of the ectoderm.
It is filled with fluid & gradually enlarges & folds around the embryo to enclose it.
The amnion forms its lining. It swells out into the chorionic cavity (formerly the blastocele) & eventually obliterates it when the amniotic & chorionic membranes came into contact.

The Yolk Sac

It lies on the side of the endoderm & provides nourishment for the embryo until the trophoblast is sufficiently developed to take over.
Part of it contributes to the formation of the primitive gut, the remainder resembles a balloon floating in front of the embryo until it atrophies & becomes trapped under the amnion on the surface of the placenta.
After birth, all that remains of the yolk sac is a vestigial structure in the base of the umbilical cord, know as the vitelline duct.

The Embryo:

This name is applied to the developing offspring after implantation & until 8 weeks of conception. Further embryo development forms a fetus.
During this period all the organs & systems of the body are laid down in rudimentary form.
So that its completion they have simply to grow & mature for a further 7 months.
The conceptus is known as a fetus during this time.

How fetus develops?

For the first 3 weeks following conception the term Fertilized Ovum or Zygote.
From 3 -8 weeks after conception is known as the period of embryo development & following this it is the fetus until birth, then it becomes a baby.

Embryo development

4 Weeks after conception:

The embryo is 4-5mm in length
Rapid growth
Formation of the embryonic plate
Chorionic villi form
Primitive central nervous system forms
Heart develops & begins to beat
Limb buds form
Rudiments of eye, ears & nose appears.

5 – 8 Weeks:

The fetus is 27-31mm in length & weighs 2-4 grams
Very rapid cell division
Head & facial features develop
Head is disproportionately large as a result of brain development
All major organs laid down in primitive form
External genitalia present but sex not distinguishable
Early movements
Visible on ultrasound from 6 weeks
Centers of bone begin to ossify.

9 – 12 Weeks:

The fetus average length is 50-87 mm & weight is 45 gms
Eyelids fuse
Kidneys begin to function & the fetus passes urine from 10 weeks
Fetal circulation functioning properly
Sucking & swallowing begin
Sex apparent
Moves freely (not felt by mother)
Some primitive reflexes present
Placenta & fetal circulation is complete

13 – 17 Weeks:

The fetus is 94-100 mm in length & weighs 97- 200 gms
Rapid skeletal development – visible on X- ray
Head is erect
Meconium present in gut
Lanugo appears
Nasal septum & palate fuse
Fingerprints are set
Coordinated limb movements are present

17 – 20 Weeks

The fetus is 150 – 190 mm in length & weighs apporoximately 260 – 460 gms
“Quickening” – mother feels fetal movements
Fetal heart heard on auscultation
Vernix caseosa appears
Fingernails can be seen
Skin cells begin to be renewed

21 – 25 Weeks:

The fetus is about 200 – 240 mm in length & weighs 499 – 910 gms
Most organs become capable of functioning
Periods of sleep & activity
Responds to sound
Skin red & wrinkled
Eyebrows & fingernails develop

26 – 29 Weeks:

The fetus is 250 – 275 mm in length & weighs about 910 – 1500 gms
Survival may be expected if born
Pupillary membrane disappears from eyes
Eyelids reopen
Rhythmic respiratory movements occurs
Skin is red

30 – 34 Weeks:

The fetus is 280 – 320 mm in length & weighs about 1700 – 2500 gms
Begins to store fat & iron
Testes descend into scrotum
Lanugo disappears from face
Skin becomes paler & less wrinkled
Toenails become visible
Vigorous fetal movement occurs

35 – 37 Weeks:

The fetus is 330 – 360 mm in length & weighs about 2700 – 3400 gms
Increased fat makes the body more rounded
Lanugo disappears from body
Head hair lengthens
Nails reach tips of fingers
Ear cartilage soft
Plantar creases visible
Face & body have a loose wrinkled appearance
Amniotic fluid decreases

38 Weeks after conception – full term

The fetus is 360 mm in length & weighs about 3400 – 3600 gms
Term is reached & birth is due
Contours rounded
Skull firm
Skin is smooth
Chest is prominent
Bones of skull are ossified & nearly together at sutures

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