Raouf Abdel-Rahman Fadel First week of development 1

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HANDOUT # 2

FIRST WEEK OF HUMAN

DEVELOPMENT

Anatomy Department

R.A. FADEL

Raouf Abdel-Rahman Fadel First week of development 2

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لَو ْدنَوا َو َو َو ْد ننَوااَو خَو يا ِّا ا نُس َو َو ٍة اْد {12}ط

{13 } َّم ياٍة َواَو اٍة ف ي نُسطْد َو ًة َو َولْدنَوااُس ُس َّم

المؤمنون سورة

Raouf Abdel-Rahman Fadel First week of development 3

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First Week of Human Development

Fertilization to implantation

The term development is not synonym to pregnancy, as the first week of development

(embryo) is approximately equal to the third week of pregnancy (mother).

Intra uterine life of human being is divided into 3 periods:

1) Pre-embryonic period: First 2 weeks of development

2) Embryonic (organogenic) period: From the beginning of the 3rd

week to the end of

the 8th

week of development (Embryo)

3) Fetal period: From the beginning of the 3rd

month to the end of the 9th

month of

development (Fetus)

Fertilization

- Fertilization is the process by which male gamete (sperm, n) and female gamete

(ovum or secondary oocyte, n) fuse to form a unicellular organism, the zygote (2n).

- It occurs in the ampullary region of the uterine tube (lateral widest part).

- About 200-300 million sperms are present in the ejaculated semen. Sperms remain

viable in the female reproductive tract for several days. Sperms begin their journey

and pass rapidly from the vagina into the uterus and thence into the uterine tube (by

Pregnancy

Development (embryonic age)

(fertilization age)

2 weeks

Ovulation

14th day

Week1 Week2 Week3 Week4

Last menstrual period Missed menstrual period

(Gestational age)

4 weeks

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the activity of the flagella and contraction of the muscles of the uterus and uterine

tube).

- Only 300-400 sperms reach the site of fertilization while the others degenerate and

die. To be able to fertilize the 2ry oocyte, the sperms start to undergo:

a) Capacitation: the glycoprotein coat and the seminal proteins are removed from

the surface of the acrosome.

b) Acrosome reaction: only for those sperms showing capacitation. Perforation of

acrosome leads to the release of enzymes (Acrosin and Trypsin like substance).

- Only one sperm is able to fertilize the ovum while the others help the fertilizing sperm

in penetrating the corona radiata and zona pellucida by their enzymatic actions. The

head and tail of the sperm enter the ovum while the plasma membrane is left behind on

the surface of the ovum. The tail then disappears while the head (nucleus) swollen and

form the male pronucleus.

- Once the sperm enters the ovum, zona reaction takes place and zona pellucida

becomes impermeable to other sperms. At that time, the 2ry oocyte which has been

arrested in metaphase of the second meiotic division, completes Meiosis II and forms a

definitive oocyte and a 2nd

polar body. The nucleus of definitive oocyte becomes the

female pronucleus.

- Zygote is the fertilized oocyte. It is a unicellular organism (single cell) containing male

and female pronuclei. During growth of the zygote the female (n) and male (n)

pronuclei replicate their DNA (2n+2n)=(4n DNA) .

- Membranes of pronuclei break down and cleavage started. Cleavage consists of

repeated mitotic divisions of the zygote, giving smaller cells called blastomeres, each

one of them containing the diploid number of chromosomes (2n,23pairs). Blastomeres

become smaller with each cell division (2,3,4,5….blastomeres) and are surrounded by

zona pellucida. Cleavage occurs as the zygote traverses the uterine tube towards the

uterus.

*The main results of fertilization:

1. Restoration of the diploid number of chromosomes (2n,46)

2. Variation of human species as chromosomes contain a new combination of genes

different from that present in both parents

3. Genetic determination of the sex of the new individual (XX or XY)

4. Initiation of cleavage (cell division by mitosis)

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- When division reaches 16-cell stage (after 3 days) a Morula (mulberry fruit like) is

formed. The blastomeres of the morula are divided into :

a) Inner cell mass (embryoblast) Embryo

b) Outer cell mass Trophoblast Placenta

- As the morula enters the uterus (3rd

or 4th

day), a cavity begins to appear (blastocele)

and the blastocyst is formed and zona pellucida soon disappears (5th

day)??

- In the blastocyst, the trophoblast (flat cells) forms its wall while the inner cell mass

(embryoblast) becomes attached to the trophoblast in one side, the embryonic pole,

while the other side is called the abembryonic pole.

- Near the end of the first week (6th

day) the blastocyst starts implantation. The

trophoblastic cells over the embryonic pole attach to the endometrial epithelium then

starts to proliferate and differentiate into:

a) Inner cytotrophoblast

b) Outer syncytiotrophoblast, a highly invasive cells which produce proteolytic

enzymes that erode the maternal tissue, enabling the blastocyst to burrow into

the endometrium.

**Remarks:

Zona pellucia prevent the dividing zygote to :

- increase in size (2ry oocyte=zygote=morula)

- stick in the wall of the uterine tube during its migration.

By the end of this week:

1- Zygote reach the blastocyst stage

2- Beginning of implantation

3- Trophoblast starts to differentiate into cyto- and syncytio-trophoblast.

Raouf Abdel-Rahman Fadel First week of development 8

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Raouf Abdel-Rahman Fadel First week of development 9

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Raouf Abdel-Rahman Fadel First week of development

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Endometrium during the menstrual cycle

There are rhythmic changes in the mucosal lining of the uterus (endometrium) known

as uterine or menstrual cycles. These cycles start at puberty and end at menopause

and are under hormonal control.

Ovarian cycle (maturation of primordial follicle until ovulation and formation of

corpus lutem) is under the control of the hormones secreted from the anterior lobe of

pituitary.

Menstrual cycle is under the hormonal control of the ovary. During the menstrual

cycle (No fertilization) the endometrium passes into three phases:

Endometrium during pregnancy

If fertilization occurs, corpus luteum in the ovary continue to develop forming the

Corpus Luteum of Pregnancy, which continue to secrete a large amount of

progesterone, which in turn increase the secretory activity of the endometrium. The

endometrium is now known as the “Decidua“ and is ready to receive the blastocyst.

Phases Hormones (secreted by ovaries)

Results

Menstrual Phase

(3-4days)

Decrease oestrogen and progesterone

due to degeneration of corpus luteum Bleeding &sheding of endometrium due

to ischaemia (constriction of spiral

arteries) followed by dilatation

Follicular Phase

(Proliferative)

(10 days)

Increase oestrogen secreted by the

growing follicles until ovulation

Repair and growth of endometrium

Luteal Phase

(Secretory)

(14days)

Increase oestrogen and progesterone

due to formation of corpus lutem

Endometrium reach its maximum

development (maximum thickness,

edematous with mucin and glycin

secretion from the gland, tortuous spiral

arteries and dilated venules and sinusoids

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Correlation between the ovarian function (hormones production) and the endometrial

changes:

1) During menstrual cycles

2) During pregnancy

1) During menstrual cycles

2) During pregnacy