first week of human development - …...raouf abdel-rahman fadel first week of development 3 _____...
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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
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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
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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.
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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