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FIRST WEEK OF HUMAN DEVELOPMENT - ... Raouf Abdel-Rahman Fadel First week of development 3 _____ First Week of Human Development (embryo) is approximately equal to the third week of

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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

  • 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 3 rd

    week to the end of

    the 8 th

    week of development (Embryo)

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

    month to the end of the 9 th

    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

  • Raouf Abdel-Rahman Fadel First week of development 4

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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 2 nd

    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 (3 rd

    or 4 th

    day), a cavity begins to appear (blastocele)

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

    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 (6 th

    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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  • Raouf Abdel-Rahman Fadel First week of development 9

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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

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