Dr Azza Zaki

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					Dr: Azza Zaki
   Introduction
To General Embryology


        Dr: Azza Zaki
  The Human Development
is a continuous process
that begins when an ovum
from a female is fertilized
by a sperm from a male.
Cell division, growth and
differentiation transform
the fertilized ovum
(zygote) into a
multicellular human being.
                   Dr: Azza Zaki
            Developmental Periods
 can be
divided
into:
prenatal
&
postnatal
periods.


                    Dr: Azza Zaki
   Normal somatic cells and primordial germ cells
    contain 46 single chromosomes (23 homologous
    pairs) and this is called diploid number. 22 pairs of
    the chromosomes are autosomes and one pair is sex
    chromosome; XX for female and XY for male.
Gametes contain 23 single chromosomes (haploid
    number).
   The male gamete (sperm) produced by meiosis in the
    testis, contains 22 autosomes and either an X or a Y
    chromosome therefore, the male gamete determines the
    genetic sex of the individual.
   The female gamete (ovum) produced by meiosis in the
    ovary, contains 22 autosomes and an X chromosome.
                          Dr: Azza Zaki
                   Chromosomes
are the carriers of genes.
 Each chromosome consists of two arms
  separated by a centromere.
 During meiosis I, single chromosomes
  undergo DNA replication, which
  essentially duplicates the arms. This
  forms duplicated chromosomes, which
  consists of two sister chromatids
  attached at the centromere
 Autosomes contain genetic information
  for most human characteristics.
 Homologous chromosomes:
     pair of similar autosomes
                            Dr: Azza Zaki
            Gametogenesis
Definition: It is a process by which mature gametes
  (sperms or ova) are formed from primitive sex
  cells.
 Site: It takes place in the testis in male and ovary in
  female.
Gametogenesis is divided into :
 Spermatogenesis : the formation of male gametes
  (sperms).
 Oogenesis: the production of female gametes (ova).
                      Dr: Azza Zaki
Gametogenesis




    Dr: Azza Zaki
         Cell Division:1- Mitosis
 It is a process by which one somatic or primitive germ
  cell divides giving rise to 2 daughter cells that are
  genetically identical to parent cell.
 Each cell contains 46 chromosomes.
 Before somatic cell enters mitosis, each chromosome
  replicates its DNA.
Mitosis is divided into phases:
A-Prophase:
Chromosomes begins to coil, contract and condense.
Each chromosome now consists of two parallel subunits,
  chromatids, that are joined at the centromere.
                        Dr: Azza Zaki
DNA Replication




     Dr: Azza Zaki
B-Prometaphase:
The chromatids become distinguishable.
C-Metaphase:
The chromosomes line up in the equatorial plane of the cell.
D-Anaphase:
The centromere of each chromosome divides longitudinally
  and each chromatid migrates to the opposite pole of the
  cell.
E-Telophase:
The chromosomes uncoil and lengthen, the nuclear
  envelope reforms and the cytoplasm divides.

                         Dr: Azza Zaki
Various stages of mitosis. In prophase, chromosomes are visible as slender threads.
Doubled chromatids become clearly visible as individual units during metaphase.
                                   Dr: Azza chromosomes.
Blue, paternal chromosomes; red, maternal Zaki
                       2-Meiosis
   It is a specialized process of cell division that
    occurs only in reproduction. It is the cell division
    that takes place in the germ (sex) cells to generate male
    and female gametes, sperm and egg cells, respectively.
    Meiosis requires two cell divisions, meiosis I and
    meiosis II, to reduce the number of chromosomes to
    the haploid number of 23.
   The first of which is a reduction division. The second
    meiotic division looks like mitosis.
   The result is the formation of four gametes; each
    contains half the number of chromosomes (23 single
    chromosomes).
                           Dr: Azza Zaki
             Meiosis I - Stages




Prophase I   Metaphase I Anaphase I   Telophase I
                   Dr: Azza Zaki
              Meiosis II - Stages




Prophase II   Metaphase II Anaphase II Telophase II
                     Dr: Azza Zaki
                              Meiosis I
(1) First meiotic division:
- The 46 chromosomes in each cells arrange themselves in 23 pairs.
- The members of each pair move apart and migrate to the opposite
   poles of the germ cell.
- The cytoplasm divides to form two cells each of which contains 23
   chromosomes.
a. DNA replication: forming a primary gametocyte (46 duplicated
   chromosomes, 4N).
b. Pairing of each two homologous chromosomes of the 46 duplicated
   chromosomes.
c. Chiasmata formation: where crossing over occurs (large chromatid
   segments are exchanged, contributing to genetic variability).
d. Arrangement: the 46 homologous chromosomes arrange at the
   metaphase plate.
                               Dr: Azza Zaki
e. Disjunction: the 46 chromosomes separate from each other,
    centromeres do not split.
f. Cell division: two secondary gametocytes (23 duplicated
    chromosomes, 2N) are formed.
(2) Second meiotic division:
- Follows the first meiotic division.
- The two chromatids of each chromosomes move apart to the
    opposite poles of the cells where they form a chromosome.
- The cytoplasm then divides to form two cells each contains 23
    chromosomes.
Results of meiotic division: A primitive germ cells with 46
    chromosomes gives four daughter cells each containing 23
    chromosomes. 1.Each germ cell contains haploid no. of
    chromosomes.
2.Genetic variability is enhanced through cross over which
    redistributes genetic material. Zaki
                             Dr: Azza
  First and second meiotic divisions. A. Homologous chromosomes
approach each other. B. Homologous chromosomes pair, and each member
of the pair consists of two chromatids. C. Paired homologous chromosomes
 interchange chromatid fragments (crossover). Note the chiasma. D. Double-
structured chromosomes pull apart. E. Anaphase of the first meiotic division.
 F and G. During the second meiotic division, the double-structured
                                 Dr: Azza
chromosomes split at the centromere. Zaki
Male Genital System




       Dr: Azza Zaki
                   1. Primary sex organs:
2 testes
 Enclosed in the scrotum.
 Each testis consists of about 250
   lobules.
 Each lobule contains 1-3
   seminiferous tubules.
 The seminiferous tubules join
   together forming a network called
   rete testis in the mediastinum testis
   (the posterior thick part of the
   testicular capsule).
 Functions of testis: formation of
   sperms & male sex hormone
   (testosterone).
                               Dr: Azza Zaki
2. Genital ducts:
a) Vasa efferentia:
 15-20 tubules connecting the rete testis to the epididymis.

b) Epididymis:
 Comma-shaped organ posterior to the testis.

 It is formed of head, body & tail which is continuous with the
   vas deferens.
c) Vas deferens:
 Enters the abdominal cavity where it forms a dilatation called
   ampulla of the vas that lies behind the urinary bladder.
d) Ejaculatory ducts:
 2, each is formed by the union of the ampulla & seminal
   vesicle of one side.
 They pierce the prostate to open in the urethra.
                           Dr: Azza Zaki
3.Glands:
1) Seminal vesicles:
 2, that lie lateral to the ampulla
   of the vas.
2) Prostate:
 Large gland that surrounds the
   beginning of the urethra.
3) Bulbourethral glands:
 2, that open into the penile
   urethra.
 4.External genitalia: the penis.


                           Dr: Azza Zaki
                 Spermatogenesis
Definition: It is the process by
  which the spermatogonia are
  transformed into
  spermatozoa (sperms).
Site: Seminiferous tubules of
  the testis.
Onset: It begins at puberty
  and continues till very old
  age.
Duration: The time required
  for a spermatogonium to
  develop into a mature
  spermatozoon is about 64
  days.                  Dr: Azza Zaki
Dr: Azza Zaki
                        Steps
- The spermatogonium which is the primitive male sex cell (46
   chromosomes) divides by mitosis into 2 daughter cells (type
   A and type B).
- The type A cell remains in contact with the basement
   membrane, while the type B continues maturation and is
   transformed into a larger cell called primary spermatocyte
   (46 chromosomes).
- The primary spermatocyte undergoes the first meiotic
   division thus giving rise to two secondary spermatocytes
   each of which has 23 chromosomes.
- Each secondary spermatocyte undergoes the second meiotic
   division thus giving rise to two spermatids each of which
   has 23 chromosomes.
- The spermatids are transformed into complete sperms by a
   process called spermiogenesis.
                           Dr: Azza Zaki
Type A spermatogonia, derived from the spermatogonial stem cell
population, represent the first cells in the process of spermatogenesis.
Clones of cells are established and cytoplasmic bridges join cells in each
                                    Dr: Azza are
 succeeding division until individual spermZaki separated from residual bodies.
           Spermiogenesis
    It is the morphological transformation of
   spermatids into spermatozoa (sperm). These
                   changes are:
1. The nucleus: enlarges and condenses to form
the head of the sperm.
2. The golgi apparatus: forms the acrosomic cap.
3. Formation of neck, middle piece, and tail.
4. Shedding of most of the cytoplasm.
5. The cell membrane: encloses the whole sperm.
.
                  Dr: Azza Zaki
The sperms are ejaculated in the seminal fluid which
  consists of secretions of the sex glands (prostate,
  seminal vesicles and bulbo-urethral glands).
The fluid of an ejaculate measures about 3.5 ml. and
  contains about 100 million sperms in each 1ml.
The sperms are genetically of two types: one carries
  X chromosome while the other carries Y
  chromosome.
All ova are of one type and carry X chromosome
                      Dr: Azza Zaki
Dr: Azza Zaki
Hormonal control of spermatogenesis:
1-Luteinizing hormone (LH):
secreted by the pituitary gland: it binds to
receptors on Leydig cells and stimulates
testosterone production, which binds to Sertoli
cells to promote spematogenesis.
2- Follicle stimulating hormone (FSH):
It binds to Sertoli cells stimulating testicular fluid
production.
Abnormal sperm:
They are frequent. Up to 10%have defects.
The head or tail may be abnormal.
Spermatozoa may be giant or dwarf and
sometimes they are joined.
Sperms with morphologic abnormality lack
normal motility and don't fertilize oocytes.
                                    Dr: Azza Zaki
Dr: Azza Zaki

				
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