Dr: Azza Zaki
Introduction
To General Embryology
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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.
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Developmental Periods
can be
divided
into:
prenatal
&
postnatal
periods.
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
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.
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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
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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).
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Gametogenesis
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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.

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DNA Replication
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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.
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Various stages of mitosis. In prophase, chromosomes are visible as slender threads.
Doubled chromatids become clearly visible as individual units during metaphase.
Blue, paternal chromosomes; red, maternal
chromosomes.
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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).
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Meiosis I - Stages
Prophase I
Metaphase I Anaphase I
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Telophase I
Meiosis II - Stages
Prophase II
Metaphase II Anaphase II Telophase II
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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.
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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
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redistributes genetic material.
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. Doublestructured chromosomes pull apart. E. Anaphase of the first meiotic division.
F and G. During the second meiotic division, the double-structured
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chromosomes split at the centromere.
Male Genital System
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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).
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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.
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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.
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