1) Human development begins with the fertilization of an egg (oocyte) by sperm, forming a single-celled zygote. Through processes such as cell division, differentiation, growth and rearrangement over 9 months, the zygote develops into a baby. 2) Development is divided into prenatal (before birth) and postnatal (after birth) periods, with major developmental changes occurring during embryonic, fetal, infant and childhood stages. 3) Both male and female gametes are produced through meiosis within the ovaries and testes, respectively, reducing the chromosome count so that fertilization restores the normal diploid number.

1. GENERAL EMBRYOLOGY
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2. THE DEVELOPING HUMAN
Introduction
• Human development is a continuous process that begins when
an oocyte (ovum) from a female is fertilized by a sperm
(spermatozoon) from a male
• Cell division, cell migration, programmed cell death,
differentiation, growth, and cell rearrangement transform the
fertilized oocyte, a highly specialized, totipotent cell, a zygote,
into a multicellular human being
• From a single cell to a baby in 9 months, the study of the
developmental processes that take place is called Embryology
Note:
• Development does not stop at birth,
• Although most developmental changes occur during the
embryonic and fetal periods,
• important changes occur during later periods of development:
infancy, childhood, adolescence, and early adulthood
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3. Terminologies
• Oocyte ( ovum /egg): refers to the female germ or sex cells
produced in the ovaries
• Sperm (spermatozoon): refers to the male germ cell produced in
the testes (testicles)
• Zygote: This cell results from the union of an oocyte and a sperm
during fertilization
 A zygote or embryo is the beginning of a new human being
Developmental Periods: can be divided into
 prenatal (before birth)
 postnatal (after birth) period.
 Stages of prenatal development:
• begins at fertilization and embryonic development ends on day 56
(8th week )
• The fetal period begins on day 57 and ends when the fetus is
completely outside the mother
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4. Prefertilization Events
 Sexual Reproduction
• Sexual reproduction occurs when female and male gametes
(oocyte and spermatozoon, respectively) unite at fertilization.
• Gametes are direct descendants of primordial germ cells, which
are first observed in the wall of the yolk sac at 4th week of
embryonic development and subsequently migrate into the future
gonad region where they arrive at the end of the 5th week
• Gametes are produced by a process called gametogenesis
(formation of gamete)
• In males, this process is called spermatogenesis
• In females, it is called oogenesis
 note :
• The sequence of gametogenesis is the same, but the timing of
events during meiosis differs in the two sexes.
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5. Primodial germ cells
in wall of yolk sac
Yolk sac
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6. • In preparation for fertilization, germ cells undergo
gametogenesis. Which include:
 meiosis, to reduce the number of chromosomes
 and cytodifferentiation to complete their maturation
 Chromosomes
• A single chromosome consists of TWO characteristic regions
called arms. These include:
 Short arm/ p arm
 Long arm/ q arm
• These two arms are separated by a centromere
• During meiosis I, single chromosomes undergo DNA replication,
which essentially duplicates the arms.
• This forms duplicated chromosomes, which consist of two sister
chromatids attached at the centromere.
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7. 7

8. 1) Ploidy and N number
• Ploidy refers to the number of chromosomes in a cell
• The N number refers to the amount of DNA in a cell
 In humans, somatic cells (cells of an organism other than the
germ cells) contain 46 single chromosomes
 The chromosomes occur in 23 homologous pairs, of which
one member (homologue) of each pair is of maternal origin,
and the other is of paternal origin to form the diploid
number of 46
Note: The term “diploid” is classically used to refer to a cell
containing 46 single chromosomes
 The 23 homologous pairs of chromosomes is made up of:
 22 pairs of matching chromosomes called autosomes
 and one pair of sex chromosomes
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9. • If the sex pair is XX, the individual is genetically female; if the
pair is XY, the individual is genetically male.
• One chromosomes of each pair is derived from the maternal
gamete, the oocyte, and one from the paternal gamete, the
sperm
• Thus, each gamete contains a haploid number of 23
chromosomes, and the union of the gametes at fertilization
restores the diploid number of 46
 Meiosis
• is the cell division that takes place in the germ 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
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10. Meiosis 1
 Events that occur during meiosis I include the following:
 synapsis
• homologous chromosomes align themselves in pairs, this
process is called synapsis
 crossingover
• Crossovers: this involve the interchange of chromatid
segments between paired homologous chromosomes
• Segments of chromatids break and are exchanged as
homologous chromosomes separate.
• As separation occurs, points of interchange are temporarily
united and form an X-like structure, a chiasma
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12. Stages A – D, prophase of 1st
meiotic division
Cells contain
23
doublestructured
chromosomes
Metaphase of 2nd
meiotic division
Disjunction
E – metaphase
F - Anaphase
G - Telophase
(of 1st meiotic division)
Cells resulting from 1st
meiotic division
2nd meiotic division
Cells resulting from 2nd meiotic
division
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13. 
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Summary of Meiosis I
Events that occur during meiosis I include the following:
Synapsis: pairing of 46 homologous duplicated chromosomes
Crossing over: exchange of large segments of DNA
Alignment: alignment of 46 homologous duplicated
chromosomes at the metaphase plate
Disjunction: separation of 46 homologous duplicated
chromosomes from each other; centromeres do not split.
Cell division: formation of two secondary gametocytes (23
duplicated chromosomes, 2N).
Meiosis II
Meiosis II. Events that occur during meiosis II include the
following:
– Synapsis: absent.
– Crossing over: absent.
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14. 
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


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•
Summary of Meiosis I
Events that occur during meiosis I include the following:
Synapsis: pairing of 46 homologous duplicated chromosomes
Crossing over: exchange of large segments of DNA
Alignment: alignment of 46 homologous duplicated
chromosomes at the metaphase plate
Disjunction: separation of 46 homologous duplicated
chromosomes from each other; centromeres do not split
Cell division: formation of two secondary gametocytes (23
duplicated chromosomes, 2N).
Meiosis II
Meiosis II. Events that occur during meiosis II include the
following:
– Synapsis: absent.
– Crossing over: absent.
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15.  Alignment: alignment of 23 duplicated chromosomes at the
metaphase plate.
 Disjunction: separation of 23 duplicated chromosomes to
form 23 single chromosomes; centromeres split
 Cell division: formation of four gametes (23 single
chromosomes, 1N).
Note:
As a result of meiotic divisions,
(a) genetic variability is enhanced through crossover, which
redistributes genetic material, and through random distribution
of homologous chromosomes to the daughter cells; and
(b) (b) each germ cell contains a haploid number of chromosomes,
so that at fertilization the diploid number of 46 is restored
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16. SPERMATOGENESIS
• Spermatogenesis is the sequence of events by which
spermatogonia are transformed into mature sperms.
• This maturation process begins at puberty
• Spermatogonia, which have been dormant in the
seminiferous tubules of the testes since the fetal period, begin
to increase in number at puberty
• After several mitotic divisions, the spermatogonia grow and
undergo changes
• Spermatogonia are transformed into primary spermatocytes,
the largest germ cells in the seminiferous tubules.
• Each primary spermatocyte subsequently undergoes 1st
meiotic division (reduction division) to form two haploid
secondary spermatocytes, which are approximately half the
size of primary spermatocytes

19. • Secondary spermatocytes then undergo a second meiotic
division to form four haploid spermatids
• these haploid spermatids are approximately half the size of
secondary spermatocytes.
• The spermatids are gradually transformed into four mature
sperm by a process known as spermiogenesis
• The entire process of spermatogenesis, which includes
spermiogenesis, takes approximately 2 months
• When spermiogenesis is complete, the sperms enter the lumina
of the seminiferous tubules
• Sertoli cells lining the seminiferous tubules support and
nurture the germ cells and may be involved in the regulation of
spermatogenesis

20. • Sperms are transported passively from the seminiferous tubules to
the epididymis, where they are stored and become functionally
mature
• The epididymis is the elongated coiled duct along the posterior
border of the testis
• It is continuous with the ductus deferens (vas deferens), which
transports the sperms to the urethra
• Mature sperms are free-swimming, actively motile cells
consisting of a head and a tail
• The neck of the sperm is the junction between the head and tail
• The head of the sperm forms most of the bulk of the sperm and
contains the haploid nucleus.
• The anterior two thirds of the nucleus is covered by the acrosome,
a caplike saccular organelle containing several enzymes

23. • When released, these enzymes facilitate dispersion of the
follicular cells of the corona radiata and sperm penetration of
the zona pellucida during fertilization
• The tail of the sperm consists of three segments:
 middle piece
 principal piece
 and end piece
• The tail provides the motility of the sperm that assists its
transport to the site of fertilization.
• The middle piece of the tail contains mitochondria, which
provide the adenosine triphosphate necessary for activity

24. OOGENESIS
• Oogenesis (ovogenesis) is the sequence of events by which oogonia are
transformed into mature oocytes
• This maturation process begins before birth and is completed after
puberty
• Oogenesis continues to menopause, which is permanent cessation of the
menses (bleeding associated with the menstrual cycles)
Prenatal Maturation of Oocytes
• Primordial germ cells (46, 2N) migrate from the wall of the yolk sac and
arrive in the ovary at 5th week and differentiate into oogonia (46, 2N),
which populate the ovary through mitotic division.
• A majority of oogonia continue to divide by mitosis but some of them
enter meiosis I and undergo DNA replication to form primary oocytes
(46, 4N)
• by the 5th month of prenatal development, the total number of germ cells
in the ovary reaches its maximum, which is estimated at 7 million

25.  Note: All primary oocytes are formed by the 5th month of fetal life
• At this time, cell death begins, and many oogonia as well as
primary oocytes become atretic
• By the seventh month, the majority of oogonia have degenerated
except for a few near the surface
 Note: No oogonia are present at birth
• All surviving primary oocytes have entered prophase of meiosis I,
and most of them are individually surrounded by a single layer of
flattened, follicular epithelial cells
• The primary oocytes enclosed by this layer of cells constitutes a
primordial follicle
• Primary oocytes begin the first meiotic division before birth, but
completion of prophase does not occur until puberty
• The follicular cells surrounding the primary oocyte are believed to
secrete a substance, oocyte maturation inhibitor (OMI) which
keeps the meiotic process of the oocyte arrested

30. Postnatal Maturation of Oocytes
• Beginning during puberty, usually one follicle matures each
month and ovulation occurs, except when oral contraceptives
are used
• The total number of primary oocytes at birth is estimated to
vary from 600,000 to 800,000
• only approximately 40,000 are present by the beginning of
puberty, and fewer than 500 will be ovulated
• As the primary oocyte enlarges during puberty, the follicular
epithelial cells become cuboidal in shape and then columnar,
forming a primary follicle
• The primary oocyte soon becomes surrounded by a covering
of amorphous acellular glycoprotein material, the zona
pellucida