1. GENERAL EMBRYOLOGY
1

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
2

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
3

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.
4

5. Primodial germ cells
in wall of yolk sac
Yolk sac
5

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.
6

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
8

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
9

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
10

11. 11

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
12

13. 





•
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.
13

14. 





•
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.
14

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
15

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

28. 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

30. Note: No primary oocytes form after birth in females, in contrast
to the continuous production of primary spermatocytes in
males
• The primary oocytes remain dormant in the ovarian follicles
until puberty
• As a follicle matures, the primary oocyte increases in size
and, shortly before ovulation, the first meiotic division is
completed giving rise to a secondary oocyte and the first
polar body.
• Unlike the corresponding stage of spermatogenesis, there is
formation 2 daughters of unequal size, this is because the
division of cytoplasm is unequal, each with 23 double
structured chromosomes
• The secondary oocyte receives almost all the cytoplasm and
the first polar body receives very little
• The 1st polar body is a small, nonfunctional cell that soon
degenerates

33. • The first polar body lies between the zona pellucida and the
cell membrane of the secondary oocyte in the perivitelline
space
• The cell then enters meiosis II but it is arrested at metaphase
approximately 3 hours before ovulation
• If a sperm penetrates the secondary oocyte, the second meiotic
division is completed, and most cytoplasm is again retained by
one cell, the fertilized oocyte (zygote), while the 2nd polar
body receive little cytoplasm
• If not fertilized, the cell degenerates approximately 24 hours
after ovulation
• The other cell, the 2nd polar body, also a small nonfunctional
cell, soon degenerates.
• As soon as the polar body is extruded, maturation of the
oocyte is complete
 Note: The first polar may also undergoes a second division

35. •
Of these, only about 400-500 become secondary oocytes and
are expelled at ovulation during the reproductive period
• Few of these oocytes, if any, are fertilized and become mature
• The number of oocytes that ovulate is greatly reduced in
women who take oral contraceptives because the hormones in
them prevent ovulation from occurring

37. Embryological terminologies
Abortion
 This is the premature stoppage of development and expulsion of
a conceptus from the uterus or expulsion of an embryo or fetus
before it is viable-capable of living outside the uterus
 The products of an abortion is called an abortus (i.e. the
embryo/fetus and its membranes)
There are different types of abortion:
 A spontaneous abortion is one that occurs naturally and is most
common during the third week after fertilization
• Approximately 15% of recognized pregnancies end in
spontaneous abortion, usually during the first 12 weeks
 A habitual abortion is the spontaneous expulsion of a dead or
nonviable embryo or fetus in three or more consecutive
pregnancies

38.  An induced abortion is a birth that is medically induced before
20 weeks (i.e., before the fetus is viable)
• This type of abortion refers to the expulsion of an embryo or
fetus induced intentionally by drugs or mechanical means
 A complete abortion is one in which all the products of
conception are expelled from the uterus
 A missed abortion is the retention of a conceptus in the uterus
after death of the embryo or fetus.
 A miscarriage is the spontaneous abortion of a fetus and its
membranes before the middle of the second trimester
(approximately 135 days)
 Threatened abortion
• This might take place before the 20th week of pregnancy
• Some pregnant women have some vaginal bleeding with or
without abdominal cramps during the first three months of
pregnancy

39. • When the symptoms indicate a miscarriage is possible, the
condition is called a "threatened abortion."
• (This refers to a naturally occurring event, not medical
abortions or surgical abortions
 Trimester: A period of three calendar months during a
pregnancy.
• Obstetricians commonly divide the 9-month period of
gestation into three trimesters. The most critical stages of
development occur during the first trimester (13 weeks) when
embryonic and early fetal development is occurring

40. C L I N I C A L C O R R E LA T E S
Birth Defects and Spontaneous Abortions
Chromosomal abnormalities
May be:
 numerical
 structural
 note
• The normal human somatic cell contains 46 chromosomes; the
normal gamete contains 23
• Normal somatic cells are diploid, or 2n; normal gametes are
haploid, or n
• Euploid refers to any exact multiple of n, e.g. diploid or triploid

41. Numerical Abnormalities
Polyploidy
• Polyploidy is the addition of an extra haploid set or sets of
chromosomes (i.e., 23) to the normal diploid set of
chromosomes (i.e., 46)
A. Triploidy
• is a condition in which cells contain 69 chromosomes
• The most common type of polyploidy is triploidy
• Triploid fetuses have severe intrauterine growth retardation with
a disproportionately small trunk
• Triploidy could result from the second polar body failing to
separate from the oocyte during the second meiotic division; but
more likely triploidy results when an oocyte is fertilized by
two sperms (dispermy) almost simultaneously

43. • Triploidy occurs in approximately 2% of embryos, but most of
them abort spontaneously
• Triploid fetuses account for approximately 20% of
chromosomally abnormal miscarriages.
• Although triploid fetuses have been born alive, this is
exceptional.
• These infants all died within a few days because of multiple
anomalies and low birth weight
B Tetraploidy:
• is a condition in which cells contain 92 chromosomes
• Usually due to failure of first mitotic division: chromosomes
replicate and divide, but all end up in the same nucleus
• Tetraploid embryos abort very early, and often all that is
recovered is an empty chorionic sac, which used to be referred
to as a "blighted embryo."

44. Aneuploidy
• Aneuploidy is the addition of one chromosome (trisomy) or loss
of one chromosome (monosomy).
 Trisomy results in spontaneous abortion of the conceptus.
 However,
I. trisomy 13 (Patau syndrome),
II. trisomy 18 (Edwards syndrome),
III. trisomy 21 (Down syndrome),
IV. and Klinefelter syndrome (47,XXY) are found in the liveborn
population
 Monosomy also results in spontaneous abortion of the conceptus
 However, monosomy X chromosome (45,X; Turner syndrome) is
found in the liveborn population
• Aneuploidy occurs as a result of non-disjunction during meiosis

45. • In meiosis, two members of a pair of homologous
chromosomes normally separate during the first meiotic
division so that each daughter cell receives one member of
each pair
• Sometimes, however,
separation
does
not
occur
(nondisjunction), and both members of a pair move into one
cell
• As a result of nondisjunction of the chromosomes, one cell
receives 24 chromosomes, and the other receives 22 instead of
the normal 23
• When, at fertilization, a gamete having 23 chromosomes fuses
with a gamete having 24 or 22 chromosomes, the result is an
individual with either 47 chromosomes(trisomy) or 45
chromosomes (monosomy)

49.  Note:
• In women, the incidence of chromosomal abnormalities,
including non-disjunction, increases with age, especially at 35
years and older
 Trisomy 21 (Down syndrome)
• Down syndrome is usually caused by an extra copy of
chromosome 21 (trisomy21)
 features of children with down syndrome include
• Physical growth retardation
• varying degrees of mental retardation (Down syndrome IQ is
around 50, compared to children without the condition with an
IQ of 100 (Mental retardation has historically been defined as
an IQ below 70)
• craniofacial abnormalities, including upward slanting eyes,
epicanthal folds (extra skin folds at the medial corners of the
eyes), flat facies, and small ears;
• cardiac defects; and hypotonia

50. •
•
•
•
•
•
•
•
Single simian crease
high incidences of leukemia,
infections,
thyroid dysfunction,
Brachycephaly
Clinodactyl of the fifth digit
and premature aging
nearly all develop signs of Alzheimer’s disease (progressive
mental deterioration manifested by loss of memory) after age
35
Incidence rate
 The incidence of Down syndrome is approximately
• 1 in 2000 conceptuses for women under age 25
• This risk increases with maternal age to 1 in 300 at age 35
• and 1 in 100 at age 40

52. A
B
(A) Child with Down syndrome, which is characterized by a flat broad face, oblique palpebral
fissures (B) broad hand with a single transverse (simian) crease

53. Anterior view of the faces of dizygotic male twins that are discordant for Down
syndrome (trisomy 21). The one on the right is smaller and hypotonic compared with
the unaffected twin. The twin on the right developed from a zygote that contained an
extra 21 chromosome. Note the characteristic facial features of Down syndrome in this
infant: upslanting palpebral fissures, epicanthal folds, and flat nasal bridge

54. Trisomy 18 (Edwards syndrome)
 Patients with trisomy 18 show the following features:
• mental retardation,
• growth retardation
• prominent occiput
• congenital heart defects
• low-set ears,
• flexion of fingers and hands
• short sternum
• Micrognathia
• renal anomalies
• Syndactyly( webbing or fusion of fingers or toes)
• and malformations of the skeletal system.
incidence rate
incidence rate of this condition is approximately 1 in 5000 newborns.
• 85% are lost between 10 weeks of gestation
• whereas those born alive usually die by age 2 months
• Approximately 5% live beyond 1 year

55. Child with trisomy 18.
Note the low sets ears,
small mouth, deficient
mandible (micrognathia)
flexion of the hands, and
absent/ or hypoplasia of
thr radius and unla

56. Photograph of child with trisomy 18. Note the prominent occiput, cleft
lip, micrognathia, low-set ears, and one or more flexed fingers

57. Trisomy 13(Patau syndrome)
 The main abnormalities of trisomy 13 are
• mental retardation
• severe central nervous system malformations
• congenital heart defects
• sloping forehead; malformed ears
• scalp defects
• bilateral cleft lip and/or palate;
• Polydactyly (presence of more than 5 digits on hand or foot)
 eye defects, such as :
• Microphthalmia
• anophthalmia (congenital absence of tissue of the eyes)
Incidence rate
• The incidence of this abnormality is approximately 1 in 20,000 live
births,
• over 90% of the infants die in the first month after birth
• Approximately 5% live beyond 1 year

58. Child with trisomy 13, note the bilateral cleft lip, the sloping
forehead, and anophthalmia

59. Trisomy 13: The syndrome is commonly accompanied by
polydactyly

60.  Turner syndrome
• Turner syndrome, with a 45,X karyotype, is the only
monosomy compatible with life
• Even then, 98% of all fetuses with the syndrome are
spontaneously aborted
• The few that survive are unmistakably female in appearance
 Characteritics features are
 the absence of ovaries (gonadal dysgenesis)
 short stature
• Webbed neck
• lymphedema of the extremities,
• skeletal deformities
• and a broad chest with widely spaced nipples

61. A
Patients with Turners syndrome (A) swelling in the hand

62. B
C
Patients with Turners syndrome (B) prominient webbed neck and widely spaced
nipples with a broad chest (C) Caused by lymphedema

63. Klinefelter's syndrome ( 47, XXY)
• found only in males
• The non disjucntion of XX homologues is the most causative event
features
• small testes
• hyalinization of seminiferous tubules
• sterility
• often tall with disproportionately long lower limbs
• Intelligence is less than in normal siblings
• Approximately 40% of these males have gynecomastia
Incidence rate:
1 in 500 males

64. Adolescent male
with Klinefelter
syndrome (XXY
trisomy)
Note
the
presence
of
breasts;
approximately
40% of males
with
this
syndrome have
gynecomastia
(development of
mammary
glands)
and
small testes

65. Structural chromosome abnormalities
• involve one or more chromosomes usually result from
chromosome breakage.
Breaks are caused by:
 Environmental factors, such as viruses, radiation, and drugs
• The result of breakage depends on what happens to the broken
pieces
• In some cases, the broken piece of a chromosome is lost, and
the infant with partial deletion chromosome is abnormal
• A well-known syndrome, caused by partial deletion is the cridu-chat syndrome
 Cri-du-chat syndrome
• caused by partial deletion of the short arm of chromosome 5
• children have a catlike cry, microcephaly, mental
retardation, and congenital heart disease

67. Microdeletions
• This takes place in a few contiguous genes
• This may result in microdeletion syndrome or contiguous gene
syndrome
• They can be detected only by high resolution banding
 (high resolution banding allows detection of very small
terminal deletions in a number of disorders)
 Examples of microdeletion include occurs on the long arm of
chromosome 15
 if the defect is inherited on the paternal chromosome, it is
called Prader-Willi syndrome
• if the defect is inherited on the maternal chromosome , it is
called Angelman syndrome

68. Prader-Willi syndrome
• affected individuals are characterized by hypotonia, obesity,
mental retardation, hypogonadism, and cryptorchidism
(undescended testes)
Angelman syndrome
• affected individuals are characterized by mentally retardation,
inability to speak, exhibition of poor motor development, and
are prone to unprovoked and prolonged periods of laughter

69. Patient with Angelman syndrome
resulting from a microdeletion on
maternal chromosome 15
Patient with prader willi syndrome
resulting from a microdeletion on paternal
chromosome 15

70. Uterus ( womb)
introduction
• is a thick-walled, pear-shaped muscular organ
dimension
 Length = approx 7-8 cm
 width = approx 5-7cm at its superior part
 thickness= approx 2-3 cm
parts
• consists of two major parts
 Body, the expanded superior 2/3
 Cervix, the cylindrical inferior 1/3
The body of the uterus
• narrows from the fundus (the rounded, superior part of the body)
to the isthmus
 Note: the isthmus is the 1cm long constricted region between the
body and cervix

72. The cervix of the uterus
• is its tapered vaginal end that is nearly cylindrical in shape
• It has a lumen called the cervical canal
• the cervical canal, has constricted openings at each end
• At the upper constricted end is the internal os, while at the
lower constricted end is the external os
• The internal os communicates with the cavity of the uterine
body,while the external os communicates with the vagina
The walls of the body of the uterus
• consist of three layers
 Perimetrium, the thin external layer
 Myometrium, the thick smooth muscle layer
 Endometrium, the thin internal layer

73. • The perimetrium is firmly attached to the myometrium
The layers of the endometrium
• It has 3 layers
 A thin superficial layer called the compact layer
 A thick middle layer called the spongy layer
 A thin deep layer called the basal layer
• . The basal layer of the endometrium has its own blood supply
and is not sloughed off during menstruation
• The compact and spongy layers collectively are called the
functional layer,
• This functional layer disintegrate and are shed during
menstruation and after parturition (delivery of a baby)

75. Uterine tubes
intro
The uterine tubes extend laterally from the horns of the uterus
Dimension
approximately 10 cm long and 1 cm in diameter
Parts
the uterine tube is divided into four parts:
 infundibulum
 ampulla
 isthmus
 uterine part
functions
 carries oocytes from the ovaries and sperms entering from the
uterus to reach the fertilization site in the ampulla of the uterine
tube
 conveys the cleaving zygote to the uterine cavity

77. Ovaries
Intro:
• The ovaries are almond-shaped reproductive glands located close
to the lateral pelvic walls on each side of the uterus
functions
 They produce oocytes
 They also produce estrogen and progesterone,
 Estrogen and progesterone are hormones responsible for the
development of secondary sex characteristics and regulation of
pregnancy
Female reproductive cycles
• females undergo reproductive cycles (sexual cycles), involving
activities of the hypothalamus of the brain, pituitary gland
(hypophysis), ovaries, uterus, uterine tubes, vagina, and mammary
glands and this commences at purberty
• These monthly cycles prepare the reproductive system for
pregnancy

78. • Gonadotropin-releasing
hormone
is
synthesized
by
neurosecretory cells in the hypothalamus and is carried by the
hypophysial portal system to the anterior lobe of the pituitary
gland
• Gonadotropin-releasing hormone stimulates the release of 2
hormones namely:
 Follicle-stimulating hormone (FSH)
 Luteinizing hormone (LH)
• These 2 hormones act on the ovaries:
 Follicle-stimulating hormone (FSH) stimulates
 the development of ovarian follicles
 and the production of estrogen by the follicular cells.
 Luteinizing hormone (LH)
 serves as the "trigger" for ovulation (release of secondary
oocyte)
 stimulates the follicular cells and corpus luteum to produce
progesterone

79. Ovarian cycle
• These are cyclic changes in the ovaries, and these changes are
produced by FSH and LH
• These changes include:
I. development of follicles
II. ovulation, and
III. Formation of the corpus luteum
 Development of follicles
• Development of an ovarian follicle is characterized by:
I. Growth and differentiation of primary oocyte
II. Proliferation of follicular cells
III. Formation of zona pellucida
IV. Development of the theca folliculi

80. • As primordial follicles begin to grow, surrounding follicular
cells change from flat to cuboidal in shape and then
columnar, forming a primary follicle
• Also, follicular (granulosa) cells and the oocyte secrete a layer
of glycoproteins on the surface of the oocyte, forming the
zona pellucida
Development of theca folliculi
• As the primary follicle continues to grow, the adjacent
(surounding) connective tissue organizes into a capsule, called
the theca folliculi
• cells of the theca folliculi then differenciate into:
 an inner layer of secretory cells, called the theca interna
 and an outer fibrous capsule,called the theca externa

83. • As development continues, fluid-filled spaces appear between
follicular (granulosa) cells
• The fluid filled space is crescent-shaped, but with time, it
enlarges which coalesce to form a single large cavity, the
antrum, which contains follicular fluid
• After the antrum forms, the ovarian follicle is called a vesicular or
secondary follicle
• The primary oocyte is pushed to one side of the follicle, where it is
surrounded by a mound of follicular cells, the cumulus
oophorus, that projects into the antrum
• The secondary/ vesicular follicular continues to grow and become
matured having a diameter of about 25mm or more
• At maturity, it is called the mature vesicular/ mature secondary
or Graafian follicle
• The early development of ovarian follicles is induced by FSH, but
final stages of maturation require LH as well

84. •
Growing follicles produce estrogen, hormone that regulates
development and function of the reproductive organs
Ovulation
 This is the release of an oocyte from the ovarian follicle
• In a few before ovulation, under the influence of FSH and LH, the
secondary follicle grows rapidly to a diameter of about 25 mm to
become mature vesicular/ mature secondary or Graafian follicle
 Coincident with final development of the vesicular follicle, there is
an abrupt increase in LH that causes the primary oocyte to
complete meiosis I and the follicle to enter the preovulatory mature
vesicular stage
 Meiosis II is also initiated, but the oocyte is arrested in metaphase
approximately 3 hours before ovulation
 In the meantime, the surface of the ovary begins to bulge locally,
and at the apex, an avascular spot, the stigma, appears

85.  For the oocyte to be released, 2 events occur which are caused
by LH surge:
I. The high concentration of LH increases collagenase
activity, resulting in digestion of collagen fibers (connective
tissue) surrounding the follicle
II. Prostaglandin levels also increase in response to the LH surge
and cause local muscular contractions in the ovarian wall
• Those contractions extrude the oocyte, which together with its
surrounding follicular (granulosa) cells from the region of the
cumulus oophorus,
• this causes ovulation in which oocyte floats out of the ovary
• Some of the cumulus oophorus cells then rearrange themselves
around the zona pellucida to form the corona radiata

88. Clinical correlates
 During ovulation, some women feel a slight pain called
mittelschmerz also known as middle pain because it normally
occurs near the middle of the menstrual cycle
 A variable amount of abdominal pain, mittelschmerz
, accompanies ovulation in some women. In these cases, ovulation
results in slight bleeding into the peritoneal cavity, which results in
sudden constant pain in the lower abdomen.
 Mittelschmerz may be used as a symptom of ovulation, but there
are better symptoms, such as the slight drop in basal body
temperature
 Some women fail to ovulate, this is called anovulation, because
of a low concentration of gonadotropins
• In these cases, administration of an agent to stimulate
gonadotropin release and hence ovulation can be employed
• Although such drugs are effective, they often produce multiple
ovulations, so that the risk of multiple pregnancies is 10 times
higher in these women than in the general population

89. antrum
Theca
interna
Luteal cells
Blood
vessels
Theca externa
A : PREOVULATORY FOLLICLE
B: OVULATION
C: CORPUS LUTEUM
fibrin

90. Corpus Luteum
• Shortly after ovulation, the walls of the ovarian follicle and theca
folliculi collapse and are thrown into folds are vascularized by
surrounding vessels
• Under LH influence, they develop into a glandular structure, these
cells develop a yellowish pigment and change into lutein cells the
corpus luteum, which secretes progesterone and some estrogen
• progesterone and estrogen, cause the endometrial glands to secrete
and prepare the endometrium for implantation of the blastocyst
• If the oocyte is fertilized, the corpus luteum enlarges to form a
corpus luteum of pregnancy and increases its hormone production.
• Degeneration of the corpus luteum is prevented by human
chorionic gonadotropin
• The corpus luteum of pregnancy remains functionally active
throughout the first 20 weeks of pregnancy

91. • If the oocyte is not fertilized, the corpus luteum involutes and
degenerates 10 to 12 days after ovulation
• It is then called a corpus luteum of menstruation
• The corpus luteum is subsequently transformed into white scar
tissue in the ovary, a corpus albicans
• Except during pregnancy, ovarian cycles normally persist
throughout the reproductive life of women and terminate at
menopause, the permanent cessation of menstruation, usually
between the ages of 48 and 55