Embryology
Textbooks
 The Developing Human by Kieth L.more
 Human Embryology by William Larsen
 Clinical Embryology by Richard Snell
 Medical Embryology Langman’s 10th edition by T.W
Sadler
Terminology
 Embryology is the study of development from fertilized egg
through the eighth week in utero
 Developmental Anatomy is the study of development from
fertilized egg to adult form
 Development is a process begins with an oocyte (ovum) is
fertilized by a spermatozoa (sperm).and ends at death.
 Teratology is the study abnormal development (congenital
malformations)
 Pre-embryonic period : first 3 weeks after fertilization
 Embryonic period ; begins 4 weeks after fertilization to the
8th week.
 Fetal period: from the 9th week until birth
Prenatal period
 in which the important changes occur before birth :
 Oocyte : female germ cell (ovum)
 Zygote : the resultant cell of fertilization and the beginning of the human being.
 Cleavage : Mitotic divisions of the zygote.
 Morula : solid ball of cells (16 or more blastomers)
 Morula turns into Blastocyst in the uterus.
 Embryo : composed of embryoblasts that forms the Bilaminar disc and extends to the
8th week.
 Fetus : from the 9th week to birth (fetal period)
Postnatal period
 Changes occur after birth :
 Infancy :
 from after birth till the first year
 Newborn (neonatal) is in the first 2 weeks
 The body grows rapidly during infancy
 Length increase by about 50%
 Weight is tripled
 Childhood :
 The period from 15 months to 12-13 years
 Teeth are replaced by permanent ones
 Active ossification of the bones
 Puberty:
 In girls between 12-15
 In boys between 13-16
 Secondary sexual characteristics develop
 Adolescence:
 The period of 3-4 years after puberty
 After sexual maturity until the attainment of physical,
mental, and emotional maturity.
 Adulthood ;
 Early adulthood 18-25 years in which ossifications and
growth is virtually completed
 After that developmental changes occur very slowly leading
to Senility (old age).
Human Development
 Development begins with
fertilization
 the process by which the male
gamete, the sperm, and the
female gamete, the oocyte,
unite to give rise to a zygote.
 Gametes are derived from
primordial germ cells
 are formed in the epiblast during
the second week and move to the
wall of the yolk sac
 During the fourth week these cells
begin to migrate from the yolk sac
toward the developing gonads and
arrive at the fifth week
 Mitotic divisions increase their
number during their migration and
also when they arrive in the gonad.
 In preparation for fertilization, germ
cells undergo gametogenesis
 which includes meiosis, to reduce the
number of chromosomes and
cytodifferentiation to complete
their maturation
Overview of the Reproductive System
Reproductive system
 In men
 The reproductive system in men has components
in the abdomen, pelvis, and perineum
 The major components are a testis, epididymis,
ductus deferens, and ejaculatory duct on each
side
 and the urethra and penis in the midline
 In addition, three types of accessory glands are
associated with the system:
 a single prostate;
 a pair of seminal vesicles; and
 a pair of bulbourethral glands.
 The design of the reproductive system in men is
basically a series of ducts and tubules
Testes
 The testes originally develop high on
the posterior abdominal wall and then
descend, normally before birth,
through the inguinal canal
 The spermatic cord is the tube-
shaped connection between the pouch
in the scrotum and the abdominal wall.
 Each testis is composed of
seminiferous tubules (400-600) and
interstitial tissue surrounded by a thick
connective tissue capsule (the tunica
albuginea).
 Spermatozoa are produced by the
seminiferous tubules
Epididymis
 The epididymis is a single,
long coiled duct that courses
along the posterolateral side of
the testis
 During passage through the
epididymis, spermatozoa
acquire the ability to move and
fertilize an egg
 The epididymis also stores
spermatozoa until ejaculation.
The end of the epididymis is
continuous with the ductus
deferens.
 The ductus deferens is a long
muscular duct that transports
spermatozoa from the tail of the
epididymis in the scrotum to the
ejaculatory duct in the pelvic
cavity
 Each seminal vesicle is an
accessory gland of the male
reproductive system
 Secretions from the seminal
vesicle contribute significantly
to the volume of the ejaculate
(semen).
 The prostate is an unpaired accessory
structure of the male reproductive
system that surrounds the urethra in
the pelvic cavity
 Secretions from the prostate, together
with secretions from the seminal
vesicles, contribute to the formation of
semen during ejaculation.
 The bulbourethral glands one on
each side, are small, pea-shaped
mucous glands
 the bulbourethral glands contribute to
lubrication of the urethra and the preejaculatory emission from the penis.
Urethra
 The urethra begins at the base of the
bladder and ends with an external
opening in the perineum
 In men, the urethra is long, about 20
cm, and bends twice along its course
 The urethra in men is divided into
preprostatic, prostatic, membranous,
and spongy parts.
 Preprostatic part is about 1 cm long,
extends from the base of the bladder
to the prostate
 The prostatic part of the urethra is
3-4 cm long and is surrounded by
the prostate
 On each side of the prostatic
urethra is the opening of the
ejaculatory duct of the male
reproductive system.
 Therefore, the connection between
the urinary and reproductive tracts
in men occurs in the prostatic part
of the urethra.
.
 The membranous part of the urethra is
narrow and passes through the deep
perineal pouch
 During its transit through this pouch, the
urethra, in both men and women, is
surrounded by skeletal muscle of the
external urethral sphincter
 The spongy urethra is surrounded by
erectile tissue (the corpus
spongiosum) of the penis
 The two bulbourethral glands in the deep
perineal pouch are part of the male
reproductive system and open into the
bulb of the spongy urethra
Reproductive system
 In women
 The reproductive tract in women is
contained mainly in the pelvic cavity and
perineum, although, during pregnancy, the
uterus expands into the abdomen
 Major components of the system consist of:
 an ovary on each side; and
 a uterus, vagina, and clitoris in the midline
 In addition, a pair of accessory glands (the
greater vestibular glands) are
associated with the tract.
Ovaries
 Like the testes in men, the ovaries develop high on the
posterior abdominal wall and then descend before birth
 Unlike the testes, the ovaries do not migrate through
the inguinal canal into the perineum, but stop short and
assume a position on the lateral wall of the pelvic cavity
 The ovaries are the sites of egg production (oogenesis).
 Mature eggs are ovulated into the peritoneal cavity and
normally directed into the adjacent openings of the
uterine tubes by cilia on the ends of the uterine tubes.
 Each of the two almond-shaped ovaries is about 3 cm
long and is suspended by a mesentery (the
mesovarium) from the posterior aspect of the broad
ligament.
 The broad ligament is a sheet-like fold of peritoneum
Uterus
 The uterus is a thick-walled muscular organ in
the midline between the bladder and rectum
 It consists of a body and a cervix, and inferiorly
it joins the vagina
 Superiorly, uterine tubes project laterally from
the uterus and open into the peritoneal cavity
immediately adjacent to the ovaries.
 has a rounded superior end (fundus of
uterus).
 Implantation of the blastocyst normally occurs in
the body of the uterus.
 During pregnancy, the uterus dramatically
expands superiorly into the abdomen.
Endometrium
 It is the inner layer of the uterus ;
 Composed of :
 Compact layer
 Thick spongy layer
 Basal layer ( has its own blood supply)
 The compact and spongy layers are the functional
ones and are shed during menses.
 Menarche : it is the first time menses occurs (age of
puberty)
 Menopause : is a variable period in which the cyclic
changes become irregular and disappear , the age of
menopause is around (45-55 years )
 Between the menarche and menopause the genital
system undergoes cyclic changes in structre and
functional activity controlled by neurohormonal
mechanisms.
Uterine tubes (Fallopian)
 The uterine tubes extend from
each side of the superior end of
the body of the uterus
 Because the ovaries are
suspended from the posterior
aspect of the broad ligaments,
the uterine tubes pass superiorly
over, and terminate laterally to,
the ovaries.
 Each uterine tube has an
expanded trumpet-shaped end
(the infundibulum),
.
 The margin of the infundibulum is
rimmed with small finger-like
projections termed fimbriae
 Medial to the infundibulum, the
tube expands to form the ampulla
and then narrows to form the
isthmus, before joining with the
body of the uterus.
 The fimbriated infundibulum
facilitates the collection of ovulated
eggs from the ovary. Fertilization
normally occurs in the ampulla
urethra
 In women, the urethra is short,
being about 4 cm long
 The inferior aspect of the urethra
is bound to the anterior surface of
the vagina
 Two small paraurethral mucous
glands (Skene's glands) are
associated with the lower end of
the urethra
The Chromosomes
 Traits of a new individual are
determined by specific genes on
chromosomes inherited from the
father and the mother.
 Humans have approximately 35,000
genes on 46 chromosomes
 Genes on the same chromosome
tend to be inherited together and so
are known as linked genes
 In somatic cells, chromosomes
appear as 23 homologous pairs
to form the diploid number of
46 (2n)
 There are 22 pairs of matching chromosomes, the autosomes, and one pair of
sex chromosomes.
 If the sex pair is XX, the individual is genetically female
 if the pair is XY, the individual is genetically male
 One chromosome 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 (1n)
 the union of the gametes at fertilization restores the diploid number of
46.
MITOSIS
 Mitosis is the process whereby one cell divides, giving rise to
two daughter cells that are genetically identical to the parent cell
 Each daughter cell receives the complete complement of 46
chromosomes
 Before a cell enters mitosis, each chromosome replicates its
deoxyribonucleic acid (DNA).
 During this replication phase the chromosomes are extremely long
 they are spread diffusely through the nucleus, and they cannot be
recognized with the light microscope
Prophase
 With the onset of mitosis the chromosomes
begin to coil, contract, and condense
 these events mark the beginning of prophase.
 Each chromosome now consists of two
parallel subunits, chromatids
 that are joined at a narrow region common to
both called the centromere.
 Throughout prophase the chromosomes
continue to condense, shorten, and thicken
 only at prometaphase do the chromatids
become distinguishable
Metaphase
 During metaphase the
chromosomes line up in the
equatorial plane, and their
doubled structure is clearly
visible
 Each is attached by
microtubules extending
from the centromere to
the centriole, forming the
mitotic spindle
Anaphase and Telophase
 Soon the centromere of each chromosome
divides, marking the beginning of anaphase
 followed by migration of chromatids to
opposite poles of the spindle
 Finally, during telophase, chromosomes
uncoil and lengthen,
 the nuclear envelope reforms, and the
cytoplasm divides
 Each daughter cell receives half of all
doubled chromosome material and thus
maintains the same number of chromosomes
as the mother cell (2n).
Mitosis
MEIOSIS
 Meiosis is the cell division that takes place in the
germ cells to generate male and female gametes, sperm
and egg cells
 Meiosis requires two cell divisions, meiosis I and meiosis
II, to reduce the number of chromosomes to the
haploid number of 23 (1n)
 As in mitosis, male and female germ cells
(spermatocytes and primary oocytes) at
the beginning of meiosis I replicate their
DNA
 so that each of the 46 chromosomes is duplicated
into sister chromatids
 In contrast to mitosis, however, homologous
chromosomes then align themselves in pairs,
a process called synapsis
 The pairing is exact and point for point except for
the XY combination
Crossover
 Crossovers, critical events in meiosis I, are 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
 The approximately 30 to 40 crossovers (one or two per
chromosome) with each meiotic I division are most
frequent between genes that are far apart on a
chromosome.
 Homologous pairs
then separate into
two daughter cells
 Shortly thereafter
meiosis II separates
sister chromatids.
 Each gamete then
contains 23
chromosomes 1n.
Meiosis
Meiosis
 As a result of meiotic divisions,
 genetic variability is enhanced through crossover, which
redistributes genetic material
 genetic variability is also enhanced through random distribution of
homologous chromosomes to the daughter cells
 each germ cell contains a haploid number of chromosomes
 so that at fertilization the diploid number of 46 is restored