B.

Mugala
BSc N, RM, RN, EN

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Human Embryology is concerned with
the study of intra-uterine life from
fertilization to birth: (9 months/ 40wks)
of gestation.
Cells of the body are single cell
(ZYGOTE) descendant gives 2 groups
of cells, 1 large group the SOMATIC
cells (that form the body) and a group
of GERM cells in the gonads for
continuation of species
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 Cont.
DEVELOPMENTAL LIFE CYCLE (I.U)
1-Pre-embryonic period:- 1st and 2nd
Wks; concerned with fertilization,
implantation & differentiation in
embryonic & non-embryonic portions.
2-Embryonic period;-3RD to 8th wk-
Rudiments of organs are formed- 20th
– 22nd wks the heart beat starts.
3-Fetal period;- end 8th wk- birth.
Rapid growth. Organs function&
assume definite human form.

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They have a diploid No. of chromosomes.
They appear at 3rd week on the yolk sac and
migrates to the undifferentiated gonads.
The gonads differentiate into tests or ovary
And these gonads give rise to primary gametes

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 Spermatogenesis

Process of sperm formation

*Occurs in testes
*Begins at puberty
*Continuous process
*Males always fertile (80+ year
olds have become fathers)
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Sequence of events of Spermatogenesis
Spermatogonia (2N): somatic like cells; stem
cells; reproduce by mitosis
1o Spermatocytes: Growth phase; double their
DNA content ; this stage represents the start meiosis
2o Spermatocytes: result of 1st Meiotic
Division; smaller than 1o spermatocytes
Spermatid: result of 2nd Meiotic Division; smaller
than 2o spermatocytes; dense nucleus; differentiate
into sperm
Spermatozoa: fully developed spermrm
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 oogenesis
PGCs- Oogonia (increase in No by
mitotic divisions)- creates primary
occytes - & these prepare them selves
for first meiotic division & and get
arrested at the completion of the 1st
meiotic stage and the start of the 2nd
meiotic division.
They become surrounded by follicular
epithelium & form primordial
follicles but they do not complete 1st
meiosis before puberty.
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 Cont.
Before ovulation, primordial follicles
form which are primary follicles. Have
zona pellucida and theca cells.
2ndary follicles are larger have the
same layers as 1st but have antrum
form.
Tertiary follicle or graafian follicle
more larger granulosa form cumulus
oophorus before ovulation complete 1 st
meiotic and starts 2nd meiotic stage
upon completion of fertilization
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 Ovulation

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EMBRYONIC LIFE CYCLE
1-Pre-Embryonic Period: (2 Weeks)
1st week events

1-FERTILIZATION
Process of fusion of male & female gametes in the
ampulla of uterine tube
Before fertilization the spermatozoa should have :
1-Capacitation; removal of seminal protein from the
acrosomal surface during the first 7 hours

2-Acrosomal reaction enzymes; after binding to

zona pellucida, zona protein induce the release of
penetrating
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enzymes (acrosin) that erode the cell
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FERTILIZATION occurs in the
ampulla of the uterine tube. Sperm
binds and then, through enzymatic
mechanisms, penetrates the zona
pellucida of an immature ovum .
Upon entrance, secondary oocyte
completes meiosis creating a
mature ovum. The nucleus of the
ovum, containing all the maternal
DNA, called the female pronucleus,
fuses with the male pronucleus that
contains all the paternal DNA,
forming the zygote
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1-Pentration of Corona-radiata
2-Pentration of Zona-pellucida
3-Fusion of cell membranes

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Cleavage
Within the zona pellucida, the zygote
undergoes multiple mitotic divisions
called cleavage. The first cleavage
results in two blastomeres, and the
second cleavage results in four
blastomeres . Blastomeres at
approximately the fifth cleavage ( 16 to
32 blastomeres) form a morula.
As cleavage progresses, the cell mass
migrates along the uterine tube toward
the uterus. The morula enters the
uterine cavity (approximately between
day 3 and day 4 post fertilization
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Blastocyst:- Cells of the morula secrete a
fluid in the blastocyst cavity (blastocoele).
The internal portion of the blasotcyst has
polarity, a cellular end and an acellular end.
The blastocyst cells form two cell masses
1- Inner cell mass
(embryoblast)
2-Trophoblast.
(extraembryonic
membranes)

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*Separation of zone pellucida.
The zona pellucida breaks apart, releasing the
blastocyst from it and allowing the blastocyst
to implant into the endometrial wall of the
uterus . The outer cell mass, or trophoblast,
initially contacts the endometrial wall.

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Implantation of the Embryo
Complex interaction between embryo
and uterine tissues
Occurs ~6-7d after fertilization

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Hormones have prepared uterus
ENDOMETRIUM to accept embryo
consists of 3 layers in secretory phase of
menstrual cycle.
compact
implantation occurs in this layer (decidua vera)
dense stromal cells, uterine gland necks,
capillaries of spiral arteries. Also called
decidua parietalis.
spongy
swollen stromal cells, uterine gland bodies,
spiral arteries (decidua capsularis) also
known as decidua reflexa.
basal
not lost during menstruation or childbirth, own
blood
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STAGES of IMPLANTATION
1. Apposition: of blastocyst to
endometrial epithelium
2. Adherence: via cell adhesion
molecules; the glycoproteins
involved are unknown.
3. Formation of
Syncytiotrophoblast: Fusion of
cytotrophoblast cells results in giant
multinucleate cells that will
surround embryo
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4. Penetration: Syncytiotrophoblast
is invasive and works its way into
uterine tissue ultimately making
contact with maternal blood vessels
5. Decidual Reaction: Uterine tissue
response to embryonic invasion by
setting up an immunological barrier,
the decidual; thus the decidua is
derived from maternal tissues

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Abnormal Implantation
Tubal pregnancy94%
often occurs if zona pellucida is lost too early
allowing premature implantation
embryo may develop through early stages.
can erode through the uterine horn and reattach
within the peritoneal cavity
Ectopic Pregnancy
external surface of uterus
Ovarian pregnancy
The recto-uterine pouch
gastrointestinal tract, mesentery
peritoneal wall
Placenta praevia – Abnormal placental
implantation
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Trophoblast
Differentiates into two layers:
Syncytiotrophoblast, an outer layer
Cytotrophoblast, an inner layer
The syncytiotrophoblast cells,
penetrate the endometrial wall of the
uterus that is rich in blood vessels.
The cells produce enzymes that
degrade endometrial stromal and
glandular cells, and invade the walls
of the maternal blood vessels (lacunar
stage).
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 Cont.
Syncytiotrophoblast produces the
human chorionic gonadotropin
(hCG), which increases the
progesterone production in the
corpus luteum of the ovary, thereby,
maintaining pregnancy.
Assaying levels of hCG in maternal
urine or blood is the basis for the
diagnosis of early pregnancy
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BILAMMINAR GERM DISC
Inner Cell Mass (ICM)
delaminates to form hypoblast
and epiblast
Occurs just prior to implantation
& gastrulation
Epiblast is 2-layered (i.e., it is
bilaminate) of cuboidal cells &
will form the embryo proper
Hypoblast flatter cells will form
yolk sac
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BILAMMINAR GERM DISC

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Yolk sac
The yolk sac give rise to EXTRAEMBRYONIC
mesoderm between trophoblast and a cavity
forms between the amniotic & yolk sac.
Chorionic cavity appears within the extra-
embryonic mesoderm divides it into SOMATIC
and SPLANCHNIC extraembryonic mesoderm
By the end of the 2nd week primitive yolk
disappear.
Syncytiotrophoblast. establish extra-placental
circulation.
cytotrophoblast penetrates syncytiotrophoblast
as primary villi

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
THE ALLANTOIS
The allantois appears on approximately
day 16 as a small, sausage-shaped
diverticulum (out pouching) from the
caudal wall of the umbilical vesicle that
extends into the connecting stalk.
Allantoic mesoderm expands beneath
the chorion and forms blood vessels that
will serve the placenta. persists
throughout much of development as a
stalk called the urachus, which extends
from the bladder to the umbilical region.
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Cont.
The urachus is represented in adults by
the median umbilical ligament. The
blood vessels of the allantoic stalk
become the umbilical arteries. The
intraembryonic part of the umbilical
veins has a separate origin.

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EMBRYOONI
C PERIOD
3 TO 8
rd th

WEEK
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3rd WEEK TRILAMINAR GERM DISC
 Gastrulation
will covert the bilaminate epiblast into
the three primary embryonic germ
layers
Ectoderm: outside; this embryonic
layer more or less surrounds the other
germ layers
Mesoderm: middle; this germ layer lies
between the ectoderm and endoderm
Endoderm: inside; this germ layer lies
at the most interior of the embryo
 Subsequently neurulation will form
epithelial and neural ectoderm from
the ectoderm
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Ectoderm
Givesrise to central nervous system &
vestibular system, sensory epithelium of
ear, nose and eye
High variety of tissues are also formed
including most of the skull, facial
skeleton, tooth structures except enamel,
cardiac septa, sensory & autonomic NS,
nerve fibers, adrenal medulla,
melanocytes, Schwann cells
leptomeninges, glial cells & C cells of
thyroid
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Surface Ectoderm
Epidermis &its appendages, nail,
hair& glands
Subcutaneous & mammary, anterior
pituitary, parenchyma of salivary
glands, oral epithelium& teeth
enamel

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MESODERM:-
Give rise to certain bone & cartilage,
muscles, dermis & its segmental nerves.
Urogenital system
Body wall + parietal membrane)
splanchnic (smooth + visceral
membranes) and also supporting tissues
such as cartilage, bone, muscle,
connective tissue, blood and lymph cells
walls of the heart, blood and lymph
vessels (Endothelium), spleen, kidneys,
gonads and associated ducts suprarenal
gland cortex

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 Endoderm
Epithelial lining of:
Gut, respiratory tract , urinary bladder
and urethra , tympanic cavity and
auditory tube

Parencchyma of:
Thyroid, parathyroid glands and liver

Stroma of tonsils and thymus

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Extraembryonic Membranes: Origins &
Functions
Chorion: covers fetus & other
membranes
Amnion: forms protective, fluid filled sac around
embryo
Yolk Sac: originally held yolk (lower animals)
Allantois: used for waste removal through its
systems of blood vessels.
Umbilical Cord: Conduit for food, oxygen, etc.

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 Teratogenes

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Teratogenesis
Signifies the production of gross
structural malformations during foetal
development, in distinction from other
kinds of drug-induced foetal damage such
as growth retardation, dysplasia (e.g.
iodide-associated goitre), or the
asymmetrical limb reduction resulting
from vasoconstriction in an otherwise
normally developing limb.

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MECHANISM OF TERATOGENESIS
The timing of the teratogenic insult in
relation to fetal development is critical in
determining the type and extent of
damage. Mammalian fetal development
passes through three phases
blastocyst formation
organogenesis
histogenesis and maturation of function.

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 Cont.
None Cell division is the main process
occurring during blastocyst formation.
During this phase, drugs can kill the
embryo by inhibiting cell division, but
provided the embryo survives, its
subsequent development does not
generally seem to be compromised.
Ethanol is an exception, affecting
development at this very early stage
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DRUG EFFECT

Thalidomide
Cont.Phocomelia, Heart defects, Gut
atresia.
Penicillamine Loose Skin
Warfarin Saddle nose, Retarded growth,
defects of
Limbs, eyes and NS
Phenytoin Cleft lip/ palate, Microcephaly, MR
Tetracycline Staining of teeth, Poor bone growth
Ethanol Foetal alcohol syndrome
ACE Drugs Renal failure
Aminoglycosides Deafness
Carbamazepine Retardation of foetal head growth
Retinoids Hydrocephaly
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 References
Rang H. P. et al (2007) Rang and Dale’s
Pharmacology. 6th Edition. Churchill Livingston,
London.
Sadler T. W.(2006) Langman’s Medical
Embryology. 10th Edition. Lippincott, Philadelphia.
USA.
Moore K. L. and Persaud T.V.N.() The Developing
Human. 8th Edition. Elsevier, New York. USA

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