1. Fertilization occurs when a sperm meets an egg in the fallopian tube. The zygote then undergoes cleavage as it travels down the tube. 2. Around 5 days after fertilization, the blastocyst implants in the uterine lining. Implantation failure can lead to ectopic pregnancy. 3. During the third week, gastrulation occurs as the embryo forms its three germ layers - endoderm, mesoderm, and ectoderm. Organogenesis then follows.

1. FERTILIZATION,
IMPLANTATION AND
EMBRYONIC WEEK
INTRODUCTION TO ANATOMY
HUMAN ANATOMY I

2. FERTILIZATION
• Union of a spermatozoon and an ovum (secondary oocyte) to
form a zygote.
• Ampulla of the fallopian tube within 24 hours after ovulation
• Sperm cells are viable within 3-5 days in the fallopian tube
• Sperm cells must be capacitated before fertilization

3. FERTILIZATION CONT.
• Phase 1: A lucky healthy capacitated sperm penetrates the corona
radiata of the secondary oocyte
• Phase 2: Acrosomal reaction occurs (enzyme acrosin lysis the
zona pellucida) and then polyspermy block (zona reaction makes
the zona pellucida impenetrable).
• Phase 3:
• The cell membranes of the gametes fuse
• male pronucleus moves into the cytoplasm of the secondary
oocyte
• the secondary oocyte completes meiosis II to become mature
ovum with a pronucleus
• the pronuclei combine to form a diploid zygote.
• No paternal mitochondrial DNA

5. CLEAVAGE
• A series of mitotic process in the
zygote to form a morula within 30
and 72 hours
• First division gives a cell (2-cell
blastula) with two blastomeres, then
4-cell, and later dividing into 16 – 32
cells.
• Until 8-cell blastula, each
blastomeres is totipotent (can
become a complete embryo)
• These cells divide one at a time; the
cells decrease in size as the number

6. IMPLANTATION
• 4th day, morula becomes fluid-
filled to become blastocyte
• Blastocyte has blastocyst
cavity, outer cell mass
(trophoblast) and pluripotent
inner cell mass (embryoblast)
• 5th day - zona pellucida
degenerates to allow
implantation

7. IMPLANTATION
CONT.
• 6th – 7th day, implantation
occurs in the stratum
functionalis of the
endometrium;
syncytiotrophoblast assists in
the adhesion
• Trophoblast must produce
sufficient immunosuppressive
factor and human chorionic
gonadotropin (hCG) to prevent
menstruation and ovulation

9. CLINICAL CORRELATES
•Abnormal Implantation – close to the internal os of the
cervix may cause placenta previa, ectopic pregnancy in the
abdominal cavity (rectouterine pouch), uterine tube (95%)
or ovary.
•Ectopic tubal pregnancy (ETP) mostly due to endometriosis
and PID, can cause rupture of the fallopian tube, uterine
bleeding, unilateral pelvic pain, increased levels of hCG,
massive bleeding in 1st trimester,
•Pluripotent embryoblasts have been experimentally used
in curing various diseases e.g., diabetes, Parkinson’s
disease, anaemia etc.

10. ECTOPIC
PREGNANCY
1 - 1.4%
2 - 80%
3 - 12%
4 - 0.2%
5 - O.2%
6 - 0.2%

11. WEEK 2 OF HUMAN
DEVELOPMENT (week of
2’s)
• Trophoblast differentiates
into inner cytotrophoblast
and outer
syncytiotrophoblast
• Embryoblast differentiates to
form bilaminar embryonic
disc: made of ventral
hypoblast and dorsal
epiblast

12. WEEK 2 cont.
• Hypoblast cells spread all through the
inner surface of the cytotrophoblast
to form a thin exocoelomic (Heuser’s)
membrane containing the primitive
yolk sac (or exocoelomic cavity).
• Within the epiblast cells, amniotic
cavity develops.
• Uteroplacental circulation by diffusion
(lacunar network)
• Chorionic villi (later become foetal
part of placenta)

13. CLINICAL CORRELATES
• Human chorionic gonadotropin (hCG) – a hormone
secreted by syncytiotrophoblast and it can be detected in
the woman’s blood and urine as early as the 8th and 10th
day respectively.
• Progesterone-receptor antagonist (blocker) -
Mifepristone – 96% efficient if used within 8-10 weeks of
the start of the last menstrual period.
• Superfecundation – 2 or more secondary oocytes are
fertilized during different coitus
• Superfetation - a secondary oocyte is fertilized when
there is already an embryo in the uterus

14. 1. Enlargement of chorionic villi
2. Hyperplastic proliferation of the cells of
the trophoblast
3. No embryo or foetus.
Clinical signs are preeclampsia in 1st
trimester, ↑ hCG, enlarged uterus with
bleeding, and pelvic pain.
Mole may later develop into malignant
Gestational Trophoblastic Neoplasia (15-
20%) or Choriocarcinoma (2%) if pieces of
trophoblast are left behind following
spontaneous abortion or dilatation and
curettage
Moles appear as clusters of grapes or
HYDATIDIFORM MOLE
(complete/partial)

15. Characterized by hypertension (˃160/110 mmHg), edema,
and/or proteinuria (˃5 g/24 hours).
It is severe if there is sudden onset of these maternal
symptoms in the third trimester.
There is generalized arteriolar constriction which affects the
brain, causing convulsion, and may cause multiple organ
failure.
Delivery of the foetus is the only remedy after the
symptoms have been treated.
Risk factors: nulliparity, diabetes, hypertension, renal
disease, twin gestation, or hydatidiform mole (produces
first trimester preeclampsia)
PREECLAMPSIA

16. 3RD WEEK
•Gastrulation begins with
primitive streak appearance
between prochordal plate
and cloacal membrane
•Epiblast invaginate to form
endoderm, intraembryonic
mesoderm and ectoderm
(trilaminar embryonic disc)

17. Transverse sections showing development of the three germ layers.
A. Day 17. B. Day 19. C. Day 20. D. Day 21.
Notochord forms
between primitive node
and prochordal plate.
Notochord induces the
ectoderm above it to
form neural plate, it also
induces the formation of
centrum of vertebrae
Neural plate undergoes
neurulation
3RD WEEK

18. VITAL GENES OF EMBRYOGENESIS
• Homeobox (Hox) genes direct the craniocaudal region
• Sonic hedgehog (SHH) gene determines anteroposterior axis and
CNS development
• PITX2 (transcription factor) is the master gene for left-sidedness.
• Serotonin (5-hydroxytryptamine; 5HT) also important in
determining laterality
• FGF gene stimulates mitosis of mesoderm causing lengthening
of the limbs
• Teratogens can cause disruption of 5HT levels or misexpression
of PITX2 results in laterality defects, such as dextrocardia, situs
inversus, and cardiac abnormalities.
• Mutation of SHH can cause holoprosencephaly

19. 3RD-8TH WEEK (ORGANOGENESIS)
• Week 4 – neural tube closes, heart begins to beat and
Upper and lower limb buds begin to form.
• Week 6 - foetal cardiac activity visible by transvaginal
ultrasound.
• Week 8 - foetal movements start.
• Week 10 - gender can be determined (genitalia have
male/female characteristics.

20. CLINICAL CORRELATES
• HOLOPROSENCEPHALY in children due to
abusive use of alcohol by the mother at the
beginning of the 3rd week. This high dose
destroys cells forming the craniofacial
structures resulting in
• little forebrain
• the two lateral ventricles form one central
ventricle
• eyes closely positioned
• cleft lip/palate
• Cyclopia in severe cases.

21. SACROCOCCYGEAL TERATOMAS
(ST)
• A tumour caused by persistent
remnant of primitive streak.
• The tumour contains derivatives of
the three germ cells derivatives
• It normally disappears shortly after
birth, but if it does not, then
surgery is done before it becomes
malignant.
• Occurs more in female neonates.

22. CHORDOMA
• A benign or malignant tumor caused
by the remnants of the notochord.
• It may be in the base of the cranium
(occipital chordoma ) or in the sacral
region (coccygeal chordoma).
• It is more common in men around
50 years.
• Chordoma presses on the brain of
spine as it grows leading to its

23. SIRENOMELIA (CAUDAL DYSGENESIS/
DYSPLASIA
• Abnormal gastrulation due to genetic
anomaly, it is extremely rare
• There is insufficient mesoderm in the caudal
region of the embryo leading to
• hypoplasia and fused lower limbs
• imperforate anus
• abnormal genital organs
• The clinical sign could also include VATER
(vertebral defect, anal atresia,
tracheoesophageal fistula, renal defects) or
VACTERL (VATER plus cardiovascular defects
and upper limb defects)
mermaid

24. DERIVATIVES OF
ECTODERM
NEUROECTODERM (NEURAL
TUBE)
• All neurons in the brain and
spinal cord
• Astrocytes, ependymal cells
and oligodendrocytes
• Neurohypophysis (posterior
pituitary) and pineal gland
• Retina and optic nerve

25. DERIVATIVES OF ECTODERM
NEURAL CREST CELLS contribute to
so many organs and tissues:
• Neurocranium, pia mater and
arachnoid, odontoblasts
• Cranial nerves, dorsal root
ganglion, Schwann cells and
autonomic ganglia
• Aorticopulmonary septum,
endocardial cushions, myenteric
(Auerbach) plexus
• Melanocytes, chromaffin cells of
adrenal medulla, parafollicular or

26. DERIVATIVES OF ECTODERM
SURFACE ECTODERM
 Epidermis, hairs, nails, sweat and sebaceous glands, and
mammary glands
 Adenohypophysis
 Sensory organs of the ear and eye lens
 Enamel of teeth, acinar cells of parotid gland and epithelium of
the mouth
 Epithelium of lower anal canal below the pectinate line, lower
male urethra and external auditory meatus
 Olfactory epithelium

27. NEURAL TUBE DEFECTS
(NTDs)
• Failure of the neural tube to close
completely and there is ↑ alpha-feto
proteins levels increase.
• It can occur in the cephalic region -
anencephaly
• It can occur in the cervical and caudal
regions - spina bifida: SB occulta, SB with
meningocele, SB with meningomyelocele,
SB with myeloschisis).
• A daily intake of 400 µg of folic acid 3
month before and during pregnancy can
reduce the occurrence of NTDs.
• Women with a history of this anomaly in

28. MESODERM
• Intraembryonic mesoderm forms thick paraxial mesoderm,
intermediate mesoderm and the thin lateral mesoderm
• Paraxial mesoderm  35 somites  sclerotome, myotome,
dermatome
• Intermediate mesoderm
• Lateral mesoderm is divided by an intraembryonic coelom into
visceral (splanchnic) mesoderm and parietal (somatic)
mesoderm

29. DERIVATIVES OF MESODERM
• Sclerotome  vertebrae, intervertebral discs and body wall
muscles
• Myotome  epimeric and hypomeric muscles
• Dermatome  dermis of the back.
• Intermediate mesoderm  urogenital ridge (kidneys and
gonads)
• Visceral (splanchnic) mesoderm  cardiovascular
structures
• Parietal (somatic) mesoderm  dermis in the ventral body
wall, bones of the limbs, sternum and connective tissue
• Other derivatives are all body muscles, covering
membranes of body cavities, blood, lymph, wall of GIT,

30. DERIVATIVES OF THE ENDODERM
• Epithelial lining of GIT, trachea, bronchi, lungs, tract,
biliary apparatus, urinary bladder, female urethra, most
of the male urethra, inferior 2/3 of vagina, auditory
tube, middle ear cavity
• Anal canal above the pectinate line
• Thyroid follicular cells, principal and oxyphil cells of
parathyroid, liver, gallbladder and pancreas
• Reticular stroma of the tonsils and the thymus
• Acinar cells of submandibular and sublingual glands