Study Notes for ANAT0001

ANAT0001- Introduction to Anatomy and Development


Fertilisation and Cleavage
HUMAN LIFE CYCLE
1. Fertilization
2. Cleavage (1-5 days)- cell division (mitotic), not linked to growth
3. Implantation (6-9 days)- within the uterus, to get nutrients
4. Gastrulation (3-4 weeks)- developing nervous system
5. Body plan (4 weeks)
6. Organogenesis (4-8 weeks)- major organ system formed within the embryo ( not
necessarily functional)
7. Foetal development (8-40 weeks)
8. Birth


OOGENESIS

 All the eggs are produced before birth
 Starts with primordial germ cells, undergo mitosis
 Meiosis begins in foetal ovary- stops at Prophase I until puberty
 Few primary oocytes then reactivate meiosis at each menstrual cycle- forming
secondary oocyte & a polar body
 Secondary oocyte held at Metaphase II until fertilised by sperm
 After ovulation, oocyte and first polar body surrounded by ZONA PELLUCIDA
(membrane)
 Within oocyte, just beneath the plasma membrane, there's small cortical granules
( required after fertilization)
 Polar body is discarded

3:54 PM
HUMAN LIFE CYCLE
1. Fertilization
2. Cleavage (1-5 days)- cell division (mitotic), not linked to growth
3. Implantation (6-9 days)- within the uterus, to get nutrients
4. Gastrulation (3-4 weeks)- developing nervous system
5. Body plan (4 weeks)
6. Organogenesis (4-8 weeks)- major organ system formed within the embryo ( not
necessarily functional)
7. Foetal development (8-40 weeks)
8. Birth


OOGENESIS

 All the eggs are produced before birth
 Starts with primordial germ cells, undergo mitosis
 Meiosis begins in foetal ovary- stops at Prophase I until puberty
 Few primary oocytes then reactivate meiosis at each menstrual cycle- forming
secondary oocyte & a polar body
 Secondary oocyte held at Metaphase II until fertilised by sperm
 After ovulation, oocyte and first polar body surrounded by ZONA PELLUCIDA
(membrane)

,  Within oocyte, just beneath the plasma membrane, there's small cortical granules
(required after fertilization)
 Polar body is discarded

FIRST WEEK OF HUMAN DEVELOPMENT
 Mammalian egg is normally fertilised in the AMPULIA of the oviduct ( fallopian tube)
 As it divides, it move down the oviduct to the uterus, where it "hatches" from the zona
pellucida and implants into the uterus wall
 Implantation takes ~6 days after ovulation
 Only some sperm can reach down the fallopian tube


ECTOPIC PREGNANCY
 ~1% of pregnancies
 Embryo implants outside the uterus
 Usually occurs in fallopian tube, but may also be at the cervix, ovary and abdomen
 Rarely viable, but a serious threat to mother, with internal bleeding and common
complication



FERTILIZATION
 Protein at zona pellucida acts as receptor for sperm (species-specificity)
 Sperm make contact with species-specific receptors , acrosome burst, release enzyme
that digest a hole in the zona pellucida
 Sperm pass through, fuse with the oocyte plasma membrane
 Polyspermy- always lethal, may result in faulty segregation of chromosome during
mitosis, leading to developmental arrest in mammals
 Calcium ions increase when sperm fuse with the plasma membrane
 Zp3 is the sperm-binding protein
 Inactivation of zp3 + zona pellucida impermeable -> avoids polyspermy


MATERNAL AND PATERNAL CHROMOSOMES ARE REQUIRED FOR NORMAL
DEVELOPMENT
 Both chromosomes are important
 Male chromosomes to produce embryonic tissue
 Female chromosomes to produce embryo itself
 It's called genetic imprinting

THE FERTILIZED EGG MUST
 Produce trillions of cells (through mitotic cell division)
 Instruct these cells on what cell-type they should turn into
 Organize them into tissues and organ
 Different cell-types must acquire the special characteristics required for their
function( cell differentiation)


CLEAVAGE DIVISION
 Polar body don't have any use ( will be discarded)
 Cells become smaller and smaller after the divisions ( knowns as CLEAVAGE
DIVISION)
 Cleavage division- not accompanied by growth
 At the morula stage (~16 cells), they undergo COMPACTION ( cells maximize their
contacts)

,  Cells at the centre of the compacted embryo are known as the inner cell mass,
peripheral cells are known as trophoblast
 Fertilised egg first divide about 24 hours after fertilization
 First cell division is made- cell from the inside is different from the outside
 Cell from outside starts to pump salt , then water diffuses in, forming a cavity in the
centre of embryo ( known as blastocyst), known as blastocoel in the centre of embryo

REGULATIVE DEVELOPMENT
 2- and 4-cell stage embryo can be divided into individual blastomeres
 Each blastomere transferred to the uterus of a surrogate female, they grow to normal
size and form genetically identical clones
 Totipotent to describe cells that are capable of producing all the tissues formed by the
fertilized egg


TETRAGAMETIC CHIMERA
 Formed by two sperm and two eggs
 Happens in human too


CELL FATE IN MAMMALIAN BLASTOCYST
 Trophoblast forms chorion, ICM divides into epiblast and hypoblast layers, the latter
differentiating as extraembryonic endoderm
 Epiblast forms the amniotic membrane and extraembryonic mesoderm as well as all
tissues (ectoderm, mesoderm and endoderm) of the foetus


EMBRYONIC STEM CELLS
 Inner cell mass isolated and cultured in vitro as embryonic stem cells
 Using different culture conditions, adding different factors, it is possible to make them
differentiate into many different cell-types.
 Hoped that these can be used to grow organs and tissues for transplantation
 Stem cells only produced after taken out from embryo and was cultured in vitro


FUNCTION OF THE TROPHOBLAST
 Implantation
 Differentiate chorion
 Immunosuppression- Embryo develops paternal antigen. Why mother's immune
system didn't attack dad's cell?
 Endocrine gland (chorionic gonadotrophin, chorionic somatomammotropin, chorionic
thyrotropin, chorionic corticotropin)
 Has several functions in the embryo and foetus


TROPHOBLAST DISEASE
 Hydatidiform moles (1/500 pregnancies) are formed when trophoblasts growing
uncontrollably
 If left untreated, miscarriage but some will form a tumour (choriocarcinoma)


IMPLANTATION
 Attached to the uterine wall, trophoblast undergoes rapid cell proliferation ( cell
dividing)
 Formed syncytiotrophoblast - outer layer

,  Has no cell boundaries, but a common cytoplasm with many nuclei
 Remaining trophoblast is known as cytotrophoblast- inner layer
 Syncytiotrophoblast is highly invasive and penetrates the uterine wall- to obtain
nutrients from the uterine wall
 ICM divides into two cell populations, epiblast and hypoblast ( known as bilaminar
embryonic disc)
 Once embryonic disc formed, cavity appear between the epiblast and cytotrophoblast,
known as the amniotic cavity


IMPLANTATION (8-11 DAYS)
 Syncytiotrophoblast (St) expands as blastocyst invades the uterus, forming lacunae (L)
that fill with maternal blood from uterine capillaries
 Beginning of the utero-placental circulation
 Amnion (A) forms in the epiblast and the hypoblast spreads along the cytotrophoblast
(Ct) to form Heuser's membrane (HM)
 Enclosed cavity is known as yolk sac(YS)
 Extracellular reticulum (ER) is secreted by both Heuser's membrane and
cytotrophoblast


STEM VILLI
 After 14 days the syncytiotrophoblast, cytotrophoblast, extraembryonic mesoderm
protrude into the lacunae, forming stem villi
 Mesoderm forms the blood vessels of the chorion, increasing the efficiency with which
O2, CO2, nutrients and waste products are transferred between maternal and
embryonic blood supplies


PLACENTAL TRANSFER
 Stem villi allow the transfer of O2 and nutritional elements from the mother to the
developing embryo
 CO2 and waste products transferred in opposite direction
 Damaging substance eg alcohol, drugs and viruses may cross the placenta into the
embryo


3 MONTHS OLD HUMAN FOETUS
 Developing extraembryonic tissues provide a survival capsule for the foetus, which is
bathed in amniotic fluid
 Foetus is connected to the chorion via the umbilical cord and its blood vessels
 As embryo grows, it will bulge into the uterine cavity and the chorion will become
restricted to the side of the embryo closest to the umbilical cord, forming discoid
placenta




Cell and Tissue

HUMAN DEVELOPMENT
 Cells -> Primary germ layers-> Tissues -> Organs