Embryogenesis and Fetal Development

Question 1

Describe the 3 phases of the development of the zygote

Answer 1

• pre-embryonic period - first 2 weeks after fertilization, includes the implantation of the zygote into the endometrium
• the embryonic period - weeks 2-8
• the fetal period - weeks 8 to birth

Question 2

Describe the pre-embryonic period of the development of the zygote

Answer 2

• during the first week the zygote travels along the uterine tube towards the uterus
• at this stage a strong membrane of glycoproteins called the zona pellucida surround the zygote
  -the zygote receives nourishment (mainly glycogen) from the goblet cells of the uterine tubes and later the secretory cells of the uterus
• CLEAVAGE
  -during the travel the zygote undergoes mitotic cellular replication and division (cleavage), resulting in the formation of blastomeres
  - the zygote divides into two cells at 1 day, then 4 at 2, 8 at 2.5 and 16 by 3 days, this is known as morula
  COMPACTATION - the cells bind tightly together
• BLASTULATION
  - cavitation occurs where the outermost cells secrete fluid into the morula and a fluid-filled cavity of blastocoele appears
  - this results in the formation of the blastocyst comprising 58 cells
  - the development of the morula to the development of the blastocyst is known as blastulation and has occured by around day 4
• the zona pellucida remains during cleavage so that despite an increase in the number of cells the overall size of the zygote stays constant
  –> it prevents the developing blastocyst from increasing in size and therefore getting stuck in the uterine tube
  –> it also prevents embedding occurring in the tube rather than the uterus (ectopic pregnancy)
• around day 4 the blastocyst enters the uterus
• endometrial glands secrete glycogen-rich fluid into the uterus which penetrates the zone pellucida
  –> this and nutrients in the cytoplasm of the blastomeres provides nourishment for the developing cells
• the blastocyst digests its way out of the zona pellucida once it enters the uterine cavity
• the blastocyst possesses an inner cell mass (embryoblast) and an outer cell mass (trophoblast)
• the trophoblast becomes the placenta and chorion
• the embryoblast becomes the embryo, amnion and umbilical cord
• during week 2 the trophoblast proliferates and differentiates into two layers, the outer syncytiotrophoblast and the inner cytotrophoblast
• simultaneous to implantation the embryo continues developing
• the cells of the embryoblast differentiation into two types of cells
  –> the epiblast (closest to trophoblasts)
  –> the hypo-blast (closest to blastocyst cavity)
• these two layers of cells form a flat disc known as the bilaminar embryonic disk
• a process of gastrulation turns the bilaminar disc into a tri-laminar embryonic disk (3 layers)
• during gastrulation cells rearrange themselves and migrate due to predetermined coding
• three primary germ layers are the main embryonic tissues from which various structure and organs will develop, the first appearance of these layers is around day 15, it is known as the primitive streak
  –> ectoderm - it is the start of tissue that covers most surfaces of the body (epidermis layer of skin, hair, nails and additionally the nervous system)
  –> mesoderm - forms the muscle, skeleton, dermis of skin, connective tissue, urogenital glands, blood vessels and the blood and lymph cells
  –> endoderm forms the epithelial lining of the digestive, respiratory and urinary systems and glandular cells of organs such as the liver and pancreas
• the epiblast separates from the trophoblast and forms the floor of the amniotic cavity
  –> the amnion forms from the cells lining the cavity
• the cavity is filled with fluid and gradually enlarges and folds around the bilaminar disc to enclose it
• the amniotic cavity fills with amniotic fluid derived initially fro maternal filtrate, later the fetus contributes by excreting urine
• at about 16 days mersodermal cells form a hollow tube in the midline called the notochordal process
  –> this becomes a more solid structure (notochord) about a week later
• specialized inducing cells and responding tissues cause development of the vertebral bodies and intervertebral discs to occur
• the neural tube is developed from further cell migration, differentiation and folding of embryonic tissue
  –> this occurs in the middle of the embryo and develops towards each end, process known as (neurulation)
• the hypoblast layer of the embryoblast gives rise to extra-embryonic structures only such as the yolk sac
• hypoblast cells migrate along the inner cytotrophoblast lining of the blastocele-secreting extracellular tissue which becomes the yolk sac
  –> the yolk sac is lined with extraembryonic endoderm which in turn is lined with extraembryonic mesoderm
  –> the yolk sac serves as a primary nutritive function, carrying nutrients and oxygen to the embryo until the placenta fully takes over this role
• after birth all that remains of the yolk sac is a vestigial structure in the base of the umbilical cord (vitelline duct)
• the endoderm and mesoderm cells contribute to the formation of some organs such as the primitive gut arising from the endoderm cells
• an outpouching of endodermic tissue forms the allantois
  –> this extends to the connecting stalk around which the umbilical cord later forms
• growth of blood vessels is induced connecting separately to vessels of the embryo and placenta
• blood islands that later go on to develop blood cells arise from the mesodermal layer, the remainder resembles a balloon floating in front of the embryo until it atrophies by the end of the 6th week when blood forming activity transfers to embryonic sites

Question 3

Describe the pre-embryonic period of the development of the zygote (implantation)

Answer 3

• implantation of the trophoblast layer into the endometrium (now known as the decidua) begins
• implantation is usually to the upper posterior wall
• at the implantation stage the zona pellucida will have totally disappeared
• the syncytiotrophoblast layer invades the decidua by forming finger like projections (villi) that make their way into the decidua and spaces (lacunae) that fill up with the mothers blood
• the villi begin to branch and contain blood vessels of the developing embryo
  - -> thus allowing gaseous exchange between the mother and embryo
• implantation is assisted by proteolytic enzymes secreted by the syncytiotrophoblast cells that erode the decidua and assist with the nutrition of the embryo
• the syncytiotrophoblast cells also produce human chorionic gonadotropin (hCG) a hormone that prevents menstruation and maintains pregnancy by sustaining the function of the corpus luteum

Question 4

What happens during weeks 0-4?

Answer 4

• rapid growth
• embryonic plate forms
• primitive CNS development
• heart develops
• heart starts to beat
• limb buds form

Question 5

What happens during weeks 5-8?

Answer 5

• rapid cell division
• head and facial features form
• major organs laid down in primitive form
• external genitalia develop -> however gender is not yet distinguishable
• early movements
• visible on ultrasound scan from 6 weeks

Question 6

What happens during weeks 9-12?

Answer 6

• eyelids fuse
• kidneys begin to function
• passes urine after 10th week
• starts to suck and swallow
• fetal circulation fully functional -> placenta fully formed at 12 wks
• gender apparent
• moves freely but not felt by mother
• some primitive reflexes present

Question 7

What happens during weeks 13-16?

Answer 7

• rapid skeletal development (important time for mothers to increase calcium and vitamin d intake)
• meconium present in gut
• lanugo appears (fine downy hair)
• nasal septum and palate fuse

Question 8

What happens during weeks 17-20?

Answer 8

• vernix appears (greasy substance covering fetus)
• fingernails seen
• skin cells begin to be renewed
• movements felt by mother (quickening)

Question 9

What happen during weeks 21-24?

Answer 9

• most organs capable of function
• periods of sleep and activity
• responds to sound
• skin wrinkled and red

Question 10

What happens during weeks 25-28?

Answer 10

• eyelids reopen
• eyebrows and eyelashes present
• respiratory movements
• fetus is viable if female

Question 11

What happens during weeks 29-32?

Answer 11

• male and female fetus are viable
• subcutaneous fat begins to form
• skin becomes thicker, paler, smoother
• begins to store iron in liver
• testes descend in male fetus
• lanugo disappears on face

Question 12

What happens during weeks 32-35 (34-37 post LMP)?

Answer 12

• further fat formation
• shape becomes rounded
• wrinkled appearance disappears
• nails reach tips of fingers
• ear cartilage still soft
• hair on head lengthens
• plantar creases visible
• some lanugo present
• abundant vernix
• blood vessels not overly apparent
• pink colour (white caucasian)

Question 13

What happens during weeks 36-38 (38-40 post LMP)?

Answer 13

• body contours rounded
• plenty of subcontaneous fat
• lanugo disappears
• vernix disappears
• skull bones firm