chapter 17 - development of an embryo

Question 1

where does fertilisation occur

Answer 1

• day 1

- occurs 1/3 of the way down the uterine tube

Question 2

what is early cleavage

Answer 2

• occurs day 2
• zygote begins to divide without any growth (number of cells increase but decrease in size)
• travels down the uterine tube towards the uterus due to muscular contractions and the beating of cilia

Question 3

what is late cleavage

Answer 3

• occurs day 4

- a solid ball of cells forms, embryo is still small but with many cells

Question 4

what is the blastocyst

Answer 4

• formed day 5
• hollow ball or cells, thin layer of cells surround a cavity filled with fluid and the inner cell mass becomes the embryo in the uterus

Question 5

what is implantation

Answer 5

• occurs day 6-7
• embryo (inner cell mass) sinks into the endometrium lining and becomes firmly attached to the wall of the uterus
• embryo grows / develops by absorbing nutrients from glands and blood vessels of lining

Question 6

what are stem cells

Answer 6

• cells that aren’t specialised for any particular role
• capable of undergoing proliferation (repeated divisions by mitosis)
• can differentiate into specialised cells

Question 7

what is differentiation

Answer 7

• process of unspecialised cells developing the characteristics of particular cell types

Question 8

what is a totipoten stem cell

Answer 8

• has the ability to form all cell types that make up the human body (including embryo and embryonic membranes)
• embryo / zygote: is totipotent, undergoes several rounds of cell division to form the blastocyst

Question 9

what is a pluripotent stem cell

Answer 9

• can form all cell types that make up the human body, but not the cell types that make up the embryonic membranes

Question 10

what is a multipotent stem cell

Answer 10

• able to develop into cells that have a specific function but not into all cell types

Question 11

what are the primary germ layers

Answer 11

• ectoderm, endoderm, mesoderm
• inner cell mass undergoes changes, resulting in the formation of three layers of cells (embryonic tissues)
• differentiate into tissues and organs of the body

Question 12

what are the stages in early embryonic development / implantation

Answer 12

• days 1-7

- fertilisation, early cleavage, late cleavage, blastocyst for,action, implantation

Question 13

what is the ectoderm

Answer 13

• central nervous system, epidermis of skin, glands of skin, hair, nails, epithelium of mouth, nostrils, glands of mouth, anal canal, enamel of teeth

Question 14

what is the mesoderm

Answer 14

• skeletal system, muscles, epithelium of blood vessels, kidneys, ureters, ovaries, testes and reproductive tracts

Question 15

what is the endoderm

Answer 15

• epithelium of alimentary canal, liver, pancreas, urinary bladder, gall bladder, pharynx, larynx, trachea, bronchi, lungs

Question 16

what are the embryonic membranes

Answer 16

• there are four embryonic membranes form that lie outside the embryo and serve to protect and nourish it as it develops
• amnion, chorion, yolk sac, allantois

Question 17

what is the amnion

Answer 17

• first membrane to develop, surrounds embryo by day 8, secrete amniotic fluid
• protects embryo against physical injury (shock absorber)
• maintains constant temperature
• allows free movement
• ruptures before childbirth

Question 18

what is the chorion

Answer 18

• forms from outer cell lay of blastocyst and a layer of mesodermal cells, surrounds embryo / 3 other layers, fuses with a,m as it enlarges
• forms main part of placenta (endocrine, immune, nutritional, respiratory, excretory)

Question 19

what is the yolk sac and allantois

Answer 19

• not important in humans, forms part of umbilical cord

Question 20

what is the corpus luteum

Answer 20

• maintains endometrium lining as nourishment of embryo (first 3 months)

Question 21

what is the placenta

Answer 21

• made up of foetal / maternal tissue, many functions, become endocrine gland

Question 22

functions of the placenta

Answer 22

• endocrine: secretes hormones necessary for maintaining the pregnancy
• excretory: transports nitrogenous wastes (Uris acid, urea, creatinine) out of foetal blood supply
• immune: transports antibodies from mother o foetus (immunity to some infectious diseases
• nutritional: transports nutrients to foetal blood, stores essential nutrients early in pregnancy, release them later when demand is greater
• respiratory: transports O to and CO2 from foetus

Question 23

what is the chorionic villi

Answer 23

• becomes surfed by pools of the mothers blood, increases surface area -> increased exchange
• fully developed it is 16m^2 in length

Question 24

what is the chorionic villus

Answer 24

• foetal and maternal blood do not mix, exchange of substance takes place by diffusion and active transport
• oxygen / nutrients: diffuse from maternal blood to foetal blood
• CO2 / waste: diffuse from foetal blood to maternal blood

Question 25

what is the umbilical chord

Answer 25

- attaches placenta to foetus - umbilical arteries (2): carry deoxygenated blood away from foetus -> capillaries of chorionic villi -> mother - umbilical vein: carry oxygenated blood to foetus, mother -> chorionic villi -> foetus - mothers blood: enters placenta through uterine arteries, leaves through uterine veins

Question 26

what happens during week 4-5 of development

Answer 26

- 4: muscle and spinal chord, brain, tail, heart and liver, pharyngeal arches with clefts, 4mm long - 5: arm and leg buds appear

Question 27

what happens during weeks 6-7 of development

Answer 27

- recognisable human form, all organs (not functioning) - head goes from bald size of body (mid week 5) to a third of the body (end week 7) - eye slits, jaw, nose and earlobes (almost fully formed - arms, legs, fingers, toes, sex organs evident - 1g and 3cm in length

Question 28

what are the sources of stem cells

Answer 28

- multipotent: - umbilical chord / placental stem cells: considered adult SC, differentiate into RBC and cells of immune system, can be removed at birth and stored, treats leukaemia - adult stem cells: occur in adult tissue, present in bone marrow, muscle and brain, replace damaged / injured tissue, pre specialised (brain cells -> nerve cells, heart cells -> cardiac muscle cells, patient would not reject own cells - pluripotent: - embryonic stem cells: day 1-7, inner cell mass, can be rejected by another's body

Question 29

what are the uses of stem cells

Answer 29

- replacement tissue: for conditions like strokes, spinal chord injuries, burns, heart disease, diabetes, arthritis, Alzheimer's, Parkinson's - cell based therapies: testing new drugs, testing toxins, understanding birth defects - identifying agents: such as teratogens, that may kill or harm an embryo