Embryogenesis Flashcards
What is the ovarian cycle?
The physiological changes that occur in the ovaries that are essential for the preparation and release of an oocyte.
What is the menstrual cycle?
The physiological changes in the endometrial layer of the uterus which are essential to receive a fertilised oocyte.
Describe the menstrual PHASE
Day 1-5
First day of menstruation, uterus sheds its endometrium down to the basal layer due to reduced level of oestrogen and progesterone which causes the endometrium to spasm and withdraw its blood supply.
50-150ml blood loss, enzyme plasmin inhibits clotting.
What is eumenorrhoea?
Normal regular menstruation
Describe the follicular phase
Day 1-13 runs alongside mestural phase but carries on longer.
Low levels of oestrogen and progesterone stumlaute the hypothalamus to produce gonadotrophin releasing hormone (GnRH).
GnRH causes production of FSH and LH by the anterior pituitary gland.
A series of endocrine functions take place to mature an oocyte within s Graafian follicle.
Maturation takes approx 7 days
Describe the ovulation phase
Days 14
High oestrogen levels cause an increase in LH around days 12-13 which lasts 48 hours. This matures the oocyte and weakens the follicle wall.
Ovulation occurs on day 14
Fimbrae guide the oocyte into the uterine tube to await fertilisation.
Meiotic cell division resumes and the diploid oocytes become haploid.
Mucus in the cervix awaits spermicide from intercourse. Following ovulation the oocyte travels to the uterus.
Describe the luteal phase?
Day 15-28
Cells from ruptured follicle produce corpus luteum.
Corpus luteum produced oestrogen, relaxin, inhibin and progesterone for 2 weeks
Relaxin creates uterine quiescence-ideal environment for fertilised oocyte to implant
Corpus luteum continues its role until the placenta is formed and mature
If oocyte is unfertilised the corpus luteum degenerates leading to low levels of oestrogen and progesterone which causes a high level of GnRH causing the release of FSH which ultimately restarts the cycle.
What forms a zygote?
The fusion of genetic material from the haploid sperm cell and the secondary oocyte
How long is the fertilisation process?
12-24 hours
How many sperm reach the oocyte?
200 out of 300 million (avg)
What is capacitation?
The removal of the glycoprotein coat and increased flagellation of the sperm
What is the zona pellucida?
The extra cellular matrix surrounding the oocyte.
Creates chemicals to attract capacitated sperm
What is an acrosome reaction?
The acrosomal layer of the sperm releases hyaluronidase to allow access to the zona pellucida. Many sperm are involved in this. The first sperm to reach the zona pellucida penetrates it
What happens to the oocyte upon penetration?
It undergoes a cortisol reaction making it impermeable to other sperm. The nuclei of the oocyte and the sperm fuse to form a zygote with a full set of 46 chromosomes
What is the blastocyst called once it has embedded into the decidua?
The embryo
When does an embryo become a foetus?
At 8 weeks gestation
Where does the embryo develop from?
The embryonic disc
What are the 3 layers of cell development?
Ectoderm - closest to the amniotic sac. Forms the skin and most of the CNS
Mesoderm - forms the bones, muscles, heart and blood vessels. Also kidneys and reproductive organs
Endoderm - forms digestive and respiratory organs
Outline what happens at day 13 of embryonic development
Chorionic villi secrete hCG which stimulates the corpus luteum to continue producing hormones to continue the pregnancy
Outline day 15 of embryonic development
Primitive streak appears in midline of dorsal surface. Blood vessels form
Outline day 18 of embryonic development
Primitive nervous system folds into position. Heart begins to twitch. Eyes and ear develop.
Weeks 3-4 of embryonic development
Heart pumps blood, brain divides into fire mid and hind
Lungs firm, gastrointestinal system identifiable
Outline week 5 of embryonic development
Rapid brain growth. Limb buds appear. Umbilical cord formed
Outline week 6 of embryonic development
Liver behind to function. Rudimentary kidneys and genitalia form. Cartilage forms skeleton
Outline week 7 of embryonic development
Eyelids, gall bladder, palate and tongue form. Diaphragm develops. Arms and legs begin movement.
Outline weeks 8 of embryonic development
Hands and feet are well formed, heart has 4 chambers and major blood circulation begins. 40-60 bpm. By the end of the 8th week all major body systems are formed and the embryo becomes a foetus
Outline weeks 8-12 of fetal development?
Head constitutes 1/2 of fetal length. Intestines established. Amniotic fluid swallowed and foetus uriniates
Outline weeks 13-16 of fetal development?
Doubles in length. Facial features migrate to correct position. Meconium develops in the gut
Outline weeks Weeks 17-24 of fetal development?
Fetal heart heard by stethoscope, movements felt. Vernix formed, hair on head
Outline weeks 25-28 of fetal development
Subcutaneous fat laid down, eyes open, lung physiology sufficient to allow gaseous exchange
Outline week 29 of fetal development
Foetus is fully formed and rig and can work to an extent. For the remainder of the pregnancy organ physiology matures
