Early Human Development

Question 1

Name the prenatal stages of human development and the time frame for each stage

Answer 1

• Zygote: 1-3 days
• Morula: 4-7 days
• Blastocyst: 8-14 days
• Embryo: 3-8 weeks
• Fetus: 9-40 weeks

Question 2

Name the postnatal stages of human development and the time frame for each stage

Answer 2

• Neonate: first month
• Infant: 2-15 months
• Child: 16 months - 12 yrs
• Adolescent: 12-20
• Adult: beyond 20
• Old age: after 40

Question 3

What day of the menstrual cycle does ovulation occur and what happens?

Answer 3

Day 14
Remains of follicle become CL which produces progesterone + oestrogen=
- endometrial glands secrete uterine milk (beginning of luteal phase)
- prepares endometrium for implantation

Question 4

How much sperm reaches the site of fertilisation?

Answer 4

• ejaculation: 200-600 million sperms deposited around external os
• moves 2-3mm/min (pH affects rate)
• reaches ampulla 5 mins after deposition
• only 200 reach site of fertilisation

Question 5

What is semen made of?

Answer 5

<10% sperm, rest is secretions from accessory sex glands = prostaglandins + fructose

Question 6

Evaluate male normal fertility

Answer 6

100 million/ml semen
20-50 million/ml = fertile
At least 40% should be motile after 2 hours, some after 24 hours

Question 7

How long after ovulation can an oocyte be fertilised?

Answer 7

24 hours

Question 8

How long does sperm stay viable within the female reproductive tract?

Answer 8

48 hours

Question 9

What is capacitation?

Answer 9

• functional change sperm undergo in female tract
• lasts 7 hours
• removal of protein from plasma membrane of sperm overlying acrosomal region
• only capacitated sperm can pass through corona cells + undergo acrosome reaction

Question 10

Where is the site of fertilisation?

Answer 10

Ampullary region of uterine tube

Question 11

What are the consequences of fertilisation?

Answer 11

1. Restoration of diploid number (2n)
2. Sex determination (X/Y)
3. Induction of cleavage = rapid division of cells to give embryo

Question 12

What is the acrosome reaction?

Answer 12

Sperm goes into coronal cells and acrosome head releases enzymes which digest through corona + wall of oocyte

Question 13

What is polyspermy?

Answer 13

Once sperm entered oocyte it prevents further sperm entry by causing cortical reaction which leads to formation of fertilisation membrane (= resistant to further sperm entry)

Question 14

What are the stages of cleavage?

Answer 14

2 blastomeres -> 4 blastomeres -> morula -> compacted morula

Question 15

What is cleavage?

Answer 15

Series of rapid mitotic divisions - characterised by lack of growth between divisions
(G1 + G2 switched off = constant division)

Question 16

How does the compacted morula convert to blastocyst?

Answer 16

By formation of inner cavity - distinguishes inner cell mass (ICM)

Question 17

How does the blastocyst attach to the uterine epithelium? And what happens next?

Answer 17

• Blastocyst hatches from surrounding Zone pellucida
• Trophoblast invade endometrial wall + drag inner cell mass in
• Due to invasion, spinal arteries of uterus erode + spill blood = establishes blood flow

Question 18

Describe what is seen at complete implantation

Answer 18

• primitive yolk sac = energy source
• fibrin clot - stops bleeding
• trophoblastic lacunae - gives placental interface
• space forms between epiblast + hypoblast = bilayer

Question 19

Where is the normal implantation site?

Answer 19

Posterior or anterior wall of body of uterus

Question 20

Name the abnormal implantation sites.

Answer 20

1. Mesentery of intestinal loop - rectouterine cavity (Pouch of Douglas)
2. Uterine tube (95% of ectopic pregnancies) - can be ampullary, tubal or interstitial region
3. Ovarian implantation
4. Implantation in internal os = placenta previa