Coitus, Fertilisation and Preimplantation

Question 1

Length of luteal phase is

Answer 1

fixed at approximately 14 days

Question 2

• Sperm are viable for

Answer 2

24 - 72 hours

Question 3

• Oocytes are viable for

Answer 3

12 - 24 hours

Question 4

So for successful fertilisation, coitus must occur

Answer 4

no more than 3 days before ovulation and no more than 1 day after ovulation

Question 5

Transport of Gametes - Ovum

Answer 5

• At ovulation - egg extruded onto surface of ovary
• Fimbriae sweep over ovary surface and pick up ovum
• Sticky cumulus cells cling to the ciliated surface of the fimbriae
• Movement of cilia and smooth muscle of fallopian tube propel ovum to uterus
• Ovum transported by ciliary current to ampulla of fallopian tube

Question 6

Transport of Conceptus to Uterus

Answer 6

• Ovum released from ovary and wafted by fimbriae into fallopian tube
• Fertilised in the ampulla of fallopian tube
• Conceptus “held” in fallopian tube as oestrogen maintains contraction of smooth muscle near where fallopian tube enters wall of uterus
• Conceptus undergoes mitotic cell divisions ie cleavage and morula is formed

Question 7

vaginal environment

Answer 7

– Vaginal environment is acidic (pH 5.7)
– Uterotubal junction anatomical barrier
– Phagocytosis by leukocytes in uterine lumen
– Length and energy requirements of trip

Question 8

sperm into the egg

Answer 8

• Passage into cervical mucus dependent on oestrogen-induced changes in mucus consistency
• Sperm can reach ampulla within 5-10 minutes after ejaculation
• Movement through uterus and fallopian tubes is via sperm’s own propulsions AND vaginal, cervical, uterine contractions and ciliary beat movement in fallopian tubes

Question 9

structure of the sperm

Answer 9

• Head
– Acrosome (contains enzymes to aid penetration of ovum)
– Nucleus (contains genetic material)
• Midpiece
– Contains mitochondria around filamentous core
– Provides energy for tail - ‘Engine room’ of sperm
• Tail
– Specialised flagellum that propels sperm forward

Question 10

Capacitation of Sperm

Answer 10

– Increase in Ca2+ permeability - rise in intracellular Ca2+
– Removal of membrane proteins (glycoprotein)
– Change in surface charge
– Depletion of cholesterol

Question 11

Capacitation (an irreversible process) occurs in female tract over 1-3 hours and results in

Answer 11

– Change from wave-like beats of sperm tail to whip-like action to propel sperm forward
– Sperm’s plasma membrane is altered so it is capable of fusing with surface membrane of egg (acrosome reaction)

Question 12

Transport of Conceptus to Uterus - hormones

Answer 12

• Plasma progesterone levels rise 3-4 days after fertilisation, smooth muscle relaxes, and conceptus passes into uterus
• Approximately 4-5 days after fertilisation, cavities develop between the cells (blastocyst)
• For approx 3 days, conceptus/blastocyst lies free in the uterine cavity supported by uterine secretions

Question 13

Blastocyst Avoids Maternal Rejection

Answer 13

• Releases immunosuppressive agents (e.g. interleukin 1α, 6 and 8)
• Also secretes human chorionic gonadotrophin (hCG)
– Closely related to LH, supports the steroid synthesis of the corpus luteum, and therefore prevents both menstruation and any further follicular development
– Maintains the corpus luteum
– Also promotes trophoblast growth and placental development

Question 14

corpus luteum

Answer 14

• In a non-fertile cycle, the corpus luteum will fail after 10 days and menstruation will occur
• hCG stimulates the Leydig cells of male fetuses to produce testosterone - important for development of the male duct system

Question 15

Implantation - Embryo hatching

Answer 15

• Day 6/7 after fertilization the zona pellucida degenerates and blastocyst is released (‘hatches’)
• Lytic factors from endometrial cavity involved
• Blastocyst probably produces factors too

Question 16

Implantation - Apposition

Answer 16

• Earliest contact between blastocyst wall, trophoectoderm and endometrial epithelium
• Occurs in crypt in endometrium

Question 17

Implantation - Adhesion

Answer 17

• Time limited window
• complex interactions between trophoblast and maternal epithelial tissue
• Trophoblast attaches to uterine epithelium via microvilli of the trophoblast
• Integrin receptor family thought to be involved

Question 18

Implantation - Invasion

Answer 18

• Syncytiotrophoblast cells ‘flow’ into the endometrium causing oedema, glycogen synthesis and increased vascularisation (decidualisation)
• Pregnant endometrium is now termed the decidua
• Nutrition still depends on uterine secretion and tissues
• Breakthrough bleeding may occur
• Growth in the embryonic disk is slow and it remains very small (0.1 - 0.2 mm

Question 19

Human Chorionic Gonadotropin (hCG)

Answer 19

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•
Human Chorionic Gonadotropin (hCG)
Maintains progesterone secretion from the corpus luteum until the placenta can synthesize its own progesterone
Syncytiotrophoblasts secrete hCG soon after implantation (peaks ~8-10 weeks of gestation) Measurable by day 7-8 post conception