Conception and nidation Flashcards
Conception and nidation
Main things to talk about
Oogenesis
Follicular development in the ovary
Hormonal events and ovulation
The endometrial cycle
Spermatogenesis
Fertilization (AKA conception)
Oogenesis
Primordial germ cells appear in the yolk sac. By 20 weeks, there are 7 million oogonia.
§ Number of oocytes falls to 1 million by birth and falls about 0.4 million by puberty.
§ Chromosome no. in gametes is half that of normal cells as oogonia undergoes meiosis to form 1ry oocyte.
§ Primary oocyte later remains in suspended prophase for 10–50 years.
§ During ovarian cycle, one oocyte is selected to complete meiosis 1 to form a secondary oocyte.
§ After ovulation, the oocyte is arrested in metaphase of meiosis 2 until fertilization.
Follicular development in the ovary
§ Most ova never reach advanced maturity, and about 800 are lost each month.
§ Aggregation of stromal cells around follicles become thecal cells and innermost form the corona radiata.
§ A fluid-filled space develops in granulosa cells and zona pellucida collects around ovum.
§ As Graafian follicle matures, theca interna and externa forms.
§ After ovulation, the corpus luteum is formed, if no implantation occurs then corpus albicans is formed.
Hormonal events and ovulation
FSH stimulates follicular growth and LH receptor development & declines by follicular estrogen production.
§ LH stimulates ovulation and stimulates and sustains development of the corpus luteum. LH surges 35-42
hours before ovulation due to positive estrogen feedback and a smaller coincidental FSH peak. Both FSH
and LH lower in second half of cycle.
ne levels are low prior ovulation then elevates throughout most of luteal phase (corpus luteum).
§ Corpus luteum produces oestrogen and progesterone, declines both FSH and LH.
§ There are feedback mechanisms that regulate the release of FSH and LH by the pituitary.
This is principally
achieved by the oestrogens and progesterone produced by the ovaries. In the presence of ovarian failure,
as seen in the menopause, the gonadotrophin levels become markedly elevated because of the lack of
ovarian oestrogen and progesterone production.
The endometrial cycle
Menstrual phase – shedding of functional layer of
endometrium (stratum compactum and spongiosum)
due to vasoconstriction of arterioles leading to the wall
necrosis and thus shedding.
§ Phase of repair – day 4–7 of cycle; regeneration of
epithelial surface from stratum basalis.
§ Follicular phase – maximum period of growth of
endometrial glands, due to oestrogen.
§ Luteal phase –14 days; ‘saw-toothed’ glands,
pseudodecidual reaction in stroma, and leukocytes
infiltration.
Spermatogenesis
§ Full maturation takes 64–70 days.
§ Mature sperm arise from haploid spermatids.
§ Structure of spermatozoon includes head (covered by
acrosomal cap), body (helix of mitochondria) and tail (two
longitudinal fibres and nine pairs of fibres).
§ Seminal plasma originates from the prostate, seminal
vesicles and bulbourethral glands.
§ High concentration of fructose provides energy for
sperm motility, high concentration of prostaglandins.
§ Sperm transport; rapid migration into receptive cervical
mucus, sperm migrate at 6 mm/min.
§ Only motile sperm reach the fimbriated ends of the tubes.
§ Capacitation; final sperm maturation occurs during
passage through the oviduct which enables penetration through zona pellucida.
Fertilization (AKA conception)
§ Small number of sperm reach oocyte and adherence of sperm initiates the acrosome reaction.
§ Sperm head fuses with oocyte plasma membrane, hhead and midpiece engulfed into oocyte
§ Fusion of male and female chromosomes is known as syngamy.
§ 36 hours after fertilization, the morula (16-cell stage) is formed. 6 days after fertilization, implantation (AKA
nidation) of blastocyst (fluid-filled cavity within morula) occurs, and decidual reactions take place- topic 5.
