Ovulation, ovarian cycle-IV Lecture – Anatomy Lectures – MBBS Lectures

Specific learning objectives:

01-2020 By the end of this session, you should be able to:
Describe the maturation of oocytes at puberty
Describe the stages of follicular maturation

1. Primary
2. Preantral
3. Secondary/Antral
4. Preovulatory.
   Define ovarian cycle and enlist its components
   Describe the hormonal regulation of ovarian cycle
   Describe the relation of ovarian cycle with maturation of follicles.
   Correlate the hormones of ovaries with changes in endometrium of uterus
   Describe the mechanism of ovulation
   Describe the middle pain (mittelshimerz) associated with ovulation
   Describe the fate of secondary oocyte
   Explain the causes, diagnostic procedures & treatment of anovulatory cycles
   Describe the structure and formation of corpus luteum and corpus albicans
   Describe the characteristics of Zona Pellucida
   Explain the abnormalities of gametes

OOGENESIS Postnatal Maturation Of Oocyte (continues)

36-48 hrs. before ovulation (12th/ 13th D of Menstrual cycle) 1st
meiotic division is completed & secondary oocyte is formed
• 3 hours before ovulation, nucleus of secondary oocyte begins
the second meiotic division but arrested at metaphase
• This division is completed only if a sperm penetrates the
secondary oocyte; otherwise cell degenerates approx. 24 hrs
after ovulation

OOGENESIS

Initially the plasmalemma of the oocyte (called oolemma) is smooth
and in close apposition with the surrounding follicular cells.
• However, as the oocyte enlarges and the zona pellucida forms
around it, a uniform cover of microvilli develops over the oolemma
which interdigitate (gap junctions) with microvilli of follicular cells thus
helping in transport of material from follicular cells to oocyte.
• As the oocyte growth advances, its cytoplasm also shows many
special features.
• The organelles increase in number and size and become dispersed
evenly in the cytoplasm.
• Lipid granules appear in the cytoplasm. This stored nutritive material
represents the yolk of the future ovum.
• The Golgi apparatus produces a special type of granules called cortical
granules. These granules measure about 500 nm in diameter, are
bounded by a membrane and are located in the peripheral part of the
oocyte, just under the oolemma. They contain an electron-dense material
rich in enzymes.
• The cortical granules play a very important role in blocking polyspermy,
i.e., fertilization of the ovum by more than one sperm.

FOLLICLES MATURATION

Maturing follicles pass through four stages:

1. Primary

2. Preantral

3. Secondary or antral (longest)

4. Preovulatory or Graafian follicle (37 hrs before ovulation)

1.Primary follicle is primary oocyte surrounded by first single layer of

simple cuboidal follicular cells & then stratified cuboidal follicular cells

(granulosa cells)

• Granulosa cells rest on a basement membrane which separate them

from surrounding stromal cells called theca folliculi

• Granulosa cells & the oocyte secrete a layer of glycoproteins on the

surface of oocyte, forming the zona pallucida

2. Pre antral follicle: Before the next phase, surrounding Stroma of

ovary condensces to form, Theca folliculi which differentiate into:

THECA INTERNA: cellular and more vascular

THECA EXTERNA: Less vascular & less cellular

3. Small fluid-filled spaces appear between the
   follicular cells which coalesce to form the single big
   cavity (follicular antrum). The follicle is now called
   Antral/ Maturing/ Secondary/ Vesicular follicle
   • Follicular antrum enlarges and becomes crescentshaped
   to encircle the primary oocyte immediately
   surrounded by few layers of cuboidal follicular cells
   (cumulus oophorus).
4. Follicular antrum enlarges sufficiently to surround
   the oocyte (Mature/Graafian/ Preovulatory
   follicle, 25 mm or more in diameter).
   • A surge in luteinizing hormone induces
   preovulatory growth phase & meiosis-I is
   completed; an unequal cytokinesis of meiosis-I
   gives rise to SECONDARY OOCYTE & 1st.Polar
   Body which lies between zona pallucida & cell
   membrane of secondary oocyte in the perivitelline
   space.

Monthly cycles & Hormones

OVULATION

OVULATED MASS = Secondary oocyte + Zona pellucida +Cumulus
oophorous (2-3 layers of follicular cells)
Fimbriae of fallopian tube sweep over the surface of ovary, & the tube itself
begins to contract rhythmically
PASSAGE: Peritoneal cavity, Sweeping action of fimbriae of uterine tube,
Fimbriated part, Infundibulum, Ampulla (within 25min of
ovulation)
Once in the tube, cumulus cell withdraw their cytoplasmic processes from
zona pellucida and lose contact with the oocyte
Ovarian cycle normally persists throughout the
reproductive life & terminates at menopause (48-
55 yrs)
The endocrine, somatic, & psychological changes
occurring at menopause are called the
climacteric.

Anovulatory menstrual cycles

Causes, signs & symptoms, diagnosis & treatment
Causes:
• Hormonal imbalance—Low concentration of gonadotropins
(hypothalamus, anterior pituitary, thyroid)
• Functional problem
• Ovaries can stop working; not producing mature oocytes
• The ovum may have matured properly, but the follicle may have failed
to burst or burst without releasing the egg. This is called luteinised
unruptured follicle syndrome (LUFS).
• Physical damage to the ovaries,
• Birth control pills
• Polycystic ovarian syndrome or Stein-Leventhal syndrome (90%)
• Weight loss
• Obesity

Sign & symptoms(Sign—–objective, Symptom—–subjective)
• Amenorrhea
• Oligomenorrhea
• Irregular menstrual cycles
• Obesity
• Facial hairs
Diagnosis
• Temperature charting
• LH levels
• Ultrasound
• Gonadotropin levels during luteal phase
Treatment
• Clomifene citrate
• gonadotropins

OVULATION (OVARIAN CYCLE)

FATE OF SECONDARY OOCYTE
Fertilizing ability of 2ndary oocyte is for 24hrs

1. Fertilization fails: 2ndary oocyte degenerates,
   resorbed and passed out in genital fluids.
2. Fertilization occurs:
   FERTILIZATION (fusion of two gametes) within 12-24hrs
   of ovulation
   • RESUMPTION of Meiosis II (metaphase) unequal
   cytokinesis
   2nd POLAR BODY
   FERTILIZED OVUM/ DEFINITIVE OOCYTE/
   OOTID (Two pronuclei)

C O R P U S L U T E U M (Formation & Fate

CELLULAR AND ENDOCRINE ORGAN
STRUCTURE:
After ovulation, walls of ovarian follicle & theca interna are thrown into
folds and vascularized by surrounding vessels
Under the influence of LH, these cells develop a yellowish pigment
and change into lutein cells which secrete estrogen & progesterone
which causes the uterine mucosa to enter the secretory phase in
preparation for implantation
If fertilization fails to occur then corpus luteum reaches full maturity
approx. 9 days after ovulation, then regresses & shrinks due to
degeneration of lutein cells(luteolysis) and known as corpus luteum
of menstruation. Estrogens & Progesterone levels in the blood drop
to regress the endometrium and menstruation starts. The corpus
luteum is transformed into a mass of fibrous scar tissue, known as
corpus albicans

Corpus luteum (continues)

If fertilization occurs, degeneration is prevented by hCG (by syncytiotrophoblast),
then corpus luteum enlarges (up to 5 cm) and is known as corpus luteum of
pregnancy (by end of 3rd month, it may be 1/3 to ½ of total size of ovary) and
secretes increased amount of progesterone & some estrogen, which:
Cause the endometrial glands to secrete
Enhances decidual changes in the endometrium which proliferates and thickens
(maternal part of placenta) thus prepare the endometrium for implantation & for
placenta formation
Prevents further ovulation
Secretes relaxin (hormone) which softens the connective tissue of symphysis pubis
for easy parturition
After 4th month, it regresses slowly as secretion of progesterone by trophoblast of
placenta becomes adequate for maintenance of pregnancy
Removal of corpus luteum before 4th month——abortion

Corpus luteum & Corpus albicans

Z O N A P E L L U C I D A

Amorphous, Acellular, PAS+ (Carbohydrates + Glycoproteins)

composed of three glycoproteins (ZPA, ZPB, ZPC) forming a network

Fenestrated (Swiss-cheese)

Semipermeable (absorption of female genital secretions) but stiff

Produced by: Primary oocyte + Follicular cells

FUNCTIONS :

 Species (Human) specific recognition of spermatozoa Glycoprotein

acts as a sperm-receptor molecule

 Filters the entry of spermatozoa with genetic aberrations

 Filters the entry of weak (abnormal) spermatozoa

 Selects the sex bearing spermatozoa X or Y (some authorities

believe)

 Prevents polyspermia (Blocking reaction) indirectly

 Prevents implantation of conceptus (prevents ectopic pregnancy)

 Prevents Growth of Conceptus till D6 of development

ABNORMALITIES OF GAMETES

Ideal maternal reproductive age is 18-35years.
Likelihood of chromosomal abnormalities & gene mutation in
the embryo increases after the mother is 35.
Abnormalities of gametes can be classified into two main
groups:

1. Chromosomal abnormalities.
2. Morphological abnormalities.

• One ovarian follicle occasionally contains two or three primary oocytes
which may give rise to twins or triplets, but they usually degenerate before
reaching maturity.
• In rare cases, one primary oocyte contains two or even three nuclei; these
usually die before reaching maturity.