Theriognology | Ovulation/ Luteogenesis and Luteolysis | Dr. Isam Bahnan Basheer Page | 1

Dr. Isam Bahnan Basheer, BVMS, MSc, PhD

Lecturer, Department of Surgery and Theriogenology

College of Veterinary Medicine, University of Mosul, Mosul, Iraq

https://orcid.org/0000-0002-6425-913X

https://www.researchgate.net/profile/isam_sharum

Theriogenology | Female Fertility and Diseases| 4th year 2019

Ovulation

Is the process by which the mature oocyte is released from the Graafian follicle. The

number of ovulated eggs varies between species. The exact timing of ovulation is difficult

to establish since the continuous observation of the ovaries would be required. Ovulation

occurs at the average value 8–12 hours after the end of estrus.

Types of Ovulation

1. Spontaneous ovulation

Is the ovulatory process in which the matured follicles secrete ovarian steroids to generate

an LH surge leading to ovulation (independent of copulation). Species in which the

females are spontaneous ovulators include rats, mice, sow, ewe, cow, mare, monkey, and

woman.

2. Induced ovulation

Is the process in which ovulation is induced by genital stimulation during coitus rather

than the increased pre-ovulatory LH surge. Ovulation stimulator might be seminal

plasma, sperms, physical mating even pheromones.

Species in which the females are spontaneous ovulators include cat, rabbit, ferret, and

she-camel.

Theriognology | Ovulation/ Luteogenesis and Luteolysis | Dr. Isam Bahnan Basheer Page | 2

Structure of the Graafian follicle

Follicle development occurs as a wave-like pattern, each wave is initiated with

follicle recruitment- selection- growth- dominance, and ovulation (or regression).

Usually, 2 to 4 follicular waves occur during the estrous cycle in cattle.

A figure demonstrating follicle development toward ovulation

Theriognology | Ovulation/ Luteogenesis and Luteolysis | Dr. Isam Bahnan Basheer Page | 3

The elevation in circulating concentrations of estradiol during the late follicular

phase leads to the preovulatory surge of LH.

Local regulation of ovulation involves the interaction of LH and intra-follicular

factors including steroids, prostaglandins, and peptides derived from endothelial

cells, leukocytes, fibroblasts, and steroidogenic cells.

The LH surge stimulates the process of ovulation by:

Activating an inflammatory reaction, which weakens and ruptures the follicle wall.

Initiates luteinization of the granulosa and theca interna cells of the follicle.

An hour before ovulation, the follicle forms an ‘apex’, the point where rupture

takes place at the most avascular point called Stigma. After ovulation, there is a

rapid collapse of the follicle wall.

Usually, ovulation occurs from 24–32 hours after the beginning of the surge (8-12

hours after end estrus).

At ovulation, the mature antral follicle ruptures, separating its content of the

follicular fluid in the abdominal cavity and releasing the unfertilized ovum, still

surrounded by cumulus cells.

The ovum is collected by the fimbria of the oviduct and transports down the

oviduct by a combination of cilia action and muscular contractions of the oviduct

wall.

At this stage, the ovulated follicle forms a temporary structure called corpus

hemorrhagicum, which is filled with blood that quickly clots.

The picture is showing the moment of ovulation

Theriognology | Ovulation/ Luteogenesis and Luteolysis | Dr. Isam Bahnan Basheer Page | 4

Mechanism of ovulation induced by LH surge

Notes:

Plasminogen is produced in the liver and it circulates freely. When a need for

plasmin occurs, plasminogen is activating by specific chemicals turning it into

plasmin.

Plasmin is a protease (enzyme) that activates collagenases and weakens the wall

of the Graafian follicle, leading to ovulation.

Theriognology | Ovulation/ Luteogenesis and Luteolysis | Dr. Isam Bahnan Basheer Page | 5

Luteogenesis and Luteolysis

The corpus luteum (CL) is a hard-yellow structure resulting from the ovulated follicle

as a temporary reproductive gland that produces and secretes progesterone responsible

for the maintenance of pregnancy.

LH is responsible for the transformation of granulosa and theca interna cells into lutein

cells (Luteinization) which secrete progesterone.

The CL is one of the most highly vascularized organs and receives the greatest rate of

blood flow per gram of tissue of any organ in the body. During the development of

CL, the blood flow surrounding an early corpus luteum gradually increases in parallel

with the increase in CL volume and plasma progesterone concentration.

The increased blood flow is closely associated with the potential to produce and release

of progesterone.

Luteolysis

Is the irreversible structural and functional degradation of the corpus luteum that

occurs at the end of the luteal phase leading to a considerable decrease in progesterone

production. The degraded corpus luteum forms a non-functional structure called

corpus albicans, that is filled with fibrous tissue.

In non-pregnant animals, regression of the CL by PGF2α is an important mechanism

to resume the estrus cycle, where luteolysis starts between days 16-17 post estrus.

Luteolysis in non-pregnant animals

Theriognology | Ovulation/ Luteogenesis and Luteolysis | Dr. Isam Bahnan Basheer Page | 6

In pregnant females, the presence of embryo blocks uterus to produce PGF2α to

maintain the CL and production of progesterone for pregnancy.

Failure of Luteolysis in pregnant animals

Mechanism of luteolysis by PGF2α

Luteolysis is tightly controlled through crosstalk between the corpus luteum and the

uterine endometrium.

The endometrium produces PGF2α, which infuses from the uterine vein into the

ovarian artery via a countercurrent exchange mechanism. Thus, PGF2α is delivered

directly to the ovary and causes luteolysis.

In the sow, ewe, cow, the ovarian artery is very tortuous and in close apposition to the

uterine vein, increasing the area of contact. The walls of blood vessels are thin where

they are in contact. Therefore, sufficient PGF2α can diffuse.

The ovarian artery of the mare is straight and caudal to the uterine vein. Thus, they are

in contact only in a limited area and PGF2α is instead transported to the ovary via the

systemic circulation.

In some species (cow and ewe), PGF2α synthesis in the uterine horn only influence the

life span of the CL in the ipsilateral (same side) ovary.

In other species (sow and mare), PGF2α synthesis from one horn is sufficient to cause

regression of CL in both ovaries.

Theriognology | Ovulation/ Luteogenesis and Luteolysis | Dr. Isam Bahnan Basheer Page | 7

Countercurrent exchange of PGF2α

The process of luteolysis by PGF2α include:

Reduction of the blood flow in the corpus luteum.

The severe increase in luteal blood flow (day17 after estrus) is a key phenomenon of the

onset of luteal regression. However, PGF2α causes a rapid reduction in luteal blood

supply (day19 after estrus) which is one of the main luteolytic actions.

Blood flow surrounding the corpus luteum in non-pregnant cow

Theriognology | Ovulation/ Luteogenesis and Luteolysis | Dr. Isam Bahnan Basheer Page | 8

The direct action of PGF2α on luteal cells.

1. The continuous exposure of the uterus to progesterone during the diestrus period

causes downregulation of the progesterone receptors in the endometrium. This

allows estrogen (secreted from waves of follicle growth during the luteal phase)

to bind to its receptor leading to stimulation of the oxytocin receptor in the

endometrium.

2. At the end of the estrous cycle, the CL produces oxytocin, which stimulates the

uterus to produce uterine PGF2α, which in turn, stimulates the CL to produce more

oxytocin.

3. Oxytocin binds to its endometrial receptor, activating the synthesis and release of

PGF2α into the utero-ovarian vein.

4. PGF2α reaches the ovary via the countercurrent exchange mechanism. Thus,

PGF2α is directly delivered to the ovary and causes luteolysis.

Note:

In pregnant females, progesterone inhibits both the estrogen and oxytocin receptors

in the endometrium; where both have the potential to stimulate the release of PGF2α.

PR (Progesterone receptor); ES (Estrogen receptor); OR (Oxytocin receptor)

Mechanism of luteolysis drive-by PGF2α