Fertilization

Question 1

What is fertilization?

Answer 1

The process of union of two gametes, ovum and sperm, whereby the somatic chromosome number is restored, and the development of a new individual exhibiting characteristics of the species is initiated.W

Question 2

What events occur before, during, and after fertilization?

Answer 2

Gametogenesis, maturation of gametes, gamete transport to the site of fertilization, interactions of gametes and their fusion, early embryogenesis

Question 3

What are the main results of fertilization?

Answer 3

Restoration of the diploid number of chromosomes, determination of the sex of the new individual, and initiation of cleavage.

Question 4

Difference between spermatogenesis and oogenesis in terms of location of process?

Answer 4

Spermatogenesis occurs entirely in the testes, oogenesis occurs mostly in ovaries

Question 5

Difference between spermatogenesis and oogenesis in terms of meiotic divisions?

Answer 5

In spermatogenesis, there is an equal division of cells. In oogenesis, there’s an unequal division of cytoplasm

Question 6

Difference between spermatogenesis and oogenesis in terms of germ line epithelium?

Answer 6

Germ line epithelium is involved in gamete production in spermatogenesis, but not in oogenesis

Question 7

Difference between spermatogenesis and oogenesis in terms of number of gametes produced?

Answer 7

Four sperm produced in spermatogenesis, one ovum produced in oogenesis (plus 2-3 polar bodies)

Question 8

Difference between spermatogenesis and oogenesis in terms of size of gametes?

Answer 8

In spermatogenesis, sperm are smaller than spermatocytes. In oogenesis, ova are larger than oocytes

Question 9

Difference between spermatogenesis and oogenesis in terms of duration?

Answer 9

Spermatogenesis is an uninterrupted process; oogenesis occurs in arrested stages

Question 10

Difference between spermatogenesis and oogenesis in terms of onset?

Answer 10

Spermatogenesis begins at puberty; oogenesis begins in foetus (pre-natal)

Question 11

Difference between spermatogenesis and oogenesis in terms of release

Answer 11

Spermatogenesis causes continuous release; oogenesis occurs monthly from puberty (menstrual cycle)

Question 12

Difference between spermatogenesis and oogenesis in terms of end?

Answer 12

Spermatogenesis is lifelong but reduces with age; oogenesis terminates with menopause.

Question 13

What is oocyte maturation?

Answer 13

A complex process involving multiple steps resulting in the production of fertilizable ovum; regulated by many molecules and signaling pathways; prerequisite to fertilization and normal development

Question 14

What are the two stages of oocyte maturation?

Answer 14

1. Period of growth: oocyte and its follicle begin their growth immediately after primordial follicle activation during folliculogenesis
2. Period of nuclear and cytoplasmic maturation: meiosis starts in the fetal ovary but is arrested before birth at prophase I (dictyate arrest); LH surge causes resumption of meiosis (meiotic progression until metaphase II and then arrested again) and ovulation; sperm penetration triggers progression and completion of meiosis II.

Question 15

What of nuclear maturation happens in the ovaries of the female fetus?

Answer 15

Oocytes are formed and enter the first meiotic phase ( prophase I). Homologous chromosomes pair and exchange DNA through meiotic recombination, generating bivalent chromosomes containing crossovers.

Question 16

What of nuclear maturation happens at birth?

Answer 16

Oocytes are arrested at the end of prophase (dictyate arrest, Germinal Vesicle stage) and associate with somatic cells, generating primordial follicles.

Question 17

What of nuclear maturation happens from puberty onwards?

Answer 17

Primordial follicles grow over several months into larger follicles. Once every menstrual cycle, one follicle matures into a Graafian follicle. The pituitary gland stimulates follicle growth by releasing follicle-stimulating hormone (FSH).

Question 18

What of nuclear maturation happens in the middle of the menstrual cycle?

Answer 18

The pituitary gland releases LH, which induces the oocyte to resume the first meiotic division. Following nuclear envelope (germinal vesicle) breakdown, the spindle assembles, relocates to the oocyte cortex, and segregates homologous chromosomes between the oocyte and the first polar body. The remaining homologous chromosomes align on a second metaphase spindle without an intervening prophase stage. The mature oocyte, now referred to as an egg, arrests in metaphase II and is ovulated into the fallopian tube.

Question 19

What of nuclear maturation happens upon fertilization?

Answer 19

Anaphase II ensues, whereby the sister chromatids are segregated between the fertilized egg and the second polar body. Following completion of meiosis, the fertilized egg, now referred to as the zygote, comprises two haploid (maternal and paternal) pronuclei.

Question 20

What does oocyte cytoplasmic maturation do?

Answer 20

Prepares the oocyte to meet the metabolic demands of fertilization and embryo growth through changes in organelles (mitochondria become associated with the smooth ER, the Golgi body is fragmented, and the polar body is extruded)

Question 21

What are the granulosa cells directly surrounding the oocyte called?

Answer 21

Cumulus cells

Question 22

What is the COC?

Answer 22

The Cumulus Oocyte Complex is the oocyte with its surrounding cumulus cells

Question 23

What does the preovulatory LH surge cause?

Answer 23

1. Cumulus expansion: formation of a unique matrix in the complex of cumulus cells surrounding the oocyte.
2. Oocyte maturation (meiotic resumption): meiotic arrest depends on a high level of cAMP as well as an inactive state of maturation promoting factor (MPF) within the oocyte. Meiosis resumes due to the degradation of cAMP and concomitant activation of MPF in response to a preovulatory surge of LH
3. Ovulation: follicular rupture and release of COCs (in most species COCs are a secondary oocyte, or an ovum in the meiosis II stage, but in dogs it’s a primary oocyte, or an ovum in meiosis I stage)

Question 24

Where are COCs moved after they leave the ovary?

Answer 24

Into the ostium of the oviduct by the action of the kinocilia of fimbriae

Question 25

What are the functions of the oviduct?

Answer 25

Ovum pickup, transport of ovum and sperm in opposite directions, functional reservoir for sperm, supports sperm capacitation, nourishes and protects gametes, site of fertilization, supports early embryogenesis, nourishes and transports the embryo into the uterus

Question 26

What is "locking" and "unlocking" of the ova in the oviduct regulated by?

Answer 26

Contractility of oviductal musculature, direction and rate of fluid flow (towards ovary), beat of kinocilia (towards uterus), secretory activity of oviductal epithelium, rheological properties of oviductal fluid.

Question 27

Describe the timing of the transport of the ova in the oviduct.

Answer 27

Ova are transported to the uterus irrespective of fertilization, but in the mare, the unfertilized ova are retained in the oviduct for several months. The rate of transport is fast from the infundibulum to the ampulla, but slow through the isthmus. Retention time in the oviduct is critical for early embryogenesis (fertilized eggs should reach the uterus at an appropriate time for implantation), so it's precisely controlled.

Question 28

What sperm-related events precede fertilization?

Answer 28

Sperm release from the oviductal epithelial cells, sperm capacitation, sperm hyperactivation, and sperm acrosome reaction

Question 29

What is the purpose of sperm capacitation?

Answer 29

Must occur in the female reproductive tract (completed in the isthmic region of the oviduct); renders sperm hyperactive and able to fertilize; enables sperm to bind to eggs and undergo acrosome reaction; prevents premature acrosome reaction until the sperm reach the site of fertilization.

Question 30

What is the difference in structure of surface molecules on epididymal sperm, ejaculated sperm, and capacitated sperm?

Answer 30

Epididymal sperm have a complement of surface molecules (proteins, carbs, and yellow T's). Ejaculated sperm have their surface molecules coated with seminal plasma proteins (decapacitating factors) that mask portions of the membrane molecules. Capacitated sperm have had their surface molecules removed in the reproductive tract.

Question 31

What is sperm hyperactivation?

Answer 31

Switching from a nearly symmetrical low-amplitude flagellar beating to an asymmetrical high-amplitude beating pattern that is required for fertilization in vivo.

Question 32

What is the role of Ca2+ signaling in sperm hyperactivation?

Answer 32

Progesterone-induced activation of the CatSper (CATion channel of SPERm) channel in an alkaline environment allows rapid Ca2+ influx which, along with the mobilization of Ca2+ from the storage organelles, elevates intracellular Ca2+, thus generating the hyperactivated sperm motility.

Question 33

What are the three sperm guidance mechanisms in the oviduct?

Answer 33

Rheotaxis (fluid flow, long-range), thermotaxis (temperature gradient, long-range), and chemotaxis (chemical concentration gradient, close-range)

Question 34

Describe the role of oviduct movement in sperm transport and retention at the fertilization site.

Answer 34

Oviduct movement mechanically propels oviductal fluid droplets containing free-swimming capacitated spermatozoa toward the ovary.

Question 35

Describe the role of chemical guidance in sperm transport and retention at the fertilization site.

Answer 35

The egg complex continuously secretes an attractant, forming a gradient in the cumulus surroundings, which guides capacitated spermatozoa toward the egg.

Question 36

What is the post-capacitation sequence of events leading to fertilization?

Answer 36

Hyperactive motility, binding to zona pellucida, acrosomal reaction, penetration of zona pellucida, sperm-oocyte membrane fusion, sperm engulfed, decondensation of sperm nucleus, formation of male pronucleus

Question 37

What is the zona pellucida?

Answer 37

An extracellular matrix that is synthesized during follicular development and finally surrounds the plasma membrane of the oocyte and preimplantation embryo of mammals. Is involved in folliculogenesis, granulosa cell organization and differentiation, recognization of and binding to the spermatozoa, induction of the acrosome reaction, block to polyspermy, and protection of the preimplantation embryo.

Question 38

What are the two zona binding sites in the sperm plasma membrane?

Answer 38

1. Primary zona binding region (ZBR): for adherence of sperm to the zona pellucida. 2. acrosome reaction promoting region (ARPR)

Question 39

What is the acrosomal reaction?

Answer 39

An orderly fusion of the spermatozoal plasma membrane and the outer acrosomal membrane. Enables sperm to penetrate the zona pellucida and modifies the equatorial segment so that it later can fuse with the plasma membrane of the oocyte.

Question 40

What happens after zona pellucida penetration and before sperm-oocyte fusion?

Answer 40

The sperm reaches the perivitelline space and settles into a bed of microvilli formed from the oocyte plasma membrane. There is a migration of cortical granules to the periphery of the oocyte.

Question 41

What is the cortical reaction and what does it do?

Answer 41

It's the fusion of the cortical granule membrane with the oocyte plasma membrane and release of cortical granule contents into the perivitelline space by exocytosis. It causes the zona block and the vitelline block

Question 42

What is the zona block?

Answer 42

A process whereby the zona pellucida undergoes biochemical changes so that further sperm cannot penetrate it (prevents polyspermy)

Question 43

What is the vitelline block?

Answer 43

A reduction in the ability of the oocyte plasma membrane to fuse with additional sperm (prevents polyspermy)

Question 44

What happens after sperm-oocyte membrane fusion?

Answer 44

Disappearance of the sperm nuclear membrane; decondensation of the sperm nucleus through reduction of disulfide cross-links; spermatozoon nucleus enters the oocyte cytoplasm

Question 45

What is syngamy?

Answer 45

fusion of male and female pronuclei