TRANSPORTATION OF GAMETES

Oocyte Transport The secondary oocyte is expelled at ovulation from

the ovarian follicle with the escaping follicular fluid

During ovulation, the fimbriated end of the uterine tube becomes closely applied to the ovary.

The fingerlike processes of the tube, fimbriae, move back and forth over the ovary. The sweeping action of the fimbriae and fluid currents produced by the cilia of the mucosal cells of the fimbriae "sweep" the secondary oocyte into the funnel-shaped infundibulum of the uterine tube.

The oocyte passes into the ampulla of the tube, mainly as the result of peristalsis-movements of the wall of the tube characterized by alternate contraction and relaxation-that pass toward the uterus.

Sperm Transport From their storage site in the epididymis,

the sperms are rapidly transported to the urethra by peristaltic contractions of the thick muscular coat of the ductus deferens.

The accessory sex glands-seminal glands (vesicles), prostate, and bulbourethral glands-produce secretions that are added to the sperm-containing fluid in the ductus deferens and urethra

From 200 to 600 million sperms are deposited around the external os of the uterus and in the fornix of the vagina during sexual intercourse.

The sperms pass slowly through the cervical canal by movements of their tails. The enzyme vesiculase, produced by the seminal glands, coagulates some of the semen and forms a vaginal plug that may prevent the backflow of semen into the vagina.

When ovulation occurs, the cervical mucus increases in amount and becomes less viscid, making it more favorable for sperm transport.

The volume of sperm or ejaculate averages 2 to 6 mL.

The sperms move 2 to 3 mm/minute, but the speed varies with the pH of the environment. They are nonmotile during storage in the epididymis, but become motile in the ejaculate. They move slowly in the acid environment of the vagina, but move more rapidly in the alkaline environment of the uterus.

It is not known how long it takes sperms to reach the fertilization site

Only approximately 200 sperms reach the fertilization site. Most sperms degenerate and are resorbed by the female genital tract.

MATURATION OF SPERMS

to fertilize an oocytes, sperms must undergo a period of conditioning-capacitation-lasting approximately 7 hours.

During this period, a glycoprotein coat and seminal proteins are removed from the surface of the sperm's acrosome.

The membrane components of the sperms are extensively altered. Capacitated sperms show no morphologic changes, but they are more active.

Sperms are usually capacitated in the uterus or uterine tubes by substances secreted by these parts of the female genital tract.

THE ACROSOME REACTION:

The intact acrosome of the sperm binds to a glycoprotein (ZP3) on the zona pellucida.

Capacitated sperms come in contact with the corona radiata surrounding a secondary oocyte

Undergo complex molecular changes that result in the development of perforations in the acrosome.

Multiple point fusions of the plasma membrane of the sperm and the external acrosomal membrane occur. Breakdown of the membranes at these sites produces apertures.

The changes induced by the acrosome reaction are associated with the release of enzymes, including hyaluronidase and acrosin, from the acrosome that facilitate fertilization

VIABILITY OF GAMETES

human oocytes are usually fertilized within 12 hours after ovulation.

In vitro observations have shown that the oocyte cannot be fertilized after 24 hours and that it degenerates shortly thereafter.

Most human sperms probably do not survive for more than 48 hours in the female genital tract.

Sperms and oocytes can be stored frozen for many years to be used in assisted reproduction.

FERTILIZATION

Site of fertilization: the ampulla of the uterine tube

If the oocyte is not fertilized here, it slowly passes along the tube to the uterus, where it degenerates and is resorbed. Although fertilization may occur in other parts of the tube, it does not occur in the uterus.

Chemical signals (attractants), secreted by the oocyte and surrounding follicular cells, guide the capacitated sperms (sperm chemotaxis) to the oocyte.

DEFINATION: Fertilization is a complex sequence of coordinated molecular events that begins with contact between a sperm and an oocyte and ends with the intermingling of maternal and paternal chromosomes at metaphase of the first mitotic division of the zygote, a unicellular embryo.

The fertilization process takes approximately 24 hours.

PHASES OF FERTILIZATION

1ST PHASE:

Passage of a sperm through the corona radiata.

Dispersal of the follicular cells of the corona radiata surrounding the oocyte and zona pellucida

enzyme hyaluronidase released from the acrosome of the sperm, tubal mucosal enzymes and movements of the tail of the sperm are important in its penetration of the corona radiata.

2ND PHASE:

Penetration of the zona pellucida.

Passage of a sperm through the zona pellucida.

The enzymes esterases, acrosin, and neuraminidase appear to cause lysis of the zona pellucida, thereby forming a path for the sperm to follow to the oocyte.

Once the sperm penetrates the zona pellucida, a zona reaction-a change in the properties of the zona pellucida-occurs making it impermeable to other sperms. The zona reaction is believed to result from the action of lysosomal enzymes released by cortical granules near the plasma membrane of the oocyte. They cause changes in the plasma membrane that make it impermeable to other sperms.

3RD PHASE:

Fusion of plasma membranes of the oocyte and sperm.

The plasma or cell membranes of the oocyte and sperm fuse and break down at the area of fusion.

The head and tail of the sperm enter the cytoplasm of the oocyte, but the sperm's plasma membrane remains behind

4th PHASE:

Completion of the second meiotic division of oocyte and formation of female pronucleus. Penetration of the oocyte by a sperm activates the oocyte into completing the second meiotic division and forming a mature oocyte and a second polar body

Following decondensation of the maternal chromosomes, the nucleus of the mature oocyte becomes the female pronucleus.

5TH PHASE:

Formation of the male pronucleus. Within the cytoplasm of the oocyte, the nucleus of the sperm enlarges to form the male pronucleus and the tail of the sperm degenerates

Morphologically, the male and female pronuclei are indistinguishable. During growth of the pronuclei, they replicate their DNA-1 n (haploid), 2 c (two chromatids). The oocyte containing two haploid pronuclei is called an ootid.

6TH PHASE

As the pronuclei fuse into a single diploid aggregation of chromosomes, the ootid becomes a zygote. The chromosomes in the zygote become arranged on a cleavage spindle in preparation for cleavage of the zygote

7TH PHASE: As the pronuclei fuse into a

single diploid aggregation of chromosomes, the ootid becomes a zygote. The chromosomes in the zygote become arranged on a cleavage spindle in preparation for cleavage of the zygote

CLEAVAGE OF THE ZYGOTE

Cleavage consists of repeated mitotic divisions of the zygote, resulting in a rapid increase in the number of cells. These embryonic cells-blastomeres-become smaller with each successive cleavage division.

Division of the zygote into blastomeres begins approximately 30 hours after fertilization. After the nine-cell stage, the blastomeres change their shape and tightly align themselves against each other to form a compact ball of cells. This phenomenon, COMPACTION, is probably mediated by cell surface adhesion glycoproteins.

When there are 12 to 32 blastomeres, the developing human is called a MORULA. Internal cells of the morula are surrounded by a layer of cells that form the outer cell layer. The spherical morula forms approximately 3 days after fertilization and enters the uterus.

FORMATION OF THE

BLASTOCYST

Shortly after the morula enters the uterus (approximately 4 days after fertilization), a fluid-filled space called the blastocystic cavity appears inside the morula. The fluid passes from the uterine cavity through the zona pellucida to form this space. As fluid increases in the blastocystic cavity, it separates the blastomeres into two parts:

A thin, outer cell layer, the trophoblast, which gives rise to the embryonic part of the placenta

A group of centrally located blastomeres, the inner cell mass, which gives rise to the embryo; because it is the primordium of the embryo, the inner cell mass is called the embryoblast

TOPICS FOR 1ST CLASS TEST ON 7TH NOVEMBER 2012:

DEFINATIONS IN GENERAL EMBRYOLOGY

MALE AND FEMALE REPRODUCTION

GAMETOGENESIS

MATURATION AND TRANSPORT OF GAMETES

FERTILIZATION, CLEAVAGE, BLASTOCYST FORMATION