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Gametogenesis and Fertilization

Dr. Lubna Nazli

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Objectives

• What is embryology?• Overview of reproductive organs.• Spermatogenesis & spermiogenesis• Oogenesis• What is fertilization, its events• Cleavage• Zona pellucida, its function• Formation of germ layers• First week changes• Clinical anatomy

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Overview of Reproductive System

• Primary sex organs (gonads)– Produce gametes (testes or ovaries) - – Gametogenesis - spermatogenesis or oogenesis

• Secondary sex organs– Male - ducts, glands, penis deliver sperm cells– Female - uterine tubes, uterus and vagina receive sperm

and nourish developing fetus• Secondary sex characteristics

– Develop at puberty• pubic, axillary and facial hair, scent glands, body morphology and

low-pitched voice in males

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Male rep system

Testes Genital ducts: Epididymis, Ductus deferens, Ejaculatory ducts Urethra Accessory glands: Seminal vesicles, Prostate, Bulbourethral gland Penis

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Testicular Capsule

Seminiferous tubule

Straight tubule

Interstitial tissue

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Limiting m.

Sertoli Cell

Spermatogenic epithelium

Interstitial tissue & Leydig cell

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Spermatogenesis

• In males, gametogenesis begins at puberty and continues into advanced age.

• Primordial germ cells (46,2N) migrate into the testes at week 4 of development and remain dormant.

• At puberty, primordial germ cells differentiate into type A spermatogonia (46,2N).

• Type A spermatogonia divide by mitosis to form either more type A spermatogonia (to maintain the supply) or type B spermatogonia.

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Spermatogenesis

The primitive sex cells appear earliest in 4th week of intra uterine life in the wall of yolk sac as primordial germ cells

They migrate to the developing testes and lie dormant among the cells lining the seminiferous tubules

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Spermatogenesis

At puberty the germ cells awaken and start the actual process of spermato genesis

These cells increase in number by simple mitosis to form cells known as spermatogonia ; type- A and type- B.

Type-B spermatogonia, replicate DNA to have 46 double structured chromosomes to begin meiosis-1 and are called primary spermatocytes.

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Spermatogenesis

Meiosis 2 follows immediately without DNA replication. Only 23 double structured chromosomes are involved

2 secondary spermatocytes quickly undergo meiosis-2 (Centromeres split in metaphase 2) and end with the formation of 4 spermatids each with 23 single structured chromosomes and1N DNA

Two spermatids bear X chromosome complement and other two bear Y chromosome complement

• As steps of spermatogenesis continue the spermatocytes progressively move from basement membrane to the lumen of seminiferous tubule

• The cells of Sertoli provide nutrition and pockets of support to developing spermatocytes

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SPERMIOGENESIS

• Spermatids are rounded cells. • They modify to assume specific

shape of the sperm.• This process is called

Spermiogenesis. In it they elongate and reorganize internal structure to acquire the particular shape.

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SPERMIOGENESIS (cont.)

• A mature sperm has head, neck and tail

• From lumen of seminiferous tubules sperms enter duct of epididymis

• They take 20 days to travel this 4-6 meter long tortuous duct

• If ejaculation does not occur they die and degenerate

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Abnormal sperms

They are with: Two heads Two tails or: Dwarf Giant

Contrary to oocytes abnormal sperms are seen more frequently

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Oogenesis

• In females, most of gametogenesis occurs during embryonic development.

• Primordial germ cells migrate into the ovaries at week 4 of development and differentiate into oogonia (46,2N).

• Oogonia enter meiosis I and undergo DNA replication to form primary oocytes .

• All primary oocytes are formed by the fifth month of fetal life and remain dormant in prophase of meiosis I until puberty.

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• During a woman’s ovarian cycle one oocyte is selected to complete meiosis I to form a secondary oocyte and a first polar body.

• After ovulation the oocyte is arrested in metaphase of meiosis II until fertilization.

• At fertilization, the secondary oocyte completes meiosis II to form a mature oocyte (23,1N) and a second polar body.

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What is fertilization ?• A sperm enters an

ovum, and the nuclei combine to form one with 46 chromosomes.

• Fertilization is a process of several events and typically takes place in the ampulla of the uterine tube.

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Fertilization: Four Major Steps

1. Sperm contacts the egg2. Sperm or its nucleus enters the egg3. Egg becomes activated and developmental

changes begin4. Sperm and egg nuclei fuse

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Capacitation• Changes take place in the glycoprotein coat of sperm as they

travel up the female reproductive tract. These changes are absolutely essential for fertilization.

Penetration of Corona Radiata• The sperm uses both chemical and physical means to penetrate

the egg’s corona radiata.

Cortical Reaction• Entry of a sperm into the egg triggers changes that prevent

polyspermy (fertilization of an egg by more than one sperm). These changes are known as the cortical reaction.

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Fusion of Pronuclei• The male and female pronuclei fuse and the

egg completes its second meiotic division, resulting in a second polar body. The fertilized egg is now known as the zygote (“together”).

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What happens now?• Development of the zygote, the study of which is known

as embryology or developmental biology. • The zygote undergoes a series of mitotic cell divisions

called cleavage.• The stages of development are: Fertilized ovum (zygote) 2-cell stage 4-cell stage 8-cell stage Morula Blastula Early Gastrula Late Gastrula

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Compaction

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Cleavage• The zygote undergoes a number of ordinary mitotic

divisions that increase the number of cells in the zygote but not its overall size.

• Each cycle of division takes about 24 hours. The individual cells are known as blastomeres.

• At the 32-cell stage the embryo is known as a morula (L. “mulberry”), a solid ball consisting of an inner cell mass and an outer cell mass.

• The inner cell mass will eventually become the embryo and fetus, while the outer cell mass will eventually become part of the placenta.

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Blastocyst Formation

Cavitation• A fluid-filled cavity forms inside the morula.

This cavity is known as the blastocyst cavity or blastocoele, and the morula is now called a blastula or blastocyst.

• The inner cell mass is now known as the embryoblast and the outer cell mass becomes the trophoblast.

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Implantation

• The blastula sheds its zona pellucida. This is required for implantation to occur. One function of the zona pellucida is to prevent premature implantation.

Attachment and Invasion• The embryo attaches to and invades into the maternal

endometrium.

• The embryo typically implants in the posterior superior wall of the uterus. The response of the maternal endometrial cells to the invading embryo is called the decidual reaction.

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Where does this all take place?

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The embryo implants in the wall of the uterus on about the 7th day of development

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Implantation

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Clinical Correlations

Ectopic pregnancy• The blastocyst implants in a location other

than the uterus. This can present as an acute surgical emergency for the mother after the fetus begins to outgrow its confines.

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Common Sites of Ectopic Pregnancy

• Site of implantation: intrauterine

• Lower part of uterus• Ant part of uterus

• Site of implantation: extrauterine• Upper and middle part of the uterine tube : Embryo probably

lost its zona pellucida prematurely.• Ovary :The egg was never released from the ovary.• Abdominal cavity

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Placenta Previa• The embryo implants in the lower part of the

uterus towards the cervix. This makes it easy for the placenta to tear, and the mother can die from hemorrhage, or the placenta may grow to obstruct the cervical canal. This is diagnosed with ultrasound, and the baby is delivered via Cesarean section.

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• What is the difference between spermatogenesis and oogenesis ?????

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Must be familiar with:/ self study topics

• Mitosis • Meiosis 1& 2• Menstrual cycle• Formation of ovarian follicles & its fate.• Structure of ovum • What is ovulation?Ref: • Textbook :Langman's Medical Embryology

Thomas W. Sadler PhD (Author)