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GametogenesisThis lecture is the preface, so to speak, to embryology; that is, it introduces the development of the specialized germ line of cells from the male and the female that are responsible for the beginnings of a new life form. You will follow the presumptive gametic cells through the development processes that give rise to the special haploid spermatozoon (sperm) and ooctye (egg) of the male and female, respectively. After this lecture you should be able to:

• Outline and illustrate the stages of spermatogenesis – the origin and development of the spermatozoon.

• Outline and illustrate the stages of oogenesis – the origin and development of the oocyte.

• Describe the structural and functional differences between the male and female gametes.

Omne vivum ex ovo — All living things come from eggs.

William Harvery, 1651

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GametogenesisThe formation of germ cells

You should understand the stages of meiosis and how it differs from mitosis. I assume you have covered this in general biology courses. If you have not you should review it in your textbook as we won’t focus on the basic tenets of meiosis in lecture. Gametogenesis is the process of giving rise to the specialized generative cells called gametes and meiosis is at the core of this process. The key features of gametogenesis are: 1) reduction of the chromosome number (in half), 2) alteration of the cell cytoplasm, and 3) modifications of the cell’s structure (shape).

SpermatogenesisDevelopment of the male gamete

This process is the series of developmental events that alters the presumptive male gamete, the spermatogonium into a mature spermatozoon. This process takes place within the epithelial lining of the seminiferous tubules of the testis.

Epithelial lining of the seminiferous tubulesCells

Sertoli cells

Blood-testis barrier

Spermatogonia line of cells

Relation to sertoli cells

Interstitial cells of testisLeydig cells

Developmental stages of spermatogoniaPrimordial germ cell

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G a m e t o g e n e s i s

Spermatogonia - type A

Spermtogonia - type B

Ploidy

N number

Primary spermatocytePloidy

N number

First meiotic division

Secondary spermatocytesPloidy

N number

Second meiotic division

SpermatidsPloidy

N number

Spermiogenesis

Mature spermatozoan (sperm)Ploidy

N number

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Anatomy of the Mature Spermatozoan (Sperm)Propulsion system to move the haploid nucleus

Basic structureHead

Nucleus

Acrosome

Neck

Middle pieceMitochondria

TailPrincipal piece

End piece

Size

Function

2.5 - 3.5 μm

4.0 - 5.0 μm5.0 - 7.0 μm

45.0 μm

5.0 - 7.0 μm

0.5 μm

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G a m e t o g e n e s i s

OogenesisDevelopment of the female gamete

This process is the series of developmental events that alters the presumptive female gamete, the oogonium into a mature oocyte. This process takes place within the confines of the ovary.

Developmental stagesPrimordial germ cell and oogonia

Investing cells

Mitotic division

Primary oocyte and primary follicleNumbers

7 million by end of 5th month in utero

700,000 to 2 million at birth

400,000 at puberty

Follicle cells

Zona pellucida

OocytePloidy

N number

Primary oocyte and growing follicleOvarian cycle beginning at puberty

5 to 12 each cycle

Follicle cells

OocytePloidy

N number

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Primary oocyte and antral follicleFollicular atresia

Half of the follicles die

Primary oocyte and mature (graafian) follicleFollicular atresia

Cumuls oophorus

Theca interna or membrana granulosa

OocytePloidy

N number

First meiotic division completed and polar body

Secondary oocyte and mature graafian follicleOocyte

Ploidy

N number

First polar body

Changes within follicle prior to ovulation

Stigma

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G a m e t o g e n e s i s

OvulationKeys to ovulation

Secondary oocyte surrounded by corona radiata and zona pellucidaLocation after ovulation

What happens after ovulation

Fertilization and completion of second meiotic divisionWhere

How

Two more polar bodies

Fertilized oocytePloidy

N number

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Anatomy of the Fertilized OocyteThe nourishing gamete

Basic structureNucleus

Cytoplasmic storehouseProteins

RNAs

Protective chemicals

Morphogenetic factors

Cortical granules

Surrounding cells and substancesZona pellucida or vitelline envelope

Corona radiata - follicular cells

Size

Function

100.0 μm

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G a m e t o g e n e s i s

Comparison of Spermatozoa and OocytesGenetic differencesSex chromosomes

Size differencesCtyoplasmic comparisons

Functional role of the egg cytoplasm

YolkPurpose of yolk

Yolk classificationMiolecithal

Medialecithal

Megalecithal

Yolk distributionIsolecithal

Centrolecithal

Telolecithal

Associated cells

Numbers

Activity levels – motility

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Comparisons of GametogenesisRemember, it is important to differentiate between the ploidy of a cell and its N number. The ploidy refers to the number of copies of each chromosome in the cell. The N number refers to the number of copies of each unique double-stranded DNA molecule in the cell. In the review of male and female gametogenesis below, you should be able to list the ploidy and N number for each stage in the processes.

SpermatogoniumPrimary oocytein primary follicle

Primary spermatocytePrimary oocytein growing follicle

Secondary spermatocyte

Primary oocytein large follicle

Spermatids

Mature sperm

Secondary oocytein mature follicle

Fertilized oocyte

Spermatogenesis Oogenesis