Gametogenesis

Omne vivum ex ovo — All living things come from eggs. William Harvery, 1651

Gametogenesis This 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.

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Gametogenesis

The 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).

Spermatogenesis

Development 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 tubules Cells

Sertoli cells

Blood-testis barrier

Spermatogonia line of cells

Relation to sertoli cells

Interstitial cells of testis Leydig cells

Developmental stages of spermatogonia Primordial germ cell

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L e c t u r e

M a n u a l

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

Spermatogonia - type A Spermtogonia - type B Ploidy

N number

Primary spermatocyte Ploidy

N number

First meiotic division Secondary spermatocytes Ploidy

N number

Second meiotic division Spermatids Ploidy

N number

Spermiogenesis Mature spermatozoan (sperm) Ploidy

N number

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Anatomy of the Mature Spermatozoan (Sperm)

2.5 - 3.5 μm

Propulsion system to move the haploid nucleus

Basic structure 4.0 - 5.0 μm

Head

Nucleus

5.0 - 7.0 μm

Acrosome

Neck

Middle piece

Mitochondria

45.0 μm

Tail

0.5 μm

Principal piece

End piece

Size

E m b r y o l o g y 10

5.0 - 7.0 μm

Function

L e c t u r e

M a n u a l

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

Oogenesis

Development 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 stages

Primordial germ cell and oogonia Investing cells

Mitotic division

Primary oocyte and primary follicle

Numbers 7 million by end of 5th month in utero 700,000 to 2 million at birth 400,000 at puberty Follicle cells

Zona pellucida

Oocyte Ploidy N number

Primary oocyte and growing follicle

Ovarian cycle beginning at puberty 5 to 12 each cycle

Follicle cells

Oocyte Ploidy N number

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Primary oocyte and antral follicle Follicular atresia Half of the follicles die

Primary oocyte and mature (graafian) follicle Follicular atresia

Cumuls oophorus

Theca interna or membrana granulosa

Oocyte Ploidy N number

First meiotic division completed and polar body

Secondary oocyte and mature graafian follicle Oocyte Ploidy

N number

First polar body

Changes within follicle prior to ovulation

Stigma

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L e c t u r e

M a n u a l

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

Ovulation

Keys to ovulation

Secondary oocyte surrounded by corona radiata and zona pellucida Location after ovulation

What happens after ovulation

Fertilization and completion of second meiotic division Where

How

Two more polar bodies

Fertilized oocyte Ploidy

N number

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Anatomy of the Fertilized Oocyte The nourishing gamete

Basic structure Nucleus

Cytoplasmic storehouse Proteins

RNAs 100.0 μm

Protective chemicals

Morphogenetic factors

Cortical granules

Surrounding cells and substances

Zona pellucida or vitelline envelope

Corona radiata - follicular cells

Size

Function

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L e c t u r e

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

Comparison of Spermatozoa and Oocytes Genetic differences Sex chromosomes

Size differences

Ctyoplasmic comparisons Functional role of the egg cytoplasm Yolk

Purpose of yolk

Yolk classification Miolecithal Medialecithal Megalecithal Yolk distribution Isolecithal Centrolecithal Telolecithal

Associated cells

Numbers

Activity levels – motility

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Comparisons of Gametogenesis

Remember, 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 doublestranded 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.

Spermatogenesis

Oogenesis

Spermatogonium

Primary oocyte in primary follicle

Primary spermatocyte

Primary oocyte in growing follicle

Primary oocyte in large follicle Secondary spermatocyte

Spermatids Secondary oocyte in mature follicle

Fertilized oocyte

Mature sperm E m b r y o l o g y 16

L e c t u r e

M a n u a l