7
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
E m b r y o l o g y L e c t u r e M a n u a l
8
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
9
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
E m b r y o l o g y L e c t u r e M a n u a l
10
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
11
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
E m b r y o l o g y L e c t u r e M a n u a l
12
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
13
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
E m b r y o l o g y L e c t u r e M a n u a l
14
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
15
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
E m b r y o l o g y L e c t u r e M a n u a l
16
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