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. 7
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 E m b r y o l o g y L e c t u r e M a n u a l 8
G a m e t o g e n e s i s Spermatogonia - type A Spermtogonia - type B Primary spermatocyte First meiotic division Secondary spermatocytes Second meiotic division Spermatids Spermiogenesis Mature spermatozoan (sperm) 9
Anatomy of the Mature Spermatozoan (Sperm) Propulsion system to move the haploid nucleus Basic structure Head Nucleus Neck Acrosome Middle piece Mitochondria Tail Principal piece 0.5 μm 2.5-3.5 μm 4.0-5.0 μm 5.0-7.0 μm 45.0 μm End piece Size Function 5.0-7.0 μm E m b r y o l o g y L e c t u r e M a n u a l 10
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 Primary oocyte and growing follicle Ovarian cycle beginning at puberty 5 to 12 each cycle Follicle cells Oocyte 11
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 First meiotic division completed and polar body Secondary oocyte and mature graafian follicle Oocyte First polar body Changes within follicle prior to ovulation Stigma E m b r y o l o g y L e c t u r e M a n u a l 12
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 13
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 E m b r y o l o g y L e c t u r e M a n u a l 14
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 15
Comparisons of Gametogenesis Remember, it is important to differentiate between the ploidy of a cell and its. The ploidy refers to the number of copies of each chromosome in the cell. The 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 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 L e c t u r e M a n u a l 16