Gametes and fertilisation Human Biology Unit 2 Gametes and fertilisation

Gametogenesis Gametogenesis is the production of sperm and egg cells by meiosis. ATAR and general

Sperm Sperm (spermatozoa) are the male gametes or reproductive cells. Sperm are formed by the process of spermatogenesis in the testes. Spermatogenesis involves meiosis – consequently, sperm cells have only 23 chromosomes. ATAR and general

A section through a testis Epididymis where sperm mature Lobules of testis consisting of seminiferous tubules where spermato- genesis takes place Lutz Slomianka, ANHB - UWA

Important structures in a spermatozoon The head - contains the nucleus which carries the 23 chromosomes. The acrosome - forms a cap over the head. The cap contains enzymes which break down the follicle surrounding the ovum, and permit fertilisation. The middle piece – contains mitochondria to provide power for the tail. The tail – is a flagellum; it acts like a propeller. ATAR and general

Structure of a human sperm Drawing by LadyofHats

Spermatogenesis Lumen of tubule Spermatids Immature sperm Germ cells undergoing meiosis Transverse section through a seminiferous tubule showing development of spermatids M I Walker, Wellcome Images

Sperm developing in the testis Sperm develop in the seminiferous tubules of the testis with their tails projecting into the lumen of the tubule. These spermatids are in the advanced stages of maturation. Yorgos Nikas, Wellcome Images

Semen Semen produced during an ejaculation consists of sperm and fluids (seminal fluid) produced by the seminal vesicle, prostate and Cowper’s glands. Seminal fluid contains a complex mixture of enzymes and nutrients. ATAR & general

Ova Ova (egg cells) are the female gametes or reproductive cells. Ova are formed by the process of oogenesis in the ovaries. Oogenesis involves meiosis – consequently, ova have only 23 chromosomes. Of the four daughter cells, only one develops into an ovum – the remaining three become polar bodies. ATAR & general

Ovary containing follicles in different stages of maturity. Corpus luteum Mature ovarian follicle Primary oocyte (developing ovum) Medical Art Services, Munich, Wellcome Images

Structures in an ovarian follicle The oocyte – is the immature ovum (nuclear division is suspended at metaphase of meiosis II). Zona pellucida and corona radiata – the outer protective layers of follicular cells surrounding the ovum. ATAR only

A primary oocyte Zona pellucida Nucleus of primary oocyte Corona radiata Nucleus of primary oocyte Fluid-filled space Glandular granulosa cells

Human egg surrounded by the zona pellucida Residual coronal cell Yorgos Nikas , Wellcome Images

Fertilisation Fertilisation typically occurs in the upper region of the Fallopian tube. It takes many sperm to break down the corona radiata. Only one sperm enters the oocyte At this point the nucleus of the oocyte completes meiosis and the two nuclei fuse. ATAR & general

Fertilisation of a human egg - a single sperm entering an ovum Wellcome Photo Library, Wellcome Images

Human egg surrounded by sperm, Joyce Harper, UCL, Wellcome Images

Sperm on the surface of a human egg trying to find their way through the zona pellucida, Yorgos Nikas, Wellcome Images