Reproduction and Development Figures 26.1 – 26.2 CHAPTER 26 Reproduction and Development Figures 26.1 – 26.2

During ejaculation, a man releases up to 500 million sperm, only one of which may fertilize an egg

You have trillions of cells in your body, and they all arose from one original cell

An American woman is 4 times more likely to deliver triplets today than 25 years ago Even though a woman is born with up to half a million developing gametes, she will only use about 500 during her lifetime

BIOLOGY AND SOCIETY: RISE OF THE SUPERTWINS On November 19, 1997, news reports heralded the arrival of the Iowa septuplets Figure 26.1

Other sets of multiple births soon followed What accounted for the sudden rash of multiple births?

All of these multiple births were by women who had taken fertility drugs because they couldn’t become pregnant naturally Couples turn to fertility drugs to overcome their natural reproductive limitations

UNIFYING CONCEPTS OF ANIMAL REPRODUCTION Reproduction is the creation of new individuals from existing ones

Asexual Reproduction In asexual reproduction One parent produces genetically identical offspring

Binary fission Is the simplest type of asexual reproduction Involves a single parent cell splitting through mitosis into two genetically identical offspring cells

Some multicellular organisms reproduce by a similar means called fission, in which one organism splits into two or more individuals Figure 26.2a

Fragmentation is the breaking of a parent body into several pieces Regeneration, which follows fragmentation, is the regrowth of a whole animal from the pieces

Budding Is the splitting off of new individuals from existing ones Figure 26.2b

Asexual reproduction has a number of advantages It allows a species to perpetuate itself if its individual members are sessile or isolated from one another It allows organisms to multiply quickly

One potential disadvantage of asexual reproduction is that it produces genetically uniform populations

Sexual Reproduction Sexual reproduction Involves the fusion of gametes (sperm and egg) from two parents Increases the genetic variability among offspring

Some animals can reproduce both sexually and asexually Figure 26.3a

Some species are hermaphrodites with both male and female reproductive systems Figure 26.3b

The mechanics of fertilization play an important part in sexual reproduction

Many organisms use external fertilization, in which parents discharge their gametes into the water, where fertilization occurs Figure 26.3c

Other organisms use internal fertilization, which occurs within the female’s body Internal fertilization requires copulation, or sexual intercourse

HUMAN REPRODUCTION Both sexes of humans have A pair of gonads, the organs that produce gametes Ducts to store and deliver the gametes Structures to facilitate copulation

Female Reproductive Anatomy The ovaries Are the site of gamete production in human females Ovaries Oviduct Follicles Corpus luteum Uterus Wall of uterus Endometrium (lining of uterus) Cervix (“neck” of uterus) Vagina Figure 26.4

The ovaries contain follicles Each follicle consists of a single developing egg cell surrounded by layers of cells that nourish and protect it The follicles also produce estrogen, the female sex hormone

Ovulation Is the process by which an egg cell is ejected from the follicle Figure 26.5

The egg enters the oviduct, which is a tube in which cilia sweep the egg toward the uterus

The uterus is the actual site of pregnancy The cervix, the narrow neck at the bottom of the uterus, opens into the vagina, or birth canal During copulation, the vagina serves as a repository for sperm

Female reproductive anatomy Oviduct Female reproductive anatomy Ovary Rectum (digestive system) Uterus Bladder (excretory system) Pubic bone Cervix Urethra (excretory system) Vagina Shaft Glans Clitoris Prepuce Labia minora Labia majora Vaginal opening Figure 26.6

Male Reproductive Anatomy The penis Contains erectile tissue

The testes Are the male gonads, enclosed in a sac called the scrotum Produce sperm

Semen Consists of this fluid and sperm Several glands Contribute to the formation of the fluid that carries, nourishes, and protects sperm

Male reproductive anatomy, side view Bladder (excretory system) Seminal vesicle Pubic bone Rectum (digestive system Erectile tissue of penis Vas deferens Urethra Prostate gland Vas deferens Glans of penis Epididymis Testis Prepuce Scrotum Figure 26.7a

Male reproductive anatomy, front view Bladder (excretory system) Seminal vesicle (behind bladder) Prostate gland Urethra Erectile tissue of penis Vas deferens Scrotum Epididymis Testis Glans of penis Figure 26.7b

Gametogenesis Gametogenesis Human gametes Is the production of gametes Are haploid cells that develop by meiosis

Oogenesis Oogenesis is the development of eggs within the ovaries Diploid cell in embryo Differentiation and onset of meiosis I Ovary Primary oocyte, arrested in prophase of meiosis I; present at birth Completion of meiosis I and onset of meiosis II Corpus luteum First polar body Secondary oocyte, Growing follicle arrested at metaphase of meiosis II; released from ovary Entry of sperm triggers completion of meiosis II Mature follicle Second polar body Ovulation Ovum Ruptured follicle (haploid) Sperm Figure 26.8

Spermatogenesis Spermatogenesis Is the formation of sperm cells

Differentiation and onset of meiosis I Seminiferous tubule Epididymis Penis Testis Scrotum Diploid cell Testis Differentiation and onset of meiosis I Seminiferous tubule Primary spermatocyte Cross section of seminiferous tubule Meiosis I completed Secondary spermatocyte Meiosis II Developing spermatids Differentiation Sperm cells (haploid) Center of seminiferous tubule Figure 26.9