Reproductive System and Development

Figure 33.3 Sexual Reproduction Requires the Production of Haploid Gametes through Meiosis (a) Spermatogenesis produces haploid sperm. (b) Oogenesis produces haploid eggs (ova).

Urinary bladder (excretory system) Figure 27.4A Urinary bladder (excretory system) Seminal vesicle (behind bladder) Prostate gland Bulbourethral gland Urethra Erectile tissue of penis Figure 27.4A Front view of male reproductive anatomy Scrotum Vas deferens Epididymis Testis Glans of penis 3

Rectum (digestive system) Figure 27.4B Rectum (digestive system) Seminal vesicle Urinary bladder (excretory system) Vas deferens Ejaculatory duct Pubic bone (skeletal system) Prostate gland Erectile tissue Figure 27.4B Side view of male reproductive anatomy (with nonreproductive structures in italic) Bulbourethral gland Anus (digestive system) Urethra (excretory system) Penis Vas deferens Epididymis Glans of penis Testis Testicle Prepuce Scrotum 4

Cross section of seminiferous tubule Figure 27.5A_1 Penis Epididymis Seminiferous tubule Testis Scrotum Testis Figure 27.5A_1 Spermatogenesis (part 1) Cross section of seminiferous tubule 5

Figure 27.5A Spermatogenesis Penis Epididymis Seminiferous tubule Testis Scrotum Testis Diploid cell 2n Differentiation and onset of meiosis I Primary spermatocyte 2n Cross section of seminiferous tubule (diploid; in prophase of meiosis I) Meiosis I completed Secondary spermatocyte n n Figure 27.5A Spermatogenesis (haploid) Meiosis II Developing sperm cells n n n n Differentiation Sperm cells n n n n (haploid) Mature sperm released into center of seminiferous tubule 6

Urethral region here expands and fills with semen Figure 27.4C Sphincter contracts Urinary bladder Urethral region here expands and fills with semen Contractions of vas deferens Contractions of seminal vesicle Contractions of prostate gland Contractions of epididymis Sphincter contracts 1 Sphincter remains contracted Semen expelled Figure 27.4C The two stages of ejaculation Contractions of muscles around base of penis Contractions of urethra Sphincter relaxes 2 7

Stimuli from other areas in the brain Figure 27.4D Stimuli from other areas in the brain Hypothalamus Releasing hormone Anterior pituitary Negative feedback FSH LH Figure 27.4D Hormonal control of the testis Androgen production Testis Sperm production 8

Endometrium (lining of uterus) Figure 27.3A Oviduct Ovaries Follicles Corpus luteum Uterus Wall of uterus Endometrium (lining of uterus) Cervix (“neck” of uterus) Figure 27.3A Front view of female reproductive anatomy (upper portion) Vagina 9

Rectum (digestive system) Urinary bladder (excretory system) Figure 27.3C Oviduct Ovary Uterus Rectum (digestive system) Urinary bladder (excretory system) Pubic bone (skeletal system) Cervix Urethra (excretory system) Vagina Figure 27.3C Side view of female reproductive anatomy (with nonreproductive structures in italic) Shaft Prepuce Clitoris Glans Vulva Labia minora Labia majora Anus (digestive system) Vaginal opening 10

Differentiation and onset of meiosis I Figure 27.5B_1 Before birth Ovary Diploid cell Differentiation and onset of meiosis I Primary oocyte within follicle Primary oocyte Figure 27.5B_1 Oogenesis and the development of an ovarian follicle (part 1) (arrested in prophase of meiosis I; present at birth) 11

Completion of meiosis I and onset of meiosis II Figure 27.5B_2 Growing follicle Completion of meiosis I and onset of meiosis II Mature follicle Ruptured follicle First polar body Figure 27.5B_2 Oogenesis and the development of an ovarian follicle (part 2) Secondary oocyte (arrested at metaphase of meiosis II; released from ovary) Ovulated secondary oocyte 12

(arrested at metaphase of meiosis II; released from ovary) Figure 27.5B_3 Ruptured follicle First polar body Secondary oocyte (arrested at metaphase of meiosis II; released from ovary) Ovulated secondary oocyte Entry of sperm triggers completion of meiosis II Second polar body Figure 27.5B_3 Oogenesis and the development of an ovarian follicle (part 3) Corpus luteum Mature egg (ovum) Degenerating corpus luteum 13

Figure 29.9 The Human Menstrual Cycle Depends on the Sequential Release of Several Hormones Follicle-stimulating hormone (FSH) launches a new menstrual cycle by stimulating the growth of ovarian follicles. Developing follicles produce estrogen. When estrogen levels reach a certain threshold, they trigger release of luteinizing hormone (LH) from the pituitary, which triggers ovulation. An estrogen, especially the large amounts of progesterone secreted by the corpus luteum, stimulates a buildup of the uterine lining in preparation for a possible pregnancy. If fertilization fails to occur, the corpus luteum dies after about 14 days, hormone concentrations crash, and the uterine lining is sloughed off.

Figure 33.4 Fertilization Occurs When a Single Sperm Enters the Egg (a) Sperm are many times smaller than eggs, as can be seen in this scanning electron micrograph of a swarm of sperm surrounding a single human egg. (b) The tail of a sperm consists of a whiplike flagellum, which helps it wriggle into the uterus and through the oviduct to meet the egg. The acrosome contains enzymes that help the sperm penetrate the egg’s outer covering. (c) Although many sperm are present on the surface of the egg, only one can penetrate to fertilize the egg.

The sperm’s acrosomal enzymes digest the egg’s jelly coat. Figure 27.9C 1 A sperm touches the egg’s jelly coat, and its acrosome releases enzyme molecules. 2 The sperm’s acrosomal enzymes digest the egg’s jelly coat. 3 Proteins on the sperm head bind to egg receptors. 4 The plasma membranes of sperm and egg fuse. Acrosomal enzymes 5 The sperm nucleus enters the egg cytoplasm. Sperm Plasma membrane 6 The vitelline layer separates and becomes impenetrable. Nucleus Acrosome Receptor protein molecules Plasma membrane Sperm nucleus Figure 27.9C The process of fertilization in a sea urchin n Vitelline layer Cytoplasm n Jelly coat Egg nucleus n 7 The nuclei of sperm and egg fuse. n Egg cell 2n Zygote nucleus 16

Figure 27.10_s1 Zygote 2 cells Figure 27.10_s1 Cleavage in a sea urchin (step 1) 17

Zygote 2 cells 4 cells 8 cells Figure 27.10_s2 Figure 27.10_s2 Cleavage in a sea urchin (step 2) 18

Many cells (solid ball) Figure 27.10_s3 Zygote 2 cells 4 cells 8 cells Figure 27.10_s3 Cleavage in a sea urchin (step 3) Many cells (solid ball) 19

Blastula (hollow ball) Cross section of blastula Figure 27.10_s4 Zygote 2 cells 4 cells 8 cells Figure 27.10_s4 Cleavage in a sea urchin (step 4) Many cells (solid ball) Blastocoel Blastula (hollow ball) Cross section of blastula 20

Table 27.11 Table 27.11 Derivatives of the Three Embryonic Tissue Layers 21

Figure 33.7 Cell Differentiation in the Blastocyst Produces the Embryo and Portions of the Placenta The mammalian blastocyst has two types of tissue, whose fates become evident after the blastocyst is implanted in the endometrium. The inner cell mass begins to differentiate into the actual embryo. The trophoblast expands into the endometrium to begin forming a placenta in conjunction with endometrial tissues.

Figure 33.8 The Three Tissue Layers Have Developmentally Distinct Fates Despite differences in the patterns of early cell division, almost all animals have recognizable layers of endoderm, mesoderm, and ectoderm cells that appear as parts of the blastocyst differentiate into specialized tissue types. During gastrulation, these tissue layers are rearranged as cells migrate to occupy their proper positions in the embryo.

Figure 33.9 The Placenta Is the Life-Sustaining Connection between Mother and Developing Fetus Through the placenta, the fetus receives oxygen and nutrients from the mother’s bloodstream and disposes of wastes. Note that there is no intermingling of the maternal and fetal blood vessels. Gases, nutrients, and wastes diffuse into and out the intervillous space, the pools of blood that bathe the chorionic villi.

Figure 33.10 Nine Months in the Womb For convenience, this period is traditionally divided into three trimesters, each 3 months long.