Chapter 47: Animal Reproduction

What are the major forms of asexual reproduction and how do they differ?

All produce offspring w/o a mate and rely largely on mitosis (highly similar genetically)

• Budding: a new individual grows from the body of the parent and eventually detaches. Seen in hydras

• Fission: an organism splits into two or more pieces that each develop into a complete individual; i.e. sea anemones

• Parthenogenesis: offspring develop from unfertilized eggs without fertilization, seen in some lizards and invertebrates

What is fertilization and how do internal and external fertilization differ?

Fusion of a haploid sperm and haploid egg to form a diploid zygote

• External: eggs and sperm are released into the environment and unite outside the body (often in aquatic animals)

  • Requires large #s of gametes and can produce large #s of ofsspring

  • Many gametes are lost/developing embryos exposed to predators

• Internal: sperm are deposited within the female reproductive tract and fertilization occurs inside the body

  • Generally more efficient, provides greater protection for gametes and embryos

How do oviparity and viviparity differ, and why might viviparity evolve?

• Oviparity: laying eggs that finish development outside the mother's body

  • Embryos obtaining nutrients primarily from yolk

• Viviparity: retaining embryos within the mother's body + giving birth to live young.

  • More protection/regulation of developmental conditions

  • Evolves b/c of cold external/environmental temperatures. Retaining embryos internally maintain favorable conditions/aid in survival

How do marsupial and eutherian mammals differ?

• Marsupials: short gestation periods; give birth to underdeveloped young that continue development while attached to a nipple (often in a pouch)

• Eutherians (placental): complex placenta supporting longer gestation period

  • Generally more developed at birth

What is gametogenesis and how do oogenesis and spermatogenesis differ?

Production of haploid gametes through meiosis

• Oogenesis: occurs in females and produces eggs

• Spermatogenesis occurs in males and produces sperm

What is oogenesis?

Process by which female gametes (eggs) are produced in the ovaries

• Both mitosis and meiosis; results in formation of a single large functional ovum

• Unequal division of cytoplasm (one cell receives more developmental resources)

• Begins before birth w/ long periods of developmental arrest

• Resulting egg contains nutrients, organelles, and genetic material needed for early development

What are the stages of oogenesis and the ploidy of each cell?

• Begins with an oogonium (2n) → mitosis, differentiation, chromosome replication → primary oocyte (2n)

• Meiosis I → secondary oocyte (n) and polar body (n)

• Meiosis II in the secondary oocyte → an ootid (n) and a polar body; meiosis II in the first polar body → two more polar bodies

• Ootid → mature egg/ovum (n)

What is spermatogenesis?

Process by which male gametes (sperm) are produced in the testes

• Begins w/ diploid stem cells → meiosis → generates haploid sperm

  • Highly efficient; each primary spermatocyte generates 4 sperm

• ~ equally divided cytoplasm among daughter cells

• Occurs continuously after sexual maturity

• Produces large #s of sperm throughout life

What are the stages of spermatogenesis and the ploidy of each cell?

• Begins w/ spermatogonium (2n) → mitosis, differentiation, chromosome replication → primary spermatocyte (2n)

• Meiosis I → two secondary spermatocytes (n)

• Meiosis II from each spermatocyte → 4 total spermatids (n)

• Spermatids develop into spermatozoa (n)

Why are polar bodies produced during oogenesis?

B/c cytoplasm is dividing unequally among daughter cells. Most is concentrated into a single developing egg to ensure the ovum revives sufficient nutrients, organelles, and cellular machinery to support early embryonic development

How do egg and sperm structure reflect their functions?

• Eggs = large cells w/ abundant cytoplasm, nutrients, organelles, and genetic material to support early embryonic development

• Sperm = smaller, specialized for mobility and delivery of genetic information

What protective structures surround sea urchin and mammalian eggs?

Protective layers help prevent physical damage and participate in sperm recognition during fertilization (help prevent polyspermy)

• Sea urchin eggs are surrounded by a jelly layer and a vitelline envelope

• Mammalian eggs possess corona radiata and zona pellucida

What are the functions of the acrosome, nucleus, mitochondria, and flagellum in sperm?

• Acrosome: contains digestive enzymes that help the sperm penetrate the egg

• Nucleus contains the haploid genetic material that combines w/ egg's genome during fertilization

• Mitochondria in the midpiece generate ATP needed for movement.

• Flagellum propels the sperm toward the egg through fluid environments

What is the step-by-step process of fertilization in sea urchins?

1. Sea urchin eggs release a chemical to help the sperm find them

2. Head of the sperm binds to the jelly layer

3. Binding triggers acrosome reaction; enzymes contained in the acrosome digest portions of the jelly layer and vitelline envelop

4. Contact w/ jelly layer increases sperm flagellum movement, powering it towards the egg’s plasma membrane

5. Plasma membrane proteins induce membrane fusion w/ the sperm

6. Sperm nucleus enters the egg, egg nuclei + sperm fuse and reestablish diploid #

7. Egg is activated + ready for development

What are bindin and bindin receptors, and why are they important?

• Bindin = protein located on the surface of sea urchin sperm

• Bindin receptors = proteins located on the egg's vitelline envelope

• Interaction b/w them allows sperm to attach to eggs of the same species, helping to prevent cross-species fertilization

What is polyspermy and why is it dangerous?

Occurs when more than one sperm fertilizes a single egg and produces an abnormal chromosome # in the zygote. This genetic imbalance disrupts normal development and is usually lethal

How is polyspermy prevented and what role does calcium play?

• Sperm enters egg → calcium ions are released throughout egg cytoplasm

• Triggers exocytosis of cortical granules (membrane-bound vesicles)

• Granules release enzymes/other molecules that modify the egg’s outer layers

• Helps to prevent additional sperm from attaching/entering the egg

How do cortical granules and the fertilization envelope prevent polyspermy?

• Following fertilization cortical granules release enzymes and solutes through exocytosis

• Proteases modify proteins within the vitelline envelope; solutes draw water into the space surrounding the egg

• Lifts the vitelline envelope away from the egg surface → converts it into a fertilization envelope which acts as a physical barrier that prevents additional sperm from entering

What is cleavage and how does it produce a blastula?

One of the stages of embryonic development that involves a series of rapid mitotic divisions that occurs immediately after fertilization

• Zygote divides repeatedly w/o significant growth

• Produces many blastomeres (smaller cells) while the embryo maintains size and eventually develops into the blastula (hollow ball of cells)

• Blastula stage = completion of early cleavage

What is a blastula?

• Early embryonic stage formed after cleavage

• Hollow ball of unspecialized cells surrounding a fluid-filled cavity

• Arrangement of cells est. the framework for later body organization

What happens during gastrulation and what are the blastopore and gastrula?

An embryonic stage in which cells migrate and reorganize to establish the basic body plan of the embryo

• Blastopore forms as cells move inward and the embryo becomes a gastrula that contains multiple layers of cells that form tissues/organs

• Germ layers (ectoderm, mesoderm, and endoderm) are established

What is neurulation?

One of the embryonic development stages that forms the neural tube

• Specialized ectodermal cells fold inward/fuse to create the neural tube which later develops into the brain + spinal cord

• Errors during neurulation can result in serious developmental abnormalities

What is organogenesis?

• Stage of development during which organs and tissues begin to form from the germ layers

• Cells become increasingly specialized

• Interactions among tissues guide formation of complex organ systems.

What is the ectoderm and what structures develop from it?

• Outermost germ layer formed during gastrulation

• Gives rise to the nervous system (brain + spinal cord), epidermis of skin, many sensory structures

• Plays a major role in communication and perception

What is the mesoderm and what structures develop from it?

• Middle germ layer formed during gastrulation

• Gives rise to muscles, bones, connective tissues, the circulatory system, and components of the reproductive + excretory systems

• Critical for body organization (support, movement, transport systems)

What is the endoderm and what structures develop from it?

• Innermost germ layer formed during gastrulation

• Lines the developing digestive tract and respiratory system

• Gives rise to the epithelial linings of GI tract/respiratory system + associated organs like liver and pancreas

• Critical role in internal physiological functions (nutrient processing and gas exchange)

What are the major structures of the female reproductive system and what are their functions?

Ovaries, oviducts, uterus, cervix, vagina, and clitoris

• Ovaries produce eggs and hormones (E2 and P4)

• Oviducts transport eggs/often site of fertilization

• Uterus houses + nourishes developing embryo and fetus

• Cervix connects the uterus to vagina

• Vagina functions in sperm reception and childbirth

• Clitoris is a sensory organ (sexual stimulation)

What are the major structures of the male reproductive system and what are their functions?

Testes, epididymis, vas deferens, seminal vesicles, prostate gland, penis, and urethra

• Testes produce sperm and testosterone

• Epididymis stores and matures sperm

• Vas deferens transports sperm toward the urethra

• Seminal vesicles and prostate gland contribute fluids that form semen/support sperm survival

• Penis delivers semen to female tract

• Urethra = passageway for semen to exit body

What are the functions of the accessory glands in semen production?

• Seminal vesicles contribute fructose that provides energy for sperm and prostaglandins that influence female reproductive tissues

• Prostate gland contributes fluids that support sperm survival

• Bulbourethral glands secrete alkaline mucus that lubricates the urethra and helps neutralize acidity

What hormones regulate the menstrual cycle?

GnRH, FSH, LH, estrogen, and progesterone

• GnRH is released by hypothalamus → stimulates secretion of FSH/LH from anterior pituitary

• FSH promotes follicle development within the ovary

• Estrogen is produced by developing follicle, influences reproductive tissues + hormone secretion

• LH triggers ovulation/formation of the corpus luteum

• Progesterone produced by the corpus luteum prepares/maintains uterine lining

What occurs during the follicular phase?

• Begins with menstruation, continues until/prepares body for ovulation

• FSH stimulates growth/development of ovarian follicles that produce increasing amounts of E2 as they grow

• Rising E2 levels promote growth of the uterine lining → trigger positive feedback on the pituitary → causes surge of LH This positive feedback results in a surge of LH secretion

What causes ovulation?

Surge in luteinizing hormone (LH)

• LH surge stimulates rupture of the mature follicle and release of the secondary oocyte and is dependent on rising E2 levels that causes positive feedback

What occurs during the luteal phase and what is the corpus luteum?

Begins after ovulation when the ruptured follicle transforms into the corpus luteum

• Corpus luteum secretes progesterone + small amount of estrogen

• P4 maintains/further develops the uterine lining in preparation for possible implantation

• High P4 suppresses GnRH, FSH, and LH via negative feedback

• If no fertilization → corpus luteum degenerates

What causes menstruation?

When fertilization and implantation do not take place

• Corpus luteum degenerates → P4 levels decline

• Causes the uterine lining to break down and detach, so blood/tissue from uterine lining are shed through the vagina

• Menstruation = beginning of a new menstrual cycle

How does pregnancy alter hormone regulation?

During pregnancy, hormones maintain the uterine lining and prevent additional ovulation

• P4 remains elevated, supports endometrium, continues to suppress GnRH, FSH, and LH to prevent development of new follicles and additional ovulation

How do barrier methods prevent pregnancy?

Physically blocking sperm from reaching the egg

• Condoms, diaphragms, and cervical caps

• Do not alter hormone levels or ovulation

How do birth control pills prevent pregnancy?

Contain synthetic hormones similar to progesterone and sometimes estrogen

• Exert negative feedback on hypothalamus/pituitary gland, decreasing GnRH, FSH, and LH

• W/o adequate LH → no ovulation

• Uterine lining develops differently, reducing the likelihood of successful implantation

How does emergency contraception prevent pregnancy?

Contains hormones similar to those found in birth control pills

• Helps to prevent or delay ovulation before fertilization occurs (acts before pregnancy is establishes)

How does the abortion pill differ from birth control and emergency contraception?

Abortion pill, mifepristone, functions as a progesterone receptor antagonist.

• Blocks P4, so it prevents maintenance of the uterine lining and terminates the pregnancy

What are the three stages of parturition in humans?

• Dilation of the cervix

  • Uterine contractions gradually widen the cervical opening

• Expulsion: strong contractions/maternal pushing deliver the baby through the birth canal

• Delivery of the placenta (remains attached to the uterine wall after birth)