Chapter 25:

Animal Reproduction

And

Development

There are two types of reproduction in animals – sexual and asexual.

• Asexual reproduction is more efficient than sexual reproduction.

• There is no waste of gametes that fail to unite to form an offspring.

• Many animals reproduce asexually, at least some of the time.

• Sexual reproduction has the advantage of genetic recombination, which can lead to more adaptation of the species.

• Fertilization may be internal, as with humans, or external, as with many fish species.

Asexual Reproduction• In asexual reproduction, there is no fusion of

sperm and egg• A common form of asexual reproduction is

budding, where a miniature version of the animal grows directly on the body of the adult, drawing nourishment from the parent. When it is large enough, it breaks off, becoming independent.

• The females of some species can reproduce by a process in which haploid egg cells develop into adults without being fertilized. Males are not necessary to the survival of the species.

Sexual Reproduction

• Spermatogenesis, or the production of sperm, does not begin until males reach puberty. At that time, cells in the testes begin producing testosterone, the hormone that initiates spermatogenesis and stimulates the development of secondary sexual characteristics.

• Oogenesis, the formation of egg cells, actually begins in the developing ovaries of the female fetus. By the time a girl is born, she has all the oocytes she will ever produce. By puberty, only about 400,000 oocytes remain of the original 2 million, and they mature into eggs.

Steps in Human Reproduction

• Copulation

• Fertilization

• Implantation

• Gestation

• Birth

• An end to life as you knew it!

Fertilization has three functions:

1. transmission of genes from both parents to offspring

2. restoration of the diploid number of chromosomes reduced during meiosis

3. initiation of development in offspring

Steps in Fertilization• Contact between sperm and egg

• Entry of sperm into the egg • Fusion of egg and sperm nuclei

• Activation of development

Cleavage is the first step in development of ALL multi-celled organisms. Cleavage converts a single-celled zygote into a

multi-celled embryo by mitosis. Usually, the zygotic cytoplasm is divided among the newly formed cells. Frog embryos divide to produce 37,000 cells in a little over 40

hours.

The blastula is produced by mitosis of the zygote, and is a ball of cells surrounding a fluid-filled cavity (the blastocoel). The decreasing size of cells increases their surface to volume ratio,

allowing for more efficient oxygen exchange between cells and their environment. RNA and information carrying

molecules are distributed to various parts of the blastula, and this molecular differentiation sets the stage for the layering of

the body in the next phases of development.

Cleavage of the zygote begins while it is still in the oviduct, producing a solid ball of cells

(morula). The morula enters the uterus, continuing to divide and becomes a blastocyst

ImplantationThe uterine lining becomes enlarged and prepared for implantation of the embryo in the trophoblast layer. Twelve days after fertilization, the trophoblast has formed a two-layered chorion. Human chorionic

gonadotropin (hCG) is secreted by the chorion, and prolongs the life of the corpus luteum until the placenta

begins to secrete estrogen and progesterone. Home pregnancy tests work by detecting elevated hCG levels

in the woman's urine.

Gastrulation involves a series of cell migrations to positions where they will

form the three primary cell layers.

•The ectoderm forms the outer layer. •The endoderm forms the inner layer.

•The mesoderm forms the middle layer.

Ectoderm

The ectoderm forms tissues associated with outer layers: skin, hair, sweat glands, epithelium. The brain and nervous system also develop from the ectoderm.

Mesoderm

The mesoderm forms structures associated with movement and support: body muscles, cartilage, bone, blood, and all other connective tissues. Reproductive system organs and

kidneys form from mesoderm.

Endoderm

The endoderm forms tissues and organs associated with the digestive and respiratory systems. Many endocrine structures, such as

the thyroid and parathyroid glands, are formed by the endoderm. The liver, pancreas,

and gall bladder arise from endoderm.

So what happens when there is no fertilization of the egg?

1. follicle stage (10-14 days average duration) - production of ova/eggs occurs in tiny cavities in the ovary called follicles; enlarging follicle produces estrogen causing the uterus to get ready for embryo implantation (uterus thickens its lining) 2. ovulation (1 day) - follicle enlarges and ruptures ovary wall; egg is released to the oviduct (usually only 1 is released at a time) 3. corpus luteum stage (10 -14 days average duration) -  yellow tissue fills the follicle after ovulation called the corpus luteum; "yellow body" secretes progesterone which maintains the thickness of the uterus in case a pregnancy occurs 4. menstruation (3-5 days average duration) - periodic shedding of the thickened lining of the uterus

Women’s Reproductive Health Concerns

• Endometriosis• Breast Cancer• Ovarian Cancer• Cervical Cancer• Premenstrual Syndrome• Premenstrual

Dysphoric Disorder• Dysmennorhea

• Polycystic Ovarian Syndrome

• Fibrocystic Disease• Abnormal Bleeding• Unplanned Pregnancy• Miscarriage• Multiple Births• Sexually Transmitted

Diseases

Sex, pregnancy, and childbirth can all be wonderful, beautiful

experiences. The trick is approaching all of them with your

eyes wide open, and having as much knowledge as possible.

The Reproductive Systemhttp://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookREPROD.html#Fertilization%20and%20Cleavage

The Visible Embryohttp://www.visembryo.com/baby/index.html

Regents Prep: The Human Reproductive Cyclehttp://regentsprep.org/Regents/biology/units/reproduction/human.cfm

Washington State Midwives Reproductive Health Care Resources and Websiteshttp://www.midwivesofwa.org//reprodhc.htm

Webshotswww.webshots.com

Chapter 25 Credits/Resources