Chapter 25: Reproduction and Development From two parents to one embryo to one baby Lecture by Jennifer Lange, Chabot College

25.1 Reproductive options (and ethical issues) are on the rise. The more we know about how it works, the more complicated it becomes.

Take-home message 25.1  Technology is making conception and pregnancy possible in many situations where previously they were not possible.  But it is simultaneously giving rise to complex legal battles and ethical dilemmas.

25.2 There are costs and benefits to having a partner: asexual versus sexual reproduction.

Reproduction without Sex

Sex = Gametes Merging

Neither Method is Perfect

Take-home message 25.2  Organisms can reproduce in two ways: asexually or sexually, or both.  Each has advantages and disadvantages, depending on the organism’s environment.

25.3 Fertilization can occur inside or outside a female’s body.

Fertilization in animals requires a watery environment, which can be found either inside or outside of the organism.

Embryonic Nourishment and Protection—Oviparity

Embryonic Nourishment and Protection— Ovoviviparity

Embryonic Nourishment and Protection— Viviparity

Take-home message 25.3  Sexual reproduction requires fertilization, which can happen externally or internally.  Among those species having internal fertilization, development of the embryo can be nourished by yolk or by nutrients in the mother’s blood.

25.4 Sperm are made in the testes.

Male Reproductive Organs The male organs are specialized for gamete production and delivery.

After puberty, sperm are constantly produced in the testis by meiosis.

Sperm: motile gametes

Take-home message 25.4  In adult men, sperm are continuously produced in the testes by meiosis.  Semen—consisting of sperm cells and fluids that nurture and aid the sperm in fertilization—is ejaculated during copulation.

25.5 There is unseen conflict among sperm cells. A male’s sperm must compete with each other, and possibly with those of other males, in order to fertilize an egg. Thus, strategies have evolved to enhance success.

Take-home message 25.5  When females mate with more than one male, sperm competition occurs and can lead to a variety of adaptations that aid in the displacement of rival males’ sperm.

25.6 Eggs are made in the ovaries (and the process can take decades).

Female Reproductive Organs The organs of the female are specialized for receiving sperm and nourishing a growing fetus.

Take-home message 25.6  Genetically, egg production and sperm production are similar, with both producing variable haploid gametes.  In the ovaries, diploid cells begin to undergo meiosis, a process that continues in the Fallopian tubes following ovulation.  A much smaller number of eggs than sperm are produced, however, and each egg is considerably larger than a sperm cell.

25.7 Hormones direct the process of ovulation and the preparation for gestation.

The ovarian cycle and menstrual cycle are coordinated by hormones produced in the pituitary gland and in the ovary itself.

Two Fates of an Egg Fertilized or Not Fertilized

Take-home message 25.7  In the ovaries, a cell within a follicle is stimulated to develop into a fertile egg by coordinated secretions of the hormones estrogen, FSH, and LH.  The preparation of the uterus for implantation of a fertilized egg is coordinated by progesterone.

25.8 In fertilization, two cells become one.

Sperm Penetrates Egg

Take-home message 25.8  At fertilization: a sperm penetrates the protective zone around the egg, the egg blocks additional sperm entry, and the sperm and egg membranes fuse.

Take-home message 25.8  The egg then completes its second meiotic division, and the haploid nuclei of the egg and sperm fuse, forming a diploid zygote.

25.9 Numerous strategies can help prevent fertilization.

Contraception Strategies Birth control can intervene at multiple stages of the reproductive process to prevent pregnancy: 1. preventing ovulation 2. preventing fertilization 3. preventing implantation

Hormonal regulation can prevent the development and ovulation of an egg. Contraception Strategies

Barrier methods prevent the sperm and egg from meeting. Contraception Strategies

Preventing the implantation of a fertilized egg is accomplished by covering or losing the endometrium. Contraception Strategies

Take-home message 25.9 Pregnancy can be prevented by numerous methods, each of which acts in one or more ways to prevent ovulation, fertilization, or implantation.

25.10 Sexually transmitted diseases reveal battles between microbes and humans.

Take-home message  Sexually transmitted diseases (STDs) are caused by a variety of organisms, including bacteria viruses, protists, fungi, and arthropods.  Worldwide, more than 300 million people are infected each year.  The effects of being infected with an STD range from non-existent to mild to extreme discomfort, sterility, or even death.

25.11 Early embryonic development occurs during cleavage, gastrulation, and neurulation.

Stages of Embryonic Development Embryonic development has three stages: 1.Cleavage: early cell division 2.Gastrulation: formation of three distinct germ layers 3.Neurulation: formation of early organ systems

Stages of Embryonic Development Cleavage: single cell divides by mitosis to form a hollow ball of approximately 1000 cells.

Stages of Embryonic Development Gastrulation: three distinct germ layers of tissue form.

Stages of Embryonic Development Neurulation: the three types of tissue created during gastrulation begin to develop into the various organs and tissues.

Take-home message  Soon after fertilization, cleavage takes place and many rapid cell divisions occur without overall growth.  Following cleavage is gastrulation, in which three distinct germ layers with specific developmental fates form.  In neurulation, mesoderm forms a supporting rod called the notochord, and above that, an infolding of ectoderm forms a neural tube, which will become the brain and spinal cord.

25.12 How does an embryo become male or female?

Take-home message  Mammalian embryos develop female internal and external reproductive organs unless a gene on the Y chromosome stimulates the fetal gonads to develop as testes, which then stimulates the development of male reproductive organs.

25.13 There are three stages of pregnancy.

Stages of Pregnancy Trimester 1: differentiation of tissues, organ formation, implantation

Stages of Pregnancy The placenta forms from the outer layers of the blastocyst—the amnion and the chorion. The amnion becomes the fluid-filled amiontic sac. The chorion joins with the endometrium to form the placenta.

Stages of Pregnancy Trimesters 2 and 3: growing larger and maturing

Take-home message  The 9-month development of a human embryo and fetus is divided into three equal periods, or trimesters.  The first trimester is primarily a time of development and differentiation of cells and tissues as the embryo implants and the placenta forms.  The second and third trimesters are characterized mostly by significant growth and rapid development of the fetus’s nervous system.

25.14 Pregnancy culminates in childbirth and the start of lactation.

Fetal hormones stimulate uterine contractions that dilate the cervix and push the fetus through the birth canal.

Hormones stimulate and coordinate lactation.

Take-home message  Birth is the culmination of pregnancy and occurs in three phases. The first is the initiation of contractions and the dilation of the cervix. The second is delivery of the baby. The third is expulsion of the placenta.

 Lactation, stimulated by prolactin and other hormones, nourishes the baby with proteins, carbohydrates, and lipids and provides the infant with protective antibodies. Take-home message 25.14

Helping Sperm Meet Egg

Take-home message  Assisted reproductive technology (ART) procedures typically involve removing eggs from a woman’s ovaries, combining them with sperm to achieve fertilization, and reinserting the fertilized eggs into the woman’s uterus or Fallopian tube.  These technologies can enable previously infertile couples to have babies.