Fertilization Fertilization is the union (fusion) of monoploid sperm nucleus (n) with a monoploid egg nucleus (n). During meiosis the chromosome number (2n) was reduced. In fertilization, the resulting diploid zygote has its species number of homologous chromosomes (2n) restored. Fertilization must occur in a moist environment.

male gamete + female gamete → zygote (n) (n) (2n)

External vs. Internal Fertilization

• External Fertilization – fertilization occurs outside the body of the female in the water. Large numbers required due to predators. Ex: fish, amphibians and other aquatic vertebrate animals.

• Internal Fertilization – the gametes fuse in the moist reproductive tract of a female. Ex: terrestrial vertebrate animals, birds, reptiles, and mammals.

Embryonic Development

• After internal or external fertilization, the zygote (fertilized egg) undergoes a series of rapid mitotic cell divisions called cleavage. Although there is very little growth in size, cleavage causes in increase in the number of cells with a decrease in the size of each cell. Once cleavage begins, the zygote becomes a developing embryo.

Stages of Development• Cleavage – a series of rapid mitotic cell divisions that

leads to the formation of a mass of cells, the morula. Additional mitotic cell divisions leads to the formation of the blastula, a single layer of cells which is a hollow ball-type structure. During cleavage, there is little or no increase in individual cell size.

• Gastrulation – one side of the blastula becomes indented forming the gastrula, which has an inner layer, the endoderm, and an outer layer, the ectoderm. The mesoderm forms between ectoderm and endoderm.

• Differentiation: Three embryonic layers differentiate and give rise to the various tissues, organs, and systems of the multicellular animal.

The nervous system and the skin originate from the ectoderm layer

The muscles, circulatory system, skeleton, excretory system, and gonads originate from the mesoderm layer.

The lining of the digestive and repiratory tracts, and portions of the liver and pancreas, originate from the endoderm.

L. variegatus zygotes, viewed from the side. A. 1-cell zygote. The fertilization envelope is visible as a large "halo" around the embryo. The arrow points to the site of sperm penetration. B. 2-cell. C. 8-cell. D. 16-cell. E. 32-cell. F

Cleavage

Gastrulation & Differentiation

• Laminins correspond to a large number of heretotrimeric glycoproteins, playing and a major role in several cell functions, including differentiation, proliferation, adhesion, and migration

Growth

• The cell number and cell size increases. Early development consists of differentiation, growth of cells, tissues and organs.

• Site of development- internal development occurs inside the female’s body while external development occurs outside the female’s body in both terrestrial and aquatic environments.

• In water: The eggs of many fish and amphibians are fertilized externally and develop externally in an aquatic environment. The survival rate is generally low, which accounts for the large number of fertilized eggs produced. The developing embryo’s food is the yolk stored in the egg.

On land : Amniotic Egg

• Eggs of birds, many reptiles, and a few mammals develop externally on a land environment after internal fertilization. Since there is a better survival rate, a somewhat fewer number of fertilized eggs are produced. The developing embryo’s source of food is the yolk.

A – Yolk

B – Shell

C – Chorion

D – Embryo

E – Amnion

F - Allantois

p. 785 textbook, Chapter 35

• Some adaptations for animals which develop externally on land are a shell which provides protection and membranes which help provide a favorable environment for embryonic development. These embryonic membranes include:

The amnion contains the amniotic fluid. This fluid provides a water environment, protects the embryo from shock, and prevents adhesion of the embryonic tissues to the shell.

The yolk sac surrounds the yolk. Blood vessels which penetrate the yolk sac transport food to the developing embryo.

The allantois functions as a respiratory membrane and a storage site for the nitrogenous waste, uric acid.

The chorion is an outer membrane surrounding the other embryonic membranes and separates them from the environment.

• Internal development involves the growth of the embryo within the body of the parent which provides nutrition and protection. Generally, a relatively high survival rate allows the production of fewer fertilized eggs.

Reproduction & Development in Human Males

• The male reproductive system performs two major functions:

1. The production of sperm cells.

2. The deposition of these cells within female reproductive tract.

p. 1007 textbook, Chapter 44

• Sperm production occurs in the testes. The testes are located in the scrotum where the temperature is 1-2 degrees Celsius cooler than normal body temperature. This provides an optimum temperature for sperm production and storage. From the testes sperm move through the sperm duct and urethra. The urethra is a tube contained within the penis. The penis is a structural adaptation for internal fertilization.

• Glands secrete a liquid into these tubes. The liquid serves as a transport medium for the sperm which is and adaptation for life on land. This liquid and the sperm cells constitute semen. *Secretions from prostate and bulbourethral glands also secrete alkaline fluid that neutralize acids in female reproductive tract.

• In addition to producing sperm, the testes also produce the male sex hormone, testosterone. This regulates the maturation of sperm and the development of secondary characteristics, such as beard development and the lower voice pitch.

Reproduction & Development in Human Females

• Ovaries are paired structures located within the lower portion of the body cavity. Ovaries produce eggs (ova) in tiny cavities called follicles. Following ovulation (release of the egg from the ovary), the egg cell is transported through an oviduct (Fallopian tube) to the uterus. At the lower end of the uterus, known as the cervix, is a muscular tube, the vagina. The vagina receives the sperm from the male and is also the birth canal.

• At the birth of a female, all of the potential eggs that she will ovulate are present in immature form. Usually only 1 egg is released at the time of each ovulation.

• In addition to eggs, the ovaries also produce the female sex hormones estrogen and progesterone. These regulate the development of secondary sex characteristics such as the development of the mammary glands and the broadening of the pelvis. The hormones also have coordinating role in the menstrual cycle.

Menstrual Cycle

• The menstrual cycle begins at puberty and ceases at menopause. Menopause is the permanent cessation of the menstrual cycle. The duration of this cycle is approximately 28 days but may vary considerably and may be interrupted by pregnancy, illness, or other factors.

• There are four stages.

Four stages of Menstrual Cycle:

• Follicle Stage – involves the maturation of an egg within the follicle and the secretion of the hormone estrogen, which initiates thickening blood tissue in uterine lining.

• Ovulation – the release of an egg from the follicle.• Corpus Luteum Stage – forms from the follicle following

ovulation. It secretes progesterone which enhances the thickening of the uterine lining.

• Menstruation – periodic shedding of the thickened uterine lining. It lasts for a few days and occurs when fertilization does not take place.

Roles of Hormones in the Menstrual Cycle

• The reproductive cycle of the human female involves the interaction of the hormones from the hypothalamus, pituitary gland, and ovaries. During the menstrual cycle, under the influence of the hormones from the hypothalamus, the pituitary gland releases hormones (FSH and LH) which influence the functioning of the ovaries. FSH stimulates follicle growth and the ovary to produce estrogen. LH stimulates the growth of the corpus luteum (in the ovary) to secrete progesterone which inhibits estrogen production. The ovaries, in turn, secrete estrogen and progesterone which produce changes in the uterus. Estrogen causes the release of FSH to be stopped. And increase in progesterone causes the reduction in the production of FSH and LH. In addition, the hormones from the ovaries regulate the secretion of hormones by the pituitary gland and the hypothalamus. This is an example of a negative feedback mechanism.

Fertilization• Fertilization usually occurs in the upper one third of the oviduct. If

the egg is not fertilized within approximately 24 hours after ovulation, it deteriorates. If fertilization occurs, cleavage of the fertilized egg (zygote) begins in the oviduct, and six to ten days later, the resulting embryo may become implanted in the uterine lining. After this stage of development, the yolk of the egg has been depleted. But with implantation, the embryo can obtain nutrients from the uterus.

• Identical twins develop from one zygote which separates during cleavage. If more than one egg is released and fertilized, multiple births may result. Fraternal twins develop from two eggs, each fertilized by separate sperm cells.

• In vitro fertilization is commonly called test tube babies.

Prenatal Development• Prenatal development includes

the following processes: Cleavage – occurs in the

oviduct. Gastrulation – occurs after the

embryo is implanted in the uterus.

Differentiation and Growth – result in the formation of specialized tissues and organs.

• The placenta is the connection between the mother and the developing embryo which allows nutrients, respiratory gases, and wastes to be exchanged. Since there is no direct connection between the embryo’s blood and the mother’s, substances move between them by active and passive transport.

• The amnion is the membrane that surrounds the embryo and contains amniotic fluid which protects the developing embryo from shock and temperature changes.

Placental Development

• The umbilical cord holds the blood vessels that carry materials between the embryo and the placenta.

• Birth usually occurs after a gestation period of approximately nine months. During birth, strong contractions of the uterine muscles forces the baby through the cervix and vagina to the outside of the mother’s body. The expulsion of the placenta occurs after the delivery of the baby.

• And now for the rest of development... Here come the organs! Week 4: How you looked at 1 month past conception....

• Week 4 Week 4-8 is when all the major organ systems of the body are formed.Week 7 / Day 48: First brain waves can be detected

Week 8

• Week 9 - Week 40: (Fetus):• After 12 weeks or so, the baby's development is

largely "finished" - except brain and lung development

• The fetus just spends much of the 2nd and 3rd trimesters just growing (and doing various flip-turns and kicks inside the amniotic fluid)

Week 16

And then, before you know it...

Week 20 - Love that thumb!

Week 36 - 40 - Partruition....