Chapter 46Animal Reproduction

Reproduction Overview Asexual (one parent; mitotic cell division)

– fission (parent separation into two individuals; sea anemone)

– budding (individuals splitting off from one; corals) – gemmules (group of cells grow into new individual;

porifera)

– fragmentation & regeneration (body parts may develop into new individual; sea star)

Sexual (fusion of haploid gametes)

– gametes (sex cells)

– zygote (fertilized egg)

– ovum (unfertilized egg)

– spermatozoon (male gamete)

Reproductive cycles Parthenogenesis

unfertilized egg development; haploid, sterile adults (honeybees)

Hermaphroditism both male & female reproductive systems; sessile & burrowing organisms (earthworms)

Sequential hermaphroditism

reversal of gender during lifetime •protogynous (female 1st)

•protandrous (male 1st)

Bluehead wrasse

Mechanisms of sexual reproduction

Fertilization (union of sperm and egg) • external

• internal

Pheromones chemical signals that influence the behavior of others (mate attractants)

Mammalian reproduction, I The Human Male

1. Testes: male gonads2. Seminiferous tubules: sperm formation3. Leydig cells: Testosterone/androgen production (next to seminiferous tubes)4. Scrotum: testes held outside body temp.5. Epididymis: 6m long; sperm development6. Vas deferens: sperm propulsion7. Seminal vesicles: 60% of semen; alkaline;

contains fructose; coagulating enzyme, and prostaglandins

8. Prostate gland: anticoagulant enzymes; citrate9. Bulbourethral glands: acid neutralizer10.Penis/urethra: semen deliveryA

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Male Reproductive Structures

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Mammalian reproduction, II

The Human Female1. Ovaries: female gonads

2. Follicle: egg capsule – one egg surrounded by layers of follicle cells, which nourish and protect the developing egg.

3. Corpus luteum: secretes estrogen and progesterone, which maintains the uterine lining during pregnancy.

4. Oviduct: fertilization

5. Uterus/endometrium: womb/lining

6. Cervix/vagina: sperm receptacle

Spermatogenesis Location: Seminiferous tubules

1. Primordial germ cell (2n) are the cells in a male embryo that will differentiate into spermatogonia (2n).2. Spermatogonium (2n): the sperm precursor; undergo meiosis to become sperm. 3. Primary spermatocyte (2n): in prophase I of meiosis.4. Secondary spermatocyte (1n): product of 1st meiotic division; undergoes 2nd meiotic division.

5. Spermatids (1n): product of 2nd meiotic division6. Sperm cells (1n): spermatids will develop flagella and is called sperm.

Oogenesis Location: Ovaries1. Primordial germ cells (2n): are the cells in a male

embryo that will differentiate into oogonia (2n).2. Oogonium (2n): cell that will undergo meiosis and

form an egg/ovum.3. Primary oocyte (2n): cell arrested in prophase I of

meiosis. The primary oocyte will remain in this state until puberty, where one primary oocyte will complete prophase I. FSH will stimulate one follicle to grow and the primary oocyte will then complete meiosis I.

4. Secondary oocyte (1n): Arrested at metaphase II until fertilization.

5. Both meiosis I and II involve unequal cytokinesis. The end product is one ovum (1n) and 3 polar bodies.

The Male Pattern

Primary sex characteristics: Controlled by androgen hormones. - Development of reproductive structures- Sperm production

Secondary sex characteristics: Controlled by androgen hormones.- Deepening of voice- Development of facial and pubic hair- Muscle growth- Aggressive behavior

The female pattern Cyclic: release of one egg at a one time during a cycle.

Called “Estrous” cycles/estrus in many mammals; if

pregnancy does not occur, the endometrium is absorbed by the body. -Estrus: females copulate during the period of ovulation

Called “Menstrual” cycle in humans and primates; if pregnancy does not occur, the endometrium is not absorbed Menstruation

Menstrual Cycle:1. Proliferation Phase: Endometrium regenerates after menstruation. 2. Secretory Phase: Endometrium continues to thicken and releases glycogen

Secretory phase Menstrual Flow Proliferation phase

Ovarian Cycle: Parallels the menstrual cycle1. follicular phase: follicle growth 2. ovulation: oocyte release 3. luteal phase: hormone release

5 Hormones coordinate the menstrual and ovarian cycles:

1. GnRH (gonadotropin-releasing hormone): secreted by the hypothalamus

2. FSH (Follicle-stimulating hormone): stimulates follicle growth; causes the follicle to secrete estrogen.

3. LH (Leuteinizing hormone): stimulates the formation of the corpus luteum.

4. Estrogen: secreted from the follicle and corpus luteum; causes the endometrium to thicken; produces the female’s secondary sex characteristics.

5. Progesterone: secreted from the corpus luteum; causes the endometrium to thicken.

Risinglevel ofestrogen keeps FSH& LH levels low

High levelsof estrogenstimulate therelease of FSH& LH

Ovulation 1 day afterLH surge

Corpus luteum secretesestrogen and progesterone

Rising levelsof estrogen &progest. inhibitFSH & LH

Estrogen/Progest. levelsdrop when corpusluteum disintegrates

Ovulation

Menopause

Menopause: The cessation of ovulation and menstruation, between ages of 46-54.

Due to: Lack of response to FSH and LH; decline in production of estrogen

Why do human females undergo menopause? Losing the ability to reproduce allows her to tend to her children and grandchildren, increasing the survival of her kin, thereby ensuring her genes are passed on successfully.

Embryonic & fetal development Gestation: pregnancy –

the condition of carrying one or more embryos.

Pregnancy occurs after conception.

Human pregnancy averages 266 days (38 weeks).

Human gestation can be divided into three trimesters.

First Trimester

Main period of organogenesis. After fertilization, the fertilized egg will divide

in a process called cleavage. Day 3 or 4: Ball of cells reaches uterus. Day 7: Blastocyst formed

Day 12: Blastocyst embeds in the endometrium; process called “implantation.” The embryo will obtain nutrients from the endometrium. Tissues grow out of the embryo and mingle with the endometrium to form the placenta (site of nutrient/gas/waste exchange).

Fourth Week: Heart beats Eighth Week: All major structures

formed; embryo now called a “fetus” Twelfth Week: Fetus is 5 cm long 1st Trimester:

-Growing fetus is sensitive to radiation & birth defects-Fetus secretes HCG (human chorionic gonadotropin) to maintain secretion of estrogen and progesteroneby the corpus luteum. Pregnancy tests test for HCG in urine. -Mother: cervical plug; growth of placenta; enlargement of uterus; cessation of ovulation; enlargement of breasts.

The Second Trimester The fetus increases in size during this trimester

– up to 30 cm. bony parts of the skeleton begin to form Fetal movements can be felt by the mother. Corpus Luteum disintegrates and placenta

secretes own progesterone. The Last Trimester

fetus increases in size – up to 50 cm. Circulatory and respiratory systems mature Fetal growth uses large parts of its mother's

protein and calcium intake Maternal antibodies pass to the fetus during

the last month (temporary immunity).

Labor In the last weeks of pregnancy, the mother will

have high levels of estrogen, which trigger the formation of oxytocin receptors on the uterus.

Oxytocin produced by the fetus and the mother’s posterior pituitary stimulates: -contractions of smooth muscle in uterus-release of prostaglandins from placenta that increase contractions Positive feedback: contractions stimulate the release of more oxytocin and prostaglandins.

The Three Stages of Labor:1. Dilation2. Expulsion3. Delivery of placenta

Birth = ParturitionLactation: 1. Unique to mammals 2. Decrease in progesterone allows for the secretion of prolactin from the anterior pituitary gland Stimulates milk production 3. Oxytocin causes the release of milk from the mammary glands

Contraception Contraception: Deliberate prevention of pregnancy Abstinence: Refrain from sexual intercourse Temporary abstinence: Rhythm method or natural family planning; no

sexual intercourse a few days before and after ovulation; pregnancy rate of 10-20%

Barrier methods:-Condom-Diaphragm-IUD (Intrauterine devices)

Birth Control pills: combination of synthetic estrogen and progestin negative feedback to stop the release of GnRH, FSH, and LH; pregnancy rate of 1%-No LH release: No ovulation-No FSH release: No follicle development

MAP: “Morning After Pill” – Birth control pills that are taken up to 3 days after intercourse; prevents implantation; 75% effective

Minipill: Releases progestin, which forms a mucus block-Norplant: under skin; effective for 5 years-DepoProvera: injection every 3 months

Contraception, continued

Sterilization: Permanent -Tubal ligation: oviducts are cauterized or ligated to prevent an egg from traveling to uterus-Vasectomy: Vas deferens is cut to prevent sperm from entering urethra

Abortion: termination of pregnancy-1/3rd of all pregnancies are spontaneously aborted or miscarried.-Mifepristone (RU486): taken within 7 weeks of pregnancy; pill that blocks progesterone from receptors in uterus; prostaglandins added for uterine contractions

Modern technologies

•Amniocentesis

•Chorionic villus sampling

•High-frequency sound waves

•Ultrasound imaging