(Bio 105) Chapter 18 Animal Reproduction and Development

Contractions of the smooth muscle in the uterus aid in forcing the baby through the vagina during labor.

Contractions

Mammalian sex is determined genetically by the combination of X and Y chromosomes. -Individuals homozygous for X (XX) are female and heterozygous individuals (XY) are male. -In mammals, the presence of a Y chromosome causes the development of male characteristics and its absence results in female characteristics. The XY system is also found in some insects and plants.

X, Y, female, male

Asexual reproduction produces offspring that are genetically identical to the parent because the offspring are all clones of the original parent.

identical, clones

The regulation occurs through signaling between cells and tissues and responses in the form of differential gene expression.

signaling

In land mammals, including humans, the pair of testes must be suspended outside the body so the environment of the sperm is about 2 °C lower than body temperature to produce viable sperm. -If the testes do not descend through the abdominal cavity during fetal development, the individual has reduced fertility.

2, descend

An ejaculate will contain from two to five milliliters of fluid and from 50-120 million sperm per milliliter.

50-120 million

The reproductive tissues of male and female humans develop similarly in utero until about the seventh week of gestation when a low level of the hormone testosterone is released from the gonads of the developing male. -Testosterone causes the primitive gonads to differentiate into male sexual organs.

7th, testosterone

Fission, also called binary fission, occurs in some invertebrate, multi-celled organisms. -It is in some ways analogous to the process of binary fission of single-celled prokaryotic organisms. -The term fission is applied to instances in which an organism appears to split itself into two parts and, if necessary, regenerate the missing parts of each new organism.•

Fission, analogous, split

Hermaphroditism occurs in animals in which one individual has both male and female reproductive systems.

Hermaphroditism

Parthenogenesis is a form of asexual reproduction in which an egg develops into an individual without being fertilized. -The resulting offspring can be either haploid or diploid, depending on the process in the species.

Parthenogenesis, haploid, diploid

The sperm cells are associated with Sertoli cells that nourish and promote the development of the sperm. -Other cells present between the walls of the tubules are the interstitial cells of Leydig, which produce testosterone once the male reaches adolescence.

Sertoli, Leydig

Fragmentation may occur through accidental damage, damage from predators, or as a natural form of reproduction.

accidental damage, damage from predators, or as a natural form of reproduction

An acrosome (acrosomal vesicle) is found at the top of the head of the sperm. -This structure contains enzymes that can digest the protective coverings that surround the egg and allow the sperm to fuse with the egg.

acrosome, digest

Some animals produce offspring through asexual reproduction while other animals produce offspring through sexual reproduction. Both methods have advantages and disadvantages.

advantages, disadvantages

External fertilization usually occurs in aquatic environments where both eggs and sperm are released into the water. -After the sperm reaches the egg, fertilization takes place.

aquatic

Asexual reproduction produces genetically identical offspring (clones), whereas in sexual reproduction, the genetic material of two individuals combines to produce offspring that are genetically different from their parents.

asexual, sexual

During differentiation, the embryonic stem cells express specific sets of genes that will determine their ultimate cell type. -For example, some cells in the ectoderm will express the genes specific to skin cells. -As a result, these cells will take on the shape and characteristics of epidermal cells.

cell type, skin, epidermal

Sexual reproduction is the combination of reproductive cells from two individuals to form genetically unique offspring.

combination

Organs develop from the germ layers through the process of differentiation.

differentiation

Some of these species live in closely coordinated schools with a dominant male and a large number of smaller females. -If the male dies, a female increases in size, changes sex, and becomes the new dominant male.

dominant, smaller

Sperm are haploid cells, consisting of a flagellum for motility, a neck that contains the cell's energy-producing mitochondria, and a head that contains the genetic material.

flagellum, neck, head

The uterus supports the developing embryo and fetus during gestation.

gestation

Only half the population (females) can produce the offspring, so fewer offspring will be produced when compared to asexual reproduction. -This is a disadvantage of sexual reproduction compared to asexual reproduction.

half

Ants, bees, and wasps use parthenogenesis to produce haploid males (drones). -The diploid females (workers and queens) are the result of a fertilized egg.

haploid, diploid

Sperm are immobile at body temperature; therefore, the testes are external to the body so that a correct temperature is maintained for motility.

immobile

In some crocodiles and some turtles, moderate temperatures produce males and both warm and cool temperatures produce females.

moderate, warm, cool

The secretions from the accessory glands provide important compounds for the sperm including nutrients, electrolytes, and pH buffering. -There are also coagulation factors that affect sperm delivery and motility.

nutrients, electrolytes, pH buffering

Internal female reproductive structures include ovaries, oviducts, the uterus, and the vagina. -The pair of ovaries is held in place in the abdominal cavity by a system of ligaments. -The outermost layer of the ovary is made up of follicles, each consisting of one or more follicular cells that surround, nourish, and protect a single egg.

ovaries, oviducts, uterus, vagina, follicles

The penis drains urine from the urinary bladder and is a copulatory organ during intercourse.

penis

During the menstrual period, a batch of follicular cells develops and prepares their eggs for release.

prepares

Asexual reproduction occurs in prokaryotic microorganisms (bacteria and archaea) and in many eukaryotic, single-celled and multi-celled organisms. -There are several ways that animals reproduce asexually, the details of which vary among individual species.

prokaryotic, eukaryotic

Individuals of some species change their sex during their lives, switching from one to the other. -If the individual is female first, it is termed protogyny or "first female," if it is male first, it is termed protandry or "first male." -Oysters are born male, grow in size, and become female and lay eggs.

protogyny, protandry

The process by which an organism develops from a single-celled zygote to a multi-cellular organism is complex and well regulated.

regulated

At ovulation, one follicle ruptures and one egg is released.

ruptures

The scrotum houses the testicles or testes (singular: testis), and provides passage for blood vessels, nerves, and muscles related to testicular function. -The testes are a pair of male gonads that produce sperm and reproductive hormones.

scrotum, testes

Sea anemones (Cnidaria), such as species of the genus Anthopleura, will divide along the oral-aboral axis, and sea cucumbers (Echinodermata) of the genus Holothuria, will divide into two halves across the oral-aboral axis and regenerate the other half in each of the resulting individuals.

sea anemones, sea cucumbers

Hermaphrodites may self-fertilize, but typically they will mate with another of their species, fertilizing each other and both producing offspring. -Self-fertilization is more common in animals that have limited mobility or are not mobile, such as barnacles and clams. -Many species have specific mechanisms in place to prevent self-fertilization, because it is an extreme form of inbreeding and usually produces less fit offspring.

self- fertilize, inbreeding

Semen is a mixture of sperm (about five percent of the total) and fluids from accessory glands that contribute most of the semen's volume.

semen

The wrasses, a family of reef fishes, are all sequential hermaphrodites.

sequential hermaphrodites

A single individual can produce offspring asexually and large numbers of offspring can be produced quickly; these are two advantages that asexually reproducing organisms have over sexually reproducing organisms.

single, quickly

In a stable or predictable environment, asexual reproduction is an effective means of reproduction because all the offspring will be adapted to that environment.

stable, predictable

More complicated chromosomal sex determining systems also exist. -For example, some swordtail fish have three sex chromosomes in a population.

three

Sterilization in women is called a tubal ligation; it is analogous to a vasectomy in males in that the oviducts are severed and sealed, preventing sperm from reaching the egg.

tubal ligation

Fisheries workers have been known to try to kill the sea stars eating their clam or oyster beds by cutting them in half and throwing them back into the ocean. -Unfortunately for the workers, the two parts can each regenerate a new half, resulting in twice as many sea stars to prey upon the oysters and clams.

twice

Some vertebrate animals—such as certain reptiles, amphibians, and fish—also reproduce through parthenogenesis.

vertebrate

In viviparity, the young are born alive. -They obtain their nourishment from the female and are born in varying states of maturity. -This occurs in most mammals, some cartilaginous fish, and a few reptiles.

viviparity

Fragmentation is the breaking of an individual into parts followed by regeneration. -If the animal is capable of fragmentation, and the parts are big enough, a separate individual will regrow from each part.

Fragmentation

Bird sex determination is dependent on the combination of Z and W chromosomes. -Homozygous for Z (ZZ) results in a male and heterozygous (ZW) results in a female. -Notice that this system is the opposite of the mammalian system because in birds the female is the sex with the different sex chromosomes. -The W appears to be essential in determining the sex of the individual, similar to the Y chromosome in mammals. -Some fish, crustaceans, insects (such as butterflies and moths), and reptiles use the ZW system.

Z, W, male, female

At the tip of the head of a sperm cell is a structure like a lysosome called the acrosome, which contains enzymes. -When a sperm binds to the zona pellucida, a series of events, called the acrosomal reactions, take place. -These reactions, involving enzymes from the acrosome, allow the sperm plasma membrane to fuse with the egg plasma membrane and permit the sperm nucleus to transfer into the ovum.

acrosome, acrosomal reactions

In mammals, the blastula forms the blastocyst in the next stage of development. Here the cells in the blastula arrange themselves in two layers: the inner cell mass, and an outer layer called the trophoblast. -The inner cell mass will go on to form the embryo. -The trophoblast secretes enzymes that allow implantation of the blastocyst into the endometrium of the uterus. -The trophoblast will contribute to the placenta and nourish the embryo.

blastocyst, trophoblast

A number of female reproductive structures are exterior to the body. These include the breasts and the vulva, which consists of the mons pubis, clitoris, labia majora, labia minora, and the vestibular glands. -The breasts consist of mammary glands and fat. -Each gland consists of 15 to 25 lobes that have ducts that empty at the nipple and that supply the nursing child with nutrient- and antibody-rich milk to aid development and protect the child.

breasts, vulva, mammary glands

Following a mating dance, the female releases eggs into the male seahorse's abdominal brood pouch and the male releases sperm into the water, which then find their way into the brood pouch to fertilize the eggs. The fertilized eggs develop in the pouch for several weeks.

broad pouch

Budding is a form of asexual reproduction that results from the outgrowth of a part of the body leading to a separation of the "bud" from the original organism and the formation of two individuals, one smaller than the other. -Budding occurs commonly in some invertebrate animals such as hydras and corals. -In hydras, a bud forms that develops into an adult and breaks away from the main body.

budding, hydras, corals

Part of the uterus, called the cervix, protrudes into the top of the vagina.

cervix

Most non-avian reptiles and insects produce leathery eggs, while birds and some turtles produce eggs with high concentrations of calcium carbonate in the shell, making them hard. -Chicken eggs are an example of a hard shell.

chicken eggs

When an egg is released at ovulation, the fimbrae help the nonmotile egg enter into the tube. -The walls of the oviducts have a ciliated epithelium over smooth muscle. -The cilia beat, and the smooth muscle contracts, moving the egg toward the uterus.

ciliated epithelium, cilia

The development of multi-cellular organisms begins from this single-celled zygote, which undergoes rapid cell division, called cleavage, to form a hollow ball of cells called a blastula. During cleavage, the zygote rapidly divides into multiple cells. -The cells rearrange themselves to form a hollow ball called the blastula

cleavage, blastula

During sexual reproduction, the genetic material of two individuals is combined to produce genetically diverse offspring that differ from their parents. -The genetic diversity of sexually produced offspring is thought to give sexually reproducing individuals greater fitness because more of their offspring may survive and reproduce in an unpredictable or changing environment.

combined, fitness

Following ovulation, the follicular tissue that surrounded the ovulated egg stays within the ovary and grows to form a solid mass called the corpus luteum. -The corpus luteum secretes additional estrogen and the hormone progesterone that helps maintain the uterine lining during pregnancy. -The ovaries also produce hormones, such as estrogen.

corpus luteum, hormones

Internal fertilization may occur by the male directly depositing sperm in the female during mating. -It may also occur by the male depositing sperm in the environment, usually in a protective structure, which a female picks up to deposit the sperm in her reproductive tract.

depositing

Invertebrates such as earthworms, slugs, tapeworms, and snails are often hermaphroditic.

earthworms, slugs, tapeworms, snails

Cells in each germ layer differentiate into tissues and embryonic organs. -The ectoderm gives rise to the nervous system and the epidermis, among other tissues. The mesoderm gives rise to the muscle cells and connective tissue in the body. The endoderm gives rise to the gut and many internal organs.

ectoderm, mesoderm, endoderm

During orgasm, the accessory organs and glands connected to the testes contract and empty the semen (containing sperm) into the urethra and the fluid is expelled from the body by muscular contractions causing ejaculation. -After intercourse, the blood drains from the erectile tissue and the penis becomes flaccid.

ejaculation

The sex of some other species is not determined by chromosomes, but by some aspect of the environment.

environment

When the sperm have developed flagella they leave the seminiferous tubules and enter the epididymis. -This structure lies along the top and posterior of the testes and is the site of sperm maturation.

epididymis

The penis contains three tubes of erectile tissue that become engorged with blood, making the penis erect, in preparation for intercourse. -The organ is inserted into the vagina culminating with an ejaculation.

erect

The oviducts, or fallopian tubes, extend from the uterus in the lower abdominal cavity to the ovaries, but they are not in contact with the ovaries. -The lateral ends of the oviducts flare out into a trumpet-like structure and have a fringe of finger-like projections called fimbrae.

fallopian tubes, fimbrae

Fertilization is the process in which gametes (an egg and sperm) fuse to form a zygote. -To ensure that the offspring has only one complete diploid set of chromosomes, only one sperm must fuse with one egg.

fertilization

The fusion of a sperm and an egg is a process called fertilization. -This can occur either inside (internal fertilization) or outside (external fertilization) the body of the female. -Humans provide an example of the former, whereas frog reproduction is an example of the latter.

fertilization, internal, external

Nearly all fish spawn, as do crustaceans (such as crabs and shrimp), mollusks (such as oysters), squid, and echinoderms (such as sea urchins and sea cucumbers).

fish

The nuclear membranes of the egg and sperm break down and the two haploid nuclei fuse to form a diploid nucleus or genome. -To ensure that no more than one sperm fertilizes the egg, once the acrosomal reactions take place at one location of the egg membrane, the egg releases proteins in other locations to prevent other sperm from fusing with the egg.

fuse, proteins

During gastrulation, the blastula folds in on itself and cells migrate to form the three layers of cells in a structure, the gastrula, with a hollow space that will become the digestive tract. Each of the layers of cells is called a germ layer and will differentiate into different organ systems. -The three germ layers are the endoderm, the ectoderm, and the mesoderm.

gastrula, germ layer(s)

The cells in the blastula then rearrange themselves spatially to form three layers of cells. -This process is called gastrulation.

gastrulation

In ovoviparity, fertilized eggs are retained in the female, and the embryo obtains its nourishment from the egg's yolk. -The eggs are retained in the female's body until they hatch inside of her, or she lays the eggs right before they hatch. -This process helps protect the eggs until hatching. -This occurs in some bony fish (like the platyfish Xiphophorus maculatus), some sharks, lizards, some snakes (garter snake Thamnophis sirtalis), some vipers, and some invertebrate animals (Madagascar hissing cockroach Gromphadorhina portentosa).

hatch

As in all animals, the adaptations for reproduction in humans are complex. -They involve specialized and different anatomies in the two sexes, a hormone regulation system, and specialized behaviors regulated by the brain and endocrine system.

hormone regulation system, brain, endocrine

The process of differentiation is regulated by location-specific chemical signals from the cell's embryonic environment that sets in play a cascade of events that regulates gene expression.

location specific chemical signals

Female seahorses produce eggs that are then fertilized by the male. Unlike with almost all other animals, the young then develop in a pouch of the male seahorse until birth.

male

Species that reproduce sexually (and have separate sexes) must maintain two different types of individuals, males and females.

males, females

Gastrulation leads to the formation of the three germ layers that give rise during further development to the different organs in the animal body. -This process is called organogenesis.

organogenesis

When testosterone is absent, the primitive gonads develop into ovaries. -Tissues that produce a penis in males produce a clitoris in females. -The tissue that will become the scrotum in a male becomes the labia in a female. -Thus the male and female anatomies arise from a divergence in the development of what were once common embryonic structures.

ovaries, penis, clitoris, scrotum, labia

In oviparity, fertilized eggs are laid outside the female's body and develop there, receiving nourishment from the yolk that is a part of the egg. -This occurs in some bony fish, some reptiles, a few cartilaginous fish, some amphibians, a few mammals, and all birds.

oviparity

Sperm form in the walls of seminiferous tubules that are coiled inside the testes. -The walls of the seminiferous tubules are made up of the developing sperm cells, with the least developed sperm at the periphery of the tubule and the fully developed sperm next to the lumen.

periphery, lumen

For example, species of turbellarian flatworms commonly called the planarians, such as Dugesia dorotocephala, are able to separate their bodies into head and tail regions and then regenerate the missing half in each of the two new organisms.

planarians

The bulk of the semen comes from the accessory glands associated with the male reproductive system. -These are the seminal vesicles, the prostate gland, and the bulbourethral gland.

seminal vesicles, prostate gland, bulbourethral gland

Each testis is approximately 2.5 by 3.8 cm (1.5 by 1 inch) in size and divided into wedge-shaped lobes by septa. -Coiled in each wedge are seminiferous tubules that produce sperm.

seminiferous tubules

Parthenogenesis has been observed in species in which the sexes were separated in terrestrial or marine zoos. -Two female Komodo dragons, a hammerhead shark, and a blacktop shark have produced parthenogenic young when the females have been isolated from males. -It is possible that the asexual reproduction observed occurred in response to unusual circumstances and would normally not occur.

separated, isolated

If fertilization does not occur, a portion of the lining of the uterus sloughs off during each menstrual period. -The endometrium builds up again in preparation for implantation.

sloughs off

Most external fertilization happens during the process of spawning where one or several females release their eggs and the male(s) release sperm in the same area, at the same time. -The spawning may be triggered by environmental signals, such as water temperature or the length of daylight.

spawning

During sexual reproduction the male gamete (sperm) may be placed inside the female's body for internal fertilization, the sperm may be left in the environment for the female to pick up and place in her body, or both sperm and eggs may be released into the environment for external fertilization.

sperm, eggs

Reproduction through fragmentation is observed in sponges, some cnidarians, turbellarians, echinoderms, and annelids. -In some sea stars, a new individual can be regenerated from a broken arm and a piece of the central disc.

sponges, cnidarians, turbellarians, echinoderms, annelids

Sex determination in alligators, some turtles, and tuataras, for example, is dependent on the temperature during the middle third of egg development. -This is referred to as environmental sex determination, or more specifically, as temperature-dependent sex determination. -In many turtles, cooler temperatures during egg incubation produce males and warm temperatures produce females, while in many other species of turtles, the reverse is true.

temperature, environmental

Internal fertilization occurs most often in terrestrial animals, although some aquatic animals also use this method.

terrestrial

The nature of the individuals that produce the two kinds of gametes can vary, having for example separate sexes or both sexes in each individual. Sex determination, the mechanism that determines which sex an individual develops into, also can vary.

two, sex determination

In an unstable or unpredictable environment, species that reproduce asexually may be at a disadvantage because all the offspring are genetically identical and may not be adapted to different conditions.

unstable, unpredictable

-The uterus is a structure about the size of a woman's fist. -The uterus has a thick muscular wall and is lined with an endometrium rich in blood vessels and mucus glands that develop and thicken during the female cycle. -Thickening of the endometrium prepares the uterus to receive the fertilized egg or zygote, which will then implant itself in the endometrium.

uterus, endometrium

The vagina is a muscular tube that serves several purposes. It allows menstrual flow to leave the body. -It is the receptacle for the penis during intercourse and the pathway for the delivery of offspring.

vagina

The sperm leave the epididymis and enter the vas deferens, which carries the sperm behind the bladder, and forms the ejaculatory duct with the duct from the seminal vesicles. -During a vasectomy, a section of the vas deferens is removed, preventing sperm (but not the secretions of the accessory glands) from being passed out of the body during ejaculation and preventing fertilization.

vas deferens, vasectomy

Parthenogenesis occurs in invertebrates such as water fleas, rotifers, aphids, stick insects, and ants, wasps, and bees.

water fleas, rotifers, aphids, stick insects, ants, wasps, bees

Fertilization usually takes place within the oviduct and the developing embryo is moved toward the uterus. -It usually takes the egg or embryo a week to travel through the oviduct.

week

In mammals, a layer called the zona pellucida protects the egg.

zona pellucida