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Lecture. Reproduction & Development. Sex in the animal kingdo m. both sexual and asexual sexual – fusion of haploid gametes to form a diploid zygote gametes – egg and sperm zygote can itself give rise to gametes via meiosis zygote develops into an embryo via mitosis

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Reproduction & Development

Sex in the animal kingdo m

Sex in the animal kingdom

  • both sexual and asexual

  • sexual – fusion of haploid gametes to form a diploid zygote

    • gametes – egg and sperm

    • zygote can itself give rise to gametes via meiosis

    • zygote develops into an embryo via mitosis

  • asexual – generation of a new individual without the fusion of gametes

    • relies upon mitotic division

Asexual reproduction

Asexual reproduction

  • mechanisms:

  • 1. fission

    • separation of the parent into two equal sized individuals

  • 2. budding

    • new individuals arise as outgrowths from the parent

  • 3. fragmentation & regeneration

    • breaking of the body into several pieces

  • 4. parthenogenesis

    • egg develops without being fertilized

    • progeny is either haploid or diploid

Sexual reproduction

Sexual reproduction

  • sexual reproduction must enhance reproductive success

  • if not successful – it would not have evolved because asexual reproduction produces more individuals

  • advantages of sex?

    • diversitydue to recombination of parental genes during meiosis and fertilization

  • sexual reproduction is advantageous when environmental conditions change rapidly and frequently

Reproductive cycles

Reproductive Cycles

  • most animals’ reproductive cycles coincide with the change of seasons

    • give birth when resources are plentiful and conditions favor the survival of their offspring

  • ovulation – mid-point in the reproductive cycle corresponding to the release of eggs

  • reproductive cycles not just restricted to animals who reproduce sexually

  • asexual reproduction cycles – some fishes, amphibians and reptiles can undergo periods of parthenogenesis

    • double their chromosome number to produce diploid offspring

  • reproductive cycles found in animals that have sexual and asexual cycles

Mammalian reproductive cycles

Mammalian Reproductive Cycles

  • most females undergo estrus = time during which the female is behaviorally and physiologically receptive to the male (fertile egg is produced)

    • complex series of hormonal changes that affect egg production, uterine and vaginal environments and sperm production

  • many mammals are monoestrus

    • only once a year

    • strictly regulated by the environment

    • e.g. wild dogs, bears and sea lions

    • e.g. domestic dogs – biestrus

    • other animals have cycles that can repeat on a cyclical basis

      • e.g. rats – every 4 to 6 days



  • happens when finding a sexual partner is difficult

  • rather common in sessile, aquatic animals and in parasites

  • both male and female reproductive organs (monoecious)

  • this means any two individuals can mate

    • one acts as a male, one acts as a female

  • some can self-fertilize



  • union of egg and sperm

  • can either be: external or internal

  • external fertilization requires a moist habitat

    • prevents dessication of gametes

    • also allows the sperm to swim to the egg

    • clustering of animals in the same area for fertilization = spawning

    • when not synchronized within a population – mating pairs will undergo courtship behaviors to stimulate release of gametes

    • e.g. frogs – external fertilization with courtship behavior – locates breeding sites and potential mates

Internal fertilization

Internal Fertilization

  • sperm deposited in or near the female reproductive tract

    • male copulatory organ deposits sperm

    • female can have a seminal receptacle for receipt and storage of sperm – then delivers it to the egg when needed

    • e.g. sharks – modified pelvic fins called claspers that are inserted into the female cloaca for sperm transfer

      • followed by internal fertilization – thus ensuring success

      • some sharks are viviparous – give birth to live young

  • fertilization occurs within the female’s reproductive tract

    • usually followed by internal embryonic development

  • evolved because of dry, terrestrial habitats

  • requires cooperative behavior

  • requires compatible reproductive structures

  • fewer gametes produced vs. external fertilization because the chances of successful fertilization are higher


  • mammalian fertilization:

    • in a few mammals – fertilization is delayed after coitus (delayed fertilization)

      • e.g. bats – coitus in fall, fertilization in spring

    • most mammals have direct fertilization after coitus

      • but development of the embryo can be arrested for one to two weeks = embryonic diapause

      • e.g. sea lions, bears and marsupials

      • may be due to presence of resources – allows mother time to feed before having to nurse

      • also allows young to be born during a time of good resources

Care of offspring

Care of Offspring

  • internal fertilization and gestation ensures better protection of offspring

    • against desiccation and physical damage

  • internal gestation done by placental mammals = eutherians

    • receive nourishment from the mother through the placenta

  • internal gestation for long periods of time is not necessary

    • marsupials – embryos crawl out of the uterus and complete development attached to a mammary gland located in a pouch

Care of offspring1

Care of Offspring

  • alternative to internal gestation and development: development of the amniotic egg

    • egg laying animals – monotremes

    • birds and reptiles

    • protection of the developing embryo by a protective layer of calcium and proteins in the form of the shell

    • eggs of fish and amphibians are quite different – surrounded by a gelatinous material and they lack internal membranes

Gamete production

Gamete production

  • gametes= egg and sperm

  • develop from a set of precursor cells called germ cells

    • develop during embryonic development and remain dormant until sexual maturity

      • e.g. humans

  • typical site for these germ cells and gamete production are thegonads

    • not required in simpler animals

  • more complex reproductive systems also have accessory organs in association with the gonads

    • seminal vesicles, prostate gland

    • seminal receptacles or spermathecae (in insects)

Reproductive adaptations

Reproductive adaptations

  • same basic plan for all vertebrates

  • in many non-mammalian vertebrates – formation of a cloaca

    • combined opening for digestive, urinary and reproductive tracts

    • female mammals have separate openings for all three

    • male mammals combine the urinary and reproductive opening

  • many vertebrates have a uterus that is divided into two chambers

  • in mammals that produce only a few offspring at a time – uterus is a single structure

  • differences in the male reproductive system – in the copulatory organs

    • human penis is quite distinct from other mammals

Male reproductive system

Male reproductive system

  • three accessory organs

    • seminal vesicle

    • prostate

    • bulbourethral glands (Cowper glands)

  • reproductive ducts

    • epididymus

    • vas deferens (ductus deferens)

    • urethra


Reproductive Ducts

-vas deferens: conducting tube from testis to urethra

-epididymus – connection between the testis and the vas deferens

-stores immature sperm

-vas deferens + blood vessels + nerve = spermatic cord (passes through inguinal canal)

-urethra:3 sections:

A. prostatic - runs through prostate

B. membranous - between prostate and penis

C. spongy - through penis

Reproductive glands

Reproductive glands


  • seminal vesicles, prostate, bulbourethral glands

  • -produce fluid that combine with sperm to make semen

  • -semen: alkaline, activates sperm cells

  • 1. prostate: surrounds the urethra

  • -secretes a thin, milky fluid that enhances sperm motility and neutralizes vaginal fluid

  • 2. seminal vesicles: connect to urethra via the ejaculatory ducts

  • -secretes an alkaline fluid that contains sugars and prostaglandins (stimulates uterine contractions)

  • 3. bulbourethral glands: 2 glands behind the prostate

  • -secrete a fluid that lubricates the penis


















Urethral Orifice


  • scrotum = supportive structure for the testes

  • hangs from the root of the penis

  • externally- single pouch separated at the midline by a raphe

  • internally – divided by a scrotal septum into two sacs each containing 1 testis

  • the scrotum contains two muscles

    • dartos muscle (smooth muscle) & the cremaster muscle – skeletal muscle that is a continuation of the internal oblique






testicular artery

testicular artery





Tunica Albuginea

Tunica Vaginalis

-testis: develop internally near the kidneys and descend during the latter half of the seventh month gestation

-covered by two protective membranes:

1. tunica vaginalis –derived from the peritoneum

2. tunica albuginea – internal to the TV

-extends inward to divide the testes into lobules (200-300)

-each lobule contains 1 to 3 coiled seminiferous tubules for sperm production

- lined with spermatogenic cells that produce sperm

-contain sperm stem cells = spermatogonium


-testis:also contain two additional cell types:

-Sertoli cells = sustenacular cells

-found in the seminiferous tubule

-secretion of growth factors that mediate spermatogenesis

-interstitial cells – located between the seminiferous tubules

-for testosterone production



  • sperm development – from sperm stem cells called spermatogonium

  • these spermatogonium develop in the embryonic testes

  • the spermatogonium remain dormant in the testes until puberty

  • mature sperm cells = spermatozoa

  • 1. differentiation of the spermatogonium into primary spermatocytes (2n)

  • 2. start of meiosis

  • 3. formation of secondary spermatocytes (n) – 23 chromosomes comprised of two chromatids

  • 4. completion of meiosis and formation of four spermatids (23 chromosomes each made up of one chromatid)

  • 5. spermiogenesis– development of spermatids into a sperm cell (head, acrosome and flagella development)



  • 300 million made each day

  • 60 um long

  • major parts

    • 1. head: contains the nucleus with 23 highly condensed chromosomes (one chromatid)

    • 2. acrosome:covers the anterior 2/3 of the head

      • contains digestive enzymes to dissolve the protective barriers of the egg (hyaluronidase and proteases)

    • 3. tail or flagellum

      • neck - constricted region just behind the head

      • iddle piece – contains mitochondria arranged in a spiral

      • principal piece – longest portion of the tail

      • end piece – terminal portion of the tail






made up of a:


body (or shaft)


-conveys urine and semen

-body is found externally, root is found internally (attached to the pubic ramus)

-body is comprised of two tissue types: erectile tissue surrounded by connective tissue

A. corpus cavernosum– larger spaces

B. corpus spongiosum - smaller spaces & surrounds the urethra

-corpus spongiosumenlargens at the tip - glans penis (sensory receptors)

-glans penis covered with a loose fold of skin = prepuce


-release of Gonadotropic releasing hormone(GnRH) from the hypothalamus

which stimulates the anterior pituitary gland

-anterior pituitary releases two gonadotropins (FSH and LH)

1. Follicle stimulating hormone - stimulates spermatogenesis

2. Leutinizing hormone - stimulates male hormone production

-male hormones: androgens (e.g. testosterone)

-testosterone: secondary sex characteristics, reproductive organ development and maintenance

-rise in testosterone causes a negative feedback: hypothalamus release of GnRH is inhibited


-uterus: receives and nourishes the embryo

-comprised of a body, a curved portion (fundus) and the cervix

-uterine wall outer perimetrium, muscular myometrium and inner


-endometrium: mucosal layer covered with epithelium

-rich blood supply, sloughed off during menstruation

-comprised of a deeper basal layer and an outer functional layer







-uterine tubes or oviducts (Fallopian tubes): conduction of egg from ovary to uterus

-expands at end near the ovary = infundibulum withfimbrae (fingers) for the “catching”

of the released egg

-fertilization occurs in the uterine tubes

-cervix: projects into the vaginal canal

-vaginal canal broadens to surround the cervix – fornix

-site for the insertion of the penis and deposition of sperm

-opens at the exterior as the vulva (female external genitalia)


Vaginal canal



urethral orifice

Labia minora

Vaginal orifice

Labia majora


-known as the vulva

-mons pubis – mass of adipose tissue that lays on top of the pubic bone

-mons pubis divides into the labia majora and minora

-labia majora: folds of adipose tissue that enclose and protect the labia minora

-labia minora: surrounds the external urethral orifice and the vaginal orifice

-clitoris: erectile tissue with a glans, body and a root


-ovary: production of oocytes

-inner medulla and outer cortex

-medulla - connective tissue with blood & lymphatic vessels and nerves

-cortex - granular tissue due to the presence of tiny ovarian follicles

containing egg stem cells (oogonium)


-ovarian follicles mature and contain the developing oocyte = oogenesis

1. growth of primary follicles into secondary folliclescontaining a primary oocyte

2. secondary follicle matures -> vesicular /tertiary follicle with a large secondary oocyte (for ovulation)

3. ruptured follicle degenerates into a corpus hemorrhagicumand then into a corpus luteum female sex hormone production (estrogen & progesterone)



  • early fetal development – primordial germ cells in the developing ovaries differentiate to form oogonia (egg stem cells)

  • at birth: meiosis in the oogonia results in the development of primary follicles containing primary oocytes– stops at prophase I of meiosis

  • at puberty - release of FSH and LH each month causes the development of a primary oocyte into a secondary oocyte followed by ovulation

  • coincides with the development of the primary follicle (primary oocyte) into a secondary follicle (primary oocyte) and finally into a tertiary follicle (secondary oocyte)

  • tertiary follicle – ovulation of secondary oocyte

  • fertilization is required for the completion of meiosis II

  • therefore during oogenesis, ONE oocyte merges with the sperm and THREE polar bodies form


-GnRH causes release of FSH and LH from anterior pituitary

-FSH causes maturation of follicles

-LHresults in development of corpus luteum

-corpus luteum produces and releases estrogen and progesterone

-estrogen and progesterone: regulate pregnancy, menstruation, secondary sex characteristics & development of sex organs at puberty


-divided into two cycles

1. uterine cycle

2. ovarian cycle

-Ovarian cycle:

-rise in FSHmatures the follicle

- day 9 - follicle begins to produce follicular estrogen

-day 14 - sudden increase in FSH and LH plus follicular estrogen coincides with ovulation

-the increase in follicular estrogen is what causes the spike in LH

-day 14 - ovulation = LH spike is the trigger


-Ovarian cycle:

-drop in LH, FSH and follicular estrogen after ovulation

-but FSH will begin to climb at the end of the cycle because of an increase in GnRH release from the hypothalamus


Uterine cycle:

-menstrual cycle initiated by increased FSH at Day 1

-rise in follicular estrogen starts thickening of uterine lining = known as the proliferative phase

-following ovulation and the development of the corpus luteum = increased secretion of luteal estrogens + synthesis of progesterone

-increased progesterone results in thickening of the stratum functionalis= known as the secretory phase


-Uterine cycle:

-estrogen and progesterone begin to drop about day 25 to 28 - disintegration of the stratum functionalis

-Menstruation (3 to 7 days)



-fertilization in the upper third of the oviduct/fallopian tube

-fertilization = union of egg and sperm to produce the zygote

-sperm must undergo capacitation after ejaculation – increase rate of tail beating

-happens in the female reproductive system

-plasma membrane of the ovulated secondary oocyte is surrounded by an extracellular matrix (glycoproteins) = zonapellucidaand a ring of follicular cells = corona radiata(for nourishment in the follicle)

-sperm must penetrate these layers before docking onto the plasma membrane


1. several sperm penetrate corona radiata and enter zonapellucida

2. Sperm bind to the zonapellucida & trigger release of acrosomal contents zonal digestion

3. ONE sperm contacts the plasma membrane of the oocyte

4. changes to the zonapellucida result (hardens) – blocks polyspermy

5. entry of sperm nucleus into egg

6. fusion of the sperm’s pronucleus with the pronucleusof the egg = zygote


embryonic stage:

1st week to week 8

-first cell division – within 24 hrs & takes 6 hrs

to complete

-second day – four cells

-end of third day – 16 cells

-fourth day – morula stage

-fourth to fifth day – blastocyst stage

-end of fifth day – hatching of blastocyst from


-6th day – implantation of blastocyst into



  • -union of sperm and egg nuclei (zygote) -> first cell division within 24 hours to form

  • the embryo

  • cell division continues -> formation of the morulaat day 4

    • morula = a mass of tiny, uniformly sized cells with equal amounts of cytoplasm

    • cells of the embryo = blastomeres


embryonic stage:

1st week to week 8

-first cell division – within 24 hrs & takes 6 hrsto complete

-second day – four cells

-end of third day – 16 cells

-fourth day – morula stage

-fourth to fifth day – blastocyst stage


-day 4 – 5: morulaforms a blastocyst (blastula) = assymetricalball of cells with a cavity called a blastocoel

-day 6: -> implantation of blastocyst into the stratum functionalis of the endometrium


-blastula = blastocyst - hollow ball of cells/blastomeres

-outer layer = trophoblast- forms extraembryonic tissues (e.g. placenta,

yolk sac)

-inner mass of cells at one end = totipotent embryonic stem cells

-second week of development (day 7 – 14) :

-amniotic cavity forms between the inner cell mass and the trophoblast

-the inner cell mass flattens = embryonic disc


Fertilized egg





  • third week of gestation in humans (days 14 – 21)

  • prior to gastrulation – the embryo is composed of an upper and lower layer that form an embryonic disk

    • the epiblast and hypoblast

    • epiblast = embryo

    • hypoblast (‘roof’ of the yolk sac) = supports embryology & forms part of the yolk sac

Primitive streak


Future ectoderm







Fertilized egg





  • during gastrulation - epiblast cells move toward the midline of the blastoderm – toward a structure called the primitive streak

  • epiblast cells migrate through the primitive streak toward the yolk

    • some cells push the hypoblast to the side to become the endoderm

    • those remaining cells  mesoderm

    • leftover non-migrating epiblast cells  ectoderm

  • animals with three germ layers are known as Triploblastic

Primitive streak


Future ectoderm











-three germ layers:

1. ectoderm  integumentary system & nervous system

2. mesoderm  mesenchyme which forms the bones, fat, cartilage, blood, muscleof the animal

3. endoderm digestive organs and their linings

Endometrial epithelium(uterine lining)

Blastocyst reaches uterus.

Inner cell mass




Blastocyst implants(7 days after fertilization).

Expanding region oftrophoblast

Maternal bloodvessel




Extraembryonic membranesstart to form (10–11 days),and gastrulation begins(13 days).

Amniotic cavity



Yolk sac (from hypoblast)

Chorion (from trophoblast)

Gastrulation has produced athree-layered embryo withfour extraembryonicmembranes.






Yolk sac


yolk sac




-yolk sac: forms blood cells, gives rise to sex cells and the stem cells of the immune system

-portion of it becomes part of the umbilical cord

-embryo’s first connection is via a connecting stalk

-allantois: tube from the yolk sac that projects into the connecting stalk - gives rise to the umbilical artery and vein


yolk sac




-chorion– forms four weeks after implantation as slender projections that grow out from the trophoblast= chorionic villi (becomes the placenta)

-produces human chorionic gondadotropin hormone – hormone of pregnancy

-amnion – surrounds the amniotic cavity (amnionic fluid + developing embryo)

-amnion enfolds the connecting stalk and remnants of the yolk sac to form the umbilical cord


-fifth week (days 28 – 35): formation of lens, beginnings of maxilla and mandible &

paddle-shaped forelimb

-embryo is 10-12 mm long

-35 + 2 days: formation of eye, ear, forebrain, nasal pit, tail

-35 + 5 days: formation of midbrain, heart, external ear, primitive fingers

-sixth week: formation of primitive toes (22-24 mm)

-seventh week: formation of eyelids, webbed fingers

-eighth week: separation of toes and fingers (34-40 mm)


-fetal stage : end of the eighth week -> birth

-third month: body lengthens and head growth slows

-ossification of bones

-fourth month: reproductive organs appear

-rapid body growth

-lower limbs lengthen

-development of hair, eyebrows, lashes, nipples

-fifth month: growth slows

-skeletal muscles are active

-fetus curls into fetal position

-sixth to ninth month: weight gain

-skin smoothens as fat is deposited beneath skin

-eyelids open

-organs elaborate and grow (digestive and respiratory are last)

-neuronal networks form

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