ANIMAL REPRODUCTION

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CHAPTER 46

• ANIMAL REPRODUCTION

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I. OVERVIEW OF ANIMAL REPRODUCTION

• A. BOTH ASEXUAL AND SEXUAL REPRODUCTION OCCUR IN
  THE ANIMAL KINGDOM
• ASEXUAL REPRODUCTION PRODUCES OFFSPRING WHOSE
  GENES ALL COME FROM A SINGLE PARENT.
• SEXUAL REPRODUCTION REQUIRES THE FUSION OF MALE
  AND FEMALE GAMETES TO FORM A DIPLOID ZYGOTE.

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B. DIVERSE MEANS OF ASEXUAL REPRODUCTION ENABLE
ANIMALS TO PRODUCE IDENTICAL OFFSPRING RAPIDLY

• FISSION BUDDING AND FRAGMENTATION WITH
  REGENERATION ARE MECHANISMS OF ASEXUAL
  REPRODUCTION IN VARIOUS INVERTEBRATES.

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C. REPRODUCTIVE CYCLES AND PATTERNS VARY
EXTENSIVELY AMONG ANIMALS

• ANIMALS MAY REPRODUCE EXCLUSIVELY SEXUALLY OR
  ASEXUALLY OR THEY MAY ALTERNATE BETWEEN THE TWO
  DEPENDING ON ENVIRONMENTAL CONDITIONS.
• VARIATIONS ON THESE TWO MODES ARE MADE POSSIBLE
  THROUGH PARTHENOGENESÍS HERMAPHRODITISM AND
  SEQUENTIAL HERMAPHRODITISM.
• REPRODUCTIVE CYCLES ARE CONTROLLED BY HORMONES
  AND ENVIRONMENTAL CUES SUCH AS CHANGES IN
  TEMPERATURE RAINFALL DAY LENGTH AND SEASONAL
  LUNAR CYCLES.

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II. MECHANISMS OF SEXUAL REPRODUCTION

• IN EXTERNAL FERTILIZATION EGGS SHED BY THE
  FEMALE ARE FERTILIZED BY SPERM IN THE EXTERNAL
  ENVIRONMENT.
• IN INTERNAL FERTILIZATION EGG AND SPERM UNITE
  WITHIN THE FEMALES BODY.

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A.    INTERNAL AND EXTERNAL FERTILIZATION BOTH
DEPEND ON MECHANISMS ENSURING THAT MATURE SPERM
ENCOUNTER FERTILE EGGS OF THE SAME SPECIES

• EXTERNAL AND INTERNAL FERTILIZATION REQUIRE
  CRITICAL TIMING OFTEN MEDIATED BY ENVIRONMENTAL
  CUES PHEROMONES AND/OR COURTSHIP BEHAVIOR.
• INTERNAL FERTILIZATION REQUIRES IMPORTANT
  BEHAVIORAL INTERACTIONS BETWEEN MALE AND FEMALE
  ANIMALS AS WELL AS COMPATIBLE COPULATORY ORGANS.

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B.    SPECIES WITH INTERNAL FERTILIZATION USUALLY
PRODUCE FEWER ZYGOTES BUT PROVIDE MORE PARENTAL
PROTECTION THAN SPECIES WITH EXTERNAL
FERTILIZATION

• GREATER PROTECTION OF EMBRYOS AND PARENTAL CARE
  OF THE YOUNG USUALLY FOLLOW THE PRODUCTION OF
  RELATIVELY FEW OFFSPRING BY INTERNAL
  FERTILIZATION.

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III. MAMMALIAN REPRODUCTION

• HUMAN REPRODUCTION INVOLVES INTRICATE ANATOMY AND
  COMPLEX BEHAVIOR
• EXTERNAL REPRODUCTIVE STRUCTURES OF THE HUMAN
  MALE ARE THE SCROTUM AND PENIS.
• THE MALE GONADS OR TESTES RESIDE IN THE COOL
  ENVIRONMENT OF THE SCROTUM.
• THEY POSSESS ENDOCRINE LEYDIG CELLS SURROUNDING
  SPERM-FORMING SEMINIFEROUS TUBULES THAT
  SUCCESSIVELY LEAD INTO THE EPIDIDYMIS VAS
  DEFERENS EJACULATORY DUCT AND URETHRA WHICH
  EXITS AT THE TIP OF THE PENIS.

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• EXTERNALLY THE HUMAN FEMALE HAS A VESTIBULE
  CONTAINING SEPARATE OPENINGS OF THE VAGINA AND
  URETHRA THE LABIA MINORA BORDERING THE
  VESTIBULE THE LABIA MAJORA AND THE CLITORIS.
• INTERNALLY THE VAGINA IS CONNECTED TO THE
  UTERUS WHICH CONNECTS TO TWO OVIDUCTS.
• TWO OVARIES (FEMALE GONADS) ARE STOCKED WITH
  FOLLICLES CONTAINING DIPLOID PRIMARY OOCYTES
  FORMED BEFORE THE WOMANS BIRTH.

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• BEGINNING AT PUBERTY ONE OR MORE FOLLICLES
  MATURE DURING EACH MENSTRUAL CYCLE.
• THE OOCYTE CONTAINED IN A MATURING FOLLICLE
  UNDERGOES THE FIRST MEIOTIC DIVISION AND A
  SECONDARY OOCYTE WHICH IS HAPLOID IS EXPELLED
  FROM THE SURFACE OF THE OVARY DURING OVULATION.
• AFTER OVULATION THE REMAINING TISSUE OF THE
  FOLLICLE FORMS A CORPUS LUTEUM THAT SECRETES
  PROGESTERONE AND ESTROGEN FOR A VARIABLE
  DURATION DEPENDING ON WHETHER OR NOT PREGNANCY
  OCCURS.

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B. SPERMATOGENESIS AND OOGENESIS BOTH INVOLVE
MEIOSIS BUT DIFFER IN THREE SIGNIFICANT
WAYS

• CYTOKINESIS IS UNEQUAL IN OOGENESIS PRODUCING
  ONE LARGE OVUM.
• PRODUCTION OF SPERM IS CONTINUOUS IN HUMANS THE
  NUMBER OF FUTURE EGG CELLS IS SET AT BIRTH.
• SPERMATOGENESIS IS AN UNINTERRUPTED SEQUENCE BUT
  THERE ARE LONG DELAYS IN OOGENESIS.

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B. A COMPLEX INTERPLAY OF HORMONES REGULATES
REPRODUCTION

• ANDROGENS FROM THE TESTES CAUSE THE DEVELOPMENT
  OF PRIMARY AND SECONDARY SEX CHARACTERISTICS IN
  THE MALE.
• ANDROGEN SECRETION AND SPERM PRODUCTION ARE BOTH
  CONTROLLED BY HYPOTHALAMIC AND PITUITARY
  HORMONES.
• FEMALE HORMONES ARE SECRETED IN A RHYTHMIC
  FASHION REFLECTED IN THE MENSTRUAL OR ESTROUS
  CYCLE.
• IN BOTH TYPES OF FEMALE CYCLES THE ENDOMETRIUM
  THICKENS IN PREPARATION FOR POSSIBLE
  IMPLANTATION.

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• THE MENSTRUAL CYCLE HOWEVER IS PUNCTUATED BY
  ENDOMETRIAL BLEEDING AND LACKS THE CLEAR-CUT
  PERIOD OF SEXUAL RECEPTIVITY LIMITED TO THE HEAT
  PERIOD OF THE ESTROUS CYCLE.
• THE HUMAN MENSTRUAL CONSISTS OF THE MENSTRUAL
  FLOW PHASE PROLIFERATIVE PHASE AND SECRETORY
  PHASE.
• THE OVARIAN CYCLE INCLUDES THE FOLLICULAR AND
  LUTEAL PHASES.
• THE FEMALE REPRODUCTIVE CYCLE IS ORCHESTRATED BY
  SECRETION OF GNRH FROM THE HYPOTHALAMUS AND OF
  FSH AND LH FROM THE ANTERIOR PITUITARY.
• THE DEVELOPING FOLLICLE PRODUCES ESTROGENS AND
  THE CORPUS LUTEUM SECRETES PROGESTERONE AND
  ESTROGEN.
• POSITIVE AND NEGATIVE FEEDBACK PRODUCE THE
  CHANGING LEVELS OF THESE FIVE HORMONES WHICH
  COORDINATE THE MENSTRUAL AND OVARIAN CYCLES.

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D.    EMBRYONIC AND FETAL DEVELOPMENT OCCUR
DURING PREGNANCY IN HUMANS AND OTHER (PLACENTAL)
MAMMALS

• HUMAN PREGNANCY CAN BE DIVÍDED INTO THREE
  TRIMESTERS.
• ORGANOGENESIS IS COMPLETED BY 8 WEEKS.
• BIRTH PARTURITION RESULTS FROM STRONG RHYTHMIC
  UTERINE CONTRACTIONS ASSOCIATED WITH LABOR.

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• POSITIVE FEEDBACK INVOLVING THE HORMONES ESTROGEN
  AND OXYTOCIN AND PROSTAGLANDINS REGULATE LABOR.
  
• THE ABILITY OF A PREGNANT WOMAN TO ACCEPT HER
  FOREIGN FETUS MAY BE DUE TO THE SUPPRESSION OF
  THE IMMUNE RESPONSE IN HER UTERUS.
• CONTRACEPTIVE METHODS INCLUDE PREVENTING THE
  RELEASE OF MATURE GAMETES FROM THE GONADS
  PREVENTING GAMETE UNION IN THE FEMALE TRACT
  PREVENTING IMPLANTATION OF THE ZYGOTE.

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