CHAPTER 46 ANIMAL REPRODUCTION

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.

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.

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.

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 FEMALE'S BODY.

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.

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.

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.

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 WOMAN'S BIRTH.

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.

CYTOKINESIS IS UNEQUAL IN OOGENESIS, PRODUCING ONE LARGE OVUM. 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.

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.

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.

HUMAN PREGNANCY CAN BE DIVÍDED INTO THREE TRIMESTERS. 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.

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.