Male Reproductive System: Zoology

• Zoology is the branch of biology devoted to the study of animals.
• It encompasses animal anatomy, physiology, behavior, ecology, evolution, and classification.
• Human reproduction is the process by which the human species creates new individuals.
• It involves sexual reproduction, requiring the fusion of gametes (sperm and egg) from male and female individuals.

Male Reproductive System

• The male reproductive system includes the testes, epididymis, vasa deferentia, seminal vesicles, prostate gland, bulbourethral glands, and penis.
• Testes are the primary sex organs in males, responsible for producing sperm and the male hormone testosterone.
• They are located within the scrotum, which helps maintain a temperature lower than body temperature for proper sperm development.
• Each testis contains seminiferous tubules where spermatogenesis (sperm production) occurs.
• Sertoli cells within the seminiferous tubules provide nourishment and support to developing sperm cells.
• Leydig cells (or interstitial cells) located outside the seminiferous tubules produce testosterone.
• Spermatozoa (sperm) are transported from the seminiferous tubules to the epididymis for maturation and storage.
• The epididymis is a coiled tube located on the posterior surface of each testis, where sperm undergo further maturation and acquire motility.
• The vas deferens (or ductus deferens) carries sperm from the epididymis to the ejaculatory duct.
• The seminal vesicles are paired glands that secrete a fluid rich in fructose, prostaglandins, and fibrinogen, which nourishes and protects sperm.
• The prostate gland secretes an alkaline fluid that neutralizes the acidity of the vaginal tract, enhancing sperm motility.
• Bulbourethral glands (or Cowper's glands) secrete a clear, mucus-like fluid that lubricates the urethra and neutralizes any acidic urine residue.
• Semen is the fluid ejaculated during orgasm, consisting of sperm and secretions from the seminal vesicles, prostate gland, and bulbourethral glands.
• The penis is the male external genital organ, responsible for delivering sperm into the female reproductive tract.
• It contains erectile tissue that becomes engorged with blood during sexual arousal, resulting in an erection.
• The urethra runs through the penis and serves as a passageway for both semen and urine.

Female Reproductive System

• The female reproductive system includes the ovaries, fallopian tubes (or oviducts), uterus, vagina, and external genitalia (vulva).
• Ovaries are the primary sex organs in females, responsible for producing eggs (ova) and the female hormones estrogen and progesterone.
• They are located in the pelvic cavity, one on each side of the uterus.
• Each ovary contains follicles, which are structures that contain developing oocytes (immature eggs).
• Oogenesis (egg production) begins during fetal development but is arrested at prophase I of meiosis until puberty.
• At puberty, one or more follicles mature each month under the influence of follicle-stimulating hormone (FSH).
• As a follicle matures, it enlarges and develops a fluid-filled cavity called the antrum.
• The mature follicle, called a Graafian follicle, ruptures during ovulation, releasing the secondary oocyte into the fallopian tube.
• After ovulation, the ruptured follicle transforms into a corpus luteum, which secretes progesterone and estrogen to prepare the uterus for implantation.
• If fertilization does not occur, the corpus luteum degenerates into a corpus albicans.
• Fallopian tubes (or oviducts) are paired tubes that extend from the ovaries to the uterus.
• They capture the ovulated oocyte and provide a site for fertilization.
• The fimbriae are finger-like projections at the end of the fallopian tube that sweep the oocyte into the tube.
• Fertilization typically occurs in the ampulla, the widest part of the fallopian tube.
• The uterus is a muscular organ where implantation of the fertilized egg occurs and where the embryo and fetus develop during pregnancy.
• It is located in the pelvic cavity between the bladder and the rectum.
• The uterus consists of three layers: the endometrium (inner lining), myometrium (muscular layer), and perimetrium (outer layer).
• The endometrium undergoes cyclical changes during the menstrual cycle in preparation for implantation.
• If fertilization occurs, the embryo implants in the endometrium. If fertilization does not occur, the endometrium is shed during menstruation.
• The myometrium is responsible for uterine contractions during labor and delivery.
• The cervix is the lower, narrow portion of the uterus that connects to the vagina.
• The vagina is a muscular canal that extends from the cervix to the outside of the body.
• It serves as the passageway for childbirth and menstruation, as well as the receptacle for the penis during sexual intercourse.
• The external genitalia (vulva) include the labia majora, labia minora, clitoris, and vaginal opening.
• The labia majora are outer folds of skin that protect the other external genitalia.
• The labia minora are inner folds of skin that surround the clitoris and vaginal opening.
• The clitoris is a small, erectile organ located at the top of the vulva, which is highly sensitive to stimulation.

Gametogenesis

• Gametogenesis is the process of forming gametes (sperm and eggs) through meiosis.
• Spermatogenesis is the production of sperm in the seminiferous tubules of the testes.
• It begins with diploid germ cells called spermatogonia, which undergo mitosis to produce more spermatogonia.
• Some spermatogonia differentiate into primary spermatocytes, which undergo meiosis I to produce two haploid secondary spermatocytes.
• Secondary spermatocytes undergo meiosis II to produce four haploid spermatids.
• Spermatids undergo spermiogenesis, a process of differentiation that transforms them into mature spermatozoa (sperm).
• Oogenesis is the production of eggs in the ovaries.
• It begins with diploid germ cells called oogonia, which undergo mitosis to produce more oogonia.
• Oogonia differentiate into primary oocytes, which begin meiosis I but are arrested at prophase I until puberty.
• At puberty, one primary oocyte each month completes meiosis I, producing a haploid secondary oocyte and a small polar body.
• The secondary oocyte begins meiosis II but is arrested at metaphase II until fertilization.
• If fertilization occurs, the secondary oocyte completes meiosis II, producing a mature ovum (egg) and another polar body.

Menstrual Cycle

• The menstrual cycle is a recurring series of physiological changes in the female reproductive system associated with menstruation.
• It is regulated by hormones, including follicle-stimulating hormone (FSH), luteinizing hormone (LH), estrogen, and progesterone.
• The menstrual cycle typically lasts about 28 days but can vary among individuals.
• The menstrual cycle consists of four phases: the menstrual phase, the follicular phase, the ovulation phase, and the luteal phase.
• The menstrual phase is characterized by the shedding of the endometrium, resulting in menstrual bleeding.
• It typically lasts for 3-7 days.
• The follicular phase is characterized by the growth and maturation of ovarian follicles.
• FSH stimulates the development of follicles in the ovary.
• As follicles develop, they produce estrogen, which stimulates the growth and thickening of the endometrium.
• The ovulation phase is characterized by the release of the secondary oocyte from the Graafian follicle.
• A surge in LH triggers ovulation, typically occurring around day 14 of the cycle.
• The luteal phase is characterized by the formation of the corpus luteum from the ruptured follicle.
• The corpus luteum secretes progesterone and estrogen, which maintain the thickened endometrium and prepare it for implantation.
• If fertilization does not occur, the corpus luteum degenerates, leading to a decline in progesterone and estrogen levels.
• The decline in hormone levels triggers menstruation and the start of a new cycle.

Fertilization

• Fertilization is the fusion of a sperm and an egg to form a zygote.
• It typically occurs in the ampulla of the fallopian tube.
• During fertilization, the sperm undergoes capacitation, a process that enhances its motility and ability to penetrate the egg.
• The sperm releases enzymes from its acrosome, which digest the outer layers of the egg, including the zona pellucida.
• Once the sperm penetrates the egg, the egg undergoes cortical reaction, which prevents polyspermy (fertilization by more than one sperm).
• The sperm and egg nuclei fuse to form a diploid zygote.

Embryonic Development

• Embryonic development is the process of growth and differentiation that transforms the zygote into a multicellular embryo.
• The zygote undergoes cleavage, a series of rapid mitotic divisions with no significant growth.
• Cleavage results in the formation of a morula, a solid ball of cells.
• The morula develops into a blastocyst, a hollow ball of cells with an inner cell mass (embryoblast) and an outer layer called the trophoblast.
• The blastocyst implants in the endometrium of the uterus.
• The trophoblast develops into the placenta, which provides nutrients and oxygen to the developing embryo.
• The inner cell mass differentiates into the three primary germ layers: the ectoderm, mesoderm, and endoderm.
• The ectoderm gives rise to the epidermis, nervous system, and sense organs.
• The mesoderm gives rise to the muscles, bones, circulatory system, and reproductive system.
• The endoderm gives rise to the lining of the digestive tract, respiratory system, and certain glands.
• Organogenesis is the process of organ formation, which occurs during the embryonic period.

Pregnancy and Parturition

• Pregnancy is the period from fertilization to birth, during which the embryo and fetus develop in the uterus.
• It is divided into three trimesters, each lasting about three months.
• During pregnancy, the placenta provides nutrients, oxygen, and hormones to the developing fetus.
• Hormones such as human chorionic gonadotropin (hCG), estrogen, and progesterone maintain the pregnancy and prepare the body for childbirth.
• Parturition (childbirth) is the process of delivering the baby from the uterus to the outside world.
• It is triggered by a complex interplay of hormonal and mechanical factors.
• Parturition involves three stages: dilation, expulsion, and placental stage.
• The dilation stage involves the widening of the cervix.
• The expulsion stage involves the delivery of the baby.
• The placental stage involves the expulsion of the placenta and fetal membranes.