🐅Animal Physiology Unit 12 – Reproduction: Gametes and Hormonal Control

Basics of Reproduction

• Reproduction is the biological process by which new individual organisms are produced from their parents
• Two main types of reproduction: asexual and sexual reproduction
  • Asexual reproduction involves a single parent and produces genetically identical offspring (clones)
  • Sexual reproduction involves the fusion of male and female gametes, resulting in genetically diverse offspring
• Reproductive strategies vary among species, including internal and external fertilization, oviparity (egg-laying), and viviparity (live birth)
• Reproductive success is crucial for the survival and continuity of a species
• Reproductive fitness refers to an individual's ability to pass on their genes to the next generation
• Trade-offs exist between reproductive effort and other aspects of an organism's life, such as growth and survival

Gamete Formation

• Gametes are specialized reproductive cells (sperm in males and eggs in females) that fuse during fertilization to create a zygote
• Gametogenesis is the process of gamete formation, which includes spermatogenesis (sperm production) in males and oogenesis (egg production) in females
• Meiosis is a key process in gamete formation, reducing the chromosome number by half to produce haploid gametes
  • Meiosis I separates homologous chromosomes, while meiosis II separates sister chromatids
• Spermatogenesis occurs in the seminiferous tubules of the testes and involves the differentiation of spermatogonial stem cells into mature spermatozoa
• Oogenesis takes place in the ovaries and involves the development of primary oocytes into mature ova (eggs)
  • Unlike spermatogenesis, oogenesis is not a continuous process and is arrested at various stages until ovulation
• Gametes contain only one set of chromosomes (haploid) to ensure the proper chromosome number is restored upon fertilization

Male Reproductive System

• The male reproductive system consists of the testes, epididymis, vas deferens, seminal vesicles, prostate gland, and penis
• Testes are the primary male reproductive organs, responsible for sperm production (spermatogenesis) and testosterone secretion
  • Seminiferous tubules within the testes are the site of spermatogenesis
  • Leydig cells in the interstitial space produce testosterone
• Epididymis is a highly coiled tube that stores and matures sperm, allowing them to develop motility and fertilizing capability
• Vas deferens is a muscular tube that transports sperm from the epididymis to the ejaculatory ducts during ejaculation
• Accessory glands, including the seminal vesicles, prostate gland, and bulbourethral glands, secrete fluids that contribute to semen formation
• Penis is the male copulatory organ, responsible for delivering semen into the female reproductive tract during sexual intercourse
• Spermatozoa are the mature, motile sperm cells capable of fertilizing an egg

Female Reproductive System

• The female reproductive system consists of the ovaries, fallopian tubes, uterus, cervix, and vagina
• Ovaries are the primary female reproductive organs, responsible for egg production (oogenesis) and estrogen and progesterone secretion
  • Follicles within the ovaries contain the developing oocytes and secrete estrogen
  • Corpus luteum forms after ovulation and secretes progesterone
• Fallopian tubes (oviducts) transport the ovulated egg from the ovary to the uterus and serve as the site of fertilization
• Uterus is a muscular organ that supports the development of the embryo and fetus during pregnancy
  • Endometrium is the inner lining of the uterus that thickens in preparation for implantation and sheds during menstruation if pregnancy does not occur
• Cervix is the lower, narrow portion of the uterus that connects to the vagina and helps to protect the uterus from bacterial infections
• Vagina is a muscular, elastic canal that serves as the receptacle for the penis during sexual intercourse and acts as the birth canal during childbirth
• Mammary glands (breasts) are accessory reproductive organs that produce milk to nourish offspring after birth

Hormonal Control of Reproduction

• Reproductive functions are regulated by a complex interplay of hormones from the hypothalamus, anterior pituitary, and gonads (HPG axis)
• Gonadotropin-releasing hormone (GnRH) is secreted by the hypothalamus and stimulates the anterior pituitary to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH)
  • FSH stimulates follicle development in females and spermatogenesis in males
  • LH triggers ovulation and formation of the corpus luteum in females and stimulates testosterone production in males
• Gonadal hormones (estrogen, progesterone, and testosterone) provide feedback to the hypothalamus and anterior pituitary, regulating the secretion of GnRH, FSH, and LH
• In males, testosterone is the primary androgen responsible for the development and maintenance of male reproductive structures and secondary sexual characteristics
• In females, estrogen and progesterone regulate the menstrual cycle, prepare the uterus for implantation, and maintain pregnancy
  • Estrogen promotes the development and maintenance of female reproductive structures and secondary sexual characteristics
  • Progesterone prepares the uterus for implantation and helps maintain pregnancy by suppressing uterine contractions
• Prolactin, secreted by the anterior pituitary, stimulates milk production in the mammary glands

Reproductive Cycles

• Reproductive cycles are periodic changes in the reproductive system that prepare an individual for potential fertilization and pregnancy
• In humans, the female reproductive cycle is called the menstrual cycle, which typically lasts 28 days and is divided into the follicular phase, ovulation, and luteal phase
  • Follicular phase: FSH stimulates follicle development and estrogen secretion
  • Ovulation: LH surge triggers the release of the mature egg from the ovary
  • Luteal phase: Corpus luteum secretes progesterone to prepare the uterus for implantation; if fertilization does not occur, the corpus luteum degenerates, and menstruation begins
• Other mammals may exhibit different types of reproductive cycles, such as estrous cycles (e.g., in dogs and cats) or seasonal breeding (e.g., in many wild species)
• Environmental factors, such as photoperiod and temperature, can influence reproductive cycles in some species
• Reproductive cycles are controlled by the complex interplay of hormones from the HPG axis

Fertilization and Early Development

• Fertilization is the fusion of a sperm cell with an egg cell to form a zygote, which marks the beginning of a new individual
• In internal fertilization, sperm are deposited into the female reproductive tract, and fertilization occurs within the fallopian tube
• The zygote undergoes cleavage, a series of rapid mitotic divisions, to form a blastocyst
  • The blastocyst consists of an inner cell mass (which will develop into the embryo) and a trophoblast (which will form the placenta)
• Implantation occurs when the blastocyst attaches to the uterine wall, initiating the development of the placenta
• The placenta is a temporary organ that facilitates the exchange of nutrients, gases, and waste products between the mother and the developing fetus
• Gastrulation is a critical stage in early development, during which the three primary germ layers (ectoderm, mesoderm, and endoderm) are formed
• Organogenesis is the process by which the germ layers differentiate into specific tissues and organs
• Fetal development continues throughout pregnancy until birth (parturition)

Reproductive Disorders and Treatments

• Infertility is the inability to conceive after 12 months of regular, unprotected sexual intercourse
  • Causes of infertility can be male factors (e.g., low sperm count, poor sperm motility), female factors (e.g., ovulatory disorders, fallopian tube blockage), or a combination of both
• Polycystic ovary syndrome (PCOS) is a common endocrine disorder in women, characterized by irregular menstrual cycles, hyperandrogenism, and polycystic ovaries
• Endometriosis is a condition in which endometrial tissue grows outside the uterus, causing pain and potential infertility
• Erectile dysfunction (ED) is the inability to achieve or maintain an erection sufficient for satisfactory sexual performance
• Sexually transmitted infections (STIs) can cause reproductive tract damage and infertility if left untreated
• Assisted reproductive technologies (ART) are medical interventions that help individuals conceive, such as in vitro fertilization (IVF), intrauterine insemination (IUI), and intracytoplasmic sperm injection (ICSI)
• Hormonal therapies, such as ovulation induction with clomiphene citrate or gonadotropins, can help treat ovulatory disorders
• Surgical interventions, such as varicocele repair or tubal surgery, may be necessary to correct anatomical issues affecting fertility