BIOL3324 Lecture 24: Reproductive Biology PDF

Overview Reproduction depends on the union of male and female gametes (haploid) to form an individual with a unique set of chromosomes (diploid) The reproductive systems are designed to enable union of genetic material from two sexual partners (1 + 1 ), and the female system is equipped...

Overview Reproduction depends on the union of male and female gametes (haploid) to form an individual with a unique set of chromosomes (diploid) The reproductive systems are designed to enable union of genetic material from two sexual partners (1 + 1 ), and the female system is equipped to house and nourish the offspring to the development point where it can survive independently in the external environment Includes the gonads, reproductive tract and accessory sex glands The significance of ploidy Chromosomal number (ploidy) All normal humans have two sets of chromosomes, one set that comes from the mother and a second set that comes from the father This pairing of chromosomes is referred to as the diploid chromosomal number and is symbolized as 2n Humans have 23 pairs of homologous chromosomes (or 46 chromosomes) – 23 chromosomes from mom and 23 chromosomes from dad The homologues carry genes that code for the same traits The gamete is the germ cell contributing half of the chromosomal pair from mom or dad to the offspring Gametes thus have 23 chromosomes and are referred to as being haploid or 1n Gamete formation requires a unique process of nuclear division called meiosis Mitosis v. Meiosis 2nx2 2nx2 2nx2 2nx2 1nx2 2n 1n Meiosis There are two nuclear divisions (referred to as meiosis I and meiosis II) Prophase I - After chromosomal replication, homologous pairs seek out and find their like partners to form tetrads After pairing up, crossover events can occur to allow for the exchange of genetic material between the matching pairs Metaphase I - Tetrads align along the midline Anaphase I - Separation of the tetrads Telophase I - division of the daughter cells Meiosis Meiosis II results in a 2nd division so that the 2 daughter cells from meiosis I become 4 daughter cells with each cell having a haploid set of chromosomes Spermatogenesis Mitotic proliferation Maintain germ cells and increase cell numbers Type A spermatogonia Divide to ensure that there is a continual supply of germ cells Differentiate into Type B to become spermatocytes Type B spermatogonia Destined to become spermatozoa Undergo mitotic divisions to produce 2 spermatocytes (per round) Meiosis (early spermatogenesis) Increase cell numbers and to produce a haploid gamete Primary spermatocytes replicate their DNA before 1 meiotic division (secondary spermatocytes) and then undergo a second division again to produce 4 haploid spermatids Spermiogenesis (late spermatogenesis) Remodeling phase and repackaging phase DNA is repackaged, organelles are lost or remodeled into new structures Spermatogenesis Mitotic proliferation Maintain germ cells and increase cell numbers Type A spermatogonia Divide to ensure that there is a continual supply of germ cells Differentiate into Type B to become spermatocytes Type B spermatogonia Destined to become spermatozoa Undergo mitotic divisions to produce 2 spermatocytes (per round) Meiosis (early spermatogenesis) Increase cell numbers and to produce a haploid gamete Primary spermatocytes replicate their DNA before 1 meiotic division (secondary spermatocytes) and then undergo a second division again to produce 4 haploid spermatids Spermiogenesis (late spermatogenesis) Remodeling phase and repackaging phase DNA is repackaged, organelles are lost or remodeled into new structures Spermiogenesis Transformation of the spermatid into a spermatozoon Stepwise process including: Formation of the acrosomal enzymes in the golgi apparatus Formation of the acrosome Formation of the flagellum Repositioning and replication of the mitochondria Condensation and repackaging of the DNA Removal of excess cytoplasm The spermatozoon The specialized haploid cell product of spermatogenesis Four parts Head: contains the repackaged nuclear material Acrosome: contains enzymes used to penetrate the ovum Midpiece: contains the mitochondria responsible for powering the tail Tail: flagella responsible for sperm motility Seminiferous epithelial cycle For a full cycle of spermatogenesis (spermatogonia → spermatozoon) the seminiferous epithelium has 4.6 cycles (~74 days) As the cycle progresses a germ cell moves away from the basement membrane towards the lumen The closer the germ cell gets to the lumen, the older (and more mature) the cell becomes Each new generation begins approximately every 16 days Oogenesis Occurs in the ovaries Mitotic division and meiotic arrest (fetal) Oogonia divide mitotically to give rise to a 6 – 7 x 106 oogonia pool Oogonia enter meiosis I (now called primary oocytes), but are arrested prior to the first meiotic division A portion of the cells (4 – 5 x 106 oogonia) entering meiosis I fail to complete the program and undergo apoptosis Primary oocytes are surrounded by a single layer of granulosa cells & incorporated into follicles Oogenesis is suspended until puberty Oogenesis Resumption of meiosis I (puberty to menopause) A portion of the resting pool of primary follicles (10-30) are induced to resume meiosis I Primary oocytes undergo the first meiotic division extruding ½ of the chromosome pairs into a polar body while retaining most of the cytoplasm and ½ of the chromosome pairs to become a secondary oocyte Secondary oocytes grow in size and follicles develop in preparation for ovulation One oocyte will become the dominant oocyte and suppress the development of the rest of the pool which will then become atretic. That one oocyte is the cell destined for ovulation Meiosis II Second meiotic division occurs as a result of fertilization The second polar body is extruded and the 23 maternal chromosomes of the secondary oocyte pair with the 23 paternal chromosomes of the sperm Male reproductive system Function Production of sperm (spermatogenesis) Delivery of sperm to the female Structures Testes (within the scrotum) Epididymis Vas deferens Seminal vesicles Prostrate gland Bulbourethral glands urethra The Scrotum Sac of skin housing the testes and epididymis Divided by a midline septum to provide a compartment for each testis Externally indicated by the raphe Provide an environment with a lower temperature than normal body temperature Penis Optimal sperm development temperature is ~34°C Cremaster Lower temps, the scrotum is pulled closer to the muscle body (becomes shorter and heavily wrinkled) Midline septum Higher temps, the scrotum hangs further from the body (loose and flaccid to increase surface area for Testis cooling) Scrotal skin Dartos muscle The Scrotum Sac of skin housing the testes and epididymis Dartos muscle Smooth muscle internal to the superficial fascia that wrinkles the scrotal skin when contracted Reduces surface area Draws testes closer to body Penis Cremaster muscle Cremaster Bands of skeletal muscle that arise from within the muscle trunk that are used to elevate the testes Midline septum Play little role in temperature regulation Play a larger role in protecting the gonads during Testis strenuous activity Scrotal skin Dartos muscle The Penis Copulatory organ designed to deliver sperm into the female reproductive tract Possesses “erectile tissue” – spongy network of CT and smooth muscle riddled with vascular spaces. During sexual arousal, the vascular spaces fill with blood causing the penis to enlarge and become rigid Corpus spongiosum – surrounds the uretha (ventral) Corpora cavernosa – paired bodies (dorsal) The Testis Covered by tunics Tunica albuginea and tunica vaginalis Septa divide the testis into several hundred lobules each containing a seminiferous tubule Seminiferous tubules (sperm factory) Site of spermatogenesis 80% of testicular mass Myoid cells (smooth muscle cells) surround the seminiferous tubules and contract rhythmically (perhaps to help propel seminiferous fluid and sperm out of the testis) Seminiferous tubules converge and meet at the Rete testis (ducts that connect the testis to the epididymis The Testis Sertoli cells Make up the lumen of the seminiferous tubules Tight junctions that serve as the blood testis barrier Provide nutrients for the maturing germ cells Phagocytic Secrete seminal fluid into the tubule Produce ABP to maintain high concentrations of testosterone with the seminiferous tubules Responsible for controlling and regulating spermatogenesis Leydig cells Located in the interstitium Produce testosterone in response to LH from anterior pituitary The Ducts (Epididymis) Sperm move through the epididymis over the course of ~20 days Three different regions with multiple roles in modifying sperms’ motility, metabolism, and morphology “Finishing school” Secretes materials that enable the sperm to penetrate the ova and become motile “Swim school” Serves as the site where sperm gain the ability to swim Concentration organ Absorbs fluids secreted by the seminiferous tubules to concentrate sperm 100x Stores sperm The Other Ducts Ductus deferens Travels up the spermatic cord to join with the seminal vesicle Thick layers of smooth muscle in its walls to propel sperm (via peristalsis) into the urethra Sperm storage site Sperm can be stored for several days Ejaculatory duct Juncture of the seminal vesicle and the ductus deferens Short duct enters into the prostate and empties into the urethra Seminal Vesicles Paired structure Located posterior to the bladder Merges with the ductus deferens to form the ejaculatory duct Secretes ~50% fluids of the seminal plasma Helps to dilute sperm in semen and promote better motility Secretions Highly viscous and alkaline Fructose as an energy source for sperm Prostaglandins which stimulate smooth muscle contraction Substances that enhance sperm motility and aid in capacitation Prostate gland Located immediately inferior to the bladder Completely surrounds the urethra to form the prostatic urethra Multiple glandular structures that open directly into the urethra Secretions account for ~35-45% of the seminal plasma Slightly acidic in nature Rich in seminalplasmin, an antibiotic Citric acid Prostate-Specific Antigen (PSA), a liquefactant Prostatic-Specific Acid Phosphatase (PSAP), potent nociceptive Zinc, protect the sperm from oxygen radicals Bulbourethral gland Also called Cowper’s gland Located within the urogenital diaphragm Paired structures Ducts travel through the bulb and empty into the spongy urethra Secretions account for ~5-15% of the seminal plasma Secretes mucus-like substance that provides lubrication of the urethra prior to ejaculation Also neutralizes the acidic environment of the urethra prior to ejaculation Semen Mixture of seminal plasma and sperm Mixture is created during early stage of ejaculation Sometimes referred to as ejaculate Charateristics Milky-white, sticky mixture 3 – 5 mL in volume Contains ~200-500x106 sperm Alkaline Semen function Alkalinity of the semen creates an environment in the acidic vagina favorable to sperm survival Clotting factors (semenogelin) in semen coagulate just after ejaculation to ensure that the sperm remain quiescent in the vaginal tract Liquefactants (like PSA) liquefy the “clot” to free and activate the sperm 20-40 minutes after copulation Prostaglandins induce reverse peristalsis of the uterus to direct sperm and semen towards the oviduct and decreases the viscosity of the cervical mucus to allow for sperm penetration into the uterus Substances help to suppress the immune response in the female reproductive tract Substances that are bacteriocidal prevent bacterial expansion in immunosuppressed environment Hormonal regulation of testicular function Function of Testosterone (T) Pre-partum Masculinization of the reproductive tract and external genitalia Promotes descent of testes into the scrotum Post-partum Promotes growth and maturation of the reproductive system at puberty Essential for spermatogenesis Maintains the reproductive tract during adulthood Responsible for male secondary sexual characteristics Male pattern of hair growth Thickening of vocal folds and enlargement of the larynx (deep voice) Promotes muscle growth and male body configuration Function of Testosterone (T) Reproduction-related Develops the sex drive at puberty (and sustains it) Promotes descent of testes into the scrotum Non-reproductive Exerts a protein anabolic effect Promotes bone growth at puberty Can be converted to estrogen