Transcript File
Reproductive Physiology What will be covered: • Gametogenesis • How gametes form • Sexual differentiation • Pituitary-gonadal axis • Female reproductive physiology • Ovarian cycle • Uterine cycle • Hormonal control and changes • Male reproductive physiology Gametogenesis • This is the formation of gamete cells via meiosis • Gametes are specialised as they contain only half of the genetic material required to reproduce; this is so they can bind with another gamete via sexual reproduction • In meiosis 4 haploid gametes are formed from a diploid oogonium (female) or spematogonium (male) • The process differs slightly between oocytes and spermatocytes Spermatogenesis • During development germ cells are produced, however these remain inactive until puberty • Hormones act on germ cells, causing them to undergo mitosis and meiosis to form sperm cells Stages of sperm production: 1. Spermatocytogenesis 2. Spermatidogenesis 3. Spermiogenesis Spermatogonium Mitosis Primary spermatocyte Meiosis I Secondary spermatocyte Meiosis II Spermatid Sperm Spermatogenesis Spermatogonium undergo mitosis to ‘double up’ DNA, ready for meiosis. The spermatocytes undergo meiosis twice to produce four spermatids each with one set of DNA, ready to mature into sperm and eventually bind with an ovum in sexual reproduction Oogenesis • Oogenesis produces a secondary oocyte which is released during the ovulatory stage of the menstrual cycle • If no fertilisation occurs it will not undergo further meiotic division (i.e. will not undergo meiosis II on diagram) Stages of oocyte production: 1. Oocytogenesis – finished by birth 2. Ootidogenesis 3. Fertilisation of ovum Oogonium Mitosis Primary oocyte Meiosis I Sperm Primary polar body Secondary oocyte Meiosis II Secondary polar body Ootid Ovum Oogenesis The primary oocyte divides by meiosis to produce a secondary oocyte and a polar body (these are non-functioning waste products). Meiosis II occurs after fertalisation. Further differences in male vs female gamete production Male Female • Continuous production of sperm from puberty indefinitely • Stem cells are retained • Sperm cells are mobile with little cytoplasm • All oocytes are produced before birth • Stem cells are used up • Sperm cells are large, and are immobile • Oocytes are surrounded by follicular cells, and together form a follicle Gender Determination • Chromosomes determine gender • Humans have 23 chromosomes donated by the egg, and 23 donated by the sperm • Two gametes fuse to form a zygote. This requires fusion of both the cytoplasm and the genetic material X X X Y XX XY XX XY Zygotes Non-disjunction • Occasionally, zygotes form without ‘normal’ separation of X/Y chromosomes. This is non-disjunction in meiosis I or II • This can result in monosomy (only one sex chromosome) or polyploidy (more than two sex chromosomes) • Monosomy: XO = Turner Syndrome • Polyploidy: XXY = Kleinfelter Syndrome XYY = Jacob Syndrome XXX = Trisomy X Turner Syndrome (XO) Females have typical phenotype of short stature, neck skin folds, primitive sexual characteristics, no menstruation and typical facial features Kleinfelter Syndrome (XXY) Males with small testes, infertility, feminine sexual characteristics, normal IQ Jacob Syndrome (XYY) Males with above-average height and reduced IQ Trisomy X (XXX) Females who are normal, and are only identifiable by karyotype Gender Determination • At week 7 of embryonic development the SRY gene is activated (SRY = sex-determining region of Y chromosome) • As the name suggests, it is only present in males • The SRY gene determines the development of sexual characteristics • Absence of SRY causes gonads to become ovaries • Presence of SRY causes gonads to descend down the Wolffian duct and become testes. • The testes produce anti-mullerian hormone which causes the mullerian duct to degenerate. The Wolffian duct becomes the vas deferens • The ovaries then produce female hormones and there is an absence of testosterone, so the Wolffian duct degenerates. An absence of anti-mullerian hormone allows the Mullerian duct continue developoing to become fallopian tubes Pituitary-Gonadal Axis • The hypothalamus produces gonadotrophin-releasing hormone (GnRH) • GnRH stimulates the release of follicle-stimulating hormone (FSH) and lutenising hormone (LH) from the anterior pituitary gland • FSH acts on ovaries/testes to stimulate them to produce gametes • LH acts on endocrine cells to stimulate them to secrete steroid and peptide hormones • This is a negative feedback system Female Reproduction • The pituitary-gonadal axis controls menstruation in females What the hormones do Oestrogen: produced by developing follicle. Stimulates LH surge. Progesterone: maintains endometrium for implantation. LH: oocyte maturation and subsequent release from follicle. FSH: stimulates a few follicles to complete meiosis. Menstrual Cycle • Lasts 28days ( range is 24-35days) • Starts with shedding of endometrium (period) and release of FSH • Involves a ovarian cycle and a uterine cycle • Ovarian cycle • Development of ovarian follicle • Production of hormones • Ovum release • Uterine cycle • Shedding of endometrium prior to ovum release • Preparation for implantation of potential embryo via hormones • These cycles are linked by hormone production/release The Cycles Ovarian cycle: follicular phase (follicle develops), ovulation phase (follicle released), luteal phase (unfertalised follicle degenerates) Uterine cycle: menses (endometrium sheds), proliferative phase (endometrium builds up), secretory phase (endometrium secretes progesterone for potential zygote) If Fertalisation Occurs… • The endometrium lining is maintained by: • Progesterone from the corpus luteum • Human chorionic gonadotrophin (hCG) until w7 • Progesterone from the placenta from w7 • The placenta also produces oestrogen (breast development) and human placental lactogen (milk production) • Birth is stimulated by: • Increased corticotrophin-releasing hormone from the placenta for 2weeks prior to birth • Reduced progesterone, increased oxytocin (uterine contraction), increased inhibin (cervical relaxation) Male Reproduction • The aim of the male reproductive system is to produce, maintain and transport viable sperm • It also produces hormones which: • Develop secondary sexual characteristics • Offer feedback for spermatogenesis • Parts of the male reproductive system: • Testes • Accessory glands • Prostate • Seminal vesicles • Bulbourethral glands Testes • Site of sperm production • Divided into lobules, each with one seminferous tubule • Seminferous tubule maintains a suitable environment for sperm to develop by… • Blood-testis barrier • Compartments, therefore low glucose/high hormone • Hormone production Testes cells Sertoli cells Anti-mullerian hormone Secreted in embryology, causes Mullerian duct degeneration Inhibin Regulates FSH release Androgen-binding hormone Binds to testosterone/DHT and reduces their loses, increasing testosterone activity Leydig cells Estradiols/Aromatase Support spermatogenesis Testosterone Spermatogenesis and secondary sexual characteristics (stimulated by LH to produce testosterone) FSH increases response to LH GnRH hypothalamus LH ant. pituitary FSH ant. Pituitary Androgen-binding peptide release Sertoli cells Inhibin release Sertoli cells -ve feedback loop Testosterone Leydig cells Spermatogenesis Secondary characteristics Spermatogenesis Hormonal Control Males and Reproduction • The role of the male in reproduction is to deliver sperm to the vagina • This requires an erection reflex • Erotic stimuli stimulate the autonomic nervous system, which vasodilates penile arterioles • This causes blood to flood the penile tissue, causing an erection • Following the erection reflex an ejaculation reflex is required • Emission (movement of sperm from vas deferens into urethra, along with seminal fluids) is stimulated by the sympathetic nervous system • Strong muscle contractions (bulbospongiosus) due to a spinal reflex cause expulsion of semen - ejaculation Problems Sometimes problems occur with this: • Erectile dysfunction = unable to initiate an ejection or maintain it until ejaculation • Premature ejaculation = male reaches orgasm and ejaculates too quickly for partner to achieve enjoyment • Prolonged ejaculation = inability for male to orgasm and ejaculate