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Human Systems Physiology 2 - Describe the role and function of the adult female reproductive organs and accessory structures Explain the process and regulation of oogenesis and follicular development Physiological Integration: Describe and differentiate between the ovarian cycle and uterine (menstru...

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Human Systems Physiology 2 - 301270

1. Describe the role and function of the adult female reproductive organs and accessory structures 2. Explain the process and regulation of oogenesis and follicular development 3. Physiological Integration: Describe and differentiate between the ovarian cycle and uterine (menstrual) cycle. Discuss how these processes are regulated via the hypothalamic-pituitary-gonadal axis 4. Physiological Integration: Discuss the regulation of female sex hormones and their influence on puberty and menopause

Amerman. Human Anatomy & Physiology – Chapter 26

 The reproductive system performs a range of functions which ensure:  Each offspring has 2 parents and receives genetic material from both  Provides genetic diversity  Foundation for survival and evolution of species  In both males and females, formation of gametes (reproductive cells) relies on the Hypothalamus – Pituitary – Gonadal (HPG) axis  Most cells of the body contain 46 chromosomes (diploid), gametes contains 23 chromosomes (haploid)

 There are a number of functions of the female reproductive system which are important for reproduction:  Create female gametes (egg cells, known as oocytes/ova)  Support a developing embryo and foetus  Provide nourishment to a growing infant

 Ovaries - female gonads  Produce female gametes (ova)  Secrete female sex hormones

Figure 26.10: Internal organs of the female reproductive system.

 Accessory organs include:     

Uterine (fallopian) tubes Uterus Vagina Greater vestibular glands Mammary glands

Figure 26.13: Internal anatomy of the female breast.

 Primary sex organ for females  Produces gametes (oocyte/ova) and sex hormones including estradiol, estrone, estriol, progesterone, inhibin and relaxin  Each egg develops in its own fluid-filled follicle and is released by ovulation Ovaries

Fallopian (Uterine) Tube

Uterus -

perimetrium (outer covering) myometrium (muscle layer) endometrium (inner lining)

Vagina Figure 26.11: Internal organs of the female reproductive system

 Ovaries consist of an outer cortex and inner medulla  The medulla contains blood vessels and nerves  The cortex contains follicles, which progress through several stages of development during the creation of female gametes  During ovulation, an oocyte gets expelled from the ovaries into the fallopian (uterine) tube

Image Source: Histology of an ovary http://medcell.med.yale.edu/histology/ovary_follicle.php

Figure: The female reproductive system Silverthorn, 2019, Human Physiology: An Integrated Approach

 The fallopian (uterine) tube connects the ovaries to the uterus  Fimbriae are finger-like projections which extend around the ovaries  The connection between ovaries and the uterine tubes are not a closed seal – fimbriae drape around each ovary and “catch” an oocyte that is released from the ovary during ovulation

Figure 26.11 Internal organs of the female reproductive system

 Fallopian tube is made of smooth muscle which can undergo peristaltic contraction  Ciliated cells work together to move oocyte toward uterus  Fertilization (when sperm cells bind to and penetrates an oocyte) typically occurs within ampulla of the uterine tube

Figure 26.11 Internal organs of the female reproductive system

 The uterus is a thick-walled, hollow organ  Sperm must make their way through the uterus and enter the uterine tube in order for fertilization to take place  The uterus is an important organ as it provides a site for newly fertilized ovum to implant itself and grow fundus

body

cervix

 The uterus is comprised of three distinct layers: 1. Endometrium – This innermost layer is shed during menstruation, varies in thickness depending on hormonal influences 2. Myometrium – The middle muscular layer is composed of smooth muscle and is important for uterine contractions during menstruation and during the process of childbirth 3. Perimetrium – the outermost layer is the serosa layer, made of connective tissue.

Figure: The female reproductive system Silverthorn, 2019, Human Physiology: An Integrated Approach

 The vagina is a thick-walled tube that extends from uterus to the body’s exterior  The vagina has three main functions: Pathway to discharge menstrual fluid Organ of copulation Birth canal

 Vaginal epithelial cells secrete glycogen  can be broken down into lactic acid. A low pH in the vagina and aids in preventing infection  The vagina is also protected by stratisfied epithelial tissue and mucous secretions from the cervix and various glands

 External genitalia of the female reproductive system lie external to the vagina and include several structures which help protect internal structures  Several glands (paraurethral gland, the greater vestibular glands) produce secretions which help with lubrication during intercourse and prevent entry of bacteria into the vagina and urethra

Figure 26.12: The female perineum.

 Mammary glands are modified sweat glands  Primary biological role is to produce milk for newborn infant, becomes physiologically important after reproduction has been accomplished  Each mammary gland is composed of 15-25 lobes, each lobe contains smaller lobules containing glandular alveoli which produce milk during lactation  Milk passes through a series of ducts, with multiple lactiferous ducts opening at the nipple More about the function of mammary glands next week!

Figure 26.13: Internal anatomy of the female breast.

 Tactile and psychological stimuli mediated by autonomic pathways promote sexual excitement  During sexual excitement:  Vaginal mucosa, vestibule, and breasts become engorged with blood  Increased secretions of vaginal mucosa and vestibular glands

 Uterus experiences peristaltic waves of contraction which can draw semen further into the reproductive tract  Unlike the male sexual response and ejaculation, in females orgasm is not required for conception

Oocyte, estrogen and progesterone production and site of ovulation

Typical site of fertilization Uterine (fallopian) tube

Broad ligament

Ovary

Lumen (cavity) of uterus

Uterine tube • Ampulla • Isthmus • Infundibulum • Fimbriae

• Mesosalpinx • Mesovarium • Mesometrium

Body of uterus

Wall of uterus • Endometrium • Myometrium • Perimetrium

Isthmus

Vagina Cervix

Site of implantation and growth of foetus

 Hormonal regulation of gamete production requires fluctuations in hormones from the pituitary gland and ovaries. This is regulated by both positive and negative feedback systems.  A single cycle in females is typically 28 days, but cycle length can vary greatly between individuals (20 -45 days)

Ovulation

Figure 26.18: Pituitary and ovarian hormone interactions during the uterine cycle.

 Differs from spermatogenesis in following ways:    

Begins before female infant is born, then suspended until puberty Once re-activated, oogenesis continues until menopause Only 1 gamete is produced by the end of the process (vs 4 sperm cells) Occurs about once per month as a part of ovarian cycle

 Sperm are produced in high numbers for two reasons:  Must travel through inhospitable female reproductive tract; most will not survive  Sperm cells produce enzyme that allows them to penetrate secondary oocyte, but one sperm cell by itself cannot produce enough enzyme— hundreds of sperm are needed

 Meiosis is an important process which ensures genetic variability amongst gametes, and ensures that a consistent number of chromosomes are maintained during reproduction  To generate gametes, stem cells must undergo mitosis, followed by Meiosis I and Meiosis II

Figure 26.1 The stages of meiosis.

 Oogenesis: a process in which female gametes, or oocytes/ova, are produced. Only 1 egg each month is typically produced  Oogenisis occurs in conjunction with development of a follicle (cells surrounding the ova)  Follicle: a small cluster of cells which surround the oocyte as it matures, capable of secreting progesterone and estrogen.

 In females, oogenesis commences before birth, where gamete stem cells called oogonia undergo and complete their mitotic division into primary oocytes  Cell division is halted at Prophase I between birth and puberty  Females produce all primary oocyte cells during foetal development. During this time about 1 -2 million primary oocytes are created.

Figure 26.14: The stages of oogenesis.

 During childhood, when levels of GnRH, LH, FSH and female sex hormones are low, primary oocytes remain in Prophase I and don’t develop any further until puberty (onset of menstruation)  During this period many primary oocytes degenerate, by the time puberty commences only around 300, 000 primary oocytes remain

Figure 26.14: Stages of oogenesis

 During a female’s reproductive years (puberty to menopause), cyclical increases in the level of FSH cause 20 – 30 primary oocytes to undergo further development  During this time, typically only one primary oocyte survive completion of Meiosis I to form a secondary oocyte

Figure 26.14: The Stages of Oogenesis.

 During meiosis of the primary oocyte:  Primary oocytes complete Meiosis I to produce two haploid cells that differ in size  The smaller cell contains DNA but little cytoplasm; called first polar body; usually degenerates  Larger cell, secondary oocyte, contains DNA & most of cytoplasm

 During ovulation, the secondary oocyte is expelled from the ovary into the uterine tube

Figure 26.14: The Stages of Oogenesis.

Figure: The menstrual cycle, Silverthorn, 2019.

 A secondary oocyte is viable for 12 -24 hours after ovulation  Female gametes do not complete meiosis, unless fertilized by a sperm cell  If fertilized, the secondary oocyte will complete Meiosis II, and divide to form a second polar body and a fertilized ovum The second polar body typically degenerates. The fertilized ovum will continue to grow and divide as it travels towards the uterus  If the secondary oocyte does not get fertilized, the cell will remain in metaphase II and be shed during the menstrual cycle

Figure 26.14: The stages of oogenesis

 The oocyte develops inside a group of cells known as a follicle, which are located within the ovarian cortex  Ovarian follicles undergo their own maturation process at the same time oocytes are developing  Follicular development is essential, as these cells are responsible for producing female sex hormones estrogen and progesterone  Estrogen and progesterone are important, as they contribute to the feedback mechanisms of the HPG axis which results in rhythmic fluctuations of hormones

Figure: The female reproductive system Silverthorn, 2019, Human Physiology: An Integrated Approach

 Just like with oogenesis, follicular development starts before birth  At the time of birth, females have about 1-2 million primordial follicles created within the ovaries. Each contains a primary oocyte  During childhood, some primordial follicles degenerate  Some primordial follicles mature into primary follicles and then into secondary follicles

BEFORE BIRTH

CHILDHOOD TO PUBERTY

 Secondary follicles are a thick layer of cells which surround the primary oocyte, and are able to produce hormones such as estrogen Figure 26.14: The stages of follicular development

 During a female’s reproductive years, rising levels of FSH and LH cause one dominant secondary follicle to undergo further maturation to become a vesicular follicle  After ovulation, follicular cells remain in the ovary, become the corpus luteum  If fertilization does not take place the corpus luteum degenerate after about 10 days  After follicular cells stop producing hormones, the cellular remains become the corpus albicans Ovulation  If pregnancy occurs, the corpus luteum is maintained and continues releasing hormones for about 3 months (until the placenta is established and takes over this role). Figure 26.14: The stages of follicular development

 Follicular development occurs at the same time as the development of the oocyte; both processes take place in the ovaries

Figure 26.14: Comparison of the stages of oogenesis and follicular development

 The HPG axis regulates female reproductive processes  The HPG axis is important regulator for gamete production (oogenesis) and other processes like follicle maturation, the menstrual cycle, and production of female sex hormones (estrogen and progesterone) Figure 26.16: Hormonal regulation of ovarian function via the HPG axis.

 Regulated by fluctuations in hormones from hypothalamus, anterior pituitary and ovaries  Ovarian cycle: ovaries develop gametes, follicular cells, and produce sex hormones  Menstrual (uterine) cycle: uterus undergoes structural changes to the endometrium Figure 26.19: The Big Picture of Hormonal Regulation of the Ovarian and Uterine Cycles.

 The ovarian cycle is a monthly series of events associated with maturation of an oocyte and its follicle in an ovary  Averages 28 days, normally lasts between 25 to 30 days  In some individuals, ovarian cycle can be as little as 20 or as long as 45 days

 The ovarian cycle includes the following three phases:

Follicular phase

Ovulation Luteal phase Figure 26.15: The ovarian cycle.

 The follicular phase of the ovarian cycle typically lasts for about 14 days, although the duration varies greatly between individuals  Variability in ovarian cycle length is usually due to the duration of the follicular phase  During this time, rising levels of FSH cause a dominant follicle to mature from a primary follicle to a vesicular follicle  Typically, a small number of primary oocytes will undergo partial meiosis, but only one develops into a secondary oocyte

Figure 26.15: The ovarian cycle.

 Ovulation is a single event where the vesicular follicle erupts from the ovary wall, ejecting its’ secondary oocyte into the peritoneal cavity  Ovulation occurs at the end of the follicular phase (about Day 14 of the ovarian cycle – although may occur between Day 10 -26 )  Fimbriae of the uterine tubes help guide the secondary oocyte further along the female reproductive tract  Some follicular cells remain with the secondary oocyte, creating the corona radiata  The vesicular follicle collapses in on itself and remains within the ovary

Figure 26.15: The ovarian cycle.

 The luteal phase is the period where the remnants of the ruptured follicle becomes an endocrine organ called the corpus luteum  The luteal phase typically lasts for 14 days (from Day 14 –Day 28)  The corpus luteum can secrete oestrogen and progesterone for up to three months  During this time the corpus luteum gradually degrades into scar tissue.  When it no longer produces hormones, it becomes corpus albicans

Figure 26.15: The ovarian cycle.

 Each month, as an oocyte is undergoing maturation and release from the ovaries, the lining of the uterine wall is preparing for a fertilized egg to arrive and embed within the lining of the uterus (endometrium).  The functional lining of the uterus is the endometrium  The menstrual cycle requires rhythmic fluctuations of estrogen & progesterone to establish, build and shed the endometrial lining

Figure 26.17: Endometrial changes during the uterine cycle.

 Menstruation is the phase of the uterine cycle where the functional layer of the endometrium is shed, resulting in bleeding  The menstrual phase occur while ovarian hormones are low and LH and FSH levels are only starting to rise  Menstruation typically lasts between 4-7 days, and aligns with the first half of the follicular phase of the ovarian cycle  Between 35 – 100 mL of blood can be discharged during the menstrual phase, but some individuals may experience heavier bleeding, or longer periods of bleeding (menorrhagia)

Figure 26.17: Endometrial changes during the uterine cycle.

 During the proliferative phase, new endometrial lining is generated, glands enlarge and blood vessels proliferate  The proliferative phase is driven by increases levels of estrogen from the ovaries and surges in LH and FSH production from the anterior pituitary gland  This phase aligns with the second half of the follicular phase of the ovarian cycle

The end of the proliferative phase coincides with ovulation taking place in the ovaries

 During the Secretory Phase, blood vessels and glands within the endometrial lining continue to proliferate  Glands start secreting a glycogen-rich fluid called uterine milk, which can help sustain embryonic growth if fertilization takes place  The secretory phase typically lasts 14 days (although duration varies amongst individuals) and aligns with the luteal phase of the ovarian cycle

If fertilisation does take place, it usually takes about 10 days for the embryo to travel through the uterine tube and implant in the endometrial lining. At this point in time, the uterus is primed for sustaining a growing embryo

 The ovarian cycle is regulated by the endocrine system via the HPG axis Hypothalamus  pituitary  ovaries  Estrogens and progesterone are involved in a multi-tiered regulation including positive & negative feedback loops

Figure 26.16 Hormonal regulation of ovarian function via the HPG axis.

 First tier control: GnRH released from the hypothalamus  Second tier control: LH and FSH is released from the anterior pituitary  Third tier control: LH causes thecal cells of the follicle to secrete estradiol and estriol. FSH causes granulosa cells of the follicle to secrete estrogen and inhibin  Eventually, the new vesicular follicle will produce enough estrogen itself to initiate a positive feedback response which causes the LH surge  The LH surge triggers ovulation and encourages the corpus luteum to produce additional progesterone, estrogen and inhibin Figure 26.18: Pituitary and ovarian hormone interactions during the uterine cycle

Figure 26.19: The Big Picture of Hormonal Regulation of the Ovarian and Uterine Cycles.

 Puberty is a stage where humans begin to reach sexual maturity and are capable of reproduction  In females, puberty results in an increase of sex hormones, onset of ovarian/uterine cycles and development of secondary sex characteristics  Puberty in females can start between 9 – 11 years of age, but age of onset varies between individuals  Maturation of the reproductive system is a gradual process, which can take up to 4 years to complete

 Prior to puberty, low GnRH levels keep LH, FSH, estrogen and progesterone levels low  At puberty, the hypothalamus becomes less sensitive to low levels of estrogen and progesterone, and GnRH levels start to increase  Elevated GnRH levels stimulate HPG axis, and blood levels of estrogen and progesterone increase

Figure Source: https://www.pediatricnursing.org/article/S0882-5963(19)30593-7/fulltext

 Female sex hormones includes estrogens (estrone, estradiol and estriol) and progesterone, both are steroid hormones derived from cholesterol  Estrogen and progesterone are essential for the maturation and development of the female reproductive organs, external genitalia, breast tissue, and results in fat accumulation around the hips and thighs  First sign of puberty in girls is bu...