Physiology of Female Reproduction helps us understand how the female reproductive system works and how its different organs coordinate during reproduction. In this article, we will learn about female reproductive physiology, the structure and functions of the female reproductive organs, ovulation, the menstrual cycle, and the hormonal control of menstrual cycle.
1.1 FEMALE REPRODUCTIVE SYSTEM
The female reproductive system is more complex structurally and functionally. Its complexity is related with the adaptation of internal fertilization and viviparity. In human, the female reproductive system consists of:
- A pair of ovaries
- A pair of fallopian tubes or oviducts or uterine tubes
- A uterus
- A vagina
- External genitalia
- Accessory genital glands
- Mammary glands
Ovaries
The primary female sex organs are the ovaries which are almond shaped and lie in the lower part of abdomen or pelvic cavity and is suspended by a fold of peritoneum, mesovarium. Each ovary is connected to the uterus by an ovarian ligament and to the lateral pelvic wall by a suspensory ligament. Each ovary consists of an outer cortex and an inner medulla; but in mature ovary there is no demarcation line between these two. The cortex contains follicles and corpora lutea and the medulla contains only the large blood vessels of it. Each ovary is a compact organ and mainly composed of stroma of fibrous connective tissue i.e., spindle -shaped fibroblasts. The tunica albuginea, another connective tissue layer covers the cortex. Outside the tunica albuginea, the germinal epithelium covers the surface of the ovary. The germinal epithelium is formed of simple squamous or cuboidal epithelial cells.
During the growth of the ovary, the germinal epithelium at various places dips into the body of the ovary. A mass of epithelial cells becomes separated from the main layer by the growth of connective tissue of stroma. One of the epithelial cells gives rise to an immature ovum or oocyte whereas others become a sac or follicle called follicular epithelium or granulosa surrounding the ovum. Both these two constitute the primordial follicle. The stroma of the ovary surrounding the follicular epithelium or granulosa becomes organized into connective tissue layers, the theca externa and theca interna. The follicular cells divide by mitosis and form a stratified follicular epithelium, the granulosa layer. A thick coat, the zona pellucida which is synthesized by both the follicle cells and the oocyte surrounds the oocyte. It is again surrounded by follicular membrane, membrane granulosa formed of cells called granulosa cells. The thecal cells formed from the stroma and are located around the follicle that persist and become active are called interstitial cells. They secrete small amount of androgen hormone.
Fallopian Tubes (oviducts or uterine tubes)
Each oviduct or fallopian tube is a muscular tube situated above and behind the urinary bladder and occupies the central position in the pelvic cavity. Its lumen is lined by ciliated epithelium and the wall is composed of three layers:
(i) a mucosa at the liminal surface consists of an epithelium of simple columnar cells that contain ciliated and secretory cells. The secretory cells produce a viscous liquid film to provide nutrition and protection to the ovum.
(ii) a muscularis.
(iii) a serosa composed of visceral peritoneum
The fallopian tube has four parts
- infundibulum.
- ampulla.
- isthmus.
- uterine part or interstitium.
Infundibulum is a broad, funnel-shaped part which bears a number of finger-like processes called fimbriae. It opens into the body cavity by an aperture called ostium which lies close to the ovary to receive the egg released from the ovary. The fimbriae bear cilia which beat toward the ostium to direct the egg into the infundibulum.
Ampulla is next to the infundibulum and is long, wide and thin walled major part of the fallopian tube.
Isthmus is very short, narrow, thick-walled straight part which follows ampulla.
Uterine part is narrow communicates with the uterine cavity passing through the uterine wall
The fallopian tube conveys the egg or ovum from the ovary to the uterus and eggs are propelled by peristaltic and ciliary action. It also provides appropriate environment to the ovum for its fertilization.
Uterus
Uterus is a pear-shaped highly elastic medium sac located between the urinary bladder and the rectum. It is specialized for the development of the embryo. It is attached to the body wall by a double fold of peritoneum, the mesometrium. The wall of uterus is formed of three layers of tissues:
(a) Perimetrium: outer covering of peritoneum.
(b) Myometrium: a middle layer of smooth muscle fibres.
(c) Endometrium: an inner lining of mucous membrane.
It consists of simple columnar epithelium of ciliated, non-ciliated cells and rich in tubular glands. During different phases of menstrual cycle, it undergoes cyclic changes.
The uterus has four parts:
- Fundus
- Cornua
- Corpus or body
- Cervix
Fundus is the upper wide, dome-shaped part which receives fallopian tubes. Cornua is the upper corner where the oviducts enter the uterus. Corpus or body is the middle, large main part. Cervix is a narrow part which communicates above with corpus or body by an aperture, the internal os, and with the vagina below by an opening, the external os. The cervical canal that is the cavity of cervix between the internal os and external os contains glands which secrete a protective mucous plug during pregnancy.
The functions of uterus are- (i) it receives the ovum from the fallopian, (ii) it forms placenta for the development of the foetus; (iii) it expels the young one at birth.
Vagina
It is a large, median elastic muscular tube and its wall consists of a mucosa, a muscular layer and an adventilia. It opens to outside of the body by an aperture called vaginal orifice. Its lining is thrown into transverse folds, the vaginal rugae which becomes smooth after the first delivery. The opening of vagina partially covered by a thin ring of tissue, hymen which is ruptured in long run due to vigorous physical activities.
Its functions are:
(i) During copulation it receives the male penis.
(ii) It allows menstrual flow.
(iii) It serves as the birth canal during delivery of young one (parturition)
External genitalia (vulva)
(a) Labia majora
(b) Labia minora
(c) Monspubis
(d) Vestibule
(e) Clitoris
(f) Perineum
Labia majora are two outer large, thick folds of skin that form the boundary of vulva and contain numerous sebaceous or oil glands and also partly covered by pubichair.
Labia minora are two inner, smaller, thin, moist, fleshy folds of skin between the labia majora. They are posteriorly fused together to form the four chette and also contain numerous sebaceous glands.
Mons pubic is a fleshy elevation above the labia majora which bears pubic hair.
Vestibule is the depression in front of the anus and is flanked by labia minora and labia majora.
Clitoris is a small erectile organ, homologous to the penis in the male but is very small and solid, having no passage through it. It lies at the anterior junction of the labia minora and is covered with squamous epithelium. It ends in a rounded glans which is covered by a small hood of skin, the prepuce.
Perineum is the area between the fourchette and the anus.
Accessory genital glands
Each side of the vaginal orifice a pair of small, rounded vestibular or Bartholin's glands are present, duct of which open just outside the hymen into the vestibule. These are homologous to the Corper's glands of the male. The glands secrete a clear, viscoid fluid under sexual excitement which serves as a lubricant during sexual intercourse.
Breasts or Mammary Glands
The breasts or mammary glands are located over the pectoral muscles on the front wall of the thorax in female. They are sweat glands or sebaceous glands as they originate as an integumentary outgrowth. Each breast has a nipple in its middle which is surrounded by a circular pigmented area called areola. The breasts contain fatty (adipose), connective tissue and mammary (milk) glands in which connective tissue supports the mammary glands and the adipose tissue covers them.
Each mammary gland consists of 15-20 lobes of the compound tubule alveolar type which secrete milk to nourish the new babies. Each lobe i.e., cluster of alveoli is drained by a lactiferous duct and are separated from each other's by dense connection and adipose tissues and represents a gland. The lactiferous ducts emerge independently from each lobe in the nipple and it has a dilation near its termination called lactiferous sinus. During lactation it acts as a reservoir of milk.
The mammary glands in the female begin to develop at puberty and during pregnancy they grow under the influence of hormones oestrogen and progesterone.
The process that leads to discharge of an ovum from the ovaries great increase in the diameter of the follicle, i.e., primary follicle. This increase in size is produced both by proliferation of the granulosa cells and by accumulation of the liquor folliculi within the follicle. Initially, the follicle cells begin to multiply, forming an increasing number of layers of cells. At the same time the ovum enlarges through the accumulation of yolk substances within its cytoplasm, and a new membrane, the zona pellucida, is deposited on the surface of the ovum.
The escape of the ovum from a mature follicle is known as ovulation which occurs due to rupturing of ovarian follicle and the wall of the ovary. Following ovulation in the mammal there is a proliferation of the granulosa and theca interna and the ovulated follicle is transformed into a new glandular structure, the corpus luteum. The life span of corpus luteum is conditioned upon the establishment of pregnancy. Normally in gravid cycle the corpus luteum lasts about 14 days, but in pregnancy it enlarges to about 2-3 cm in diameter and persists for 5-6 months. The large corpus luteum associated with the gravid state is called the corpus luteum of pregnancy (or corpus luteum verum) in contrast to the corp corpus luteum spurium in which the conception does not occur. Finally, the corpus luteum degenerates. At this stage of degeneration, the corpus luteum has a whitish colour and is termed as the corpus albicans.
The reproductive life of a human female lasts about 40-50 years of age and only about 450 ova are produced by her over the entire reproductive life.
HORMONAL CONTROL OF MENSTRUAL CYCLE
In the female of human, the uterine lining rapidly deteriorates resulting in the rupture of its blood vessels, the flow of blood (which is of both arterial and venous origin) and other fluid, and the sloughing away of most of the endometrium, phenomenon collectively called the menstruation. The period of menstruation phase which lasts about 4-5 days, occurs between the periods of ovulation. The cycle is called menstrual cycle and in human female it averages about 28 days from the start to the end, before the next cycle begins. Repair of the uterine lining follows, and with the beginning of the next cycle the endometrium begins to thicker progressively once more. If fertilization occurs, the endometrium is maintained and undergoes further changes. Menstruation only occurs if the released ovum is not fertilized. Lack of menstruation may be indicative of pregnancy. It may also be for some reasons such as stress, poor health etc.
The menstrual phase is followed by the follicular phase. During follicular phase, the primary follicles in the ovary grow to become a fully mature Graafian follicle. Simultaneously, the endometrium of uterus regenerates through proliferation. The changes in the levels of pituitary and ovarian hormones, induce the changes in the ovary and the uterus. During the follicular phase, the secretions of gonadotropins (LH and FSH) increases gradually and stimulates follicular development as well as secretion of oestrogens by the growing follicles.
In the middle of cycle i.e., about 14th day both LH and FSH attain a peak level and rapid secretion of LH leads to its maximum level called LH surge. This induces rupture of Graafian follicle and thereby the release of ovum. This phase is called ovulation phase.
The ovulation phase is followed by the luteal phase. In this phase the remaining parts of the Graafian follicle transform as the corpus luteum which secretes large amounts of progesterone, essential for maintenance of the endometrium. Such an endometrium is required for implantation of the fertilised ovum and other events of pregnancy. All the events of the menstrual cycle stop and there is no menstruation during pregnancy.
But in absence of fertilization, the corpus luteum degenerates which causes disintegration of the endometrium leading to menstruation and a new cycle starts.
The menstrual cycles cease around 50 years of age in human beings which is called as menopause. The first menstruation begins at puberty which is called menarche. So, the menstruation cycle, the indicator of normal reproductive phase extends between menarche and menopause.
So, the FSH produced by the pituitary gland, which is triggered by follicle stimulating hormone releasing factor (FSH-RF) from hypothalamus initiates the development of follicles in the ovary. Oestrogen, mainly oestradiol, synthesized by the cells of follicles stimulate the uterine epithelium to proliferate.
When oestrogen reaches a certain concentration, it acts on pituitary gland to shut down FSH production and on the hypothalamus to result in discharge of luteinizing hormone releasing factor(LH-RF) to the pituitary. The LH released by the pituitary in combination with FSH causes ovulation. LH causes the cells of follicle to alter, change and fill the old follicular cavity and form the corpus luteum.It starts secreting hormones-progesterone and oestrogen. Increase of these inhibit production of gonadotropic hormones from the pituitary. The production of ovarian hormones drops and support of uterine lining disappears leading to menstruation. If implantation does not occur then in response to low level of ovarian hormones, the level of pituitary gonadotropic hormones begins to rise again, followed by development of a new follicle and arise in oestrogen as the next monthly cycle begins.
