Reproductive Physio 1 Flashcards
Differentiate between the development of the male and female repro systems
The multiple differences depend primarily on:
a single chromosome (the Y chromosome) and
a single pair of endocrine structures, the testes in the male and the ovaries in the female.
The differentiation of the primitive gonads into testes or ovaries in utero is genetically determined in humans, but the formation of male genitalia depends on the presence of a functional, secreting testis.
In the absence of testicular tissue, development is female.
After birth, the gonads remain quiescent until adolescence, when they are activated by gonadotropins from the anterior pituitary.
Hormones secreted by the gonads will cause the appearance of features typical of the adult male or female and the onset of the menstrual cycle in the female.
In human females, ovarian function regresses after a number of years and menstrual cycles cease (the menopause).
In males, gonadal function slowly declines with advancing age, but the ability to produce viable gametes persists
What are the functions of the gonads?
In both sexes, the gonads serve dual functions:
1. Gametogenesis: the production of the reproductive cells, or gametes; these are spermatozoa (singular, spermatozoan, usually shortened to sperm) by males and ova (singular, ovum) by females.
2. Secretion of particular steroid hormones, often termed sex hormones; the androgens and the ooestrogens
The secretory and gametogenic functions of the gonads are both dependent on the secretion of the anterior pituitary gonadotropins, follicle stimulating hormone (FSH), and luteinizing hormone (LH).
The sex hormones feed back to inhibit gonadotropin secretion
Describe the hormones of the reproductive systems
The androgens are the steroid sex hormones that are masculinizing in their action; the oestrogens are those that are feminizing.
Both types of hormones are normally secreted in both sexes.
The primary sex hormones are testosterone in the male and oestradiol and progesterone in the female.
Testosterone is the androgen secreted in large amounts by the testes but they also secrete small amounts of oestrogens.
The ovaries secrete large amounts of oestrogens and small amounts of androgens
Androgens are secreted from the adrenal cortex in both sexes, and some of the androgens are converted to oestrogens in fat and other extragonadal and extra adrenal tissues.
The ovaries also secrete progesterone, a steroid that has special functions in preparing the uterus for pregnancy.
Particularly during pregnancy, the ovaries secrete relaxin (polypeptide hormone), which loosens the ligaments of the pubic symphysis and softens the cervix, facilitating delivery of the foetus.
In both sexes, the gonads secrete other polypeptides, including inhibin B, a polypeptide that inhibits follicle-stimulating hormone (FSH) secretion.
Outline the general pattern of reproduction control
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As a result of changes in the amount and pattern of hormonal secretions, reproductive function changes markedly during a person’s lifetime.
In males, gonadotropin secretion is noncyclic.
In post pubertal females an orderly, sequential secretion of gonadotropins is necessary for the occurrence of menstruation, pregnancy, and lactation.
Outline the stages in reproductive function
- Initial stage – begins during foetal life and ends in the first year of life (infancy) :: GnRH, the gonadotropins, and gonadal sex hormones are secreted at relatively high levels
- Infancy to puberty :: The secretion rates of these hormones are very low, and reproductive function is quiescent
- Puberty :: Hormonal secretion rates increase markedly, ushering in the period of active reproduction.
Cyclical swings in women during the menstrual cycle - Later in life :: Reproductive function diminishes, largely because the gonads become less responsive to the gonadotropins. The ability to reproduce ceasing entirely in women.
Describe the cellular composition of the testis of the male repro system
In the centre of each seminiferous tubule is a fluid-filled lumen containing spermatozoa.
The tubular wall is composed of developing germ cells and Sertoli cells.
Each Sertoli cell extends from the basement membrane all the way to the lumen in the centre of the tubule and is joined to adjacent Sertoli cells by means of tight junctions forming a Sertoli-cell barrier.
The Leydig cells, which lie in small connective tissue spaces between the tubules, are the cells that secrete testosterone.
What is the function of the Sertoli-cell barrier?
Sertoli-cell barrier serves to:
1. Prevents the movement of many substances from the blood into the lumen of the seminiferous tubule and thereby ensures proper conditions for germ-cell development and differentiation in the tubules;
2. Ensures that the different stages of spermatogenesis take place in different compartments and, hence, in different environments
Describe the functional relationship between Sertoli and Leydig cells
Sertoli cells serve as the route by which nutrients reach developing germ cells, and they also secrete most of the fluid found in the tubule lumen.
Nourish developing spermatozoa and receive stimulation by testosterone and FSH to secrete paracrine agents that stimulate sperm proliferation and differentiation. Also secrete paracrine agents that influence the function of Leydig cells.
The luminal fluid contains androgen-binding protein, which binds the testosterone that is secreted by the Leydig cells and crosses the Sertoli-cell barrier to enter the tubule.
This protein maintains a high concentration of testosterone in the lumen of the tubule.
Thus, Sertoli cells act as intermediaries between the germ cells and the hormones—FSH from the anterior pituitary and testosterone from the Leydig cells—that stimulate spermatogenesis.
In other words, these hormones do not act directly on the germ cells but rather on the Sertoli cells, which respond by secreting a variety of chemical messengers that function as paracrine agents to stimulate proliferation and differentiation of the germ cells
Outline the development of the male reproductive system from conception
7 weeks gestation: 1st identifiable step differentiating ovarian from testicular pathways is movement of primordial germ cells into medullary cords.
SRY (Sex-determining Region on Y c’some) controls early testis differentiation
SRY gene product acts to initiate male sexual differentiation
10% of 46 XX males have no identifiable SRY gene
Sertoli cells secrete MIS (Mullerian Inhibiting Substance, aka anti-Mullerian hormone) causing female reproductive structures to regress.
Leydig cells secrete testosterone which induces differentiation of the Wolffian duct system (epididymis, vas deferens, sex accessory glands).
Outline spematogenesis
Begins with spermatogonia (singular: spermatogonium), undifferentiated germ cells and concludes with the production of sperm.
The spermatogonia begin to divide mitotically only at puberty and then throughout life
Describe the process of puberty in males
A burst of testosterone secretion occurs in male foetuses before birth.
In the neonatal period there is another burst, with unknown function, but thereafter the Leydig cells become quiescent.
A period follows (in all mammals) when the gonads of both sexes are quiescent until they are activated by gonadotropins from the pituitary to bring about the final maturation of the reproductive system – adolescence.
Often called puberty (however, strictly defined as the period when the endocrine and gametogenic functions of the gonads have first developed to the point where reproduction is possible)
Describe testosterone
Synthesized from cholesterol in the Leydig cells.
Also formed from androstenedione secreted by the adrenal cortex.
Its secretion is under the control of LH, and the mechanism by which LH stimulates the Leydig cells involves increased formation of cAMP via the G protein-coupled LH receptor and Gs.
Cyclic AMP increases the formation of cholesterol from cholesteryl esters and the conversion of cholesterol to pregnenolone via the activation of protein kinase A
Ninety-eight percent of the testosterone in plasma is bound to protein.
Approx 65% to gonadal steroid-binding globulin (GBG) or sex steroid-binding globulin, and 33% to albumin.
A small amount of circulating testosterone is converted to oestradiol, but most of the testosterone is converted to 17-ketosteroids, principally androsterone, and excreted in the urine
What are the effects of testosterone in the male?
- Required for initiation and maintenance of spermatogenesis (acts via Sertoli cells)
- Induces differentiation of male accessory reproductive organs and maintains their function
- Induces and maintains male secondary sex characteristics; opposes action of oestrogen on breast growth
- Required for sex drive and may enhance aggressive behaviour
- Stimulates protein anabolism, bone growth, and cessation of bone growth
- Decreases GnRH secretion via an action on the hypothalamus
- Inhibits LH secretion via an action on the anterior pituitary
- Stimulates erythropoietin secretion by the kidneys
Describe the MoA of testosterone in the reproductive system
Like other steroids, binds to an intracellular receptor, and the receptor–steroid complex then binds to DNA in the nucleus, facilitating transcription of various genes.
In addition, testosterone is converted to dihydrotestosterone (DHT) by 5a – reductase in some target cells and DHT binds to the same intracellular receptor as testosterone.
Testosterone–receptor complexes are less stable than DHT–receptor complexes in target cells, and they conform less well to the DNA-binding state.
Thus, DHT formation is a way of amplifying the action of testosterone in target tissues.
DHT also circulates, with a plasma level that is about 10% of the testosterone level
