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Human Reproductive System
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REPRODUCTION

Introduction

Organisms have a varied life span varying from a few hours to many years. Each organism undergoes a life-cycle from the time of its formation, either from one or two parents, until its death.

Reproduction is a characteristic of all living forms that results in the continuity of life from one generation to another. Reproduction is not only a means of continuity for the species, it is also a process concerned with maintenance of the species.

There are two main types of reproduction:

(1) Asexual reproduction is a rapid method, involves a single parent and is a characteristic of some plants and simple animals. The offsprings have identical inherit characteristics from the parent.

(2) Sexual reproduction is a slower method, involving two parents, and the offsprings share hereditary characteristics of both parents. The reproduction involves the fusion of nuclei of specialized sex cells called gametes (sperm and egg), leading to the formation of zygote or fertilized egg.

Human Reproduction

(A) Male reproductive system

It consists of following structures:

(2) Ducts and glands

Ducts, rete testis and epididymis. The spermatozoa from the seminiferous tubules are transported to the exterior by ducts. The seminiferous tubules open by number of vasa-efferentia that join to form a network known as the rete testis. About 20 tubes from the rete testis lead into the long (about 20 feet) coiled tube, the epididymis, located on the surface of each testis. The epididymis stores sperm before ejaculation, and secretes a small portion of the semen. As the epididymis emerges from the testes, it forms the vas deferens (ductus deferens). Each vas deferens enters the abdominal cavity through the inguinal canal, arches over the bladder and opens into the back side of the urethra. Since the urethra carries both urine and semen (sperm) it is also called the ureto--genital canal.

SEMINIFEROUS TUBULES

(1) Sometimes the testes do not descend down into the scrotum during development and no functional sperm are produced. This condition is to known as cryptorchidism and results in sterility. In elephants and whales, temperature is not a critical factor for functional sperm and the testes are permanently located in the abdominal cavity.

(2) The human penis does not contain a bone, though it is common in many mammals. The penis bone is called a baculum.

Each testis is covered by a connective tissue covering (tunica albuginea), and send partitions inside, dividing it into lobules. Each lobule contains many seminiferous tubules, lined by germinal epithelial cells. These cells undergo the process of spermatogenesis (forming spermatogonia, primary and secondary spermatocytes, spermatids and sertoli or nurse cells), to produce spermatozoa. Between the seminiferous tubules are numerous microscopic interstitial cells (Leydig cells) which produce the male hormone, testosterone. The main functions of testosterone are development of male secondary sex characteristics, stimulation of protein anabolism and inhibition of anterior pituitary secretions of gonadotropins.

B) Female reproductive system

The female reproductive system consists of following parts:

(1) Ovaries. The ovaries are two oval bodies (about 30-40 mm long) located below and behind the uterine tubes. Each ovary is supported by a series of ligaments. The main function of the ovary is to produce eggs (ova) and the female sex hormones, estrogen and progesterone. They are attached by a membrane to the uterus and supplied with blood vessels.

(2) The Uterine Tubes. The uterine tubes consist of the proximal part called oviducts and the distal part called Fallopian tubes. The uterine tubes are attached to the uterus at its upper outer angles. The tubes are lined with cilia and extend to the ovaries. At the distal end each fallopian tube expands into funnel like infundibulum, having the fringe like projections called fimbriae. When ova are released from the ovary they pass down the fallopian tube. Fertilization normally occurs in the Fallopian tube.

(3) The Uterus. Both oviducts open into a wider tube, the uterus, or womb, which measures about 3 inches in length, 2 inches in width and 1 inch in depth. The uterus consists of a dome-shaped portion, called the fundus, a central tapering portion- the body and a narrow portion opening into the vagina, called the cervix.

(4) The vagina is a collapsible muscular tube, about 3 inches long, and capable of great distention. At the lower end of the vagina normally there is a fold of mucous membrane in the virginal state, the hymen, which partially closes the orifice. The vagina receives the seminal fluid, serves as the lower part of the birth canal and acts as an excretory duct for uterine secretions and menstrual flow.

(5) The vulva is a collective name for female external genitalia. Two folds of skin, the labia majora and the labia minor, enclose the vulva at the entrance to the vagina.

(6) The Bartholin’s or vestibular glands are present on either side of the vaginal orifice, and secrete a lubricating fluid.

(7) The mammary glands, or breasts, are functionally related to the female reproductive system, since they secrete milk for the nourishment of the young.

A transverse section of an ovary shows that it consists of an outer cortex and inner medulla. The medulla contains the connective tissue called stroma, blood vessels and nerve fibers. The cortex is lined by germinal epithelium, below which there are groups of follicles, each enclosing an egg.

The cortex is lined externally by a dense fibrous tissue, the tunica albuginea. Out of many follicles present at birth, only 300 to 400 follicles reach maturity and liberate their ova (ovulation) during the reproductive years of a human female (puberty to menopause). One of the developing follicles (primary follicle, secondary follicle, maturing follicle) form a mature follicle called the Graafian follicle.

Each Graafian follicle contains the ovum. Immediately surrounding the ovum is a layer of follicle cells, the inner zona pellucida and outer layer of columnar cells, the corona radiata. The follicle contains a fluid filled space called the antrum. The antrum is lined by follicle cells which from two zones, the membrana granulosa and the cumulus oophorus. The Graafian follicle is surrounded by two connective tissue cell layers, the outer theca externa and the inner theca interna. The follicular cells secrete the hormone estrogen which controls the menstrual cycle.

Female Reproductive cycle

The Menstrual Cycle

From puberty (menarche-onset of the menses) until menopause a woman’s reproductive system undergoes many cyclic changes are related to the changes in the endometrium (inner lining of the uterus), breasts, ovaries, vagina, and hormone secretions.

The cyclic reproductive changes of the human female is marked by menstruation, during which some cells, unclotted blood from ruptured blood vessels, other fluids and the uterine endometrium is released through the cervix and vagina. Each menstrual cycle occurs about every 28 days and lasts 4-5 days. The menstruation occurs 12 to 14 days after an ovum is released from the ovary (ovulation), about once in four weeks. The periodicity of the cycle varies with different individuals. After fertilization, menstruation ceases, and is the first indication of pregnancy.

The menstrual cycle (of 28 days) is generally divided into four phases, with major events occurring in each phase. The 4 phases are:

(1) The menstrual (destructive) phase,

(2) The proliferative (follicular) phase,

(3) The ovulatory phase, and

(4) The secretory (luteal) phase

  1. The menstrual phase. The uterus lining
  2. (i.e., endometrium) and its blood vessels slough off ,

    and is discharged with blood, mucus, cell debris and

    other fluid as the menses , through the vagina.

    This may last for 4-5 days. Menses occur when

    fertilization does not take place.

  3. The proliferative phase occurs between the end

of menses, and ovulation. In this phase, under the

stimulation of estrogen, the uterine endometrium

undergoes a process of growth (proliferation) and a

new thick endometrium is formed. In the ovary,

the follicle begins to develop into a Graafian follicle.

This phase cycle days 6 to 13 or 14 in a 28 day cycle.

3) The ovulatory phase indicates the rupture of the Graafian follicle and release of the ovum (ovulation). It occurs some 14 days after the start of menstruation. During this phase the concentration of estrogen is high in blood and it stimulates the ovulation. The blood vessels enlarge and grow in the endometrial wall, and some secretory cells or glands are formed.

(4) The secretory phase occurs between ovulation and the onset of menses, i.e., the phase lasts about 14 days (cycle days 15 to 28). The endometrium which is under the influence of progesterone and estrogen, increases in size, becomes thick, the endometrial glands become enlarged, undergo maximum secretory activity and its blood vessels become coiled and enlarged.

The ovum released in the ovulatory phase may or may not be fertilized. If the ovum is fertilized, it becomes embedded in the endometrium (implantation). In this case the corpus luteum remains to secrete progesterone which help the embryo to grow within the uterus. If the ovum is not fertilized, the corpus luteum disintegrates and the progesterone level falls sharply. Fourteen days after ovulation, unless fertilization occurs, menstruation begins again.

Embryonic Development

  1. Structure of spermatozoa

Typically, each sperm consists of an anterior head, an intermediate middle piece, and a posterior tail. The head contains a nucleus with haploid chromosomes, and the acrosome which effects penetration of the sperm into the egg. The middle piece has proximal and distal centrioles, the axial filament and numerous mitochondria which provide energy for locomotion of the sperm. The tail is long and consists of the main piece and end piece, with central axial filament, fibrils, and partly covered by protoplasmic membrane. The human sperm is about 0.05 mm in length , and a single ejaculation (about 4 ml) contains some 300 million cells. To fertilize the egg, more than 60 to 80 million sperm cells are required per ejaculation. Such high sperm count is necessary to induce pregnancy.

Structure of ovum

When the Graafian follicle is ripe (10-20 mm in diameter), it projects from the ovarian surface, finally bursts and releases the mature ovum (ovulation) into the funnel of the oviduct. The mature ovum is roughly spherical,about 100 to 200 microns in diameter. The mature ovum has a nucleus (eccentric in position) and nucleolus, commonly called the germinal vesicle and germinal spot, respectively. The protoplasm of the ovum, called ooplasm (yolk or vitellus) provides nutrition during early embryonic development. The cell membrane of the ovum is called vitelline membrane or oolemma. Since it appears as a colorless zone, it is also called the zona pellucida. The space between the vitellus or ooplasm and the vitelline membrane is called perivitelline space. (Polor bodies are lodged in this space.) When the mature ovum is discharged,some of the follicle cells are still attracted radially around the ovum, called corona radiata. The ovum is propelled down the oviduct to the uterus by ciliated cells; this transfer takes several hours.

During copulation, the spermatozoa are deposited in the vagina, which may result in pregnancy. Pregnancy entails a sequence of many events, including fertilization, cleavage, implantation, embryonic growth (up to the end of 2nd month), and fetal growth (2nd month to the birth of the fetus) that ends with the birth of the baby.