Human reproduction

Introduction

Human reproduction is probably the most complex, lengthy process, and biologically it is an essential aspect of life. Unlike other organisms, human being`s reproduction is a unique process encompassing a series of physiological changes in women. Human reproduction is purely sexual in nature. The process takes place with the help of androgens- sex hormones and the reproductive organs. Gametogenesis is the process of formation of male gametes (sperms) or the female gametes (ovum). The whole process of pregnancy is broadly categorized into pre-fertilization, fertilization and post-fertilization phases. After the gametogenesis, male gametes are transferred into the female reproductive system followed by fertilization that assists in forming a fused product- a zygote. After the fertilization, a series of cell differentiations leads to the development of blastocyst. The blastocyst undergoes series of cell divisions and differentiation that can ultimately transfer and install the mass of cells on the inner uterine-wall; this is known as implantation. Next phase is the pregnancy, divided into 2 phases,  the embryonic development and the fetal development.

 

Fertilization of sperm with the ovum    

Some key points 

  1.  Although sexuality is present in all the individuals right from the conception, they become dormant as the individual attains sexual maturity. 

  2. Sexual maturity varies between the individuals based on the genetic, geographic, environmental and dietary factors. General health status of the individual is also important for normal and healthy sexuality.

  3. Although sexual development is present by birth itself, active sexual changes start from teenage. For boys, it starts generally between 14-18 years and for girls, it’s as early as 11-13  years.

  4. Sexuality starts with certain noticeable changes in the physical, behavioural and psychological aspects of an individual. Some of the changes are the growth of hair on the unusual parts of the body including armpits, around the testes area for boys and pubic area for girls.

  5. The facial skin becomes oily with the symptoms of acne (pimples). Acne is due to the abrupt hormonal changes among boys.

  6. In the case of girls, most noticeable changes occur as they grow is,  the growth of breast tissues, darkening of the skin around the nipples. Furthermore, they begin to menstruate. The first menstrual period is called menarche.

  7. Voice changes are seen in boys. The penis occasionally begins to become enlarged and erect indicating the sign of sexual maturity influenced by the secretion of testosterone- male sex hormones.

 

Anatomy and physiology of the reproductive system

Male Reproductive System

The male reproductive system consists of parts that help in the production and transfer of sperms into the female reproductive system . Anatomically, male sexual organs are classified into internal and external genitalia. External genital structures include the penis, scrotum, epididymis, and testes whereas the internal organs are vasa-deferens, seminal vesicles, prostate gland, and bulbourethral glands. Detailed explanation with an image is shown below.

 

Brief anatomy of male reproductive parts

Parts of the male reproductive system 

  1. Testes are important parts help in the formation of male germ-cells (sperms). The testes are situated outside the abdominal cavity freely floating in a pouch called the scrotum. Scrotum is a sac-like structure made up of fold like creases visible outside called rugae. The scrotum regulates the temperature of the testes between 2 to 2.5o C which is much lower than the normal internal body temperature to preserve the sperm cells. Testis is oval in shape, measuring about 4 to 5 cm length and 2- 3 cm wide. The testis is covered by a unique dense covering tissues. An individual testis has many sub-divisions called lobules. Each testis comprises of 250 lobules.

  2. Each testicular lobule contains 1 to 3 highly coiled tubules called seminiferous tubules. They are the primary sites of the production (germination), maturation, and transportation of the sperm cells. Seminiferous tubules are made up of long columnar cells known as Sertoli cells. Sertoli cells are further enclosed by spermatogenic cells(sperm-producing cells).

  3. Spermatogenesis is the process of production of sperms. It is supported by the meiosis inside the  Seminiferous tubules that are arranged in a loop-like fashion inside the testes. The interstitial spaces found in between many seminiferous tubules contain tiny blood vessels as well as interstitial cells - Leydig cells. Leydig cells helps to synthesize and secrete male sex hormones - testosterone (androgens).

  4. Many accessory organs also aid in the production of sperms. Some of them are rete-testis, vasa efferentia, epididymis and vas deferens. The seminiferous tubules of each testis is channelled into the vasa efferentia through rete testis then the vasa efferentia opens into epididymis which is located along the posterior surface of each testis. The epididymis further descend into vasa deferens which loops over the urinary bladder. Epididymis has a duct-like structure called seminal vesicle connecting the urethra with the ejaculatory duct.

  5. Ejaculatory ducts are responsible for storage and transportation of the sperms from the testis towards the tip of urethra. Urethra is an exit for urine as well as semen. Urethra extends through the penis to reach the extreme end of opening- urethral meatus.

  6. The penis is the male external genital organ made up of special tissues capable of expanding during sexual intercourse as it facilitates insemination. The enlarged end of the penis is the glans penis covered by a loose fold of skin called the foreskin. The male accessory glands surrounding the glans are paired seminal vesicles, a prostate gland and paired bulbourethral glands. Secretions from these glands constitute the seminal plasma rich in fructose, calcium and certain enzymes. The secretions of bulbourethral glands help in nourishing the sperms as well as the lubrication.

 

Male reproductive system- image from freepik.com

 

Female Reproductive System

The female reproductive system is divided into external and internal genitalia. External organs include the mons pubis, hymen, labia majora, labia minora, vagina, cervix, and the internal organs are uterus, ovaries, and fallopian tubes.
 

External genitalia

The female external genitalia is an outer portion extend between the internal organs and the urethral opening. The mons pubis is the rounded eminence, made up of fatty tissue enclosing around the prominence of the symphysis pubis. Females develop fine hair around the pubic area as they attained maturity (puberty). Following is the anatomical and physiological description of the female reproductive canal

  1. The labia majora: They are the fatty folds of skin starting from the mons pubis reaching downward and backwards to merge with the perineal region (the area around anus). Labia majora protects the mucous layers present inside by forming the lateral boundaries. The external layer of each labium is strong and pigmented whereas the inner surface is smooth having sebaceous glands that secrete sebum. The labia majora correspond to the scrotum in a male.

  2. The labia minora located just below the 2 labia majora. Labia minora has a very rich blood supply. They are the 2 small folds of skin, with no fatty tissues inside. Labia minora further extends backwards on each side of the external opening into the vagina. On the upper portion of the opening, the labia minora fuses with clitoris which corresponds to the penis (excluding the urethra) in the maleOn its lower portion forms frenulum.

  3. The clitoris is highly sensitive part having 2 corpora cavernosa separated by a partition. It possesses a sensitive tip of spongy erectile tissue, the glans clitoris having rich nerve supply. The opening of the urethra measures 2.5 cm located behind the clitoris and immediately in front of the vaginal opening.

  4. The next part behind the clitoris is the vestibule - a space into which the urethra and vagina opened. Vestibule is both sexual and urinary in function as it allows both urethra and vagina to pass through it.

  5. The hymen is a thin layer, very delicate and usually broken before maturity hence, it is not a good indicator of virginity of a female. If the hymen is open then it will be torn during the first coitus (intercourse). However, any athletic activities like cycling, trekking, etc can also tear this layer. In few women the hymen there is a chance that hymen can still persist even after the intercourse

  6. The vagina connects the uterus to the outside world. The vulva and labia form the entrance, and the cervix junction between the cervix, uterus, fallopian tubes and the external parts like labia, clitoris. The vagina receives the penis during sexual intercourse and also serves as a conduit for menstrual flow from the uterus.

 

Diagram showing anatomy of vagina

 

Diagram showing female reproductive system

Female reproductive system- credit to Vecteezy

 

Internal genital structures

1. Ovaries

Ovaries are the female gonads responsible for the production of eggs (ovum). They also perform 2 other major functions; secretory function by secreting sex hormones as well as the protective function by protecting the fertilized ovum. Ovaries are present by birth but they are not activated until a girl reached puberty. Matured ovaries measures about the size of a large grape. They are located on either side of the uterus against the wall of the pelvis. They are held firmly by a pair of ligaments called the infundibulum. Each ovary measures about 2 to 4 cm in length at maturity. The ovaries perform 3 major functions. Firstly, they provide nourishment, shelter, and protection to the eggs –ovum. Secondly, they assist in the production of hormones; estrogen, progesterone, relaxin, and inhibin. Thirdly, ovaries produce eggs through ovulation. Menstruation is the cyclic process help in the production of eggs every 28 days. At birth, a female has around 150,000 to 500,000 immature follicles in her ovaries,

2. Fallopian tubes

They are otherwise known as oviducts, there are 2 oviducts measuring about 10-12 cm long. The fallopian tubes stick to the peripheral layer of each ovary as shown in the picture. Each fallopian tube connects with the uterus through a funnel-shaped infundibulum( ligaments). Each fallopian tube has fine projections at their tip, they are known as fimbriae assisting in supporting and collecting the ovum after ovulation.

3. Uterus

The uterus is a hollow space supplied with spongy material rich with blood supply. Uterus provides space and nourishment for the growing fetus. The uterus is an active muscular structure, strong, elastic and highly responsive. It is usually single but in rare cases, it has 2 clefts known by the term bicornuate uterus. The uterus is an inverted pear-shaped hollow organ having 3 layers; inner endometrium, myometrium in the middle and the perimetrium at the outermost area attached to the anterior abdominal wall. The uterus lies in front of the abdominal cavity and it is supported by strong ligaments attached to the pelvic wall. On its lower side, uterus opens into the vagina through a narrow cervix and on the upper side, it receives ovum by providing connection into the fallopian tubes. The cervical canal forms a cavity into the cervix which along with the vagina forms the complete birth canal. The endometrium sheds its surface every 28 days in healthy women .This happens as a result of the menstrual cycle. The menstrual cycle is an indication of the reproductive ability of women.

 

Questions

  1. Enumerate the phases of development of human beings?

  2. What is embryonic phase , explain the majr changes during embryogenesis?

  3. Explain the anatomy of external genetalia of a female.

  4. What is implantation?

  5. What major changes are expected during fetal development?

  6. Briefly explain the male reproductive system.

 

Further reading

1.      

  Infertility 

2.

Fetal development

3.

reproductive health                              

4.

Sexual reproduction in plants

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