REPRODUCTIVE SYSTEMS AND REPRODUCTION IN HUMANS

Subject: 

BIOLOGY

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Term:

FIRST TERM

WEEK 3

TOPIC:

REPRODUCTIVE SYSTEMS AND REPRODUCTION IN HUMANS

CONTENTS

1.   Reproductive system in males and female

i.                   Function of various parts

ii.                 Structure of a male sperm and ovum

2.   Fertilization

i.                   Fusion of gametes

ii.                 Implantation

3.   Development of embryo

i.                   Selective exchange between mother and child and removal of excretory product from foetus; survival conditions like food, oxygen, ammonic fluid, placenta and umbilical cord

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SUB-TOPIC 1: REPRODUCTIVE SYSTEM IN HUMANS

The reproductive system in human are the most highly developed among the animals. Basically, the male and the female reproductive system consist of the gonad where the sex cell or gametes, as well as glands that are involved in secretion of hormones and fluids essential for development and sexual process.

The reproductive system consists of the internal genitalia which are contained within the abdominal cavity and the external genitalia which is found outside the body in the groin region. In the male, the gonads or testes produce sperm and their secretions in conjunction with other accessory glands that produce semen to facilitate reproduction.

1. THE REPRODUCTIVE SYSTEM IN A MALE HUMAN

The male reproductive system consists of the following organs

i. The testes: these are two ovoid structures situated in the scrotal sac. They are external and suspended in the posterior end of the abdominal cavity. They have descended into the scrotum at birth. They require temperature range of below 35⁰C to function effectively. The testes contain coiled tubules called seminiferous tubes. This contains actively dividing cells that produce sperm cells. The seminiferous tubes continue and emerge into a spongy structure called epididymis atop the testes. This structure stores sperms produced in the testes. The cells of these tubules divide repeatedly to form sperm cells in a process called spermatogenesis. The testes produce the sperm-the male gametes. It also secretes the hormones testosterone, which is responsible for producing secondary sex characteristics in males as well as stimulating sperm production

ii.Vas deferens: this is called the sperm duct. It is a continuation of the epididymis. It serves as the pathway through which sperm cells run from the epididymis and open into the urethra. e

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Male Reproductive System

The organs of the male reproductive system enable a man to have sexual intercourse and to fertilize female sex cells (eggs) with sperm. The gonads, called testicles, produce sperm. Sperm passes through a long duct called the vas deferens to the seminal vesicles, a pair of sacs that lies behind the bladder. These sacs produce seminal fluid, which mixes with sperm to produce semen. Semen leaves the seminal vesicles and travels through the prostate gland, which produces additional secretions that are added to semen. During male orgasm the penis ejaculates semen.

iii.  Penis: this is a sensitive organ of copulation transverse by urethra. The urethra is a common duct for sperms and urine in the male, thus the penis is described as a urinogenital organ. It consists of spongy erectile tissue. Blood flows into the dilated erectile tissue when stimulated. The flow of blood into the tissue contributes to its turgidity; hence the erection of the penis is no stimulation.

The penis is covered with a sensitive skin called glans penis and retractable foreskin called the prepuce. This can be surgically removed during circumcision,

iv.  Epididymis: A set of profusely coiled tubules known as epididymis connects each testis to the vas deferens, which conveys the sperm cells to the seminal vesicle for storage until there is the need for ejaculation.

v.  Other accessories and glands: the male reproductive system also consists of certain other accessories and glands whose secretions play important roles in reproduction. This includes the prostate gland whose secretion helps to energize and transport the sperm; the concentration of the sperm so as to normalize acidic matters along the wall of the urethra. The mixture of sperm and those secretions is known as semen. Seminal fluid secreted by the prostate gland provides a medium for sperm cells to swim and be nourished. On ejaculation, the sperms are discharged to the outside through the urethra.

2. THE REPRODUCTIVE SYSTEM IN FEMALE HUMAN

The female reproductive system consists of the following:

i. Ovary: the female human has two ovaries each located below the kidneys on both sides. The ovaries which are supported by ligaments contain several thousands of potential eggs called primary oocytes. They are covered by a layer called germinal epithelium: the ovaries produce the female gamete i.e ovum (plural Ova) matured ovum is secreted approximately once in every 28days. The shedding of mature ovum is called ovulation.

ii.  Oviduct: it has a funnel shaped structure known as the fallopian funnel which opens to each ovary. The opening is lined with ciliated musculat finger-like structures. The egg released by the ovary is received by the fallopian funnel. This is made possible by the beating of cilia to create a gentle current wafting the released ova into the oviduct or fallopian tube, a pathway for the passage of ovum fertilization occurs in the oviduct which opens to the uterus

Female Reproductive System

A component of the female reproductive system, the uterus is a muscular organ with an expandable neck called the cervix. Two main types of cancer arise in the uterus—endometrial cancer originates in the lining of the uterus, while cervical cancer begins in the epithelial cells of the cervix.

iii.  Uterus (womb): it is a pear –shaped structure (muscular organ) widest at the upper end and narrow at the lower end. Each of the left and right side of the widest end of the uterus is connected to a fallopian tube (oviduct). It opens to the vaginal through muscular ring called cervix. It has a glandular lining membrane for nourishing the embryo in the early stage of development. It also has smooth muscles in it concentration ultimately expels the foetus and its placenta.

iv.   Cervix: this lies the ventral ends of the uterus. It closes after fertilisation to avoid further entrance of sperm and foreign bodies.

v. Vagina: this part serves as the receptor of sperm cells ejaculated by male. It opens posteriorly

to the vulva. There is a film of connective tissue called hymen that cover the vagina opening.

This tissue can be easily removed by sexual activities or physical exercises.

vi.   Clitoris; this is a rod-like rudimentary structure analogous to the penis in the male. It is sensitive and erectile. It does not play any serious role in reproduction other than arousal.

The urinary and the reproductive passage of human female are separate and they open separately to the exterior through external genitalia called vulva. Hence urine-genital organ is present in all mammals except in human female.

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The structure of a male sperm cell (spermatozoon)

The male gametes, the spermatozoon is shaped like tadpole. It consists of a head with a nucleus having an apical acrosome, a middle piece containing mitochondria and a tail or flagellum. The whole sperm including the tail with which it moves inside the seminal –fluid is about 60µm long. A sperm cell is microscopic and is usually smaller than an egg (Ovum).

See diagram of the human sperm in page 451 of your Modern Biology text book.

Structure of Human Gonads

Gonads—in the male, the testes (singular, testis), and in the female, the ovaries—are the organs that produce gametes and sex hormones. The male gamete is the spermatozoan, produced by cell division in the seminiferous tubules of the adult testes. Typically, several hundred million sperm reach maturity in the epididymis and are stored in the vas deferens each day. Whatever is not released in ejaculation is reabsorbed, part of a continuous cycle. In the female, the ovaries produce eggs, or ova. At birth, about 2 million oocytes, or immature eggs, are present in the ovaries. Once the female reaches puberty, one egg matures approximately every 28 days inside a saclike Graafian follicle. Ovulation occurs when the mature egg bursts from the follicle and the ovary, beginning its journey down the fallopian tube toward the uterus.

Structure of a human egg (ovum)

The human female gametes (egg) are also microscopic but are larger than the sperm cell. Each ovum is about 0.1mm. in diameter. It consist of a cytoplasm, a nucleus I the centre, granules and yolk droplets. The yolk provide sources of nourishment for the embryo at least in its early stage of development

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The ovum’s cytoplasm is surrounded by two membranes. The inner one is the plasma membrane, while the out one is the vitelline membrane. The ovum is bounded on the outside by a jelly coat of variable thickness made up of glycoprotein. The structure of ovum is analogous to the egg cell of birds and reptiles. The cells of ovary are known as ova or eggs.

See diagram of the human ovum in page 452 of your Modern Biology text book.

The nuclei of the sperm cell and the ovum contain chromosome which are treated like the material that carry genes. The genes are responsible for passing on the parents’ characteristics to the offspring. Each of the two gametes (sperm and ovum) is haploid.

EVALUATION

1.                 State the function of the various parts of 1a. Male reproductive organ

1b. Female reproductive system

2.                 Draw and label the structure of male and female gametes

3.                 State ten differences between the male and the female reproductive organs.

4. Explain how human reproduction is possible despite the fact that eggs and sperm are microscopic in size.

5. Explain the role of hormones in human reproduction.

6. Based on what you have learned about human reproductive organs, describe the process of human reproduction in detail. How does conception occur, and how is the fertilized egg transported to the uterus?

SUB-TOPIC 2: FERTILISATION

Fertilization is the fusion of the spermatozoon and an ovum. It takes place in the Fallopian Tube. In human, fertilization is preceded by the process of sexual intercourse or coition, where a man discharges semen into te vagina. The semen of about 2cm3 containing about 100million sperms is deposited in the vagina near the cervix of the uterus.

FUSION OF GAMETES

Spermatozoa discharged into the vagina during copulation swim up to the cervix into the uterus and up the fallopian tube where fertilization takes place.

When the sperm encounters the ovum, it usually digests the jelly covering of the egg by means of the enzyme contained in the acrosome. The sperm itself to the layer of follicle cells outside the secondary oocyte as corona radiate. There is a fusion of egg plasma and sperm plasma membrane. The yolk at the point where the sperm penetrate the egg bulges out to form a cone. Once the sperm penetrate the egg, the yoke change to a membrane known as vitelline membrane. One other very important event is that the tail of the sperm is lost and the middle piece disintegrates. The egg now travels down the fallopian tubes and the second meiotic division takes place consequently upon fusion of sperm with egg. It is noteworthy that once a sperm has entered the ovum, another sperm cannot enter it because of the development of a protective cortical layer.

IMPLANTATION

The zygote resulting from the fusion of the sperm cell and the egg contains the characteristic

features of both the man and the woman involved in the sexual intercourse. The zygote under a number of cell division into two cells, four cells, eight cells, etc. until a ball of cell known as blastocyst is obtained as it passed further down the fallopian tube on it way to the uterus. On reaching the uterus, it becomes embedded in the uterus wall which is thick and highly vascularized. This process is known as implantation and as a result the woman is pregnant Implantation is the process by which the fertilized egg or zygote enter into an intimate relationship which leads to its embedment in the uterine wall such that it cannot be flushed out. It is the first stage of development of placenta. It involves the adhesion of the embryonic trophoblast (mitotically divided mass of cell at the early stage of embryonic development) and microvilli of the mother and embryonic cells.

EVALUATION

1.     Define fertilization and implantation

2.     Explain the process involved in fusion of gametes and how it results in fertilization.

3. Describe the role of hormones in human reproduction, and how they impact the process of fertilization, implantation, and pregnancy.

4. Analyze the factors that can affect human reproduction, including environmental, genetic, social, and other factors.

5. Discuss the importance of understanding human reproduction for topics such as birth control, family planning, and other reproductive health issues.

6. Explore the challenges that can arise in human reproduction, such as infertility, miscarriage, or other complications during pregnancy and childbirth.

SUB-TOPIC 3:

DEVELOPMENT OF EMBRYO

The cells of the embryo grow and divide constantly. At first, they appear similar but later develop differing structures and functions. They eventually form various specialize tissue of a new individual. These tissues grow and extend in relation to each other, thus giving rise to the body organs which become easily recognizable in pregnancy. For example, the blood, blood vessel and the heart develop within a month in the developing embryo.

Conditions necessary for the development of embryo include

i.                   The Placenta,

ii.                 The Amniotic fluid and

iii.               The Umbilical cord.

Selective exchange of the materials between the mother and the child takes place in the placenta. In the course of the development of the embryo in pregnancy, an organ called placenta in formed. T is a disc of tissue with projections adhering to the living of the uterus. It is formed between the embryo and the uterine wall. Through the placenta, nourishment and oxygen are obtained by the embryo from the mother and waste products such as carbon (IV) oxide and urea are removed.

Primate Gestation

Gestation is the period that elapses between the time a baby is conceived and when it is born. Compared to other mammals, primates have unusually long gestation periods and primate babies are born in a more mature state than other mammals. For example, nearly all primates are born with their eyes open, while most mammals do not open their eyes for days, or even weeks, after birth. Human gestation, which lasts about nine months, is divided into three trimesters. The developing baby is called an embryo for the first eight weeks, after which it is called a fetus. All of its major organs develop in the first trimester. The second trimester fetus shows obvious human features and grows quickly. In the third trimester, the fetal organs mature.

All the exchanges between the embryo and the blood circulation of the mother take place by diffusion through the placenta

Functions of the placenta include the following:

i.         It anchors the embryo in the uterus

ii.       It serves as the conveyor of water, dissolved oxygen, nutrient, hormone and salt from the blood of the mother to that of the embryo

iii.     Excretory product such as urea, salt and carbon (IV) oxide from the embryo’s blood diffuse into the mother’s blood through it.

iv.     It also produces hormones which help the mother to adapt to pregnancy.

Although there is exchange of materials between the embryo and the maternal blood streams, the blood of the mother and that of the embryo never mix, otherwise the blood pressure of the mother would kill the embryo. Also the blood group of the embryo may not be the same as the mother and agglutination of the embryo blood could occur.

The embryo is attached to the placenta by a tube known as umbilical and which contain the blood vessel of the embryo. It contains two umbilical arteries that carry deoxygenated blood from the foetus to the placenta. it also contains the umbilical vein that transport oxygenated

blood and food substances from the placenta to the foetus.

The foetus has a system of membrane and fluids which help to protect and cushion it against shock.

These membrane system include

(a)   The amnion: it is the innermost membrane that is filled with amniotic fluid which serve the following functions.

i.     Protects the embryo from rocking, shock and desiccation

ii.   it helps to maintain the internal temperature

(b)  The chorion: it is the outermost membrane. It absorb oxygen through it porous walls.

(c)   The alantois: it is involved in respiration, nutrition and excretion through the placenta. The foetus continues to grow in the uterus. At the end of the nine month of pregnancy, the baby is fully formed and is ready to be born. The period between fertilization and birth is called gestation or pregnancy. At the end of the gestation (nine months) the foetus comes to lie with its head downward just above the cervix of the uterus.

Day One

The first cell of a new human being, called a zygote, is formed when a sperm fertilizes an egg. In the egg cell shown here, the nuclei of the sperm and egg appear as two yellow-brown, irregular shapes. When these two nuclei fuse, fertilization is accomplished.

EVALUATION

1.     State three function of the placenta

2.     State the function of the umbilical cord

3. Describe the role of allantois in respiration, nutrition and excretion through the placenta

GENERAL EVALUATION

OBJECTIVE TEST

1.     Which of the following structure is absent in the reproductive system of human female?

(a) ovary (b) uterus (c) fallopian tubes (d) prostate gland

2.     The sperm cells in human male are stored in the — (a) seminal vessels (b) scrotum (c) testes (d) epididymis

3.       fertilisation of the gametes takes place in the (a) Ovary (b) testes (c) oviduct (d) vagina

4.     Which of the following statement bellow is not true? (a) fertilisation of humans occurs in the oviduct (b) fertilisation in humans occur in the uterus (c) fertilisation results in formation of a zygote (d) fertilisation of the egg can only be effected by a sperm cell

5.     Exchange of materials between the embryo and the mother occurs by diffusion through which of the structure below? (a) umbilical cord (b) amnion (c) placenta (d) oviduct

6. The passage of the baby through the birth canal is called (a) delivery (b) conception (c) fertilization (d) gestation

7. Which of the following is not a characteristic of human embryos? (a) They are about 25cm long at birth (b) They are about 1cm long at birth (c) They have 18 pairs of ribs (d) They weigh about 3kg at birth

8. Which of the following statement about milk secretion is not correct? (a) The mammary glands are made up of 15 to 25 lobes (b) The milk is produced in the alveoli in response to stimulation by oxytocin (c) The milk is produced by both the lobes of the alveoli located in the mammary glands (d) The milk is secreted into a network of small canals called the lactiferous ducts, which empty into the nipple

9. Which of the following statements is not true about the reproductive system in males? (a) the penis is the organ that carries sperm to the female genital tract for fertilization (b) the prostate gland produces a milky fluid that forms part of semen (c) the bulbourethral glands produce alkaline mucous (d) the testes produce sperm and testosterone, a hormone that supports male sexual development

10. The process of releasing mature eggs from ovaries is called (a) menstruation (b) ovulation (c) fertilization (d) gestation

Answers:

1. (c) fallopian tubes; 2. (a) seminal vessels; 3. (d) vagina; 4.(b) 1cm long; 5.(c) both the lobes; 6. (b) birth; 7.(a) 25cm long; 8.(c) The oxytocin stimulates the let down reflex; 9.(b) ovulation; 10. (b) ovulation.

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ESSAY TEST

State the function of the following reproductive organs (a) testes (b) fallopian tube (c) prostate gland (d) uterus

1.     State five similarities and seven differences between the reproductive organ of male and female. Human

2.     Name the structure that offer survival condition for the embryo during pregnancy

WEEKEND ASSIGNMENT

Read about fertilization in plants and draw the sexual life cycle of a flowering plant.