Fertilization and fetal development.docx

FETAL DEVELOPMENT

THE GENESIS OF LIFE

INTRODUCTION

  • The process of human development begins with the fertilization of an ovum (female gamete) by a spermatocyte (male gamete).
  • The fusion of the ovum and sperm marks the beginning of series of events that progress until a new human being is formed
  • These series of changes occur for 37 to 42 weeks of gestation.

FERTILIZATION

  • Fertilization is the process by which a sperm fuses with an ovum to form a new diploid cell called zygote.
  • Zygote begins life as a single cell with a complete set of genetic material; 23 chromosomes from the ovum and 23 chromosomes from the sperm, a total of 46 chromosomes.
  • The process of fertilization takes place in the ampulla of the fallopian tube.

FERTILIZATION PROCESS

  • Large number of spermatozoa are necessary to increase the chances for conception, this is because the spermatozoa must traverse the cervical canal, the uterus and the fallopian tube to get to the ovum.
  • Approximately 200 to 600 million spermatozoa are deposited into the vagina in a single ejaculation out of which only about 300-500 actually reach the ovum and only a spermatozoon fertilizes the ovum.
  • The mature ovum and the spermatozoa have only a brief time to unite.
  • Ova are considered fertile for about 12 to 24 hours after ovulation, sperm can survive in the female reproductive tract for 48 to72 hours but are believed to be healthy and highly fertile for only about 24hours.

FERTILIZATION PROCESS

  • The cell membrane of the ovum is surrounded by two layers of tissue which forms a barrier around the secondary oocytes; the zone pellucida and corona radiata and are held together by hyaluronic acid.
  • The zona pellucida is closest to the cell membrane of the ovum, it is a clear non cellular layer whose thickness influences fertilization rate.
  • The corona radiata is a ring of elongated cells surrounding the zona pellucida.
  • The ovum has no inherent power of movement, high estrogen levels during ovulation helps to increase peristalsis within the fallopian tubes, which helps propel the ovum through the tube towards the uterus, the spermatozoa on the other hand derive energy for movement from the fructose in semen secreted by seminal vesicles.

FERTILIZATION PROCESS

  • The spermatozoa propels themselves up the female tract by the
flagellar movement of their tails.
  • Muscular contraction of the uterus and fallopian tube from the effect of prostaglandin (from the sperm) on smooth muscles and estrogen effect of thinning out the cervix also aid the movement of the sperm upward.
  • Transit time of the sperm from the cervix into the fallopian tube can be as short as 5minutes but usually takes an average of 2-7hrs after ejaculation.

FERTILIZATION PROCESS

  • The sperm must undergo two physiological process before it can penetrate the corona radiata and zona pellucida for fertilization to take place.
  • These processes are capacitation and acrosomal reaction.
  • Capacitation is an enzymatic reaction that removes the glycoprotein coating from the spermatozoa and plasma protein from the seminal fluid
  • If the glycoprotein coat is not removed, the sperm will not be able to fertile the ovum.
  • Capacitation occurs in the uterus or uterine tube and takes about 7hrs.

FERTILIZATION PROCESS

Characteristics of the sperm after capacitation

After capacitation, the sperm takes on 3 characteristics, these are:

  • Ability to under acrosomal reaction
  • Ability to bind to the zona pellucida
  • Acquisition of hypermotility

FERTILIZATION PROCESS

  • Acrosomal reaction follows capacitation
  • The acrosome of the sperms surrounding the ovum release their enzymes (hyaluronidase and trypsinlike substances) to break down the hyaluronic acid in the ovum’s corona radiata.
  • Hundreds of acrosomes rupture before enough hyaluronic acid is cleared for a single sperm to penetrate the zona pellucida successfully.
  • At the moment of penetration by the fertilizing sperm, the zona pellucida undergoes a reaction known as block of polyspermy that prevents additional sperm from entering a single ovum.
  • After the sperm penetrates the ovum, the true moment of fertilization occurs as the nuclei of the ovum and the sperm units to produce a diploid zygote.
  • The zygote contains a new combination of genetic material that results in an individual different from either parent and from anyone else.

FERTILIZATION PROCESS

  • The sex of the zygote is determined at fertilization by the contribution of the father.
  • Females have two X chromosomes and males have an X and a Y chromosome.
  • If the spermatozoon contains an X chromosome, the offspring is female (46, XX), if the spermatozoon has one Y chromosome, the offspring will be male (46, XY).

Stages of fetal development

  • The embryonic stage
  • The fetal stage

Embryonic stage

  • The embryonic period is the first 8 weeks of fertilization.
  • The first 14days of development from the day the ovum is fertilized is the pre embryonic period
  • This stage is majorly for formation of all major organs (organogenesis).
  • The embryo is most venerable to teratogens during this period.
  • The shape of the embryo changes as the organs develops, taking a more human shape by the end of 8th week
  • Development at this stage is divided into two phases:
cellular multiplication and cellular differentiation.
  • These phases are characterized by rapid cellular multiplication, differentiation and establishment of the primary germ layers and embryonic membranes.

Cellular multiplication

  • Cellular multiplication begins as the zygote moves through the fallopian tube toward the cavity of the uterus.
  • It takes about 3days or more to reach the uterine cavity.
  • The zygote undergoes a mitotic division called cleavage.

Cleavage

  • Cleavage is a term used to describe the mitotic cell division of the zygote.
  • Mitotic cell division occurs after fertilization as the zygote passes down the uterine tube, resulting in the formation of two blastomeres.
  • The cells continue to divide, increasing in number although decreasing in size.
  • When the number of cells reaches approximately 16 (usually on the 3rd day, when it reaches the uterus) the zygote is called a morula.
  • The morula consist of groups of centrally located cells called the inner cell mass and an outer cell layer.
  • The inner cell mass called the embryoblast gives rise to the embryo, the outer cell layer forms the trophoblast from which the placenta develops.
  • After the morula penetrates the uterine cavity, fluid enters through the zona pellucida into the intercellular space of the inner cell mass, the morula is now called the blastocyst, the fluid filled space fuse, forming a large cavity the blastocyst cavity about the 4th day after fertilization.
  • The blastocyst develops into a double layer of cells called the embryonic disk from which the embryo and amnion develops.

Cellular differentiation

  • About the 10th to 14th day after fertilization, the blastocyst differentiates into the primary germ layers:
  • the ectoderm
  • the mesoderm
  • the endoderm
  • From which all tissues, organs and systems develops.

Fetal stage

  • The fetal stage starts from the end of the 8th week till birth.
  • It is characterized by the growth and development of the organs formed during the embryonic stage

Developmental activities by weeks

Week three
  • Embryonic disk becomes elongated and pear shaped
  • Broad cephalic end and a narrow caudal end
  • Ectoderm form cylindrical tube for brain and spinal cord development
  • The endoderm gives rise to the gastrointestinal tract which appears as a tube-like structure
  • The most advanced organ at week 3 is the heart

4-5weeks

  • Development of vertebrae that form the spinal cord
  • Development of the pharyngeal arches which will form the lower jaw, hyoid bone and larynx
  • Development of the pharyngeal pouches which forms the Eustachian tube and cavity for the middle ear, the tonsils and the parathyroid and thymus glands
  • The primordia of the ear and eye are also present.

4-5weeks

  • By the end of 28 days, the tubular heart is beating at a regular rhythm and pushing its own primitive blood cells through the main blood vessels.
  • During the fifth week, the optic cups and lens vessels of the eye form and the nasal pits develop.
  • The embryo has a marked C-shaped body, with rudimentary tail and the large head folded over a protuberant trunk
  • By day 35, the arm and leg buds are well developed, with paddle-shaped hand and foot plates.
  • The heart, circulatory system, and brain show the most advanced development.

Six Weeks

  • At 6 weeks the head structures are more highly developed and the trunk is straighter than in earlier stages.
  • The upper and lower jaws are recognizable, and the external nostrils are well formed.
  • The trachea has developed, and its caudal end is bifurcated for beginning lung formation.
  • The upper lip has formed, and the palate is developing.
  • The ears are developing rapidly.
  • The arms have begun to extend ventrally across the chest, and both arms and legs have digits, although they may still be webbed.
  • There is a slight elbow bend in the arms, which are more advanced in development than the legs.
  • Beginning at this stage, the prominent tail will recede.
  • The heart now has most of its definitive characteristics, and fetal circulation begins to be established. The liver starts to produce blood cells.

Seven Weeks

  • At 7 weeks the head of the embryo is rounded and nearly erect
  • The eyes have shifted and are closer together, and the eyelids are beginning to form.
  • The palate is near completion, and the tongue is developing in the formed mouth.
  • The gastrointestinal and genitourinary tracts undergo significant changes during the seventh week.
  • Before this time the rectal and urogenital passages formed one tube that ended in a blind pouch; they now separate into two tubular structures.
  • The beginnings of all essential external and internal structures are present.

Eight Weeks

  • At 8 weeks the embryo’s crown-rump (C-R) length is approximately 3 cm (1.2 in.) and it clearly resembles a human being.
  • Facial features continue to develop. The eyelids begin to fuse.
  • Auricles of the external ears begin to assume their final shape, but they are still set low
  • External genitals appear, but the embryo’s sex is not clearly identifiable.
  • The rectal passage opens with the perforation of the anal membrane.
  • The circulatory system through the umbilical cord is well established.
  • Long bones are beginning to form, and the large muscles are now capable of contracting
  • By the end of the eighth week, the embryo is sufficiently developed to be called a fetus.
  • Every organ system and external structure that will be found in the full-term newborn is present.
  • The remainder of gestation is devoted to refining structures and perfecting function.

Nine to Twelve Weeks

  • By the end of the ninth week the fetus reaches a C–R length of 5 cm (2 in.) and weighs about 14 g.
  • The head is large and comprises almost half of the fetus’s entire size.
  • At 12 weeks, the fetus reaches 8 cm (3.2 in.) C–R length and weighs about 45 g.
  • The face is well formed, with the nose protruding, and the ears acquiring a more adult shape.
  • The eyelids close at about the 10th week and will not reopen until about the 26- to 29-week period.
  • Tooth buds appear for all 20 of the child’s first teeth (baby teeth). The

limbs are long and slender, with well-formed digits.

  • The fetus can curl the fingers toward the palm and begins to make a tiny fist.
  • The legs are still shorter and less developed than the arms.
  • The urogenital tract completes its development, well-differentiated genitals appear, and the kidneys begin to produce urine.
  • Fetal heart rates can be ascertained by electronic devices between 8 and 12 weeks.

Thirteen to Sixteen Weeks

  • This is a period of rapid growth.
  • At 13 weeks, the fetus weighs 55 to 60 g and has a C–R length of about 9 cm (3.6 in.).
  • Lanugo begins to develop, especially on the head.
  • The skin is so transparent that blood vessels are clearly visible beneath it.
  • More muscle tissue and body skeleton have developed and hold the fetus more erect
  • Active movements are present; the fetus stretches and exercises its arms and legs.
  • It makes sucking motions, swallows amniotic fluid, and produces meconium in the intestinal tract.
  • Bronchial tubes are branching out in the primitive lungs, and sweat glands are developing.
  • The liver and pancreas now begin production of their appropriate secretions.
  • By the beginning of week 16, skeletal ossification is clearly identifiable.

Sixteen to Twenty Weeks

  • The fetus doubles its C–R length and now measures 19 cm (8 in.) long.
  • Fetal weight is between 435 and 465 g.
  • Lanugo covers the entire body and is especially prominent on the shoulders.
  • Subcutaneous deposits of brown fat, which has a rich blood supply, make the skin less transparent.
  • Brown fat is found chiefly at the root of the neck, posterior to the sternum and in the perirenal area.
  • Nipples now appear over the mammary glands.
  • The head is covered with fine, “woolly” hair, and the eyebrows and

eyelashes are beginning to form.

  • Nails are present on both fingers and toes.
  • Muscles are well developed, and the fetus is active
  • The mother feels fetal movement, known as quickening. The fetal heartbeat is audible through a fetoscope.

Twenty-Four Weeks

  • The fetus at 24 weeks reaches a crown-to-heel (C–H) length of 28 cm (11.2 in.) and weighs about 780 g.
  • The hair on the head is growing long, and eyebrows and eyelashes have formed. The eye is structurally complete.
  • The fetus has developed a reflex hand grip (grasp reflex) and a startle reflex.
  • Skin covering the body is reddish and wrinkled, with little subcutaneous fat. Skin on the hands and feet have thickened, with skin ridges on palms and soles forming distinct footprints and fingerprints.
  • The skin over the entire body is covered with vernix caseosa, a protective cheeselike, fatty substance secreted by the sebaceous glands. The alveoli in the lungs are just beginning to form.

Twenty-Five to Twenty-Eight Weeks

  • At about 25 weeks, the fetal skin is still red, wrinkled, and covered with vernix caseosa.
  • The brain is developing rapidly, and the nervous system is complete enough to

provide some degree of regulation of body functions.

  • The eyelids, under neural control, open and close. The fetus has nails on both fingers and toes. In the male fetus, the testes begin to descend into the scrotal sac.
  • Even though the lungs are still physiologically immature, they are sufficiently

developed to provide gas exchange.

  • A fetus born at this time will require immediate and prolonged intensive care to survive and then to decrease the risk of major handicap. The fetus at 28 weeks is about 27 cm long C–R or 35 to 38 cm (14 to 15 in.) long C–H and weighs 1200 to 1250 g.

Twenty-Nine to Thirty-Two Weeks

  • At 30 weeks the pupillary light reflex is present
  • The fetus weighs about 2000 g with a C–R length of 31 cm (12.4 in.) or with a C–H length of about 38 to 43 cm (15 to 17 in.), by 32 weeks of age.
  • The central nervous system (CNS) has matured enough to direct rhythmic breathing movements and partially control body temperature, however, the lungs are not yet fully mature.
  • Bones are fully developed but soft and flexible.
  • The fetus begins storing iron, calcium, and phosphorus.
  • In males the testicles may be located in the scrotal sac but are often still high in the inguinal canals.

Thirty-Five to Thirty-Six Weeks

  • The fetus begins to get plump, and less wrinkled skin covers the deposits of subcutaneous fat. Lanugo begins to disappear, and the nails reach the edge of the fingertips. By 35 weeks of age the fetus has a firm grasp and exhibits spontaneous orientation to light.
  • By 36 weeks of age the weight is usually 2500 to 2750 g, and the C–H length of the fetus is about 42 to 48 cm (17 to 19 in.) or C–R length is about 35 cm (14 in.).
  • An infant born at this time has a good chance of surviving but may require special care.

Thirty-Eight to Forty Weeks

  • The fetus is considered term at 37 weeks and up to 40 weeks after conception.
  • The C–H length varies from 48 to 52 cm, with males usually longer than females.
  • Males also usually weigh more than females.
  • The skin is pink and has a smooth, polished look.
  • The only lanugo left is on the upper arms and shoulders.
  • Vernix caseosa is present, with heavier deposits remaining in the creases and folds of the skin.
  • The body and extremities are plump, with good skin turgor, and the fingernails extend beyond the fingertips.
  • The chest is prominent but still a little smaller than the head, and mammary glands protrude in both sexes.
  • In males, the testes are in the scrotum or palpable in the inguinal canals.
  • As the fetus enlarges, amniotic fluid diminishes to about 500 ml or less, and the fetal body mass fills the uterine cavity.
  • The fetus assumes what is called its position of comfort or lie.
  • The head is generally pointed downward, following the shape of the uterus (and possibly because the head is heavier than other parts).
  • The extremities, and often the head, are well flexed.