PREGNANCY, GROWTH AND DEVELOPMENT
Zygote is formed when a sperm cell fertilizes an egg cell, and it is the beginning of prenatal development. A new human is born after 38 weeks of cell division, growth and specialization into distinctive tissues and organs.
Humans grow, develop, and age.
-Growth is an increase in size. It involves increase in cell number as a result of mitosis, followed by enlargement of the newly formed cells and of the body.
-Development, which includes growth, is the continuous process by which an individual changes from one life phase to another. These life phases include a Prenatal period, which begins with fertilization of an egg cell and ends at birth and a Postnatal period, which begins at birth and ends with death.
Pregnancy:
Pregnancy, which is also known as Gestation lasts for 266 days (38 weeks) from conception to childbirth. It is the presence of a developing offspring in the uterus.
-Pregnancy occurs as a result from the union of the genetic packages of an egg cell and a sperm, which is called Fertilization.
Transport of Sex Cells: Before fertilization can take place, a secondary oocyte must be ovulated and enters a uterine tube. During sexual intercourse, the male deposits semen containing sperm cells in the vagina near the cervix.
-To reach the egg cell, the followings: a) The lashing of sperm flagellum and muscular contraction within the walls of the uterus, and uterine tube, stimulated by Prostaglandins in semen, play important role the swimming of the sperm cell to the uterus.
-b) Also, due to high concentration of estrogens, the uterus and cervix contain a thin, watery secretion that promotes sperm transport and survival.
-In contrast to estrogen effects, high concentration of progesterone seen at the latter of the cycle, makes female reproductive tract secretes a viscous fluid that hampers sperm transport and survival.
-Sperm movement is inefficient because out of 200 to 600 million sperm cells deposited in the vagina by a single ejaculation, only a few hundred reach the secondary oocyte.
-It takes sperm cells 5 to 10 minutes to reach the upper portions of the uterine tube, but they cannot fertilize an egg for about 10 hours.
-Out of the few hundred sperm cells (Spermatozoa) that reach the secondary oocyte, usually only one sperm cell fertilizes it.
-For the sperm cells to fertilize the egg cell, they must first of all undergo a process called Capacitation, which will allow them to penetrate an egg.
-Most sperm cells are fertile for up to 72 hours within the female reproductive tract after ejaculation, while the secondary oocyte may survive for only 12 to 24 hours after ovulation.
-Consequently, sexual intercourse must occur earlier than 72 hours before ovulation or within 24 hours following ovulation if fertilization is to take place.
Fertilization: When the sperm cell encounters an egg cell, it undergoes an Acrosomal Reaction – exocytosis of the acrosome, releasing the enzymes needed to penetrate the secondary oocyte.
-But the firs sperm cell to reach an egg cell is not the one to fertilize it.
-The sperm cell must penetrate the Granulosa cells (Corona Radiata) and Zona Pellucida that surround the egg cell and it may require hundreds of sperm cells to clear a path for the one that penetrates the secondary oocyte.
-Two of the acrosomal enzymes are Hyaluronidase, which digests the Hyaluronic acid that binds follicular cells together, and Acrosin, a protease similar to the trypsin of pancreatic juice.
-When the cell membranes of the sperm cell and the egg cell fuse, the sperm head and midpiece enter the egg and also it sheds its tail.
-At the same time, the oocyte cell membrane becomes unresponsive to other sperm cells caused by the fusing of both the sperm and egg cell membranes, which triggers some lysosome-like granules beneath the egg cell membrane to release enzymes that harden the zona pellucida.
-Polyspermy is a process by which the secondary oocyte is fertilized by two or more sperm cells and to stop this happening, the egg cell uses two mechanisms:
1)The Fast Block. The binding of the sperm to the egg opens Na+ channels in the egg membrane that leads to rapid inflow of Na+ which depolarizes the membrane and inhibits the binding of any more sperm to it.
2) The Slow Block. It involves lysosome-like granules (Cortical Granules) beneath the egg cell membrane.
-When the sperm cell fuses with the egg cell, it triggers an inflow of Ca+ ; this in turn , triggers a cortical reaction in which the cortical granules release their secretion beneath the zona pellucida.
-The secretion swells with water, pushes any remaining sperm away from the egg, and creates an impenetrable Fertilization Membrane.
-Because the sperm cell and egg cell each provide 23 chromosomes, the product of fertilization is a cell with 46 chromosomes-the usual number in a human cell.
The fertilized egg, now called a Zygote, is ready for its first Mitotic division.
Prenatal Period:
This period of development usually lasts for 38 weeks from conception, and it involves three major processes: Cleavage, an Embryonic stage and a Fetal stage.
Period of Cleavage: Conception occurs when the genetic packages of sperm and egg merge, forming a zygote.
-Cleavage refers to mitotic divisions that occur in the first 3 days.
-The first cleavage occurs 30 hours after fertilization and produces the first two daughter cells called Blastomeres.
-Blastomeres divide simultaneously at a shorter and shorter time intervals, doubling the number of blastomeres each time.
-By the time the fertilized egg arrives in the uterus, about 72 hours after ovulation, it consists of 16 or more cells, a structure that resembles a solid ball of cells called Morula.
-Morula remains free within the uterine cavity for about three days while the cell division continues, and the solid ball of cells gradually hollows out.
-The morula lies free in the uterine cavity for 4 to 5 days and divides into 100 cells or so.
-During this time, it is nourished by nutrients that were stored in the egg cytoplasm during oogenesis and by endomertrial secretion called Uterine Milk, which accumulates in a cavity in the morula.
-During this stage, the zona pellucida disintegrates and releases the fertilized egg cell or Conceptus, which is now at a stage called the Blastocyst.
-Within the blastocyst, cells in one region group to form an inner cell mass called the Embryoblast and the cells that form the outer wall of the blastocyst make up the Trophoblast.
-The trophoblast is destined to form part of the placenta and play an important role in nourishment of the embryo, whereas the embryoblast is destined to become the embryo itself.
-About 6 days after ovulation, blastocyst attaches to the endometrium, usually on the posterior wall of the Fundus or body of the uterus.
-This process of the blastocyst nestling into the uterine lining is called the Implantation.
-The trophoblast secretes the called Human Chrionic Gonadotropin (HCG), which keeps the immune system from rejecting the blastocyst and stimulates the corpus luteum to secrete Estrogen and Progesterone, and progesterone suppresses Menstruation.
-HCG stimulate these hormones from the developing Placenta.
-The placenta is a vascular structure, formed by the cells surrounding the embryo and cells of the endometrium, that attaches the embryo to the uterine wall and exchanges nutrients, gases and wastes between the maternal blood and the embryo’s blood.
Hormonal Changes During Pregnancy: Spontaneous abortion or lost of the embryo during a typical reproductive is due to a rapid decrease in the concentrations of estrogens and progesterone.
-HCG, which is produced by embryonic cells called trophoblast, normally helps to prevent spontaneous abortion, and maintains corpus luteum, which continues to secrete estrogens and progesterone.
-Secretion of HCG continues at a high level for about two months, then declines to a low level by the end of four months.
-For the remainder of the pregnancy, the placenta tissue produces high concentration of estrogens and progesterone and other hormones that play different roles during the pregnancy such as:
1) Estrogens and progesterone maintain the uterine wall and inhibit secretion of LH and FSH.
2) Progesterone and Relaxin inhibit contractions of uterine muscles
3) Estrogens cause enlargement of the vagina
4) Relaxin helps relax the ligaments of the pelvic joints.
-The placenta secretes placental lactogen that stimulates the development of the breasts and mammary glands.
-During pregnancy, increasing secretion Aldosterone promotes retention of sodium and body fluid and increasing secretion of parathyroid hormone helps maintain a high concentration of maternal blood calcium.
Other Changes During Pregnancy: Other changes that occur during pregnancy including the followings:
- a) The uterus enlarges greatly. b) The woman’s blood volume, cardiac output, breathing rate and urine production increase. c) The woman’s dietary needs increase, but if intake is inadequate, fetal tissues have priority for use of available nutrients.
Embryonic Stage: The embryonic stage extends from the beginning of the second week through the eighth week of prenatal development. During implantation, the embryoblast undergoes Embryogenesis-arrangement of the blastomeres into three Primary Germ Layers: Ectoderm, Mesoderm and Endoderm.
-Embryogenesis begins with the formation of a narrow space called the Amniotic Cavity between the embryoblast and cytotrophoblast.
-The embryoblast flattens into an Embryonic Disc composed of ectoderm and endoderm.
-As the disc elongates, a raised groove called the primitive streak forms along the midline of the ectoderm.
-Cells on the surface migrate medially toward this groove, down into it and then laterally between the ectoderm and endoderm.
-This forms the middle germ layer called mesoderm.
-All organs form from the primary germ layers and when this is formed, the embryo is called Gastrula.
-Also, during this time, another structure that appears is called a Connecting Stalk, which attaches the embryo to the developing Placenta.
-Gastrulation is an important process in prenatal development a cell’s fate is determined by which it is in.
1) Ectoderm gives rise to the nervous system, potions of skin, the lining of the mouth, and lining of the anal canal.
2) Mesoderm gives rise to muscles, bones, blood vessels, lymphatic vessels, reproductive organs, kidneys and linings of body cavities.
3) Endoderm gives rise to linings of digestive tract, respiratory tract, urinary bladder and urethra.
One of the important changes to occur in the embryonic phase is the mode of Nutrition.
-As the embryo implants in the uterus, proteolytic enzymes from the Trophoblast break down endometrial tissue, providing nutrients for the developing embryo.
-During this phase, Chorionic Villi develop and are surrounded by spaces called Lacunae, which are filled with maternal blood.
-By the end of week four, the embryo develops head, face, upper and lower limbs, and the heart beats and forces blood through blood vessels.
-During the fifth to the seventh week the embryo’s head grows rapidly and become rounded and erect. The face, which is developing the eyes, nose, and mouth, appears more humanlike.
-The upper and lower limbs elongate, and fingers and toes form and at the end of week seven, all the main internal organs are established, and these structures enlarge, the body takes on a humanlike appearance.
-From the placenta that is formed, is a membrane called the placenta membrane, which consist of the epithelium of the villi and the epithelium of the capillaries (blood vessels) inside the villi.
-Through the placenta membrane, oxygen and nutrients diffuse from the maternal blood into fetal blood, while carbon dioxide and other wastes diffuse from the fetal blood through the placenta membrane and into the maternal blood.
-When the placenta is fully formed, it appears as a reddish-brown disc, and the surface facing the fetus is smooth and gives rise to the Umbilical Cord.
The placenta and umbilical cord are not the only accessory organs of the fetus. There are also four membranes namely: the Amnion, York Sac, Allantois and Chorion.
The Amnion: The amnion is a transparent sac that develops from cells of embryonic disc and in between the amnion and embryonic disc is a fluid called Amniotic Fluid.
-The amniotic fluid protects the embryo from trauma and temperature fluctuations, allows the freedom of movement important to muscle development, enables the embryo to develop symmetrically, and prevents adhesion of, for example, an arm to the trunk.
The York Sac: It is formed during the second week, and it arises partly from cells of the embryonic disc opposite the amnion. It is a small sac suspended from the ventral side of the embryo.
-It contributes to the formation of the Digestive Tract and produces the first Blood Cells and future Egg and Sperm cells.
The Allantois: It is formed during the third week as a tube extending from the early yolk sac into the connecting stalk of the embryo.
-It, too, forms the blood cells, and gives rise to the umbilical arteries and vein.
The Chorion: It is the outermost membrane, enclosing all the rest of the membranes and embryo.
-The chorion forms the fetal portion of the placenta, and it fuses with the amniotic cavity membrane to form Amniochorion membrane.
-The function of the chorion involves the following roles: 1) Nutritional roles, 2) Excretory roles, 3) Respiratory roles 4) Endocrine roles and 5) Immune role.
By the end of week 8, embryo is recognizable as a human and that concludes the end of embryonic stage.
-The embryonic stage is the most critical period of development, because during it, the embryo implants within the uterine wall, and all the essential external and internal body parts formed.
–Disturbances to development during the embryonic stage can cause major malfunctions and this why early prenatal care is very important.
Fetal Stage: The fetus is the final stage of prenatal development, extending from the end of the eighth week until birth.
-The organs that formed during the embryonic stage now undergo growth and cellular differentiation and acquire the functional capability to support life outside the mother.
-At the beginning of the fetal stage, the head is disproportionately large, and the lower limbs are relatively short.
During the third month, the body lengthening accelerates, but growth of the head slows.
-The upper limbs achieve the relative length they will maintain throughout development and ossification centers appear in most of the bones.
-By the time the fetus is 12 weeks, the external reproductive organs a distinguishable as male or female.
In the fourth month, the body grows very rapidly and reaches a length of up to 20 cm and weighs about 170 grams.
-The fetus has hair, nipples and nails and may even scratch itself.
In the fifth month, growth slows, and the lower limbs achieve their final relative proportions.
-Skeletal muscles contract and the pregnant woman may feel fetal movements for the first time.
-Some hair grows on the fetal head, and fine, downy hair called Lanugo covers the skin.
-The skin is covered with Sebum from the sebaceous glands and dead epidermal cells also coats the skin.
During the sixth month, the fetus gains a substantial amount of weight.
-Eyebrows and eyelashes appear, the skin is quite wrinkled and translucent and blood vessels in the skin cause a reddish appearance.
In the seventh month, the skin becomes smoother as fat is deposited in the subcutaneous tissues.
-The fused eyelids open and the fetus is bout 40 cm long.
In the final trimester, fetal brain cells rapidly form networks, as organs elaborate and grow.
-A layer of fat I laid down beneath the skin and testes of males descend from regions near the developing kidneys, through the inguinal canal and into the scrotum.
-The digestive and respiratory systems mature last, which is why infants born prematurely often have difficulty digesting milk and breathing.
Fetal Blood and Circulation: Throughout fetal development, the maternal blood supplies oxygen and nutrients and carries away wastes.
–