Lesson 2 Chapter 27/28 – Human Development

Lesson 2 Chapter 27/28 – Human Development 1 Unit 11 Study Guide Excluded: The neonatal period – pages # 765 - 798 Human Development  Prenatal development occurs in the period from conception to delivery.  The term embryology refers to the study of the developmental events that occur during prenatal development.  The period of prenatal development can be further subdivided.  Pre-embryonic development begins at fertilization and continues through cleavage and implantation.  Pre-embryonic development is followed by embryonic development, which extends from implantation to the end of the eighth developmental week.  Fetal development begins at the start of the ninth developmental week and continues up to the time of birth. Lesson 2 Chapter 27/28 – Human Development Developmental Anatomy 1. Pre-embryonic period a. 2 weeks period i. Fertilization ii. transportation of zygote iii. implantation iv. formation of embryonic tissue 2. Embryonic period a. 6 weeks period i. differentiation of germ layers into body organs ii. formation of embryonic membranes iii. formation of placenta, umbilical cord 3. Fetal period a. from 9 weeks until birth i. growth and specialization of body structures From Fertilization to Implantation 2 Lesson 2 Chapter 27/28 – Human Development 3 1. Fertilization: the fusion of male and female gametes to form a single cell that contains 23 chromosomes from each parent = 46 chromosomes. o In the female body, an ovum is released from the ovary and it is transported through the fallopian tube (oviduct) towards the uterus o Fertilization must occur within the fallopian tubes due to the short life span of the ovum (within 12-24 h of its release) o Sperm cells enter the females vagina to the cervix, through the uterus & then finally the oviduct o Many sperms do not survive the journey: i. Many are destroyed by the acidic environment of the vagina ii. Other sperm enter the oviduct that does not have an egg o Only 1 sperm penetrates through the outer layer of the ovum i. Acrosome reaction: when the sperm comes in contact with the coats surrounding the egg, the acrosome (an enzyme filled vacuole) bursts open to release enzyme that digest the jelly-like coating of the egg. ii. Penetrates of egg membrane: when the sperm membrane comes in contact with the egg membrane, the sperm nucleus & egg nucleus fuse, completing fertilization. iii. Cortical reaction: Once the membrane have fused, the egg’s plasma membrane depolarizes preventing other sperms from binding with and entering it. Lesson 2 Chapter 27/28 – Human Development 4 o Once the sperm’s nucleus combines with the egg’s nucleus, a diploid zygote has formed. 2. Cleavage and Implantation: the zygote continues to grow and move down the fallopian tube into the uterus. o Within 30 h of being fertilized, the zygote divides by mitosis creating 2, which will divide o o o o o o o o again to make 4 cells, and then divide to form 8, and so on. Cleavage is the process of cell division without enlargement of the cells. By the time the zygote is a sphere of 16 cells, it is called a morula. 3-5days after fertilization, the morula reaches the uterus & begins to fill the uterine fluids. It soon develops into a hollow fluid-filled ball of cells called a blastocyst. The trophoblast (outer layer) forms and will soon become the chorion layer, which will eventually become the placenta (a structure that provides nutrients & oxygen to, and removes wastes from, the developing offspring.) The inner cell mass of the blastocyst will develop into the embryo. Between 5-7 days, the blastocyst attaches to the thickened endometrium (implantation) with the help of the enzymes produced by the trophoblast. Implantation is completed by the 10th to 14th day, woman is now said to be pregnant. OMG! Around the time of implantation, the trophoblast starts to secrete hCG hormone (human chorionic gonadotropin) i. hCG maintains the corpus luteum, like LH psat the time the corpus luteum would normally degenerate, so estrogen and progesterone continue to “maintain pregnancy” and prevent menstruation. ii. Increased hCG production for 2 months, then it will drop to a low level by the end of 4 months iii. Placenta will then take over to produce estrogen and progesterone to maintain the endometrium 3. Gastrulation: After implantation, the inner cell mass changes: o Space forms between the trophoblast and inner cell mass creating the amniotic cavity which will soon fill with amniotic fluid in the amniotic sac & is the place where the baby will develop. o The inner cell mass flattens into the embryonic disk which is supported by short stalk that connects the blastocyst with the endometrium. o The embryonic disk will form 3 layers: endoderm (in), ectoderm (out), mesoderm (middle); it is called gastrulation as the 3 layers are called the primary germ layers (embryo is now a gastrula) i. Ectoderm (outer layer): epidermis & nervous system ii. Mesoderm (middle layer): skeleton, muscles, and reproductive structures iii. Endoderm (inner layer): linings of the digestive and respiratory tracts and the endocrine glands Lesson 2 Chapter 27/28 – Human Development 5 o Gastrulation marks the start of Morphogenesis: the series of events that form distinct structures of the developing organism o Differentiation: process that enables a cell to develop a particular shape & allow certain cell to perform specific functions. 4. Neurulation: the embyro’s nervous system begins to develop from the ectoderm that is located above the notcochord (rod-like structure) o The neural plate folds creating the neural tube o The neural tube pinches off from the ectoderm, which develops into the spinal cord and brain. The outer cells of the blastocysts form the extra-embyronic membranes (trophoblast) surround and support the embryo tissues of the embryo: - - - - Chorion: o Forms the fetal portion of the placenta o Secretes hCG; maintains corpus luteum and pregnancy Amnion: o Transparent sac, protection, contains amniotic fluid o Prevents shock to the embryo, helps regulate body temp., prevents infection and allows movement Yolk sac: o Sac suspended from abdominal area of embryo o Serves no nutritive function o Early red blood cells, formation of digestive tract and future egg or sperm cells Allantois: o Foundation of the umbilical cord (contains 2 arteries, 1 vein) o During 2nd month, most of the allantois degenerates and the remainder becomes part of the urinary bladder The placenta is usually fully formed by the end of the 2 nd month. It is the site of gas, nutrient, mineral, hormone, antibody and waste exchange. It produces the hormones (hCG, estrogen, progesterone, and relaxin). It acts as a barrier to most but not all teratogenic agents (environmental agents such as drugs) Lesson 2 Chapter 27/28 – Human Development Embryo vs Fetus The embryo is defined as the developing pregnancy from the time of fertilization until the end of the eighth week of gestation, when it becomes known as a fetus (8-23 weeks). 3 stages of labour: • • • Stage of dilation • 6-12 hours • from onset of labor to the complete dilation of the cervix • rupture of amniotic sac Stage of expulsion • from complete cervical dilation to delivery of the baby • 10 minutes to hours Placental stage 6 Lesson 2 Chapter 27/28 – Human Development • 5 – 30 minutes Abnormalities of the Placenta • Placenta previa – placenta implantation at the lower segment of the uterus – obstructing the descent of the baby’s head – presented with antepartum hemorrhage • Abruptio placentae – premature separation of the placenta – bleeding may be concealed or revealed 7 Lesson 2 Chapter 27/28 – Human Development 8