Anatomy and Physiology Biology 2401 Chapter-29 Development and Inheritance

An Introduction to Development and Inheritance Gradual modification of anatomical structures and physiological characteristics from fertilization to maturity Inheritance Transfer of genetic material from generation to generation

This chapter on the development includes: Fertilization – union of an egg and sperm Gestation - fertilization  birth; 38 weeks of pregnancy Prenatal period – time before birth Embryo development (first 8 weeks) Fetal development (week 9 till birth) Neonatal period – first 42 days after birth Labor - events that lead to birthing Parturition - birthing process Obstetrics: branch of medicine that deals with pregnancy, labor and delivery

Fertilization Fertilization Capacitation Occurs in uterine tube within a day after ovulation Secondary oocyte travels a few centimeters Spermatozoa must cover distance between vagina and ampulla Capacitation Must occur before spermatozoa can fertilize secondary oocyte Contact with secretions of seminal glands Exposure to conditions in female reproductive tract

Fertilization Sperms: They have Head – with haploid nucleus (22X or 22Y) and acrosome Midpiece – with mitochondria for producing energy (ATP) Flagella – tail for swiming About 300-500 million sperms are deposited in the vagina They undergo changes and develop the capacity to fertilize (capacitation) They move through the uterus and uterine tube with the help of their flagella and muscular contractions of the uterus.

Fertilization Secondary oocyte: It is a haploid cell (22X) that is released after meiosis I (ovulation) It has a glycoprotein layer – zona pellucida It has multiple layers of follicular/granulosa cells – corona radiata It moves into the uterine tube with the help of peristalsis and ciliated epithelial cells It remains active for about 12-24 hours after ovulation

Fertilization Sperms penetrate corona radiata layer Acrosomal enzymes digest their way through zona pellucida One sperm fuses with the secondary oocyte (syngamy) Oocyte membrane depolarizes Intracellular release of calcium causes zona pellucida to harden (cortical reaction) Blocks fusion of any other sperm (polyspermy is prevented) Syngamy triggers secondary oocyte to undergo meiosis II Ovum and second polar body are formed Male pronucleus and female pronucleus fuse (fertilization) Diploid zygote (44XX or 44XY) is formed

Gestation First Trimester Second Trimester Third Trimester Period of embryological and early fetal development Rudiments of all major organ systems appear Second Trimester Development of organs and organ systems Body shape and proportions change Third Trimester Rapid fetal growth and deposition of adipose tissue Most major organ systems are fully functional

The First Trimester First Trimester Includes four major stages Cleavage Implantation Placentation Embryogenesis

Early Embryonic/Cleavage Divisions Zygote (surrounded by zona pellucida)  divides by mitosis (cleavage divisions) 2 identical blastomeres are formed 4 blastomeres are formed by the 4th day, a ball of genetically identical blastomeres is formed – an embryo stage called the morula blastomeres secrete a fluid in the center By the 5th day, a hollow ball of cells is formed Blastocyst: - trophoblast cells are the peripheral cells which will form the future embryonic membranes - inner cell mass or embryoblast which will form the future embryo - blastocele is the internal fluid-filled cavity

Location of Events Syngamy takes place in the oviduct Fertilization takes place in the oviduct Early embryonic/cleavage divisions take place in the oviduct while the embryo is moving towards the uterus Blastocyst reaches the uterus by the 5th day Blastocyst implants in the uterus Ectopic pregnancy: development of an embryo or fetus outside the uterine cavity : mostly occur in the uterine tube

Twins Fraternal twins (dizygotic – from two zygotes) Release of 2 oocytes  fertilized by 2 sperms  two zygotes are formed  two embryos genetically as different as any 2 siblings Identical twins (monozygotic – from one zygote) Release of 1 oocyte  fertilized by 1 sperm  one zygote is formed  zygote divides to form 2 blastomeres  blastomeres separate to independently develop into 2 embryos genetically identical & always the same sex Conjoined twins If the blastomeres do not completely separate  newborns remain attached to different extents  share some body structures

Implantation – Attachment of the Embryo to the Uterus Trophoblast cells: - Secrete enzymes to digest the uterus wall  the entire embryo slowly becomes embedded into the endometrium of the uterus - Secrete human chorionic gonadotropin (hCG) that maintains the uterine lining

Embryo Development Gastrulation Cells of embryonic disc produce 3 distinct layers Endoderm  epithelial lining of GI & respiratory Mesoderm  muscle, bone & other connective tissues Ectoderm  epidermis of skin & nervous system

The First Trimester Formation of the Extraembryonic Membranes Support embryological and fetal development Yolk sac Amnion Allantois Chorion

Figure 29-5 Extraembryonic Membranes and Placenta Formation The embryonic disc bulges into the amniotic cavity at the head fold. The allantois, an endodermal extension surrounded by mesoderm, extends toward the trophoblast. Week 3 Amniotic cavity (containing amniotic fluid) Allantois Head fold of embryo Syncytial trophoblast Chorion Chorionic villi of placenta Yolk sac 16

Placenta Formed during 3rd month Maternal & fetal blood vessels do not join  blood does not mix Allows diffusion of O2, nutrients, wastes Does not allow most microbes (exceptions) Not a barrier to drugs such as alcohol Secretes several hormones to support pregnancy Detaches from the uterus at birth time (afterbirth) Umbilical cord 2 arteries that carry deoxygenated blood to the maternal vessels 1 umbilical vein that carries oxygenated blood to the fetus Has embryonic connective tissue Cut at the time of delivery Stub drops off in 2 weeks  leaving a scar (umbilicus)

The Endocrine Placenta Synthesized by placenta and released into maternal bloodstream Human chorionic gonadotropin (hCG) Human placental lactogen (hPL) Placental prolactin Relaxin Progesterone Estrogens

The First Trimester Embryogenesis Body of embryo begins to separate from embryonic disc Body of embryo and internal organs start to form Folding, differential growth of embryonic disc produces bulge that projects into amniotic cavity Projections are head fold and tail fold Organogenesis Process of organ formation

The Second and Third Trimesters Second Trimester Fetus grows faster than surrounding placenta Third Trimester Most of the organ systems become ready Growth rate starts to slow Largest weight gain Fetus and enlarged uterus displace many of mother’s abdominal organs

Fetal Development

Labor Parturition: giving birth Labor: process of expelling the fetus : initiated by a decrease in progesterone levels and elevated levels of estrogens, prostaglandins, oxytocin, and relaxin : Uterine contraction forces fetal head into cervix (stretch)  nerve impulses reach hypothalamus  oxytocin is release  increased contractions  further stretching of cervix True labor: uterine contractions occur at regular intervals  dilation of the cervix  “show” (discharge of blood-containing mucus from the cervical canal) False labor: contractions and pain at irregular intervals : no cervical dilation

Stages of Labor 1. Dilation - 6 to 12 hours - regular contractions of the uterus - rupture of amniotic sac & dilation of cervix (10cm) 2. Expulsion - 10 minutes to several hours - baby moves through birth canal 3. Placental - 30 minutes - placenta (afterbirth) is expelled by uterine contractions - constriction of blood vessels that were torn  reduce the possibility of hemorrhage Cesarean section (C-section): incision through lower abdominal wall and uterus to deliver the baby

29-7 Postnatal Life Duration of Life Stages Neonatal Period: extends from birth to 1 month Infancy: 1 month to 2 years of age Childhood: 2 years until adolescence Adolescence: period of sexual and physical maturation Senescence: process of aging that begins at end of development (maturity)

Lactation Lactation: production & release of milk During pregnancy  anterior pituitary secrets increased levels of prolactin  progesterone inhibits the effect of prolactin After delivery  progesterone levels drop  prolactin stimulates milk production Suckling  nerve impulses to hypothalamus  increased release of oxytocin  milk ejection reflex If suckling stops, milk secretion stops Colostrum: a cloudy fluid released by mammary gland during late pregnancy . : contain antibodies that protect the infant.

Inheritance Inheritance: passing of traits from one generation to another. Genetics: branch of biology that deals with inheritance. Genetic counselor: health care personnel that offers advice on genetic problems Human somatic cells: - diploid cells - 44 autosomes + 2 sex chromosomes - Males: 44XY - Females: 44XX

Determination of Sex

Inheritance

Inherited Disorders

Testing for Genetic Disorders Ultrasound: - Transducer emits high-frequency sound waves  reflected sound waves are converted to on-screen image called sonogram Amniocentesis: - usually done at 14-16 weeks gestation - amniotic fluid around the fetus is extracted  fetal cells are examined for genetic defects Chorionic Villus Sampling: - withdrawal of chorionic villi for chromosomal analysis