Animal Development Chapter 44

Fertilization The union of sperm and egg to form a zygote. It requires three main events occur.

Stage 1- Attachment Sperm reach the egg membrane by burrowing through the corona radiata and bind to the zona pellucida. The acrosome of the sperm releases enzymes to digest a portion of zona pellucida.

Stage 2- Prevention of Polyspermy The second a sperm touches the plasma membrane of an egg, the membrane depolarizes to temporarily repel sperm. Vesicles in the egg cell release enzymes that turn the zona pellucida impenetrable.

Stage 3- Becoming Diploid The sperm releases its chromatin into the cell. The chromatin reorganizes into chromosomes in the sperm pronucleus. While this happens, the secondary oocyte completes ovulation. A nuclear envelope surrounds both the sperm and egg pronuclei. The basal body of the sperm forms a spindle apparatus of the zygote.

Embryonic Development An embryo goes through 3 development stages ●Cellular ●Tissue ●Organ

Cellular Stages of Development Part 1- Cleavage Cleavage is division without growth. It involves DNA replication and mitosis. The volume of cytoplasm does not change, only the number of cells. The cells get smaller and smaller. 16 uniform cells form a morula. The morula continues dividing until it forms a blastula.

Cellular Stages of Development Part 2- Blastula Formation A blastula is hollow ball of cells with a blastocoel(fluid filled center). Blastula has two main parts Inner cell mass- becomes the embryo Trophoblast- becomes the placenta and aids in enodmetrium attachment In amphibian, reptile, and bird embryos cleavage is not uniform because yolk is present.

Tissue Stages of Development Part 1- Early Gastrula When the embryo attaches to the uterine wall, the inner cell mass divides into: the epiblast (forms germ layers) and hypoblast (forms yolk-produces first blood cells) An early gastrula has 2 layers of cells. ●Ectoderm (outer layer)- nervous system, epidermis and derivatives, epithelial linings of oral cavity and rectum ●Endoderm (inner layer)- epithelial linings of digestive and respiratory tracts and bladder, thyroid glands

Tissue Stages of Development Part 2- Late Gastrula ●Gastrulation is complete when the mesoderm is developed. The mesoderm is the middle layer and forms musculoskeletal, cardiovascular, urinary, reproductive systems The three layers are called the embryonic germ layers.

Organ Stages of Development Mesoderm forms the notochord which is replaced by the vertebral column in chordates. The ectoderm cells above the notochord form the neural plate  neural groove  neural tube  CNS. The nervous system forms from midline ectoderm Midline mesoderm form axial muscles. The endoderm forms a gut tube by folding. Somite- gives rise to muscles and vertebrae in mammals

Extraembryonic Structures 1.Yolk- derived from the hypoblast; site of early blood cells creation in humans. Source of nutrients for bird and reptile embryos. 2.Chorion- formed from the trophoblast; outer membrane of the embryo. Site of implantation onto the endometrium. Contributes to formation of placenta in mammals 3.Allantois- mammalian waste transporter. Later it becomes the umbilical cord which carries oxygen, food, and wastes (including CO 2 ) back and forth from placenta to embryo 4.Amnion- formed from epiblast. Surrounds fluid-filled cushion that protects the developing embryo.

Developmental Processes 1.Growth 2.Cellular Differentiation- cells become specialized in structure and function 3.Morphogenesis- produces shape and form of body a. Pattern formation- how tissues and organs are arranged b. Apoptosis- programmed cell death

Cellular Differentiation Cells are totipotent- each contain instructions needed for every specialized cell type in the body. Scientists believe differential gene expression causes specialization.

Cytoplasmic Segregation The egg cell contains maternal determinants which help influence the path of development. Cytoplasmic segregation helps determine how the morula cells will develop.

Induction Induction is the ability of an embryonic cell to influence the development of another tissue by touch or chemical signaling This works with maternal determinants to influence organ development.

Morphogenesis Pattern formation determines the placement of axes, organs, and appendages. Homeoboxes are conserved genes that act as main switches. Homeotic genes code for transcription factors and determine how the segments of an organism will develop. Apoptosis determines the shape of hands and feet in human development.

Human Embryonic and Fetal Development Conception to parturition is 266 days in humans. Human development can be be divided by embryonic development (months 1 and 2 when major organs are developed) and fetal development (months 3-9 when structures are refined).

Trimesters First trimester- embryonic and early fetal development occur Second trimester- development of organs and organ systems Third trimester- fetus grows rapidly and major organ systems become functional

Embryonic Development Week One- fertilization occurs in the upper third of the oviduct. Cleavage begins 30 hours after fertilization while the cell moves to the uterus. On day three, the embryo is a morula. Day 5 is the transformation into a blastocyst.

Embryonic Development Week Two- Embryo begins implantation into uterus wall. The trophoblast (single layer of cells on blastocyst) secretes human chorionic gonadotropin (HCG) to maintain the endometrium.The inner cell mass detaches from the trophoblast and flattens into a the embryonic disk (endoderm and ectoderm). Gastrulation occurs.

Embryonic Development Week Three- Nervous system and heart development occur Weeks Four and Five- Umbilical cord forms and limb buds appear. Sense organs become more prominent

Embryonic Development Weeks Six through Eight- Head achieves its normal relationship with the body, nervous system allows for reflex actions, and all organs are established. Embryo is the size of an aspirin tablet.

Placenta Structure and Function Placenta functions in gas, nutrient, and waste exchange between embryonic and maternal cardiovascular systems. Chemicals can cross the placenta. Each organ has a “sensitive period” when normal development can be altered.

Fetal Development ●Weight increases 600 times ●Genitalia appears in month 3 ●Initially flexes limbs and nods its head, but eventually moves to avoid discomfort ●Heartbeat can be heard at 16 weeks with a stethoscope ●Can survive at 24 weeks outside the womb

Stages of Birth Stage One- Cervix dialation Stage Two- Baby is born and umbilical cord cut Stage Three- Placenta is delivered.