Animal DevelopmentChapter 44

FertilizationThe union of sperm and egg to form a zygote.

It requires three main events occur.

Stage 1- AttachmentSperm 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 PolyspermyThe 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 DiploidThe 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 DevelopmentAn embryo goes through 3 development stages● Cellular ● Tissue● Organ

Cellular Stages of Development

Part 1- CleavageCleavage 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 DevelopmentPart 2- Blastula FormationA 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 DevelopmentPart 1- Early GastrulaWhen 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 DevelopmentPart 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 plateneural 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 Structures1. 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 CO2) back and forth from placenta to embryo

4. Amnion- formed from epiblast. Surrounds fluid-filled cushion that protects the developing embryo.

Developmental Processes1. Growth2. Cellular Differentiation- cells become

specialized in structure and function3. Morphogenesis- produces shape and form of

bodya. Pattern formation- how tissues and organs are

arrangedb. Apoptosis- programmed cell death

Cellular DifferentiationCells 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).

TrimestersFirst trimester- embryonic and early fetal development occurSecond trimester- development of organs and organ systemsThird trimester- fetus grows rapidly and major organ systems become functional

Embryonic DevelopmentWeek 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 DevelopmentWeek 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 DevelopmentWeek Three- Nervous system and heart development occur

Weeks Four and Five- Umbilical cord forms and limb buds appear. Sense organs become more prominent

Embryonic DevelopmentWeeks 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 BirthStage One- Cervix dialationStage Two- Baby is born and umbilical cord cutStage Three- Placenta is delivered.