Animal Development Chapter 50

Learning Objective 1 Analyze the relationship between cell determination and cell differentiation, and between pattern formation and morphogenesis

Cell Differentiation Process by which a cell becomes specialized to carry out specific functions Cell determination molecular events leading to cell differentiation Stem cells relatively undifferentiated cells

Morphogenesis The development of form In pattern formation, cells may occurs through pattern formation In pattern formation, cells may communicate by signaling migrate undergo changes in shape undergo apoptosis (programmed cell death)

KEY CONCEPTS The development of form requires not only cell division and growth but also cell determination and cell differentiation, and pattern formation and morphogenesis

Learning Objective 2 Relate differential gene expression to nuclear equivalence

Gene Expression Principle of nuclear equivalence Usually no genetic changes occur in cell determination and cell differentiation Differential gene expression Various types of differentiated cells express different subsets of their genes

Learning Objective 3 Describe the four processes involved in fertilization

Fertilization 1. Contact and recognition 2. Sperm entry is regulated between noncellular egg coverings and sperm 2. Sperm entry is regulated prevents interspecific fertilization prevents polyspermy (fertilization of egg by more than one sperm)

Fertilization 3. Fertilization activates egg triggering events of early development 4. Sperm and egg pronuclei fuse initiates DNA synthesis

Fertilization SF SM FC SN 1 µm Fig. 50-1, p. 1082 Figure 50.1: Fertilization. In this TEM, a fertilization cone (FC) forms as a sperm enters a sea urchin egg. (SN, sperm nucleus; SM, sperm mitochondrion; SF, sperm flagellum) 1 µm Fig. 50-1, p. 1082

KEY CONCEPTS Fertilization includes contact and recognition between egg and sperm, regulated sperm entry, activation of the fertilized egg, and fusion of egg and sperm pronuclei

Learning Objective 4 Describe fertilization in echinoderms Point out some ways in which mammalian fertilization differs

Egg Coverings Echinoderms In mammals vitelline envelope jelly coat zona pellucida

Acrosome Reaction Facilitates penetration of egg coverings when sperm first contacts egg In mammals, acrosome reaction is preceded by capacitation maturation process results in ability of sperm to fertilize egg

Effects of Capacitation on Sperm Increased rate of metabolism Flagellum beats more rapidly; Result: Sperm are more motile (hyperactivated) Changes in sperm glycoproteins Allow sperm-egg binding Pro-Acrosin (inactive) is converted to acrosin (active) Able to digest zona pellucida proteins

Capacitation These are monitor screen images from an instrument which records the movement paths of the sperm cells heads (white points) during a certain time span and displays them with a green line. UPPER PANEL: Before capacitation the majority of the lines are straight. LOWER PANEL: After capacitation almost all the sperm cells have now gone over to swinging their heads strongly as indicated by the jagged lines.

Polyspermy Echinoderms In mammals sea urchin fertilization is followed by a fast block to polyspermy (depolarization of plasma membrane) and a slow block to polyspermy (cortical reaction) In mammals changes in zona pellucida prevent polyspermy

http://biology.kenyon.edu/courses/biol114/Chap13/Chapter_13B.html

Slow Block to Polyspermy The left image shows the approach of the sperm at about 2 o'clock and the rising of the vitelline membrane. Intracellular Ca++ is monitored by an indicator that becomes more fluorescent when it binds Ca++. Slow Block

Cortical Reaction

Learning Objective 5 Trace the generalized pattern of early development of the embryo from zygote through early cleavage and formation of the morula and blastula

The Zygote Undergoes cleavage a series of rapid cell divisions without a growth phase partitions zygote into many small blastomeres

Cleavage Morula a solid ball of cells Blastula a hollow ball of cells

KEY CONCEPTS Cleavage, a series of rapid cell divisions without growth, provides cellular building blocks for development

Learning Objective 6 Contrast early development, including cleavage in the echinoderm (or in amphioxus), the amphibian, and the bird, paying particular attention to the importance of the amount and distribution of yolk

Invertebrates and Simple Chordates Have isolecithal eggs (evenly distributed yolk) undergo holoblastic cleavage (division of entire egg)

Cleavage in Sea Stars

Nucleus 100 µm 50 µm 50 µm (a) Unfertilized egg (b) 2-cell stage (c) Figure 50.3: LMs showing sea star development. (a) The isolecithal egg has a small amount of uniformly distributed yolk. (b–e) The cleavage pattern is radial and holoblastic (the entire egg becomes partitioned into cells). (f, g) The three germ layers form during gastrulation. The blastopore is the opening into the developing gut cavity, the archenteron. The rudiments of organs are evident in the sea star larva (h) and the young sea star (i). All views are side views with the animal pole at the top, except (c) and (i), which are top views. Note that the sea star larva is bilaterally symmetrical, but differential growth produces a radially symmetrical young sea star. 100 µm 50 µm 50 µm (a) Unfertilized egg (b) 2-cell stage (c) 4-cell stage Fig. 50-3 (a-c), p. 1084

Blastocoel Archenteron Blastopore 50 µm 50 µm 50 µm (f) Early gastrula Figure 50.3: LMs showing sea star development. (a) The isolecithal egg has a small amount of uniformly distributed yolk. (b–e) The cleavage pattern is radial and holoblastic (the entire egg becomes partitioned into cells). (f, g) The three germ layers form during gastrulation. The blastopore is the opening into the developing gut cavity, the archenteron. The rudiments of organs are evident in the sea star larva (h) and the young sea star (i). All views are side views with the animal pole at the top, except (c) and (i), which are top views. Note that the sea star larva is bilaterally symmetrical, but differential growth produces a radially symmetrical young sea star. Archenteron Blastopore 50 µm 50 µm 50 µm (f) Early gastrula (d) 16-cell stage (e) Blastula Fig. 50-3 (d-f), p. 1084

Archenteron Mouth Anus Stomach Blastopore 1 mm 50 µm 50 µm (h) Figure 50.3: LMs showing sea star development. (a) The isolecithal egg has a small amount of uniformly distributed yolk. (b–e) The cleavage pattern is radial and holoblastic (the entire egg becomes partitioned into cells). (f, g) The three germ layers form during gastrulation. The blastopore is the opening into the developing gut cavity, the archenteron. The rudiments of organs are evident in the sea star larva (h) and the young sea star (i). All views are side views with the animal pole at the top, except (c) and (i), which are top views. Note that the sea star larva is bilaterally symmetrical, but differential growth produces a radially symmetrical young sea star. 1 mm 50 µm 50 µm (h) Sea star larva (i) Young sea star (g) Middle gastrula Fig. 50-3 (g-i), p. 1084

Cleavage in Amphioxus

Polar body Figure 50.4: Cleavage and gastrulation in amphioxus. As in the sea star, cleavage is holoblastic and radial. The embryos are shown from the side. (a) Mature egg with polar body. (b–e) The 2-, 4-, 8-, and 16-cell stages. (f) Embryo cut open to show the blastocoel. (g) Blastula. (h) Blastula cut open. (i) Early gastrula showing beginning of invagination at vegetal pole. (j) Late gastrula. Invagination is completed, and the blastopore has formed. Fig. 50-4 (a-d), p. 1085

Blastocoel Fig. 50-4 (e-g), p. 1085 Figure 50.4: Cleavage and gastrulation in amphioxus. As in the sea star, cleavage is holoblastic and radial. The embryos are shown from the side. (a) Mature egg with polar body. (b–e) The 2-, 4-, 8-, and 16-cell stages. (f) Embryo cut open to show the blastocoel. (g) Blastula. (h) Blastula cut open. (i) Early gastrula showing beginning of invagination at vegetal pole. (j) Late gastrula. Invagination is completed, and the blastopore has formed. Fig. 50-4 (e-g), p. 1085

Archenteron Ectoderm Endoderm Blastopore Fig. 50-4 (h-j), p. 1085 Figure 50.4: Cleavage and gastrulation in amphioxus. As in the sea star, cleavage is holoblastic and radial. The embryos are shown from the side. (a) Mature egg with polar body. (b–e) The 2-, 4-, 8-, and 16-cell stages. (f) Embryo cut open to show the blastocoel. (g) Blastula. (h) Blastula cut open. (i) Early gastrula showing beginning of invagination at vegetal pole. (j) Late gastrula. Invagination is completed, and the blastopore has formed. Endoderm Blastopore Fig. 50-4 (h-j), p. 1085

Amphibians Have moderately telolecithal eggs concentration of yolk at vegetal pole slows cleavage (only a few large cells form) large number of smaller cells form at the animal pole

Cleavage in Frogs Animal pole Vegetal pole

Reptiles and Birds Have highly telolecithal eggs large concentration of yolk at one end undergo meroblastic cleavage (restricted to the blastodisc)

Cleavage in Birds

Blastodisc Yolk Fig. 50-7a, p. 1086 Figure 50.7: Cleavage in a bird embryo. Fig. 50-7a, p. 1086

Epiblast Hypoblast Blastocoel Yolk Fig. 50-7b, p. 1086 Figure 50.7: Cleavage in a bird embryo. Fig. 50-7b, p. 1086

Cytoplasm Distribution Animals whose zygotes have relatively homogeneous cytoplasm exhibit regulative development (embryo develops as a self-regulating whole) Animals with unequal distribution of cytoplasmic components have relatively rigid developmental patterns (mosaic development)

The Gray Crescent Of an amphibian zygote determines body axis of embryo

Cytoplasmic Determinants First cleavage (control) in Frogs First cleavage (control) (a) The first cleavage is allowed to occur normally, and each separated blastomere includes half of the gray crescent. Figure 50.8: Cytoplasmic determinants in frog development. Fig. 50-8a, p. 1087

First cleavage (experimental) (b) The plane of cleavage is altered by the experimenter such that only one blastomere contains the gray crescent. Figure 50.8: Cytoplasmic determinants in frog development. Fig. 50-8b, p. 1087

Learning Objective 7 Identify the significance of gastrulation in the developmental process, and compare gastrulation in the echinoderm (or in amphioxus), the amphibian, and the bird

Germ Layers During gastrulation, basic body plan is laid down as 3 germ layers 1. outer ectoderm 2. middle mesoderm 3. inner endoderm

The Archenteron Forerunner of digestive tube Blastopore in some groups opening to the exterior

Gastrulation 1 In sea star and amphioxus cells from blastula wall invaginate eventually meet opposite wall, forming archenteron

Gastrulation 2 In the amphibian invagination at vegetal pole obstructed by large, yolk-laden cells cells from animal pole move down over yolk-rich cells and invaginate, forming dorsal lip of the blastopore

Gastrulation 3 In the bird invagination occurs at the primitive streak no archenteron forms

Learning Objective 8 Define organogenesis Summarize the fate of each of the germ layers

Organogenesis 1 The process of organ formation Ectoderm becomes nervous system sense organs outer layer of skin (epidermis)

Organogenesis 2 Mesoderm becomes Endoderm becomes notochord skeleton muscles circulatory system inner layer of skin (dermis) Endoderm becomes lining of the digestive tube

KEY CONCEPTS Gastrulation establishes ectoderm, mesoderm, and endoderm; each gives rise to specific types of tissues

Learning Objective 9 Trace the early development of the vertebrate nervous system

Early Vertebrate Nervous System Development Developing notochord is responsible for induction causes ectoderm to differentiate and form central nervous system Brain and spinal cord develop from the neural tube

Human Nervous System Development

KEY CONCEPTS Neurulation, the origin of the central nervous system, is one of the earliest events in organogenesis (organ development)

Learning Objective 10 Give the origins and functions of the chorion, amnion, allantois, and yolk sac

Extraembryonic Membranes 1 Chorion derived from ectoderm and mesoderm used in gas exchange Amnion fluid-filled sac that surrounds the embryo keeps embryo moist acts as a shock absorber

Extraembryonic Membranes 2 Allantois derived from endoderm and mesoderm stores nitrogenous wastes Yolk sac makes food available to the embryo

4 day-old chick Yolk Chorion Yolk sac Fig. 50-13a, p. 1091 Figure 50.13: Extraembryonic membranes. The formation of the extraembryonic membranes of the chick is illustrated at (a) 4 days, (b) 5 days, and (c) 9 days of development. Each of the membranes develops from a combination of two germ layers. The chorion and amnion form from lateral folds of the ectoderm and mesoderm that extend over the embryo and fuse. The allantois and the yolk sac develop from endoderm and mesoderm. Chorion Yolk sac Fig. 50-13a, p. 1091

5 day-old chick Chorion Allantois Yolk Amnion Amniotic cavity Yolk sac Figure 50.13: Extraembryonic membranes. The formation of the extraembryonic membranes of the chick is illustrated at (a) 4 days, (b) 5 days, and (c) 9 days of development. Each of the membranes develops from a combination of two germ layers. The chorion and amnion form from lateral folds of the ectoderm and mesoderm that extend over the embryo and fuse. The allantois and the yolk sac develop from endoderm and mesoderm. Amniotic cavity Yolk sac Fig. 50-13b, p. 1091

9 day-old chick Amniotic cavity Amnion Chorion Allantois Yolk sac Figure 50.13: Extraembryonic membranes. The formation of the extraembryonic membranes of the chick is illustrated at (a) 4 days, (b) 5 days, and (c) 9 days of development. Each of the membranes develops from a combination of two germ layers. The chorion and amnion form from lateral folds of the ectoderm and mesoderm that extend over the embryo and fuse. The allantois and the yolk sac develop from endoderm and mesoderm. Yolk sac Allantois Fig. 50-13c, p. 1091

KEY CONCEPTS Extraembryonic membranes (amnion, chorion, allantois, and yolk sac) have evolved in terrestrial vertebrates as adaptations to reproduction on land

Learning Objective 11 Describe the general course of early human development, including fertilization, the fates of the trophoblast and inner cell mass, implantation, and the role of the placenta

Early Human Development Fertilization occurs in the oviduct Cleavage takes place as embryo is moved down the oviduct Blastocyst develops in the uterus

Cleavage in Humans

(a) Male and female pronuclei prior to fusion Figure 50.14: Cleavage in a human embryo. (From Lennart Nilsson, Being Born, pp. 14, 15, 17. Putnam Publishing Group, 1992.) 50 µm (a) Male and female pronuclei prior to fusion Fig. 50-14a, p. 1092

24 hours 50 µm (b) 2-cell stage Fig. 50-14b, p. 1092 Figure 50.14: Cleavage in a human embryo. (From Lennart Nilsson, Being Born, pp. 14, 15, 17. Putnam Publishing Group, 1992.) 24 hours 50 µm (b) 2-cell stage Fig. 50-14b, p. 1092

50 µm (c) 8-cell stage Fig. 50-14c, p. 1092 Figure 50.14: Cleavage in a human embryo. (From Lennart Nilsson, Being Born, pp. 14, 15, 17. Putnam Publishing Group, 1992.) 50 µm (c) 8-cell stage Fig. 50-14c, p. 1092

(d) Cleavage continues giving rise to a morula Figure 50.14: Cleavage in a human embryo. (From Lennart Nilsson, Being Born, pp. 14, 15, 17. Putnam Publishing Group, 1992.) ~ 3 days 50 µm (d) Cleavage continues giving rise to a morula Fig. 50-14d, p. 1092

Blastocyst Outer trophoblast Inner cell mass gives rise to chorion and amnion Inner cell mass becomes the embryo proper Blastocyst undergoes implantation in the endometrium

The Placenta Organ of exchange Umbilical cord between maternal and fetal circulation derives from embryonic chorion and maternal tissue Umbilical cord connects embryo to placenta

Uterine epithelium Uterine gland Uterine blood vessel Trophoblast Inner cell mass Fertilization Inner cell mass Implantation Endometrium Figure 50.15: Implantation and early development in the uterus. Trophoblast Uterine gland Uterine blood vessel (a) 7 days. Fig. 50-15a, p. 1093

Healing site of implantation Uterine epithelium Yolk sac Chorion Figure 50.15: Implantation and early development in the uterus. Embryonic disc Maternal vessel Amniotic cavity (b) 10 days. Fig. 50-15b, p. 1093

Site of implantation Epithelium of uterus Embryonic disc 0.1 mm Figure 50.15: Implantation and early development in the uterus. 0.1 mm (c) 12 days. Fig. 50-15c, p. 1093

Chorionic villi (area of future placenta) Maternal blood Amniotic cavity Amnion Embryo Yolk sac Figure 50.15: Implantation and early development in the uterus. Chorionic cavity Chorionic villi (area of future placenta) Maternal blood (d) 25 days. Fig. 50-15d, p. 1093

Umbilical arteries and vein Chorion Amnion Umbilical arteries and vein Figure 50.15: Implantation and early development in the uterus. Umbilical cord Amniotic cavity Placenta (e) 45 days. Fig. 50-15e, p. 1093

2nd Month of Development

Fetus (12 to 20 Weeks)