Developmental Anatomy (Embryology) ANAT 203 Lecture Notes PDF

DEVELOPMENTAL ANATOMY (EMBRYOLOGY) ANAT 203 Lecture # 01 Professor BALA Usman [email protected] Usman BALA, PhD 1  Introduction to Embryology Usman BALA, PhD...

DEVELOPMENTAL ANATOMY (EMBRYOLOGY) ANAT 203 Lecture # 01 Professor BALA Usman [email protected] Usman BALA, PhD 1  Introduction to Embryology Usman BALA, PhD 2  Lecture Outline 1. Definition Embryology and its terminology 2. Understand the process of gastrulation and formation of the trilaminar disc 3. Understand the derivatives of each primary germ layer 4. Understand gametogenesis; oogenesis and spermatogenesis  Introduction The life span is usually considered the period between birth and death; however, the 9 months before birth are a critical part of a person’s existence. What happens in these 9 months profoundly affects the rest of a person’s life. Although most people develop normally and are born without defects (anomalies), approximately 3% of people are born with a birth defect so severe that it requires medical attention during the first year of life. Later in life, many more people discover previously unknown problems, such as the tendency to develop asthma, certain brain disorders, or cancer.  Introduction Embryology (from Greek, embryo, the unborn, and , -logia) Therefore, embryology is the science of the development of an embryo from the fertilization of an ovum to the fetus stage. It may also refer to the structure and development of the embryo of a particular organism. – Embryonic period: from conception (zygote formation) to the third month of development. – Fetal period: from third month of development to birth (266 days post- fertilization): total of about 6 months. – Teratogen: substance that causes abnormality in physiological development  Terminology There are several terms commonly used in discussions of developing humans Several of these terms are used in the Timetable of Human Prenatal Development and Most terms have Latin (L.) or Greek (Gr.) origins.  Terminology Zygote: The earliest development stage of embryo; formed when sperm and egg fused. It is also known as fertilized egg Morula: Compact ball of 16 cells formed on Day 3 Embryoblast: Inner cell mass (ICM), also known as pluriblast. It gives rise to three primary germ layers – Endoderm, Ectoderm, and Mesoderm  Terminology Blastocyst: Forms on Day 4; consists of trophoblast surrounding blastocoele and embryoblast Trophoblast: Cells forming outer layer of blastocyst (Day 4); forms placenta Blastocoele is the cavity of the blastocyst.  Terminology Hypoblast: forms on Day 8 from inner cell mass; They are precursors to extra-embryonic tissues, such as yolk sac, chorion, amnion, allantois Epiblast: forms on Day 8 from ICM; above the hypoblast; They are pluripotent, - meaning they can differentiate into any cell type in the body. – The epiblast undergoes a series of complex cellular movements and differentiations, eventually forming the entire embryo. It gives rise to the three primary germ layers. Terminology  Terminology Amniotic cavity: formed by end of Week 2 from epiblast cells Yolk sac: formed by end of Week 2 from hypoblast cells (ventral); vestigial in humans, though perhaps early nutritional value Yolk sac: Provides nutrients to the embryo Chorion: Forms part of the placenta Amnion: Surrounds the embryo, forming a protective cavity Allantois: Contributes to umbilical cord formation  Terminology Gastrulation: embryonic development phase whereby bilaminar germ disc acquires a third germ layer and becomes Trilaminar Trilaminar germ disc: ectoderm, endoderm, mesoderm (arise during gastrulation) Three primary germ layers: Ectoderm (outer layer): Forms skin, nervous system, hair, and sensory organs. Mesoderm (middle layer): Develops into muscles, bones, connective tissue, and circulatory system. Endoderm (inner layer): Gives rise to internal organs (e.g., lungs, liver, pancreas).  Terminology Foregut: anterior part of alimentary canal, from mouth to duodenum – esophagus, stomach, duodenum, liver, gallbladder, pancreas, spleen) Midgut: part of alimentary canal (forms intestines) between foregut (at opening of bileduct) and hindgut. – Includes latter parts of duodenum, jejunum, ileum, cecum, ppendix, ascending colon, hepatic flexure of colon, transverse colon (proximal2/3) Hindgut: posterior part of alimentary canal – Includes distal 1/3 of transverse colon, splenic flexure, descending colon, sigmoid colon and rectum  Gametogenesis The process by which gametes (sperm and egg cells) are produced in living organisms. Involves the development of immature cells into mature gametes, which are necessary for sexual reproduction. It is complex process that involves multiple stages, including cell division, differentiation, and maturation. Gametogenesis occurs in the following ways: – Spermatogenesis: The production of sperm cells in the testes of males. – Oogenesis: The production of egg cells in the ovaries of females.  Gametogenesis Oogenesis is the process of forming an ovum (egg) by meiosis (in animals, by mitosis in the gametophyte in plants) in specialized gonads known as ovaries. Spermatogenesis is the process of forming sperm cells by meiosis (in animals, by mitosis in plants) in specialized organs known as gonads (in males these are termed testes). After division the cells undergo differentiation to become sperm cells.  Oogenesis Development of a mature ovum (egg cell) in the ovaries of females. The germ cells undergo rapid mitotic division and differentiate into oogonia Stages involved in oogenesis: – Oogonium: The immature cell that develops into an ovum. – Primary follicle: The oogonium surrounded by a layer of cells, forming a follicle. – Growing follicle: The follicle increases in size and the ovum grows.  Oogenesis Stages involved in oogenesis: – Primary oocyte: The ovum within the growing follicle undergoes meiosis (cell division) but arrests in prophase I. – Secondary follicle: The follicle continues to grow and the ovum remains in meiosis. – Tertiary follicle: The largest follicle, containing a mature ovum. – Ovulation: The mature ovum is released from the tertiary follicle into the oviduct. – Mature ovum: The released ovum is now ready for fertilization.  Oogenesis Note: – At birth each female carries a lifetime supply of developing oocytes, each of which is in Prophase I. – A developing egg (secondary oocyte) is released each month from puberty until menopause, a total of 400-500 eggs. – Primary oocytes do not finish the first meiotic division until puberty is reached – Secondary oocyte completes the 2nd meoitic division after fertilization by the sperm in fallopian tube - ovum and second polar body. – In the absence of fertilization the secondary oocyte does not complete the second meiotic division and degenerate  Oogenesis During oogenesis, the ovum undergoes significant changes, including: – Growth and maturation – Meiosis (reduction of chromosome number) – Development of the zona pellucida (a protective layer) – Formation of the corona radiata (a layer of cells surrounding the ovum)  Ovulation Is the process whereby secondary oocyte is released from the ovary following rupture of mature graaafian follicle and become available for conception. Only one secondary oocyte is likely to rupture in each ovarian cycle which starts at puberty and ends in menopause. In relation to the menstrual period event occurs about 14 days prior to the expected period.  Ovulation Menstrual Phase (Days 1-5) – Uterine lining sheds, resulting in menstrual bleeding – Body prepares for a new cycle – Hormone levels drop Follicular Phase (Days 6-12) – Ovaries produce follicles containing eggs – Estrogen levels rise, causing: Uterine lining to thicken Cervical mucus to increase Body temperature to rise slightly  Ovulation Ovulation (Day 14) – Mature egg is released from dominant follicle – Egg travels through fallopian tube – Fertilization can occur\ Luteal Phase (Days 15-28) – Empty follicle produces progesterone – Progesterone prepares uterus for fertilized egg – Uterine lining thickens further – Body temperature rises slightly  Ovulation Premenstrual Phase (Days 26-30) – If egg isn't fertilized: – Hormone levels drop – Uterine lining sheds – Menstruation occurs Ovulation Ovulation  Changes in the follicle before ovulation Graafian follicle becomes enlarged ( 20mm ) Follicular wall near the ovarian surface becomes thinner Increase in cytoplasmic volume Changes in the number and distribution of mitochondria and in the golgi apparatus Completion of arrested first meiotic division occurs with formation of secondary oocyte and first polar body each containing haploid number of chromosomes (23X )  Cause of ovulation Combined FSH/LH mid cycle surge is responsible for the final stage of maturation, rupture of the follicle and expulsion of the oocyte – Follicle-Stimulating Hormone (FSH): FSH stimulates the growth and maturation of follicles in the ovary, preparing them for ovulation. – Estrogen peak: A rapid increase in estrogen levels, produced by the growing follicles in the ovary, triggers the LH surge.  Cause of ovulation – Luteinizing Hormone (LH) surge: The pituitary gland releases a sudden surge of LH, which stimulates the dominant follicle in the ovary to release the egg. – Gonadotropin-Releasing Hormone (GnRH): GnRH is released by the hypothalamus Stimulating the pituitary gland to release LH and FSH.  Cause of ovulation – Hormonal feedback loop: A delicate balance of hormones, including estrogen, progesterone, and inhibin, regulates the ovulatory process.  Structure of a mature ovum Largest cell in the body Consists of cytoplasm and a nucleus with its nucleolus in eccentric position Contains 23 chromosomes (23 X) Surrounded by a cell membrane called a Vitelline membrane Outer transparent mucoprotein envelope is called zona pellucida  Structure of a mature ovum Tiny channels in zona pellucida are for the transport of the materials from the granulosa cells to the oocyte Space between the vitelline membrane and zona pellucida is called perivitelline space which accommodate the polar bodies Oocyte after its escape from the follicle, retains a covering of granulosa cells known as corona radiata derived from the cumulus oophorus  Spermatogenesis Development of spermatids from the primodial male germ cells and their differentiation into spermatozoa In man time required for a spermatogonium to develop into a mature spermatozoon is about 61 days The two phases include: 1. Formation of spermatids (Spermatocytogenesis) 2. Spermiogenesis.  Spermatocytogenesis This phase of spermatogenesis is further subdivided into three phases. 1. Multiplication phase: 2. Growth phase 3. Maturation phase  Spermatocytogenesis Multiplication phase Also known as proliferation phase During this phase the diploid spermatogonia multiply mitotically to form spermatocytes It also to give rise to new spermatogonia (stem cells and enter the phase of growth)  Spermatocytogenesis Growth phase A limited growth of spermatogonium takes place during this phase Their volume becomes double They are now called primary spermatocytes which are still diploid in number. The primary spermatocytes now enters maturation phase.  Spermatocytogenesis Maturation phase The primary spermatocyte enter into the prophase of meiotic – Meiotic prophase is a very complex process characterised; – An ordered series of chromosal rearrangements – First nuclear DNA duplicates, – Chromosomes of each homologous pair (tetrad) migrate towards opposite poles of the primary Spermatocytogenesis  Spermiogenesis The final stage of spermatogenesis, Morphological changes occurs that transform the round spermatids into the mature spermatozoa The two major parts of the sperm - the head and tail are formed during this process  Spermiogenesis Head formation Changes in the nucleus – the nucleus of spermatid shrinks by losing much of its – chromosomes become closely packed into a small volume – The nucleus of the spermatids is compacted and acquires an oval shape. – The spherical shape of the nucleus also becomes elongated and narrow. Acrosome formation: – The acrosome is a cap covering the head’s apical area. – It contains hydrolytic enzymes which, after the acrosomal reaction, will allow the sperm to pass through the zona pellucida and fertilize the egg.  Spermiogenesis Tail formation Reduction of the cytoplasm Most cytoplasm is eliminated - by its displacement towards the end piece of the flagellum, originating the so-called cytoplasmic drop, The later eliminated at the end of the sperm maturation process in the tail of the epididymis. The presence of spermatozoa with cytoplasmic droplet in an ejaculate is a sign of immaturity and is classified as a secondary malformation. Formation of Spermatids  Structure of a mature spermatozoon The sperm consists of the; Head Neck Middle piece Tail The entire body of the sperm is covered by a plasma membrane.  Structure of a mature spermatozoon The head consists of: Nucleus - contains a haploid (23) chromosomes A cap-like structure called the acrosome which contains digestive enzymes that help in the fertilization of the ovum.  Structure of a mature spermatozoon The middle piece consists of many mitochondria which provide energy to the sperm for locomotion, it also contains the centrioles. The tail is for the motility of the sperm.  Thank You For Listening QUESTIONS Usman BALA, PhD 45