Specific learning objectives:
By the end of this session you should be able to:
- Define Embryology and general terminology
- Quote references from Quran to verify embryology
- Define GAMETOGENESIS, comprehend its objectives & its differences in the two sexes
- Explain the process of SPERMATOGENESIS
- Human Embryology is that branch of medical sciences, which deals with the study of human development before birth.
- Developmental Anatomy
- From a single cell to a baby in 9 months.. developmental process that represents an amazing integration of increasingly complex phenomena.
- Union of a male and a female gamete in the genital tract of a female results in the onset of pregnancy and marks the beginning of development of a new individual.
- In general terms, the event of a woman becoming pregnant is called conception.
- However, in technical terms, the union of male and female gametes is called fertilization.
- It results in the formation of a single cell called fertilized ovum or zygote.
- The zygote progressively transforms into a multicellular mature member of its species.
- These progressive changes are collectively called development.
- The complete developmental history of an individual organism is called ontogeny.
- The human development (ontogeny) is divided into two main parts:
(1) Prenatal development, i.e., development before birth, and
(2) Postnatal development, i.e., development after birth.
- The prenatal development normally occurs within the uterus of the female; for the mother it forms the period of pregnancy or gestation, and for the developing individual it constitutes the intra- uterine life (IUL).
- Taking the day of fertilization as day 0, the average length of intra-uterine life of a developing human is 266 days (38 weeks).
- First 8 weeks of human development—– the period of embryogenesis or the period of organogenesis ——-during which time different organ primordia (beginnings) are established, the developing human is known as embryo.
- From 9th week on until birth —— the fetal period——— a time when differentiation continues while the fetus grows and gains weight, the developing individual is referred to as fetus.
- Conceptus is another title that is given to the developing human at all stages of development and includes extra-embryonic membranes as well as the embryo or fetus.
- Teratology—- the study of the embryological origins and causes for birth defects or development of malformations or serious deviations from the normal develpmental process
- Obstetricians commonly divide the nine months of gestation into three equal parts, each part consisting of three calendar months.
- The initial three months of pregnancy are known as the 1st trimester, the middle three months as the 2nd trimester, and the last three months as the 3rd trimester.
EMBRYOLOGY IN THE HOLY QUR’AN
إِنَّا خَلَقْنَا الإِنسَانَ مِنْ نُطْفَةٍ أَمْشَاجٍ نَبْتَلِيهِ فَجَعَلْنَاهُ سَمِيعًا بَصِيرً))
“Verily We created man from a drop of a mingled fluid-drop (nutfa amshaj), in order to try him: so We gave him (the gifts), of hearing and sight.” (76:2).
“Nutfa” in Arabic means “very little water”
“Amshaj” means “mingling ” as man’s water mingles with that of the woman
(( أَيَحْسَبُ الْإِنسَانُ أَن يُتْرَكَ سُدًى ، أَلَمْ يَكُ نُطْفَةً مِّن مَّنِيٍّ يُمْنَى ) (القيامة 36-37
Does man think that he will be left uncontrolled (without purpose)? Was he not once a fluid-drop of ejected semen? (75:37-37).
( الَّذِي أَحْسَنَ كُلَّ شَيْءٍ خَلَقَهُ وَبَدَأَ خَلْقَ الْإِنسَانِ مِن طِينٍ . ثُمَّ جَعَلَ نَسْلَهُ مِن سُلَالَةٍ مِّن مَّاء مَّهِينٍ ) (السجدة :7-8).
“He It is Who created all things in the best way and
began the creation of man from clay, and made his progeny from an extract of despised (disliked) fluid (Sulalah) ” (32: 7-8)
( مَّا لَكُمْ لَا تَرْجُونَ لِلَّهِ وَقَارًا .وَقَدْ خَلَقَكُمْ أَطْوَارًا )
( نوح 13-14)
“What is the matter with you, that ye are not conscious of Allah’s majesty,- Seeing that it is He that has created you in diverse stages” (71: 13-14)
( فَلْيَنظُرِ الْإِنسَانُ مِمَ خُلِقَ. خُلِقَ مِن مَّاء دَافِقٍ) (الطارق 5-6)
“So, let man consider of what he was created . He was created of gushing water ” (86: 5-6)
What is the significance of embryology ?????
- Satisfy curiosity about the beginning of human individual
- Help to understand the normal relationships in gross anatomy and causes of variation
- Help obstetrician
- Help peadiatician
- The process of formation of gametes is called gametogenesis.
- In the male the process of gametogenesis is known as spermatogenesis, while in the female it is called oogenesis.
- The gametes develop from the primordial germ cells (also called primitive germ cells) PGCs.
- In the male the primordial germ cells differentiate into spermatogonia, whereas in the female into oogonia.
- The primitive germ cells originate from epiblast during second week
- Move through primitive streak during third week & migrate to the wall of yolk sac
- During week 4, these cells begin to migrate towards the developing gonads, where they arrive by the end of week five.
- Within the gonads the primordial germ cells develop further and ultimately give rise to gametes.
Purpose of gametogenesis
- To achieve the following two objectives:
1. To reduce the number of chromosomes from diploid (i.e., 46) to haploid (i.e., 23). This is achieved through meiotic cell division.
2. To change the shape and size of the gametes so as to make them suitable for fertilization.
- Meiosis is a specialized process of the cell division which occurs only in developing germ cells.
- It results in the formation of four daughter cells from one ancestor cell.
- The basic pattern of gametogenesis is the same in the male and the female.
- The main differences are:
1. The timing of different processes of gametogenesis are different in the two sexes.
2. In the male each primordial germ cell normally gives rise to four functional gametes, whereas in the female only one functional gamete is obtained from one primordial germ cell.
3. In the female the functional cell resulting from the second meiotic division serves as the final mature gamete, while in the male the cells obtained as a result of meiosis-II have to pass through a further stage of transformation to become the final functional gametes, this stage of transformation is called spermiogenesis.
- The spermatozoa develop from male germ cells.
- In the testes, they increase in number by mitotic division and finally settle down in the sex cords as large pale cells surrounded by supporting cells(derived from surface epithelium of testes).
- They remain dormant during rest of the intrauterine life and also during infancy and childhood of the individual.
- Shortly before puberty, sex cords acquire lumen and become seminiferous tubules.
- At puberty, the interstitial cells of the testes begin to secrete testosterone; under the influence of this hormone the process of spermatogenesis is started, and PGCs give rise to spermatogonial stem cells
- With the onset of puberty the spermatogonia begin to divide mitotically and soon three varieties of male primordial germ cells can be identified in the seminiferous tubules:
- (i) type A dark spermatogonia,
- (ii) type A pale spermatogonia, and
- (iii) type B spermatogonia.
- Each type A dark spermatogonium contains a darkly staining nucleus with an eccentrically located nucleolus.
- These cells are considered to be the reserve cells.
- They occasionally divide mitotically to maintain their own number and to give rise to type A pale spermatogonia.
- Each type A pale spermatogonium contains a lightly staining nucleus with an eccentric nucleolus.
- These cells divide by mitosis regularly to give rise to other type A pale spermatogonia as well as type B spermatogonia.
- Each type B spermatogonium (12µm)possesses a darkly staining nucleus with a centrally placed nucleolus.
- Each type B spermatogonium divides mitotically a few times to produce a number of daughter cells which do not divide further but enlarge to become primary spermatocytes (which constitute the next stage of the spermatogenesis & are the largest (16-18µm) germ cells in seminiferous tubules).
- The spermatogonia and the early spermatocytes have the same chromosomal configuration as that of all ordinary somatic cell (i.e., diploid and 2n).
- However, each primary spermatocyte replicates its DNA (4n) and divides by meiosis-I (prolonged prophase of 22 days followed by rapid completion of meiosis-I) to give rise to two daughter cells called secondary spermatocytes (which are haploid but 2n cells).
- Each secondary spermatocyte undergoes meiosis-II to produce two daughter cells called spermatids.
- Each of the four spermatids (resulting from meiosis-II) is a haploid, 1n cell.
- The spermatids come to lie within grooves on the luminal surface of the Sertoli cells where they enter the next stage of spermatogenesis which is called spermiogenesis.
- A special feature of spermatogenesis
- With each successive nuclear division, the progeny cells of the individual type A spermatogonia remain attached to one another by thin intercellular bridges.
- It is claimed that these bridges serve to maintain cytoplasmic continuity between the members of entire clone of differentiating progeny cells, due to which these cells proliferate and differentiate in a synchronized manner and only the spermatozoon stage becomes a completely independent cell.